Chapter II Biosorbents for the Remediation of Environment - An Overview
Treat the earth well: it was not given to you by your parents, it was loaned to
you by your children. We do not inherit the earth from our ancestors: we
borrow it from our children Ancient proverb
II. A. Introduction
Pictures of snow topped mountains and a lake in front, in brilliant sunshine
evoke the impression of not only a beautiful but also a clean environment.
However such an environment is not as clean as many people like to think it
is. In fact it never was, due to natural turnover of the elements. Much of the
ill health which affects humanity especially in the developing countries can
be traced to lack of safe and wholesome water supply. Usually water
contains two types of impurities, natural and manmade. The natural
impurities are not essentially dangerous. Pollution that is caused by human
activity includes industrial wastes which contain toxic agents like metal ions.
These chemical pollutants may accumulate in aquatic life like fish which is
used as human food, and affect man’s health. Studies on preconcentration
and determination of trace heavy metal ions are an important part of
environmental chemistry. Solvent extraction, ion-exchange process,
membrane filtration, electro deposition and solid phase extraction based on
12 Chapter 2
adsorption are important techniques for preconcentration and separation of
trace metal ions [Peñaranda and Sabino 2010].
II. B. Why remediation of the environment?
Any metallic element that has relatively high density and is toxic or
poisonous even at low concentration is termed as a heavy metal [Duruibe et
al. 2007]. Industrial revolution accelerated developments in all fields like
technology, automobile and textile. As a result a large amount of metal ions
are being released into the environment on a daily basis. Mining operations,
metal-plating facilities, power generation facilities, electronic device
manufacturing units, and tanneries release toxic metal ions into waste
streams. Contamination of aquatic media by heavy metal ions is a serious
environmental problem and a matter of great concern to scientists,
environmentalists, governments and researchers all over the world. The
presence of heavy metal ions in various water resources has stirred great
concern because of their high toxicity and non biodegradability [Monier
2012]. They leach out from waste dumps and pollute soils thereby entering
the food chain. Bio-accumulation, a phenomenon by which metals increase
in concentration at every level of food chain and are passed onto the next
higher level, may also result [Monier and Abdel-Latif 2013]. The toxicity of
metal ion is owing to their ability to bind with protein molecules and prevent
replication of DNA and subsequent cell division. In view of the toxicity of
heavy metals, various countries put forward stringent regulations for the
discharge of effluents containing heavy metal ions, into the environment
[Wilson et al. 2012]. It is well known that heavy metals can damage the
nerves, liver and bones and they block functional groups of essential
enzymes [Hadi 2012]. Heavy metal toxicity can cause chronic and
degenerative conditions. General symptoms include: headache, short-term
Biosorbents for Remediation of Environment - An Overview 13
memory loss, mental confusion, sense of unreality, distorted perception, pain
in muscles and joints, and gastro-intestinal upsets, food intolerances,
allergies, vision problems, chronic fatigue, fungal infections etc. Sometimes
the symptoms are vague and difficult to diagnose [Sahni 2011]. Among the
heavy metal ions, chromium, iron, cobalt, nickel, copper and zinc ingestion
beyond permissible quantities, cause various chronic disorders in human
beings. Copper ions although known to be an essential trace element to
humans, have fatal effect if induced at high dosage. Nickel at trace
concentrations acts as both a micronutrient and a toxicant in marine and fresh
water systems. A very small amount of nickel is needed by plants and it
becomes toxic at a higher levels. At these levels, nickel binds to the cell
membrane and hinders the transport process through the cell wall [Moghimi
and Abdouss 2012].
The removal of toxic metal ions from waste water is a crucial issue as it is
necessary to protect public health. Prolonged exposure to heavy metal ions
can cause permanent harm to the ecosystem. Treatment at source is usually
the practical source for controlling metal ion pollution.
II. C. Why biosorbents? Biosorption of heavy metal ions
Several methods have been used to remove metal ions from waste water
[Tabakci and Yilmaz 2008]. Each method has been found to be limited by
cost, complexity and by the production of secondary waste. Of the various
methods and techniques that have been used for the removal of pollutants
from contaminated water, sorption is considered as an efficient, effective,
and economic method [Zhao et al. 2011]. The absorbent materials must have
high specific surface area, many adsorption sites, and chemical stability [ aLi
et al. 2013]. Although activated carbon is the most commonly used sorbent, it
14 Chapter 2
is expensive. So there has to be other adsorbent materials, particularly low-
cost adsorbents and there has been a growing interest in using biological
materials in place of synthetic materials.
Adsorbents containing natural polymers and waste biomaterials have been
under focus recently. Biosorbents could be categorised into (i) active
biomass belonging to algae, bacteria or fungi, (ii) non active kind of
biosorbents, and (iii) abundant natural materials or polymers [Shetty 2006].
The biomass used, could be a rather abundant raw material which is either, a
waste from another industrial operation or could be cheaply available.
