c2ee21818f

Dynamic Article LinksC<Energy &Environmental Science

Cite this: Energy Environ. Sci., 2012, 5, 8075

www.rsc.org/ees REVIEW

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View Article Online / Journal Homepage / Table of Contents for this issue

A review on nanomaterials for environ
mental remediation
Mya Mya Khin,a A. Sreekumaran Nair,*b V. Jagadeesh Babu,b Rajendiran Murugana

and Seeram Ramakrishna*ab

Received 31st March 2012, Accepted 24th May 2012

DOI: 10.1039/c2ee21818f

This article gives an overview of the application of nanomaterials in environmental remediation. In the

area of environmental remediation, nanomaterials offer the potential for the efficient removal of

pollutants and biological contaminants. Nanomaterials in various shapes/morphologies, such as

nanoparticles, tubes, wires, fibres etc., function as adsorbents and catalysts and their composites with

polymers are used for the detection and removal of gases (SO2, CO, NOx, etc.), contaminated chemicals

(arsenic, iron, manganese, nitrate, heavy metals, etc.), organic pollutants (aliphatic and aromatic

hydrocarbons) and biological substances, such as viruses, bacteria, parasites and antibiotics.

Nanomaterials show a better performance in environmental remediation than other conventional

techniques because of their high surface area (surface-to-volume ratio) and their associated high

reactivity. Recent advances in the fabrication of novel nanoscale materials and processes for the

treatment of drinking water and industrial waste water contaminated by toxic metal ions,

radionuclides, organic and inorganic solutes, bacteria and viruses and the treatment of air are

highlighted. In addition, recent advances in the application of polymer nanocomposite materials for the

treatment of contaminants and the monitoring of pollutants are also discussed. Furthermore, the

research trends and future prospects are briefly discussed.

Introduction

The rapid pace of industrialization and its resulting by-products

have affected the environment by producing hazardous wastes

aDepartment ofMechanical Engineering, National University of Singapore,117574, SingaporebNUS Centre for Nanofibres and Nanotechnology (NUSCNN),Healthcare and Energy Materials Laboratory, National University ofSingapore, 117584, Singapore. E-mail: [email protected]; [email protected]

Broader context

Environmental pollution is a global menace and the magnitude of

trialization and the changing lifestyles of people. In view of this, pro

a challenging task. The advent of nanotechnology has given imm

nanomaterials with large surface-to-volume ratios (and hence exc

pollutants. The nanomaterials play major roles in environmental r

natural waters, soils, sediments, industrial and domestic waste water

gives an extensive view of the roles of nanomaterials in environme

metal oxide nanomaterials, dendrimers, carbon nanomaterials an

catalysis) and sorption are discussed in detail in addition to water pu

section on nanomaterials in the sensing of heavy metals and poiso

reference guide for scientists in the area and the background infor

solutions for environmental remediation.

This journal is ª The Royal Society of Chemistry 2012

and poisonous gas fumes and smokes, which have been released

to the environment. Conventional technologies have been used to

treat all types of organic and toxic waste by adsorption, bio-

logical oxidation, chemical oxidation and incineration. Super-

critical water oxidation (SCWO) has been proposed as a

technology capable of destroying a wide range of organic

hazardous waste. It has been receiving attention due to its ability

to destroy a large variety of high-risk wastes resulting from

munitions demilitarization and complex industrial chemical

processing. In the concentration range of 1% to 20% of organic

it is increasing day-by-day due to urbanization, heavy indus-

viding clean air and water and a clean environment for people is

ense scope and opportunities for the fabrication of desired

ellent chemical reactivities) and unique functionalities to treat

emediation and are used for purposes such as the treatment of

, mine tailings and the polluted atmosphere. The present review

ntal remediation. Environmental remediation using metal and

d polymer nanocomposites by chemical degradation (photo-

rification by nanofibre media. The review article also features a

nous gases. We believe that this in-depth review will serve as a

mation will help in fuelling further innovations on sustainable

Energy Environ. Sci., 2012, 5, 8075–8109 | 8075

http://dx.doi.org/10.1039/c2ee21818f






http://pubs.rsc.org/en/journals/journal/EE

http://pubs.rsc.org/en/journals/journal/EE?issueid=EE005008

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pollutants, SCWO is far less costly than incineration or active

carbon treatment. In parallel, the rapid growth of nanotech-

nology has gained a great deal of interest in the environmental

Mya Mya Khin

Mya Mya Khin is currently

working as a research engineer at

the National University of Singa-

pore (NUS). She finished her oral

defence recently and she is on

course to obtain her PhD degree

fromNUS.ShegraduatedwithM.

Eng. degree from NUS in Chem-

ical Engineering. She worked as a

lecturer and scientist at Ngee Ann

Polytechnic and Food & Nutrition

in Singapore (2008–2011). Her

current research involves environ-

mental remediation using electro-

spun nanofibres.

A: Sreekumaran Nair

Dr A. Sreekumaran Nair worked

as a research fellow (2008–2012)

at the Healthcare and Energy

Materials (HEM) Laboratory of

the National University of Singa-

pore. He graduated with a PhD in

Chemistry from the Indian Insti-

tute of Technology (IIT),Madras

(2006)and subsequently becamea

JSPS postdoctoral fellow (2006–

2008) in Japan. His research

interests include the fabrication of

materials for energy conversion

and storage, catalysts for solar

hydrogen, nanotechnology-based

environmental remediation and

probing the charge transport mechanism in monolayer-protected clus-

ters and 3-D superlattices.

V: Jagadeesh Babu

DrV.JagadeeshBabu is currently a

research fellow at the NUS Centre

for Nanofibres & Nanotechnology

(NUSCNN) of the National

University of Singapore. He gradu-

atedwith aPhD inPhysics from the

Indian Institute of Technology

(IIT), Madras (2008) and subse-

quently became a research assistant

in the Department of Physics IIT-

Madras from 2008–2009. From the

year 2009 to January 2010, he

worked as a postdoctoral fellow in

theGwangjuInstituteofScienceand

Technology (GIST), Gwangju,

South Korea. His research interests

include the fabrication of electrospun nanostructured materials for photo-

catalysis, photovoltaics, the splitting of water for hydrogen energy and

probing electrical charge transport in conducting polymers.

8076 | Energy Environ. Sci., 2012, 5, 8075–8109

applications of nanomaterials. The treatment of pollutants in

water and air is a great challenge and nanomaterials are impor-

tant for the environmental remediation. Nanomaterials are

excellent adsorbents, catalysts and sensors due to their large

specific surface areas and high reactivities. The high surface area-

to-mass ratio of nanomaterials can greatly improve the adsorp-

tion capacities of sorbent materials. Due to its reduced size, the

surface area of nanomaterials grows exponentially at the same

density as the diameter shrinks. In addition, the mobility of

nanomaterials in solution is high and the whole volume can be

quickly scanned with small amounts of nanomaterials due to

their small size. Because of their reduced size and large radii of

curvature, the nanomaterials have a surface that is especially

reactive (mainly due to the high density of low-coordinated

atoms at the surface, edges and vortices). These unique proper-

ties can be applied to degrade and scavenge pollutants in water

and air.1 The species adsorbed onto the nanomaterials can be

removed by applying mild (and affordable) gravitational

(centrifugal) or magnetic force (in the case of magnetic

Rajendiran Murugan

Dr Rajendiran Murugan is

currently a research fellow at the

NUSCNN of the National

University of Singapore. He

graduated with a PhD in organic

chemistry (CLRI–CSIR Lab)

from the University of Madras

and subsequently became an

associate scientific manager at

the Synthetic Chemistry Depart-

ment in Biocon. His research

interests include nanocatalysis,

the synthesis of small/conjugated

molecules and the fabrication of

chemosensors.

Seeram Ramakrishna

Prof. Seeram Ramakrishna,

FREng, FNAE, FAAAS is a

Professor of Materials Engi-

neering and Director of the HEM

Labs (http://serve.me.nus.edu.sg/

seeram_ramakrishna/) at the

National University of Singapore.

He is an acknowledged global

leader for his pioneering work on

the science and engineering

of nanofibres (http://research

analytics.thomsonreuters.com/m/

pdfs/grr-materialscience.pdf). He

has authored five books and five

hundred peer reviewed papers,

which attracted�16 000 citations

and h-index of 65. Various international databases includingThomson

Reuters Web of Science, Elsevier Scopus and Microsoft Academic

place him among the top twenty five authors and most cited materials

scientists in the world. He is an elected International Fellow ofMajor

Professional Societies in Singapore, ASEAN, India, the UK and the

USA.



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nanoparticles). Nanomaterials in various shapes/morphologies/

forms have a significant impact on water and air quality in the

natural environment.2 Magnetic nanoadsorbents are particularly

attractive as they can be easily retained and separated from

treated water.3Nanomaterials also have different distributions of

reactive surface sites and disordered surface regions. In addition,

several natural and engineered nanomaterials have also been

proved to have strong antimicrobial properties, including chi-

tosan, silver nanoparticles (nAg), photocatalytic TiO2, and

carbon nanotubes (CNT).4–8 Nanotechnology is also used for the

detection of pesticides and heavy metals (e.g. cadmium, copper,

lead, mercury, arsenic, etc.). Furthermore, nanomaterials have

enhanced redox and photocatalytic properties.9,10 The fabrica-

tion of nanomaterials can be achieved by (1) grinding, milling

and mechanical alloying techniques; (2) physical or chemical

vapour deposition or vacuum evaporation; (3) sol–gel chemical

synthesis methods; (4) gas-phase synthesis techniques, such as

flame pyrolysis, electroexplosion, laser ablasion and plasma

synthesis, and (5) microwave techniques or combustion methods

or delamination of layered materials (the controlled crystalliza-

tion from amorphous precursors).11 The functionalization

process is applied to nanomaterials by a coating technique or

chemical modification in order to (1) improve the surface and

optical properties, (2) to avoid aggregation and (3) to eliminate

the interaction between the nanomaterials and biological

substances. For example, doping with an appropriate dopant can

improve the photocatalytic activity and cause a red-shift in the

band-gap of TiO2, which leads to its capability to absorb light in

the visible range.12 The small particle size of nanoparticles (NPs)

brings excessive pressure drops when the NPs are applied in a

fixed bed or any other flow-through system, as well as certain

difficulties in their separation and reuse and even a possible risk

to ecosystems and human health caused by the potential release

of nanoparticles into the environment. Therefore, hybrid nano-

composites have been fabricated by impregnating or coating the

fine particles onto solid particles of a larger size to overcome the

limitations of NPs. The resultant polymer-based nanocomposite

(PNC) retains the inherent properties of the nanoparticles;

however, the polymer support materials provide higher stability,

processability and improvements thanks to the nanoparticle–

matrix interaction. In addition, the incorporation of nano-

particles (NPs) into polymeric nanocomposites leads to an

enhancement of the mechanical, electrical and optical properties.

NP-based membranes can be fabricated by assembling NPs on

porous membranes13,14 or blending them with polymeric or

inorganic membranes.15 The fabrication of membranes with

metal oxide NPs could increase the permeability and fouling-

resistance as well as the quality of the permeate.13 The

improvements to membranes or membrane surfaces using

nanomaterials provide several changes in the properties, such as

the porosity, the hydrophilicity of the surface properties, its

electropositivity or electronegativity and the surface catalytic

properties. The possible size grading due to the incorporated

nanoporous materials can prevent the passage of a range of

contaminants and microorganisms through the membrane.

Nanofibres can also provide a better filtration with a much

smaller porosity and have the capability to trap much smaller

contaminants. The internal surface areas of nanofibres are much

higher than conventional filter materials. Furthermore,


nanofibrous materials can have interconnected open pore

structures and can potentially allow high flow rates. This paper

gives an overview of the application of nanomaterials in the

purification of water and air contaminated with toxic metal ions,

greenhouse gases, organic and inorganic solutes, bacteria and

viruses and their performance in environmental remediation,

pollutant sensing and detection, cleaner production and so on.

Environmental remediation technologies

(A) Environmental remediation by chemical degradation

One of the widely used environmental remediation methods is

chemical degradation. Chemical degradation methods include

(1) ozone/UV radiation/H2O2 oxidation, (2) photocatalytic

degradation, (3) supercritical water oxidation, (4) the Fenton

method, (5) sonochemical degradation, (6) the electrochemical

method, (7) the electron beam process, (9) solvated electron

reduction, (9) permeable reactive barriers of iron and other zero-

valent metals and (10) enzymatic treatment methods. Ozone or

UV radiation-based technologies (O3/UV/H2O2) are chemical

oxidation processes applicable to water treatment for the

degradation of individual pollutants or the reduction of the

organic load (chemical oxygen demand, COD) and the biode-

gradability of waste water could be enhanced by using these

techniques. Ozone and UV radiation alone can be used for

disinfection purposes. O3/UV/H2O2 techniques generally involve

two oxidation/photolysis routes to remove foreign matters

present in water. Thus, ozone, hydrogen peroxide and/or UV

radiation can react individually or photolyze the organic matters

directly in water. However, when ozone or hydrogen peroxide

are used in combination with UV radiation, the pollutants can be

degraded by an oxidation process through hydroxyl free radicals

generated in situ. Hydroxyl radicals have the largest standard

redox potential, with the exception of fluorine. Among the most

common water pollutants, phenols and some pesticides are

substances that react rapidly with hydroxyl radicals, whereas

organochlorine compounds are less reactive. Another feature of

this oxidation process is that it is a destructive type of water/air

pollution removal due to the reaction of the pollutants with

hydroxyl radicals.16,17

The term ‘‘photocatalytic degradation’’ involves photons and a

catalyst. Electrons pertaining to an isolated atom occupy discrete

energy levels. In a crystal, each of these energy levels is split into

many energy levels as there are atoms. Consequently, the

resulting energy levels are very close and can be regarded as

forming a continuous band of energies. For a metal (or

conductor), the highest energy band is half-filled and the corre-

sponding electrons need only a small amount of energy to be

raised into the empty part of the band, which is the origin of the

electrical conductivity at room temperature. In contrast, in

insulators and semiconductors, valence electrons completely fill a

band, which is thus called the valence band, whereas the next

highest energy band (termed the conduction band) is empty, at

least at 0 K. In liquid water, two HO2_ radicals can combine if

their concentrations allow them to react significantly, yielding

H2O2 and O2 (in a disproportionation reaction). In turn, H2O2

can scavenge an electron from the conduction band or from the

superoxide and accordingly can be reduced to a hydroxyl radical



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(_OH) and a hydroxide ion (OH�). Because these reactions are

known to take place in homogeneous aqueous phases, they are

believed to occur on the TiO2 surface. In other words, the

very oxidizing _OH might be produced, in principle, by the

three-electron reduction of O2:

O2 + 2H+ + 3e� / OH_+ OH�

HO2 + R–H / H2O2 + R_

where R–H is an organic species with a labile H atom; however,

this reaction would also compete with H-atom abstraction from

R–H by the _OH radical. A much more direct way of forming the

_OH radical is through the oxidation of an adsorbed water

molecule or an OH� ion by a valence band hole (h+), (i.e., by an

electron transfer from these entities to the photoexcited semi-

conductor). The chemistry occurring at the surface of a photo-

excited semiconductor is based on the radicals formed from O2,

H2O and electron-rich organic compounds. In addition, cations

in aqueous solution can be directly reduced by conduction band

electrons provided that the redox potentials of these cations are

adequate (i.e., lying below the conduction band energy).18 The

photocatalytic reaction occurs, at least principally, in the

adsorbed phase and the overall process can be formally divided

into five steps:

1. The transfer of the reactants from the fluid phase to the

surface;

2. The adsorption of at least one of the reactants;

3. The reaction in the adsorbed phase;

4. The desorption of the product(s); and

5. The removal of the product(s) from the interfacial region.

As the adsorption and desorption rates are temperature-

dependent, temperature can have an effect on the photocatalytic

reaction rates. Increased rates upon raising the temperature

above the ambient temperature have been reported for the gas-

phase removal of some pollutants and, above all, for their

mineralization rate.19,20

The supercritical water oxidation (SCWO) process involves

bringing together an aqueous waste stream and oxygen in a heated

pressurized reactor operating above the critical point of water

(374 �C, 22.1 MPa or 218 atm). Under these conditions, the

solubility properties of water are reversed (i.e., increased organic

solubility and decreased inorganic solubility) and the viscosity of

the media is decreased to a value similar to gas-like values, thus

enhancing the mass transfer properties. These unique properties of

hot pressurized water allow oxygen and organics to be contacted

in a single phase in which the oxidation of the organics proceeds

rapidly. At 400–650 �C and 3750 psi, SCWO can be used to ach-

ieve complete oxidation of many organic compounds with

destruction rate efficiencies of 99.99% or higher. Small nano-

particles of metal oxides can be produced by hydrothermal

synthesis, which is performed in supercritical water above a critical

temperature and pressure due to the properties of supercritical

water. The supercritical hydrothermal synthesis of metal oxide

nanoparticles is green not only because it requires only water as a

solvent but also because the resulting nanomaterials contribute to

a sustainable society. During the SCWO of organic acids in waste

water containing benzoic acid, p-tolualdehyde and p-toluic acid,

and in organic compounds, such as cobalt and manganese acetate,


cobalt manganese oxide nanoparticles, synthesized in situ in the

reactor, act as an oxidation catalyst to enhance the oxidation rate

of organic compounds. Consequently, either the reaction

temperature can be reduced or the residence time can be short-

ened. The TOC of the waste water was found to be 37 480 ppm

and decreased to 200 ppm after the reaction.21–23

The term ‘‘Fenton reagent’’ refers to aqueous mixtures of Fe(II)

and hydrogen peroxide via the Fenton method. This method has

indicated the following net reaction as the predominant process:

Fe2+ + H2O2 / Fe3+ + HO_+ OH� (1)

where Fe2+ and Fe3+ represent the hydrated species, Fe(H2O)62+

and Fe(H2O)63+, respectively. Reaction (1) is often referred to as

the Fenton reaction, although many other reactions occur in

Fenton systems. The primary utility of the Fenton reagent in the

degradation of pollutants is the formation of _OH. The hydroxyl

radical is a very strong, nonselective oxidant capable of

degrading a wide array of pollutants.24,25

Sonochemical reactions are related to new chemical species

produced during acoustical cavitation, whereas the enhancement

of heterogeneous reactions can also be related to mechanical

effects induced in the fluid system by sonication. These effects

include an increase in the surface area between the reactants, a

faster renovation of catalyst surfaces and accelerated dissolution

and mixing. The peculiar nature of sonochemical reactions offers

alternative pathways, providing a faster or environmentally safer

degradation of contaminants. Sonochemistry provides a unique,

high-temperature gaseous environment inside the cavitation

bubbles, where the thermolysis of CCl4 molecules takes place at

fast rates, yielding _Cl and _CCl3 radicals as primary intermediate

products. These radicals further react with _OH radicals or O2,

yielding stable HCl, HOCl, Cl2 and CO2 as the final degradation

products. Au–TiO2 nanoparticles were prepared by sonicating

(20 kHz) an aqueous solution of HAuCl4$3H2O containing

polyvinylpyrrolidone, 1-propanol and TiO2 (Deguzza P-25) at

room temperature under a nitrogen atmosphere. The waste water

solution was irradiated with light and ultrasound was used for

the degradation via sonophotocatalysis.26–28

Fig. 1 shows a schematic diagram for the different electro-

chemical treatments. Fig. 1(A) represents direct electrolysis by

anodic oxidation in which the pollutant reacts at the electrode

surface with adsorbed _OH produced from water oxidation at a

high O2� overpotential anode. Fig. 1(B) represents indirect

electrolysis where the pollutant reacts in the solution with an

irreversibly electrogenerated reagent (B+) produced from the

oxidation of inactive B at the anode. The direct electrolytic

processes includes conventional procedures of cathodic reduc-

tion and anodic oxidation. The indirect methods deal with the

use of redox mediators as reversibly electrogenerated reagents, as

well as oxidants as irreversibly electrogenerated reagents at the

anode (e.g., O3, ClO, Cl and ClO2) or the cathode (e.g., H2O2).

