Accepted ManuscriptReviewRecent progress and future challenges on the use of high performance magneticnano-adsorbents in environmental applicationsJenifer Gmez-Pastora, Eugenio Bringas, Inmaculada OrtizPII: S1385-8947(14)00881-XDOI: http://dx.doi.org/10.1016/j.cej.2014.06.119Reference: CEJ 12376To appear in: Chemical Engineering JournalReceived Date: 25 April 2014Revised Date: 27 June 2014Accepted Date: 28 June 2014

Please cite this article as: J. Gmez-Pastora, E. Bringas, I. Ortiz, Recent progress and future challenges on the useof high performance magnetic nano-adsorbents in environmental applications, Chemical Engineering Journal(2014), doi: http://dx.doi.org/10.1016/j.cej.2014.06.119

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Recent progress and future challenges on the use of high performance

magnetic nano-adsorbents in environmental applications

Jenifer Gmez-Pastora, Eugenio Bringas and Inmaculada Ortiz*

Dept. Chemical and Biomolecular Engineering, ETSIIT, University of Cantabria, Avda. Los Castros s/n,

39005 Santander, Spain

*Correspondence to: Dept. Chemical and Biomolecular Engineering, ETSIIT, University of

Cantabria, Avda Los Castros s/n, 39005 Santander, Spain.

E-mail: ortizi@unican.es; Phone: +34 942201585; Fax: +34 942201591

1

ABSTRACT

The application of magnetic nanoparticles (MNPs) as adsorbent materials in solving

environmental problems has recently received great attention due to their unique

physical and chemical properties, which make them superior to traditional adsorbents.

The ability of functionalization by anchoring specific functional groups on their surface

makes possible the synthesis of different types of engineered MNPs for the removal of a

large number of both organic and inorganic contaminants. However, the successful

implementation of the MNPs-based adsorption technology needs of the evaluation and

optimization of the magnetic recovery stages, the regeneration process and the

management of both the spent regeneration solution and the exhausted adsorbent. This

work presents a comprehensive review on the use of MNPs in the treatment of polluted

wastewaters with toxic metals and dyes. In addition, the magnetic recovery options and

the possible strategies that can be employed for the nanomaterials regeneration and

reuse are analyzed.

Keywords: magnetic nano-adsorbents; heavy metals removal; dyes removal;

magnetic recovery;

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1. INTRODUCTION

In recent years, magnetic nanomaterials have attracted considerable attention because of

their unique properties that make them very useful in different fields. Magnetic

nanomaterials have at least one dimension smaller than 1 micron and are possible to be

manipulated under the influence of an external magnetic field [1]. In addition, below

certain critical dimensions, that vary with the material parameters, magnetic materials

become superparamagnetic [2,3] thus the nanomaterials exhibit no magnetic properties

upon removal of the external field and therefore have no attraction for each other,

offering the advantage of reducing risk of particle aggregation, and more importantly,

they provide a strong response to an external magnetic field [3-5].

Advances in the synthesis methods over the past decade have led to availability of

superparamagnetic nanoparticles with different shell and surface modifications [6-11].

Magnetic nanoparticles (MNPs) are generally composed of magnetic elements, such as

iron, cobalt, nickel, or their oxides like magnetite (Fe3O4), maghemite (-Fe2O3), nickel

ferrite (NiFe2O4), cobalt ferrite (CoFe2O4), etc. [4]. Usually MNPs are coated with

organic layers (e.g., surfactants or polymers such as dextran and polyethylene glycol) or

inorganic components, such as metallic elements (e.g., gold or platinum), metal oxides

(aluminum oxide, cobalt oxide), activated carbon, silica, etc. [2,7-9] in order to make

them stable against oxidation, corrosion and spontaneous aggregation, to increase their

physico-chemical stability and to provide a functionalizable surface [2].

Despite the significant advantages offered by MNPs, it was not until the last half of the

20th century when scientists began to study their characteristic behavior [3]. MNPs

exhibit great potential for their applications as catalytic materials, pigments, coatings,

gas sensors, magnetic recording devices, magnetic data storage devices, magnetic

resonance imaging, drug delivery, magnetic hyperthermia, bioseparation, etc. [1,4,5,7-

3

14]. On the other hand, the application of MNPs in solving environmental problems has

recently received great attention [12,15-18] due to several reasons first, the higher

economical and environmental efficacy of those processes based on the enhanced

physical and chemical properties of MNPs (i.e. high surface area, ease of

functionalization, chemical stability, etc.) [18] and second, the advantages that the

superparamagnetic behavior of MNPs provide to the design of separation and recovery

steps in complex multiphase systems [19].

The integrated design of an adsorption process based on the use of MNPs must take into

account different stages: i) adsorption separation, ii) magnetic recovery of the adsorbent

for further reuse, iii) adsorbent regeneration and, iv) management of both the spent

regeneration solution and the saturated adsorbent. Although the technical feasibility of

MNPs as adsorbent materials for water treatment processes has been widely reported in

the literature, the adsorbent regeneration stage and material reusability have received

much less attention in spite of their great importance for the process economy.

Moreover, the design of suitable magnetic separation devices, which allow the magnetic

recovery of the adsorbents should be considered as a crucial stage since it is required for

the further re-use of MNPs. Finally, the management of both the solid and liquid wastes

generated in the process by either material recovery or by final disposal is an issue of

concern which has not been previously reviewed in the literature.

Hence, this work aims at the analysis of the different individual stages taking part in the

design of MNPs-based adsorption process with the purpose of detecting their level of

development and bottlenecks thus, providing useful guidelines to conceive novel

integrated separation processes based on the use of nanoadsorbents and able to mitigate

environmental problems of concern. In particular the integration of MNPs in the design

of novel adsorption processes in either wastewater treatment or water remediation is

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analyzed in this work through two different applications focused on the removal of two

groups of inorganic and organic model pollutants namely, heavy metals and dyes.

2. ADSORTION OF HEAVY METALS AND DYES BY MNPs

Water pollution by heavy metals and dyes has become a serious problem because of

their adverse effects on ecological systems and human health [19,20]. Heavy metals like

copper, chromium, cadmium, lead, zinc, mercury, nickel or arsenic, are released into the

environment as consequence of different natural or human induced activities such as

mining, metal finishing, painting and printing processes, pesticides and fertilizers

manufacture, etc. [21-23]. The major concern about heavy metals is that most of them

are highly toxic, carcinogenic and mutagenic at even relatively low concentrations [23-

25]. Also, heavy metals are non-biodegradable and thus they tend to accumulate in

living organisms [19]. Their deleterious effects depend on the specific metal

characteristics. Arsenic for example is commonly known as a deadly poison since

ancient times due to its lethality, but others like lead, cadmium, chromium and mercury

have also serious toxic effects [15].

