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Prog. Polym. Sci. 33 (2008) 399447

Application of chitosan, a natural aminopolysaccharide, for dye

removal from aqueous solutions by adsorption processes using

batch studies: A review of recent literature

Gre gorio Crini, Pierre-Marie Badot

Department of Chrono-Environment, University of Franche-Comte , UMR UFC/CNRS 6565, Place Leclerc, 25000 Besanc-on, France

Received 21 December 2006; received in revised form 9 November 2007; accepted 9 November 2007

Available online 17 November 2007

Abstract

Application of chitinous products in wastewater treatment has received considerable attention in recent years in the

literature. In particular, the development of chitosan-based materials as useful adsorbent polymeric matrices is an

expanding field in the area of adsorption science. This review highlights some of the notable examples in the use of chitosan

and its grafted and crosslinked derivatives for dye removal from aqueous solutions. It summarizes the key advances and

results that have been obtained in their decolorizing application as biosorbents. The review provides a summary of recent

information obtained using batch studies and deals with the various adsorption mechanisms involved. The effects of

parameters such as the chitosan characteristics, the process variables, the chemistry of the dye and the solution conditions

used in batch studies on the biosorption capacity and kinetics are presented and discussed. The review also summarizes and

attempts to compare the equilibrium and kinetic models, and the thermodynamic studies reported for biosorption onto

chitosan.

r 2007 Elsevier Ltd. All rights reserved.

Keywords:Chitosan; Biosorption; Dyes; Batch process; Modeling and thermochemistry of biosorption

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www.elsevier.com/locate/ppolysci

0079-6700/$- see front matterr 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.progpolymsci.2007.11.001

Abbreviation:AB, acid blue; AB 1, acid black 1; AB 15, acid blue 15; AB 25, acid blue 25; AB 40, acid blue 40; AB 62, acid blue 62; AB

113, acid blue 113; AG 25, acid green 25; AG 27, acid green 27; AO 7, acid orange 7; AO 10, acid orange 10; AO 12, acid orange 12; AO

51, acid orange 51; AR, acid red; AR 1, acid red 1; AR 14, acid red 14; AR 18, acid red 18; AR 73, acid red 73; AR 27, acid red 27; AR 87,acid red 87; AR 88, acid red 88; AR 138, acid red 138; AV 5, acid violet 5; AY 25, acid yellow 25; BB, basic blue; BB 1, basic brown 1; BB

3, basic blue 3; BB 9, basic blue 9; BR, brilliant red M5BR2; BY 45, basic yellow 45; CV, crystal violet; DB, direct blue; DB 14, direct blue

14; DB 71, direct blue 71; DO, direct orange; DR, direct red; DR 2, direct red 2; DR 81, direct red 81; DS, direct scarlet B; DY 4, direct

yellow 4; IC, indigo carmine; IR, iragalon rubine RL; MB, maxilon blue 4GL; MB 29, mordant blue 29; MB 33, mordant brown 33; MO,

methyl orange; MO 10, mordant orange 10; MY, metanil yellow; MY 30, mordant yellow 30; O II, orange II; Rb 5, reactive blue 5; RB,

reactive blue RN; RB 5, reactive black 5; RB 2, reactive blue 2; RB 15, reactive blue 15; RB 19, reactive blue 19; RB 222, reactive blue 222;

RO, reactive orange; RO 16, reactive orange 16; R 6G, rhodamine 6G; RR, reactive red; RR B, reactive red RB; RR 2, reactive red 2; RR

141, reactive red 141; RR 189, reactive red 189; RR 195, reactive red 195; RR 222, reactive red 222; RTB, reactive T-blue; RY, reactive

yellow GR; RY 2, reactive yellow 2; RY 86, reactive yellow 86; RY 145, reactive yellow 145.Corresponding author. Tel.: +33 3 81 66 57 01; fax: +33 3 81 66 57 97.

E-mail address: [email protected] (G. Crini).
http://www.elsevier.com/locate/ppolyscihttp://localhost/var/www/apps/conversion/tmp/scratch_6/dx.doi.org/10.1016/j.progpolymsci.2007.11.001mailto:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_6/dx.doi.org/10.1016/j.progpolymsci.2007.11.001http://www.elsevier.com/locate/ppolysci

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400

2. General considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402

2.1. Batch experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402

2.2. Why to use chitosan as raw material? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403

2.3. Considerations on dye adsorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4052.4. Why to use chitosan as a biosorbent for dye removal? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407

2.5. Raw chitosan and chitosan-based materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409

3. A brief review of the recent literature on the adsorption of dyes by chitosan . . . . . . . . . . . . . . . . . . . . . . . 412

4. Control of adsorption performances of chitosan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415

4.1. Influence of the chitosan characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415

4.1.1. Chitosan origin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415

4.1.2. Physical nature of the chitosan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415

4.1.3. Chemical structure of chitosan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418

4.2. Activation conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419

4.2.1. Chitosan preprotonation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419

4.2.2. Grafting reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419

4.2.3. Influence of crosslinking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4204.2.4. Chitosan-based composite beads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422

4.3. Influence of process variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422

4.3.1. Effect of chitosan dosage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422

4.3.2. Effect of initial dye concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422

4.3.3. Effect of contact time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423

4.3.4. Effect of stirring rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424

4.3.5. Effect of dryness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424

4.4. Chemistry of the dye. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424

4.5. Effect of the solution conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425

4.5.1. Effect of pH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425

4.5.2. Effect of pH variation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427

4.5.3. pH sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427

4.5.4. Effect of ionic strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427

4.5.5. Effect of competitive molecules and ions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428

4.6. Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428

4.7. Desorption of dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429

5. Adsorption mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429

6. Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432

6.1. Equilibrium isotherm models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432

6.2. Kinetic modeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435

6.3. Thermochemistry of biosorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438

6.3.1. Effect of temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438

6.3.2. Thermodynamic parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 440

7. Economic aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442

8. Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444

1. Introduction

Many industries, such as textile, paper, plastics

and dyestuffs, consume substantial volume of water,

and also use chemicals during manufacturing and

dyes to color their products. As a result, they

generate a considerable amount of polluted waste-

water [15]. For example, pulp and paper mills

generate varieties of pollutants depending upon the

type of the pulping process. Their toxic effluents

are a major source of aquatic pollution and will

cause considerable damage to the receiving waters if

discharged untreated [1]. This specific type of

pollution is characterized by high biochemical

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oxygen demand (BOD), chemical oxygen demand

(COD), suspended solids (mainly fibers), bad smell,

toxicity (high concentration of nutrients, presence

of chlorinated phenolic compounds, sulfur and

lignin derivatives, etc.), and especially color [1,2].

Color is the first contaminant to be recognized

in wastewater and the presence of very small

amounts of dyes in water is highly visible andundesirable [4,5].

During the past three decades, several wastewater

treatment methods have been reported and at-

tempted for the removal of pollutants from textile,

pulp and paper mill effluents. The technologies can

be divided into three main categories: (i) conven-

tional methods, (ii) established recovery processes

and (iii) emerging removal methods (seeTable 1). In

the literature, there are a great number of feasibility

studies concerning the treatment of dyeing effluents

by these methods [28].

It is known that wastewaters containing dyes are

very difficult to treat, since the dyes are recalcitrant

molecules (particularly azo dyes), resistant to

aerobic digestion, and are stable to oxidizing agents.

Another difficulty is treatment of wastewaters

containing low concentrations of dye molecules. In

this case, common methods for removing dyes are

either economically unfavorable and/or technically

complicated. Because of the high costs associated

with their practical applications to remove trace

amounts of impurities, many of the methods for

treating dyes in wastewater (Table 1) have not been

widely applied on a large scale in the paper and

textile industries. In practice, no single process

provides adequate treatment and a combination of

different processes is often used to achieve the

desired water quality in the most economical way.

