Application of-chitosan-a-natural-aminopolysaccharide-for-dye-removal-from-aqueous-solutions-by-adsorption-processes-using-batch-studies-a-review-of-r

Prog. Polym. Sci. 33 (2008) 399–447
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 CriniÃ, 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
ARTICLE IN PRESS
www.elsevier.com/locate/ppolysci
0079-6700/$ - see front matter r 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.
ÃCorresponding author. Tel.: +33 3 81 66 57 01; fax: +33 3 81 66 57 97.
E-mail address: gregorio.crini@univ-fcomte.fr (G. Crini).

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405
2.4. 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420
4.2.4. 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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 [1–5]. 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
ARTICLE IN PRESS
G. Crini, P.-M. Badot / Prog. Polym. Sci. 33 (2008) 399–447400

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 and
undesirable [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 (see Table 1). In
the literature, there are a great number of feasibility
studies concerning the treatment of dyeing effluents
by these methods [2–8].
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
ARTICLE IN PRESS
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 (minÀ1
)
k1 equilibrium rate constant of pseudo-first-
order adsorption (minÀ1
)
k2 equilibrium rate constant of pseudo-
second-order adsorption (g/mg min)
ki intraparticle diffusion rate constant
(mg/g minÀ1/2
)
qe amount of dye adsorbed at equilibrium
(mg/g)
qt amount of dye adsorbed at time t (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) 399–447 401

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 or
bacterial 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 characteristics
that 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 chitosan’s 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 ‘‘adsorption
from 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 measuring
the 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
ARTICLE IN PRESS

conditions and found reproducible:
qe ¼
VðCo À CeÞ
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 adsorbent
concentration, 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
[9–11]. This low-cost material is a linear homo-
polymer composed of b(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,
ARTICLE IN PRESS
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)).

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 helpful
not 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 deacetylated
product 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-forming
ability, bioadhesivity, polyfunctionality, hydrophi-
licity and adsorption properties (Table 2). Most of
the properties of chitosan can be related to its
cationic nature [9–12], 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.

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 [11,21,23–29],
biotechnology [10,21,30], chemistry [21,31–34], 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 [13–15], floc-
culating [16,17] and chelating agents [19–22]. 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*, donor–acceptor, 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. 4 gives 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 with
high nitrogen content
Rigid D-glucosamine structure; high
crystallinity; hydrophilicity
Capacity to form hydrogen bonds
intermolecularly; high viscosity
Weak base; the deprotonated amino
group acts a powerful nucleophile
(pKa 6.3)
Insoluble in water and organic
solvents; soluble in dilute aqueous
acidic solutions
Numerous reactive groups for
chemical activation and crosslinking
Forms salts with organic and
inorganic acids
Chelating and complexing properties
Ionic conductivity
Polyelectrolytes (at
acidic pH)
Cationic biopolymer with high
charge density (one positive charge
per glucosamine residue)
Flocculating agent; interacts with
negatively charged molecules
Entrapment and adsorption
properties; filtration and separation
Film-forming ability; adhesivity
Materials for isolation of
biomolecules
Biological
properties
Biocompatibility
J Non-toxic
J Biodegradable
J Adsorbable
Bioactivity
J Antimicrobial activity (fungi,
bacteria, viruses)
J Antiacid, antiulcer, and
antitumoral properties
J Blood anticoagulants
J Hypolipidemic activity
Bioadhesivity

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
Numberofarticles
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.

