Renault 2009
Renault 2009
Renault 2009
Review article
a r t i c l e
i n f o
Article history:
Received 30 October 2008
Received in revised form 11 December 2008
Accepted 17 December 2008
Available online 25 December 2008
Keywords:
Biopolymer
Chitosan
Coagulation
Flocculation
Wastewater treatment
Bioocculant
a b s t r a c t
Chitosan is a partially deacetylated polymer obtained from the alkaline deacetylation of
chitin, a biopolymer extracted from shellsh sources. Chitosan exhibits a variety of physico-chemical and biological properties resulting in numerous applications in elds such
as cosmetics, biomedical engineering, pharmaceuticals, ophthalmology, biotechnology,
agriculture, textiles, oenology, food processing and nutrition. This amino-biopolymer has
also received a great deal of attention in the last decades in water treatment processes
for the removal of particulate and dissolved contaminants. In particular, the development
of chitosan-based materials as useful coagulants and occulants is an expanding eld in
the area of water and wastewater treatment. Their coagulation and occulation properties
can be used to remove particulate inorganic or organic suspensions, and also dissolved
organic substances. This paper gives an overview of the main results obtained in the treatment of various suspensions and solutions. The effects of the characteristics of the chitosan
used and the conditions in solution on the coagulation/occulation performance are also
discussed.
2008 Elsevier Ltd. All rights reserved.
Contents
1.
2.
3.
4.
5.
6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Categories of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Why use coagulants and flocculants based on biopolymers? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1. Coagulation/flocculation process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. Chitosan as bioflocculant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mechanisms of coagulation/flocculation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chitosan for coagulation and flocculation a review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
Water pollution results from all Human activities:
domestic, industrial and agricultural, and is not only due
to petroleum, minerals, sewage treatment sludge or persistent organic pollutants produced by the incineration of
* Corresponding 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).
0014-3057/$ - see front matter 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.eurpolymj.2008.12.027
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waste, but also to synthetic substances produced by chemistry (dyes, fertilisers, pesticides, and so on) [1]. When
water is polluted, decontamination becomes necessary.
The best purication approach are sought to reach the
decontamination objectives required by law. The literature
reports a multitude of processes for the decontamination
of contaminated water and wastewater such as coagulation, precipitation, extraction, evaporation, adsorption on
activated carbon, ion-exchange, oxidation and advanced
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2. Categories of materials
The use of organic polyelectrolytes in water treatment
was recently reviewed by Bolto and Gregory [16], with
emphasis on the types of polymers commonly available
and the nature of the impurities to be removed. Polyelectrolyte applications in industrial wastewater treatment
have been reviewed by Bratby [17] and Trkman [22].
Examples include efuents from the dye, textile and milk
industries.
There are two major classes of materials used in coagulation/occulation processes [11,17]:
(1) inorganic and organic coagulants including mineral
additives (lime, calcium salts, etc.), hydrolysing
metal salts (aluminium sulphate, ferric chloride, ferric sulphate, etc.), pre-hydrolysed metals (polyaluminium chloride, polyaluminosilicate sulphate,
etc.) and polyelectrolytes (coagulant aids);
(2) and organic occulants including cationic and anionic polyelectrolytes, non-ionic polymers, amphoteric and hydrophobically modied polymers, and
naturally occurring occulants (starch derivatives,
guar gums, tannins, alginates, etc.).
Coagulation is mainly induced by inorganic metal salts,
e.g., aluminium and ferric sulphates and chlorides. The
most common additives are aluminium sulphate (generally known as alum), ferric chloride and ferric sulphate
[11,17]. The addition of these cations results in colloidal
destabilization, as they specically interact with and neutralise the negatively charged colloids. For example, once
the Fe(III) coagulant has been added to the solution to be
treated, the Fe(III) ions hydrolyse rapidly in an uncontrollable manner, forming a range of hydrolysis species which
play an essential role in the coagulation process. Metal
speciation in solution has been well documented [11,17].