Thus, there is a need to develop new decolorization

methods that are effective and acceptable in

industrial use.It is now recognized that adsorption using

low-cost adsorbents is an effective and economic

method for water decontamination. A large variety

of non-conventional adsorbents materials have been

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Nomenclature

aL Langmuir isotherm constant (l/mg)

C intercept of the intraparticle diffusion

equation (mg/g)

Ce liquid-phase dye concentration at equili-

brium (mg/l)

Co initial dye concentration in liquid phase

(mg/l)

DG Gibbs free energy change (kJ/mol)

DH enthalpy change (kJ/mol)

DS entropy change (J/mol K)

Ea activation energy (kJ/mol)

KF Freundlich isotherm constant (l/g)

KL Langmuir isotherm constant (l/g)

k0 frequency factor (min1)

k1 equilibrium rate constant of pseudo-first-order adsorption (min1)

k2 equilibrium rate constant of pseudo-

second-order adsorption (g/mg min)

ki intraparticle diffusion rate constant

(mg/g min1/2)

qe amount of dye adsorbed at equilibrium

(mg/g)

qt amount of dye adsorbed at timet (mg/g)

qmax maximum adsorption capacity of the

adsorbent (mg/g)

m mass of adsorbent used (g)

nF Freundlich isotherm exponent

R universal gas constant (8.314 J/mol K)

T absolute temperature (1K)

t time (min)

te equilibrium time (min)

V volume of dye solution (l)

x amount of dye adsorbed (mg)

Table 1

Principal existing and emerging processes for dyes removal

Conventional treatment

processes

Coagulation/floculation Precipitation/floculation Electrocoagulation/

electroflotation Biodegradation Adsorption on activated carbon

Established removal

methods

Oxidation Electrochemical treatment Membrane separation Ion-exchange Incineration

Emerging recovery

technologies

Advanced oxidation Selective bioadsorption Biomass

G. Crini, P.-M. Badot / Prog. Polym. Sci. 33 (2008) 399447 401

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proposed and studied for their ability to remove

dyes [6]. However, low-cost adsorbents with high

adsorption capacities are still under development to

reduce the adsorbent dose and minimize disposal

problems. Much attention has recently been focused

on various biosorbent materials such as fungal orbacterial biomass and biopolymers that can be

obtained in large quantities and that are harmless to

nature. Special attention has been given to poly-

saccharides such as chitosan, a natural aminopoly-

mer. It is clear from the literature that the

biosorption of dyes using chitosan is one of the

more frequently reported emerging methods for

the removal of pollutants.

Chitosan has been investigated by several re-

searchers as a biosorbent for the capture of

dissolved dyes from aqueous solutions. This natural

polymer possesses several intrinsic characteristicsthat make it an effective biosorbent for the removal

of color. Its use as a biosorbent is justified by two

important advantages: firstly, its low cost compared

to commercial activated carbon (chitosan is derived

by deacetylation of the naturally occurring biopo-

lymer chitin which is the second most abundant

polysaccharide in the world after cellulose); sec-

ondly, its outstanding chelation behavior (one of the

major applications of this aminopolymer is based

on its ability to tightly bind pollutants, in particular

heavy metal ions).In this paper, we review the use of chitosan for

dye removal from aqueous solutions. Since the

review only presents data obtained using raw,

grafted and crosslinked chitosans, the discussion

will be limited to these chitosan-based materials and

their adsorption properties. The main objectives are

to summarize some of the developments related to

the decolorizing applications of these polymeric

materials and to provide useful information about

their most important features. We give an overview

of several recent batch studies reported in the

literature, with the various mechanisms involved.

To do so, an extensive list of recent literature has

been compiled. The effects of various parameters

such as chitosans characteristics, the activation

conditions, the process variables, the chemistry of

the dye and the experimental conditions used in

batch systems, on biosorption are presented and

discussed. The review also summarizes the equili-

brium and kinetic models, and the thermodynamic

studies reported for biosorption onto chitosan,

which are important to determine the biosorption

capacity and to design treatment processes.

2. General considerations

2.1. Batch experiments

The change in the concentration of a pollutant

(adsorbate) in the surface layer of the material(adsorbent) in comparison with the bulk phase with

respect to unit surface area is termed adsorption.

The term biosorption is given to adsorption

processes, which use biomaterials as adsorbents

(or biosorbents). The assessment of a solid-liquid

adsorption system is usually based on two types of

investigations: batch adsorption tests and dynamic

continuous-flow adsorption studies. The present

review only presents data obtained using batch

studies. When studying adsorption from solutions

on materials it is convenient to differentiate between

adsorption from dilute solution and adsorptionfrom binary and multicomponent mixtures covering

the entire mole fraction scale. To judge by the

number of papers published annually on adsorption

from dilute solution, this subject is more important

than adsorption from binary mixtures. Therefore,

reference will be made hereafter to adsorption from

dilute aqueous solutions.

Batch studies use the fact that the adsorption

phenomenon at the solid/liquid interface leads to a

change in the concentration of the solution.

Adsorption isotherms are constructed by measuringthe concentration of adsorbate in the medium

before and after adsorption, at a fixed temperature.

For this, in general, adsorption data including

equilibrium and kinetic studies are performed using

standard procedures consisting of mixing a fixed

volume of dye solution with an known amount of

chitosan in controlled conditions of contact time,

agitation rate, temperature and pH. At predeter-

mined times, the residual concentration of the dye is

determined by spectrophotometry at the maximum

absorption wavelength. Dye concentrations in solu-

tion can be estimated quantitatively using linear

regression equations obtained by plotting a calibra-

tion curve for each dye over a range of concentra-

tions. The adsorption capacity (adsorption uptake

rate) is then calculated and is usually expressed in

milligrams of dye adsorbed per gram of the (dry)

adsorbent. For example, the amount of dye

adsorbed at equilibrium, qe, is calculated from

the mass balance equation given by Eq. (1). The

symbols used in the equation are defined in the

Nomenclature section. In general, the experi-

ments are conducted in triplicate under identical

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conditions and found reproducible:

qe VCo Ce

m . (1)

The equilibrium relationship between adsorbent

and adsorbate, i.e. the distribution of dye molecules

between the solid adsorbent phase and the liquid

phase at equilibrium, which are the basic require-

ments for the design of adsorption systems, are

described by adsorption isotherms using any of the

mathematical models available. The equilibrium

adsorption isotherm, usually the ratio between the

quantity adsorbed and that remaining in solution at

a fixed temperature at equilibrium, is fundamentally

important since the equilibrium studies give the

capacity of the adsorbent and describe the adsorp-

tion isotherm by constants whose values express the

surface properties and affinity of the adsorbent (i.e.to study the interaction between the adsorbate and

the surface and to know about the structure of the

adsorbed layer).

In the literature, batch methods are widely used

to describe the adsorption capacity and the adsorp-

tion kinetics. These processes are cheap and simple

to operate and, consequently, often favoured for

small- and medium-size process applications using

simple and readily available mixing tank equipment.

Simplicity, well-established experimental methods,

and easily interpretable results are some of the

important reasons frequently evoked for the ex-

tensive usage of these methods. Another interesting

advantage is the fact that, in batch systems, the

parameters of the solution such as adsorbentconcentration, pH, ionic strength, temperature,

etc. can be controlled and/or adjusted.

2.2. Why to use chitosan as raw material?

The majority of commercial polymers and ion-

exchange resins are derived from petroleum-based

raw materials using processing chemistry that is not

always safe or environmental friendly. Today, there

is growing interest in developing natural low-cost

alternatives to synthetic polymers[6].Chitin, found in the exoskeleton of crustaceans,

the cuticles of insects, and the cells walls of fungi, is

the most abundant aminopolysaccharide in nature

[911]. This low-cost material is a linear homo-

polymer composed ofb(1-4)-linked N-acetyl gluco-

samine (Fig. 1). It is structurally similar to cellulose,

but it is an aminopolymer and has acetamide groups

at the C-2 positions in place of the hydroxyl groups.

The presence of these groups is highly advantageous,

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O

NHCOCH3

OH

CH2OH

O

O

NH2

OH

CH2OH

O

n n

Chitin Chitosan

DA 1-DA

O

NHCOCH3

OH

CH2OH

O

O

CH2OH

NH2

OH

Commercial Chitosan

N-acetyl glucosamine unit glucosamine unit

O

Fig. 1. Chemical structure of chitin [poly(N-acetyl-b-D-glucosamine)], chitosan [poly(D-glucosamine)] and commercial chitosan (a copolymer

characterized by its average degree of acetylation (DA)).


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providing distinctive adsorption functions and

conducting modification reactions. The raw poly-

mer is only commercially extracted from marine

crustaceans primarily because a large amount of

waste is available as a by-product of food proces-

sing [9]. Chitin is extracted from crustaceans(shrimps, crabs, squids) by acid treatment to

dissolve the calcium carbonate followed by alkaline

extraction to dissolve the proteins and by a

decolorization step to obtain a colorless product

[10,11] (Fig. 2).