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 adsorbents
with 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 interesting
selectivity, 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), 1–2 mmol metal
per gram for Pt and Pd, and up to 7–10 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 treated
solution. 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 polymers
are 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 materials
Matrix 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)

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 new
complexing 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.

be a serious drawback for long-term applications.
These problems can rebut industrial users. Readers
interested in a detailed discussion of these problems
should 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 [43–57]. For example, as long ago as
1958, Giles et al. [43] investigated the binding
behavior of dyes to chitin. In 1982–1985, extensive
studies on the adsorption of dyes on chitin by
McKay et al. [44–48] also revealed that chitin can
adsorb substantial quantities of dyestuffs from
aqueous solutions. The interaction of chitosan with
dyes was studied by several workers [49–57]. 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] and
No 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 [58–116].
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 which
will 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 hydrophilic
biopolymer
Very abundant material
and 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 range
of dyes
Fast kinetics
High selectivity in
decolorizing both very
dilute or concentrated
solutions
Versatile biosorbent
Variability in the polymer
characteristics
The performance depends
of the origin and treatment
of the polymer, and also its
degree of N-acetylation
Nonporous sorbent
Requires chemical
derivatization to improve
its performance
Not effective for cationic
dyes (except after
modification)
pH sensitivity
Its use in sorption columns
is limited (hydrodynamic
limitations and column
fouling)
Non-destructive process

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 [61–63]
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
[66–70]
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
[79–82]
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 [86–93]
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 [97–99]
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 [103–105]
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 [112–115]
Wen YZ. China RR 195 [116]

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 been
developed 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]. 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 restric-
tions, and the decrease observed in the intraparticle
diffusivity.
Table 7 outlines 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

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 form
hydrogels, 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 Szeto’s
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 of
the 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 the
evaluation 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 [89–93] 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
[112–115] 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. McKay’s group [97–99] recently pub-
lished a series of papers on the ability of chitosan to
ARTICLE IN PRESS
hardening of
droplets
chitosan aqueous
solution
oil phase
emulsification
crosslinking agent
stirring
particles
separation
Fig. 5. Schematic representation of preparation of chitosan
particles by emulsion crosslinking.