A broad range of biosorbents are there which can collect all heavy metals
from the solution. Agricultural products proven as good biomass sources
include wool, straw, coconut husks, coconut fibre, peat moss, exhausted
coffee, waste tea, walnut skin, cork biomass, defatted rice bran, rice hulls,
wheat bran, soybean hulls and cotton seed hulls, sawdust, pea pod, cotton
and mustard seed cakes. Agricultural wastes like maize bran and sugar beet
pulp are also used to make biosorbents [Copello et al. 2008]. Biosorbents
interact efficiently with metal ions, are capable of removing even trace levels
of heavy metal ions and are inexpensive. Hence they could be used in waste
water treatment technology [Guibal 2004] and biosorption is considered
effective in removing contaminants from aqueous effluents [Blázquez et al.
2012, Hemalatha et al. 2011].
Biosorption can be divided into two main processes: adsorption of the ions on
cell surface and bioaccumulation within the cell. The term ‘bioaccumulation’
has been proposed for the sequestering metal ions by metabolically mediated
processes (living microorganisms), and the term ‘biosorption’ for the
nonmetabolically mediated processes (inactive microorganisms). The
Biosorbents for Remediation of Environment - An Overview 15
mechanistic differences between biosorption and bioaccumulation are so
significant that the use of the two terms has become a necessity. The two
processes can coexist and can also function independently when a consortium
of microorganisms is exposed to metal-bearing solutions. Advantages of
biosorption include low-cost, high efficiency, minimisation of chemical and/or
biological sludge, regeneration with a suitable eluent allowing reuse of the
biosorbent, no additional nutrient requirement, employment of harsher reaction
environments, immobilisation in a matrix which allows the use in conventional
ion exchange systems and possibility of metal recovery [Norton et al. 2004].
Biosorption mechanisms vary, and in some cases they are still not very well
understood. Yet they may involve various phenomena like complexation,
coordination, electrostatic attraction, and ion exchange [Jeon and Holl 2004].
Physical mechanisms such as adsorption or precipitation may also occur.
Any of these mechanisms may be important in immobilizing the metal on the
biosorbent. Since the biomaterials that are used for sorption are complex, a
number of these mechanisms could be occurring simultaneously [Volesky
2001]. There are several ligand functions in biomass that could potentially
attract and sequester metal ions. The acetamido groups in chitin, amino and
phosphate groups in nucleic acids, amino, amido, sulfhydryl and carboxyl
groups in proteins and hydroxyl groups in polysaccharides are some of the
ligand functions present in natural polymers [Volesky and Holan 1995].
II. C. 1. A brief account of biosorbents used for toxic metal ion removal
The usage of natural materials that are available in large quantities or certain
waste from agricultural operations as low cost adsorbents may be
advantageous as they are widely available and are nature friendly. Different
researchers have used different biomass such as azadirachta indica bark
16 Chapter 2
[King et al. 2007], neem biomass [Arshad et al. 2008], citrus pectin [Ankit
and Schiewer 2008], banana peel [Hossain et al. 2012], bengal gram husk
(husk of channa dal) [Ahalya et al. 2005] and cupressus lusitanica bark
[Netzahuatl-Muñoz et al. 2012]. Terminalia catappa (almond tree) biomass
acts as an efficient biomass for the sorption of Al(III) and Cr(VI) ions [Edith
and Osakwe 2012]. The binding ability of Cd(II) ion has been studied on
seven different species of brown, red and green seaweeds [Hashim and Chu
2004]. Hypnea valentiae biomass binds cadmium metal ion from waste water
[Horsefall and Spiff 2005]. Lignin isolated from black liquor (a waste
product of paper industry) could be used to adsorb heavy metal ions [Guo et
al. 2008]. Grapefruit peel has been recognised as an effective biosorbent for
cadmium and nickel [Torab-Mostaedi et al. 2013]. Copper ion biosorption in
the presence of complexing agents onto orange peel and chemically modified
orange peel was investigated and was found that copper ion uptake was
reduced in the presence of complexing agents. Chemically modified orange
peel showed a higher Cu(II) ion uptake capacity and that too in the presence
of complexing agents in solution [Izquierdo et al. 2013]. The biosorption
behaviour of Araucaria heterophylla (green plant) biomass was investigated
and it was found that this biomass could be used as an effective, low cost
biosorbent for the removal of Pb(II) from aqueous solution [Sarada et al.
2013] . Sawdust can be used as a low-cost adsorbent to remove heavy metal
ions from water [Ahmed 2011, Naiya et al. 2008]. Natural bamboo sawdust
can also be an efficient Cu(II) ion adsorbent [Zhao et al. 2012]. Investigation
of the sorption of Zn(II), and Pb(II) ions on coir revealed that Pb(II) ion had
higher sorption affinity than Zn(II) ions [Conrad and Hansen 2007].
Coconut fibre which is an agro-industrial waste could be converted into an
efficient biosorbent [Gopalakrishnan and Jeyadoss 2011]. Coir pith is a
Biosorbents for Remediation of Environment - An Overview 17
waste material from the process of separating coir fibre from coconut husks
for use in mattress padding. It is found to have a high capacity of Cr(VI) ion
adsorption [Suksabye et al. 2007]. The sorption capacity of raw rice husk is
seen to be greatly enhanced by some simple and low-cost chemical
modifications [Kumar and Bandyopadhyay 2006]. Chemically modified coir
pith could be used for the adsorptive removal of Cr(VI) ions from
electroplating waste water [Suksabye and Thiravetyan 2012]. Physical and
chemical properties of the cellulose can be modified by graft copolymerization
and the grafted cellulose could be considered as an excellent candidate for
waste water treatment process [Wen et al. 2012]. Starch and certain other
materials have been investigated as polymer supports for the preparation of
adsorbents with different functional groups for metal adsorption [Feng et al.