Since the biodegradation of chitin is very slow in

crustacean shell waste, accumulation of large

quantities of discards from processing of crusta-

ceans has become a major concern in the seafood

processing industry. So, there is a need to recycle

these by-products. Their use for the treatment of

wastewater from another industries could be helpfulnot only to the environment in solving the solid

waste disposal problem, but also to the economy.

However, chitin is an extremely insoluble material.

Its insolubility is a major problem that confronts the

development of processes and uses of chitin [11],

and so far, very few large-scale industrial uses have

been found. More important than chitin is its

derivative, chitosan (Fig. 1).

Partial deacetylation of chitin results in the

production of chitosan (Fig. 2), which is a

polysaccharide composed by polymers of glucosa-

mine and N-acetyl glucosamine. The chitosan

label generally corresponds to polymers with less

than 25% acetyl content. The fully deacetylatedproduct is rarely obtained due to the risks of side

reactions and chain depolymerization. Copolymers

with various extents of deacetylation and grades are

now commercially available. Chitosan and chitin

are of commercial interest due to their high

percentage of nitrogen compared to synthetically

substituted cellulose. Chitosan is soluble in acid

solutions and is chemically more versatile than

chitin or cellulose. The main reasons for this are

undoubtedly its appealing intrinsic properties, as

documented in a recent review [11], such as

biodegradability, biocompatibility, film-formingability, bioadhesivity, polyfunctionality, hydrophi-

licity and adsorption properties (Table 2). Most of

the properties of chitosan can be related to its

cationic nature [912], which is unique among

abundant polysaccharides and natural polymers.

These numerous properties lead to the recognition

of this polyamine as a promising raw material for

adsorption purposes.

ARTICLE IN PRESS

Shellfish wastes

demineralization

deproteinization

decoloration

hydrolysis

glucosamines

oligosaccharides

Chitin

deacetylation carb oxymethylchitin

carb oxymethylation

chitosan derivatives

derivatization

Chitosan

salts

acetylation

oligosaccharides

glucosamines

N-acetyl-D-glucosamines

Fig. 2. Simplified representation of preparation of chitin, chitosan and their derivatives.


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The interest in chitin and chitosan is reflected by

an increasing number of articles published (Fig. 3),

and of meetings in Europe, Asia and America on

this topic. Table 3 summarizes the main applica-

tions of chitin and chitosan. Currently, these

polymers and their numerous derivatives are widely

used in pharmacy[21,36,37], medicine[11,21,2329],

biotechnology [10,21,30], chemistry [21,3134], cos-

metics and toiletries [11,21], food technology [35],

and the textile[21], agricultural [12,20,21], pulp and

paper industries[21]and other fields [21,38,39]such

as enology, dentistry and photography. The poten-

tial industrial use of chitosan is widely recognized.

These versatile materials are also widely applied in

clarification and water purification, and water and

wastewater treatment as coagulating [1315], floc-culating [16,17]and chelating agents [1922]. How-

ever, despite a large number of studies on the use of

chitosan for pollutant recovery in the literature, this

research field has failed to find practical applica-

tions on the industrial scale: this aspect will be

discussed later.

2.3. Considerations on dye adsorption

Synthetic dyes are an important class of recalci-

trant organic compounds and are often found in the

environment as a result of their wide industrial use.These industrial pollutants are common contami-

nants in wastewater and are difficult to decolorize

due to their complex aromatic structure and

synthetic origin. They are produced on a large

scale. Although the exact number (and also the

amount) of the dyes produced in the world is not

known, there are estimated to be more than 100,000

commercially available dyes. Many of them are

known to be toxic or carcinogenic.

Generally, dyes can be classified with regard to

their chemical structure (e.g. azo, anthraquinone,indigo, triphenylmethane), with regard to the

method and domain of usage (e.g. direct, reactive,

chromic, metal-complexes, disperse, mordant, sul-

fur, vat, pigments), and/or with regard to their

chromogen (e.g. n-p*, donoracceptor, cyanine,

polyenes). Mishra and Tripathy [40] proposed a

simplified classification as follows: anionic (direct,

acid and reactive dyes), cationic (basic) dyes and

non-ionic (disperse) dyes. As mentioned, there are

many structural varieties such as acidic, disperse,

basic, azo, diazo, anthraquinone-based and metal

complex dyes. Azo and anthraquinone colorants are

the two major classes of synthetic dyes and

pigments. Together they represent about 90% of

all organic colorants.

Fig. 4gives some examples of dyes currently used

in the textile industry. Reactive Black 5, a diazo dye,

has two sulfonate groups and two sulfatoethylsul-

fon groups in its molecular structure that have

negative charges in aqueous solution. Basic Blue 3, a

monoxazine dye, possesses an overall positive

charge because it tends to ionize in solution. The

anthraquinonic dyes Reactive Blue 19 and Disperse

ARTICLE IN PRESS

Table 2

Intrinsic properties of chitosan

Physical and

chemical properties

Linear aminopolysaccharide withhigh nitrogen content

Rigid D-glucosamine structure; highcrystallinity; hydrophilicity

Capacity to form hydrogen bondsintermolecularly; high viscosity

Weak base; the deprotonated aminogroup acts a powerful nucleophile

(pKa 6.3)

Insoluble in water and organicsolvents; soluble in dilute aqueous

acidic solutions

Numerous reactive groups forchemical activation and crosslinking

Forms salts with organic andinorganic acids

Chelating and complexing properties

Ionic conductivity

Polyelectrolytes (at

acidic pH)

Cationic biopolymer with highcharge density (one positive charge

per glucosamine residue)

Flocculating agent; interacts withnegatively charged molecules

Entrapment and adsorptionproperties; filtration and separation

Film-forming ability; adhesivity Materials for isolation of

biomolecules

Biological

properties

BiocompatibilityJ Non-toxicJ BiodegradableJ Adsorbable

BioactivityJ Antimicrobial activity (fungi,

bacteria, viruses)J Antiacid, antiulcer, and

antitumoral properties

J Blood anticoagulantsJ Hypolipidemic activity

Bioadhesivity


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Blue 14 have an anionic and non-ionic character,

respectively. Basic Green 4 is an N-methylated

diaminotriphenyl methane dye, which has a cationic

character. It is important to note that dye molecules

have many different and complicated structures,

and their adsorption behavior is directly related to

the chemical structure, the dimensions of the dye

organic chains, and the number and positioning of

the functional groups of the dyes. This is one of the

most important factors influencing adsorption.

However, to the we ay adsorption is affected by

the chemical structure of the dyes was not clearly

identified: this aspect will be discussed in the

following sections.

Generally, a suitable adsorbent for adsorption

process of dye molecules should meet several

conditions:

low cost,

readily available,

large capacity and rate of adsorption, high selectivity for different concentrations, and efficient for removal of a wide variety of

target dyes.

Recently, numerous low-cost adsorbents have

been proposed for dye removal. Among them,

non-conventional activated carbons from solid

wastes, industrial by-products, agricultural solid

wastes, clays, zeolites, peat, polysaccharides and

fungal or bacterial biomass deserve particular

attention as recently summarized in a review by

Crini [6]. Each has advantages and drawbacks.

However, at the present time, there is no single

adsorbent capable of satisfying the above require-

ments. Thus, there is a need for new systems to be

developed. In addition, the adsorption process

provides an attractive alternative treatment, espe-

cially if the adsorbent is selective and effective for

removal of anionic, cationic and non-ionic dyes.

ARTICLE IN PRESS

4%7%

3%

28%

1%4%

53%

coagulation

precipitation

adsorption

membranes

flocculation

flotation

filtration

0

50

100

150

200

250

300

1998 1999 2000 2001 2002 2003 2004 2005

Numberofartic

les

Fig. 3. A Scopus database literature survey of the wastewater applications of chitosan and chitin: (a) research articles published from 1998

to 2005 (the survey did not include patents) and (b) main domains of chitosan and chitin in the removal of pollutants from solutions.


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Now, the amounts of dyes adsorbed on the above

adsorbents are not very high, some have capacities

between 100 and 600 mg/g and some even lower

than 50 mg/g [6]. To improve the efficiency and

selectivity of the adsorption processes, it is essential

to develop more effective and cheaper adsorbentswith higher adsorption capacities.