ARTICLEINPRESS
Table 8
Results of batch studies for various dyes using chitosan
Dye Chitosan Effective pre-
treatment of
chitosan
Particle size Sspa
pH T (1C) Equilibrium
time
Equilibrium
model
qm
b
Kinetic model Adsorption
mechanism
Reference
AB protonation 450–900mm 3.6 20 4 h 296 Diffusionc
[65]
AG 25 Crab shell 355–500 mm 4 25 24 h Langmuir 645.1 Lagergren [97–99]
AG 25 Protonation 3 4 days Langmuir 525 [81]
AG 27 Nanoparticle 180 nm 25 24 h Langmuir 2103.6 [110]
AO 7 Bead (crab) Crosslinking 4 30 5 days Langmuir 1940 Ho and McKay [67]
AO 10 Crab shell 355–500 mm 4 25 24 h Langmuir 922.9 Lagergren [97–99]
AO 12 Bead (crab) Crosslinking 3 30 5 days Langmuir 1954 Ho and McKay [67]
AO 12 Crab shell 355–500 mm 4 25 24 h Langmuir 973.3 Lagergren [97–99]
AO 51 Wet bead 30 3 days Langmuir 656 Elovich Chemisorption [86]
AO 51 Dried bead 30 3 days Langmuir 494 Elovich Chemisorption [86]
AR 14 Bead (crab) Crosslinking 3 30 5 days Langmuir 1940 Ho and McKay [67]
AR 18 Crab shell 355–500 mm 4 25 24 h Langmuir 693.2 Lagergren [97–99]
AR 73 Crab shell 355–500 mm 4 25 24 h Langmuir 728.2 Lagergren [97–99]
AR 87 Bead (shrimp) 6 30 Langmuir 76 Ho and McKay Chemisorption [83]
BB Protonation 450–900 mm 9.6 20 4 h 50 Diffusionc
[65]
BB 3 Powder (crab) Grafting 25 3 25 40 min Langmuir 166.5 Ho and McKay Chemisorption [73]
BB 9 Wet bead 30 3 days Langmuir 222 Elovich Chemisorption [86]
BB 9 Dried bead 30 3 days Langmuir 202 Elovich Chemisorption [86]
BB 9 Powder (crab) Grafting 25 3 25 40 min Langmuir 121.9 Chemisorption [72]
BB 9 0.177 10 9.5 60 24 h Lagergren Diffusionc
[59]
BB 9 Grafting 0.99 5.5 25 3 h Langmuir [58]
DB 6 26 5 h Langmuir Lagergren [75]
DR 81 Bead (crab) Crosslinking 3 30 5 days Langmuir 2383 Ho and McKay [67]
DS 0.206 8.47 47.5 24 h 37.18 [60]
IC Shrimp shell 60–100 mesh 6 35 2 h Langmuir [63]
MY Bead (crab) Crosslinking 4 30 5 days 1334 Ho and McKay Diffusionc
[66]
RB Bead Crosslinking 0.24 2 60–200 min Avrami [62]
RB 2 Bead (crab) Crosslinking 3 30 5 days Langmuir 2498 Ho and McKay [67]
RB 5 3 2 days Langmuir 1100 [79]
RB 5 Flake Crosslinking 2 mm 350 6 25 5 days Freundlich [71]
RB 15 Bead (crab) Crosslinking 4 30 5 days 722 Ho and McKay Diffusionc
[66]
RB 222 Swollen bead 2.8 mm 30 4 days Langmuir 1009 Ho and McKay Chemisorption [90]
RB 222 Flake 1–1.41 mm 11.8 30 4 days Langmuir 199 Ho and McKay Chemisorption [90]
RB 222 Bead (lobster) 0.715 mm 12.3 30 Diffusionc
