journal homepage: www.elsevier.com/locate/europolj
F. Renault, B. Sancey, P. -M. Badot, G. Crini *
European Polymer Journal
of contaminated water and wastewater such as coagulation, precipitation, extraction, evaporation, adsorption on
Water pollution results from all human activities: water is polluted, decontamination becomes necessary.
reports a multitude of processes for the decontamination waste, but also to synthetic substances produced by
chemical wives (dyes, fertilizers, pesticides, and so on) [1]. when
to petroleum, minerals, sewage treatment sludge or exactly the organic pollutants produced by the incineration of
decontamination objectives required by law. The literature 1.Introduction
domestic, industrial and agricultural, and is not only due
activated carbon, ion-exchange, oxidation and advanced The best purification approach is sought to reach the
article info abstract
Review articles
Received in revised form 11 December 2008 Received 30 October 2008
Article history:
Contents lists available at ScienceDirect
Flocculation Coagulation
European Polymer Journal 45 (2009) 1337–1348
Chitosan
doi:10.1016/j.eurpolymj.2008.12.027
Université de Franche-Comté, Laboratoire Chrono-environnement, UMR 6249 UFC/CNRS usc INRA, Place Leclerc 25030 Besançon cedex, France
Biopolymers
Bioflocculant Keywords:
E-mail address: gregorio.crini@univ-fcomte.fr (G. Crini).
0014-3057/$ - see front matter 2008 Elsevier Ltd. All rights reserved.
Wastewater treatment
Available online 25 December 2008
*Corresponding author. Tel.: +33 3 81 66 57 01; fax: +33 3 81 66 57 97.
Accepted 17 December 2008
. . . .
4. Mechanisms of coagulation/flocculation. . . .
00 chitin, a biopolymer extracted from shellfish sources. Chitosan exhibits a variety of physico- chemical and biological properties resulting in numerous applications in fields such as
for the removal of particulate and dissolved contaminants. In particular, the development
2008 Elsevier Ltd. All rights reserved.
used and the conditions in the solution on the coagulation/flocculation performance are also
00 agriculture, textiles, oenology, food processing and nutrition. This amino-biopolymer has
. . . . the area of water and wastewater treatment. Their coagulation and flocculation properties
1. Introduction 2.
Categories of materials 3.
Why use coagulants and flocculants based on biopolymers? . . . 00
00 00 . . . . 3.2. Chitosan as bioflocculant
Chitosan is a partially deacetylated polymer obtained from the alkaline deacetylation of
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
. . . . 5. Chitosan for coagulation and flocculation – a review 6.
Conclusions. . . .
00 as cosmetics, biomedical engineering, pharmaceuticals, ophthalmology, biotechnology,
discussed.
00 also received a great deal of attention in the last decades in water treatment processes
00 References
Contents
of chitosan-based materials as useful coagulants and flocculants is an expanding field in
00
. . . . can be used to remove particulate inorganic or organic suspensions, and also dissolved
3.1. Coagulation/flocculation process. . . .
Chitosan for coagulation/flocculation processes – An eco-friendly approach
(which may have human health implications); (ii) production of large volumes of (toxic) sludge; (iii) dispersion of
have been reviewed by Bratby [17] and Türkman [22].
or immiscible liquids, and eliminates the solid particles
optimum pH range in which metal hydroxide precipitates guar gums, tannins, alginates, etc.).
– a secondary treatment or purification step using chemical or biological methods;
metal salts (aluminum sulphate, ferric chloride, ferric sulphate, etc.), pre-hydrolysed metals (polyalu minium chloride, polyaluminosilicate sulphate,
A general scheme of industrial water treatment involves
their concentration, but also on the temperature and nature of the solution. Therefore, new types of reagents have
some cases in secondary and tertiary treatment) [8–15].
coagulation/flocculation materials [17].
be used on an industrial scale for technological and especially economic reasons. Complete treatment will clearly
[16–22]. Biopolymers may be of great interest since they
(COD). However, the significant disadvantage of these conventional coagulants is the inability to control the nature was recently reviewed by Bolto and Gregory [16], with oxidation, incineration, electrofloatation, electrochemical
play an essential role in the coagulation process. Metal the molecules produced during the secondary purification
including metal salts such as polyaluminium chloride and reagents which destabilizes the colloidal particles, leading
[11,17]. The addition of these cations results in colloidal
stage of the coagulants, and not after their addition to and its drawbacks. An industrial effluent treatment
or damage the purification equipment.
lation/flocculation processes [11,17]:
mechanical, physical and chemical methods;
most common additives are aluminum sulphate (generally known as alum), ferric chloride and ferric sulphate
usually involves the dispersal of one or several chemical
naturally occurring flocculants (starch derivatives,
more effective than the traditional additives. Their significant advantage is that their hydrolysis occurs under specific experimental conditions during the preparation
introduced in the solution [17]. As a result, their performance is dependent not only on the pH of the water and
However, many of the available processes proposed cannot
organic matter). Primary treatment only concerns
heterogeneous effluent, ie, effluent containing suspended solids
and the nature of the impurities to be removed. Polyelec trolyte applications in industrial wastewater treatment
chemical substances may have several environmental con sequences (i) an increase in metal concentration in water
The use of organic polyelectrolytes in water treatment
In wastewater treatment using inorganic coagulants, an
simple separation step then eliminates the floc. The coagu lants and flocculants frequently used are mineral additives also be required to remove the remaining pollutant or
treated, the Fe(III) ions hydrolyse rapidly in an uncontrollable manner, forming a range of hydrolysis species which is obtained, knowing that each method has its advantages
secondary treatment because otherwise particulate pollution would hinder later treatment, making it less efficient
There are two major classes of materials used in coagu
levels have a strong positive effect on the reduction of turbidity, suspended solids (SS) and chemical oxygen demand
have been considered for environmental applications large aggregates, thereby facilitating their removal in subsequent sedimentation, floatation and filtration stages. it
(2) and organic flocculants including cationic and anionic polyelectrolytes, non-ionic polymers, amphoteric and hydrophobically modified polymers, and
eg, aluminum and ferric sulphates and chlorides. the
pre-hydrolysed forms of aluminum (such as polyaluminum chloride or PAC) and iron (polyferric sulfate or PFS) are industries.