2009, Shibi and Anirudhan 2005, Castro et al. 2011]. Starch phosphate
carbamate is reported to have high adsorption capacity for Cu(II) ions and it is
pointed out that it may act as a super absorbent in many applications [Heinze
et al. 2003]. Inspired by this discovery Guo et al studied the removal of Pb(II)
ions from aqueous solution by crosslinked starch phosphate carbamate.
Crosslinked starch phosphate carbamate was found to be an effective
adsorbent for Pb(II) ions [Guo et al. 2006]. Succinylated and oxidised corn
starch was used in the adsorption of divalent metal ions [Kweon et al. 2001].
Crosslinked amphoteric starch with quaternary ammonium and carboxymethyl
groups was prepared and investigated for Pb(II) ion uptake [Xu et al. 2005].
Use of polycarboxylated starch-based adsorbent for Cu(II) ions was also
studied [Chauhan et al. 2010].
II. D. Importance of chitosan
One of the commonly used biosorbents is fungal biomass because these
microorganisms are commonly used for the production of industrial enzymes
18 Chapter 2
[Guibal 2004]. The metal sorption sites on the constituents of cell walls has
been located with the help of transmission electron microscopy [Tsezos and
Volesky 1982]. Chitin was first discovered in mushrooms by the French
Professor, Henrni Braconnot, in 1811. In 1820s chitin was also isolated from
insects [Bhatnagar and Sillanpää 2009]. It is one of the most representative
polymers in fungal cell walls. The strong metal-sorbent Rhizopus arrhizus
belonging to the Rhizopus species have both chitin and chitosan as cell-wall
components, like other members of the same species and this is regarded as
responsible for the uptake of heavy metal ions. In some Mucorales species
chitin is replaced by chitosan and this led researchers to use chitin/chitosan
material for the uptake of metal ions [Baik et al. 2002, Gyliene et al. 2002].
Chitin is the main structural component of molluscs, insects, crustaceans,
fungi, algae, and marine invertebrates like crabs and shrimps [Gonil and
Sajomsang 2012, Yadav et al. 2012]. The solid waste from processing of
shellfish, crabs, shrimps, and krill constitutes large amounts of chitinaceous
waste. Chitin is very similar in structure to cellulose being composed of
poly-2-acetamido-2-deoxy-D-glucose. Chitosan, like chitin is a natural
polysaccharide found in a wide range of natural sources [Donia et al. 2008]
including plant cell walls [Yoshizuka et al 2000]. Chitosan was discovered
in 1859 by Professor C. Rouget [Bhatnagar and Sillanpää 2009]. Possibility
of various chemical modifications widens the application of chitosan.
Chitosan is the partially deacetylated chitin prepared by the alkaline
deacetylation (Fig.II.1) [Kocak et al. 2012]. It is a copolymer of β-[1→4]
linked 2-acetamido-2-deoxy-D-glucopyranose and 2-amino-2-deoxy-D-
glucopyranose. It is the world’s second most abundant natural hydrophilic
biomacromolecule [Wang et al. 2013]. It is important and interesting to note
that the term “chitosan” does not refer to a single well defined structure, and
Biosorbents for Remediation of Environment - An Overview 19
chitosans can differ in molecular weight, degree of acetylation, and sequence
(i.e., whether the acetylated residues are distributed along the backbone in a
random or block manner). American
Fig. II. 1: Alkaline deacetylation of chitin to chitosan
Chitosan is currently at the focus of increasing scientific and economic interest
due to easy availability among all polysaccharides and also owing to its
significance in nature and technology [Liu et al. 2008]. It is an economical
and attractive biosorbent [Laus et al. 2010]. This polysaccharide is unique in
nature because of the presence of amino groups in its backbone. The main
parameters influencing the characteristics of chitosan are its molecular weight
and its degree of deacetylation (DD) representing the proportion of
deacetylated units. Degree of deacetylation is the same as the relative amount
of free amine [Croisier and Jérôme 2013]. Molecular weight and its degree of
deacetylation are determined by the conditions selected during preparation but
can be further modified at later stage. For example, the DD can be lowered by
reacetylation and the MW can be lowered by acidic depolymerisation.
Chitosan is currently receiving a great deal of attention for medical and
pharmaceutical applications. The main reasons for this increasing interest are
undoubtedly due to its appealing intrinsic properties. Chitosan is
metabolised by certain human enzymes, eg lysozyme, and is considered
biodegradable [Muzzarelli 1997]. In addition it has been reported that
chitosan acts as a penetration enhancer by opening epithelial tight-junctions.
Due to its positive charges at physiological pH, chitosan is also a
20 Chapter 2
bioadhesive. Chitosan is non-toxic, has antibacterial properties and affinity
towards proteins [Chamundeeswari et al. 2010]. Properties like
biocompatibility, hydrophilicity, high chemical reactivity, chelation, and
adsorption make chitosan a good work horse for many applications [Laus
and de Favere 2011, Higazy et al. 2010, Yuan et al. 2013]. The variety of
current and potential applications include those in biomedicine,
pharmaceutical systems, cosmetics, food processing, medicine, agriculture,
biochemical separation systems, tissue engineering, biomaterials, and drug
controlled release systems [Vashist et al. 2013, Chatterjee et al. 2009].