[89]
RO 16 Crosslinking 25 mm 2 25 24 h Langmuir 30.4 [74]
RR 2 Bead (crab) Crosslinking 3 30 5 days Langmuir 2422 Ho and McKay [67]
RR 141 Shrimp shell 850 mm–1 mm 11 60 24 h Langmuir 156 [109]
RR 189 Bead Crosslinking 2.3–2.5 mm 3 30 5 days Langmuir 1936 Ho and McKay Chemisorption [94]
RR 189 Bead Crosslinking 2.3–2.5 mm 3 30 5 days Langmuir 1834 Ho and McKay Diffusionc
[68]
RR 189 Bead 2.3–2.5 mm 6 30 5 days Langmuir 1189 Ho and McKay Chemisorption [94]
G.Crini,P.-M.Badot/Prog.Polym.Sci.33(2008)399–447413

ARTICLEINPRESS
Table 8 (continued )
Dye Chitosan Effective pre-
treatment of
chitosan
Particle size Sspa
pH T (1C) Equilibrium
time
Equilibrium
model
qm
b
Kinetic model Adsorption
mechanism
Reference
RR 189 2.3–2.5 mm 6 30 5 days Langmuir 950 Ho and McKay Diffusionc
[68]
RR 222 Bead Crosslinking 3 30 2 days Langmuir 2252 Ho and McKay Chemisorption [69]
RR 222 Bead 30 5 days Freundlich 1965 Lagergren Diffusionc
[87]
RR 222 Swollen bead 2.8 mm 30 4 days Langmuir 1653 Ho and McKay Chemisorption [90]
RR 222 Wet bead 30 3 days Freundlich 1498 Elovich Chemisorption [86]
RR 222 Dried bead 30 3 days Freundlich 1215 Elovich Chemisorption [86]
RR 222 Bead (crab) 3.11 mm 30–40 30 5 days Langmuir 1106 Diffusionc
[91]
RR 222 Bead (shrimp) 2.39 mm 30–40 30 5 days Langmuir 1026 Diffusionc
[91]
RR 222 Bead (lobster) 2.93 mm 30–40 30 5 days Langmuir 1037 Diffusionc
[91]
RR 222 Flake (shrimp) 16–30 mesh 4–6 30 5 days Langmuir 494 Diffusionc
[91]
RR 222 Flake (lobster) 16–30 mesh 4–6 30 5 days Langmuir 398 Diffusionc
[91]
RR 222 Flake 1–1.41 mm 11.8 30 4 days Langmuir 339 Ho and McKay Chemisorption [90]
RR 222 Flake (crab) 16–30 mesh 4–6 30 5 days Langmuir 293 Diffusionc
[91]
RR 222 Bead Crosslinking 4.01 30 3 days Freundlich [88]
RR 222 Bead (lobster) 0.715 mm 12.3 30 Diffusionc
[89]
RY Bead Crosslinking 0.24 2 60–200 min Avrami [62]
RY 2 Bead (crab) Crosslinking 4 30 5 days Langmuir 2436 Ho and McKay [67]
RY 86 Bead (crab) Crosslinking 3 30 5 days Langmuir 1911 Ho and McKay [67]
RY 145 Swollen bead 2.8 mm 30 4 days Langmuir 885 Ho and McKay Chemisorption [90]
RY 145 Flake 1–1.41 mm 11.8 30 4 days Langmuir 188 Ho and McKay Chemisorption [90]
RY 145 Bead (lobster) 0.715 mm 12.3 30 Diffusionc
[89]
a
Speciﬁc surface area in m2
/g.
b
Adsorption capacities in mg/g.
c
Intraparticle diffusion model.
G.Crini,P.-M.Badot/Prog.Polym.Sci.33(2008)399–447414