2.4. Why to use chitosan as a biosorbent for dye

removal?

As already mentioned, a growing number of

papers have been published since the 1980s con-

cerning chitosan for wastewater treatment. In

particular, chitosan has received considerable inter-

est in heavy metal chelation due to its relatively low

cost compared with commercial activated carbon,

its excellent metal-binding capacities and interestingselectivity, as well as its possible biodegradability

after use. It is frequent to reach adsorption

capacities as high as 3 mmol metal per gram

chitosan for Cu (i.e. 200 mg/g), 12 mmol metal

per gram for Pt and Pd, and up to 710 mmol metal

per gram for Mo and V[18,19]. In accordance with

the very abundant data in the literature, liquid-

phase adsorption using chitosan is one of the most

popular methods for the removal of heavy metals

from wastewater since proper design of the adsorp-

tion process will produce a high-quality treatedsolution. Readers interested in a detailed discussion

of the interaction of metal ions with chitosan should

refer to the excellent comprehensive review by

Guibal [18].

Besides being natural and plentiful, chitosan

possesses interesting characteristics that also make

it an effective biosorbent for the removal of color

with outstanding adsorption capacities. Compared

with conventional commercial adsorbents such as

commercial activated carbons (CAC) for removing

dyes from solution, adsorption using chitosan-based

materials as biosorbents offers several advantages

(Table 4). In particular, three factors have specifi-

cally contributed to the growing recognition of

chitosan as a suitable biomaterial for dye removal:

First is the fact that the chitosan-based polymersare low-cost materials obtained from natural

resources and their use as biosorbents is extre-

mely cost-effective. In many countries, fishery

wastes were used as excellent sources to produce

chitosan. Since such waste is abundantly avail-

able, chitosan may be produced at relatively low

ARTICLE IN PRESS

Table 3

Applications of chitin and chitosan

Fields Applications

Agriculture Protection of plants

Increase of crop yields (reduces the growth

of phytopathogenic fungi)

Seed and fertilizer coating; soil treatment

Biomedical

engineering

Biological activities (antifungal,

antimicrobial, antiinfectious); antitumor

agent

Hemostatic effects; enhances blood

coagulation

Promotes tissue growth; stimulates cell

proliferation; artificial skin

Sutures/bandages

Ophthalmology, contact lenses

Biotechnology Enzyme and cell immobilization

Cell-stimulating materialsMatrix for affinity chromatography or

membranes

Chemical

industry

Water purification (metal chelation); water

engineering (flocculation, filtration,

adsorption); sludge treatment

Reverse osmosis, filtration membranes; gas

separation

Production of biodegradable packaging

films

Catalysis

Cosmetics and

toiletries

Hair spray, lotion; hand and body creams;

shampoo, moisturizer

Food industry Diet foods and dietary fiber;

hypocholesterolemic activity (binds

cholesterol, fatty acids and

monoglycerides)

Preservation of foods from microbial

deterioration

Bioconversion for the production of value-

added food products

Recovery of waste material from food-

processing discards

Clarification and deacidification of fruit

juices and beverages

Emulsifying agent; colour stabilization

Animal feed additive

Pharmaceutics Controlled drug delivery carriers

Microcapsules (forming gels and capsules

with anionic polymers)

Dermatological products (treats acne)

Others Textiles (anti-bacterial properties)

Pulp and paper (wet strength)

Enology (clarification, deacidification)

Dentistry (dental implants)

Photography (paper)


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cost. The volume of biosorbent used is also

reduced as compared to conventional adsorbents

since they are more efficient.

Second is the high adsorption capacities re-ported. The biosorbents posses an outstanding

capacity and high rate of adsorption, and also

high selectivity in detoxifying both very diluted

or concentrated solutions. They also have an

extremely high affinity for many varieties of dyes.

The third factor is the development of newcomplexing materials because chitosan is versa-

tile: it can be manufactured into films, mem-

branes, fibers, sponges, gels, beads and

nanoparticles, or supported on inert materials.

The utilization of these materials presents many

advantages in terms of applicability to a wide

variety of process configurations.

Of course, there are, also disadvantages of using

chitosan in wastewater treatment (Table 4). This

research field fails to find practical application at the

industrial scale. There are several reasons for

explaining this difficulty in transferring the process

to industrial applications [10,11,18,20]. The adsorp-

tion properties depend on the different sources of

chitin (the quality of commercial chitin available is

not uniform) and performance is also dependent on

the type of material used. Another important

criterion to be taken into account concerns the

variability and heterogeneity of the polymer (the

difficulty of controlling the distribution of the acetyl

groups along the backbone makes it difficult to get

reproducible initial polymers). There is a need for a

better standardization of the production process to

be able to prepare reproducible initial polymers

having the same characteristics. Changes in the

specifications of the polymer may significantly

change adsorption performance. Another problem

with chitosan derivatives is their poor physicochem-

ical characteristics, in particular low surface area

and porosity. In addition, although chitosan is

much easier to process than chitin or other low-cost

adsorbents, the stability of chitosan materials is

generally lower, owing to their more hydrophilic

character and, especially, pH sensitivity. Being a

biopolymer, chitosan is biodegradable and this may

ARTICLE IN PRESS

HO

H2N

N

N SO3Na

SO3Na

N

N

Reactive Black 5

NaO3SOCH2CH2O2S

NaO3SOCH2CH2O2S

O

N

(C2H5)2N N(C2H5)2

Cl-

+

Basic Blue 3

O

O

NH2

SO3Na

HN

SO2CH2CH2OSO3NaReactive Blue 19

N

N(CH3)2

+ O

O

HO

-O

Basic Green 4

O

O

NHCH3

NHCH3

Disperse Blue 14

Fig. 4. Examples of commonly used dyestuffs in the textile industry.


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be a serious drawback for long-term applications.

These problems can rebut industrial users. Readers

interested in a detailed discussion of these problemsshould refer to the work of Guibal [18]. However,

the opportunity now exists to consider chitosan for

emerging applications where other technologies

would be unsuitable.

Different reviews of chitosan-based biomaterials

have been reported concerning adsorption and

separation, including metal complexation [18,19],

complexing adsorbent matrices [21,22,41,42], and

membranes [33]. Obviously, chitosan has also been

investigated as a biosorbent for the capture of

dissolved dyes from aqueous solutions in numerous

articles. The effectiveness of chitin and chitosan to

adsorb dye molecules has been reported by numer-

ous workers [4357]. For example, as long ago as

1958, Giles et al. [43] investigated the binding

behavior of dyes to chitin. In 19821985, extensive

studies on the adsorption of dyes on chitin by

McKay et al. [4448] also revealed that chitin can

adsorb substantial quantities of dyestuffs from

aqueous solutions. The interaction of chitosan with

dyes was studied by several workers [4957]. These

earlier papers clearly demonstrated that raw materi-

als have an intrinsically high affinity and selectivity

for a wide range of dyes, although several contra-

dictory observations have been reported. However,

a few review articles on the potential of chitosan for

dye removal have been published. The application

of the adsorption of pollutants including dyes onto

chitosan has been reviewed by Ravi Kumar[21]andNo and Meyers [22]. Various chitosan-based com-

posites and membranes have been also developed

and proposed for adsorption and separation pur-

poses [33,42]. To avoid repetition, in the following

chapters, only raw, grafted and crosslinked chit-

osans will be discussed. This review focuses on the

recent developments related to decolorizing applica-

tions of the chitosan-based materials and reports the

main advances published over the last 10 years. This

is an ambitious project since the very large number

of groups working around the world forces us to

make a selection from the most significant results.Table 5 lists some of the researchers whose results

are discussed in this review and the dyes they

investigated [58116].

2.5. Raw chitosan and chitosan-based materials

Practical use of chitosan has been mainly

confined to the unmodified forms. For a break-

through in its utilization, chemical derivatization

onto polymer chains has been proposed to produce

new materials. Derivatization is a key point whichwill introduce the desired properties to enlarge the

field of its potential applications. Chitosan has three

types of reactive functional groups, an amino group

as well as both primary and secondary hydroxyl

groups at the C-2, C-3 and C-6 positions, respec-

tively (Fig. 1). Its advantage over other polysac-

charides is that its chemical structure allows specific

modifications without too many difficulties, espe-

cially, at the C-2 position [11]. These functional

groups allow direct substitution reactions and

chemical modifications, yielding numerous useful

materials for different domains of application.