Halogenated organics are usually toxic and their electro-

reduction can make them easily biodegradable. This is also a

common goal in the electrochemical decontamination of organic

pollutants in waste waters by anodic oxidation. Although

modern processes have seriously reduced the operation costs for

cathodic reduction and anodic oxidation, both methods are still

too costly to be competitive with biological treatment.29–31



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Sources of free radicals, principally hydroxyl radicals (_OH),

oxidatively decompose pollutants. An excellent source of free

radicals for water treatment is ionizing radiation. The irradiation

of water produces both reducing and oxidizing species, which

allow for a versatile approach for the ultimate treatment of

a variety of pollutants. Machine-generated electron beams

(e-beams) provide reliable and safe radiation sources for the

treatment of flowing waste streams on a process-size scale.

Process versatility is provided by the continuous, rapid treatment

potential and a tolerance for feedstocks of varying qualities.

Additionally, modern e-beams have excellent operational reli-

ability. The energy of the electron determines its depth of pene-

tration in water. The number of electrons is referred to as the

beam current and is controlled by the cathode size and config-

uration. The e�aq reacts with numerous organic compounds. Of

particular interest to the application in waste treatment are the

reactions with halogenated compounds. A generalized reaction is

shown below:

e�aq + RCl / R_+ Cl�

Thus, reactions involving the e�aq often result in the dechlori-

nation of organochlorine compounds. This result may be

sufficient for waste treatment purposes. However, further reac-

tion of the resulting organic radical (R_) may also be desirable to

mineralize the compound. The e�aq also reacts with many other

organic compounds and contributes to the removal of these

compounds from aqueous solution. The process is nonselective

in the destruction of organic chemicals because strongly reducing

reactive species (e�aq/H_) and strongly oxidizing reactive species

(_OH) are formed at the same time and in approximately the same

concentration in solution.32,33

In the solvated electron reduction method, deep blue solutions

of solvated electrons are formed when Li, Na, K, Ca or other

group I and group II metals are dissolved in liquid ammonia

(eqn (1)). These media have long been used to reduce organic

compounds. Among the many functional groups reduced by this

process, chloroorganic compounds are the ones reduced with the

highest rates (eqn (2)).

Na / Na+ + e� (1)

RCl + e� / R_+ Cl� (2)

Fig. 1 A schematic diagram for the different electrochemical treatments

of organic pollutants.29 (A) represents direct electrolysis by anodic

oxidation and (B) represents indirect electrolysis.


Most solvated electron-treated wastes require post treatment.

The first post treatment involves removing and recovering

ammonia from the matrix. This is accomplished by passing hot

water or steam through the jacket of the treatment cell and by

condensing the ammonia for reapplication.34–36

The core function of permeable reactive barriers (PRBs) and

many related technologies is to bring the contaminated material

in contact with a reactive material that promotes a process that

results in decontamination. The processes that are responsible

for contaminant removal by zero-valent metals (ZVMs) and

PRBs include both a ‘physical’ removal from solution to an

immobile phase and a ‘chemical’ removal by reaction to form less

hazardous products. Sequestration by Fe0 occurs mainly by

adsorption, reduction and co-precipitation, although other

processes may be involved, such as pore diffusion and poly-

merization. In most cases, adsorption is the initial step and

subsequent transformations help ensure that the process is irre-

versible. In some cases, however, adsorption is the sequestration

process of primary importance. This is certainly true with metals

that occur as soluble cations, which can be expected to adsorb

fairly strongly to iron oxides, but cannot be reduced to insoluble

forms by Fe0: e.g., Mg2+, Mn2+ and Zn2+. The lifetime of PRBs

using Fe0 as a reactive medium is limited by precipitation at the

barrier.37–39

Chemical processes often require the presence of excess

quantities of reagents to accomplish transformation to the

desired extent. In addition, particularly harsh conditions (e.g.,

high temperatures or extremes of pH) are sometimes required to

facilitate the chemical transformations. This can present a

problem once the desired transformation has taken place because

the resulting stream may be a low-quality mixture that cannot be

released to the environment or reused without subsequent

treatment. Finally, many chemical treatment processes are not

highly selective in terms of the types of pollutants that are

transformed during treatment. Consequently, such processes are

usually more economical for the treatment of dilute waste water

and are often used as a polishing step before waste discharge into

the environment. Biological processes are designed to take

advantage of the biochemical reactions that are carried out in

living cells. Such processes use the natural metabolism of cells to

accomplish the transformation or production of chemical

species. The metabolic processes occur as a result of a sequence

of reactions conducted inside the cell that are catalyzed by

proteins called enzymes. An important advantage of biological

systems is that they can be used to carry out processes in which

no efficient chemical transformations have taken place. In

addition, biological processes can often be conducted without the

harsh conditions that are necessary during chemical trans-

formations. However, the use of microorganisms provides many

rate-limiting factors. For example, costly and time-consuming

methods may be necessary to produce microbial cultures that can

degrade the targeted pollutant. Furthermore, severe conditions,

such as chemical shock, extremes of pH and temperature, toxins,

predators and high concentrations of the pollutants, intermedi-

ates and products may irreversibly damage or metabolically

inactivate the microbial cells. Thus, the sensitivity of microor-

ganisms to changes in their environment can make these

processes difficult to control over the long term. They also

require a supply of macro- and micro-nutrients for the support of



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microbial growth and often result in the formation of large

quantities of biomass that ultimately must be discarded into the

environment. In addition, the biochemical reactions occur at a

rate that is limited by the metabolism of the microorganism and,

thus, are often slower than chemical processes. Moreover,

whereas biological systems are commonly used to remove the

bulk organic load in waste waters, these systems often have

difficulty in removing toxic pollutants to consistently low levels.

Therefore, conventional biological processes may not be able to

improve water quality sufficiently to meet the waste water

discharge criteria. In an attempt to overcome some of the

problems associated with chemical and biological systems, recent

research has focused on developing the environmental applica-

tions of enzymes that have been isolated from their parent

organisms. Enzymes can be applied to transform targeted

contaminants, including many of those that may resist biodeg-

radation. This catalytic action can be carried out on, or in the

presence of, many substances that are toxic to microbes. In

addition, some enzymes can operate over relatively wide

temperatures, pH values and salinity ranges compared to

cultures of microorganisms. They can also be used to treat

contaminants at high and low concentrations and are not

susceptible to shock loading effects associated with changes in

contaminant concentrations that can often irreversibly damage

or metabolically inactivate microbial cells. Consequently, there

are fewer delays associated with shutdown/startup periods that

are normally required to acclimatize biomass to waste streams.

Importantly, the catalytic action of enzymes enables the devel-

opment of smaller systems of lower capital cost due to the high

reaction rates associated with enzymatic reactions. In addition,

because bacterial growth is not required to accomplish waste

transformations, sludge production is reduced because no

biomass is generated. The following situations are those where

the use of enzymes might be most appropriate: (i) the removal of

specific chemicals from a complex industrial waste mixture

before on-site or off-site biological treatment; (ii) the removal of

specific chemicals from dilute mixtures, for which conventional

mixed-culture biological treatment might not be feasible; (iii) the

polishing of a treated waste water or groundwater to meet limi-

tations on specific pollutants or to meet whole effluent toxicity

criteria; (iv) the treatment of wastes generated infrequently or in

isolated locations, including spill sites and abandoned waste

disposal sites; and (v) the treatment of low-volume, high-

concentration waste water at the point of generation in a

manufacturing facility to allow reapplication of the treated

process waste waters to facilitate recovery of soluble products or

to remove pollutants known to cause problems downstream

when mixed with other wastes from the plant.40,41

(B) Environmental remediation by metals/metal oxides

For environmental remediation, the most widely studied nano-

scale metals (NMs) and metal oxides (NMOs) include silver,

iron, gold, iron oxides, titanium oxides, etc. The size and shape of

NMs and NMOs are both important factors which affect their

performance. Efficient synthetic methods to obtain shape-

controlled, highly stable and monodisperse metal/metal oxide

nanomaterials have been widely studied during the last decade.

Generally, the synthesis methods can be classified into two


categories: (1) physical approaches, including inert gas conden-

sation, severe plastic deformation, high-energy ball milling,

ultrasound shot peening; and (2) chemical approaches, including

reverse micelle (or microemulsion), controlled chemical co-

precipitation, chemical vapor condensation, pulse electrode

position, liquid flame spray, liquid-phase reduction, gas-

phase reduction, etc. Among these synthesis protocols, co-

precipitation,42,43 thermal decomposition and/or reduction and

hydrothermal synthesis techniques are used widely and are easily

scalable with high yields. In this section, the applications of

nanoscale metals and NMOs in environmental remediation

are presented.

Silver nanoparticles are known for their strong antibacterial

effects against a wide array of organisms (e.g., viruses, bacteria,

fungi). Therefore, silver nanoparticles are widely used for the

disinfection of water.44–46 The different mechanisms of antimi-

crobial activity by nAg+ are shown in Fig. 2. Ag+ ions interact

with thiol groups in proteins, which leads to inactivation of

respiratory enzymes and the production of reactive oxygen

species (ROS).47 It was also shown that Ag+ ions can prevent

DNA replication and affect the structure and permeability of the

cell membrane.48 Silver ions are also photoactive in the presence

of UV irradiation, causing an improvement in the UV inactiva-

tion of bacteria and viruses.49,50 To date, several mechanisms

have been postulated for the antimicrobial property of silver

nanoparticles: (1) the adhesion of nanoparticles to the surface

altering the membrane properties. Nano Ag (nAg) particles have

been reported to degrade lipopolysaccharide molecules, accu-

mulating inside the membrane by forming ‘‘pits’’ and causing an

increase in the membrane permeability;51 (2) nAg particles could

penetrate the bacterial cell, resulting in DNA damage; and (3) the

dissolution of nAg releases antimicrobial Ag+ ions.52 Particles of

Ag less than 10 nm are more toxic to bacteria such as E. coli and

P. aeruginosa.53,54 Silver nanoparticles ranging from 1 to 10 nm

inhibit certain viruses from binding to host cells by preferentially

binding to the virus’ glycoproteins. Furthermore, triangular nAg

nanoplates were found to be more toxic than nAg rods, nAg

spheres and even Ag+ ions. The incorporation of nAg into

polymer materials, such as polymethoxybenzyl and poly(L-lactic

acid)-co-poly(3-caprolactone) nanofibres, have also shown anti-

microbial properties against E. Coli, A. Niger, S. aureus and

Salmonella entrica.55–58 Several materials containing iron, such as

iron sulfide, iron bearing oxyhydroxides and aluminosilicate

minerals, were successfully used in the reduction and precipita-

tion of metal ions. Out of all the iron-based materials, elemental

iron was found to be the most successful for ground water

remediation.59 With the advent of nanotechnology, iron nano-

particles replaced the use of bulk iron-based systems for water

purification.60 After a few years, noble metal nanoparticles were

also shown to degrade halocarbons by the same mechanism of

reductive dehalogenation.61–64 In an investigation by Lisha et al.,

Hg2+ ions were reduced to a zero-valent state, followed by the

alloying of reduced mercury on gold nanoparticle surfaces.64,65

The application of noble metal nanoparticles for the removal of

halogenated organics and pesticides from drinking water has also

been patented.66,67 Gold nanoparticles also exhibit potential to

remove inorganic mercury from drinking water.63

Sulphur dioxide (SO2) is frequently released to the atmosphere

by the combustion of fossil-derived fuels in factories, power



Fig. 2 The different mechanism for the antimicrobial activity of Ag+

ions.47

Fig. 3 A nanoscale bimetallic particle for chlorinated solvent removal.75

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plants, houses and automobiles. The corrosion of buildings due

to acid rain by SO2 is a serious challenge. TiO2 is the most

commonly used catalyst to convert SO2 to sulfur through the

reaction: SO2 + 2H2S / 2H2O + 3Ssolid. Rodriguez68,69 found

that the combination of a gold (Au) and TiO2 system produced

highly efficient desulfurization. Metallic gold has a very low

chemical and catalytic activity.68,69 However, when gold was

dispersed on some metal oxides (MgO, TiO2, MnOx, Fe2O3,

Al2O3), it gave the positive effects of catalytic activity due to

charge transfer between the oxide and gold and a limited nano-

scale size (usually less than 10 nm).69Rodriguez69 investigated the

dissociation effects of SO2 using the combined systems of TiO2/

Au andMgO/Au. On both oxide supports, the largest activity for

the full dissociation of SO2 was observed in systems containing

Au coverages that were less than 1 mL when the size of the Au

nanoparticles was below 5 nm. In addition, the combined system

of TiO2/Au provided a more effective dissociation of SO2 than

that of MgO/Au so that TiO2 played a direct active role for the

dissociation of SO2 as well as modified chemical properties of the

supported Au nanoparticles.70 Furthermore, catalytic perfor-

mance tests, such as (1) tests for the reduction of SO2 by H2S

through the reaction: SO2 + 2H2S / 2H2O + 3Ssolid and (2) by

carbon monoxide through the reaction: SO2 + 2CO / 2CO2 +

Ssolid indicated that the combination of Au/TiO2 is 5–10 times

more active than pure TiO2.

Zero-valence state metals, such as Fe0, Zn0, Sn0 and Al0, are

effective for the remediation of contaminants in contaminated

groundwater.59,71 Bimetallic combination (Fe0/Ni0: the ratio of

Fe to Ni being 3 : 1) has provided better degradation rates.72

Bimetallic particles are composed of two types of zero-valent

iron (ZVI). The structure of bimetals includes cluster-in-cluster

and core–shell structures for nanoscale particles.73 As iron

corrodes, protons from water are reduced to adsorbed H atoms

and to molecular hydrogen at the catalytic Ni surface. Fig. 3

shows a schematic diagram of the reaction of a chlorinated

organic molecule with a bimetallic nanoparticle. Fe or Zn

serves as an electron donor while another species (Pt or Pd)

serves as a catalyst.74,75 TCE is adsorbed onto the surface of

the Ni–Fe particles where the C–Cl bond is broken and the

chlorine atom is replaced by hydrogen. A similar mechanism

was applied by Cheng et al.76 for the dehalogenation of

4-chlorophenol to phenol on palladized graphite and iron

electrodes.76


Dabro et al. investigated the dehydrochlorination of penta-

chlorophenol to phenol or cyclohexanol on supported palladium

electrodes.77 The rapid and complete dechlorination of all the

chlorinated contaminants was achieved for the water and

groundwater slurries using bimetallic nanoparticle systems

composed of Pd/Fe.78 Contaminants, such as tetrachloroethane

(C2Cl4), could be transformed to ethane by accepting electrons

from the oxidation of iron through the following reaction:

C2Cl4 + 4Fe0 + 4H+ / C2H4 + 4Fe2+ + 4Cl�

The removal efficiency was found to be greater than 99% using

iron nanoparticles without a palladium coating after 24 h. In

addition, bimetallic coupling with a second catalytic metal has

also been widely applied for the degradation of contaminants in

contaminated water. The degradation rate by bimetallic combi-

nations was found to be faster than that observed for metal iron

alone.79,80 Bimetallic Pd/Au nanoparticles consist of two catalytic

metals, whereas bimetallic nanoparticles of iron consists of a

catalytic material (Pd or Ni) and an electron donating material

(i.e. Fe). Bimetallic Pd/Au nanoparticles can increase the cata-

lytic activity by a factor of 15 as compared to Pd nanoparticles,

Al-supported Pd nanoparticles and Pd black.81,82

Reducing the size of iron to the nanoscale has been very

efficient for enhancing the rate of reaction and reducing the

formation of by-products.83 However, the material became less

stable with an increase in the reaction activity as zero-valent iron

nanoparticles are easy to oxidize in air and hydrolyze in water.

Therefore, the efficiency of Fe0 particles could be significantly

reduced in large scale environmental remediation. To maintain

the stability of Fe0 nanoparticles, immobilization of Fe0 in the

membrane or bimetallic Fe0 nanomaterials have been used.

Meyer et al. in 2004 found that membrane immobilization for

nanoparticles of Fe0 and bimetallic Ni0/Fe0 offered two advan-

tages in the removal process of toxic organic compounds. Those

advantages were (1) the polymer membrane controlled growth of

zero-valent iron metal nanoparticles and (2) the localization and

higher concentration of organic substances in the membrane

domain led to a significant enhancement of the reaction rates.84

Although bimetallic particles of Ni/Fe showed a relatively high

reactivity towards chlorinated compounds,85,86 there is still an

environmental concern over the toxicity of Ni. Particles of nZVI

have been applied for the efficient removal of different metals,

such as Cr(VI),87 U(IV) and U(VI)88 and Co(II).89 Choe et al. (2001)

reported the reductive denitrification by nanoscale zero-valence



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iron.90 Ghauch et al.91 reported interesting results on the elimi-

nation of antibiotics from water using iron NPs.91 Bimetallic

nanoparticles of ZVI and Al have been utilized for the removal of

chlorinated organic solvents, nitrate and heavy metals, such as

Cr6+ and Cu2+, as well as perchlorate ions from waste water.92–99

Bimetallic particles of Fe and Al were found to degrade CCl4 by a

factor of 6 as compared to micro-sized Fe ZVI. Bimetallic

nanoparticles of Pd/Fe were found to provide a higher degra-

dation of CCl4 than one dimensional ZVI Fe nanoparticles100 due

to the catalytic effects of metals (Pd or Ni) through a direct

hydrogen reduction101 and the prevention of oxide formation at

the iron surface.72 Complete degradation could be obtained with

a high surface reactivity, the elimination of toxic intermediates

and a durable and stable performance by using bimetallic Pd/Fe

nanoparticles.101–105 Bimetallic Cu/Al particles have shown a

better degradation of CCl4 and CH2Cl2, which was not degraded

by most of the ZVI.106 Martin et al. have reported the use of

ferric-oxide NPs to remove and recover phosphate from

municipal waste waters.107

TiO2 nanoparticles have been extensively used for the oxida-

tive and reductive transformation of organic and inorganic

contaminants in air and water.108 TiO2 photocatalysts have been

a popular choice for much of the published photocatalysis work.

Its large bandgap energy (3.2 eV) requires UV excitation to

induce charge separation within the particle. TiO2 is a semi-

conductor and has three distinct polymorph crystalline struc-

tures: rutile, anatase and brookite. Both rutile and anatase have a

tetragonal structure, containing 6 and 12 atoms per unit cell with

an axial ratio of c/a (c ¼ perpendicular axis, a ¼ horizontal axis)

0.64 and 2.51, respectively. The brookite has an orthorhombic

structure. Both rutile and anatase have been applied in photo-

catalytic and photoelectrochemical processes due to their easy

synthesis by various techniques. TiO2 has been widely applied in

heterogeneous photocatalysis to decompose a host of organic

pollutants, such as phenolic compounds,109 metal ethylene

diamine tetra acetate complexes,110 airborne microbes and

odorous chemicals.111 Most of this research involved UV

photons as the major exciting light sources. There is 5% of solar

irradiation within UV range; therefore, it is necessary to enhance

the performance of TiO2 by using photons from the near visible

to visible region. This can be achieved by manipulating the

particle size of the photocatalyst or by doping the TiO2 with

foreign ions.112–115

The modification TiO2 with noble metal ions could decrease

the TiO2 band gap, which benefits the electron transfer from

the valence band to the conduction band, which facilitates the

formation of oxidative species, such as _OH.116 In addition, the

modified surface properties of noble metal-doped TiO2 photo-

catalysts could attract more cationic dye, i.e. rhodamine B (RB),

which could be adsorbed on the Ag-doped TiO2 surface due to an

increased number of electron traps. One dimensional TiO2

nanomaterials and doped TiO2 nanomaterials are widely used for

the degradation of halogenated compounds, the removal of dyes

via photocatalytic oxidation, the removal of metals and some

disinfection processes for drinking water and waste water. TiO2

nanoparticles are activated by UV irradiation (radiation at

wavelength 320–400 nm) and its photocatalytic properties have

been utilized in various environmental applications to remove

contaminants from both water and air.117 Recent research has


shown that the desired band gap narrowing of TiO2 can be better

achieved by using nonmetal elements, such as N, F, S and C.