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 between
chitosan and anionic dyes has also been intensively
investigated by Guibal and co-workers [79–82].
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 900 mmol/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 chitosan
depends 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 [10–12,18]. 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 studies
have 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 40–50% aqueous alkali at 110–115 1C for a
few hours [12]. Chitin occurs in a wide variety of
species, 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 [89–93]. 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
ARTICLE IN PRESS

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 easily
accessible 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 demonstrated
that 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 the
polymer 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 great
impact 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 adsorption
than the flake type by a factor of 2–4 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 1103 mg/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 of qmax for
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
chitosan’s 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
ARTICLE IN PRESS

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] showed
that 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 and
the 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 [80–82] 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 is
usually 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 250–420 mm, the values
were 380, 179 and 87 mg/g for RR 222, RY 145 and
RB 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 2.3–2.5, 2.5–2.7 and 3.5–3.8 were
1936, 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)
ARTICLE IN PRESS

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 [80–82].
Adsorption performance (in particular intrapar-
ticle diffusion) is also controlled by polymer
porosity (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 solution
followed by a coagulation. Recently, Kim and Cho
[71] proposed a sol–gel 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 by
alkaline 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 polymer’s 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 of N-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 polymer’s
performance. Guibal et al. [82] observed that
ARTICLE IN PRESS