The most commonly used chemical activations are

carboxymethylation, acetylation and grafting. The

variety of groups which can be attached to

the polymer is almost unlimited. To control both

the physical, mechanical and chemical properties,

various techniques can be used, and often, the

methods are adapted from the cellulose world [11].

The chitosan derivatives can be classified into four

main classes of materials: modified polymers, cross-

linked chitosans, chitosan-based composites and

membranes (Table 6).

ARTICLE IN PRESS

Table 4

Advantages and disadvantages of chitosan and chitosan-based

materials used as biosorbent for the removal of dyes from

aqueous solutions

Advantages Disadvantages

Low-cost hydrophilicbiopolymer

Very abundant materialand widely available in

many countries

Renewable resource Cationic polysaccharide (in

acidic medium)

Environmentally friendly,publicly acceptable

material

Extremely cost effective Outstanding dye-binding

capacities of a wide rangeof dyes

Fast kinetics High selectivity in

decolorizing both very

dilute or concentrated

solutions

Versatile biosorbent

Variability in the polymercharacteristics

The performance dependsof the origin and treatment

of the polymer, and also its

degree ofN-acetylation

Nonporous sorbent Requires chemical

derivatization to improve

its performance

Not effective for cationicdyes (except after

modification)

pH sensitivity Its use in sorption columnsis limited (hydrodynamic

limitations and column

fouling)

Non-destructive process


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An important class of chitosan derivatives are the

crosslinked materials, from gel types to bead types

or particles (including microparticles, microspheres

and nanoparticles). Gels are often divided into three

classes depending on the nature of their network,

namely entangled networks, covalently crosslinked

networks and networks formed by physical interac-

tions. Berger et al. [26] suggested the following

modified classification for chitosan gels; i.e. a

separation of chemical and physical gels. Physical

gels are formed by various reversible links and

chemical gels are formed by irreversible covalent

links, as in covalently crosslinked chitosan gels.

Hydrogels and beads can be formed covalently

crosslinking polymer with itself. In this chemical

type of crosslinking reaction, the crosslinking agents

are molecules with at least two reactive functional

groups that allow the formation of bridges bet-

ween polymer chains. To date, the most common

crosslinkers used with chitosan are dialdehydes such

as glyoxal, formaldehyde and in particular glutar-

aldehyde (GLU)[26]. GLU reacts with chitosan and

it crosslinks in inter and intramolecular fashion

through the formation of covalent bonds mainly

with the amino groups of the polymer. Its reaction

with chitosan is very well documented. The main

drawback of GLU is that it is considered to be

toxic, even if the presence of free unreacted GLU in

gels is improbable since the materials are purified

before use. Other crosslinkers of chitosan are

epoxides such as epichlorohydrin (EPI) and ethy-

lene glycol diglycidyl ether (EGDE), isocyanates

(hexamethylenediisocyanate) and other agents (car-

boxylic acids, genipin). Covalent crosslinking, and

therefore the crosslinking density, is influenced by

various parameters, but mainly dominated by the

concentration of crosslinker. It is favoured when

chitosan molecular weight (MW) and temperature

ARTICLE IN PRESS

Table 5

Authors of recent research on dye removal by chitosan (selected papers)

Corresponding author Country Dye(s) Reference(s)

Airoldi C. Brazil BB 9 [58]

Annadurai G. Iran BB 9, DS [59,60]

Cestari AR. Brazil IC, RB, RN, RR, RY [6163]

Chen DH. Taiwan AG 25, AO 12 [64]

Chen L. China AB, BB [65]

Chiou MS. Taiwan AO 7, AO 12, AR 14, DR 81 MY, RB 2, RB 15, RR 2,

RR 189, RR 222, RY 2, RY 86

[6670]

Cho SY. Korea RB 5 [71]

Crini G. France BB 3, BB 9 [72,73]

de Favere VT. Brazil RO 16 [74]

Dutta PK. India DB [75]

El-Tahlawy KF. Egypt BR, IR, MB [76,77]

Fahmy HM. Egypt DR [78]

Guibal E. France AB 1, AB 113, AG 25, AV 5, AY 25, DB 14, DB 71,

DY 4, MB 29, MB 33, MO 10, RB 5

[7982]

Guha AK. India AR 87 [83]Hebeish R Egypt AR, BY 45, DO, RO [84,85]

Juang RS. Taiwan AO 51, BB 9, RB 222, RR 222, RY 145, R 6G [8693]

Li HY. Taiwan RR 189 [94]

Martel B. France AB 15, AB 25, AB 62, DR 81, MY 30, RB 5, RB 19 [95]

Manolova N. Bulgaria RR [96]

McKay G. Hong Kong AG 25, AO 10, AO 12, AR 18, AR 73 [9799]

Miyata K. Japan AB 40, AR 18, AR 88, DR 2 [100]

Prado AGS. Brazil IC [101]

Saha TK. Bangladesh azo dye [102]

Shimizu Y. Japan AO 7, AR 1, AR 88, AR 138, BB 9, CV [103105]

Shyu SS. Taiwan BB 1, BB 3 [106]

Stevens WF. Thailand BB 9, CV, MO, O II [107,108]

Thiravetyan P. Thailand RR 141 [109]

Szeto YS. Hong Kong AG 27 [110,111]Uzun I. Turkey CV, O II, Rb 5, RB 5, RY 2 [112115]

Wen YZ. China RR 195 [116]


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increased. Moreover, since crosslinking requires

mainly deacetylated reactive units, a high degree

of deacetylation (DD) of chitosan is favorable.

The crosslinked polymeric materials have a three-

dimensional network structure and can swell con-

siderably in aqueous medium without dissolution.

Their synthesis and properties have been recently

described in detail [41]. Various methods have beendeveloped for the chemical crosslinking of chitosan,

which commonly result in gel formation. The

crosslinking step is a well-documented reaction

and an easy method to prepare chitosan-based

materials with relatively inexpensive chemicals.

Generally, a crosslinking step is required to

improve mechanical resistance and to reinforce the

chemical stability of the chitosan in acidic solutions,

modifying hydrophobicity and rendering it more

stable at drastic pH, which are important features to

define a good adsorbent. However, it decreases the

number of free and available amino groups on the

chitosan backbone, and hence the possible ligand

density and the polymer reactivity. It also decreases

the accessibility to internal sites of the material and

leads to a loss in the flexibility of the polymer

chains. So, the chemical step may cause a significant

decrease in dye uptake efficiency and adsorption

capacities, especially in the case of chemical reac-

tions involving amine groups, since the amino

groups of the polymers are much more active

than the hydroxyl groups and can be much more

easily attacked by crosslinkers. Consequently, it is

important to know, control and characterize the

conditions of the crosslinking reaction since they

determine and allow the modulation of the cross-

linking density, which is the main parameter

influencing interesting properties of gels [26]. Theseconditions 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 restric-

tions, and the decrease observed in the intraparticle

diffusivity.

Table 7outlines various methods and approaches

which have been proposed for the preparation of

chitosan particles including microspheres/micropar-

ticles, and nanoparticles. Selection of any of the

methods depends upon factors such as particle

size requirement, thermal and chemical stability. In

practice, the methods are often combined and

different follow-up treatments are carried out [33].

The emulsion crosslinking method is widely used for

the synthesis of microspheres. This method is

schematically represented in Fig. 5. With this

method, the size of the particles can be controlled

by modifying the size of the aqueous droplets.

Another interesting method is spray drying. This is

a complex operation with the movement of count-

less droplets/particles in turbulent drying medium

flows under changing temperature and humidity

ARTICLE IN PRESS

Table 6

The four main classes of chitosan derivatives

I. Modified polymers

Carboxymethylchitosans Alkylated chitosans Chitosan sulfate derivatives Carbohydrate-branched chitosans Grafted chitosans Ligand-bound chitosan

II. Crosslinked chitosan

Covalently crosslinked particles Ionically crosslinked particles Nanoparticles Physical gels

III. Chitosan-based composites

Chitosan-dendrimer hybrids Chitosan-supported on inert materials (silica gel, glass beads,

alumina, etc.)