Such modified TiO2 materials exhibited larger absorption in the

visible region and enhanced the degradation of organic dyes

under visible light irradiation, especially under natural solar light

irradiation.118 The relevant mechanisms for the role of nonmetals

in modified TiO2 materials in its visible light photoactivity are

due to the substitution of oxygen for nitrogen in the TiO2 lattice.

The corresponding N(2p) states are located above the valence

band edge. Mixing of the N(2p) states with the O(2p) states

results in a reduction of the band gap in N-doped TiO2 and the

photocatalyst can be active under visible light irradiation.119

Nano-sized TiO2 was also reported to kill viruses, including

poliovirus 1,120 the hepatitis B virus,121 the Herpes simplex

virus122 and MS2 bacteriophage.123 The concentration of TiO2

required to kill bacteria varies between 100 and 1000 ppm

depending on the size of the particles and the intensity and

wavelength of the light used.124 The antibacterial activity of TiO2

is related to the ROS production, especially hydroxyl free radi-

cals and peroxide formed under UV-A irradiation via oxidative

and reductive pathways, respectively.125 A strong absorption of

UV-A activates TiO2 under solar irradiation and significantly

enhances solar-triggered disinfection. In a study by Gelover

et al., the complete inactivation of fecal coliforms was achieved in

15 min at an initial concentration of 3000 cfu per 100 mL by

exposing water in TiO2-coated plastic containers to sunlight,

whereas the same inactivation required 60 min with uncoated

containers.126 An attractive characteristic of TiO2 photocatalytic

disinfection is its capability of activation by visible light, e.g.

sunlight. Metal doping has been shown to improve the visible

light absorbance of TiO2127 and increase its photocatalytic

activity under UV irradiation.128 Noble metals, especially silver,

have received much attention for this purpose. Silver has been

shown to enable the visible light excitation of TiO2.129 Recently,

it was demonstrated that doping TiO2 with silver greatly

improved the photocatalytic inactivation of bacteria130 and

viruses.131 Reddy et al. demonstrated that 1 wt% Ag in TiO2

reduced the reaction time required for complete removal of 107

cfu of E. coli per mL from 65 to 16 min under UV-A radiation.132

Silver enhances the photoactivity by facilitating electron–hole

separation and/or providing more surface area for adsorp-

tion.116,133 Visible light absorption by silver surface plasmons is

thought to induce electron transfer to TiO2 resulting in charge

separation and, thus, activation by visible light.129,134 Ag/TiO2 or

Au/TiO2 therefore shows a great potential as a photocatalytic

material due to its photoreactivity and visible light response.

Table 1 summarizes the application of TiO2 nanomaterials for

environmental remediation. TiO2 is widely used for NOx control,

whereby NOx is reduced back to N2 or oxidized to NO2 and

HNO3. The oxidation of nitric acid converts it into a raw

material with useful applications, such as in fertilizers. The

activity of TiO2 is improved by the addition of an adsorbate, such

as zeolites, which concentrate NO on the surface. A highly

selective photoreduction of NO to N2O and N2 is observed on

the surface of TiO2. In the work of Skubal et al. the surfaces of

titanium dioxide NPs were modified with a bidentate chelating

agent, thiolactic acid (TLA), to remove aqueous cadmium from

simulated waste waters.135 Amezaga-Madrid et al. tested the

effect of TiO2 thin films deposited on soda lime glass slides by



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sol–gel techniques.136 After 40 min, a 70% reduction of P. aeru-

ginosa cells was observed. Kuhn et al. reported that bacteria

placed on TiO2 coated surfaces were killed in the following order:

E. coli > P. aeruginosa > S. aureus > Enterobacter faecium > C.

albicans.137 While pure TiO2 had no antibacterial effect in free

solution or in agar, silver-doped TiO2 particles were more

effective than pure silver nanoparticles of a similar size.138

Inorganic nanoscale ZnO was mostly applied to eliminate H2S

as an adsorbant. Recently, nanostructured ZnO could efficiently

remove heavy metals.147 Lee et al. prepared nanometer size zinc

oxide (ZnO) powders by a ‘solution–combustion method

(SCM)’. Compared with two TiO2 powders, P25 and one

prepared by a homogeneous precipitation process at low

temperature, the zinc oxide nanopowder showed a higher

removal rate of Cu2+ ions from the solution.148 ZnO nanosheets

prepared via a hydrothermal approach were used to adsorb Pb2+

and then hydrothermally treated in an aqueous solution con-

taining a sulfur source. Due to the surface hydroxy groups, the

resultant ZnO nanosheets exhibited a good capacity of Pb2+,

which was found to be 6.7 mg g�1.149 The Pb2+-preloaded ZnO

nanosheets were put into a Teflon-lined stainless steel autoclave

containing a sulfur source at 120 �C for 12 h and the resulting

ZnO/PbS nanocomposite exhibited a potential use in photo-

catalytic remediation. ZnO nanoparticles exhibit strong anti-

bacterial activities on a broad spectrum of bacteria.150 The

penetration of the cell envelope and the disorganization of the

bacterial membrane upon contact with ZnO nanoparticles were

also indicated to inhibit bacterial growth.151 Other NPs, such as

cerium oxide, have been applied for the removal of Cr(VI)

through the adsorption of the metal onto the surface of

the NPs.152

Various dopants, such as transition metal ions (Fe, Co, Cu),

have been used to enhance the photocatalytic activity of TiO2.

Metal-doped TiO2 was found to induce the desired red-shift of

the adsorption spectrum. However, it has also been documented

that the presence of transition metals may increase the proba-

bility of electron–hole recombination, resulting in a reduction of

the semiconductor’s photocatalytic efficiency. Titania doping

with nonmetal elements (N, C, S) has also been considered as an

effective approach to extend the spectral response of TiO2

towards visible energy wavelengths. In this approach, single and

co-doped TiO2 catalysts with Fe and S have been reported to

exhibit enhanced photocatalytic activity under visible light.

Table 1 Photocatalytic processes of environmental remediation by one dime

Category Target material

Disinfection Water-borne pathogenic virudrinking water

Metal removal ArsenicDye removal/destruction ofbiological toxins and inactivation ofbacteria

Methylene blue, creatinine,biological toxins (microcystiE. Coli

Metal removal LeadDye removal Methylene blueAir treatment NOx and toluene in airDisinfection S. aureus and E. Coli in watNutrient removal NitrateNutrient removal NitrateOrganic removal 2,4,6-Trichlorophenol


Recently, emphasis has been given to co-doped TiO2 systems,

involving cations and anions, in order to improve the photo-

catalytic efficiency of TiO2. It has been suggested that charge

separation between electrons and holes is improved by the

co-doping of TiO2 with Fe and S. Under both UV and sunlight

irradiation, all synthesized S single or co-doped catalysts showed

larger catalytic activities compared to the commercial TiO2, P25.

Toluene photo-excitation appears to be favoured at a low S

concentration (0.2%) in the nanomaterial but it provided a

detrimental effect at 0.4% of S in TiO2 nanomaterials. Co-doping

with S and Fe favours the photocatalytic activity of the nano-

material when it is compared to that of single Fe-doped TiO2. In

addition, the single S-doped and co-doped S/Fe–TiO2 catalysts

showed high selectivity (>90%) toward the partial oxidation of

toluene (production of benzaldehyde).153 The photocatalytic

activity of the prepared TiO2 catalysts doped with Li+, Rb3 and

Y3+ under sunlight irradiation was evaluated using 2-naphthol as

a pollutant model. The results showed a great enhancement in

the photocatalytic efficiency with the incorporation of Y3+ in

samples synthesized by solid grinding, while in samples synthe-

sized by sol–gel process either Rb+ or Y3+ dopants greatly

improved the photocatalytic activity. The increase in photo-

activity may be due to: (i) a decrease of the energy gap, which

favors a higher photoexcitation efficiency under solar radiation

and provides a larger population of excited species (hole–electron

pairs); (ii) the small particle size favors the increase in the surface-

to-volume ratio and the scavenging action of the photogenerated

electrons by the Y3+ or Rb+ ions results in a prevention of the

recombination of electron–hole pairs and increases the lifetime of

the charge carriers; (iii) the doping ions (Rb+ or Y3+) can act as

electron traps, thus facilitating the electron–hole separation and

subsequent transfer of the trapped electron to the adsorbed O2,

which acts as an electron acceptor on the surface of the TiO2; (iv)

the dopant prevents the recombination of electron–hole pairs

and increases the lifetime of the charge carriers. Therefore,

photocatalysis activity under sunlight can be improved.154

Increasing amounts of silver in Ag-doped TiO2 significantly

increases the rate of degradation of a model dye, rhodamine.

Two possible parallel mechanisms, which may result in electron

population of the conduction band of the material, have been

suggested: (i) rhodamine absorbs visible light and injects an

electron into the conduction band of the TiO2 material or (ii) the

material itself absorbs visible light, which is probably facilitated

nsional or ion doped-TiO2 nanoparticles

Type of nanomaterial Ref.

s in Ag-doped TiO2 139

One dimension TiO2/Fe-doped TiO2 140

n-LR),Crystalline TiO2 thin film/thecomposite material of TiO2 andAl2O3

141

One dimension TiO2 142Ag-doped TiO2 nanofibres 143Ag-doped TiO2 nanofibres 143

er Ag-doped TiO2 nanofibres 143Cu/Fe/Ag-doped TiO2 144TiO2 NPs doped with Bi3+ 145Ag-doped TiO2 146



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by the surface plasmon absorption of the silver-doped materials.

It is probable that the two mechanisms are happening in parallel,

indeed some commentators have questioned the suitability of

dyes, such as methylene blue, for photocatalytic studies. The role

of silver clearly controls the recombination rate and subsequently

allows a greater proportion of hydroxyl radicals to form.155

Table 2 The typical properties of pressure-driven NF membranes162,163

Pore size (nm) �2 nmWater permeability (L m-2 h-1 bar-1) 5–50Operating pressure (bar) 2–10Molecular weight cut off (Da) >100

(C) Nanofiltration

(C-1) Nanofiltration by membranes. Membrane technologies

are more efficient nowadays due to their reliable contaminant

removal without the production of any harmful by-products,

especially in water and waste water treatment processes. The

basic principle of membrane filtration is to apply semi-permeable

membranes to remove fluids, gases, particles and solutes. For the

separation of materials from water, membranes must be water

permeable and less permeable to solutes or other particles.

Pressure-driven membrane processes, such as microfiltration

(MF), ultrafiltration (UF), nanofiltration (NF) and reverse

osmosis (RO), have been applied for water treatment, reuse and

desalination systems throughout the world. Nanofiltration (NF),

as a promising membrane technology, is a method for removing

low molecular weight solutes, such as salts, glucose, lactose and

micro-pollutants, in contaminated water.156,157The availability of

clean water has emerged as one of the most serious problems

facing the global economy in the 21st century. Water treatment

systems typically involve a series of coupled processes, each

designed to remove one or more different substances in the

source water, with the particular treatment process being based

on the molecular size and properties of the target contaminants.

RO is very efficient for retaining dissolved inorganic and small

organic molecules. NF can effectively remove hardness (i.e.,

Ca(II)) and natural organic matter. However, a limitation of

both RO and NF processes is the requirement of high pressures

(100–1000 psi) to operate the water treatment. Conversely, UF

and MF membranes require lower pressures (5–60 psi) but those

membranes cannot retain dissolved ions and organic solutes.

Advances in nanochemistry can provide enhancement of UF and

MF processes for recovering dissolved ions from aqueous solu-

tions.158–160 Both cellulose acetate and polyamide can be used to

form NF membranes. In addition, other polymers (e.g., poly-

vinyl alcohol (PVA) and sulfonated polysulfone) and inorganic

materials (e.g., some metal oxides) can also be used for NF

membrane synthesis.161 The typical properties of NF membranes

are shown in Table 2. NF membranes selectively reject

substances as well as enable the retention of nutrients in water.

Therefore, the advantages of NF are comparable to the RO

process. The adsorption of pollutants onto the membrane can be

(1) physical in nature, which is a completely reversible process, or

(2) chemical in nature, which is irreversible for strong chemical

bonds, such as polymerization, or reversible for weak secondary

chemical bonds, such as hydrogen bonds and complexation or (3)

both. NF is capable of removing hardness, natural organic

matter, particles and a number of other organic and inorganic

substances via one single treatment. Water hardness is caused by

calcium and magnesium ions, while strontium and barium rarely

occur in substantial concentrations. NF membranes can reject

bivalent ions in significantly high amounts.163,164 The calcium ion

rejection was found to be 74.9–78.9% higher than the expected


results of the manufacturer (in Germany) for a thin film

composite of a polyamide membrane with molecular weight cut-

off of 300 Da (NF 200B).165 A better rejection of magnesium ions

was also found to be 86.7–90.3% using the same type of

membrane due to the stronger hydration of the Mg2+ ion. Van

der Bruggen et al.166 observed that a spiral-wound membrane

made of polyamide materials with a molecular weight cut-off of

250 Da (NF 70) could remove the major fraction of hardness

from groundwater. When groundwater was treated with a NF 70

membrane for the production of drinking water, it was suggested

that the permeate should be mixed with streams that have been

treated by traditional methods or, alternatively, hardness should

be re-added to the drinking water in order to obtain the desired

hardness of 1.5 mmol L�1. The separation performance of NF

membranes for cations is 60%.166 For anions, the separation

efficiency obtained with NF was larger. For NF, the ion size

plays a role for membranes with small pores, leading to a large

selectivity.167 Nitrate rejection was 76% for NF70, which is better

than expected anticipated by the manufacturer of NF 70. The

higher retention of multivalent ions could be obtained over a

wide range of concentrations using a membrane with small pore

diameters.168 The pore size distribution of NF membranes have a

common feature: the largest fraction of medium-size pores is

approximately 0.85 nm, which is primarily responsible for the

sieving effect of the membranes. In addition, the NF membranes

contain larger fractions of the smallest pores (0.22–0.254 nm)

and there is a significant fraction of wider pores (1.55 nm) in

the skin of the NF membranes.169 Mass transport through a

nanofiltration membrane can be achieved by two mechanisms:

diffusion and convection. When the transmembrane pressure is

high, the diffusion mechanism becomes less influential than the

convection mass transport mechanism and, therefore, a better

retention of Ca2+ ions is found. The retention of Ca2+ was found

to be very high for NF 70 and a membrane made of poly-

piperazineamide (UTC20) and remained constant when the

pressure was above 10 bar. A 90% retention was found for

multivalent ions, such as sulfate, calcium and magnesium ions,

and 60–70% was found for monovalent ions, such as sodium and

chloride.168 In NF membranes, electrostatic interactions between

the negatively charged membrane and the charged species appear

as an important feature over the size effect and salts containing

divalent sulfate anions are better rejected by the membrane than

salts with a monovalent chloride anion. However, the differences

in the retention of the examined chlorides are mostly controlled

by the size of the cations. The rejection behaviour of the NF

membranes for multivalent ions is due to charge interactions

with the membrane and the size exclusion of hydrated ions.170

Monovalent ions tend to have lower rejections unless they are

retained to maintain charge neutrality with multivalent counter

ions.169 Ko�suti�c et al., observed the order of chloride retention as

follows: RMgCl2> RCaCl2

> RNaCl, which holds for the two types



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of thin film polyamide NF membranes with slightly lower

retention values for the NF-type membranes. This is caused by

the presence of a fraction of large pores in the surfaces of the

membrane.169 Such an order was not found for sulfates and this

may be due to the ion pairing effect, which is more prevalent in

MgSO4 than in Na2SO4 solutions. Orecki et al. observed that the

NF process of surface water provided a retention coefficient

of 90–99% for sulfate and 82% for carbonate, while it rejected

40–55% of the covalent salts.171 The total hardness of the surface

water could be reduced by 85.2%. The rejection of both divalent

dibasic arsenate, HAsO42� and sulfate (SO4

2�) ions was found to

be notably higher than the rejection of chloride ions by the

charged NF membranes due to the stronger electrostatic repul-

sion of the former ions than the repulsion of the lesser charged,

monovalent chloride ion. This shows that the charge exclusion

influences the retention of ions by the negatively charged NF

membranes.172

The NF process has many advantages, including ease of

operation, reliability, no requirement of additives and a modular

construction that is easy to upscale.166 However, the limitation in

NF is membrane fouling. Scaling or the formation of precipitates

takes place when the concentration of the ionic salt exceeds its

solubility in water. Inorganic foulants in the NF process are

found to be carbonate, sulphate and phosphate salts of divalent

ions, metal hydroxides, sulphides and silica. Fouling leads to

physical damage to the membrane due to the plugging of pores

and the inability to remove the scales from the membrane

surface.172 However, the addition of hydrochloric or sulphuric

acid can prevent the formation of calcium carbonate during the

NF process.168 An alternative method to prevent membrane

fouling is the addition of antiscalants, i.e. surface active materials

which can interfere with precipitation during the NF process via

threshold inhibition, crystal modification or dispersion.

Common surface active agents are sodium hexametaphosphate,

diethylenetriamine-penta-methyl phosphoric acid and

1-hydroxyethylidene-1,1-diphosphonic acid. The high rejection

of antiscalants by membranes takes place during the NF process

due to their high molecular weight and negative charge.172

The NF process is commonly used for the removal of natural

organic matter (NOM) from water. NOM is a polydisperse

mixture of individual particles in natural water originating from

degraded and partly re-synthesized plant residues. Humic and

fulvic acid seem to contribute to the natural color of water, which

becomes visible if the concentration of the dissolved organic

carbon (DOC) is higher than 5 ppm.172 In addition, a large

variety of dyes and chemical additives from textile plant are

environmental pollutants.173 The results observed by researchers

include the findings that hydrophobic fractions of NOM appear

to be retained best, acidic pH favours a lower rejection of NOM

(60–75%), while a neutral pH favors a high rejection of NOM

(90%). Rejection has also been found to be concentration

dependent and ionic species may promote conformational

changes in the NOM fractions, which can improve rejection via

the NF process.174,175 Braghetta et al. observed a decrease in the

rejection of DOC at a low pH and high ionic strength using a

sulfonated polysulfone hollow fibre NF membrane. The inter-

action between the membrane polymer (the hydrophobic

domain) and the organic solute can influence the rejection

behavior as the hydrophobicity of the organic polymer affects the


sorption of organics onto the membrane surface.176 When

adsorption takes place inside the membrane or on the surface, the

pore size can become reduced as molecules that have a similar

size to the pores block them through permeation. The interac-

tions that occur in a multi-component solution may improve the

rejections of NOM by the membrane due to friction coupling; i.e.

the effect of coupling the different components. Solutions con-

taining electrolytes, non-electrolytes or solutions of weak acids

tend to exhibit strong interactions.177 The total organic carbon

(TOC) content of surface water can be reduced by 93.5% via

the NF process and removes turbidity by 85.5%.171 However, the

gradual plugging of the membrane has been observed in the

treatment of surface water with a low initial turbidity and high

TOC conducted by the NF process. Flushing of the membrane

with permeates from the RO process could prevent membrane

plugging and allow a recovery of the initial flux.