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, especially
with 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 polymer
were 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 pKa of 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
and Roberts [50]. The pH-dependent solubility of
chitosan provides a convenient tool to improve its
performance although solubility is a very difficult
parameter to control [11]. In fact, the solubility and
its extent depends on the concentration and on the
type of acid. The polymer dissolves in hydrochloric
acid and organic acids such as formic, acetic, lactic
and oxalic acids. However, solubility decreases with
increasing concentrations of acid. Solubility is also
related to the DA, the ionic concentration, as well as
the conditions of isolation and drying of the
polymer [11]. In particular, the distribution of acetyl
groups along the chain (random or blockwise) can
strongly influence the solubility of the polysacchar-
ide and also the interchain interactions due to
H-bonds and the hydrophobic character of the
acetyl groups.
Trung et al. [108] proposed a pretreatment using
citric acid to produce decrystallized chitosan with a
low degree of crystallinity and a high anionic dye-
binding capacity. The percentage crystallinity of
decrystallized chitosan was 10%, significantly lower
than that of raw chitosan (32%). This reduction is
attributed to a probable rearrangement of polymer
chains during precipitation in the presence of citrate
ions. They also indicated that the decrystallized
chitosan had the same degree of DD and MW as the
original chitosan. Decrystallized chitosan adsorbed
anionic dyes almost twofold more efficiently than
raw chitosan, due to its more amorphous character,
but showed decreased adsorption for cationic dyes.
However, the presence of ash in decrystallized
chitosan could also play a role in increased dye-
binding capacity. Gibbs et al. [81] showed that the
preliminary protonation of amine groups, obtained
by contact with a sulfuric acid solution, reduced
the variation of solution pH following adsorbent
addition. Crini et al. [72] found that a homogeneous
chemical treatment such as a solubilization–
reprecipitation process could give a chitosan pro-
duct with a higher adsorption level for dyes than
one prepared by a heterogeneous process with the
same DD. They attributed this to an increase of the
surface area due to the conversion of the chitosan
flakes into a powder.
4.2.2. Grafting reactions
Several workers have suggested that although
chitosan as such is very useful for treating con-
taminated solutions, it may be advantageous to
chemically modify chitosans, e.g. by grafting reac-
tions [72,73,77,95,103,106,113]. The modifications
can improve chitosan’s removal performance and
selectivity for dyes, alter the physical and mechan-
ical properties of the polymer, control its diffusion
ARTICLE IN PRESS

properties and decrease the sensitivity of adsorption
to environmental conditions. Chemical grafting
of chitosan with specific ligands has been reviewed
by Jayakumar et al. [23] and by Prabaharan and
Mano [41].
It is known that the only class for which chitosan
[106] and crosslinked chitosan [85] have low affinity
is basic (cationic) dyes. To overcome this problem,
Crini et al. [72,73] suggested the use of N-benzyl
mono- and disulfonate derivatives of chitosan in
order to enhance its cationic dye hydrophobic
adsorbent properties and to improve its selectivity.
The maximum adsorption capacities of these
adsorbents for BB 9 and BB 3 were 121.9 and
166.5 mg/g, respectively. These derivatives could be
used as hydrophobic adsorbents in acidic media
without any crosslinking reaction. To fully develop
the high potentials of chitosan, it is necessary to
introduce chemical substituents at a specific position
in a controlled manner as suggested by Lima et al.
[58] and Chao et al. [106]. Lima et al. [58] proposed
the use of chitosan chemically modified with
succinic anhydride in the BB 9 adsorption. This
chemical derivatization provides a powerful means
to promote new adsorption properties in particular
towards basic dyes in acidic medium. Chao et al.
[106] suggested enzymatic grafting of carboxyl
groups onto chitosan as a means to confer the
ability to adsorb basic dyes on beads. The presence
of new functional groups on the surface of beads
results in increases in surface polarity and the
density of adsorption sites, and this improves the
adsorption selectivity for the target dye. Other
studies showed that the ability of chitosan to
selectively adsorb dyes could be further improved
by chemical derivatization. Shimizu et al. [103]
proposed novel chitosan-based materials by react-
ing chitosan with a higher fatty acid functionalized
with a glycidyl moiety in order to introduce long
aliphatic chains. They observed that these products
could be used as effective adsorption materials for
both anionic and cationic dyes. Martel et al. [95],
and El-Tahlawy and co-workers [76,77] proposed
the use of cyclodextrin-grafted chitosan as new
chitosan derivatives for the removal of dyes. Martel
et al. [95] demonstrated that these materials are
characterized by a rate of adsorption and a global
efficiency greater than that of the parent chitosan
polymer. Uzun and Gu¨ zel [113,114] reported that
carboxymethylated chitosan is a rather better
adsorbent than raw chitosan for acidic dyestuffs,
and its production is not costly.
4.2.3. Influence of crosslinking
Raw chitosan powders also tend to present some
disadvantages such as unsatisfactory mechanical
properties and poor heat resistance. Another
important limitation of the raw material is that it
is soluble in acidic media and therefore cannot be
used as an insoluble adsorbent under these condi-
tions, except after physical and chemical modifica-
tion. One method to overcome these problems is to
transform the raw polymer into a form whose
physical characteristics are more attractive. So,
crosslinked beads have been developed and pro-
posed. After crosslinking, these materials maintain
their properties and original characteristics [62],
particularly their high adsorption capacity,
although this chemical modification results in a
decrease in the density of free amine groups at the
surface of the adsorbent in turn lowering polymer
reactivity towards metal ions [80].
An important work on crosslinked chitosan was
done by Chiou and co-workers [66–70]. Chitosan
beads were crosslinked with GLU, EPI or EGDE.
The results showed that the chitosan-EPI beads
presented a higher adsorption capacity than GLU
and EGDE resins [68,69]. They reported that these
materials can be used for the removal of reactive,
direct and acid dyes. It was found that 1 g chitosan
adsorbed 2498, 2422, 2383 and 1954 mg of RB 2,
RR 2, DR 81 and AO 12, respectively [67]. It is
important to specify that the adsorption capacities
of CAC for reactive dyes generally vary from 278 to
714 mg/g [6]. Another advantage of EPI is that it
does not eliminate the cationic amine function of
the polymer, which is the major adsorption site to
attract the anionic dyes during adsorption [69]. The
crosslinking of chitosan with GLU (formation of
imine functions) or with EDGE decreases the
availability of amine functions for the complexation
of dyes and with a high crosslinking ratio the uptake
capacity drastically decreases. They also indicated
that the crosslinking ratio slightly affected the
equilibrium adsorption capacity for the three cross
linkers under the range they studied [68]. The
amount of dye adsorbed was found to be higher in
acidic than in basic solution. This was explained by
considering the rate of diffusion from the swollen
beads in acidic and basic media. In basic medium, a
limited swelling of the beads inhibited the diffusion
of dyes at a faster rate as it occurred in acidic
medium. Among the conditions of the crosslinking
reaction that have a great impact on dye adsorption
are the chemical nature of the crosslinker, as
ARTICLE IN PRESS