IV. Membranes

Table 7

Some methods for preparation of chitosan particles

Crosslinking with chemicals

(Single) emulsion crosslinking Multiple emulsion Precipitation/crosslinking

Crosslinking and interactions with charged ions, molecules and

polymers

Ionotropic gelation Wet-phase inversion Emulsification and ionotropic gelation Emulsification and solvent evaporation Simple or complex coacervation (precipitation, complexation)

Miscellaneous methods

Thermal crosslinking Solvent evaporation method Neutralization method

Spray drying Freeze drying Reverse micellar Coating Interfacial acylation


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conditions. Chitosan microspheres obtained by this

technique are characterized by a high degree of

sphericity and specific surface area, parameters that

are important for application as adsorbents.

Ionic crosslinking reactions have also been

employed by using ionotropic gelation to formhydrogels, beads and nanoparticles. Aside from its

complexation with negatively charged ions or

molecules, an interesting property of chitosan is its

ability to gel on contact with specific polyanions.

This gelation process is due to formation of inter

and intramolecular crosslinks mediated by these

polyanions. Tripolyphosphate (TPP) is commonly

used to provoke the ionotropic gelation of chitosan.

The particles can be obtained by the addition of a

chitosan solution to a solution of TPP or vice versa,

under strirring. In either case, the size of the

particles is strongly dependent on the concentration

of the solutions. Chiou and Li [68] and Szetos

group [110,111] recently reported the ionotropic

gelation of chitosan with TPP. They prepared

chitosan particles by adding an alkaline phase

containing TPP into an acidic phase containing

chitosan. (Nano)particles are formed immediately

upon mixing the two phases through molecular

linkages created between TPP phosphates and

chitosan amino groups. The solution of TPP was

used to produce more rigid materials. They reported

that TPP had no effect on dye adsorption. To

stabilize chitosan in acid solutions, Chiou and Li

[68] also proposed an ionotropic gelation process

followed by a chemical crosslinking step.

Chitosan is usually used in a flaked or powdered

form that is both soluble in acidic media and non-

porous. Moreover, the low internal surface area ofthe non-porous polymer limits access to interior

adsorption sites and hence lowers dye adsorption

capacities and adsorption rates. To overcome this

obstacle, porous beads were synthesized. Indeed

an interesting characteristic of the chitosan is its

excellent ability to be processed into porous

structures.

3. A brief review of the recent literature on the

adsorption of dyes by chitosan

There is abundant literature concerning theevaluation of adsorption performances of raw

chitosan, especially in terms of adsorption capacity

(amount of dye adsorbed) or uptake. In a batch

system, the determination of the dye uptake rate by

a chitosan-based material is often based on the

equilibrium state of the adsorption system. At least

100 dyes, mainly anionic dyes, have been so far

studied. Chitosan has an extremely high affinity for

many classes of dyes (Table 8). In particular, it has

demonstrated outstanding removal capacities for

anionic dyes such as acid, reactive and direct dyes.This is due to its unique polycationic structure.

The effectiveness of chitosan for its ability to

interact with dyes has been studied by numerous

workers. Juang and co-workers [8993] demon-

strated the usefulness of chitosan for the removal of

reactive dyes. They found that the maximum

adsorption capacities of chitosan for RR 222, RB

222 and RY 145 were 1653, 1009 and 885 mg/g,

respectively [90]. Annadurai [59,60]and Crini et al.

[72] also reported that chitosan may be a useful

adsorbent for the effluent of textile mills because of

its high adsorption capacity. Uzun and Gu zel

[112115] noted that chitosan can be used in the

studies of dyestuff adsorption in comparison with

most other adsorbents. This polysaccharide showed

a higher capacity for adsorption of dyes than CAC

and other low-cost adsorbents, as reviewed by Crini

[6]. Kim and Cho [71] also indicated that the

amount of RB 5 adsorbed on chitosan beads is

much greater than on CAC. Similar conclusions

were reached by Lima et al. [58] for the BB 9

adsorption. McKays group [9799] recently pub-

lished a series of papers on the ability of chitosan to

ARTICLE IN PRESS

hardening of

droplets

chitosan aqueous

solutionoil phase

emulsification

crosslinking agent

stirring

particles

separation

Fig. 5. Schematic representation of preparation of chitosan

particles by emulsion crosslinking.


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Table 8 (continued)

Dye Chitosan Effective pre-

treatment of

chitosan

Part ic le size Sspa pH T(1C) Equilibrium

time

Equilibrium

model

qmb Kinetic

RR 189 2.32.5 mm 6 30 5 days Langmuir 950 Ho and

RR 222 Bead Crosslinking 3 30 2 days Langmuir 2252 Ho and

RR 222 Bead 30 5 days Freundlich 1965 Lagerg

RR 222 Swollen bead 2.8 mm 30 4 days Langmuir 1653 Ho and

RR 222 Wet bead 30 3 days Freundlich 1498 Elovich

RR 222 Dried bead 30 3 days Freundlich 1215 Elovich

RR 222 Bead (crab) 3.11 mm 3040 30 5 days Langmuir 1106

RR 222 Bead (shrimp) 2.39 mm 3040 30 5 days Langmuir 1026

RR 222 Bead (lobster) 2.93 mm 3040 30 5 days Langmuir 1037

RR 222 Flake (shrimp) 1630 mesh 46 30 5 days Langmuir 494

RR 222 Flake (lobster) 1630 mesh 46 30 5 days Langmuir 398

RR 222 Flake 11.41 mm 11.8 30 4 days Langmuir 339 Ho and

RR 222 Flake (crab) 1630 mesh 46 30 5 days Langmuir 293

RR 222 Bead Crosslinking 4.01 30 3 days Freundlich

RR 222 Bead (lobster) 0.715 mm 12.3 30

RY Bead Crosslinking 0.24 2 60200 min Avram

RY 2 Bead (crab) Crosslinking 4 30 5 days Langmuir 2436 Ho and

RY 86 Bead (crab) Crosslinking 3 30 5 days Langmuir 1911 Ho and

RY 145 Swollen bead 2.8 mm 30 4 days Langmuir 885 Ho and

RY 145 Flake 11.41 mm 11.8 30 4 days Langmuir 188 Ho and

RY 145 Bead (lobster) 0.715 mm 12.3 30

aSpecific surface area in m2/g.bAdsorption capacities in mg/g.cIntraparticle diffusion model.

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act as an effective adsorbent for the removal of acid

dyestuffs from aqueous solution. The monolayer

adsorption (saturation) capacities were determined

to be 973.3, 922.9, 728.2 and 693.2 mg of dye per

gram of chitosan for AO 12, AO 10, AR 73 and AR

18, respectively [99]. The interaction betweenchitosan and anionic dyes has also been intensively

investigated by Guibal and co-workers [7982].

Their investigations clearly indicated that chitosan

had a natural selectivity for dye molecules and was

very useful for the treatment of wastewater. They

reported that adsorption capacities ranged between

200 and 2000 mmol/g for chitosan and between 50

and 900mmol/g for CAC[82]. They concluded that

chitosan exhibited a twofold or more increase in the

adsorption capacity compared to CAC in the case of

acid, direct, reactive and mordant dyes. The best

choice for the adsorbent between CAC and chitosandepends on the dye, however, it was impossible to

determine a correlation between the chemical

structure of the dye and its affinity for either carbon

or chitosan.

It is evident from this brief literature survey that

chitosan can be utilized as an interesting tool for the

purification of dye-containing wastewater because

of its outstanding adsorption capacity.

4. Control of adsorption performances of chitosan

The data from the literature show that the control

of adsorption performances of a chitosan-based

material in liquid-phase adsorption depends on the

following factors:

(i) the origin and nature of the chitosan such as

its physical structure, chemical nature and

functional groups;

(ii) the activation conditions of the raw polymer

(physical treatment, chemical modifications);

(iii) the influence of process variables such as

contact time, initial dye concentration, polymer

dosage and stirring rate;

(iv) the chemistry of the dye (e.g. its pKa, polarity,

MW size and functional groups);

(v) and finally, the solution conditions, referring to

its pH, ionic strength, temperature and presence

of impurities.

These aspects will be described in the following.

However, the reader is encouraged to refer to the

original papers for complete information on experi-

mental conditions in the batch studies used.