Chitosan has been used as a raw material for medical applications such as
surgical sutures, artificial skin and immunosuppressants. Chitosan and its
derivatives received considerable attention as antitumor, antiulcer,
immunostimulatory, anticoagulant and antimicrobial agents [Ramachandran
et al. 2011]. As the degradation products of chitosan are non-toxic, non-
immunogenic and non-carcinogenic [Alves and Mano 2008] chitosan finds
application in many fields such as waste water treatment, functional
membranes and flocculation. Different from most other natural polymers,
chitosan has high reactivity and processability for its specific molecular
structure and polycationic nature. Finally chitosan is abundant in nature, and
its production is of low cost and is ecologically interesting.
II.D.1. Chitosan as the most promising biosorbent
Chitosan among other biosorbents is one of the most promising alternative
adsorbents for the recovery of heavy metals from waste water [Kavianinia et
al. 2012, Prado et al. 2011]. Chitosan chelates five to six times greater
amounts of metal ions than chitin and this is attributed to the presence of free
amino groups and hydroxyl groups in it [Cestari et al. 2010, Ghaee et al.
2012]. These groups function as the coordination sites for heavy metal ions
Biosorbents for Remediation of Environment - An Overview 21
[Wong et al. 2004]. The physicochemical properties of chitosan related to the
presence of amine functions make it very efficient for binding anionic dyes
such as Reactive Black 5 (RB 5) [Gibbs et al. 2004] and anionic dyes [Wong
et al. 2008].
Chitosan is a weak base and is insoluble in water and organic solvents,
however, it is soluble in aqueous acidic solution (pH < 6.5), which can
convert the glucosamine units into a soluble form R-NH3+ (Fig.II.2).
Fig. II. 2: Conversion of the glucosamine units into the soluble form
The property of dissolving in weak organic acids is actually a blessing in
disguise. A number of different physical conditionings could be done in lieu
of this property [Ma et al. 2012]. Indeed, polymer solubilization is a
necessary step that allows preparation of chitosan hydrogels in the form of
films membranes fibers and even hollow fibers [Peirano et al. 2008, Vieira et
al. 2007]. Chemical or physical modification including chemical crosslinking
of the surface of the chitosan with crosslinking agents have been performed
to improve its chemical stability, mechanical strength, pore size,
hydrophilicity and biocompability, resistance to biochemical and
microbiological degradation, selectivity and capacity for the adsorption of
metal ions and dyes from industrial effluents [Rosa et al. 2008, Osifo and
Masala 2010, Tungtong et al. 2012]. Adsorption capacity could be improved
by crosslinking, insertion of new functional groups [Yadav et al. 2012, Liu et
22 Chapter 2
al. 2013], and conditioning of chitosan beads or resins [Miretzky and Cirelli
2009]. The presence of amino groups in chitosan opens up the possibility of
several chemical modifications including the preparation of Schiff’s bases by
reaction with aldehydes and ketones [Sun et al. 2003]. In spite of producing
mechanically and chemically stable beads, crosslinking has been found to
have negative effect on the adsorption capacity of chitosan. The main reason
for the loss of adsorption capacity is that amine groups are involved in the
crosslinking reaction [Martinez et al. 2007]. This leads to decrease in the
number of free and available amino groups on the chitosan backbone, and
hence the possible ligand density and the polymer reactivity as the flexibility
of the polymer chains is lost. As a result the accessibility to internal sites of
the material is decreased [Crini and Badot 2008]. As amine groups in
chitosan are considered to be the most important feature in the adsorption of
metal ions especially transition metal ions this is really embarrassing. It is
important to know, control and characterise the conditions of the
crosslinking reaction since they determine and allow the modulation of the
crosslinking density, which is the main parameter influencing interesting
properties of gels. The crosslinking reaction is mainly influenced by the size
and type of crosslinker agent and the functional groups of chitosan. The
smaller the molecular size of the crosslinker, the faster the crosslinking
reaction. Then its diffusion is easier. Depending on the nature of the
crosslinker the main interactions forming the network are covalent or ionic.
Degree of crosslinking is the main parameter influencing important
properties such as mechanical strength and swelling [Moore and Roberts
1981]. These conditions are useful for a better comprehension of the
adsorption mechanisms. For example the loss in flexibility of the polymer
resulting from the crosslinking may explain some diffusion restrictions, and
Biosorbents for Remediation of Environment - An Overview 23
the decrease observed in the intraparticle diffusivity [Crini and Badot 2008].
Several crosslinking agents such as glyoxal [Martinez et al. 2007],
epichlorohydrin [Kim et al. 2012] and ethylene glycol diglycidyl ether [Li
and Bai 2006] have been proposed but glutaraldehyde is the most widely
used because it does not have much diminishing adsorption capacity [Hu et
al. 2011]. Structural formulae of chitosan crosslinked with epichlorhydrin
(chitosan-EPI), glutaraldehyde (chitosan-GLA) and ethylene glycol
diglycidyl ether(chitosan-EGDE) are shown in Fig.II.3.