The fouling of membranes by NOM and a decline of the flux

are the major problems associated to the NF techniques in the

treatment of drinking water.178 Polysulfone membranes have

been used in some studies and have caused adsorptive fouling.

The fouling mechanisms are adsorption, precipitation, gel

formation and an interaction with multivalent ions. Thus, pre-

treatment with a coagulant has been suggested to prevent these

adverse effects and a combination of a coagulation process and

membrane filtration can significantly reduce the necessary

amount of coagulant required, while reducing the turbidity and

DOC. Moreover, coagulation has been found to provide an

efficient hygienic barrier in combination with membranes.179

However, the removal of NOM has been significantly affected by

the type of coagulant, the coagulation conditions, the type of

membrane, the filtration conditions and the characteristics of the

treated water. In addition, cartridge filtration and activated

carbon adsorption can be used to prevent membrane fouling by

NOM.172 New ways of reducing membrane fouling have been

suggested for the immobilization of TiO2 photocatalysts on the

membrane surface and these have been called composite

membranes. The use of nanoparticles, such as nano-alumina,

silica, silver, iron oxide, etc., in membrane structures has gained a

lot of interest, but their application in water treatment and the

possible release of nanoparticles into the water has remained

open to debate whilst the application of NF has progressed in the

treatment of water.180

Organic micro-pollutants present in natural waters used for

drinking water production lead to negative effects on human

health. Many pesticides and organohalide compounds have been

found to be carcinogenic, even in very low concentrations. The

removal of micro-pollutants is traditionally done by activated

carbon adsorption. This method is very expensive when large

fractions of NOM are present because the adsorption sites are

mainly taken by NOM due to its high concentration.181 The

removal efficiencies largely depend on the membranes used and

on the pesticides that have to be removed. Montovay et al. found

an 80% removal of atrazine and a 40% removal of metazachlor,

which is significantly lower than the expected result.183 Kiso

et al.157 studied the removal of pesticides, such as acaricides,

fungicides, insecticides, herbicides and rodenticides, such as

atrazine and simazine, together with pyridine and a chlorinated

pyridine compounds. Four different (unspecified) membranes

produced by Nitto–Denko were used. The rejections obtained



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with three of these membranes were too low and the rejections

with the fourth membrane were very high (over 95%); however,

this membrane seemed to be an RO membrane and provided a

high NaCl rejection.157 Van der Bruggen et al.184 obtained good

results for the removal of atrazine, simazine, diuron and iso-

proturon. Rejections were all over 90% with NF70 (Dow/Film-

tec), but for two other membranes (NF45, a Dow/Filmtec

membrane, which is a thin film composite on a polyester support

viable for operation at pH from 2 to 10 and temperatures up to

45 �C, and UTC-20, a Toray membrane) relatively low rejections

were found for the removal of diuron and isoproturon. This may

be due to the high polarity of these compounds, which causes

interactions with the charged membrane.183 Van der Bruggen

et al.167 also proved that there was no significant effect of

concentration on the rejection of pesticides in water. The rejec-

tion of pesticides was higher in river water and tap water than in

distilled water, but the water flux was lower.167 This was mainly

due to ion adsorption inside the membrane pores. Narrower pore

sizes counteracted the effect of the presence of NOM. The NOM

was assumed to enhance the adsorption of pesticides onto the

membranes surface and an increased size exclusion and electro-

static repulsion was also observed during the NF process.182,184

The rejection of pesticides is reasonably high and correlates

positively with the order of their molecular sizes.169 Important

factors that must be considered for the selection of an appro-

priate membrane are (1) the molecular weight cut-off (MWCO),

which is expressed in Dalton, (2) the membrane porosity, (3) the

degree of membrane desalination, and (4) the charge of the

membrane. Membranes with a MWCO varying between 200 and

400 Da are considered to be suitable for the adequate removal of

pesticides from water. The rejection of uncharged pesticide

molecules has been positively correlated with the pore size

distribution and the number of pores on the surface of the

membrane. The highest desalting membrane was found to

effectively reject almost all pesticides. However, rejection was

again found to be a function of the properties of the pesticide,

such as hydrophobicity and charge, regardless of the membrane

salt rejection performance. In general, the reliability of the

membrane desalination degree is not an accurate indicator to

assess the removal of hydrophobic organic micro-pollutants. A

number of studies confirm that composite polyamide membranes

indicate far better rejection performances for several mixtures of

micro-pollutants, including pesticides, compared to cellulose

acetate membranes (CA).185,186 This behavior may be due to the

higher polarity of CA membranes, which is responsible for the

poor rejection of the highly polar pesticides. The electrostatic

repulsion between negatively charged pesticides and the negative

charge of the TFC membrane surface could enhance the overall

rejection performance.187 The removal of pesticide is dependent

not only on the properties of the membrane but also on the

properties of the pesticides, such as (1) the molecular weight and

size of the pesticide, (2) the hydrophobicity, and (3) the

polarity.156,165,188,189 Ben�ıtez et al.189 investigated the application

of NF for the removal of phenyl-urease in natural waters. Two

types of membrane were used in their investigation: (1) a thin film

composite (a polypyperazinamide skin layer on a polyester

support), which is negatively charged because of its active

nanopolymer layer; and (2) a cellulose acetate polymer, which is

negatively charged. It was found that the retention order for the


thin film membranes (with a hydrophilic character) was iso-

proturon > linuron > chlortoluron > diuron, which showed that

the molecule polarity is the main parameter that influences the

retention. On the contrary, the retention efficiency for

the cellulose acetate membrane (with a hydrophobic character)

followed the sequence: linuron > diuron > chlortoluron > iso-

proturon, which indicates that the main factor responsible for the

retention is the MW for the adsorption. Moreover, the mass

adsorbed sequence found in both membranes for both mineral

and reservoir water was: linuron > diuron > chlortoluron >

isoproturon. From the results obtained, it can be concluded that

the thin film membrane was the most adequate for the removal of

phenylureas from natural waters, especially for the most polar

compounds because higher retention coefficients as well as higher

retentions are obtained for DOC and aromatic compounds. They

also observed that the additional presence of humic acids and

calcium ions increases membrane fouling, which is a consequence

of the adsorption of various species (NOM and ions on the

membrane) as well as pore blocking and the formation of a cake

layer on the membrane surface. However, the effect of humic

acids and calcium ions on the retention coefficients is not

significant, especially for the less hydrophobic compounds

(chlortoluron and isoproturon).189 Manttari et al.190 investigated

the combined treatment of paper mill effluents with the activated

sludge process and NF using a membrane made of piperazine

and benzene tricarbonyl trichloride, which produced a negatively

charged, hydrophilic membrane with a smooth surface and a

MWCO of 200–300 Da. It was reported that a well operated

activated sludge process can be a good pretreatment prior to the

NF operation as a high COD removal can be achieved via

the activated sludge process and the resulting concentrate, after

the NF process, had a lower amount of organic pollutant in the

waste effluent compared to the NF operation without the

pretreatment. The color rejection of NF after treatment was

almost complete and the permeate color was always lower

than 10% Pt–Co. Similar to the color, quite high COD rejections

(80–100%) were found with NF. Furthermore, the conductivity

rejection was around 65% and the permeate conductivity was

between 1.98 and 2.67 mS cm�1.190 Ducom and Cabassud studied

the removal of trichloroethylene, tetrachloroethylene and chlo-

roform by NF. The removal efficiency of the former two

compounds was satisfactory, but the chloroform rejection was

significantly lower. This effect was attributed to the selective

adsorption of chloroform into the membrane structure, which

led to higher concentrations in the permeate.191 However, good

removal efficiencies of chloroform were obtained by Waniek

et al.192 Pang et al.193 evaluated the removal efficiency of

1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT) in

contaminated water using a NF unit of a polyvinylidene fluoride

(PVDF) membrane. The DDT concentration decreased signifi-

cantly from 77.4 mg L�1 to 52.2 mg L�1, while the cumulated

quantity adsorbed reached 506 mg m�2. The results show that

DDT was easy to adsorbed on the polyvinylidene fluoride

membrane.192 The removal mechanism of DDT by PVDF could

be due to two possible mechanisms: adsorption on the membrane

or repulsion (steric and electrostatic) by the membrane. The

increase of the initial DDT concentration had a negative effect

on the DDT removal and the influence of pH on the rejection is

not obvious. In addition, the rejection of DDT was lower with a



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higher flux. It was difficult to simultaneously achieve a high

permeate flux and high DDT rejection; however, DDT can be

easily adsorbed by humic acid and can be removed altogether.

With the blocking effect of the pores by the ions and the presence

of organic matter (humic acid) and inorganic matter (NaCl,

CaCl2 and CaSO4), the elimination of DDT can be enhanced.

The removal of viruses and bacteria is extremely important in

drinking water purification. The removal of the protozoa giardia

and cryptosporidium from surface water sources is a main priority

for many governments and drinking water companies. Tradi-

tional chlorination is still often applied as a disinfection method,

but the disadvantages of chlorination were found to be (1) the

formation of disinfection by-products (DBPs) and (2) the resis-

tance of cryptosporidium to chlorine and chloramines.

Membrane filtration may facilitate an improvement in the

disinfection process because it is an extra barrier for viruses and

bacteria.181 Bacteria (0.5–10 mm) and protozoan cysts and

oocysts (3–15 mm) are larger and their removal can be guaran-

teed with at least 4 log units by using UF membranes. The

smaller viruses may be rejected by NF membranes, which have

pore sizes below 1 nm.

Filtration processes by a membrane with carbon nanotube

walls have been reported.193The filtration membrane consisted of

a hollow cylinder with radially aligned carbon nanotube (CNT)

walls. Srivastava et al.194 efficiently conducted the removal of

Escherichia coli from drinking water using the CNT-aligned

membrane. Membranes that have CNTs as pores could be used

in desalination and demineralization. Those tubes act as the

pores in the membrane. A membrane filter possessing both super

hydrophobicity and superoleophilicity was synthesized from

vertically aligned multi-walled carbon nanotubes on a stainless

steel mesh for the possible separation of oil and water. Both

super hydrophobicity and superoleophilicity could be obtained

due to the dual-scale structure and needle-like nanotube geom-

etry of the mesh with micro-scale pores combined with the low

surface energy.194 The nanotube filter could separate diesel and

water layers and even surfactant-stabilized emulsions. The

successful phase separation of the high viscosity lubricating oil

and water emulsions was also carried out. The separation

mechanism can be readily expanded to a variety of different

hydrophobic and oleophilic liquids, such as brewery waste water,

which may have a high content of organic matter in terms

of COD (from 1000 mg L�1 to 4000 mg L�1) and BOD (up

to 1500 mg L�1).

Polymeric membranes have many advantages, such as their

easy formation, the selective transfer of chemical species and

their relatively low cost.195 However, inorganic membranes are

currently in competition with organic membranes for commer-

cial applications because they have a better resistance to chemical

attack, goodmechanical strength and a high tolerance of extreme

pH conditions and oxidation. Inorganic membranes composed

of metal oxides have higher durability in many water purification

applications. In addition, the increased conversion, better selec-

tivity, milder operating conditions and decreased separation load

are some other attractive features which promote inorganic

membranes as chemical reactors in many established and novel

reaction systems.196 In many of the harsh operational environ-

ments, only inorganic membranes can offer advantages.

However, inorganic membranes are not used extensively because


of the high costs and relatively poor control in pore size distri-

bution. Besides, the effective membrane layer is very thick in

comparison to the mean pore size, which results in a reduced flow

rate. Thus, the incorporation of inorganic nanoparticles into the

polymeric membranes has been considered as a way to make

polymeric membranes more attractive for commercialization.

Silver nanoparticles (1–70 nm) have been blended into poly-

sulfone membranes by dispersing nanoparticles in a casting

solution before dissolving them in the polysulfone resin (PSf).

The addition of silver nanoparticles did not alter the membrane

structure. The impregnation of nAg (0.9% by weight) signifi-

cantly decreased the number of Escherichia coli that were able to

grow on the membrane surface by 99% after filtration with a

dilute bacteria suspension. Furthermore, the silver nanoparticles

reduced the attachment of an E. coli suspension to the surface of

the immersed membrane by 94%.197 The antibacterial mechanism

of silver is related to its interaction with sulfur and phosphorous,

most notably the thiol groups (S–H) present in cysteine and other

compounds. The interaction of ionic silver (which can be released

from the silver nanoparticles) with thiol groups and the forma-

tion of S–Ag or disulfide bonds can damage bacterial proteins,

interrupt the electron transport chain and dimerize deoxy-

ribonucleic acid (DNA).198,199

Singapore began reclamation of domestic sewage in 1998

through the NEWater study. Indirect portable reuse is practised

through the introduction of a suitable amount of NEWater into

reservoirs that can be further purified by water treatment tech-

niques before supplying to the public. To produce NEWater,

primary sedimentation and secondary treatment by activated

sludge were first applied for the clarification of used domestic

water. Then, biofiltration via MF or UF, reverse osmosis (RO)

and UV disinfection processes were applied for applications,

such as in manufacturing or industry or recharging into

groundwater or blending into reservoirs. The main problem in

the reclamation of domestic sewage is biofouling of the RO

membranes, which reduces the flux and increases the required

frequency of cleaning. Pretreatment prior to RO is needed to

prevent biofouling. Recently, an increased number of investiga-

tions on the use of NF in reducing dissolved organic matters with

molecular weights greater than 200 Da have been attempted.

Choi et al.206 used a polyamide NF membrane bioreactor for the

reclamation of water from domestic sewage and 0.5–2.0 mg of

TOC per L was found in the permeate. The amount of TOC was

much less than the average amount of TOC (5 mg L�1) fromMF

membrane bioreactors. Therefore, the NF membrane provided a

better retention of organic matter than theMFmembrane, which

led to a reduced TOC in the permeate or feed of the RO

process.200 An average of 89% rejection for high organic acids

from anaerobic effluent was observed using polyamide NF

membranes.201 Therefore, application of NF membranes is

becoming a potential pretreatment process that reduces the

problem of biofouling in the RO process. The salt rejections and

organics removal by NF membranes could provide the required

water quality for groundwater recharging.202 In addition, higher

rejections of emerging contaminants were observed using poly-

piperazineamide thin film composite membranes as shown in

Table 3. The permeate flux when using NF membranes at 415–

485 kPa was three times greater than that of RO membrane.203

Furthermore, energy cost was significantly reduced by 2–4 times



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using NF at 415–485 kPa compared to ROmembranes operating

at 1000–2100 kPa to produce the same permeate flux. The

advantages of using NF as a pretreatment prior to RO for the

desalination of sea water over sand filters are the use of fewer

chemicals and the more consistent quality of the permeate. In

addition, NF could reduce fouling in RO as other pretreatments

using UF and MF membranes can cause fouling in RO due to

their inability to remove TDS. It was observed that NF can

remove almost all turbidity and microorganisms and reduces

TDS by 37.3%.204 More than an 80% rejection of divalent ions

and an 86.5% total rejection have been found (Table 3). Rejec-

tion rates for NFmembranes for the treatment of tertiary treated

effluent201 hardness were achieved using NF prior to the RO

process.204

Furthermore, NF pretreatment before the RO of sea water

could reduce biofouling and scaling due to the high removal of

microorganisms and hardness and the lower required pressure

for operating the RO process due to the high removal of TDS,

which in turn saves the cost of power consumption.204

The polyamide NF membrane bioreactor was observed to

have a high treatment efficiency and a satisfactory stability for

long term operation compared to NF membrane bioreactors

made from cellulose acetate, although the permeate productivity

was found to be lower.200 The concentrations of DOC in the

permeate of the cellulose acetate NF membrane bioreactor

deteriorated after 130 days operation, after which time only 10%

of TOC was removed by the membrane. Similarly, the salt

rejection rates of monovalent and divalent ions were reduced

from 40–60% and 70–90%, respectively, to less than 10% after 80

days of operation. The increased presence of DOC in the

permeate was due to the hydrolysis of cellulose acetate, which

may have led to an increase in the membrane pore size and a

decrease in the surface charge and hydrophobicity of the

membrane. After 71 days of operations, large voids were found

on the surface of the cellulose acetate membrane via an AFM

image, which showed the deterioration of the membrane.204 The

deterioration of the membrane may have been due to biodegra-

dation. Therefore, the selection of a suitable type of membrane is

important.

NF is widely used for industrial waste water treatments, such

as (1) color removal and the recovery of salts from waste water in

Table 3 The typical properties of pressure-driven NF membranes162,163

ConstituentsRejection rate(%)

TOC 96Conductivity 48Sodium 40Potassium 40Calcium 40Magnesium 70Chloride 80Pharmaceutically activecompounds

25

Salicylic acid 100Naproxen 90Gemfibrozil 90Ketoprofen 78Carbamazepine 90


the textile industry; (2) the recovery of heavy metals and eluents

from waste water in metal processing plants; (3) the removal of

organics and the recovery of proteins, enzymes and aromatic

compounds from waste water in the food processing industry; (4)

the reclamation and reuse of water from waste water pulp and

the paper industry; and (5) the removal of organics and multi-

valent ions from waste water at landfill leachate plants.172,205

Fersi et al.207 observed that a 90% retention of bivalent cations

and a 60% retention of monovalent cations can be achieved using

a NF membrane (pore size � 2 nm) in the treatment of waste

water from the textile industry. In addition, the retention of

TDS, the turbidity and the color were found to be greater than

90%. Thus, NF membranes are suitable for removing water

soluble dyes with molecular weights ranging from 200 to 1000 Da

and divalent salts for softening effects.206,207 The limitation of the

NF membrane to remove color from industrial waste water is the

declination of the flux due to the combined effect of the

concentration polarization, the adsorption and/or the pore

blocking caused by high COD and salt concentrations. However,

pore enlargement (to 56–95%) of the NF membrane was found

on the surface of a composite membrane of polyamide and

polysulfone in the presence of a dye solution containing 10–15 g

of Na2SO4 L�1 and this was found to due to SO42� pushing

through the membrane via transmembrane pressure. To prevent

this pore enlargement problem, an optimum operating pressure

and the proper selection of the membrane must be carried out to

ensure the integrity of the membrane for a long period.

(C-2) Nanofiltration by nanofibre media. Nanofibres are

essentially defined by their effective diameters (in the range of

1 to 200 nm) and they can be used for fine filtration. The synthetic

materials, both organic and inorganic, that are spun from the

molten state into the finer fibres differ in terms of the equipment

downstream of the spinneret, which enables a wide range of fibre

diameters to be produced. Because these media are capable of

removing contaminants to below 0.1 mm, they are usually known

as nanomembranes. Fig. 4 shows thiol-functionalized zonal

mercaptopropyl silica nanofibres. Those nanofibres were

obtained by the dissolution of electrospun polyacrylonitrile

(PAN) nanofibres with dimethylformamide. PAN nanofibres

were prepared by coating with 3-mercaptopropyl trimethoxy-

silane via a sol–gel process. The uniform porous channels

between the nanofibres facilitated Hg2+ transportation in the

adsorption process.208

Fine spinning techniques have been used for the production of

carbon and ceramic fibres as well as the fibres of other materials

and are obviously used as a filter medium, particularly for air

filtration.209 The introduction of an antimicrobial functionality

agent in the particulate filters has been explored and it has been

found that most microorganisms often become resistant, which

could limit the benefit of an antimicrobial functionality.