several methods of purification before maximal efficiency
(eg, the removal of salts produced by the mineralization of
emphasis on the types of polymers commonly available
speciation in solution has been well documented [11,17].
synthetic polymers such as polyacrylamide. Using these – a primary treatment or pre-treatment step using
the procedure. It is known that PAC-based products provide better coagulation than alum at low temperatures
the Fe(III) coagulant has been added to the solution to be additives (lime, calcium salts, etc.), hydrolysing objectives: for instance lowering the levels of pollution, return
to a water course or recycling.
chitosan may be considered as one of the most promising in the primary purification of industrial wastewater (and in
decreasing the pH from the alkaline levels to near neutral the effluent. This pre-treatment stage is mandatory before
For these reasons, alternative coagulants and flocculants forms larger, denser flakes that are easier to separate. A
2. Categories of materials
In certain cases, a tertiary treatment of the water can
Examples include effluents from the dye, textile and milk require several steps and it is often appropriate to combine
acrylamide oligomers which may also be a health hazard.
and suspended substances (colloids or dispersions) from
occur, should be determined. The addition of metals depresses the wastewater pH to a lower value. in general,
Coagulation using chemical coagulants consists of combining insoluble particles and/or dissolved organic matter into three main stages [1]:
etc.) and polyelectrolytes (coagulant aids);
Coagulation is mainly induced by inorganic metal salts, the process line must also be designed according to the quality
been developed [11,23]. Alternative coagulants based on
are natural low-cost products, characterized by their environmentally friendly behavior. Among these biopolymers,
(1) inorganic and organic coagulants including minerals
Coagulation/flocculation is a frequently applied process
of the hydrolysis species formed when the coagulant is treatment, biodegradation and membrane filtration [2–7].
example).
– and treatment of the sludge formed (incineration for
the raw solution. This results in a much tighter control of to the formation of micro-floc. Bonding the micro-floc particles
together by the addition of a flocculation additive
destabilization, as they specifically interact with and neutralize the negatively charged colloids. For example, once
However, it produces abundant sludge that is difficult to dehydrate, its efficiency is entirely dependent on the pH and when formed in cold water alum flocs are not very
mechanically resistant. In addition, the use of alum is a source of concern and the debate about its possible toxicity is still open. Since high aluminum concentrations in water may have human health implications, environmentally (PAM) is a commonly used organic polymeric flocculant be cause it is possible to synthesize polymers with various functions (positive, negative or neutral charge) which can be used to produce a good settling performance at relatively low cost. Although extensive work has been done, future research needs to look into how molecular weight and charge density distribution affect the flocculation performance to produce a better choice of flocculants for specific industrial applications [11,17,22].
As mentioned, inorganic polymeric flocculants such as pre- hydrolysed PFS or polyferric chloride (PFC) have been recently proposed. These materials contain complex poly nuclear ions formed by OH bridging having high molecular weight and high cationic charge. These compounds become more effective at a comparatively lower dose than the conventionally applied reagents. They can be used over a wide range of pH and temperature due to their high level of hydrolysis.
As previously reported, the inorganic salt aluminum sulphate (alum) is one of the most widely used coagulants in conventional water and wastewater treatments. The performance of alum no longer needs to be proved and is rewarded for its low cost, ease of use and availability.
bridges, high removal efficiency can be achieved even with a small amount of flocculant, which generates a small volume of sludge [11]. Furthermore, the polymer performance is less dependent on pH (for example polyamines are effective over a wide range of pH). There are no residuals or metals added such as Al(III) and Fe(III), and the alkalinity is maintained.
The flocculation performance primarily depends on the type of flocculant used, how much is used, its molecular weight, ionic nature, the type of material in suspension wastewater and the type of wastewater [11,15–22] . Table 1 shows some examples of commercial polymeric flocculants. For example, poly(acrylamide)
Flocculants are used in fast solid–liquid separations involving the aggregation of particles. Flocculating agents are classified into inorganic and polymeric materials [11,17].
Inorganic flocculants have almost been abandoned because they have numerous disadvantages such as the large amounts required for efficient flocculation and subsequently the large volume of sludge produced. They are also highly sensitive to pH, inefficient towards very fine particles, and applicable only to a few disperse systems [17,22].
and also produce lower volumes of sludge. Because they are already partially neutralized, they have less effect on the pH of the water and so reduce the need for pH correction.
However, a detailed understanding of the flocculation mechanisms (in particular the mode of action) of these coagulants is still lacking [11,16,17,22].
3.1. Coagulation/flocculation process
Polymeric flocculants are easy to handle and immediately dissolve in aqueous systems. They can also reduce the sludge volume. Because cationic polymeric flocculants destabilize particles and coloring matter through the
compression of electrical double layers, charge neutralization, adsorption and subsequent formation of particle–polymer–particle
The coagulation process is not always perfect as it may result in small flocs when coagulation takes place at low temperatures or produces fragile flocs which break up when subjected to physical forces. It is not only necessary to overcome these problems but also to improve the process to obtain good quality effluent and rapid sedimentation of the flocs formed. To do so, several products known as
coagulation aids or polymeric additives can be used to bring together and agglomerate the flocs formed by the coagu lant [11,17]. These water-soluble polymers, regularly used in water treatment, are mainly synthetic, although a few natural products may be of interest [16,20,21]. The poly meric additives are broadly characterized by their ionic structure:
cationic, anionic and non-ionic [20,21]. Ionic polymers or ''polyelectrolytes'' of various structures are usually used as coagulant aids to enhance the formation of larger flocs in order to improve the rate of sedimentation [17,22]. Coagulant aids can act either by polymer bridging or by charge neutralization [11,17,20,21].