In wastewater treatment using inorganic coagulants, an
optimum pH range in which metal hydroxide precipitates
occur, should be determined. The addition of metals depresses the wastewater pH to a lower value. In general,
decreasing the pH from the alkaline levels to near neutral
levels has a strong positive effect on the reduction of turbidity, suspended solids (SS) and chemical oxygen demand
(COD). However, the signicant disadvantage of these conventional coagulants is the inability to control the nature
of the hydrolysis species formed when the coagulant is
introduced in the solution [17]. As a result, their performance is dependent not only on the pH of the water and
their concentration, but also on the temperature and nature of the solution. Therefore, new types of reagents have
been developed [11,23]. Alternative coagulants based on
pre-hydrolysed forms of aluminium (such as polyaluminium chloride or PAC) and iron (polyferric sulfate or PFS) are
more effective than the traditional additives. Their signicant advantage is that their hydrolysis occurs under specic experimental conditions during the preparation
stage of the coagulants, and not after their addition to
the raw solution. This results in a much tighter control of
the procedure. It is known that PAC-based products provide better coagulation than alum at low temperatures
1339
Table 1
Examples of polymeric occulants used in water and wastewater
treatment.
Cationic polyelectrolytes
Poly(diallyldimethyl ammonium chloride)
Epichlorohydrin/dimethylamine polymers
Cationic polyacrylamides
Poly(alkylamines) [poly(ethyleneimine), poly(vinylamine)]
Poly(styrene) derivatives
Ionenes
Sulphonium polymers
Natural cationic polymers (chitosan, cationic starches)
Anionic polyelectrolytes
Anionic polyacrylamides
Carboxylic acid polymers
Phosphonic acid polymers
Sulphonic acid polymers
Natural anionic polymers (sulphated polysaccharides, modied
lignin sulphonates)
Non-ionic polymers
Polyacrylamide
Natural non-ionic polymers (starch, cellulose derivatives)
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Table 2
Principal properties of chitosan in relation to its use in water and waste
treatment application.
Principal characteristics
Potential applications
Non-toxic
Biodegradable
Renewable resource
Ecologically acceptable polymer
(eliminating synthetic polymers,
environmentally friendly)
Efcient against bacteria, viruses,
fungi
Formation of salts with organic
and inorganic acids
Ability to form hydrogen bonds
intermolecularly
Ability to encapsulate
Removal of pollutants with
outstanding pollutant-binding
capacities
membranes, bers, etc.). Chitosan has been used in the solid state for the chelation of metal ions in near-neutral
solution, the complexation of metal anions in acidic solution and the dye complexation using adsorption processes
[5357]. This biopolymer has been used in gel-bead form
for adsorption in batch or xed-bed column systems,
deposited on a suitable support (e.g., ceramic), or in a
water soluble form in polymer-enhanced ultraltration
processes and solvent extraction processes [5762].
Chitosan also possesses several intrinsic characteristics
that make it an effective coagulant and/or occulant for the
removal of contaminants in the dissolved state [57,59]. It
has characteristics of both coagulants and occulants, i.e.,
high cationic charge density, long polymer chains, bridging
of aggregates and precipitation (in neutral or alkaline pH
conditions). Its uses are justied by two important advantages: rstly, its non-toxicity and biodegradability [17];
secondly its outstanding chelation behaviour [53,55,56].
Its unique physico-chemical properties render it very efcient in interactions with various contaminants including
both particulate and dissolved substances. These properties have been exploited for the design of coagulation/occulation processes applied to the treatment of various
efuents (see Table 3). For example, chitosan has been successfully used, for precipitative occulation at pH above
the pKa of the macromolecule, in the treatment of mineral
and organic suspensions [7375] and the coagulation of
negatively charged contaminants in acidic solutions containing dyes [85] or humic acid [82,83]. Other examples
can be found in the review of No and Meyers [57].
The main reasons for the success of biopolymers such as
chitosan in wastewater treatment using coagulation/occulation processes are: chitosan has the advantage of being
a non-toxic material, non corrosive and safe to handle well
(non hazardous product, not irritating for skin and eyes. . .)
[16,17]; Hirohara et al. patented a chitosan-based material
safe to animals and plants without causing environmental
pollution [39]; chitosan is efcient in cold water and at
much lower concentrations than metal salts; it does not
leave residual metals that can cause secondary contamination problems; the low concentrations of polymers reduce
Table 3
Examples of efuents treated by coagulation/occulation using chitosan.
Efuent
Reference(s)
[6368]
[69]
[70]
[71,72]
[7378]
[7981]
[82,83]
[84,85]
[8690]
[9193]
[94]
[95]
[9698]
[99]
[100]
[101]
[102]
1341
the volume of sludge produced compared to the sludge obtained with alum for example; chitosan considerably increases the density of the sludge and facilitates its drying
compared to the sludge produced with metal salts; in addition, as biopolymers are biodegradable the sludge can be
efciently degraded by micro-organisms; two studies reported that the sludge produced from the treatment of
milk processing plant wastewater [69] and kaolinite suspensions [77] was non-toxic and could be used to stimulate growth in plants; chitosan does not add much to the
salinity of the treated water and is useable at alkaline pH.