mentioned above, but also the extent of the
reaction. In general, the adsorption capacity de-
pends on the extent of crosslinking and decreases
with an increase in crosslinking density. When
chitosan beads were crosslinked with GLU under
heterogeneous conditions, it was found that the
saturation adsorption capacity of RR 2 on cross-
linked chitosan decreased exponentially from 200 to
50 mg/g as the extent of crosslinking increased from
0 to 1.6 mol GLU/mol of amine. This is because of
the restricted diffusion of molecules through the
polymer network and reduced polymer chain
flexibility. Also the loss of amino-binding sites by
reaction with aldehyde is another major factor in
this decrease. However, Chiou and co-workers
indicated that the crosslinking step was necessary
to improve mechanical resistance, to enhance the
resistance of material against acid, alkali and
chemicals, and also to increase the adsorption
abilities of chitosan. The removal performance of
crosslinked chitosan and CAC for anionic dyes were
compared: the adsorption values were 3–15 times
higher at the same pH. Chiou and co-workers
[66–70] concluded that chitosan chelation was the
procedure of choice for dye removal from aqueous
solution. However, Kim and Cho [71], studying the
adsorption of RB 5 on crosslinked chitosan beads,
arrived at contrasting conclusions. They demon-
strated that the adsorption capacity of non-cross-
linked beads was greater than that of crosslinked
beads in the same experimental conditions.
The materials, mainly crosslinked using GLU,
have been also proposed as effective dye removers
by several other workers [62,77,84,85,88,94,105]. All
these studies showed that the reaction of chitosan
with GLU leads to the formation of imine groups,
in turn leading to a decrease in the number of amine
groups, resulting in a lowered adsorption capacity,
especially for dyes sorbed through ion-exchange
mechanisms. However, this limiting effect of a
chemical reaction with GLU significantly depends
on both the procedure used and the extent of
crosslinking, as reported by Hebeish et al. [84,85]. In
heterogeneous conditions, chitosan (solid state) was
simply mixed with GLU solution, while in homo-
geneous conditions chitosan was mixed with GLU
solution after being dissolved in acetic acid solution.
An optimum aldehyde/amine ratio was found for
dye adsorption, which depended on the crosslinking
operation mode (water-soluble or solid-state solu-
tion). The initial increase in dye adsorption was
attributed to the low levels of crosslinking in the
precipitates preventing the formation of closely
packed chain arrangements without any great
reduction in the swelling capacity. This increase in
adsorption was interpreted in terms of the increases
in hydrophilicity and accessibility of complexing
groups as a result of partial destruction of the
crystalline structure of the polymer by crosslinking
under homogeneous conditions. At higher levels of
crosslinking, the precipitates had lower swelling
capacities, and hence lower accessibility because of
the more extensive three-dimensional network and
also because of its more hydrophobic character with
increased GLU content. Juang et al. [88], studying
the adsorption of RR 222 on crosslinked chitosan
beads, also observed that the adsorption capacity
depends on the extent of crosslinking and decreases
as crosslinking density increases. This result was
mainly interpreted by the fact that the crosslinking
reaction with GLU decreases the availability of
amine functions for the complexation of dyes. The
results showed that the chitosan-GLU beads pre-
sented a higher adsorption capacity than glyoxal
beads. Gaffar et al. [77] and Shimizu et al. [105]
reported that the extent of crosslinking showed a
significant influence on adsorption properties. These
authors noted that the increase in the extent of
crosslinking is accompanied by a decrease in dye
uptake, confirming the results of Hebeish et al.
[84,85]. The adsorption capacity increased greatly at
low degrees of substitution but decreased with
increasing substitution. This phenomenon is inter-
preted in terms of increased hydrophilicity caused
by the destruction of the crystalline structure at low
crosslinking densities, while this can be associated
with an accompanying decrease in active sites,
accessibility, and swellability of the adsorbent by
increasing the level of crosslinking. On the contrary,
Chiou and Li [94], studying the adsorption of RR
189 on EPI-crosslinked chitosan beads, reported
that the crosslinking ratio did not affect the
adsorption capacity.
Another study showed that the physical and
mechanical properties of chitosan could be further
improved by crosslinking. Chitosan forms gels
below pH 5.5 and acid effluents could severely limit
its use as an adsorbent in removing dyes from acid
effluent. To solve this problem, Cestari et al. [62]
proposed the use of homogeneously crosslinked
beads. They reported that the beads were not only
insoluble in acid solution but also presented higher
specific surface areas (0.1 and 0.24 m2
/g before and
after the crosslinking reaction, respectively) and
ARTICLE IN PRESS