3. Why use coagulants and flocculants based on biopolymers?
Technological progress in polymer chemistry has also improved flocculant technology to provide organic polymers and polyelectrolytes with greater purification efficiency [16,24,25]. Commercial organic flocculants are basically of two types: synthetic materials based on various monomers (acrylamide, acrylic acid, diallyldimethylammonium chloride, etc.) and natural organic materials based on polysaccharides or natural polymers (starch, cellulose, alginate, natural gums, etc. ) [26–28]. The advantage of polymeric flocculants is their ability to produce large, dense, compact flocs that are stronger and have good settling character characteristics compared to those obtained by coagulation.
Cationic polyelectrolytes
Anionic polyelectrolytes
Natural non-ionic polymers (starch, cellulose derivatives) Poly(diallyldimethyl ammonium chloride)
Anionic polyacrylamides F. Renault et al. / European Polymer Journal 45 (2009) 1337–1348
Carboxylic acid polymers
Epichlorohydrin/dimethylamine polymers Cationic polyacrylamides
Phosphonic acid polymers
Poly(alkylamines) [poly(ethyleneimine), poly(vinylamine)]
Sulphonic acid polymers Poly(styrene) derivatives
Natural anionic polymers (sulphated polysaccharides, modified lignin sulphonates)
Sulphonium polymers Ionenes
Non-ionic polymers
Natural cationic polymers (chitosan, cationic starches)
Polyacrylamide Table 1
Examples of polymeric flocculants used in water and wastewater treatments.
the normally used anionic and non-ionic polymers are generally of low toxicity, but cationic polyelectrolytes are polymers have been utilized in coagulation/flocculation
of commercial polymers are also derived from petro leum- based raw materials using chemical processing that aluminum in the treated water. Polymer-based products
primary amino group and it is a commercially interesting the potential problems associated with their use are high
it is known that bioflocculants can play this role because
on polyelectrolyte toxicity, it is believed that the use of chemicals used to produce the monomer units (such as
wastewater treatment (Table 2) because it can be conditioned and used for a different pollutant complexation chitin, a natural polymer of major importance [43,44]. Chi tin is the second most abundant biopolymer in the world, perform less well at low temperatures), a smaller volume
from the additives. Bolto and Gregory [16] reported that
conventional materials [32–36]. Some of the reported
the molar fraction of deacetylated units, and crystallinity.
used in water and wastewater treatment generally arise
promising bioflocculant for environmental and purification
engineering, pharmacy, dentistry, ophthalmology,
biotechnology, chemistry, cosmetics, textile, pulp and paper,
and produce no secondary pollution [17,30,31,38,39]. They
nearly always carry a negative surface charge and because and wastewater treatment. Because of the above concerns
Increasing use is also being made of synthetic coagu lants of organic polymeric origin. Commercial synthetics
ethyleneimine, and trimethylolmelamine), unreacted
[46–52]. Chitosan is also widely applied in water and
Increasing the ionic strength (giving some reduction in
weight (MW), degree of deacetylation (DD), representing
biological products have recently been proposed and studied as effective coagulants and flocculants for replacing a wide range of applications as coagulants and flocculants,
counterions to neutralize the particle charge. It is well
Over the usual range of water pH (5–9) particles
Chitosan is a linear copolymer of D-glucosamine and N acetyl-D-glucosamine produced by the deacetylation of
For example, acrylamide is extremely toxic producing se vere neurotoxic effects [17]. Commercial forms of synthetic organic flocculants may also contain toxic products water phases arising from larger agglomerate sizes, efficiency at low temperatures (hydrolysing metal coagulants for the purification of wastewater [17]. The use of inorganic polymeric coagulants has also been questioned.
is effective and competitive. In particular, chitosan is a
The potential industrial use of chitosan is widely recognized. This versatile material is used in biomedical a source of debate. Contaminants of synthetic polymers
yeast [37]. Compared with conventional chemical flocculants, bioflocculants are safe and biodegradable polymers,
part of the electrical double layer) or specifically adsorbing
from residual unreacted monomers (such as acrylamide,
purposes, as reported in recent patents [39–42].
may potentially be applied not only in food and fermentation processes, downstream processing but also in water
oenology, food industry, agriculture and photography reaction by-products of the polymers in water [11,16,17].
[11,16,17]. In comparison with alum, some of the advantages of these polymers are: lower coagulant dose requirements, increase in the rate of separating the solid and
characteristics and properties of chitosan are its molecular However, although synthetic water-soluble polymers find
is growing interest in developing natural low-cost alternatives to synthetic polyelectrolytes [29–31]. Numerous
variety of functional groups which can interact with
contaminants [30,31]. Bioflocculation is a novel approach that
(starches, chitosan, alginates) and microbial materials produced by micro-organisms including bacteria, fungi and it is important to note that the use of polyelectrolytes is also
3.2. Chitosan as bioflocculant
water, a less pH-dependent process and a reduced level of friendly coagulants will present an interesting alternative
more toxic, especially to aquatic organisms. the majority
crustaceans, shrimps and crabs. Chitosan has unique properties among biopolymers especially due to the presence of
during preparation [44,45].
is not always safe or environmentally friendly. Today, there also improve settleability and increase the floc toughness.
compound because of its high nitrogen content in comparison to cellulose [43]. The main parameters influencing the
the zeta potential and a decreased thickness of the diffuse
forms, from water soluble forms to solid forms (gels, beads,
bioflocculants will increase [17,30,39].