In the coagulation/occulation process, the settling
speed of the ocs formed is also important since it inuences the overall cost and efciency [17]. It is known that
the addition of occulant has a signicant effect on the settling time when alum and/or PAC are used as coagulants. In
the case of chitosan, the increase of oc size favours the
oc settling speed and therefore reduces the settling time.
There are, of course, disadvantages which must be balanced against the benets: chitosan is only efcient over a
limited pH range and when present in excess, has a negative effect on performance (overdosing can restabilize a
dispersion and affect other process aspects); the coagulation/occulation properties depend on the different
sources of chitin/chitosan (the quality of commercial chitin
available is not uniform); another important criterion to be
taken into account concerns the variability and heterogeneity of the biopolymer chitosan: changes in the specications of the macromolecule may change coagulation
properties; each chitosan must be characterized in terms
of fraction of deacetylation, polymer weight and crystallinity because these parameters signicantly inuence its
physico-chemical properties (solubility, viscosity); there
is a need for a better standardization of the production process to be able to prepare biopolymers having the same
characteristics. These problems can rebut industrial users
[55].
The main parameter which must be taken into account
for the design of the experimental mode is solubility. However, the solubility is a very difcult parameter to control
[45]. Chitosan is a linear hydrophilic copolymer with a rigid structure containing both glucosamine and acetylglucosamine units. It is insoluble in either water or organic
solvents. However, in dilute organic acids such as acetic
acid and formic acid and inorganic acids (with the remarkable exception of sulphuric acid), the free amino groups are
protonated and the biopolymer becomes fully soluble. The
pKa of the amino group of glucosamine residues is about
6.3 and, at acidic pH (below pH 5), chitosan becomes a
soluble cationic polymer with high charge density
[44,45]. So, treatment of chitosan with acids produces protonated amine groups along the chain and this facilitates
electrostatic interactions between polymer chains and
the negatively charged contaminants (metal anions, dyes,
organic compounds, etc.). However, its solubility depends
on several parameters such as the DD and MW of the polymer, the distribution of acetyl groups along the macromolecular chain, the type and concentration of the acid used
for dissolving the polymer, the polymer concentration,
and the ionic strength. It is important to note that the DD
affects the apparent pKa and thus charge, viscosity and
1342
(related to the high molecular weight of biomacromolecules) and electrostatic patch. The mechanism is the following: coagulation by charge neutralization destabilizes
colloidal impurities and transfers small particles into large
aggregates (bridge formation) and adsorbs dissolved organic substances onto the aggregates by an adsorption
mechanism which can then be removed easily by ltration
and sedimentation. For example, cationic chitosan derivatives can be easily adsorbed onto the colloid surface of anionic inorganic (bentonite) suspensions due to electrostatic
attraction. Adsorbed macromolecules tend to form loops
and extend some distance from the particle surface into
the aqueous phase. Their ends also dangle and get adsorbed
by another particle forming a bridge between particles. For
effective bridging occur, the length of the biopolymer
chains should be sufcient to extend from one particle surface to another. Hence a polymer with longer chains should
be more effective than one with shorter chains. Here, occulation is interpreted as being a result of charge neutralization, patch occulation, and/or polymer bridging. The
chitosan characteristics that are important for occulation
are therefore charge density (related to its DD), molecular
weight (MW) and molecular structure. Literature data
show that the type of mechanism also depends on different
factors such as pH, ionic strength of the solution, and coagulant concentration. For example, the main mechanism for
dye coagulation with chitosan appears to be charge neutralization at acidic pH [85] and increasing chitosan dosage
increases dye removal up to a concentration resulting in
complete neutralization of anionic charges. Above that concentration, the excess of cationic charges leads to suspension re-stabilization. Numerous works claim that chitosan
is involved in a dual mechanism including coagulation by
charge neutralization and occulation by bridging mechanisms [57,85]. It should be also noted that one of the greatest differences between metal salts and cationic polymers
concerns their hydrolysis. Aluminium sulphate hydrolyses
immediately on contact with water giving rapid adsorption
reactions while chitosan is not hydrolysed [17]. A mechanism of action of polymeric occulating agents was described in detail by OMelia [106].