Furthermore, most of the microorganisms enter the filter with

airborne particulate matter and they grow in size due to accu-

mulation and build up on the filter surface. Metallic silver and

silver oxides are safe and effective antimicrobial agents at low

levels. Positively charged silver ions attract electronegative

bacterial cells and bind with the sulfhydryl group on the cell

membrane or bacterial DNA and result in the prevention of

proliferation of the microorganisms.210,211 Ionic plasma



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processing (IPD) is a suitable method for the coating of surface

engineered nanosized silver particles on polymeric surfaces for

use in heating, ventilation and air conditioning (HVAC) air

filters. Porous electrospun nanofibrous scaffolds (porosity larger

than 70%) can be used to replace flux limiting asymmetric porous

ultrafiltration membranes (of porosity in the range of 34%).

Yoon et al. synthesized polyacrylonitrile nanofibrous layers

supported on nonwoven microfibrous substrates (a melt-blown

polyethylene terephthalate mat) and applied a water resistant,

but water permeable, coating of chitosan over the nanofibrous

layer. A high flux and low degree of fouling resulted with the use

of this nanofibre media.212 It was observed that an electrospun

membrane conveniently rejects the microparticles and acts as a

screen filter without fouling the membrane, especially when the

particles are larger than the largest pore size of the nanofibrous

membrane. A high surface-to-volume ratio of the nanofibrous

media increased the degree of fouling. Therefore, the surface

modification of the nanofibrous screen filter with a suitable

hydrophilic or hydrophobic oligomer is often recommended to

reduce the fouling effect. A novel alumina nanofibre filter, which

consists of a single layer of an alumina nanofibre grafted onto a

microglass fibre backbone has shown potential for the effective

removal and retention of aerosols from polluted air. The

extraction fraction of the nanofibre filter was found to be three

orders lower than the HEPA filters, demonstrating that viruses

were effectively retained in the nanofibre filter due to an elec-

trostatic attraction between the electropositive fibre surface and

the electronegative aerosol particles.212

The addition of polystyrene nanofibres to the coalescence

filters (glass fibres) have shown that the addition of small

amounts of polystyrene nanofibres significantly improve the

coalescence efficiency of the filter but also significantly increase

the pressure drop of the filters.213 Conventional ion exchange

resins are normally either a gel structure or a granular structure

and are typically made of styrene or acrylic as the structural

materials. Granular resinous materials have larger pore volumes

and low ion-exchange capacities than gel type materials and

additionally have a better mechanical strength. The fibrous

materials are applied as a support for the ion-exchange func-

tionality due to their ease of preparation, contact efficiency and

the physical requirement for strength and dimensional

stability.214 Polymeric nanofibre-based ion exchangers have

higher swelling behaviors compared to other media because of

their high surface area, porosity and capillary motion.215 In

Fig. 4 The assembly of a zonal mercaptopropyl silica nanofibre.208


addition, polymer nanofibre ion exchangers are found to have

extremely rapid kinetics and higher ion-exchange capacities.215

A major trend with regards to filter media is the development

of the combination filter, capable of performing two separation

tasks at once, such as solid–gas separation and the adsorption of

a gaseous impurity. These combination filters employ a chemi-

cally active agent embedded in the filter medium material, the

most common of which is activated carbon as an adsorbent.

Recent developments in nanofibre filters offer a range of

adsorptive and chemical treatments and antimicrobial action via

silver zeolite technology. Titanate nanofibres have been synthe-

sized via a hydrothermal reaction between a concentrated NaOH

solution and TiOSO4. The titanate fibres were dispersed in

ethanol to form a suspension containing 0.2 wt% titanate

nanofibres. The suspension was sonicated for 10 min to achieve a

homogeneous dispersion, which was used to apply thin layers on

the porous substrate using a spin-coater. The separation effi-

ciency of the ceramic membranes can be significantly improved

by constructing a top separation layer with TiO2 nanofibres.

These improvements are due to the radical changes in the texture

of the top-layer. It is also found that the top layers of the TiO2

nanofibres on the porous glass and alumina support were similar

in terms of their structure and performance. They are able to

retain more than 95% of 60 nm particles at a very high flux rate of

about 900 L m�2 h�1. Moreover, the fabrication of these

membranes is relatively simple and economical with low rejection

rates, compared to the conventional ceramic membranes. This

approach can be scaled-up for the fabrication of ceramic

membranes in practical applications.216

For the filtration of liquids, conventional porous polymeric

membranes have their intrinsic limitations; e.g. low flux rates and

high fouling. These drawbacks are due to the geometric structure

of the pores, the corresponding pore size distribution217 and the

formation of undesirable macro-voids across the whole

membrane layer.218 It appears that electrospun nanofibrous

membranes can overcome some of these limitations and, conse-

quently, a polyethersulfone (PES) electrospun nanofibre mat has

been applied as a membrane for liquid filtration. To increase

the mechanical strength of the PES fibre, a poly(ethylene

terephthalate) (PET) non-woven sub-layer was also used. The

composite membrane (PES–PET) is illustrated in Fig. 5. The

PES/PET electrospun nanofibrous membranes (ENMs) indi-

cated that the membranes possess a high initial flux. The

turbidity of the permeated suspension was found to decrease

significantly with time.219 Polysulfone (PSU) nanofibrous

membranes possess much higher porosity and have a high

surface area, which results in high flux pre-filters with even higher

loading capacities. Such pre-filters can be used in various appli-

cations, such as the removal of micro-particles from waste water

and in ultrafiltration, or with nanofiltration membranes, to

prolong the life of these membranes.220,221 A coat of TiO2/poly-

vinyl alcohol (PVA) on the polyester filter effectively modifies the

matrix, narrows the pore size to 10 mm and reduces the contact

angles by 40� because of the hydroxyl groups from PVA

and TiO2. The improved hydrophilicity and anti-fouling prop-

erties of the composite membranes enables a highly stabilized

(10 m3 m�2 h�1 or 10 kPa) pure water flux and a high effluent flux

during long term filtration tests in membrane reactor systems for

the treatment of simulated waste water in the removal of nitrate/



Fig. 5 The structure of PES/PET composite membranes.220

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ammonium to allow for water reuse in a polyester fibre

production plant.222

(D) Sorbents

Water remediation techniques include (1) adsorption, (2)

biotechnology, (3) catalytic processes, (4) membrane processes,

(5) ionizing radiation processes, and (6) magnetically assisted

processes. In addition, nanomaterials-based processes are

becoming promising options for applications in water treatment.

The adsorption technique is the most frequently studied method

to purify water.221–224 Sorption is the transfer of ions from the

solution phase to the solid phase. Sorption actually describes a

group of processes, which include adsorption and precipitation

reactions. Basically, adsorption is a mass transfer process by

which a substance is transferred from the liquid phase to the

surface of a solid and becomes bound by physical and/or

chemical interactions.225 Various low cost adsorbents, derived

from agricultural waste, industrial by-products, natural mate-

rials, or modified biopolymers, have been developed recently and

applied in the removal of heavy metals from metal-contaminated

waste water. In general, there are three main steps involved in

pollutant sorption onto solid sorbents: (i) the transport of the

pollutant from the bulk solution to the sorbent surface; (ii)

adsorption on the particle surface; and (iii) transport within the

sorbent particle. Technical applicability and cost-effectiveness

are the key factors that play major roles in the selection of the

most suitable adsorbent to treat waste water.226,227

Magnetic sorbents or magnetic ion exchange (MIEX) resins

have been introduced for the removal of natural organic matter

(NOM) from ambient raw water and was found to be better

than coagulation processes. The ion exchange resin beads

contain a magnetized component within their structure, which

allows the beads to act as individual magnets. The magnetic

component results in the beads forming agglomerates that can

settle rapidly or fluidize at high hydraulic loading rates. The

very small resin bead size could provide a high surface area to

allow the rapid exchange of selective ions. Resins with a poly-

acrylic skeleton with macroporous properties are more suitable

for the removal of NOM than gel resins.228,229 In addition, the

MIEX process involves adsorbing the DOC onto the MIEX

resin in a stirred contactor that disperses the resin beads to

achieve a maximum surface area. The magnetic part of the resin


allows the resin to agglomerate into larger and faster settling

particles, which allow a recovery rate of greater than 99.9% to

be obtained. The MIEX process has been applied for the

removal of humic acid,230 low to moderate organic concentra-

tions over wide range of alkalinities and bromide concentra-

tions.231,232 Coagulation removed 60% of the dissolved organic

carbon (DOC) associated with the 1–10 k fraction but had little

impact on the DOC concentration of the <1 k fraction. Treat-

ment with MIEX removed approximately 80% of the DOC

associated with the 1–10 k fraction and almost 60% of the DOC

associated with the <1 k fraction. The nature of water was

varied with differing types of NOM and passed through the

MIEX resin. It was observed that an increase in the hydro-

phobicity of the resin reduced the DOC. Furthermore, the

MIEX resin was used for the removal of some specific inorganic

ions, such as As(V) and Cr(VI). The MIEX process has also been

applied in a pre-treatment step for catalytic processes, such as

effective debromination using ozonation.233

Magnetic chitosan gel particles, a magnetite bearing covalently

immobilised copper phthalocyanine dye, magnetic charcoal and

magnetic alginates have all been applied in the removal process

of polycyclic dyes, malachite green, crystal white and other

organic dyes from waste waters.234–238 Magnetotactic bacteria

naturally occur as magnetic sorbent sources in nature. These

bacteria have the ability to orient themselves in the direction of

the magnetic field. Magnetotactic bacteria have been applied in

the removal of organic pollutants from water by enzymatic

reactions.239 Surfactant-coated magnetite nanoparticles have

been applied in the extraction of the organic contaminant,

2-hydroxyphenol.240 Floating magnetic sorbents, in the form of

polymer-coated vermiculite iron oxide composites, have been

formulated. The composites float on the surface of water and can

easily remove spilled oil from oil contaminated water.241

In addition, zero valent iron (nZVI) nanoparticles with a

diameter of 1–120 nm are able to remove As(III) and As(V) by

rapid adsorption followed by precipitation and result in surface

corrosion byproducts.242–244 The adsorption of As(III) and As(V)

by iron nanoparticles is due to the weak electrostatic attraction

between the adsorbed species and the binding sites.245 Nano-

particles of nZVI have been prepared by adding freshly made

ferric chloride to a reaction vessel containing solid NaBH4.246

Polyvinyl alcohol-co-vinyl acetate-co-itaconic acid has been

found to be an effective surfactant for the stability of nZVI

nanoparticles. Initially the sites are amorphous Fe(II)/Fe(III)

magnetite. As the treatment progresses, the initial reactive sites

gradually transform into lepidocrocite and the more crystalline

magnetite. Heavy metal ions (e.g. Ni(II)) are adsorbed on the

oxide shell as corrosion proceeds and precipitates on the iron

core of the nanoparticle. Nanoparticles of nZVIs have higher

reactivities due to their larger surface area (average area: 35.5 m2

g�1) than commonly used microparticles (average area: 0.9 m2

g�1) and they also have reaction rates that are 100 times higher

than those of microparticles.247 Since the reactions with orga-

nohalides are considered as ‘‘inner-sphere’’ surface-mediated

processes, the application of iron nanoparticles is becoming a

real potential.246 Fig. 6 shows the reduction of organic pollutants

by chemisoprtion on nZVI nanoparticles. Iron can reduce water

and form hydrogen gas under anaerobic conditions as

follows:248,249



Fig. 6 A schematic diagram of the reduction of perchloroethylene on the

surface of a nZVI particle.74

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Fe0 + 2H2O / Fe2+ + H2 + 2OH�

In addition, iron can remove chlorine by hydrogenolysis as

follows:

Fe0 + R–Cl + H+ / Fe2+ + R–H + Cl�

where Fe0 is oxidized to Fe2+, while perchloroethylene is dech-

lorinated. The application of bulk zero valent iron metal in

environmental remediation has limitations, such as slow reaction

rates and the formation of more toxic intermediates resulting

from the dechlorination process, which are difficult to destroy.250

Iron nanoparticles have been widely applied in the trans-

formation and detoxification of a wide variety of common

environmental contaminants, such as chlorinated organic

solvents, organochlorine pesticides and PVBs, due to their large

surface area and surface activity.251 The degradation rates of

trichloroethylene (TCE) has been significantly increased to 3 �10�3 L m�2 h�1 from 6.3 � 10�5 L m�2 h�1, which was obtained

when bulk Fe0 particles (>1 micrometer) were used to remove

TCE.252 The feasibility of applying nZVI particles in water

remediation showed that up to 90% TCE was degraded within 30

min.253 In addition, non-toxic final products were formed. The

capacity of transforming TCE into harmless compounds was

found to be higher than that of micro ZVI particles due to the

higher specific area. The content of Fe0, the solution pH, the

concentration of TCE and the presence of anions, such as nitrate,

can significantly affect the degradation of TCE. Furthermore,

the application of nZVI particles could reduce the presence of

perchlorate ions in contaminated media by 66% within 336 h via

a reduction process on the surface of nZVI and the association of

an oxygen molecule from the perchlorate ions with nZVI.254,255

The amount of Ni metal adsorbed by iron-core nanoparticles

is higher than by iron oxide nanoparticles (i.e. gFe2O3). The

addition of functional materials, such as stable noble metals,

metal oxides and low molecular weight organic molecules and

polymers, in the preparation of nZVI nanoparticles could deliver

higher adsorption capacities for heavy metals that will lead to

better remediation.245 Modified nZVI nanoparticles that have

been applied in the removal of metal ions from water are shown

in Table 4. Hu et al. reported that modified jacobsite (manganese

iron oxide) nanoparticles can be applied and regenerated many

times without a decrease in their adsorption performance.256 The

study by Bezbaruah et al. shows that nZVI entrapment in a


biopolymer matrix (alginate) may increase their overall efficacy

in groundwater remediation. The authors have shown that nZVI

can be effectively trapped in Ca-alginate beads and the reactivity

of the entrapped nZVI toward a model contaminant (nitrate) was

comparable to that of one dimensional ZVI.257 The reduction in

the nitrate concentration using one dimensional nZVI and

trapped nZVI in Ca-alginate beads were 55–73% and 50–73%,

respectively, over a 2 h remediation period. In addition, Ca-

alginate is suitable for the entrapment of nZVI to make the

nanoparticles relatively stationary in aqueous media (e.g.,

groundwater). Thus, the mobility and settlement problems

associated with one dimensional nZVI can be overcome and

alginate-trapped nZVI can be effectively used in permeable

reactive barriers for groundwater remediation. Keum and Li

reported the application of Fe0 nanoparticles in the reductive

debromination of polybrominated diphenyl ethers (PBDEs). The

debromination of PBDEs by nZVI particles has high potential

value for the remediation of PBDEs in the environment.263 Kim

et al. applied Fe0 in the removal of alachlor and pretilachlor.264

Wang and Zhang reported the utilization of Fe0 nanoparticles in

the dechlorination of several chlorinated aliphatic compounds

and a mixture of PCB at relatively low metal-to-solution ratios

(2–5 g per 100 mL).265 Up to 25% of PCB could be removed

within 17 h by using fresh nZVI particles. Fe0 nanoparticles in

environmental remediation has great potential; however, the

limitations of using Fe0 nanoparticles are: (1) the high activity

leads to the storage of freshly synthesized iron particles becoming

a significant problem; (2) the activity of the iron particles

decreases as the reaction proceeds due to the formation of

Fe(OH)3, which forms on the surface of the Fe0 nanoparticles

and may make the iron core unreactive, therefore blocking

further reaction; (3) the freshly fabricated Fe0 can easily form

aggregates and this decreases the dispersion ability. Recently,

research focus have been on solving the above problems. To

improve the stability of Fe0 nanoparticles against aggregation,

He and his group members developed palladized iron (Fe–Pd)

nanoparticles stabilizing on starch265 or sodium carboxymethyl-

cellulose (CMC).266 The results showed that the dispersibility of

the nanoparticles increased and, thus, the degree of dechlorina-

tion greatly increased. The starch-coated Fe0 nanoparticles

exhibited markedly greater reactivity during the dechlorination

of TCE or PCB in water. About 98% of TCE and 80% of PCB

were found to be decomposed after 1 h and 100 h, respectively. In

addition, the degradation rate of TCE by CMC-stabilized

nanoparticles was 17 times faster than that of Fe0 nanoparticles.

Ag-modified Fe0 nanoparticles (1% Ag)267 could facilitate the

dechlorination of tetra-, tri- and di-chlorobenzenes (TeCB, TCB

and DCB, respectively) within 24 h at a metal loading of 25 g L�1

and the dechlorination rate was found to positively correlate with

the amount of silver loaded on the bimetallic particles.

Researchers268,269 have done lots of work on the dechlorination of

PCBs using nanoparticles with activated carbon and bimetallic

nanoparticles. Several kinds of assemblies and bimetallics have

been investigated, such as GAC (granular activated carbon)/Fe/

Pd bimetallics, GAC/ZVI, GAC/ZVI/Pd, Pd/Mg bimetallics, etc.

The GAC/ZVI/Pd system showed an efficient dechlorination of

2-chlorobiphenyl (2-ClBP) at 90% after 2 days. The high degra-

dation ability of the GAC/ZVI/Pd system is caused by the

synergistic and simultaneous function of adsorption and



Table 4 Previous studies of the environmental remediation using modified nZVI

Type of nanoparticleParticle size(nm) Targeted heavy metal ion

Major binding sites or functionalgroups for the removal of the heavymetal

Modified jacobsite (MnFe2O4)255 10 Cr(VI) MnO2 and Fe2O3 are major

adsorptive componentsFerrites (MeFe2O4, where Me ¼ Mn,Co, Cu, Mg, Zn or Ni)257

20 Cr(VI) The major adsorption driving force isthe redox reaction between Mn(II)and incoming Cr(VI)

Magnetic nanoparticles encapsulatedby poly(3,4-ethylenedioxythiophene)(PEDOT)258

11 Ag(I), Hg(II), Pb(II) Major binding sites are the O-donoratoms and S-donor atoms of PEDOT

Magnetite nanoparticles modifiedwith dimercaptosuccinic acid259,260

6 Hg(II), Co(II), Cu(II), As(V), Ag(I),Cd(II), Ti(III), Pb(II)

Thiol groups fromdimercaptosuccinic acid play a majorrole in the binding site

Maghemite (-Fe2O3)261 50 Mo(VI) Anionic adsorption between

positively charged maghemite andMoO4

2� at pH values below 6 andelectrostatic repulsion betweennegatively charged maghemite andMoO4

2�

Silica-coated magnetite (Fe3O4)262 50–80 Hg2+ The dithiocarbamate group after the

derivitization of magnetite plays amajor role in the removal of Hg2+

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dechlorination of PCB. The perchlorate ion was significantly

reduced by 1.8 and 3.3 fold with starch and CMC-modified

nZVI, respectively, when it was compared with nZVI.270

Ge et al. applied modified Fe3O4 magnetic nanoparticles

(MNPs) (15–20 nm) with 3-aminopropyltriethoxysilane and

copolymers of acrylic acid and crotonic acid in the removal of

Cd2+, Zn2+, Pb2+, Cu2+ from metal contaminated water.272,273 It

was observed that the MNPs could efficiently remove the metal

ions with a high maximum adsorption capacity at pH 5.5 and

could be used as a recyclable adsorbents under convenient

conditions.271 Mak and Chen found that methylene blue could be

recovered fast and efficiently from an aqueous solution by

polyacrylic acid-bound iron oxide magnetic nanoparticles (12 nm

average diameter) due to their large specific surface area and the

absence of internal diffusion resistance.274 The impact of pH on

the removal of Mo(VI) from contaminated waste water was

studied by Afkhami and Norooz-Asl.275The surfaces of the metal

oxides (Fe2O3 suspension) are generally covered with hydroxyl

groups that vary in their form at different pHs. The surface

charge is neutral at pHzpc (the zero point charge pH, where the

pHzpc of maghemite nanoparticles is around 6.3). Below the

pHzpc, the adsorbent surface is positively charged and anion

adsorption occurs. With an increase in the pH up to pH 4, the

uptake of MnO42� ions increases and remains constant in the pH

range of 4–6. Then, the uptake of Mo(VI) in the form of MoO42�

ions decreases at pH values higher than 6 as the adsorption

surface becomes negatively charged at pH > pHzpc; this leads to

an increasing electrostatic repulsion between the negatively

charged species (MoO42�) and the negatively charged adsorbent,

which releases the adsorbed MoO4. The removal efficiency was

highly pH dependent and the optimal adsorption was found to be

pH 4.0–6.0.273

The sorbents can also be of mineral, organic or biological

origin (e.g. activated carbons, zeolites, clays, silica beads), low

cost adsorbents (e.g., industrial by-products, agricultural wastes

and biomass) and polymeric materials (e.g., organic polymeric


resins and macroporous hyper-crosslinked polymers).279 In

recent years, natural polymer adsorbents have been used (e.g.,

chitin and starch and their derivatives, chitosan and cyclodex-

trin).274,275 Polysaccharide-based materials can be used as

sorbents in waste water treatment.276 Chitosan has been applied

extensively in various research areas of water/waste water treat-

ment.277,278 Chang and Chen modified the chitosan polymer and

grafted carboxylic groups. Then, the carboxylated chitosan was

covalently bound to magnetic nanoparticles. Afterwards, modi-

fied chitosan nanoparticles were used for the removal of metals

fromwaste water.280Qi and Xumodified the chitosan polymer by

ionic gelation with tripolyphosphate (TPP) as an ionic cross

linker and both the research groups found that modified chitosan

nanoparticles could provide high adsorption capacities of

Pb2+.281 However, the limitation of modified chitosan nano-

particles is disintegration in aqueous solution or aggregation in

pH 9 alkaline solution due to weak electrostatic interactions

between chitosan and TPP molecules. Some NPs are powerful

adsorbents due to their unique structure and electronic proper-

ties. Dissolved organic carbon and organic colloids in the sub-

micron size range have been recognized as a distinct non-aqueous

organic phase to which organic pollutants are adsorbed,280 which

leads to a reduction in their bioavailability.