(a) (b)
(c)
Fig. II. 3: Structural formulae of (a) chitosan – EPI, (b) chitosan – GLA, and (c) chitosan- EGDE
24 Chapter 2
A spectroscopic study on the effect of glutaraldehyde on the chitosan
adsorption properties shows that as the concentration of glutaraldehyde
increases, the crystallinity and adsorption capacity of the crosslinked
chitosan is affected [Monteiro and Airoldi 1999]. An increase in the degree
in crosslinking results in an increased crystallinity of the beads and possibly
negatively affects the mobility of the metal ions in the beads. It also causes
reduced adsorption capacities due to a weaker interaction of the metal ions
with the chitosan or the loss of active sites.
II.D.2. Some selected modifications of chitosan used as biosorbents
Recently much attention has been paid to chemical modification of chitosan
[Abdelaal et al. 2013]. The modifications can alter the physical and
mechanical properties of the polymer. Several workers have suggested it
may be advantageous to chemically modify chitosan by grafting reactions
[Shimizu et al. 2005]. Carboxymethylated chitosan was reported to be a
rather better adsorbent than raw chitosan for acidic dyestuffs [Uzun and
Güzel 2004]. A chitosan biopolymer derivative was synthesized by
anchoring a new ligand, namely 4-hydroxy-3-methoxy-5-[(4-methyl
piperazin-1-yl) methyl] benzaldehyde, with chitosan. Equilibrium adsorption
studies of Mn(II), Fe(II), Co(II), Cu(II), Ni(II), Cd(II), and Pb(II) ions on
this derivative were conducted. This derivative is claimed to show good
adsorption capacity for these metal ions [Krishnapriya and Kandaswamy
2010]. Another chitosan derivative has been synthesized by crosslinking a
metal complexing agent, [6, 6’- piperazine-1,4-diyldimethylenebis (4-
methyl-2-formyl) phenol], with chitosan [Krishnapriya and Kandaswamy
2009]. Adsorption experiments of this chitosan derivative toward Mn(II),
Fe(II), Co(II), Cu(II), Ni(II), Cd(II), and Pb(II) ions were carried out and the
results showed that the adsorption was dependent on the pH of the solution.
Biosorbents for Remediation of Environment - An Overview 25
The maximum adsorption capacity was 1.21 mmol g-1 for Cu(II) ions. The
higher adsorption capacity of this chitosan derivative is attributed to the
additional coordination sites available in the crosslinker.
The effects of various parameters, such as pH, contact time, initial
concentration, and temperature on the adsorption of Hg(II) by
ethylenediamine-modified magnetic crosslinked chitosan microspheres have
been examined. These microspheres exhibited good adsorption capacity for
Hg(II)ions [Zhou et al. 2010]
The performance of a crosslinked magnetic modified chitosan, which has
been coated with magnetic fluids and crosslinked with glutaraldehyde, has
been investigated for the adsorption of Zn(II) ions from aqueous solutions
[Fan et al. 2011]. The maximum adsorption capacity was estimated to be
32.16 mg g-1 at 298 K. The cross-linked magnetic modified chitosan was
stable and easily recovered.
Crosslinked magnetic chitosan-2-aminopyridine glyoxal Schiff’s base resin
act as an efficient adsorbent for Cu(II), Cd(II) and Ni(II) ions from aqueous
solution. 2-Aminopyridine-glyoxal Schiff’s base (APG) was prepared first
through the reaction between the amino group of 2-aminopyridine and the
aldehyde group of glyoxal. Further the modification of chitosan with 2-
aminopyridine-glyoxal Schiff’s base was carried out via Schiff’s base
formation between the amino group in chitosan and the active aldehyde
group of APG [Monier et al. 2012].
Magnetic chitosan nanocomposites claim to be a very efficient, fast, and
convenient tool for removing Pb(II), Cu(II), and Cd(II) ions from water.
They can be used as a recyclable tool for the separation of these metal ions
[Liu et al. 2009].
26 Chapter 2
Chitosan crosslinked with epichlorohydrin efficiently remove Cr(VI) and
display a high uptake capacity [Kavianinia et al. 2012].
Raw chitosan beads have been chemically modified into protonated chitosan
beads, carboxylated chitosan beads and grafted chitosan beads. These
modified forms showed a significant sorption capacity compared to raw
chitosan beads. Among the sorbents studied, grafted chitosan beads showed
a higher sorption capacity. The copper uptake obeys the Freundlich isotherm.
The pH of the medium influences the sorption of Cu(II) ions onto modified
chitosan beads. Modified forms of chitosan removes copper selectively from
other common ions present in water. The mechanism of copper sorption
onto all the modified forms of chitosan beads is governed by adsorption, ion-
exchange and chelation [Gandhi et al. 2011].