5. Chitosan for coagulation and occulation a review
Different reviews of chitosan-based materials have appeared concerning separation and complexation, including
membrane ltration [50] and adsorption [5356,59]. Obviously, chitosan has also been investigated as a coagulant
for the capture of contaminants from aqueous solutions
in numerous articles [57,107]. However, the studies mainly
focused on the recovery of suspended solids (SS) and colloids; in the case of dissolved contaminants there are many
fewer studies [84].
In 19751978, extensive studies by Bough and coworkers [63,64,108115] demonstrated the effectiveness
of chitosan for coagulation and recovery of SS in processing
wastes from a variety of food processing industries including poultry, eggs, cheese, meat, fruit cakes, seafood and
vegetables. These studies indicated that chitosan can reduce the SS of such processing waste by as much as 65%
to 99% and good results were also obtained for the reduction of turbidity (TB) and chemical oxygen demand
(COD). In some instances, chitosan can be used as a coagulant aid in conjunction with a synthetic polyelectrolyte or
an inorganic salt to increase treatment performance. A
mechanism of action of chitosan as occulating agent
was described by which the polymer destabilizes the colloidal suspension by adsorption of particles with subsequent formation of particlepolymerparticle bridges.
The effectiveness of chitosan as coagulant has also been
reported by Johnson and Gallanger [116], Senstad and
Almas [117], Moore et al. [118], No et al. [57,65,119], and
Sievers et al. [120]. These authors clearly demonstrated
that chitosan has an intrinsic capacity to be used as a coagulant to reduce SS, TB and COD. These works also reported
that positively charged cationic macromolecules can destabilize the negative colloidal suspension by charge neutralization as well as by bridge formation. In addition, another
important advantage must be cited: after being used the
sludge may be disposed of with a lower environmental
impact than common metal- and synthetic polymer-based
systems [69,70,77,122]. Chi and Cheng [69] reported that
the sludge from one coagulation process could be used directly as a feed supplement. Divakaran and Pillai [77] suggested that the sludge produced during the occulation of
kaolinite suspensions could be safely disposed off in landlls. However, such re-use needs to be carefully assessed
and its harmlessness must be checked.
There is recent literature concerning the evaluation of
the coagulation/occulation performance of chitosan. This
biopolymer has an extremely high afnity for many classes
of contaminants: it has demonstrated outstanding removal
properties for natural organic matter [123], humic substances [82,124,125], inorganic suspensions [74,75], dye
molecules [84,85], metal cations [97,98], proteins [70],
phenolic and aromatic derivatives [99], oil and grease
[9194], bacterial [81,126] and algal [127] suspensions.
For humic materials, Guibal et al. [84] showed that chitosan can be used as a primary coagulant or as a occulant
after coagulation: it has characteristics of both coagulants
and occulants. Ahmad et al. [91] demonstrated it was a
very effective coagulant to remove the residual oil content
from palm oil mill efuent compared to alum and PAC.
Divakaran and Pillai [127] showed that chitosan reduced
algal contents effectively by occulation and settling. However, they noted that the occulation was very sensitive to
pH. Chitosan can be used not only as a coagulant and/or
occulant but also as a bactericide. For example, Chung
[95] showed that this biopolymer was useful not only for
the removal of SS, organic and inorganic compounds, TB,
and COD, but also for the removal of pathogens. Huang
and co-workers [121,122] showed that chitosan could be
a promising substitute for alum and PAC in the coagulation
of colloidal particles because of its suitability for coagulation without posing any health threats as residual aluminium and other synthetic polymers do. To reach the same
level of turbidity removal, the required amount of chitosan
is only half that of PAC. Chitosan coagulants also produced
larger ocs of better quality and faster settling velocity
[122]. They also indicated that replacing PAC with chitosan
in the water treatment process can be cost effective.
1343
1344
Fig. 1. Photograph of samples analysed (a) efuent after PAC treatment; (b) raw efuent; (c) efuent after chitosan-based material treatment (experiments
were performed using a Jar-Test equipment; DCO and turbidity values were obtained after a 2-min settling time).
and increasing the speed during rapid mixing can also reduce the optimum dosage. The authors supposed that
destabilization of particles was enhanced by the increase
in charged groups (in acidic solutions, there is an increase
in the number of protonated amine groups on chitosan)
followed by charge neutralization, resulting in a decrease
in optimum dosage. The authors also found a linear relationship between the DD and the optimal chitosan dosage,
which indicates that amino groups of macromolecules are
the active site for coagulation [121]. There is a relationship
between the DD and the treatment time. The speed of mixing may affect the coagulation only before the optimal dosage is reached. Huang and co-workers [121,122] concluded
that the optimal pre-treatment condition to prepare chitosan coagulant and its dosage were the key parameters to
evaluate the coagulation performance.