4.1. Influence of the chitosan characteristics

It is very important to note that tuning the chitosan

manufacturing process can ernable the production of

polymers with varying chemical characteristics and

MW distributions. As stated in the introduction,chitosan is a collective term applied to deacetylated

chitins in various stages of deacetylation and

depolymerization[37]. Commercial chitosan is usual-

ly offered as flakes or powders. Products of various

companies differ in purity, salt-form, color, granula-

tion, water content, DD or degree of acetylation

(DA), amino group content, MW, crystallinity and

solubility[1012,18]. These parameters determined by

the conditions selected during the preparation are

very important because they control the swelling and

diffusion properties of chitosan and also influence its

characteristics [117]. In particular, numerous studieshave demonstrated that the MW and DD influence

the adsorption properties of this polymer. Therefore,

these factors must be considered carefully during the

adsorption optimization process.

4.1.1. Chitosan origin

From a practical viewpoint, crustaceans shells are

the potential sources for chitin production. Chit-

osan is commonly prepared by deacetylating chitin

using 4050% aqueous alkali at 110115 1C for a

few hours [12]. Chitin occurs in a wide variety ofspecies, from fungi to animals. Depending on the

chitin source, chitosan varies greatly in its adsorp-

tion properties and solution behavior, as reported

by Juang and co-workers [8993]. For example, the

adsorption capacities of RR 222 on different types

of chitosan prepared from three fishery wastes

(shrimp, crab and lobster shells) were compared.

The monolayer adsorption capacities were deter-

mined to be 293, 398 and 494 mg of dye per gram of

flake-type of chitosan for crab, lobster and shrimp,

respectively[91]. This demonstrates that the adsorp-

tion capacity of chitosan depends on its origin.

Rinaudo [11] also reported in a recent review that

the origin of chitin influences not only its crystal-

linity and purity but also its polymer chains

arrangement, and hance its properties. In particular,

the chitin resulting from crustaceans needs to be

graded in terms of purity and color since residual

protein and pigment can cause problems [10,11].

4.1.2. Physical nature of the chitosan

The adsorption capacity of chitosan also depends

on its physical structural parameters such as

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crytallinity, surface area, porosity, particle type,

particle size and water content. These parameters

are determined by the conditions selected during the

preparation and polymer conditioning.

Three crystalline forms are known for chitin:

a-, b- and g-chitins. The most abundant and easilyaccessible form is a-chitin [11,91]. Chitosan is also

crystalline and shows polymorphism depending on

its physical state. Depending on the origin of the

polymer and its treatment during extraction from

raw resources, the residual crystallinity may vary

considerably. Crystallinity is maximum for both

chitin (i.e. 0% deacetylated) and fully deacetylated

chitosan (i.e. 100%). Generally, commercial chit-

osans are semi-crystalline polymers and the degree

of crystallinity is a function of the DD. Crystallinity

plays an important role in adsorption efficiency as

reported by Trung et al. [108]. They demonstratedthat decrystallized chitosan is much more effective

in the adsorption of anionic dyes. Crystallinity

controls polymer hydratation, which in turn deter-

mines the accessibility to internal sites. This para-

meter strongly influences the kinetics of hydratation

and adsorption. Dissolving the polymer breaks the

hydrogen bonds between polymer chains. The

reduced polymer crystallinity can be maintained

through freeze-drying of the chitosan solution,

while air-drying or oven-drying partially reestab-

lishes polymer crystallinity. The conditioning of thepolymer and physical modification can strongly

reduce the influence of this important parameter

and improve diffusion properties [18]. The gel

formation procedure also allows an expansion of

the polymeric network, a decrease in steric hin-

drance phenomena and a decrease in the crystal-

linity of raw materials which enhance mass

transport. The case of dye adsorption with cross-

linked chitosan is a typical example of the influence

of particle size. When crosslinked with GLU, the

network formed makes the sorption performances

become dependent on the size of particles. This

dependence disappears when chitosan particles are

modified by gel formation. Hebeish et al. [84,85]

indicated that the crosslinking step changes the

crystalline nature of chitosan and decrease the

particle size of the crystallites, enhancing its

adsorption capacity. The crosslinking reaction

destroys the crystalline structure at low levels of

crosslinking. The authors assumed that more

accessible domains are created as a result of changes

in the physical and chemical structures of chitosan

during the modification by GLU, and consequently

these effects increased dye adsorption [85]. How-

ever, Cestari et al. [62] recently noted that after the

crosslinking reaction, there is a small increase in the

crytallinity of chitosan beads with increased access

to the small pores of the material.

Among the other parameters that have a greatimpact on dye adsorption is particle type. Chitosan

can be presented as gels, flakes, powders and

particles. Chitosan beads are preferred since flake

and powder forms of polymer are not suitable for

use as adsorbents due to their low surface area and

lack of porosity, as indicated by Varma et al. [19].

Beads are usually prepared by dropping high-

viscosity chitosan salt solutions into a basic solution

with slow stirring. The diameters of the drops as

well as the solution flow rate control the diameter of

the beads. Wu et al. [91] reported that bead-type

chitosan gives a higher capacity for dye adsorptionthan the flake type by a factor of 24 depending on

the source of fishery waste. For example, a

comparison of the maximum adsorption capacity

(qmax) for RR 222 by chitosan flakes and beads

prepared from a crab source showed 293 mg/g for

flakes and 1103mg/g for beads. The authors

explained this result by the fact that the beads

possessed a greater surface area (i.e., more loose

pore structure) than the flakes. They also reported

that the adsorption capacity of chitosan depends on

its source. The qmax were determined to be 1106,1037 and 1026 mg of dye per gram of bead-type of

chitosan for crab, lobster and shrimp, respectively

[91]. Again, it can be noted that the order ofqmaxfor

the different sources is exactly identical to that of

the surface area of the whole animal, i.e., crab4

lobster4shrimp. Chang and Juang [86] also noted

that chitosan in the bead form significantly im-

proves the adsorption performance of RR 222, AO

51 and BB 9 compared to that in the flake form.

Guibal et al. [82] indicated that it would be

interesting to use chitosan gel beads instead of

flakes since the production of gel beads decreases

the residual crystallinity of polymer which enhances

both the porosity and the diffusion properties of the

material, due to the expansion of the chitosan

network and the increase in the specific surface area.

Crini et al.[72]observed that compared to chitosan

flakes, chitosan beads exhibited a twofold or more

increase in the adsorption capacity for BB 9. One of

chitosans most promising features is its excellent

ability to be processed into nanostructures. These

nanochitosans can also be used in batch studies, as

reported by Hu et al. [110]. They noted that an

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adsorption capacity of 2103.6 mg of AG 27 per

gram chitosan was achieved, which was significantly

higher than that of the chitosan microparticles.

Previously, it has been demonstrated that the

particle size of chitosan also influences its adsorp-

tion profile. For example, Park et al. [56] showedthat of the smaller particle size, the more dye was

absorbed. As adsorption is a surface phenomenon,

this can be attributed to the relationship between

the effective specific surface area of the adsorbent

particles and their sizes. The surface area values

usually increased as the particle size decreased and,

as a consequence, the saturation capacity per unit

mass of adsorbent increased. Decreasing the size of

particles improves the adsorption properties of the

chitosan, especially when chitosan is crosslinked.

However, small particle sizes are not compatible

with large-scale applications. For example, in fixed-bed columns, small particles are inappropriate since

they induce head loss and column blocking and

cause serious hydrodynamic limitations [32]. There

are a large number of studies that highlight the

correlation between adsorption performance and

size of particle. Annadurai[59,60]used chitosan for

the removal of basic and direct dye from solutions.

The results indicated that the adsorption efficiency

depends upon the particle size, dosage and tem-

perature. In particular, the adsorption capacity

increased with a decrease in the particle size andthe dye molecules were preferably adsorbed on the

outer chitosan surface. The author suggested that

this observation can be attributed to the larger total

surface associated with smaller particles [60]. In

contrast to the findings of Annadurai, Guibal and

co-workers [8082] observed that the adsorption

occurred not only at the surface of the material

due to rapid surface adsorption but also in the

intraparticle network of the polymer. In particular,

the large external surface area for small particles

removes more dye in the initial stages of the

adsorption process than the large particles, con-

firming the previous results reported by McKay

et al.[44,45]. They studied the adsorption of AG 25

on chitosan and reported that the size of adsorbent

particles influenced both the adsorption kinetics and

equilibrium [81] because of the resistance to

intraparticle diffusion. The greater the particle size,

the greater the contribution of intraparticle diffu-

sion resistance to the control of the adsorption

kinetics for materials of low porosity. In other

works [80,82], they indicated that the time required

to reach equilibrium increased on increasing the size

of the adsorbent particles. This means that intra-

particle diffusion greatly influences the accessibility

of dye molecules to internal sites. With raw

chitosan, the differences were more marked than

with protonated material [80]. Due to resistance to

intraparticle mass transfer in raw chitosan, it isusually necessary to use very small particles to

improve adsorption kinetics. When the dyes have

strong interactions with chitosan, this allows larger

adsorbent particle sizes to be used to get the same

adsorption rate. They concluded that this was

especially interesting for large-scale applications

since it was easier to manage large adsorbent

particles rather than fine powders [82]. Juang et al.