The adsorption properties of glycine modified crosslinked chitosan polymer
has been investigated [Ramesh et al. 2008]. The parameters studied include
the effects of pH, contact time, ionic strength and the initial metal ion
concentrations by batch method. The optimal pH for the adsorption of gold,
platinum and palladium was found to range from 1.0 to 4.0. The maximum
adsorption capacity of the derivative for Au(III), Pt(IV) and Pd(II) was found
to be 169.98, 122.47 and 120.39 mg g-1, respectively at pH 2.0. This glycine-
modified crosslinked chitosan polymer had acted as an efficient adsorbent
for the removal of Au(III), Pt(IV) and Pd(II) ions.
II.D.3. Semi-interpenetrating and interpenetrating polymer networks based on chitosan
Hydrogels are crosslinked macromolecular networks swollen in water or
biological fluids. The major disadvantage of hydrogels is their relatively low
mechanical strength. This can be overcome by various methods such as
crosslinking, copolymerization with hydrophobic monomers, formation of
Biosorbents for Remediation of Environment - An Overview 27
interpenetrating networks (IPNs), or crystallization that induces crystallite
formation and drastic reinforcement of their structure [Tang et al. 2009]. IPN
has been considered to be most useful in improving the mechanical strength of
hydrogels [Wang et al. 2011].
An interpenetrating polymer network (IPN) is a combination of two
polymers, in network form, of which at least one is synthesized and/or cross-
linked in the immediate presence of the other without any covalent bonds
between them [Klempner and Sperling 1994].
The interlocked structures of the crosslinked components are believed to
ensure the stability of the bulk and surface morphology [Liu and Sheardown
2005]. The SIPNs represent a system in which only one of the polymer
networks is covalently crosslinked [Mahdavinia et al. 2008]. The SIPN
hydrogels find extensive application in the recovery of precious metals,
removal of toxic or radioactive elements from various effluents, and in the
preconcentration of metals for environmental sample analysis. It has been
shown that metal uptake is generally limited by metal diffusion into the
hydrogel and the hydrogel water interfacial area [Andreopoulos 1989]. The
main disadvantage of coordination polymers that have been widely used in
metal extractions is poor swelling in water which limits the mobility of the
ligands. Hence it may be advantageous to use hydrophilic polymeric
networks based on polyacrylamide, polymethacrylic, and polyacrylic acids.
They can absorb a large amount of water and aid easy diffusion of metal ions
into the polymer networks. Polyacrylamide is a water-soluble polymer with
a hydrophobic main chain and a hydrophilic side group.
Crosslinked polyacrylamide copolymers have found widespread applications
in bioengineering, biomedicine, food industry, and in water purification and
28 Chapter 2
separation processes [Kasgoz et al. 2003]. The networks are composed of
homopolymers or copolymers and are insoluble due to the presence of
chemical or physical crosslinks. Polymer gels have been studied for their
applications in a variety of fields, such as chemical engineering, food stuffs,
agriculture, medicine, and pharmaceuticals.
Interpenetrating polymer network (IPN) hydrogels have been used in a
number of biotechnological and biomedical applications. IPN structures are
also used for the control of overall hydrogel hydrophilicity and drug release
kinetics. A wide range of so called semi-IPN (SIPN) has been prepared by
dissolving a performed linear polymer in a hydrophilic monomer and
crosslinking agent mixture which is subsequently polymerized. In this way a
synthetic hydrogel network is formed around a primary polymer chain which
modifies the behaviour of the hydrogel.
II.E. Biosorption and solid-phase extraction : Tool for metal ion separation
Many of the traditional methods of preconcentration and separation for metal
ions often require large amounts of high purity organic solvents, some of
which are harmful to health and cause environmental problems [Chang et al.
2007]. Solid phase extraction technique has been widely used in analytical
chemistry for preconcentration and separation of trace metal ions in complex
matrices in recent years. The preconcentration and separation methods based
on the sorption are considered to be superior to the liquid-liquid extraction
because of the reduced usage and exposure to organic solvents [Pyrzynska
2012]. SPE is similar to liquid-liquid extraction (LLE), which involves
partitioning of solutes between two phases. In LLE two immiscible liquid
phases are involved, whereas SPE involves partitioning between a liquid
(sample matrix) and a solid (sorbent) phase. Thus, SPE is based on the
Biosorbents for Remediation of Environment - An Overview 29
distribution of analyte between an aqueous solution and sorbent by
mechanisms, such as adsorption, co-precipitation, complex formation and
other chemical reactions on or in the sorbents. These analytes must have
greater affinity for the solid phase than for the sample matrix [Fontanals
2005]. The analyte is transferred to the active sites of the adjacent solid
phase; the choice of sorbent is therefore a key point in SPE because it can
control parameters such as selectivity, affinity and capacity [Dean 1998].
This choice depends strongly on the analytes of interest and the interactions
of the chosen sorbent through the functional groups of the analytes.
However, it also depends on the kind of sample matrix and its interactions
with both the sorbent and the analytes. This sample treatment technique
enables the concentration and purification of analytes from solution by
sorption on a solid sorbent. The analyte after sorption on the solid phase is
either desorbed with a suitable eluate or the analyte along with the sorbent is
dissolved in a suitable solvent and further analyzed [Rao et al. 2004].