Guibal et al. [84,129] showed the characteristics of
chitosan (mainly DD and MW) slightly affected the coagulationocculation performance. The DD had more effect
on bentonite suspensions and dye solutions than on kaolin
and humic acid suspensions. Very low doses of biopolymer
were required for the treatment of concentrated suspensions of bentonite; sedimentation was fast and very low
turbidity was obtained within a few minutes settling time;
the doses were signicantly lower when the pH of the suspension was less than the intrinsic pKa of chitosan. In contrast to the ndings of Guibal and co-workers, Chung [95],
studying the removal of SS, COD, TB and organic compounds from aquaculture wastewater by chitosan, observed that the treatment efciency of chitosan was
highly dependent on its DD and on the pH of the solution.
A high DD and low pH value improved the performance of
the coagulation process. Huang et al. [121] also showed
that the charge density of chitosan, and its coagulation performance, was directly proportional to the DD.
Wibowo et al. [71] studied the inuence of the MW and
DD of chitosan on the removal efciency of proteins. They
concluded that the difference in MW and DD values between samples could not explain the signicant differences
in protein recovery. No meaningful correlation was apparent. Kvinnesland and degaard [124], studying the effects
of polymer characteristics on separation in humic substance removal by cationic polymer coagulation, showed
that polyelectrolyte MW did not show any signicant effect on the coagulation of humic substances. Chen et al.
[135] reported that the DD of chitosan had limited effect
on the occulation performance while its MW played a
key role. The higher the MW, the better the occulation.
Large differences were found by Strand et al. [80,81,126]
in the efciency of chitosan materials to occulate bacterial suspensions, both regarding the effective biopolymer
concentrations and the type of chitosan giving the best
performance. In particular, the effect of MW on occulation performance was found to be of importance. The
choice of the optimal chitosan type for a given application
is a tricky task [126].
The interaction between chitosan and mineral colloids
(bentonite) has been investigated by Roussy et al. [75].
They showed that it was not necessary to add large
amounts of chitosan: doses as low as 0.20.5 mg/L were
sufcient to achieve the complete coagulationocculation
1345
1346
results for metal ion adsorption and occulation than chitosan. The grafted copolymers generally possess the main
properties of both initial components, and they are chemically stable and usually biodegradable [131].
6. Conclusions
Chitosan possesses several intrinsic properties such as
its non-toxicity, its biodegradability and its outstanding
chelation behaviour that make it an effective coagulant
and/or occulant for the removal of contaminants in the
dissolved state. It has the physico-chemical characteristics
of both coagulants and occulants, i.e., high cationic charge
density and long polymer chains, leading to bridging of
aggregates and precipitation. Numerous works have demonstrated that chitosan and its derivatives (in particular
grafted biopolymers) can be a potential substitute for
metallic salts and synthetic polyelectrolytes in the treatment of wastewater for the removal of both particulate
and dissolved substances. However, more studies are required to rene the optimisation of the properties of chitosan such as the degree of deacetylation which can
inuence coagulation and the molecular weight which
affects occulation.
Acknowledgements
Authors thank OSEO ANVAR (Besanon, France) and
INRA Transfert (Dpartement Valorisation, Paris, France)
for nancial support (Programme Chitodex Project Development of biocoagulants). The research grants given by
the French Ministry of Research and Education, the CNRS
and the Rgion of Franche-Comt which provide nancial
support for the Ph.D. students F. Renault and B. Sancey
are gratefully acknowledged. Thanks are due to the three
groups involved in our research program on pollutant complexation by chitosan-based materials: that of Dr. Giangiacomo Torri from G. Ronzoni Institute (Milan, Italy), of Prof.
Bernard Martel from University of Lille (France) and of
Prof. Yayha Lekchiri from University of Oujda (Morocco).
We acknowledge the constant contribution of Dr. Nadia
Morin-Crini (Chrono-environment Laboratory, Besanon,
France) to this research program. The authors also thank
Dr. Peter Winterton (University of Toulouse, France) for
its critical reading of this review, G. Ronzoni Institute
(Milan, Italy) for providing of chitosan samples, and Mr.
Jean-Claude Jeune (ARIST, Besanon, France) for providing
of patents.
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