Zeolites are the microporous materials (pore size < 2 nm) and

consist of a 3-dimensional arrangement of [SiO4]4� and [AlO4]

5�

polyhedra connected through their oxygen atoms to form large

negative lattices with Brønsted and Lewis acid sites. If cations are

exchanged by protons, the zeolite acquires considerable Brønsted

acidic properties. The application of zeolite materials for envi-

ronmental remediation has gained great attention due to their

selective sorption capacities, non-toxic nature, availability and

low cost. Zeolites have been widely applied in the removal of

heavy metals, such as Cr3+, Ni2+, Zn2+, Cu2+, Fe2+, Pb2+ and Cd2+

from waste water in the mining industries.281–283 The stability of

zeolites is high and disintegration was found only at pH values

below 2. In addition, zeolites have been applied in the retention



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of methyl-tert-butyl ether, chloroform and TCE in water and

they were found to be 8–12 times more efficient than activated

carbon. Mesoporous silica-based materials (pore size 2–50 nm)

have been widely used to remove heavy metals present in waste

water. Mesoporous silica with functionalized monolayers have

been useful in the removal of mercury and other heavy metals

from waste water. Better adsorption capacities were found for

amino-functionalized silica in the capture of Cu2+, Zn2+, Ni2+ and

Cr2+, whereas Hg2+ was better adsorbed on thio-functionalized

silica.284 Activated alumina, a porous form of Al2O3, has a high

porosity so it is widely used in filtering components for drinking

water purification. Mesoporous alumina, aminated mesoporous

alumina and alumina-supported MnO can remove As(V), As(III),

Cu(II) and TCE from the contaminated water.285–287

Nanoscale diboron trioxide/titanium dioxide composite

materials have been used for the separation of trace cadmium

ions from polluted water containing cadmium ions. The other

pollutant matrix ions had no negative effect on the removal of

the cadmium ions. The removal of organic pollutants using

inorganic nanoparticles and mesoporous structures can be

carried out in two ways: (1) static force (including Lewis

adsorption); and (2) weak and chemical bonding through

hydrogen bonds/p bonds with the surface functional groups.

Modification and chemical treatment of the nanomaterials are

essential to enhance the target adsorption ability. The removal

efficiency of As(III) and As(V) was higher for nanocrystalline

TiO2 than fumed TiO2.288,289 Alvaro et al. synthesized meso-

porous TiO2 nanoparticles in association with tetraethyl ortho-

silicate using neutral pluronics as templates. The photocatalytic

activity for the degradation of phenol was much higher for

mesoporous TiO2 nanoparticles compared to standard P-25

TiO2.291

Many types of polymers have been assembled into nano-

particles via polymerization techniques. PolyN-iso-

propylacrylamide nanoparticles remove Pb2+ and Cd2+ from

waste water. The adsorption of Pb2+ and Cd2+ by the polymer is

due to the Coulombic attraction between the carboxylate group

on the polymer and the positive charged metal species fromwaste

water.292 However, the utilization of polyN-isopropylacrylamide

in waste water treatment is not widespread because the main

functional group, isopropylacrylamide, does not show a favor-

able ability to remove metals. Polymeric nanoparticles synthe-

sized by the copolymerization of a pyridyl monomer with styrene

have been used for the removal of metal ions. It was observed

that the removal rate of metal ions from the aqueous solution

was fast due to the presence of bipyridine-based metal-chelating

groups on the surface of the nanoparticles.290 It was found that

the poly(vinylpyridine) nanoparticles could selectively remove

Cu2+ despite the presence of other competing ion species. Chen

et al.293 modified polystyrene nanoparticles with a specific dye

molecule called an azo-chromophore. It was observed that the

adsorption efficiency for the removal of Pb2+ increased due to a

modification of the nanoparticle ligand and the nanoparticles

could retain their adsorptive capacity after 3 cycles of adsorp-

tion/desorption.293 Bell et al. demonstrated that the sequestration

selectivity for heavy metals could be altered by grafting a

macrocyclic ligand of the polymeric nanoparticles with a core–

shell structure. The original core–shell structure without grafting

can adsorb Hg only, while the modified nanoparticles could


adsorb Co2+ selectively in the presence of other heavy metal

ions.294 Therefore, polymeric nanoparticles can be tailored for

the remediation of selective heavy metals. Tungittiplakorn et al.

found that polyurethane-based nanoparticles could be applied in

the desorption and transportation of organic pollutants with the

hydrophobic core of the nanoparticles.295,296

(E) Dendrimers

Metal nanoparticles (NPs) have received great scientific and

technological interest in environmental remediation due to their

size, unusual crystal shapes and lattice orders.295 Nano-sized

metal particles are expected to exhibit much higher reactivity

because of their larger surface area than bulk particles. However,

the preparation of functionalized NPs using different methods

still remains a great challenge. One of the unique approaches

used to prepare inorganic NPs is through the use of dendrimers.

Dendritic nanopolymers are highly branched 3D globular

nanoparticles with controlled compositions and architectures.

Dendrimers are relatively monodisperse and highly branched

nanoparticles with controlled compositions and design and their

sizes are in the range of 1–100 nm. Dendrimers are built from a

starting atom, such as nitrogen, to which carbon and other

elements are added by a repeating series of chemical reactions

that produce a spherical branching structure, as shown in Fig. 7,

in which divergent or convergent hierarchical assembly strategies

are involved. As the process repeats, successive layers are added

and the sphere can be expanded to the size required by the

investigator. Dendrimers consist of three components: (1) a core,

(2) interior branched cells and (3) terminal branched cells.297,298

Ammonia is used as the core molecule and it reacts with meth-

ylacrylate in the presence of methanol and then ethylenediamine

is added:

NH3 + 3CH2CHCOOCH3N(CH2 CH2COOCH3)3 /

3NH2CH2CH2NH2N(CH2CH2CONHCH2CH2H2)3 + 3CH3OH

At the end of each branch there is a free amino group that can

react with 2 methylacrylate monomers and 2 ethylenediamine

molecules. Each complete reaction sequence results in a new

dendrimer generation as shown in Fig. 8. Dendrimers are a novel

class of polymers with a compact spherical structure and unique

behavior and a narrow size distribution that can be used as

templates or stabilizers to form relatively monodispersed

organic/inorganic hybrid NPs. The crucial role played by den-

drimers is in the synthesis of dendrimer-stabilized NPs, in which

metal ions are usually complexed with dendrimer ligands (e.g.

interior tertiary amines, terminal functional groups) through

coordination, electrostatic interaction, etc., followed by a

reduction or other reactions to form inorganic NPs stabilized by

dendrimers. They are routinely synthesized from a central

polyfunctional core by the repeated addition of monomers. The

core is characterized by a number of functional groups. The

dendrimer generation is created by adding monomers to each

functional group in turn, leaving the end groups able to react

again. The structure of the polymer is determined by the number

of reactive groups of the core, the branch lengths and surface

group dimensions. The maximum size is limited by the genera-

tion at which the dendrimer becomes tightly packed.300



Fig. 7 The dendritic structure.298

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Poly(amidoamine), or PAMAM, dendrimers have been devel-

oped for applications in the remediation of waste water

contaminated with a variety of transition metal ions, such as

copper (Cu(II)). Diallo et al. first reported the use of PAMAM

dendrimers for copper removal in 1999. Dendritic nanopolymers

can encapsulate a broad range of solutes in water, including

cations (e.g., copper, silver, gold, iron, nickel, zinc and uranium)

by attachment to the functional groups of dendrimers, such as

primary amines, carboxylates and hydroxymates.303 They also

can deactivate bacteria and viruses after binding. Poly(amido-

amine) dendrimers can remove metal ions (Cu(II), Ag(I), Fe(III)

and others) by functioning as chelating agents and ultrafiltraters

and the removal capacity can be improved by attaching metal

ions to the functional groups of dendrimers, such as primary

amines, carboxylates and hydroxymates.299 Dendritic nano-

polymers, such as PAMAM dendrimers, have much less

tendency to pass through the pores of ultrafiltration membranes

Fig. 8 A graphical presentation of PAMAM dendrimers (a) PAMAM,

(b) poly(glycerol-succinic acid) dendrimer, (c) Boltorn� and (d) hyper-

branched polyglycerol.301


than linear polymers of a similar molar mass because of their

much smaller polydispersity and globular shape. Therefore,

dendritic nanopolymers have been used to enhance UF and MF

processes for the recovery of dissolved ions from aqueous solu-

tions. First, contaminated water is mixed with a solution of

functionalized dendritic nanopolymers and then the mixtures of

nanopolymers and bound contaminants is transferred to UF or

MF units to recover the clean water. In those units, the bound

target substance is separated from the nanopolymers by

changing the acidity (i.e., the pH) of the solution. Finally, the

recovered concentrated solution of contaminants is collected for

disposal or the nanopolymers may be recycled.302 Diallo et al.

have combined bench scale measurements of metal ion binding

to Gx-NH, PAMAM dendrimers with dead-end UF experiments

to assess the performance of dendritic polymer filtration to

recover Cu(II) from aqueous solutions. The Cu(II) binding

capacities of the PAMAM dendrimers seem to be much larger

and more sensitive to the pH of the solution than those of linear

polymers with amine groups. Furthermore, metal ion (Cu(II))

trapped PAMAM dendrimers can be regenerated by decreasing

the solution pH to 4.0–5.0, thus enabling the recovery of the

bound Cu(II) and the recycling of the dendrimer. PAMAM

dendrimers can also be applied in the recovery of perchlorate

anions and uranium metals from contaminated groundwater.

Polyamidoamine dendrimers (PAMAM), after surface modifi-

cation with benzoylthiourea groups, are a new and excellent

water-soluble chelating ion exchange material with a distinct

selectivity for toxic heavy metal ions. Investigations on the

removal of Co(II), Cu(II), Ni(II), Pb(II) and Zn(II) have been

performed using the PAMAM-supported filtration method. The

results showed that all metal ions could be retained almost

quantitatively at pH 9. Cu(II) could form the most stable

complexes with the benzoylthiourea-modified PAMAM deriva-

tives and complete retention was achieved at pH>4, and could be

separated selectively from the other heavy metal ions

investigated.303

The key novelty of the dendritic polymer filtration process is

the combination of dendritic polymers with multiple chemical

functionalities with UF and MF. This may enable the develop-

ment of a new generation of water treatment processes that are

flexible, reconfigurable and scalable.300 Dendritic polymer

filtration processes are scalable and could be used to develop

small and mobile water treatment systems as well as large and

fixed treatment systems. Dendritic nanopolymers have also much

smaller intrinsic viscosities than linear polymers with the same

molar mass because of their globular shape.300 Thus, compara-

tively lower operating pressures, energy consumption and the

loss of ligands by shear-force induced during filtration could be

achieved with dendritic polymers in cross-flow UF systems and

this dendritic filtration could be applied in industrial water

treatment.304

PAMAM dendrimers have great potential as templates for

metal composite nanoparticles due to their low toxicity and

highly regular, branched and three-dimensional structure, which

can host inorganic nanoclusters and form stable dendrimer

complexes and nanocomposites.305 In general, the reduction of

silver cations with NaBH4 and the template role of the den-

drimers ensure well dispersed silver nanoparticles with a rela-

tively small size distribution.306 Investigations have already



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indicated that dendrimer-encapsulated silver nanoparticles

possess antimicrobial activity.307 Strong bonding is achieved

through a mechanism that involves an interaction with noble

metals, such as catalytic palladium nanoparticles in films of

linear poly(ethyleneimine) (PEI) and silver. Platinum and palla-

dium nanoparticles have also been successfully encapsulated

within poly(propyleneimine) dendrimers. All of these polymers

possess some strong electron-donating centers, such as amino or

carboxylate groups, which facilitate metal–polymer complex

formation. In these studies, the preparation of polymer/metal

nanoparticles needed extra reducing agents, including sodium

borohydride (NaHB4), formaldehyde, sodium citrate or hydra-

zine. An amine-terminated hyper-branched poly(amidoamine)

(HPAMAM–NH2) has been used to produce antimicrobial silver

nanocomposites.308 More importantly, this hyperbranched

polymer was found to serve as a highly effective self-reducing

agent. The advantages of this method are: (i) no extra reducing

agent was needed; (ii) the process was conducted at room

temperature and under normal pressure and in an aqueous

solution, making it a green route; and (iii) the obtained silver

nanoparticles have several excellent properties, such as a long-

term dispersion stability, small particle size, a narrow size

distribution controlled by the composition and a good antimi-

crobial activity as tested against E. coli, S. aureus, B. subtilis and

K. mobilis. The activity was enhanced with an increasing silver

concentration. The bacterial inhibition ratio of the HPAMAM–

NH2/Ag nanocomposites reached up to 95% at a silver concen-

tration of 2.7 mg mL�1. Furthermore, pure HPAMAM–NH2 also

indicated some limited antimicrobial ability with an inhabitation

ratio less than 10%. To increase the metal adsorption capacity

and form metal nanoparticles, it is imperative that metal ions are

trapped in the interior structure of the dendrimer (to offer

protection against agglomeration). This is usually attempted by

selective protonation or hydroxylation of the surface amines

Table 5 The applications of dendrimers in environmental remediation

Type of composite dendrimer Thickness of membrane/disc (

Silica nanoparticles prepared bymixing salicylic acid and hyper-branched poly(propylene imine)

300 � 76

PAMAM dendrimer compositemembrane with hyaluronic acid ina chitosan gutter layer

300

PAMAM dendrimer compositemembrane consisting of chitosan anda dendrimer

300

Ni loaded hydrogel PAMAM 2 mm disc

PAMAM dendrimer compositemembrane consisting of chitosan anda dendrimer

300

Composite membrane PAMAMdendrimer and trimesoyl chloride onpoly(ether ketone) (TMC)

The diameters of the pores incomposite membrane are in thof nanometers

Impregnation of cross-linkedsilylated and cyclodextrin dendriticpoly(propylene imine) andpoly(ethylene imine) on ceramicmembranes, such as TiO2, Al2O3 andSiC

8 mm pore size for the TiO2 fil3 mm pore size for the Al2O3 fi


(the surface amine is more basic as it is primary in nature).308 The

interaction of Cu2+ with PAMAM leads to two changes in the

absorption spectrum: a band due to the d–d transition gains

prominence and shifts to 605 nm and a new band, due to a strong

ligand-to-metal interaction, appears at 300 nm. Some effort has

been made to synthesize dendrimer-protected silver nanodots

exhibiting fluorescence.309 It is quite clear that dendrimers play a

critical role in stabilizing the cluster/nanoparticle surface and

thus dramatically increase the stability of such species. Table 5

shows some applications of dendrimers for environmental

remediation.

(F) Carbon nanomaterials

The use of nanoscale activated carbon may have advantages over

conventional materials due to the much larger surface area of the

nanoparticles on a mass basis. In addition, their unique structure

and electronic properties can make them especially powerful

adsorbents.317 Many materials have properties that are depen-

dent on size. The advantages for processes involved in environ-

mental remediation are due to (1) their great capacity to adsorb a

wide range of pollutants, (2) their fast kinetics, (3) their good

surface area, and (4) their selectivity towards aromatic solutes.276

Activated carbon from various sources, such as coconut coir, jute

stick, rice husk, etc., is the most popular of the adsorbents. The

treatment of water by adsorption methods uses specific ion

exchangers or extractants and a combination of adsorption with

catalytic treatment methods, redox processes and magnetic

processes. Recently, a new technique in adsorption has been

reported that applies carbon nanotube clusters. The unique

property of these clusters is their ability to remove bacteria from

water by an adsorption method. Adsorption on sorbents has

become one of the preferred methods to remove toxic contami-

nants from contaminated water. Adsorption-based separation

nm) Targeted contaminants Ref.

Removal of polycyclic aromatichydrocarbons (PAH), such as pyreneand phenanthrene, and Pb2+, Cd2+,Hg2+, Cr2O7

2� from contaminatedaqueous solutions

310

Separation of CO2 from a feed gasmixture of CO2 and N2 on poroussubstrates

311

Separation of CO2 from fossil fuelemission on porous substrates

312

Separation of Cu2+, Co2+ and Cr3+

from aqueous solutions313

Separation of CO2 from a feed gasmixture of CO2 and N2 on poroussubstrates

314

thee range

Rejection of salts: MgCl2, MgSO4,NaCl and Na2SO4

315

ter andlter

Removal of organic pollutants, suchas polycyclic aromatic hydrocarbons,trihalogen methane, pesticides andmethyl-tert-butyl ether

316–319



Fig. 9 (Super) structure representations of (a) a MWCNT and (b) a

SWCNT.326

Table 6 The porosity and specific surface area of as-grown CNTs andoxidized CNTs325

Type of sampleSpecific surfacearea (m2 g�1)

Pore volumeVp (cm

3 g�1)

As grown 122 0.28H2O2 130 0.36KMnO4 128 0.32HNO3 154 0.58

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processes are widely applied in the purification of drinking water

and natural gas contaminated air. Nanoscale carbon black, with

a particle size of 20–70 nm, can be modified via oxidized with

65%HNO3 by refluxing 10 g of carbon black with 150 mLHNO3

(65%) in a conical flask at 110 �C for 120 min. The modified

carbon black is filtered and washed with deionized water until the

pH of the filtrate is stable and the product is finally dried in a

vacuum oven at 110 �C for 24 h. The increased adsorption of

Cu2+ and Cd2+ on the modified carbon black is due to the

increased amount of functional groups after oxidation of the

carbon black surface. Adsorption of Cu2+or Cd2+ on the modified

carbon black increases with an increasing pH of the solution.