On the other hand, dyes can be toxic to the aquatic life in receiving waters, being some

of them mutagenic and carcinogenic. There are more than 100,000 types of dyes which

are used in different industries including paper, plastic, leather, pharmaceutical, food,

cosmetic and textile [26-29]. In particular, textile industries produce large amounts of

colored effluents which are directly discharged into surface water [30] without any

treatment due to technological and economical limitations [31]. In addition, the

degradation by-products of some organic dyes such as synthetic azo-dyes represent a

potential environmental hazard since they contain aromatic amine compounds that are

toxic to many organisms [32]. Although some dyes are banned in many countries

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because of health concerns [32], in other regions they are still used, generating

hazardous effluents that are hardly remediated using conventional biological treatments

owing to their stability to light, heat and oxidizing agents thus being difficult to be

mineralized in conventional water treatment plants [32,33].

As consequence of the environmental hazards and health effects, drinking water and

wastewater regulations have been toughened thus promoting the development of

efficient processes able to accomplish with the concentration values imposed. Various

technologies have been developed for the removal of heavy metals and dyes from

industrial wastewater, such as coagulation/precipitation [34], ion exchange [35], solvent

extraction [36], electrodeposition [34], membrane filtration [34], electrodyalisis [37],

advanced oxidation processes [29], etc. However, the application at large scale must

face and solve many drawbacks shown by most of those methods such as their cost,

complexity, efficiency, or sludge generation [33,38]. Adsorption is considered as one of

the most promising technologies owing to its simplicity of design, ease of operation,

low cost, potential for regeneration, sludge free operation and high retention efficacy

when applied with the proper adsorbent [18,33].

MNPs have been studied as nanoadsorbent materials for heavy metals and dyes removal

and their superior characteristics in comparison with traditional adsorbents (i.e. large

surface area, high number of active surface sites, low intraparticle diffusion rate and

high adsorption capacities), make their use very promising for the treatment of polluted

waters, reducing costs and producing less contamination [39,40]. Moreover, the recent

advances in the synthesis methods allow the easy anchorage of different functional

groups on the surface of the nanoadsorbents which take part in the adsorption process as

specific binding sites, increasing the adsorption capacity and improving the selectivity

of the process for each specific pollution problem. Furthermore, their recovery can be

6

easily performed with magnetic separators, overcoming the pressure drop developed in

traditional fixed bed adsorption columns.

To provide a global overview of the state of the art, nearly two hundred scientific

publications dealing with the removal of heavy metals [18,19,21-24,38,40-152] and

dyes [20,26-28,30-33,39,153-188] by magnetic nanoadsorbents have been reviewed in

this work. Figure 1 shows the distribution of works classified by heavy metal (Figure

1a) and groups of MNPs (Figure 1b).

FIGURE 1 (a and b)

Figure 1. Literature references focused on the removal of heavy metals by adsorption

with MNPs: a) number of references per heavy metal; b) percentage of references per

type of material.

In the case of dyes, the number of studies related to their removal from wastewaters

using magnetic nanoadsorbents is limited. Dyes can be classified in two groups: acid or

anionic dyes, and basic or cationic dyes. Figure 2 illustrates the molecular structure of

methylene blue and methyl blue taken as model compounds of cationic and anionic dyes

mostly studied in the scientific literature focused on magnetic adsorbents.

FIGURE 2

Figure 2. Molecular structure of methylene blue and methyl blue.

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Figure 3 shows the magnetic nanoadsorbents reported in the literature for dyes removal

and the number of reviewed studies, classified by the type of dye removed (cationic or

anionic).

FIGURE 3

Figure 3. Studies about the removal of dyes with MNPs.

In order to provide a simplified summary, Tables 1 and 2 present the most

representative examples of the analyzed publications, in which MNPs are employed for

the removal of heavy metals and dyes from polluted waters, respectively. The

information reported includes the material characteristics (size, surface area, saturation

magnetization and point of zero charge (pHpzc)), the working conditions and the most

relevant conclusions obtained by their application on the specific pollution problem.

Next, the information compiled in the tables is discussed in a comprehensive way.

Table 1. Magnetic nanoparticles for heavy metals removal.

TABLE 1

Table 2. Magnetic nanoparticles for dyes removal.

TABLE 2

A vast number of studies has shown the applicability of nano zero-valent iron (nZVI) as

an effective heavy metal remediation technology. Although most studies are performed

at bench scale, several pilot tests and full scale applications have been reported as well

8

[17,189]. When nZVI is exposed to air or water, it is oxidized forming a layer of iron

oxides or hydroxides on the surface that is responsible for the adsorption process

[45,46,48]. The possible adsorption mechanisms for heavy metals removal by nZVI

include adsorption and/or surface precipitation, redox reduction and simultaneous co-

precipitation as metal hydroxide or metal-iron hydroxides [42,51]. This material has

been widely reported in literature for the removal of chromium and arsenic from

polluted waters. Figure 4 shows the adsorption mechanisms of Cr(VI) that is first

reduced to Cr(III) and then is incorporated to the particle surface forming a Cr-Fe

(oxy)hydroxide layer [49,51]. For arsenate, Liu et al. [44] concluded that the main

removal mechanism was adsorption onto iron corrosion products. On the other hand,

Yan et al. [41] studied the chemical transformation of arsenite and observed that nZVI

was capable of inducing As(III) oxidation and reduction, due to the core-sell structure of

nZVI. As(III) oxidation occurred at the surface of the iron oxide shell, while the

reduction process was carried out at a subsurface layer near the iron core. nZVI has

reported adsorption capacities larger than 100 mg g-1 for the removal of chromium and

arsenic compounds from synthetic waters free of competing species as shown in Table 1

[43,49,51], although the presence of humic acid and competing anions (phosphate,

sulfate, carbonate, etc.) in the solution reduces the separation efficacy [44-47,50].

FIGURE 4

Figure 4. Removal mechanisms of hexavalent chromium by nZVI [51].