epichlorohydrin, formaldehyde and dimethylamine) and processes for water purification for at least four decades
These parameters are determined by the conditions set they have particular macromolecular structures with a
products named ''bioflocculants” include biopolymers cost, lack of biodegradability and polymer toxicity. it is
of these, are often colloidally stable and resistant to aggregation. Coagulants are then needed to destabilize the particles. Destabilization can be brought about by either of sludge, a smaller increase in the ionic load of the treated
after cellulose. The main sources exploited are two marines
environmentally friendly)
processing effluents
charged molecules Non-toxic
Polymer assisted ultrafiltration (proteins...)
mineral and organic suspensions
fungi
adsorption processes Ability to form hydrogen bonds
Removal of pollutants with Biodegradable
potential applications
capacities
(drinking water, pools)
Ability to encapsulate Ecologically acceptable polymer
Sludge treatment
Coagulation of suspended solids,
Efficient against bacteria, viruses,
Recovery of valuable products Chelation of metal ions Principal characteristics
Flocculation of bacteria
Formation of salts with organic
Flocculant to clarify water
Renewable resources
intermolecularly
Removal of dye molecules by suspensions
and inorganic acids
Reduction of turbidity in food
(eliminating synthetic polymers,
Interactions with negatively
Filtration and separation Reduction of odors outstanding pollutant-binding
Table 2
treatment application.
Principal properties of chitosan in relation to its use in water and waste
F. Renault et al. / European Polymer Journal 45 (2009) 1337–1348
Effluent containing metal ions Surimi wash water
[73–78]
effluent
[99]
[82,83]
Bacterial suspensions
Partially purified sewage
[101]
[91–93]
[63–68]
Reference(s)
Pulp and paper mill wastewater Wastewater from milk processing plants
Oil-in-water emulsions
[70]
[95]
Inorganic suspensions (bentonite, kaolinite)
Effluent containing phenol derivatives
[79–81]
[100]
[84,85]
[102]
Effluents containing humic substances
Brackish water
Food, seafood and fish processing wastes
Olive oil wastewater
[69]
[86–90]
Effluents containing dyes
Raw drinking water Brewery wastewater
Aquaculture wastewater
[71,72]
[96–98]
[94]
compared to the sludge produced with metal salts; in addition, as biopolymers are biodegradable the sludge can be
for adsorption in batch or fixed-bed column systems,
Chitosan also possesses several intrinsic characteristics
(non-hazardous product, not irritating to skin and eyes...)
milk processing plant wastewater [69] and kaolinite sus pensions [77] were non-toxic and could be used to stimulate late growth in plants; chitosan does not add much to the
[55].
the addition of flocculant has a significant effect on the settling time when alum and/or PAC are used as coagulants. in
solvents. However, in dilute organic acids such as acetic available is not uniform); another important criterion to be
electrostatic interactions between polymer chains and physico-chemical properties (solubility, viscosity); there
for dissolving the polymer, the polymer concentration, the pKa of the macromolecule, in the treatment of minerals
the volume of sludge produced compared to the sludge consumed with alum for example; chitosan considerably increases the density of the sludge and facilitates its drying
high cationic charge density, long polymer chains, bridging
much lower concentrations than metal salts; it does not
6.3 and, at acidic pH (below pH 5), chitosan becomes a Its unique physico-chemical properties render it very efficient in
interactions with various contaminants including
membranes, fibers, etc.). Chitosan has been used in the solid state for the chelation of metal ions in near-neutral
limited pH range and when present in excess, has a negative effect on performance (overdosing can stabilize a
The main reasons for the success of biopolymers such as effluents (see Table 3). For example, chitosan has been used
successfully, for precipitative flocculation at pH above sources of chitin/chitosan (the quality of commercial chitin
[44,45]. So, treatment of chitosan with acids produces pro-toned amine groups along the chain and this facilitates it
[53–57]. This biopolymer has been used in gel-bead form
has characteristics of both coagulants and flocculants, ie,
[45]. Chitosan is a linear hydrophilic copolymer with a rigid structure containing both glucosamine and acetylglu cosamine units. It is insoluble in either water or organic
pollution [39]; chitosan is efficient in cold water and at
the speed of the flocs formed is also important since it influences the overall cost and efficiency [17]. It is known that
can be found in the review of No and Meyers [57].
of fraction of deacetylation, polymer weight and crystallinity because these parameters significantly influence its
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
processes and solvent extraction processes [57–62].
pKa of the amino group of glucosamine residues is about secondly its outstanding chelation behavior [53,55,56].
There are, of course, drawbacks which must be balanced against the benefits: chitosan is only efficient over a
In the coagulation/flocculation process, the settling
both particulate and dissolved substances. These proper ties have been exploited for the design of coagulation/flocculation processes
applied to the treatment of various dispersion and affect other process aspects); the coagulation/
flocculation properties depend on the different
soluble cationic polymer with high charge density water soluble form in polymer-enhanced ultrafiltration
properties; each chitosan must be characterized in terms
safe to animals and plants without causing harm to the environment
for the design of the experimental mode is solubility. How ever, the solubility is a very difficult parameter to control
removal of contaminants in the dissolved state [57,59]. it
floc settling speed and therefore reduces the settling time.
protonated and the biopolymer becomes fully soluble. the conditions). Its uses are justified by two important advantages:
firstly, its non-toxicity and biodegradability [17];
organic compounds, etc.). However, its solubility depends negatively charged contaminants in acidic solutions containing
dyes [85] or humic acid [82,83]. Other examples
affects the apparent pKa and thus charge, viscosity and a non-toxic material, non-corrosive and safe to handle well
efficiently degraded by micro-organisms; two studies reported that the sludge produced from the treatment of
characteristics. These problems can seize industrial users [16,17]; Hirohara et al. patented a chitosan-based material
the salinity of the treated water and is usable at alkaline pH.