[93] also observed that the adsorption capacity

strongly depended on the particle size of chitosan.

At a chitosan particle size of 250420 mm, the values

were 380, 179 and 87 mg/g for RR 222, RY 145 andRB 222, respectively. These results were signifi-

cantly greater than those obtained using adsorbents

such as CAC, natural clay, bagasse pith and maize

cob, in which the capacity for reactive dyes was

often less than 30 mg/g. They concluded than the

smaller the chitosan particles, the greater the

capacity for dye. Li and co-worker [94] reported

similar conclusions for the adsorption of basic dyes

on the adsorption of RR 189 on crosslinked beads.

For example, the adsorption capacity of particles

with diameters 2.32.5, 2.52.7 and 3.53.8 were1936, 1686 and 1642 mg/g, respectively, at pH 3 and

30 1C. They also concluded that the dye uptake

increased with a decrease in the particle size

since the effective surface area was higher for the

same mass of smaller particles. Chiou and Chuang

[66], using crosslinked chitosan for the removal of

dye from solutions, indicated that the increase in

adsorption capacity with decreasing particle size

suggests that the dye preferentially adsorbed on the

outer surface and did not fully penetrate the particle

due to steric hindrance of large dye molecules.

Recently, Trung et al. [108]reported that no effect

of the difference in particle size of decrystallized

chitosan on the decolorization capacity was ob-

served. The size of particles has been shown to be a

key parameter in the control of adsorption perfor-

mances of several dyes on chitosan, in particular

this may be the main parameter to control dye

adsorption equilibrium. However, the relationship

of adsorption capacity to particle size also princi-

pally depends on two criteria: (i) the chemical

structure of the dye molecule (its ionic charge) and

its chemistry (its ability to form hydrolyzed species)

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and (ii) the intrinsic characteristic of the adsorbent

(its crystallinity and porosity, the rigidity of the

polymeric chains, the degree of crosslinking), as

shown by Guibal and co-workers [8082].

Adsorption performance (in particular intrapar-

ticle diffusion) is also controlled by polymerporosity (i.e. porous volume, porous distribution

and pore size). CAC are well-known conventional

porous adsorbents and are characterized by a large

specific surface area and a great porosity that limits

the resistance to intraparticle diffusion. The aggre-

gation of dye molecules may involve a strong

increase in the size of the diffusing molecule, and

this effect may be reinforced by the influence of pore

size in controlling intraparticle diffusion properties

and accessibility to internal sites. Thus, the effi-

ciency in adsorbing dyes onto a material such as

CAC can be correlated to its surface characteristics.However, chitosan is known as a non-porous

polymer. It is characterized by a low surface area

and a low porosity that control the diffusion to the

center of the particles, especially with large mole-

cules. These features generally limit access to

interior adsorption sites. So, polymer porosity may

affect the dye adsorption capacity of chitosan. In

crosslinked chitosan beads, usually prepared by a

chemical treatment with GLU, the materials are

submicron to micron-sized, and need large internal

pores to ensure adequate surface area for adsorp-tion. Indeed these chemical treatments involve

supplementary linkages that limit the transfer of

solute molecules. In general, diffusion limitation

within particles leads to the decreases in adsorption.

These limiting effects can be compensated for by the

physical modification of the polymer. As already

mentioned, an interesting characteristic of chitosan

is its excellent ability to be processed into porous

and nanoporous structures. Gel bead conditioning

in addition to the decrease of polymer crystallinity,

improves both swelling and diffusing properties, but

also allows expansion of the porous structure of the

network, which in turn enhances the transport

of dyes. This physical modification allows both

the polymer network to be expanded (enhancing

the diffusion of large sized molecules) and the

crystallinity of the polymer to be reduced. Porous

structures can be formed by freeze-drying chitosan-

acetic acid solutions in suitable molds. Exclusion of

chitosan acetate salt from the ice crystal phase and

subsequent ice removal by lyophilization generates

a porous material with a mean pore size that can be

controlled by varying the freezing rate and hence the

ice crystal size. Pore orientation can be directed by

controlling the geometry of thermal gradients

during freezing. The mechanical properties of the

resulting material are mainly dependent on the pore

sizes and pore orientations. Another process con-

sists in dissolving the polymer in acid solutionfollowed by a coagulation. Recently, Kim and Cho

[71] proposed a solgel method to prepare porous

chitosan beads with interesting high internal specific

surface areas, allowing better accessibility of dyes to

interior adsorption sites. Nanotechnology has been

also proposed to prepare porous materials

[110,118,119]. Compared to the traditional micron-

sized materials, nano-sized adsorbents possess quite

good performance due to high specific area and

porous structure, and the absence of internal

diffusion resistance.

4.1.3. Chemical structure of chitosan

The properties of chitosan also depend on its

chemical nature (MW, DD), functional groups

(ionic charge, variety, density, accessibility) and

solution behavior (purity, water content, salt-form,

affinity for water). These parameters are also

determined by the conditions selected during the

preparation.

It is known that chitin samples have different DD

depending on their origin and mode of isolation

[12]. Deacetylation takes place during isolation byalkaline treatment to remove proteins. To prepare

chitin with a fully N-acetylated polymer or a

uniform structure, selective N-acetylation of the

free amino groups is necessary. Chitosan is prepared

by deacetylating chitin. Depending on the chitin

source and the methods of hydrolysis, commercial

chitosan also varies greatly in its MW and distribu-

tion, and therefore its solution behavior. The MW

of chitosan is a key variable in adsorption properties

because it influences the polymers solubility and

viscosity in solution. It is an important factor for

characterization, but poor solubility and structural

ambiguities in connection with the distribution of

acetyl groups are major obstacles to quantitatively

determining MWs [11]. It is also difficult to

determine the MW of native chitin.

Another important characteristic of chitosan is

the degree ofN-acetylation (DA) or DD. The DD

parameter is essential since, though the hydroxyl

groups on the polymer may be involved in attracting

dye molecules, the amine functions remain the main

active groups and so can influence the polymers

performance. Guibal et al. [82] observed that

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increasing the DD involved an increase in the

relative proportion of amine groups, which were

able to be protonated, favoring dye adsorption.

However, they indicate that the variation in

adsorption properties was not proportional to

DD, but changed with the type of dye, especiallywith chitin. Saha et al. [102], studying the adsorp-

tion of an azo dye onto chitosan flakes, also

reported that the results were found to be strongly

dependent on the DD of the polymer. The higher

DD chitosan provided a better adsorption. Re-

cently, it has been reported that the solution

properties of a chitosan depend not only on its

average DA but also on the distribution of the

acetyl groups along the main chain [11]. However,

Chiou and Li [68], studying the adsorption

of RR 189 on crosslinked chitosans reported

that both the MW and the DD of the polymerwere almost without effect on the adsorption

capacities.

An additional advantage of chitosan is the high

hydrophilic character of the polymer due to the

large number of hydroxyl groups present on its

backbone. Depending on its MW and DD, chitosan

in aqueous solution is expected to have the proper-

ties of an amphiphilic polymer. With an increase in

DD, the number of amino groups in the polymer

increases, and with an increase of MW, the polymer

configuration in solution becomes a chain or a ball.In addition, adsorption is known to change the

conformation of the chitosan polymer. The viscosity

of chitosan also greatly influences the chitosan

conditioning processes.

4.2. Activation conditions

4.2.1. Chitosan preprotonation

Because of its stable, crystalline structure, the

polyamine chitosan is insoluble in either water or

organic solvents. However, in dilute aqueous acids,

the free amino groups are protonated and the

polymer becomes fully soluble below pH 5. Since

the pKaof the amino group of glucosamine residues

is about 6.3, chitosan is extremely positively charged

in acidic medium. So, treatment of chitosan with

acid produces protonated amine groups along the

chain and this facilitates electrostatic interaction

between polymer chains and the negatively charged

anionic dyes, as previously observed by Maghami

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