Iron oxide nanoparticles such as maghemite (-Fe2O3) and magnetite (Fe3O4) have

shown great applicability in the remediation of polluted waters with metallic oxyanions,

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like arsenate and chromate, as well as cationic metal ions, like copper, nickel, zinc, lead

or cadmium due to their high adsorption capacities and their superparamagnetic

behavior which greatly simplifies their collection from the solution under magnetic

fields. According to Chowdhury et al. [64], the surfaces of iron oxides are covered with

hydroxyl groups due to the adsorption of water molecules or by structural reasons thus

being the surface functionality modified as follows:

FeOH + H+ FeOH2+ (1)

FeOH + OH- FeO- + H2O (2)

The surface functionality of iron oxides varies in form depending on both the type of

iron oxide (maghemite or magnetite) and the solution pH. However, the dominant

functional groups of most iron oxides surfaces are FeOH+ or FeOH2+ under acid

conditions, while Fe(OH)20, FeO- and Fe(OH)3- are the predominant forms at basic pH

values [64,135]. Therefore, iron oxide nanoparticles can interact with anionic metallic

pollutants such as arsenate and chromate oxyanions at low pH, mainly through

electrostatic interaction with the positively charged groups on the surface [65,67].

However, ion exchange mechanisms have been reported in the literature for the removal

of metallic oxyanions at high pH values [65,66]. A study carried out by Hu et al. [65]

demonstrated that the removal of chromate anions by maghemite nanoparticles at pH

values above the zero point charge (pHzpc) of the material occurred through an ion

exchange reaction between the OH- functional groups and the chromate anions

according to the following reaction:

(FeOH)2 + CrO42- Fe2CrO4 + 2OH- (3)

On the contrary, under alkaline conditions iron oxide materials are effective in the

uptake of metal cations such as Cu2+, Ni2+, Zn2+, Pb2+, or Cd2+ [72,66,69]. Iron oxides-

based nanomaterials have reported similar or higher adsorption capacities than nZVI

10

[23,68], showing successful removal efficacies in the remediation of natural waters and

real wastewaters, Table 1 [70,73]. In addition, due to their large saturation

magnetization values, they can be separated from the solution with a permanent magnet

in a relatively short time [67,74].

Different nanoscale ferrites have been also tested for the removal of metals such as

arsenic [75,78,80,81], chromium [76,77] and cadmium [79]. Among them, manganese

and copper ferrites have shown the best performance, with adsorption capacities as high

as 100 mg g-1 [81]. As reported by Tu et al [80] (See Table 1) copper ferrites have

reported high removal efficacies for the arsenate present in natural waters and real

wastewaters.

The use of bare magnetic nanoparticles for dye remediation has been scarcely reported

in the literature. However, maghemite, magnetite and ferrite nanoparticles have been

reported as novel nanoadsorbents for dyes separation. Maghemite nanoparticles have

proved to be effective material adsorbents for the removal of anionic dyes such as Acid

Red 27 and Congo red [32,39] as shown in Table 2, reaching adsorption capacities

higher than 200 mg g-1. Figure 5 shows the main removal mechanism of anionic dyes by

iron oxide nanoparticles which is the electrostatic attraction between the positively

charged surface of the maghemite (below pHpzc) and the sulfonate group of the anionic

dyes. Thus, the adsorption efficacy increased as the pH value decreased, with high

removal efficacies in synthetic waters free of competing species. However, the presence

of coexisting anions in the solution, such as sulfate and bicarbonate, significantly

reduces the separation efficacy [39]. Iron oxides have also been applied to the removal

of cationic dyes such as Neutral Red, Acridine Orange or Methylene Blue showing

promising results [31,154,155], as shown in Table 2.

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FIGURE 5

Figure 5. Removal mechanism of anionic dyes by maghemite nanoparticles [39].

Although pure metal nanoparticles and their metal alloys are effective adsorbents, they

suffer the disadvantage of having low stability in suspension medium, affecting the

long-term performance and applicability of the separation process. The most

straightforward strategy to make them stable against oxidation, corrosion and

spontaneous aggregation is their coating with organic or inorganic materials. This

protection method results in MNPs with a core-shell structure [7]. In general, the

external coating also extends the possibility of surface functionalization thus increasing

the adsorption capacity and selectivity towards the target pollutants. On the other hand,

the mass ratio between the magnetic core and the non-magnetic shell should be

controlled to preserve the magnetic properties (e.g. magnetic saturation) of the

composite material which are essential in the magnetic recovery stage.

Natural organic macromolecules, mostly biopolymers such as chitosan, cyclodextrin,

arabic gum, kondagogu gum, alginate, etc. are used as effective coatings due to the

presence of different functional groups in their structure which provides them high

capacity and selectivity towards heavy metals and dyes. Among these coatings, chitosan

is probably the best example being widely used in many studies.

Chitosan (poly--(14)-2-amino-2-deoxy-D-glucose) is a natural polysaccharide

produced by the N-deacetylation of chitin [21]. This biopolymer possesses good

sorption capacity for several heavy metal ions and almost all kinds of dyes due to its

high amino content on the polymer matrix which provides selectivity to the adsorption

process [89,92,159]. To improve its chemical stability in acid media, chitosan is usually

cross-linked introducing another compound (e.g. glutaraldehyde, ethylenediamine, -

12

cyclodextrin, epichlorohydrin, etc.) into the chain [21,159,163]. However, the coating of

magnetic materials by non-magnetic polymeric compounds decreases the saturation

magnetization value (since this property is defined on a per gram basis), and therefore

polymer coatings adversely affect the magnetic separation performance.

The amino and hydroxyl groups within the structure of chitosan can interact with heavy

metals by ion exchange or complexation reactions [21,91], although electrostatic

interactions can also contribute to the metallic removal [90]. Xiao et al. [91] studied the

removal mechanisms of dichromate and copper by MnFe2O4@Chitosan and their results

suggested that the chelation between chitosan and metal ions played a much important

role than the electrostatic interactions in the process. They also carried out competitive

adsorption experiments of Cu2+ and Cr2O72- by MnFe2O4@Chitosan leading to the

conclusion that the affinity of the adsorbent towards copper is much higher than the

observed towards chromium. According to the hard and soft acids and bases theory

introduced by Pearson in 1963, amino and hydroxyl groups (hard bases) form a stable

complex with hard Lewis acids such us Cr3+, Co3+, As3+, Cr6+, Co2+, Ni2+, Cu2+, Zn2+,

Pb2+ etc. [190]. Thus, the functional groups located on the surface of the adsorbent are

hard bases and tend to form strong bonds with hard acids such as the copper cation.