The main parameter which must be taken into account that makes it an effective coagulant and/or flocculant for the
the negatively charged contaminants (metal anions, dyes, and organic suspensions [73–75] and the coagulation of
deposited on a suitable support (eg, ceramic), or in a
taken into account concerns the variability and heterogeneity of the biopolymer chitosan: changes in the specifications of the macromolecule may change coagulation
of aggregates and precipitation (in neutral or alkaline pH
leave residual metals that can cause secondary contamination problems; the low concentrations of polymers reduce
the case of chitosan, the increase of floc size favors the
acid and formic acid and inorganic acids (with the remarkable exception of sulfuric acid), the free amino groups are solution, the complexation of metal anions in acidic solution and
the dye complexation using adsorption processes
is a need for a better standardization of the production process to be able to prepare biopolymers having the same
and the ionic strength. It is important to note that the DD chitosan in wastewater treatment using coagulation/flocculation
processes are: chitosan has the advantage of being
Table 3
Examples of effluents treated by coagulation/flocculation using chitosan.
adsorption and charge neutralization (including electro static patch effects), depletion flocculation, displacement
the aqueous phase. Their ends also dangle and get adsorbed
weight (MW) and molecular structure. Literature data the importance of these mechanisms depends on pH and
coagulant dose. Aluminum and iron salts give cationic hydrolyzing products that are strongly adsorbed on negatives
[11,16,17,35]. Patch flocculation occurs when macromole molecules with a high charge density adsorb to particles and locally form positively and negatively charged areas on the
wastes from a variety of food processing industries including poultry, eggs, cheese, meat, fruit cakes, seafood and (related to the high molecular weight of biomacromole cules) and electrostatic patch. The mechanism is the follow low:
coagulation by charge neutralization destabilizes
increases dye removal up to a concentration resulting in 4. Mechanisms of coagulation/flocculation
charge neutralization (electrostatic patch effect). The action of hydrolysing metal coagulants can involve something similar
membrane filtration [50] and adsorption [53–56,59]. Obviously, chitosan has also been suspected as a coagulant
be more effective than one with shorter chains. Here, floc culation is interpreted as being a result of charge neutralization, patch flocculation, and/or polymer bridging. the
terms of two distinct mechanisms: charge neutralization
particles. At higher coagulant dosages bulk precipitation
Several mechanisms such as polymer bridging, polymer
For the case where the polymer and the adsorption site
and sedimentation. For example, cationic chitosan derivatives can be easily adsorbed onto the colloid surface of an ionic inorganic (bentonite) suspensions due to electrostatic
their hydrolysis concerns. Aluminum sulfate hydrolyses formed, although more detailed studies are needed [29].
amine groups), precipitative coagulation, bridge formation
few studies [84].
and extend some distance from the particle surface into
reactions while chitosan is not hydrolyzed [17]. A mechanism of action of polymeric flocculating agents was described in detail by O'Melia [106].
of particles and may act either by polymer bridging or
Different reviews of chitosan-based materials have apeared concerning concerning separation and complexation, including at low dosages, it is well established that charge neutralization
can be an effective means of destabilizing colloidal
particle, thereby forming strong aggregates of large flocs.
5 concerning the impact of chitosan's characteristics.
dye coagulation with chitosan appears to be charge
neutralization at acidic pH [85] and increasing chitosan dosage
of Duan and Gregory [105], and Bratby [17].
focused on the recovery of suspended solids (SS) and colloids;
in the case of dissolved contaminants there are many
chains should be sufficient to extend from one particle sur face to another. Hence a polymer with longer chains should substances. Their mode of action is generally explained in
coagulation/flocculation procedures using chitosan are related to the intrinsic physical and chemical characteristics
mechanism which can then be removed easily by filtration
charge neutralization and flocculation by bridging mechanisms [57,85]. It should be also noted that one of the greatest differences between metal salts and cationic polymers is probably due to the different nature of the precipitate
charge neutralization, adsorption (related to protonated performance of chitosan. Several examples are given in part
attraction. Adsorbed macromolecules tend to form loops
immediately on contact with water giving rapid adsorption Polymeric additives can also be used to cause aggregation
5. Chitosan for coagulation and flocculation – a review effective bridging occur, the length of the biopolymer widely used as coagulants in water and wastewater treatment
for removing a broad range of impurities from effluent, including colloidal particles and dissolved organics
factors such as pH, ionic strength of the solution, and coagulant concentration. For example, the main mechanism for
well understood [11,17,103–106]. For simple metal salts
between particles). Polymer bridging occurs when long chain polymers adsorb onto the surface of more than on
aggregates (bridge formation) and adsorbs dissolved or ganic substances onto the aggregates by an adsorption
is involved in a dual mechanism including coagulation by [11,15,17]. The improved performance of these materials
of coagulation by hydrolysing metal salts and the mecha nisms involved should refer to the comprehensive reviews
in numerous articles [57,107]. However, the studies mainly solubility. Other important parameters for the design of
are therefore charge density (related to its DD), molecular hydroxide precipitate (sweep flocculation). The relative
flocculation, etc. have been proposed to explain the destabilization of colloids and suspensions by polymers
particulate contaminants using chitosan often cited are
of chitosan for coagulation and recovery of SS in processing show that the type of mechanism also depends on different particles and can give effective destabilization. The principles
governing the action of hydrolysing coagulants are
particle surface (this results in strong electrical attraction
vegetables. These studies indicated that chitosan can reduce the SS of such processing waste by as much as 65%
of the macromolecule (ie, crystallinity, purity, hydrophilicity, charge density). All these characteristics may affect the
mechanisms. Readers interested in a detailed discussion As already mentioned, aluminum and iron salts are
for the capture of contaminants from aqueous solutions by another particle forming a bridge between particles. For
are of opposite signs, it is postulated that charge neutralization is the major mechanism. Mechanisms of coagulation/
flocculation involved in the removal of dissolved and
colloidal impurities and transfers small particles into large
complete neutralization of anionic charges. Above that concentration, the excess of cationic charges leads to suspension re-stabilization. Numerous works claim that chitosan of metal oxide hydroxide occurs. Pre-hydrolysed coagu lants
are often more effective than simple metal salts
In 1975–1978, extensive studies by Bough and co workers [63,64,108–115] demonstrated the effectiveness
chitosan characteristics that are important for flocculation in negatively charged colloids by cationic hydrolysis products
and incorporation of impurities in an amorphous
F. Renault et al. / European Polymer Journal 45 (2009) 1337–1348
that positively charged cationic macromolecules can desta bilize the negative colloidal suspension by charge neutralization as well as by bridge formation. In addition, another
they showed that color can be removed either by adsorption onto solid-state chitosan or by coagulation/floccula tion using dissolved-state chitosan. The reactivity of
much greater when using chitosan in the dissolved state
previous results reported by Huang et al. [121]. These sludge may be disposed of with a lower environment
from palm oil mill effluent compared to alum and PAC.