Even though the first experimental applications of SPE started fifty years
ago, it started the development as an alternative to liquid–liquid extraction
for sample preparation only during the 1970s. It has been the most common
technique in environmental, biological and food analyses. It is particularly
used for preconcentration or separation of metal ions due to high enrichment
factors, absence of emulsion, safety with respect to hazardous samples, low
cost because of lower consumption of reagents, rapid phase separation,
ability of combination with different detection techniques, speed and
simplicity, flexibility and ease of automation [Bartyzel and Cukrowska
2011]. Among different types of solid phase extraction (SPE), chelating SPE
(in which the sorbent is functionalized by a ligand) is the best method for
metal ion extraction in the aqueous phase because it takes advantage of
30 Chapter 2
complexation phenomenon between the ligand and the metal ions. An
efficient adsorbing material should possess a stable and insoluble porous
matrix having suitable active groups (typically organic groups) that interact
with metal ions.
The biosorption process involves a solid phase (sorbent or biosorbent; usually
a biological material) and a liquid phase (solvent, normally water) containing a
dissolved species to be sorbed (sorbate, a metal ion). Due to higher affinity of
the sorbent for the sorbate species the latter is attracted and bound with
different mechanisms. The process continues till equilibrium is established
between the amount of solid-bound sorbate species and its portion remaining
in the solution. While there is a preponderance of solute (sorbate) molecules
in the solution, there are none in the sorbent particle to start with. This
imbalance between the two environments creates a driving force for the solute
species. The heavy metal ions adsorb on the surface of biomass thus, the
biosorbent becomes enriched with metal ions in the sorbate.
II.F. Physicochemical characterisation of biosorbents
II.F.1. Fourier-transform infra-red spectroscopy (FT-IR)
Infrared spectroscopy being a universal technique, is valued as the simple
and most useful tool for the determination of functional groups on polymers.
The coordination sites in a biopolymer can be located by this method. FTIR
spectrometry has been widely used to study structural changes in polymers
and polymer complexes. Participation of the functional groups in the
crosslinking process can be confirmed likewise. The FT-IR spectra obtained
for the chitosan polymer before crosslinking with epichlorohydrin recorded
characteristic peaks at 1381 and 3438 cm-1 corresponding to the -OH
bending and vibration respectively, indicating the existence of hydroxyl
Biosorbents for Remediation of Environment - An Overview 31
groups in chitosan. Shifts of the peak at 3438 to 3434 cm-1 and at 1381 to
1350 cm-1 after crosslinking, indicate the participation of hydroxyl functional
groups in the crosslinking process [Wu et al. 2012]. The characteristic
absorptions of the new functional group may appear upon functional
transformation. Thus the course of the reaction can easily be followed by
scanning the IR spectra of the starting compound and the product [Zheng et
al. 2011]. The formation of polymer metal complexes can be followed by
comparing the characteristic bands with the corresponding low molecular
weight complexes. Shifting of the IR absorptions of the ligand after complex
formation is a clear indication of the formation of the complex [Qu et al.
2011]. The entanglement of two networks in IPN or SIPN could also be
confirmed by IR spectral analysis [Luo et al. 2010].
II.F.2. Powder XRD
Most of the polymers exhibit a semi crystalline morphology, forming mixed
regions of crystalline and amorphous domains. The XRD technique has been
widely utilized to detect crystallinity in polymer blends. The nature of the
crystal structure of a sorbent plays an important role in describing the
sorption capability. Decrease in crystallinity of chitosan is showed to
enhance metal uptake [Kamari et al. 2011]. Characteristic peaks of chitosan
in the X-ray diffractograms show a reduction after crosslinking treatments.
Substitution of amino groups by the crosslinking reagents deform the
hydrogen bonds thereby leading to the formation of amorphous structure in
crosslinked treated chitosans [Wang and Kuo 2008]. Crystallinity of
modified chitosan is less compared to chitosan. The semi-crystalline nature
of semi-IPN is evident in Fig. II.4 [Mishra et al. 2008]
32 Chapter 2
Fig.II.4: X-ray diffractograms of (a) native PVA, (b) PVA-acrylamide semi-IPN, and (c) native PAM
II. F. 3. Scanning electron microscopy / Energy dispersive analysis of X-rays
Biosorbents with high surface area are usually preferred. Scanning electron
micrographs provide insight into the morphology and physical state of the
surface. SEM coupled with energy dispersive analysis of X-rays (EDAX) is
used to determine the metal uptake mechanism on chitosan [Varma et al.
2004]. The effect of increasing amount of crosslinking agent on the
morphology also can be followed using this technique. Natural chitosan
displayed a dense and smooth surface, while treated chitosans have a rough
surface texture. Crosslinked chitosan amino acid beads had rough, rubbery,
fibrous and folded surfaces. As the concentration of crosslinker
glutaraldehyde increases, the chains come closer to each other and exhibit a
regular fibrous structure. On decreasing the concentration of glutaraldehyde
Biosorbents for Remediation of Environment - An Overview 33
the structural morphology changes to layered and big fibrous bunches [Rani
et al. 2011].
II.F.4. Electron spin resonance spectroscopy
Electron spin resonance (ESR) has been used as a technique to extract
information on the electronic structure of organic, inorganic, biological, and
surface molecular species. The ESR spectral pattern of paramagnetic Cu(II)
complexes is influenced by the number of coordinating ligands as well as the
geometry of the complex [George and Mathew 2001]. The ESR parameters
give an indication about the nature of the bond between metal ion and ligand.