Nevertheless, chitosan coated MNPs have proved to be effective adsorbents for the

removal of metallic oxyanions too. For example, Gupta et al. [89] applied nZVI coated

with chitosan to the removal of arsenite and arsenate attaining maximum adsorption

capacities ranging from 94 to 119 mg As g-1 as shown in Table 1. Also, the adsorbent

was capable of operating in a wide range of pH values and interfering species like

sulfate, silicate or phosphate marginally affected the adsorption performance, reducing

the total arsenic concentration down to the World Health Organization drinking water

standards (10 g L-1) for arsenic contaminated natural groundwater.

13

Chitosan and chitosan derivatives coated MNPs have been successfully applied for acid

dyes such as Acid Orange 7 and 10, Alizarin Red, Methyl Blue, Crocein Orange G,

Acid Green 25, etc., showing adsorption capacities larger than 3000 mg g-1 as depicted

in Table 2, and, in comparison with traditional adsorbents, these nanocomposites have

reported higher adsorption capacity and faster adsorption rate of anionic dyes

[157,159,160,163]. The main removal mechanism is expected to be electrostatic

attraction at low pH [162]. The amino groups of chitosan are easily protonated under

acidic conditions, thus binding anionic dyes through ionic interaction with the sulfonate

groups of these dyes [160,163], according to the following reactions:

MNPs@Chitosan-NH2 + H+ MNPs@Chitosan-NH3+ (4)

MNPs@Chitosan-NH3+ + Dye-SO3- MNPs@Chitosan-NH3+ -- SO3--Dye (5)

Besides chitosan, other organic coatings based on natural or synthetic polymers such as

cyclodextrins, arabic gum, kondagogu gum, alginate, poly-L-cysteine, polyacrylic acid,

polysiloxanes, polypyrrole, polyrhodanine, poly(-glutamic acid), etc. have reported

good results as heavy metal and dyes adsorbent materials due to the high content of

functional groups on their structure including hydroxyl, carboxyl, amine, ether, acetyl,

aliphatic, carbonyl, thiol, sulfur, oxygen and nitrogen groups [19,94-101,164-171]. The

performance of these polymers and other organic coatings, mainly surfactants and

organic acids, is summarized in Tables 1 and 2.

Inorganic coatings without further functionalization have been scarcely applied for

heavy metal and dye remediation. Only few studies reported the use of unmodified

inorganic compounds as coating materials and most of them are based on metal oxides

or activated carbon. In general, inorganic coatings need of further surface

functionalization to create specific binding sites improving the selectivity of the

adsorption processes. Mesoporous silica is one of the most promising coating materials

14

since it provides high adsorptive surface area with well-defined pore size and shape,

avoids the magnetic attractions between magnetic nanoparticles and protects the inner

magnetic core from leaching at low pH values [134,137,139,187]. In addition, the

existence of a uniform distribution of silanol groups on the silica surface facilitates the

grafting of selective functional groups [137,184]. Although silica by itself exhibits some

interesting applications, including the removal of harmful dyes from wastewater [186],

in order to achieve higher adsorption capacities and selectivities towards heavy metals

and dyes, various functional groups have been used to modify the surface of either

coated or uncoated MNPs.

Amine-functionalized silica coated magnetic nanoparticles (MNPs@SiO2-NH2) and

amine functionalized MNPs (MNPs-NH2) have demonstrated outstanding capability to

remove a wide variety of heavy metals [132,133,136]. The removal mechanisms of

heavy metals by amine-functionalized nanoadsorbents include electrostatic interactions,

ion exchange and coordination interactions due to the metal complexing capability of

amino groups [130,138]. In most cases, the removal efficiency is pH dependent. Heavy

metal cations are preferentially adsorbed at high pH values due to the deprotonation of

the amine groups, while anionic species are removed from the solution in acidic

conditions owing to the protonation of the functional groups at low pH values

[130,135,136].

As the amino groups serve as chelation sites, it is expected that the higher the content of

amino groups on the surface the higher the adsorption capacity. However, it has been

found that sorption capacities are not always proportional to the number of surface

functional groups. In fact, a study carried out by Chung et al. [139] (Table 1)

demonstrated that high amine density on silica coated magnetite nanoparticles had an

adverse effect on the sorption process of Pb2+ and Cu2+ cations. They observed that di-

15

and triamino groups caused pore blockage and limited the mass transfer into the silica

mesopores and found that the available adsorptive sites decreased due to changes in the

surface charge.

Although the presence of competing ions in the solution can negatively affect the

adsorption performance of the target metals [134], amine functionalized MNPs have

reported good performance in treating natural waters and industrial wastewaters. Tan et

al. [40] (Table 1), used Fe3O4-NH2 nanoparticles for the removal of Pb(II) from

industrial wastewater and tap water spiked with 1 mg L-1 of Pb(II), and the removal

efficacies obtained were about 98% and hardly influenced by the water matrix.

As amine functionalization, thiol-functionalized magnetic mesoporous silica

(MNPs@SiO2-SH) and thiol-functionalized MNPs (MNPs-SH) have been considered

promising materials in heavy metal remediation by many authors [143-145]. Thiol-

functionalized MNPs have been successfully employed in the removal of divalent heavy

metal cations, mainly Hg2+ and Pb2+ ions as shown in Table 1. According to Pearsons

theory, thiol groups show high affinity towards soft acids, such as Cd2+ and Hg2+ [190].

In fact, numerous studies reveal that thiol functionalized MNPs have exerted good

adsorption for Hg2+ [144,146], even in natural waters [142,145,147]. Therefore, it is not

surprising that thiol based magnetic nanoadsorbents have been presented as an

alternative treatment technology for mercury polluted waters since their adsorption

capabilities have proved to be superior to other commercial adsorbents [142].

Amino-functionalized silica coated magnetic nanoparticles (MNPs@SiO2-NH2) have

been tested as well for the separation of acid dyes such as Acid Orange 10 and Congo

Red, as is illustrated in Table 2 [184,185]. The removal mechanism of dyes by amine-

functionalized nanoadsorbents is electrostatic attraction, being the removal efficacy pH

dependent [184]. For removal of basic dyes, the use of carboxylic functionalized silica

16

coated MNPs has exhibited high adsorption capacity and rapid adsorption rate for

Methylene Blue and Acridine Orange [186].