Most of the metals were adsorbed on the modified carbon black

when the pH was above 5.5. This might be caused by the surface

charge development of the modified carbon black and the

concentration distribution of Cu2+or Cd2+, which are both pH

dependent. At low pH, the adsorption of Cu2+ or Cd2+ on

modified carbon black is low because of the competition between

H+ and Cu2+ or Cd2+ for the adsorption sites. The surfaces of

modified carbon black have negative charges in a wide pH range

and Cu2+ or Cd2+ carry a positive charge, existing as either Cu2+

and Cd2+. When the pH level of the solution increases, the

concentration of competitor H+ ions decreases and Cu2+ or Cd2+

adsorption increases. Zhou and coworkers320 investigated the

adsorption of Cu(II), Zn(II), Pd(II) and Cd(II) on nanoscale

hydroxyapatite and carbon black. The adsorption isotherms

indicated that different kinds of heavy metals have different

affinities for black and activated carbon. The smaller nanoscale

carbon particles had a higher adsorption capacity than larger

carbon black and activated carbon because the micropores at the

internal surface of the activated carbon are not accessible to

humic acid, whereas the nanosized pores of carbon black are

more accessible to humic acid.319

Carbon nanomaterials (CNMs) have unique properties for

sorption processes. CNMs may exist in several forms, such as

single-walled carbon nanotubes (SWCNTs), multi-walled carbon

nanotubes (MWCNTs), carbon beads, carbon fibres and nano-

porous carbon. CNTs can be considered as cylindrical hollow

micro-crystals of graphite.320,324 Because they have a large

specific area, CNTs have attracted the interest of researchers as a

new type of adsorbent. CNTs are graphitic carbon needles and

have an outer diameter ranging from 4–30 nm and a length of up

to 1 mm.321 MWCNTs are made of concentric cylinders with

spacings between the adjacent layers322 of about 3.4 �A, as shown

in Fig. 9(a). SWCNTs (Fig. 9(b)) were discovered by Iijima.323 Li

et al. found that oxidized CNTs can be good Cd2+ adsorbents and

have great potential applications in environmental protection.

The specific area and pore specific volume of CNTs were found

to be increased after the oxidation of CNTs with H2O2, KMnO4

and HNO3 as shown in Table 6. The adsorption capacities of all

adsorbents were observed to be increase with an increase of the

CNT dosage. But it increases very slowly for the as-grown CNTs

and is 3.5 mg g�1 at a CNT dosage of 0.2 g per 100 mL. The

increasing trend for H2O2 and HNO3 oxidized CNTs is almost

identical and the adsorption capacities are 8.4 and 11.8 mg g�1,

respectively, at a CNT dosage of 0.2 g per 100 mL. The obvious

larger adsorption degree takes place at CNT dosages of 0.03 to

0.08 g per 100 mL and was found for KMnO4 oxidized CNTs

(19 mg g�1). The removal efficiency for KMnO4 oxidized CNTs


almost arrives at 100% at a CNT dosage of 0.08 g per 100 mL,

which suggests that the treatment of CNTs with KMnO4 is an

effective method to improve their Cd(II) adsorption

capabilities.325

Due to the large specific area, CNTs have shown exceptional

adsorption capabilities and high adsorption efficiencies for

various organic pollutants, such as benzene, 1,2-dichloroben-

zene325 and ethyl benzene.326 Generally, adsorption has a long

residence time for activated carbon, which is the most commonly

used adsorbent to achieve equilibrium conditions (i.e., it took

20 h for an adsorption equilibrium to be reached for phenol from

water).327

In contrast, Peng et al. observed that less time (40 min) was

required for CNTs to adsorb dichlorobenzene. This may be

because CNTs have no porous structure as traditional adsor-

bents do (e.g., activated carbon), in which the adsorbate has to

move from the exterior surface to the inner surface of the pores to

achieve the equilibrium.327 The short time needed to achieve

equilibrium also suggests that CNTs have very high adsorption

efficiencies and the potential to remove dichlorobenzene from

water. It was also found that CNTs grown by pyrolysis with a

mixture of propylene–hydrogen and a nickel catalyst at 750 �C in

a ceramic furnace were better at the adsorption of dichloroben-

zene when compared to graphitized CNTs. The reason for this

was the rough surface, which makes the adsorption of dichlo-

robenzene much easier for the as-grown CNTs. For the graphi-

tized CNTs, the heat treatment of the CNTs at 2200 �C for 2 h in

an inert N2 atmosphere eliminated the defects and the surface of

the graphitized CNTs became smooth after the treatment at this

high temperature. Thus, the adsorption of dichlorobenzene by



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graphitized CNTs was decreased. The results from Bina et al.,328

as shown in Fig. 10, indicated that the equilibrium adsorbed

amount for SWCNTs is higher than for hybrid carbon nanotubes

(HCNTs) and MWCNTs. With a C0 of 10 mg L�1, the SWCNTs

showed the greatest adsorption capacity for ethylbenzene (eth-

ylbenzene: 9.98 mg g�1). The adsorption of ethylbenzene on

CNTs is dependent on the chemical nature of the surface and its

porosity characteristics. HCNT associated with silica could

result in a more porous structure for MWCNTs and produce a

sheet of carbon nanotubes, which have a greater area than

MWCNTs for ethylbenzene adsorption. This was the main

reason for the enhanced removal of ethylbenzene by HCNTs

than MWCNTs. Furthermore, due to the electrostatic interac-

tion between the ethylbenzene molecules and the SWCNT

surface, higher ethylbenzene adsorption through the single-wal-

led CNTs than through MWCNT has been observed.327 Because

the ethylbenzene molecules are positively charged, the adsorp-

tion of ethylbenzene is thus favored for adsorbents with a

negative surface charge. This results in more electrostatic

attraction and thus leads to a higher ethylbenzene adsorption.

Single-walled carbon nanotubes (SWCNTs) and multi-walled

carbon nanotubes (MWCNTs) were purified by sodium hypo-

chlorite solutions and were applied as sorbents to study the

sorption of Zn2+ from aqueous solutions.328 The amount of Zn2+

adsorbed on the CNTs increased with an increase in the

temperature. Using the same conditions, the Zn2+ sorption

capacity of the CNTs was much greater than that of commer-

cially available powdered activated carbon, indicating that

SWCNTs and MWCNTs are effective sorbents. In addition, the

sorption/desorption study showed that the Zn2+ ions could be

easily removed from the surface site of SWCNTs and MWCNTs

by a 0.1 mol L�1 nitric acid solution and the sorption capacity

was maintained after 10 cycles of the sorption/desorption

process. Therefore, it was suggested that CNTs can be reused

several times in water treatment and regeneration. The activation

of CNTs plays an important role in enhancing the maximum

sorption capacity. Activation causes a modification of the

surface morphology and surface functional groups and causes

removal of amorphous carbon.326

The activation of CNTs under oxidizing conditions with

chemicals such as HNO3, KMnO4, H2O2, NaOCl, H2SO4, KOH

and NaOH has been widely reported. During activation, the

Fig. 10 The equilibrium amount of ethylbenzene adsorbed on CNTs

with a C0 of 10 mg L�1 (adapted from ref. 328).


metallic impurities and catalyst support materials are dissolved

and the surface characteristics are altered due to the introduction

of new functional groups. After oxidation with nitric acid,

adsorption isothermal experiments showed that the CNTs had

more defects and they have more functional groups on their

surfaces when they are prepared at 650 �C. They additionally hadhigher lead adsorption capabilities and are promising adsorbents

for use in waste water treatment. The increased capacity of

adsorption was found for the removal of Cd2+ (ref. 325), Ni2+ and

Cu2+ by other researchers.328 The amount of cationic dyes, such

as methyl violet and methylene blue, adsorbed increased with the

pH due to the electrostatic attraction between the negatively

charged surface of the CNT adsorbents and the positively

charged cationic dyes.329,330 The oxidation of the CNTs with

KMnO4 and H2O2 exhibited little enhancement in the specific

area, while HNO3 oxidation provided a larger specific area.325 A

few studies are available detailing SWNTs with antimicrobial

activity towards Gram-positive and Gram-negative bacteria due

to either a physical interaction or oxidative stress that compro-

mises the cell membrane integrity.331,332 Carbon nanotubes may

therefore be useful for inhibiting microbial attachment and

biofouling formation on surfaces. However, the degree of

aggregation,333 the stabilization effects by NOM334 and the

bioavailability of the nanotubes will have to be considered for

these antimicrobial properties to be fully effective.335

The separation of metal ion carriers (i.e. nanoparticles with

metal ions) from water after water treatment is a challenging

problem. In order to improve the separation of carriers of metal

ions from treated water, the metal ions can be bound to poly-

meric molecules and/or carbon nanoparticles forming nano-

carbon conjugates or polymer nanocomposites in water that are

able to precipitate rapidly. This leads to a significant increase in

the size of the nanocomposites with the formation of precipitates.

The precipitates can be easily removed from water by filtration or

centrifugation with the subsequent extraction of the metals.

Table 7 shows the application of nanocomposites of carbon

materials and the advantages of using them for environmental

remediation. Multi-walled carbon nanotube–TiO2 composite

catalysts can be used as catalysts in photocatalytic processes for

water treatment. The introduction of increasing amounts of

CNTs into the TiO2 matrix prevents particles from agglomer-

ating, thus increasing the surface area of the composite materials.

A synergy effect on the photocatalytic degradation of phenol was

found mostly for the reaction activated by near-UV to visible

light irradiation. This improvement on the efficiency of the

photocatalytic process appeared to be proportional to the shift of

the UV–vis spectra of the CNT–TiO2 composites for longer

wavelengths, indicating a strong interphase interaction between

carbon and semiconductor phases. This effect was explained in

terms of CNTs acting as photosensitizer agents rather than an

adsorbents or dispersing agents. Surface defects at the surfaces of

carbon nanotubes provide advantages not only for the anchoring

of the TiO2 particles but also for the electron transfer process to

the semiconductor. Original carbon nanotubes, containing

moderate amounts of oxygen surface groups, produced the

highest synergistic effect for the degradation of phenol under

near-UV to visible irradiation. The efficiency of CNT–TiO2

catalysts in the photocatalytic oxidation of mono-substituted

organic compounds under visible irradiation was dependent



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from the ring activating/deactivating properties of the aromatic

molecules. A higher kinetic synergy effect was observed for

compounds presenting electron donor groups, such as phenol

and aniline. For nitrobenzene and benzoic acid a synergy factor

near to 1 was obtained, indicating the inexistence of any synergy

effect between the CNTs and TiO2 in the photocatalytic degra-

dation of these pollutants.348 A comparison of the photocatalytic

activity of TiO2 and TiO2/CNTs composites for acetone degra-

dation in air shows that the presence of a small amount of CNTs

can enhance the photocatalytic activity of TiO2 greatly.345 Elec-

trons excited by TiO2 may easily move to the nanostructure of

the CNTs due to the strong interaction between TiO2 and CNTs.

Then, CNTs raise the band gap of TiO2, which can prevent

recombination of the e�/h+ pairs. Moreover, the abundant

hydroxyl groups adsorbed on the large surface of the composites

can lead to the formation of more_OH radicals, which result in an

enhancement of the photocatalytic activity of TiO2.349

In the tricomponent mixture of AgNPs–CNTs–PAMAM

prepared by Yuan et al. the disinfection effect on E. coli cells was

observed to be greater than both acid (–COOH) modified and

PAMAMmodified MWCNTs. It was observed that the strength

of the antimicrobial effect on bacteria follows the order: AgNPs–

MWCNTs–PAMAM mixture > MWCNTs–PAMAM >

MWCNTs–COOH. The tricomponent mixture showed the

highest disinfection activity due to favoring of the debundling

(dispersivity) of MWCNTs by PAMAM and increasing the

accessible surface area for bacterial interaction. Moreover, the

PAMAM-grafted MWCNTs contain abundant amine termi-

nated groups which leads to the reduction of Ag+ ions and

regeneration of the AgNPs once Ag+ ions penetrate through the

cell walls of the bacteria.350

(G) Application of polymer supported nanocomposites

The application of the NPs in environmental remediation

provided excessive pressure drops during operating in fixed bed

or any other flow-through systems, difficult separation and

Table 7 The applications of nanocomposites of CNTs for environmental re

Material blends in thenanocomposite Benefits

Nanocarbon colloids andpolyethylenimine

Sorption of metal ions takes plsimultaneously with the formata nanocomposite and the coaguand filtration of nanocompositwith metal ions can capture othcontaminated materials

CNT with silver ions and coppernanoparticles

The improvement was observedantimicrobial properties due toincreased contact surface area

CNT with TiO2 nanoparticles and P-25 TiO2

Providing high surface area, hiquality active sites, the retardatelectron–hole recombination anvisible light catalysis by modificof the band gap and/or sensitiz

CNT with iron oxide magneticcomposites

Improvement in the surface areadsorption capacity

Hybrid diatomite/carbon composites Providing an adequate open ponetwork comprised of transporpores and micropores and a fasremoval rate


limitation to reuse and possible risk to ecosystems and human

health caused by the potential release of nanoparticles into the

environment. In addition, the use of aqueous suspensions limits

their wide applications because of the problems for the separa-

tion of the fine particles and the recycling of the catalyst.

Immobilization of these nanoparticles onto polymer matrix, such

as porous resins, ion exchangers, and polymeric

membranes,350,351 has been available to solve the problems to

considerable extent, serving for the reduction of particle loss,

prevention of particles agglomeration and potential application

of convective flow occurring by free-standing particles. The

widely used host materials for nanocomposite fabrication include

carbonaceous materials, such as granular activated carbon,

silica, cellulose, sands and polymers, and polymeric host mate-

rials must possess excellent mechanical strength for long term

use.351–354 The generally used NPs include zero valent metals,

metallic oxides, biopolymers and single-enzyme nanoparticles

(SENs).355–358 These nanoparticles could be loaded onto porous

resins, cellulose or carboxymethyl cellulose, chitosan, alginate,

etc.359–363 The choice of the polymeric support is influenced by

their mechanical and thermal behaviour, hydrophobic/hydro-

philic balance, chemical stability, bio-compatibility, optical and/

or electronic properties and their chemical functionalities (i.e.

solvation, wettability, templating effect, etc.).364 The common

catalytic nanoparticles include nanosized semiconductor mate-

rials (such as nano-TiO2, ZnO, CdS), zero valence metals (such as

Fe0, Cu0 and Zn0) and bimetallic nanoparticles (such as Fe/Pd,

Fe/Ni, Fe/Al, Zn/Pd).365–375 They are usually applied as catalysts

or redox reagents for degradation of a large variety of environ-

mental contaminants, such as PCBs (polychlorinated biphenyls),

azo dyes, halogenated aliphatics, organochlorine pesticides,

halogenated herbicides and nitroaromatics. Nanocomposite

adsorbents were designed by impregnating the inorganic nano-

particles into conventional polymers, namely, alginate,376 cellu-

lose,377,378 porous resins360 and ion-exchangers,359,379 to avoid

issues caused by the ultrafine particle size, such as transition loss

and excessive pressure drops. Porous polymeric adsorbents or

mediation

Target material Reference

aceion oflationeser

Removal of Zn2+, Cd2+, Cu2+, Hg2+,Ni2+, Cr6+ from waste water

335

forthe

Removal of E. coli and S. aureusfrom contaminated water

336 and 337

ghion ofdationation

Removal of organic dyes, phenol andphenol derivatives, humic substances,and metallic ions from contaminatedwater

338–345

a and Removal of Co2+, Sr2+ and Ni2+ fromthe aqueous solution

346

roustter

Removal of polar aromaticcompounds (p-cresol) from aqueoussolutions

347



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ion exchangers have proved to be ideal alternatives to fabricate

similar hybrid adsorbents when considering their excellent

mechanical strength and the adjustable surface chemistry of the

polymeric supports.359,379 The immobilized charged functional

groups bound to the polymeric matrix are believed to enhance

the permeation of inorganic pollutants of counter charges. Table

8 summarizes some of immobilized nanoparticles in polymer

matrix. Due to the high photocatalytic activity of titanium

dioxide nanoparticles, the polymer substrates of the nano-

composite catalysts are expected to be antioxidative under UV or

visible light illumination. The reported polymeric substrates are

usually saturated carbon chain polymers or fluoropolymers,

such as poly(dimethylsiloxane) (PDMS),380 polyvinylpyrrolidone

(PVP),355 polyethylene (PE),381 polypropylene (PP),382 poly-

(3-hexylthiophene) (P3HT),383 polyaniline (PANI),384 poly-

(tetrafluoroethylene),385 and Nafion. Ameen et al. prepared

poly-1-naphthylamine (PNA)/TiO2 nanocomposite by in situ

polymerization and observed an enhanced photocatalytic

activity for the degradation of methylene blue (MB) dye under

visible light illumination. The high photodegradation efficacy of

the MB dye may be attributed to the efficient charge separation

of the electrons (e�) and hole (h+) pairs at the interfaces of PNA

Table 8 The application of polymer supported nanomaterials for environm

Type of nanoparticle Polymer matrix Prep

Fe0/Pd Polyacrylic acid (PAA)/polyvinyl alcohol (PVA)

Dippelectferripalla

TiO2 Polyaniline Polythe ppart

TiO2 Poly (tetrafluoroethylene) ElecFe0 Polystyrene–divinylbenzene Dipp

matsolu

TiO2 Poly(3-hexylthiophene)(P3HT)

Addthe P

Fe0 Alginate In sialgin

Fe0 Carboxymethyl cellulose In siFeSO

Fe0 Poly(vinyl pyrrolidone) ElecFe0/Pd Sodium carboxymethyl

celluloseIn siFeSOprec

Cu0 Chitosan In siCu(Sprec

Ni/Fe Cellulose acetate SolvPd/Sn Resin In si

Sn0

Pb0

Pb4+

Magnetite Montmorillonite Co-phydr

Hydrated ferricoxide Polymeric anion exchangers Prechydr

Hydrated ferric oxide Polymeric cation exchanger Prechydr

Hydrous Manganese oxide(HMO)

Polymeric cation exchanger OxidMn(

Fe3O4 Alginate Mix


and TiO2, as suggested by the slightly high red shift in the UV–vis

spectra.388 The schematic illustration of MB dye degradation

over the surface of PNA/TiO2 nanocomposites catalyst is shown

in Fig. 11. Some nanoscale metals and bimetals, such as Fe0, Cu0,

Zn0, Fe/Pd, Fe/Ni, Pd/Zn, etc., are very effective in destroying

various organic contaminants,386,387 such as chlorinated meth-

anes, brominated methanes, trihalomethanes, chlorinated

ethenes, chlorinated benzenes, other polychlorinated hydrocar-

bons, pesticides and dyes. Magnetite (Fe3O4), maghemite

(Fe2O3) and jacobsite (MnFe2O4) nanoparticles can be loaded on

or in the polymer matrix, such as alginate beads. A series of

magnetic alginate polymers were prepared and batch experi-

ments were conducted to investigate their ability to remove heavy

metal ions403 (Co(II), Cr(VI), Ni(II), Pb(II), Cu(II), Mn(II)) and

organic dyes402 (methylene blue and methyl orange) from

aqueous solutions. Magnetic particles in the nanocomposites

allowed easy isolation of the beads from the aqueous solutions

after the sorption process. The montmorillonite-supported

magnetite nanoparticles synthesized via a hydrosol method

exhibited a better adsorption capacity per unit mass of magnetite

and a better stability for storage than their unsupported coun-

terparts. During the adsorption of Cr(VI) onto magnetite

ental remediation

aration method Target pollutant Ref.

ing cross-linkedrospun polymer mat inc trichloride anddium chloride solution

Trichloroethylene 389

merization of aniline inresence of TiO2 nano-icles

Phenol 384

trophoretic deposition Trichlorobenzene 385ing cross-linked polymerin ferric trichloridetion

Nitrate 390

ing TiO2 nonpartisan to3HT solution

Methylene orange 391

tu synthesis of Fe0 inate bead from Fe3+


tu synthesis with

4$7H2O as a precursorCr6+ 393

trospinning Bromate 355tu synthesis with

4$7H2O and K2PdCl6 asursors

para-Nitrochlorobenzene 394

tu synthesis withO4)2$5H2O as aursor

Cr6+ 395

ent cast Trichloroethylene 396tu reduction of Sn2+ toand then deposition ofthrough the reduction of


recipitation andosol method

Cr6+ 398

ipitation of iron(III)oxides from FeCl3

As3+ and As5+ 399

ipitation of iron(III)oxides from FeCl3

Pb(II), Cu(II), Cd(II) 400

ation of the pre-loadedII)

Pb(II), Cd(II), Zn(II) 401

ing Methylene blue, methylorange

402



Fig. 11 A schematic illustration of the photocatalytic activity of PNA/

TiO2 nanocomposites.386

Fig. 12 A diagram of the synthesis process for PEI/TiO2

nanocomposites.410

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nanoparticles, Cr(VI) can not only be reduced to Cr(III), which

has less toxicity than Cr(VI), but it can also be fixed into the iron

oxide. This is of high importance for the application of magnetite

in the environmental remediation.398 Water pollution is certainly

one of the major problems faced by the world today. Metals,

such as Hg, Pb, Cr, Cd and As, in diverse forms constitute some

of the major inorganic pollutants and have many harmful effects

on humans and environment.404 Mercury exists in three chemical

forms, namely elemental (Hg0), inorganic mercurous and

mercuric forms (Hg1+ and Hg2+) and organic alkyl mercury.