Coagulation/flocculation processes have been widely
charge neutralization, precipitative coagulation, bridging,
pollutant concentration in the solution. The effect of pH The effectiveness of chitosan as a coagulant has also been
molecules [84,85], metal cations [97,98], proteins [70],
level of turbidity removal, the required amount of chitosan
compared to the solid state [85]. Comparison of saturation
structure of the flocs. In neutral solutions, because of the
in a more acidic solution and therefore lowers the viscosity that chitosan has an intrinsic capacity to be used as a
coagulant to reduce SS, TB and COD. These works also reported
kaolinite suspensions could be safely disposed off in land fills.
However, such reuse needs to be carefully assessed
flocculant but also as a bactericide. For example, Chung
published a series of papers on the ability of chitosan to
pH. They observed that an excess of cationic charge contributed to re-stabilizing the suspension and reducing the
et al. [84] showed that the MW of chitosan did not affect biopolymer has an extremely high affinity for many classes
after coagulation: it has the characteristics of both coagulants
in the water treatment process can be cost effective.
suspensions but also dissolved substances. in particular,
optimum dosage correlated well with the initial concentration of the pollutant indicating that the addition of the bio polymer could be easily monitored by determining the
HA may limit the influence of MW. In the case of bentonite properties for natural organic matter [123], humic substances
[82,124,125], inorganic suspensions [74,75], dye
of colloidal particles because of its suitability for coagulation without posing any health threats as residual aluminum and other synthetic polymers do. To reach the same
also indicated that the chitosan becomes more compact 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 flocculation of
pH. Chitosan can be used not only as a coagulant and/or was described by which the polymer destabilizes the colloidal suspension by adsorption of particles with subsequent formation of particle–polymer–particle bridges.
flocculation process involved several mechanisms such as Sievers et al. [120]. These authors clearly demonstrated
For humic materials, Guibal et al. [84] showed that chito san can be used as a primary coagulant or as a flocculant
[122]. They also indicated that replacing PAC with chitosan
improves the accessibility and availability of reactive sites
of the macromolecule chain (chain repulsion) and the
and COD, but also for the removal of pathogens. Huang (COD). In some instances, chitosan can be used as a coagulant aid in conjunction with a synthetic polyelectrolyte or
reactive dyes [89], cannot be easily removed by conventional coagulants. Guibal's group [73–75,84,85,128–130]
of mechanism depends mainly on coagulant dosage and
to be considered for optimizing the use of chitosan in coagulation/flocculation. However, the results and their interpretation also depend on the type of contaminant. Guibal the coagulation/flocculation performance of chitosan. this
mechanism of action of chitosan as a flocculating agent
act as an effective coagulant to treat not only particulate matter
efficiency of the process. The authors also noted that the
the coagulation/flocculation of humic acid (HA). They as sumed that the more complex and flexible structure of of contaminants: it has demonstrated outstanding removal
a promising substitute for alum and PAC in the coagulation
biopolymer was used [84]. The authors explained their results by the fact that using chitosan in the dissolved state
biopolymer amine groups, changes in the conformation
be the key parameter in the comparison of removal performance [84]. All these works showed that the coagulation/
is neutralized by the negative ion in acid solution, the conformation of the biopolymer changes. Huang et al. [121]
systems [69,70,77,122]. Chi and Cheng [69] reported that
algal contents effectively by flocculation and settling. How ever, they noted that the flocculation was very sensitive to
and coloring materials in primary treatment prior to bio logical treatment. However, dye molecules, in particular
is sufficient to explain the results [130]. However, the type
the MW of the biopolymer) is also an important parameter Almas [117], Moore et al. [118], No et al. [57,65,119], and
[91–94], bacterial [81,126] and algal [127] suspensions.
larger flocs of better quality and faster settling velocity to 99% and good results were also obtained for the reduction of turbidity (TB) and chemical oxygen demand
that the molar ratio between the amine groups of the bio polymer and the sulfonic groups of the dye molecules was
larger and denser flocs. In acidic solutions, chitosan becomes a more extended chain (more charged) and there fore produces smaller, looser flocs, confirming the
important advantage must be cited: after being used the
There is recent literature concerning the evaluation of
very effective coagulant to remove the residual oil content
the removal of SS, organic and inorganic compounds, TB,
(coagulation/flocculation) than in the solid state (adsorption).