From the analysis reported above it is concluded that the integration of MNPs in

environmental technologies offers many advantages owing to their high affinity towards

both organic and inorganic pollutants and their superior characteristics in comparison

with traditional adsorbents. In fact, the ability of functionalization by anchoring specific

functional groups on their surface makes possible the synthesis of different engineered

nanoadsorbents for the removal of a broad range of contaminants. In particular, coated

and functionalized MNPs have shown excellent structural and chemical characteristics

to carry out the removal of heavy metals and dyes from aqueous solutions, reporting

lower adsorbent doses and faster adsorption kinetics in comparison with bulk adsorbent

materials. However, the main benefit of employing MNPs, which is related to the

potential handling of these materials by the application of external magnetic fields, is

rarely discussed in the literature that is mainly focused on the synthesis and

performance of the magnetic nano-adsorbents. The following section analyzes the state

of the art on the design of magnetic separators.

3. MAGNETIC RECOVERY OF THE NANOADSORBENTS

Conventional adsorption/desorption processes are usually carried out by flowing the

fluid phase either the feed or the regeneration solutions through a fixed bed column

where the adsorbent material is packed. In the case of processes incorporating MNPs

with small particle size, fix bed configuration is not suitable due to the high pressure

drop caused by the fluid flow. Therefore, different contact modes such as in-series

stirred tanks or fluidized beds that allow the solid to be suspended in the liquid are

needed to overcome the contingency caused by the fluid pressure drop. However, the

17

solid suspension contact requires the separation of the adsorbent from the solution when

either the adsorption or the desorption stage are concluded. In comparison with non-

magnetic nanoparticles, where the separation is a difficult task owing to their small size,

the main advantage of MNPs materials is their magnetic nature, which greatly simplifies

their collection by applying an external magnetic field.

The manipulation of nanoparticles by the use of magnetic fields, which is called

magnetophoresis, has attracted great attention in recent years due to the high potential of

nanotechnologies and the several advantages offered by magnetic systems. On the one

hand, magnetic separation is more selective, efficient and generally much faster than

centrifugation or filtration processes which are conventionally applied for solid-liquid

separations [191]. On the other hand, the use of an external magnetic field provided by a

permanent magnet requires no power consumption. Furthermore, magnetic separations

are less sensitive to factors such as surface charge, pH and ionic concentration [192].

The motion of MNPs in a fluid under the influence of an applied magnetic field is

affected by several factors, including the magnitude and the gradient of the applied

magnetic field, the fluidic drag, gravity and buoyancy forces, particle-fluid and particle-

particle interactions, etc. [193]. All these factors, which in turn will depend on the

operation conditions and the particle parameters, should be taken into account for the

selection and design of the magnetic separator.

As mentioned previously, the particles employed in these applications are usually

superparamagnetic, with diameters ranging from a few tens to hundreds of nanometers.

Since the magnetic force is proportional to the particle volume, MNPs with larger

diameters are preferred [194]. However, it is necessary to balance this increase on the

magnetic recovery efficacy with the reduction on the surface area, which implies a

reduction on the adsorption performance.

18

The recovery of MNPs by the use of magnetic gradients has been widely reported in the

literature [195]. Traditionally, the separation of magnetic materials is carried out by

batch magnetic filters (High Gradient Magnetic Separators, HGMS) where the particle

suspension is pumped through a column filled with ferromagnetic filaments. These

wires generate the high magnetic gradients inside the separator when an external

magnetic field is applied, producing large field gradients around the wires that attract

and trap the MNPs to their surfaces, as is depicted in Figure 6 [196-198]. MNPs will be

efficiently separated if the magnetic force, which attracts the particles to the filaments,

dominates the fluid drag, gravitational, inertial, and diffusional forces which act on the

MNPs as the solution flows through the column [197]. This technology has

demonstrated to be able to capture MNPs with sizes larger than 10 nm [198].

FIGURE 6

Figure 6. MNPs capture by HGMS filter [197].

However, the most important disadvantage of the HGMS is the uncertainty over the

magnetic conditions under which the MNPs are separated, due to the inhomogeneous

magnetic gradients generated inside the column. This uncertainty results in a limited

comprehension of the magnetophoretic mechanisms and restricts the modelling and

optimization of the separation process [196,198]. In addition, there is a high risk of

particle aggregation on the surface of the wires, which could reduce the available

adsorptive surface area of the MNPs decreasing the adsorption performance in

following cycles, or even could permanently retain the materials in the column. In fact,

a study carried out by Mayo et al. [199] demonstrated that Fe3O4 nanoadsorbents that

19

were used for arsenic removal, could not be recovered from the HGMS filter, which was

employed after the sorption process. The commercial adsorbents used were irreversibly

adsorbed to the column packing, and could not be released when the magnetic field was

turned off, because their size (20 nm) and their agglomeration promoted a large

magnetic moment that provided a remanent magnetization at zero fields.

The next section analyses the potential regeneration and reusability of MNPs employed

in the removal of metallic pollutants and organic dyes from wastewaters. In addition,

the management of both the spent regeneration solutions and the loaded adsorbent

materials is also highlighted.

4. MANAGEMENT OF SPENT MAGNETIC NANOADSORBENTS

In general, an ideal adsorbent material should be stable, highly effective, cost effective

and reusable. Under the selected operation conditions, adsorbents have a finite removal

capacity, and when it is reached, the material should be regenerated for reuse or

managed at the end of its life depending on the technical feasibility and process

economy. In this section the regeneration and reusability of MNPs adsorbents for heavy

metals and dyes removal is reviewed. Additionally, different options for the

management of the spent regeneration solutions are explored. Finally, the possibilities

for the end-of-life management of the spent nanoadsorbents are analyzed.

4.1. Regeneration and reuse of MNPs

Desorption and adsorbent regeneration is a critical step, which contributes to the process

costs and pollutant recovery. A successful regeneration process should restore the initial

characteristics of the adsorbent, allowing the solid reuse during the maximum number

of cycles and thus decreasing the costs of the overall separation process. However, in a

20

large number of sorption studies available in the literature dealing with the use of

nanoparticles for the removal of heavy metals and dyes, desorption and reuse of the

adsorbents has not been accurately analyzed.