Methyl mercury and dimethyl mercury are the most toxic and

stable forms of organomercury. Due to the high toxicity effects,

the World Health Organization (WHO) has set the limit of

mercury in drinking water as 0.001 mg L�1. Sumesh et al.407

found that water soluble silver nanoparticle composites of

9 � 2 nm and 20 � 5 nm core diameter, protected by mercap-

tosuccinic acid (MSA) supported on alumina is an effective

system to remove mercuric ions from contaminated water at

room temperature (28 � 1 �C). Preparation and performance

evaluation of silver nanoparticles were done using two different

ratios of silver to MSA: 1 : 3 and 1 : 6. The solution with a

concentration 2 ppm Hg2+ was used to evaluate the degree of

removal of Hg2+ ions from the solution.

The percentage of removal by both the nanoparticle composites

is high at pH 5–6 and the performance decreases with an

increasing pH of the solution.405 The reason for the decrease of

performance of nanoparticle composite at higher pH was due to

forming of stable mercuric hydroxo complexes, which may not

interact with the surface of nanoparticle composite. Among these

two materials, the 1 : 6 silver nanoparticle composite showed

better performance than the 1 : 3 Ag nanoparticle composite.

Lisha et al.408 reported that gold NPs supported on alumina were

used as adsorbents in order to remove inorganic mercury from

drinking water. The coupling of cellulose acetate membrane and

Fe0 has contributed the degradation rate of 0.17 L m�2 h�1 for

removal of tetrachloroethane and the rate of degradation was

found to be 0.12 L m�2 h�1 for the removal of TCE. Cellulose

acetate membranes were used for this investigation and were

25 cm � 25 cm � 100 mm with a metal (Fe0) concentration of 1%

by weight. In addition, the performance testing of coupling of

cellulose acetate and the bimetallic system (the Fe0/Ni0 ratio was


4 : 1) showed that the degradation rate of TCE was found

to be 0.028 L m�2 h�1, which was slower than that of TCE

(0.098 L m�2 h�1) using bimetallics (the Fe0/Ni0 ratio was 3 : 1).406

The limitations of the coupling of a cellulose acetate membrane

withmetallic nanoparticles for the removal of chlorinated ethanes

were found to be (1) the incorporated membrane captured Fe0 or

bimetallic (Fe0/Ni0) nanoparticles and prevented them from being

released to the environment, and (2) a loading time was required

for loading nanoparticles into the membrane before they reached

the peak reaction rate. The reduction efficiency of nitrobenzene in

groundwater by iron NPs immobilized in a PEG/nylon66

membrane was investigated by Tong et al. It was found that the

iron NPs immobilized in PEG/nylon66 membrane exhibited a

high reactivity towards the removal of nitrobenzene. The

concentration of nitrobenzene quickly decreased by 68.9% in the

first 20 min and was moderately decreased by 15% from 20 to 80

min. The decrease in reduction efficiency was due to the reaction

between iron NPs immobilized in PEG/nylon66 membrane and

nitrobenzene as well as H2O, which reduced the reactive sites and

led to the oxidation of the Fe0 and Fe2+ during the first several

minutes.407,409 In a biocatalytic enzyme nanocomposite or single

enzyme nanocomposites (SENs), each enzyme molecule is sur-

rounded with a porous composite organic/inorganic network of

less than a few nm in thickness, as shown in Fig. 12. The fabri-

cation of PEI/TiO2 bionanocomposites has been performed by

ultrasonic irradiation techniqus, as shown in Fig. 12. Under

ultrasonic conditions, the coupling agent (ɣ-amido-propyl-trie-

thoxyl silicane) hydrolyzes to form hydroxyls and then poly-

condensation occurred to form Si–O–Si bonds. Commonly, the

main effects of sonication are because of cavitation or the growth

and explosive disintegration of microscopic bubbles on a micro-

second timescale. At the same time, ultrasonic cavitation can

generate a rigorous environment of local temperature up to

5000 K and local pressure up to 500 atm. Under such conditions

the modified TiO2 nanoparticles, which have polar group of

coupling agent and OH group on the surface of TiO2 could be

dispersed completely in polymer matrix via different interactions

with the functional groups of the obtained PEI. The heat stability

of the nanocomposite was improved in the presence of TiO2

nanoparticles. Several polymer supported nanomaterials have

been investigated and further studies of interaction between the

host polymers and the encapsulated NPs are still required.

(H) Nanosensors

Environmental monitoring requires rapid and reliable analytical

tools that can perform sample analysis with minimal sample



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handling. Nanoparticle (NP)-based environmental sensors have

the potential to detect toxins, heavy metals and organic pollut-

ants in air, water and soil and are expected to play an increasingly

important role in environmental monitoring. They can both

improve detection and sensing of pollutants and can be used to

develop new remediation technologies. Compared to traditional

detection methods, NP sensors may have higher selectivity,

sensitivity and stability and a lower cost.411 The measurement of

harmful gases ,such as NOx, CO2, CO, methanol, CH4, etc. is

desirable in environmental monitoring, chemical process

controlling and personal safety. Gas sensor devices are tradi-

tionally comprised of thin films of metal oxides, with tin oxide,

zinc oxide and indium oxide, etc. With the recent discovery of

novel metal oxide nanostructures, sensors comprising nano-

arrays or single nanostructures have shown improved perfor-

mance over the thin films. The improved response of the

nanostructures to different gases has been due to the highly single

crystalline surfaces as well as large surface area of the nano-

structures. A number of studies support the application of ZnO

1D nanostructures as nanosensors. The studies by researchers

indicate that NH3 and CO behave as charge donors, transferring

charge from the adsorbate to the surface while NO2, O2 and

dioxin behave as charge acceptors, withdrawing charge from the

ZnO surface.412–414 Nanotube-based sensors include metal oxide

tubes, such as Co3O4, Fe2O3, SnO2, and TiO2, and metal tubes,

such as Pt nanosensor. Jang et al. applied a poly(3,4-ethylene-

dioxythiophene) (PEDOT) nanorods nanosensor for the detec-

tion of HCl and NH3 vapor. The PEDOT nanorode sensors gave

a measurable response to NH3 and HCl vapor concentration as

low as 10 and 5 ppm, respectively.415 Comini et al. observed the

high sensitivity of a MoO3 nanorods film sensor to 30 ppm

carbon monoxide and 100 ppm ethanol.416 Kim et al. studied a

gas sensor based on a non-stoichiometric tungsten oxide nano-

rod film. The sensor was fabricated on Si wafers as the substrates

by using a microelectromechanical system (MEMS) and silicon

technology. The sensor responses were observed to be 2% N2 (or

air), 1000 ppm ethanol, 10 ppm NH3 and 3 ppm NO2 in both dry

air and a nitrogen atmosphere at room temperature.417 A highly

selective and stable ethanol sensor based on single-crystalline

divanadium pentoxide nanobelts was reported by Liu and co-

workers. The V2O5 nanobelts showed greater sensitivity to

ethanol of either low (<10 ppm) or high (1000 ppm) concentra-

tions. The response and recovery times were 30–50 s.418 Kong

and Li found a highly sensitive and selective CuO–SnO2 sensor to

H2S gas based on SnO2 nanoribbons.418,419 Comini et al. applied

a SnO2 nanobelts film for gas sensing and proved its capability to

sense gases at 30 ppm CO (350 �C), 200 ppb NO2 (300�C) and 10

ppm ethanol (350 �C).416 Gao and Wang found that the SnO2

nanobelt/CdS nanoparticle core–shell heterostructured sensor

had high sensitivity to 100 ppm ethanol vapors in air at 400 �C.420

The authors suggested that the CdS nanoparticles may be served

as additional electron sources that greatly improved the electron

conduction in the SnO2 nanobelts. Tao et al. demonstrated

the capability of silver nanowire substrates for the detection of

2,4-dinitrotoluene (2,4-DNT), the most common nitroaromatic

compound for detecting buried landmines, and other explosives

by utilizing vibrational signatures. A sensitivity of about 0.7 pg

was achieved.421 Yang et al. demonstrated the capability of

silver nanowire substrates for the detection of 2,4-dinitrotoluene


(2,4-DNT) and other explosives by utilizing vibrational signa-

tures. A sensitivity of about 0.7 pg was achieved.422

Zhang and co-workers also continuously demonstrated

detection of NO2 down to ppb levels using transistors based on

both single and multiple In2O3 nanowires operating at room

temperature. The multiwire sensor showed an even lower detec-

tion limit of 5 ppb, compared to the 20 ppb limit of single

nanowire sensors. This room temperature detection limit is the

lowest level so far achieved with all metal oxide film or nanowire

sensors.423 This improved sensitivity was due to the formation of

nanowire/nanowire junctions between the metal electrodes, a

feature available in the multiple nanowire devices but junctions

are not available in the single nanowire devices. Such junctions,

when exposed to NO2, should form a depleted layer around the

intersection and thus block the electron flow in a way more

prominent than the surface depletion of the single nanowires

with metal contacts. When detecting NO2 among other common

gases, such as O2, CO and H2, using the multiple nanowire

devices, selective response to NO2 was also observed. On the

basis of their previous study, it was suggested that a large group

of In2O3 nanowires with an appropriate doping level distribution

could have two opposite sensing responses cancelling out each

other and resulting in the immunity to NH3. This unique prop-

erty of In2O3 nanowires offers a new way to achieve selectivity, as

compared to the conventional technique of using permeable

polymer coatings. Chu et al. also evaluated the gas sensing

properties of In2O3 nanowires films. The results revealed that the

sensors exhibited higher response and good selectivity to

C2H5OH at 370 �C. The response time was about 10 s and

recovery time was shorter than 20 s.424 Functionalization of

MWCNTs multiple-films with nominally 5 nm thick Pt- and

Pd-nanoclusters prepared by magnetron sputtering provided

higher sensitivity of significantly enhanced gas detection for

NO2, H2S, NH3 and CO, up to a low limit of sub-ppm level.425

Titanate nanotubes (TNT) were proven to be an efficient

supports for the immobilization of methylene blue (MB) for the

detection of dopamine.420 Porous TiO2 sol–gel matrix can be

used to construct nitrite sensors by immobilizing partially

quaternized poly(4-vinylpyridine) complexed with PVP-Os on an

electrode.426

The limitation of nanoparticles in practical applications, such

as slow diffusion and aggregation, still exists. Immobilization of

nanoparticles by polymer matrix is one of the most efficient

approaches to overcome such limitations. Since the chemical and

physical properties of polymers may be tailored, they gained

importance in the construction of sensor devices.427 Conductive

polymer nanomaterials have attracted particular interests as

sensors for air-borne volatiles428–434 (alcohols, NH3, NO2, CO)

because of large surface area, adjustable transport properties and

chemical specificities, easy processing and scalable productions.

Polyaniline nanofibres were developed by interfacial polymeri-

zation to sense hydrazine gas and it was found that performance

of sensing was better than conventional thin film due to its high

surface area, porosity and small diameter.435 Polyaniline–SnO2/

TiO2 nanocomposite ultra thin films have also been fabricated

for CO gas sensing.428 The range of the biosensor was found to

be 6.9 � 10�14 to 8.6 � 10�13 mol L�1 and the detection limit is

2.3 � 10�14 mol L�1. A Pd–polyaniline nanocomposite was

developed as a selective methanol sensor.432 The synthesized



Table 9 Polymer-based nanocomposites for the sensing and detection of pollutants

Type of nanoparticle Polymer matrix Preparation method Target pollutant Ref.

SnO2 Polystyrene/polyaniline (PSS/PANI) In situ self-assembly CO 428SnO2 Polyaniline (PANI) Hydrothermal method Ethanol, acetone 429TiO2 Polyaniline (PANI) Chemical polymerization and a sol–gel method Trimethylamine 430Iron oxide Polypyrrole Simultaneous gelation and polymerisation CO2, N2, CH4 431Pd Polyaniline Oxidative polymerization of solution with Pd NPs Methanol 432Au Chitosan Mixed in solution Zn2+, Cu2+ 433

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nanocomposite sensor showed high selectivity and sensitivity to

methanol vapors with rapid and reverse response. Some appli-

cations of polymer-based nanocomposite sensors are shown in

Table 9. Nanosensors have also been applied as biosensors. The

application of silica-coated nanosilver as biosensors for the

biochemical compounds, such as glucose and hydrogen peroxide,

because of their superior properties, such as their nontoxic

nature, high surface area, high adsorptivity, high uniformity and

excellent biocompatibility. TiO2 nanomaterials have been

frequently proposed as a prospective interface for the immobi-

lization of biomolecules. Moreover, titanium forms coordination

bonds with the amine and carboxyl groups of enzymes and

maintains the enzyme’s biocatalytic activity.436,437,439–441 Form-

aldehyde is a hazardous air pollutant and prolonged exposure to

formaldehyde can cause a nervous system damage as well as

asthma. When sensor strip made of nylon 6 nano-fibre nets

(NFN) was exposed to formaldehyde, the methyl yellow on the

tape reacted with sulfuric acid produced by the reaction of

hydroxylamine sulfate with formaldehyde to produce a yellow-

to-red color change as shown in Fig. 13.438

Fig. 14 shows the fabrication of the biosensor, which was done

by coating graphene–gold nanocomposites (G-AuNP), CdTe–

CdS core–shell quantum dots (CdTe–CdS), gold nanoparticles

(AuNPs) and horseradish peroxidase (HRP) in a sequence on the

surface of a gold electrode (GE). It was found that sensitivity of

the biosensor is more than 11-fold better if G-AuNP, CdTe–CdS

and AuNPs are used. This could be ascribed to the improvement

of the conductivity between the graphene nanosheets in the

Fig. 13 An illustration of the colorimetric detection of formaldehyde

based on the nylon 6 NFN membranes.438

Fig. 14 The fabrication of AuNPs/Cd-Te-CdS/G-AuNPs/GE.442


G-AuNP due to introduction of the AuNPs. The electrocatalytic

synergy of G-AuNP, CdTe–CdS and AuNPs remarkably

improves the electron relay and accelerates the electrochemical

reaction and the AuNPs–CS film offers a favorable microenvi-

ronment to keep the bioactivity of the HRP. This biosensor

provided the best sensitivity in all biosensors based on graphene

materials for detection of hydrogen peroxide. This study remains

open as a new challenge and approach to explore the electro-

chemical features of graphene or its nanocomposites for the

potential utilizations.442

Conclusions and future prospects

The application of nanomaterials in the detection and removal

of pathogens provides greater sensitivity, a lower cost, shorter

turn-around times, smaller sample sizes, in-line and real-time

detection, higher throughput and portability in environmental

remediation. In addition metal and metal oxide nanomaterials

can be used to remove organic pollutants and metals by reduc-

tion or oxidation of nanomaterial and degree of removal can be

enhanced through functionalization with chemical groups that

can capture selectively target pollutants in water and air media.

This method is effective and promising and can be used in the

engineering of water and air improvements. Nanomembranes

have found applications in the production of potable water,

water reclamation, the removal of metals, dyes, NOM and the

removal of pesticides from contaminated water. Further

improvements must be made in the application of environmental

remediation to selectively remove materials, have a greater

resistance to changes in pH and the concentrations of chemicals

present in the contaminated water, greater stability for a longer

period of time and cost optimization. Nanofibrous media have a

low basis weight, high permeability and small pore size that make

them appropriate for a wide range of filtration applications. In

addition, nanofibre membranes offer unique properties, such as a

high specific surface area (depending on the diameter of fibres

and intrafibre porosity), good interconnectivity of the pores and

the potential to incorporate active chemistry or functionality at

a nanoscale. A high flux could be produced via nanofibrous pre-

filters with even higher loading capacities. Such pre-filters can be

used in various applications, such as the removal of micropar-

ticles from waste water and with ultrafiltration or nanofiltration

membranes to prolong the life of these membranes. On-going

investigations are under way to develop engineered nano-

materials of various fibre diameters and morphologies to identify

their effects on the performance of nanofibres.

The environmental applications of polymer supported nano-

composites in photocatalytic/chemical catalysis degradation, the



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adsorption of pollutants and pollutant sensing and detection

result in a greener environment. However, the study of the

interaction between the host polymers and the encapsulated NPs

and its effect on the dispersion in polluted air and water is

necessary. In addition, the large scale production of polymer-

supported nanocomposites and more practical applications

remain open. The extensive application of sorbents in environ-

mental remediation have shown the capability of adsorbing

metals and organic pollutants from contaminated water and air.

Iron-based nanomaterials, TiO2 nanomaterials and polymeric

adsorbents have shown high adsorption capacities and selectiv-

ities. The surface modification of sorbents are being studied for

process optimization. Enhancing the reusability of sorbents and

the extension of their lifespan must be explored to reduce the cost

in environmental remediation. Sensors have been developed for

sensing gases, chemicals and volatile organic compounds and the

detection and identification of bacteria. Further development is

necessary in the functional properties of nanomaterials to meet

the need for trace detection and the treatment of pollutants in

water and air and important fundamental and mechanistic

studies are required in order to fully explore their real potentials.

One dimensional CNTs with single and multiple layers have

shown superior adsorption capacities in the removal of diverse

range of biological and chemical contaminants due to their

fibrous shape with high aspect ratio and provision of large

external surface area. Small size nanoscale particles composed of

CNTs are difficult to separate from aqueous solution. Ultra

centrifugation separation method is efficient to separate CNTs.

However, high energy is necessary for this method. The

membrane filtration method is an alternative and efficient tech-

nique to separate CNTs from aqueous solutions. However, the

membrane can easily be blocked. The CNTs/metal oxide or

magnetic composites are promising materials in environmental

pollution management at a large scale. More efforts for the

development of practical applications of these CNT composites

are required in the future. Dendritic nanopolymers have been

developed for low pressure filtration processes to remove

perchlorate and uranium from contaminated water and recover

metal ions (e.g., copper, silver, nickel and zinc) from industrial

waste water. The long-term efficiencies of dendritic nanopolymer

composites as an important practical aspect have not been

reported and should be addressed in the future.

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