The bonding parameter (α2Cu) of the Cu (II) ion complex is a measure of the
inplane σ-bonding. It is calculated using the expression given by Kivelson
and Neiman [Kivelson and Neiman 1961].
II.G. Application of IPN and SIPN as biosorbent for heavy metal ions
When the hydrogel network is prepared in the presence of a previously made
polymer such as poly(ethylene glycol), polyacrylamide, poly(N-isopropyl
acrylamide), poly(vinyl pyrrolidone), poly(vinyl alcohol), or polyacrylic
acid, semi-interpenetrated networks with improved mechanical properties are
formed. Changes in the swelling in response to external stimuli, such as
temperature, pH, and ionic concentration, make useful many of these SIPN
hydrogels as novel modulation systems in biomedical fields [Kim et al. 2004,
Zaldivar et al. 2011].
A semi-interpenetrating polymer network (SIPN) hydrogel composed of
crosslinked chitosan and polyacrylamide (PAAm) shows intelligent response
to pH which makes it an excellent candidate to design novel drug delivery
systems [Mahdavinia et al. 2008].
34 Chapter 2
Water-soluble cationic dye such as Janus Green B could be adsorbed on
ternary semi-interpenetrating polymer networks containing acrylamide/
sodium acrylate, poly(ethylene glycol) because of the presence of many
ionic groups that can increase the interaction between the cationic dye
molecules and anionic groups of hydrogels [Karadağ and Üzüm 2012].
Scheme II. 1: Synthesis of chitosan/ PAAm SIPN
Successful recognition between haemoglobin and bovine serum albumin at
the same condition has been achieved by using semi-interpenetrating
polymer network hydrogel prepared to recognize haemoglobin, by
molecularly imprinted method. This SIPN was synthesised using chitosan
and acrylamide in the presence of N, N-methylenebisacrylamide as the
crosslinking agent (Scheme II. 1) [Xia et al. 2005].
Biosorbents for Remediation of Environment - An Overview 35
The SIPN hydrogels find extensive application in the recovery of precious
metals, removal of toxic or radioactive elements from various effluents, and
in the preconcentration of metal ions for environmental sample analysis
[Katime and Rodriguez 2001]. 2-hydroxyethyl acrylate (HEA) and
2-acrylamido-2-methylpropane sulfonic (AMPS) acid-based hydrogels were
prepared by radiation-induced copolymerization. The HEA/AMPS copolymer
hydrogel was found to be an effective adsorbent for heavy metal ions and has
great potential applications in environmental work as smart adsorbent
materials. The technique could be used for the decontamination of
wastewater [bLi et al. 2012].
Blend hydrogels composed of carboxymethyl chitosan and poly(acrylonitrile)
(PAN) were synthesized and sorption for different dyestuff and various metal
ions were studied. Antibacterial characteristics of these hydrogels were also
investigated towards Escherichia coli (E. coli) [Mohamed et al. 2012].
Chitosan-g-polyacrylamide semi-IPN synthesised through UV irradiation in
the absence of any photoinitiator or catalyst showed uptake capacity of 0.636
meq g-1 for Zn(II) ions from water. The experimental data of the adsorption
equilibrium from Zn(II) ion solution fit well with the pseudo-second-order
model [Saber-Samandari et al. 2012].
The preparation of polyacrylamide-chitosan and the adsorptive features of
chitosan and polyacrylamide-chitosan have been investigated for Pb(II),
UO2(II), and Th(IV) ions in terms of dependency on ion concentrations,
temperature, and adsorption kinetics [Akkaya and Ulusoy 2008]. The results
show that polyacrylamide-chitosan has greater adsorption capacity than
chitosan for all the studied ions.
36 Chapter 2
Super absorbent hydrogels based on κ-carrageenan and polyacrylamide
showed varying percentage swelling in equilibrium depending upon the
preparation conditions and nature of the swelling medium such as pH and
temperature. Decrease in the initial swelling rate with decrease in κ-
carrageenan content in the gel matrix is evident from the swelling rate
curves. These systems are found to be suitable for the uptake of Cu(II) and
Ni(II) ions from aqueous solutions [Mohanan et al. 2011].
IPN hydrogels based on poly(ethylene glycol diacrylate) and poly(methacrylic
acid) were synthesized by sequential interpenetrating technology. Adsorption
properties of the IPN hydrogels were examined for the removal of Cu(II),
Cd(II), and Pb(II) ions from aqueous solutions under the non-competitive
condition and are found to be fast-responsive, high capacity, and renewable
sorbent materials in heavy metal removing processes [Wang et al. 2011].
An interpenetrating network synthesised from 2-hydroxyethyl methacrylate
(HEMA) and chitosan was supposed to contain the useful properties of both
the components and it is found to be an efficient adsorbent for Cd(II), Pb(II),
and Hg(II) ions [Bayramoglu et al. 2007]. Various IPNs and SIPNs have
been prepared and investigated for a variety of applications. They play an
important role in the field of metal recovery. In this study we attempt to
synthesise interpenetrating networks based on chitosan and acrylamide and
investigate the metal ion uptake behaviour so that it could be used as a
biosorbent.
Biosorbents for Remediation of Environment - An Overview 37
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