The conformation of polymer chains also seems to
authors showed that when the positive charge of chitosan impact than common metal- and synthetic polymer-based
Divakaran and Pillai [127] showed that chitosan reduced
and co-workers [121,122] showed that chitosan could be an inorganic salt to increase treatment performance. A
reported by Johnson and Gallanger [116], Senstad and
can be attributed to differences in the protonation of the phenolic and aromatic derivatives [99], oils and greases
is only half that of PAC. Chitosan coagulants are also produced
amine groups was significantly increased when dissolved
and its harmlessness must be checked.
[95] showed that this biopolymer was useful not only for
used as pre-treatments to remove suspended particles
electrostatic patch and aggregation phenomenon. In general, a charge neutralization associated to bridging effect
of the solution. The length of polymer chains (related to and flocculants. Ahmad et al. [91] demonstrated it was a
values for adsorption and coagulation/flocculation proved
the more coiled structure, the biopolymer is able to produce
were performed using a Jar-Test equipment; DCO and turbidity values were obtained after a 2-min settling time).
Fig. 1. Photograph of samples analyzed (a) effluent after PAC treatment; (b) raw effluents; (c) effluent after chitosan-based material treatment (experiments
Overall, these studies report that the binding mechanism of and co-workers [73,84,128] concluded that chitosan offers
and TOC as compared to synthetic polymers (poly(acryl mide) or PAM, poly(ethyleneimine) or PEI) and a chemical
inorganic solutions. Acetic acid is a common solvent for
The data from the literature show that the control of the
(i) the origin and the nature of the chitosan (ie, its
It is generally expected, and often found, that the coagulation–flocculation efficiency of chitosan is proportional acidic and direct dyes. They also reported that reactive dyes
bonding, hydrophobic or electrostatic interaction.
(alum or ferric salts) or synthetic polymers as coagulants
Rodrigues et al. [86] and Wang et al. [87] for the treatment
(iii) the chemistry of the pollutants (type of pollutants
complete information on experimental conditions in the
acid as an alternative solvent. In addition, they also indicated that it was important to search for the optimal acid
the activation conditions of the raw biopolymer);
the chitosan, the reaction time, the rate of rotation
contradictory observations have been reported and the
proposed modified chitosan-based biopolymers as adsorbent and/or coagulants for the removal of SS; DCO and our color from pulp and paper mill effluent [53,136–138] (Fig. 1).
(iv) and finally the solution conditions referring to its Other studies of dye-chitosan interactions have been
Chitosan can dissolve in carboxylic acid solutions or in
coagulants decrease with increasing concentrations of
acidic solutions, although smaller flocs were produced, the performances of chitosan depend on the process variables such as the dosage of chitosan, the speed of mixing this biomacromolecule. However, Huang et al. [121] reported that this organic solvent increased the organic content of suspensions. They suggested using hydrochloric dyes by polymers can be described as adsorption, hydrogen
suspensions, they noted that the performance decreased
chitosan, as a coagulant aid, is very effective for decolouring
the concentration of the colloidal particles, the presence of impurities (ie, dissolved salts or trace elements such as ions and chemicals) and temperature.
the performance of chitosan in the coagulation/flocculation
process depends on the following factors: concentrations since the viscosity of dissolved chitosan with anthraquinone groups were the most difficult to decol
us. Gandjidoust et al. [90] reported that the natural coagu lant chitosan resulted in the highest removal in both color
of the coagulant, the type of acid used to dissolve the impact of MW was not very marked. Increasing the bio-
polymer concentration reduced the impact of MW. Guibal
to its charge and accordingly highly N-deacetylated samples are usually applied as coagulant and flocculant materials.
However, it is important to note that several
intrinsic characteristics such as DD and MW, and
readers are encouraged to refer to the original papers for and the speed gradient;
a promising alternative to the use of mineral reagents
coagulant (alum). Similar conclusions were reported by
AC ID. In another work [122], the authors confirmed that
(ii) the influence of process variables such as the
equipment installed, the addition of reagents, the dosage
coagulation studies used.
of paper pulp and paper mill wastewater. Our group has also
and their physico-chemical properties such as polarity and hydrophobicity);
or coagulant aids.
with decreasing MW while for kaolinite suspensions the
and pH. The optimum chitosan dosage was smaller in pH, the ionic strength, the zeta potential, the color, carried out. Sanghi and Bhattacharya [89] showed that
F. Renault et al. / European Polymer Journal 45 (2009) 1337–1348
between the DD and the treatment time. The speed of mixing may affect the coagulation only before the optimal
dosage age is reached. Huang and co-workers [121,122] concluded in the number of protonated amine groups on chitosan)
respect to kaolinite is different from that with bentonite.
et al. [122] in the removal of colloidal particles by chitosan, evaluate the coagulation performance.
that polyelectrolyte MW did not show any significant effect on the coagulation of humic substances. Chen et al.
and humic acid suspensions. Very low doses of biopolymer
Large differences were found by Strand et al. [80,81,126]
[134], and Crini et al. [136]. For example, Mishra et al. [132]
SS, and reported that charge neutralization, flocculation the coagulation process. Huang et al. [121] also showed
sufficient). With optimized selection of polymer characteristics, the required dosage can be reduced to 0.1 mg/L or
concluded that the difference in MW and DD values between samples could not explain the significant differences
of chitosan influences the coagulation and its MW the floc culation mechanism. Similar conclusions were reported by
residual oil suspended in the effluent (by a charge neutralization mechanism) and adsorbs it by an adsorption mechanism, while alum and PAC just agglomerate and bridge the active site for coagulation [121]. There is a relationship
They showed that it was not necessary to add large
Ahmad et al. [91], studying the removal of residual oil in
and Kasaeian [93]. Strand et al. [81], applying chitosan to
choice of the optimal chitosan type for a given application
chitosan. Chitosan also reacts faster to residual oil compared to inorganic coagulants: the flocs produced by chito san appear rapidly and grow fast to form larger flocs which and increasing the speed during rapid mixing can also
reduce the optimum dosage. The authors supposedly that
studying the removal of SS, COD, TB and organic com pounds from aquaculture wastewater by chitosan, observed that the treatment efficiency of chitosan was
the presence of sodium chloride in the solution aids the coagulation phenomenon and they conclude that the major adsorption. The results can be mainly explained on the basis of the higher charge density of chitosan requiring lower
reactions. These modifications can improve the removal the authors suggest that the biopolymer agglomerates the
as flocculant for coprecipitation of Mn(II) and removal of A high DD and low pH value improved the performance of
of the solutions in a very short time (a few minutes were
in charged groups (in acidic solutions, there is an increase
et al. [132], Wang et al. [133], Chavasit and Antonio Torres (bentonite) has been investigated by Roussy et al. [75].