The selection of a suitable eluent depends on the adsorbate and the adsorbent, but other

operation variables, such as pH, temperature, contact time between the solid and liquid

phases and the presence of competitive ions in the solution, may also affect the efficacy

of the desorption process [129]. However, the desorption efficacy might be enhanced by

gaining insight into the adsorption mechanisms [200]. Particularly, the desorption of

heavy metals and dyes from loaded adsorbents has been carried out with different

solutions, being most of them selected according to the influence of the pH value on the

adsorption process. Figures 7 and 8 depict a classification of the regeneration studies

that have been reported in the literature dealing with the desorption of heavy metals and

dyes from loaded MNPs, respectively. These figures show the number of regeneration

studies related to each pollutant and the eluent employed in the desorption stage. Also,

Table 3 summarizes the most relevant information obtained from those applications,

such as the adsorbent used, the target pollutant, the regeneration solution employed in

the desorption stage, the re-adsorption capacity and the number of cycles that the MNPs

could be reused in each specific case.

Table 3. Desorption and reusability of MNPs for heavy metals and dyes removal.

TABLE 3

Aqueous solutions of sodium hydroxide and strong acids are the most commonly used

regeneration solutions to elute heavy metals from loaded MNPs, as shown in Figure 7.

Since in most of the studies previously reported the adsorption of heavy metals is pH

21

dependent, the desorption stage is usually carried out by controlling the pH of the

eluent. Reverse to the adsorption process, metallic oxyanions are usually desorbed with

basic solutions, being NaOH the most preferred desorption agent in most of these

studies, as depicted in Figure 7a for arsenic and chromium. NaOH solutions with

concentrations ranging from 0.01 to 0.1 mol L-1 have been successfully applied in the

regeneration of MNPs loaded with heavy metal anions with no appreciable loss in the

sorbent capacities as reported in Table 3 [65,74,76,81]. The reusability of the materials

regenerated with NaOH solutions has been demonstrated in some of the related studies

during successive adsorption-desorption cycles [67,89,108,128,129]. Hu et al [66]

analyzed the desorption of chromate anions from loaded maghemite nanoparticles using

different basic eluents (NaOH, NaHCO3, Na3PO4, etc.) concluding that the most

effective agent was 0.01 mol L-1 NaOH. Besides, the CrO42- adsorption capacity of the

regenerated MNPs remained almost constant after six cycles, and the chromate ions

were concentrated 10 times during the regeneration process into a smaller volume.

On the other hand, the regeneration of the MNPs loaded with heavy metal cations is

normally conducted with acid solutions. As presented in Figure 7b, desorption eluents

based on HNO3 and HCl are widely reported in the literature due to their high

desorption efficacies [116,125,134,137]. Hao et al [22] (Table 3) employed Fe3O4-NH2

nanoparticles for the removal of Cu(II) ions being the reusability of the adsorbent

studied during successive sorption-desorption cycles. The results indicated that Cu(II)

ions could be desorbed completely in 1 minute in the presence of 0.1 mol L-1 HCl and

no differences in the adsorption capacity of the adsorbent were observed after 15 cycles

of operation. However, the use of strong acids is limited by the possible damage caused

to the adsorbent material since it promotes the magnetic core dissolution, especially for

bare magnetic nanoparticles. To avoid this problem, ethylendiamine tetraacetic acid

22

(EDTA), which is a very strong chelating agent for many heavy metal ions, could be

used as the eluent for metal cations desorption [92]. The use of EDTA solutions has

exhibited similar or superior desorption performance in comparison with strong acids

such as HCl or HNO3 for MNPs regeneration, as reported in Table 3 [38,92,136].

FIGURE 7

Figure 7. Regeneration solutions reported in the literature for anionic (a) and cationic

metallic pollutants (b).

Regarding dyes, acid or basic organic solutions using methanol (MetOH) or ethanol

(EtOH) as solvents are usually reported in the literature for the MNPs regeneration step

as reported in Figure 8, since these pollutants are easily dissolved in organic solvents.

Considering that most of dyes are adsorbed onto MNPs through electrostatic interaction,

the dyes desorption is successfully achieved by changing the pH of the solution. For

desorption of cationic/basic dyes, acid solutions with concentrations of 4-6% (v/v)

acetic acid in methanol (HAcMetOH) have reported high regeneration capacities of the

nanoadsorbents, with desorption percentages ranging from 80 to 100%

[27,155,164,171]. In addition, the reusability studies carried out suggest that the

adsorption capacity of the nanoadsorbents had no significant loss after being recycled

several times using these eluents (see Table 3) [27,165,174]. The regeneration study

carried out by Afkhami et al. [20] showed that, for Brilliant cresyl blue desorption from

sodium dodecyl sulfate coated maghemite nanoparticles, a mixture of acetic acid in

methanol could completely desorb the cationic dye from the loaded MNPs. Also, the

reusability of the adsorbent was longer than 15 cycles without any loss in its sorption

23

capacity. Besides HAcMetOH, HCl aqueous solutions (HClaq) and HCl ethanolic solutions

(HClEtOH) have reported good desorption performance for cationic dyes as shown in

Table 3 [168,175,177,186].

To elute anionic/acid dyes from exhausted MNPs, different eluents have been tested

showing the NaOH aqueous solutions (NaOHaq) and NaOH ethanolic solutions

(NaOHEtOH) higher desorption efficacy compared to other eluents [20,39,159,177].

Regeneration solutions based on NaOH, at concentrations ranging from 0.001 to 2 mol

L-1, have reported desorption efficacies ranging from 80-100% with negligible loss of

the materials properties through successive adsorption-desorption cycles

[159,162,184,185]. As representative example Fan et al [160] reported that the

desorption efficacy of Alizarin Red from loaded Fe3O4@Chitosan was higher than 90%

using 0.1 mol L-1 NaOHaq as eluent. Also, the adsorption percentages obtained with the

regenerated MNPs remained constant at 90% after 5 cycles of operation. Anionic dyes

have also been desorbed using basic solutions of NH4OH/NH4Cl in methanol as

illustrated in Figure 8, maintaining the adsorbent its initial adsorption capacity in

successive cycles [163,176].

FIGURE 8

Figure 8. Regeneration solutions reported in the literature for the anionic and cationic

dyes desorption stage.

