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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