stronger mechanical resistance than the raw chit-
osan powder. The chemical, physical and mechan-
ical behavior of the beads and also adsorption
properties were enhanced by crosslinking with
functional groups. The materials had a strong
adsorption capacity for RY, RB and RR below
pH 5.5. The authors also noted that crosslinking can
change the crystalline nature of chitosan, as
suggested by the XRD diffractograms. After the
crosslinking reaction, there was a small increase in
the crytallinity of the chitosan beads and also
increased accessability to the small pores of the
material.
4.2.4. Chitosan-based composite beads
Practical industrial applications of raw chitosan
in fixed-bed systems or packed in adsorption
columns are also limited. The characteristics of the
polymers can introduce hydrodynamic limitations
and column fouling, which limits their use for large-
scale columns. For example, the flaked or powdered
form swells (the crosslinked beads have lower
swelling percentage [120]) and crumbles easily, and
does not function ideally in packed-column config-
urations common to pump-and-treat adsorption
processes. Various chitosan-based composites have
been designed to overcome these problems. Chang
and Juang [87] proposed the addition of activated
clay to chitosan to prepare composite beads in order
to improve its mechanical properties. Cestari et al.
[61] also proposed the use of silica/chitosan hybrid
for the removal of anionic dyes from aqueous
solutions: these materials are of interest because
they combine the structure, strength and chemical
properties of the silica with the specific character-
istics of chitosan. Chang and Chen [64] proposed
the use of chitosan-conjugated Fe3O4 nanoparticles
for the removal acid dyes from aqueous solutions.
The adsorption capacities were 1883 and 1471 mg of
dye/g of chitosan for AO 12 and AG 25. Paneva
et al. [96] also proposed a novel effective route for
incorporating magnetic material into chitosan beads
by capillary extrusion. They concluded that the
material might be used for wastewater treatment in
the textile industry.
4.3. Influence of process variables
The amount of dye that can be removed from a
solution by chitosan also depends on process
variables used in batch systems such as chitosan
dosage, initial dye concentration, contact time,
agitation rate and dryness.
4.3.1. Effect of chitosan dosage
Of all the above factors, chitosan dosage is
particularly important because it determines the
extent of decolorization and may also be used to
predict the cost of chitosan per unit of solution to be
treated. As expected, the adsorption density in-
creases significantly as adsorbent dosage decreases.
This is due to the higher amount of the dye per unit
weight of adsorbent. Wen et al. [116] showed that
the increasing chitosan dose had a dramatic positive
impact on color removal and there was an
approximately linear relationship between chitosan
dose and color removal of the dye. Crini et al.
[72,73] also observed that the increase in adsorption
with adsorbent dosage can be attributed to in-
creased adsorbent surface and availability of more
adsorption sites. However, if the adsorption capa-
city was expressed in mg adsorbed per gram of
material, the capacity decreased with the increasing
amount of sorbent. This may be attributed to
overlapping or aggregation of adsorption sites
resulting in a decrease in total adsorbent surface
area available to the dye and an increase in diffusion
path length. It was also indicated that the time
required to reach equilibrium decreased at higher
doses of adsorbent.
4.3.2. Effect of initial dye concentration
Park et al. [56] and Knorr [121] previously found
significant correlations between dye concentration
and the dye-binding capacity of chitin or chitosan.
The amount of the dye adsorbed onto chitosan
increased with an increase in the initial concentra-
tion of dye solution if the amount of adsorbent was
kept unchanged. This is due to the increase in the
driving force of the concentration gradient with the
higher initial dye concentration. In most cases, at
low initial concentration the adsorption of dyes by
chitosan is very intense and reaches equilibrium
very quickly. This indicates the possibility of the
formation of monolayer coverage of the molecules
at the outer interface of the chitosan. At a fixed
adsorbent dose, the amount adsorbed increased
with increasing concentration of solution, but the
percentage of adsorption decreased. In other words,
the residual concentration of dye molecules will be
higher for higher initial dye concentrations. In the
case of lower concentrations, the ratio of initial
number of dye moles to the available adsorption
ARTICLE IN PRESS

Application of-chitosan-a-natural-aminopolysaccharide-for-dye-removal-from-aqueous-solutions-by-adsorption-processes-using-batch-studies-a-review-of-r

Application of-chitosan-a-natural-aminopolysaccharide-for-dye-removal-from-aqueous-solutions-by-adsorption-processes-using-batch-studies-a-review-of-r

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (11)

Similar to Application of-chitosan-a-natural-aminopolysaccharide-for-dye-removal-from-aqueous-solutions-by-adsorption-processes-using-batch-studies-a-review-of-r

Similar to Application of-chitosan-a-natural-aminopolysaccharide-for-dye-removal-from-aqueous-solutions-by-adsorption-processes-using-batch-studies-a-review-of-r (20)

Recently uploaded

Recently uploaded (20)

Application of-chitosan-a-natural-aminopolysaccharide-for-dye-removal-from-aqueous-solutions-by-adsorption-processes-using-batch-studies-a-review-of-r