As seen in Figure 8, in some of the reviewed works the desorption process of cationic

and anionic dyes was successfully carried out with pure methanol (MetOH)

[30,156,172,173,187]. As reported in Table 3, methanol solutions have been employed

24

to desorb Thionine and Janus Green (as models of cationic dyes) from chitosan coated

magnetite nanoparticles with regeneration percentages ranging from 93 to 97%, with a

negligible reduction of the adsorption capacity after 15 adsorption-desorption cycles

[20]. Besides the pH conditions, it has been demonstrated that high ionic strength of the

regeneration solution positively affects the desorption process [157].

The analysis reported above leads to the conclusion that the regeneration of magnetic

nanoadsorbents loaded with cationic or anionic metallic pollutants has been performed

with aqueous solutions of acids or bases, respectively. In the case of dye removal, acid

or basic solutions in alcohols such as ethanol or methanol have been reported as

effective regeneration solutions. Although most of the regenerated materials keep their

adsorption capacity, the concentration of the regeneration agent and the contact time

during the desorption process should be controlled to avoid modifications of the

morphological and chemical structure of the MNPs.

In spite of the adsorbent regeneration benefits the long-term performance of adsorption

processes, the desorption process generates waste solutions containing the pollutant in a

concentrated form, which have to be managed. On the other hand, the reusability of the

material may be limited because the nanoadsorbents generally loss their adsorptive

abilities after multiple cycles, generating a solid waste which needs also to be handled.

Different end-of-life scenarios for the aforementioned wastes are presented in the

following sections.

4.2. Management of spent regeneration solutions

The regeneration of the exhausted adsorbents produces an output stream of the eluent

containing the target pollutants, which should be managed in a proper way. However,

25

very few works have addressed the management of the spent desorption solutions.

There are three main alternatives for handling these effluents: i) the recovery of the

desorbed species from the eluent solutions for reuse, ii) the degradation of the pollutant

by destructive technologies such as incineration and, iii) the disposal of the solution

after its treatment by solidification/stabilization process.

In the case of heavy metals, the recovery and purification processes could be a

promising alternative for those materials with relatively high market prices such as

nickel, copper, palladium, platinum, etc. Different technologies have been reported in

the literature for the recovery of heavy metals from various liquid wastes including ion

flotation, electrodeposition, electrodialysis, membrane-based solvent extraction, etc.

[201-203]. The technology chosen depends on several factors, such as the composition

of the solution, the initial concentration of the material to be recovered, the capital

investment, the operational cost, etc. Ion flotation may be a promising technology for

the recovery of heavy metal ions from dilute solutions such as regeneration solutions,

where the metal concentration is in the order of mg L-1. This process involves the

addition of a surfactant into the solution, which is attached to the metal ions by

electrostatic or chelating interactions, and then both are separated due to the creation of

a foam phase when a gas is sparged into the mixture [201]. The surfactant can be later

recovered and recycled [202]. This technology has been successfully applied for the

recovery of nickel, cadmium, zinc, copper, etc. On the other hand, electrodeposition

could be also applied for the recovery of valuable metals from desorption solutions.

This technology has been able to recover zinc and manganese from chloride media with

conversion values close to 100% and current efficiencies as high as 90% [204,205].

Although these technologies have not been widely studied for the recovery of valuable

compounds from desorption solutions, these methods are considered as an attractive

26

option for the management of these wastes because they not only provide an interesting

alternative from the economic point of view generating a marketable product, but also

avoid environmental problems associated to the solution disposal, contributing to

resource and environmental sustainability at the same time.

In the case of more problematic metal wastes like arsenic, where recovery is not

profitable owing to its limited markets and other conventional treatment methods such

as incineration are not feasible because of the possible escape into the atmosphere of

hazardous products, an alternative method is the material encapsulation through

solidification/stabilization (S/S) processes followed by the disposal in secure landfills

[200,206,207]. The S/S methods aim to make hazardous liquid/solid wastes safe prior to

disposal. Several studies reported the incorporation of hazardous wastes such as arsenic

into storable solids through different S/S processes, being the cement-based S/S process

the technology mostly investigated, because it seems to be the most successful

technique [206]. In addition, the treated wastes can be incorporated into bricks or

concrete for construction purposes [208].

Regarding dyes desorption solutions, various authors have proposed thermal methods

such as distillation or evaporation to the recovery of the different components of the

regeneration solutions [154,174]. For eluents based on organic solvents such as acetone

or methanol, which have a low boiling point (56 and 65C respectively), these

technologies can be straightforward methods for the recovery of dyes, and also the

regeneration media for recycling purposes. However, these technologies are

energetically expensive, and the recovery and purification processes are not always

economically feasible. In those cases, degradation processes are mostly preferred.

However, most dyes are persistent organic molecules and conventional methods such as

biological degradation are not effective, and destruction methods such as incineration

27

are not appropriate solutions for treating chloride-containing dyestuffs owing to the

possible production of dioxins and furans, which remain during the process as thermally

persistent species [209]. An attractive approach to decolorize those solutions may be the

application of electrochemical technologies including electrocoagulation and

electrochemical oxidation, which have been considered as cost-effective dye treatment

methods in different works [210,211]. Electrochemical oxidation, which is considered

the most popular electrochemical method for degrading organic compounds, has been

able to completely remove a wide variety of dyes from dilute solutions in short times

with current densities ranging from 0.5 to 50 mAcm-2 [211]. Also, in the case of

chloride-containing solutions, such as the HCl based eluents, the presence of chloride

anions in the solution promotes the electrogeneration of active chlorine species such as

hypochlorite ions, which reduce the electrolysis time and enhance the dye oxidation

capability of these systems [211,212]. Although these techniques have not been directly

employed to remove organic dyes from desorption solutions, their integration with the

adsorption technology could lead to a more sustainable process.

4.3. Management of the spent nanoadsorbents

Reuse of regenerated nanoadsorbents in adsorption-desorption cycles, may lead to a

decrease of the adsorption capacity of the materials. The loss of efficacy in the

separation performance could be attributed to modifications in the adsorbent structure

derived from the intensive contact with acid or basic regeneration solutions. When the

irreversible exhaustion of the adsorbent occurs it should be replaced by fresh material

and the spent MNPs are converted to solid wastes with potential hazardous character

that should be managed in a proper way.

28

The management of the exhausted MNPs is an issue, which has been scarcely studied.