key roles. The higher the MW, the better the flocculation.
suspensions in water.
have been reported by Strand et al. [81], and Meysami on bentonite suspensions and dye solutions than on kaolin
binding of pollutants either by chemical and/or physical
as such is very useful as a coagulant, it may be advantageous geous to chemically modify the biopolymer, eg, by grafting DD of chitosan on the removal efficiency of proteins. They
for mineral colloids. However, they reported that the DD
which indicates that amino groups of macromolecules are
an olive oil water suspension by chitosan, noted that the
performance. In particular, the effect of MW on floccula tion performance was found to be of importance. the
needed for alums and PACs were 10 times more than for
the doses were significantly lower when the pH of the sus pension was less than the intrinsic pKa of chitosan. In contrast to the findings of Guibal and co-workers, Chung [95], chitosan (mainly DD and MW) slightly affected the
coagulation–flocculation performance. The DD had more effect
sediment easily. The mechanisms are different: for chitosan,
mechanism is chemical bridging rather than charge neutralization. Wu et al. [98] investigated the use of chitosan highly dependent on its DD and on the pH of the solution.
is a tricky task [126].
in optimum dosage. The authors also found a linear
relationship between the DD and the optimal chitosan dosage,
Wibowo et al. [71] studied the influence of the MW and
of natural organic matter and dyes. Different conclusions
The interaction between chitosan and mineral colloids on the flocculation performance while its MW played a
However, in contrast to this, Divakaran and Pillai [77]
concluded that chitosan was a useful flocculant for kaolinite.
turbidity was obtained within a few minutes of settling time;
concentrations and the type of chitosan giving the best
by charge neutralization mechanisms, while the dosages clearly indicated that chitosan had a natural selectivity
Ahmad et al. [92] confirmed that chitosan derivatives possess a number of functional groups responsible for the indicated that the coagulation behavior of chitosan with
was also introduced by Huang and Chen [76] and Pan that the optimal pre-treatment conditions to prepare chito
san coagulant and its dosage were the key parameters to
of polymer characteristics on separation in humic substance removal by cationic polymer coagulation, showed
Several workers have suggested that although chitosan Meyssami and Kasaeian [93], studying the treatment of
sufficient to achieve the complete coagulation–flocculation
and Bolto et al [123] and Szygula et al. [130] in the removal
reported that chitosan-gN-vinyl formamide showed better Guibal et al. [84,129] showed the characteristics of
[135] reported that the DD of chitosan had limited effect
Chitosan failed to form a good aggregate with kaolinite.
and adsorption occurs simultaneously.
below depending on the biopolymer. their investigations
the residual oil (they do not adsorb it). in another work, followed by charge neutralization, resulting in a decrease
that the charge density of chitosan, and its coagulation performance, was directly proportional to the DD.
palm oil mill effluent by chitosan, showed that chitosan requires lower dosages to destabilize the oil residue mainly
flocculate bacterial suspensions, pointed out the predominant role of bridging in the flocculation mechanism.
were required for the treatment of concentrated suspensions of bentonite; sedimentation was fast and very low
in the efficiency of chitosan materials to flocculate bacterial suspensions, both regarding the effective biopolymer
amounts of chitosan: doses as low as 0.2–0.5 mg/L were the destabilization of particles was enhanced by the increase
performance of the chitosan as patented by Laue and Hun keler [42] and also reported by Wang et al. [131], Mishra Huang and Chen [76] for the removal of bentonite. They
in protein recovery. No meaningful correlation was apparent.
Kvinnesland and Ødegaard [124], studying the effects
doses to destabilize the solution. The charge mechanism
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6. Conclusions
Bernard Martel from University of Lille (France) and of Prof.
Yayha Lekchiri from University of Oujda (Morocco).
Authors thank OSEO ANVAR (Besançon, France) and INRA Transfert (Département Valorization, Paris, France) for financial support (Programme Chitodex – Project ''Devel opment of biocoagulants"). The research grants given by the French Ministry of Research and Education, the CNRS and the Région of Franche-Comté which provide financial 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. Giangia como Torri from G. Ronzoni Institute (Milan, Italy), of Prof.
Jean-Claude Jeune (ARIST, Besançon, France) for providing of patents.
Chitosan possesses several intrinsic properties such as its non-toxicity, its biodegradability and its outstanding chelation behavior that makes it an effective coagulant and/
or flocculant for the removal of contaminants in the dissolved state. It has the physico-chemical characteristics of both coagulants and flocculants, ie, high cationic charge density and long polymer chains, leading to bridging of aggregates and precipitation. Numerous works have devised 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 refine the optimization of the properties of chitosan such as the degree of
deacetylation which can influence coagulation and the molecular weight which affects flocculation.
References Acknowledgments
We acknowledge the constant contribution of Dr. Nadia Morin- Crini (Chrono-environment Laboratory, Besançon, 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.
results for metal ion adsorption and flocculation than chito san. The grafted copolymers generally possess the main properties of both initial components, and they are chemically stable and usually biodegradable [131].