However, MNPs may have some potential risks to humans, environment and biological

systems, owing to their mobility and their increased reactivity, and therefore, the

management of these materials should be taken into consideration. Since nanoparticles

can interact with the environment through multiple ways and can alter the behavior or

react with a variety of chemicals, their handling and management must be controlled to

ensure the environmental welfare.

Little information is available in the literature on how to handle discarded nanoparticles.

The best end-of-life scenario is the recycling of the nanomaterials [15]. This approach

has been considered for noble-metal nanoparticles due to their high prices [213]. Other

methods which have been suggested to handle nanomaterial wastes include their storage

or destruction. Storage of MNPs should be carried out in a safe manner to avoid the

release of MNPs into the environment, and it should be noted that standard tests, which

are suitable for normal wastes to predict the fate of the materials disposed in landfills, in

this case are not applicable owing to their different behavior [214]. On the other hand,

destruction of MNPs by irreversible aggregation or dissolution could be an efficient

way to recover or process the materials in a conventional way [213].

However, any approach proposed for the management of these wastes requires

understanding of their chemical, physical and biological properties [213,214]. Much

work is still needed to understand the potential risks and toxicity of these materials and

although they have not been classified as hazardous wastes by the actual regulation, it is

essential to set specific legislation for their proper disposal [1,215].

The previous sections discussed the benefits and limitations of employing nano-

adsorbents to tackle environmental problems taking into account the management of

both the spent regeneration solutions and the spent adsorbents.

29

5. CURRENT CHALLENGES AND FUTURE ROLE OF THE MNPs

In spite of the promising future of the magnetic nanomaterials in environmental

technologies, there are some issues and challenges that need to be overcome before their

large scale development. As mentioned above, one of the most important issues related

to the use of MNPs is their toxicity. MNPs can pose potential threats to the environment

and human beings if they are released during their synthesis, use or disposal, since their

low size facilitates their entry into the living systems [15]. Also, they can act as

contaminant carriers or interact with natural elements, transforming them into a more

hazardous form owing to their reactivity. A possible way to reduce their toxicity may be

the surface modification using biodegradable or biocompatible materials. However,

these measures can sacrifice adsorptive surface sites and diminish the recovery

efficiency through magnetic fields [17]. Hence, under the current uncertainty the

handling of these materials must be taken into consideration, and specific legislation

should be set in a near future. This work reports different methods to manage the

exhausted nanoadsorbents in a proper way, being the recycling methods the most

sustainable alternatives However, the lack of knowledge regarding to their

environmental effects needs to be addressed in order to bring nanotechnologies a step

forward.

Concerning to the economic issue, it is clear that the required dose of MNPs in the

adsorption stage is low, and the contact times for the pollutant removal are short in

comparison with conventional adsorbents. In this sense, their application seems to be

profitable. However, a comparison of MNPs performance with other materials, such as

the low cost adsorbents, should be carried out in each particular scenario. In the case

of commercial bare nanoparticles, their cost oscillates from 225 $/kg (for iron oxides

30

such as maghemite and magnetite) to 2255 $/kg (for nZVI) [54], which are higher than

the cost of novel adsorbents based on agricultural or industrial wastes. However, it

should be noted that the cost per volume of treated water could be lower due to several

reasons. On the one hand, since both the required dose of the MNPs and the time

needed to carry out the separation are very low, the volume of treated water will be

much greater for MNPs than for other methods when using the same amount of

materials. On the other hand, the reusability of the MNPs could be satisfactorily carried

out with a simple regeneration step, unlike other adsorbents, and thus the cost of the

treatment would be reduced. Also, due to the small amounts of the material used, the

wastes generated during the desorption stage and those due to the exhausted MNPs will

be minimal in comparison with the use of other materials. This work also promotes the

so called zero-discharge processes, through the use of MNPs coupled with other

technologies, aiming to recover marketable products from the regeneration solutions,

thus being the global process more sustainable. Finally, although direct comparison is

not possible due to the different working conditions, it should be noted that low cost

adsorbents are generally effective in the removal of pollutants whose permissible limits

are at milligram per liter level. On the contrary, MNPs have reported outstanding

capability for removing hazardous contaminants with permissible levels in the order of

microgram per liter [15]. For these reasons, the use of MNPs in adsorption processes

seems to be environmentally and economically beneficial.

6. CONCLUSIONS

Water pollution by inorganic and organic compounds such as heavy metals and dyes has

become a serious problem because of their extremely hazardous effects on humans and

the ecological systems. The use of MNPs as adsorbent materials for environmental

31

remediation has a great potential due to their superior physical and chemical properties

in comparison to bulk materials. The easy functionalization of MNPs allows the

synthesis of specific nanoadsorbents to perform the selective removal of a large variety

of pollutants. However, the regeneration process of loaded MNPs that needs of a first

step for the recovery of the MNPs and the final management of both the spent

regeneration solution and the exhausted adsorbent are crucial stages that should be

evaluated in order to bring the MNPs-based adsorption technology a step forward. For

the first time, in this work the viability of these individual stages is reviewed and

analyzed in order to contribute to the development of novel integrated separation

processes based on the use of nanoadsorbents.

As illustrated in this review, the use of engineered MNPs for heavy metals and dyes

removal has numerous benefits including higher adsorption capacities and faster

removal rates in comparison to traditional sorbent materials. In addition, the magnetic

separation alternatives are presented, showing the advantages and limitations of

conventional separators. Also, the regeneration of MNPs for further reuse can be

successfully achieved by adequately contacting the MNPs with the proper eluent.

Moreover, the management alternatives of both the solid and liquid wastes generated in

the process by either material recovery or final disposal are reviewed, promoting the

zero-discharge processes for ensuring the environmental welfare.

Concluding, there is a growing interest in the use of MNPs for environmental

remediation. However, the application of MNPs in environmental technologies is still in

the early stage and much work is still needed for the establishment of the

nanotechnologies. Nevertheless, the future problems related to poor water quality and

water scarcity in many countries of the world make their future quite promising. In this

regard, this work aims to provide valuable guidelines to accomplish the successful

32

integration of magnetic nanoadsorbents in water treatment technologies, thus trying to

offer useful information for the implementation of the nanotechnologies in a near future.

ACKNOWLEDGEMENTS

Financial support from the Spanish Ministry of Economy and Competitiveness under

the project CTQ2012-31639 (FEDER 2007-2013) is gratefully acknowledged.

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