Applications of chitin- and chitosan-derivatives for the detoxification of water and wastewater —...

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Applications of chitin- and chitosan-derivatives for the detoxification of water andwastewater — A short review

Amit Bhatnagar ⁎, Mika SillanpääLaboratory of Applied Environmental Chemistry, Department of Environmental Sciences, University of Kuopio, FI-50100 Mikkeli, Finland

a b s t r a c ta r t i c l e i n f o

Available online 1 October 2009

Keywords:Water treatmentAdsorptionChitinChitosanReview

Chitin and chitosan-derivatives have gained wide attention as effective biosorbents due to low cost and highcontents of amino and hydroxyl functional groups which show significant adsorption potential for theremoval of various aquatic pollutants. In this review, an extensive list of chitin- and chitosan-derivativesfrom vast literature has been compiled and their adsorption capacities for various aquatic pollutants asavailable in the literature are presented. This paper will give an overview of the principal results obtainedduring the treatment of water and wastewater utilizing chitin and chitosan-derivatives for the removal of:(a) metal cations and metal anions; (b) radionuclides; (c) different classes of dyes; (d) phenol andsubstituted phenols; (e) different anions and other miscellaneous pollutants. The review provides asummary of recent information obtained using batch studies and deals with the various adsorptionmechanisms involved. It is evident from the literature survey that chitin- and chitosan-derivatives haveshown good potential for the removal of various aquatic pollutants. However, still there is a need to find outthe practical utility of such developed adsorbents on commercial scale.

© 2009 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262. Chitin- and chitosan-derivatives for detoxification of water and wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.1. Chitin- and chitosan-derivatives for metals (cations and anions) and radionuclides removal . . . . . . . . . . . . . . . . . . . . . . 272.2. Chitin- and chitosan-derivatives for dyes removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.3. Chitin- and chitosan-derivatives for phenols removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.4. Chitin- and chitosan-derivatives for anions removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.5. Chitin- and chitosan-derivatives for miscellaneous pollutants removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3. Conclusions and future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

1. Introduction

Water is one of the basic necessities required for the sustenanceand continuation of life. It is therefore important that supply of goodquality water should be available for various activities. However, thisis becoming increasingly difficult in view of large scale pollutioncaused by industrial, agricultural and domestic activities. Theseactivities generate wastewater which contains both inorganic andorganic pollutants. Some of the common pollutants are phenols, dyes,detergents, insecticides, pesticides and heavymetals [1]. The nature of

pollutants in wastewater depends on the source of generation andvaries from place to place. These pollutants are often toxic and causeadverse affects on human and animal life if present above certainconcentration levels. In order to avoid pollution of natural waterbodies, it is essential to treat wastewater for the removal of pollutantsbefore being discharged into them. A number of methods such ascoagulation, membrane process, adsorption, dialysis, foam flotation,osmosis, photocatalytic degradation and biological methods havegenerally been used for the removal of toxic pollutants from waterand wastewater [2]. The type of the process to be employed maydepend on nature of pollutant. However, adsorption process is oftenconsidered most appropriate as it can remove both inorganic andorganic pollutants and the operation of the process is convenient.

Advances in Colloid and Interface Science 152 (2009) 26–38

⁎ Corresponding author. Tel.: +358 40 355 3405.E-mail addresses: [email protected], [email protected] (A. Bhatnagar).

0001-8686/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.cis.2009.09.003

Contents lists available at ScienceDirect

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Activated carbon has been found to be a universal adsorbent foreffluent treatment and is commonly used for the removal of variouspollutants from water [3]. However, its widespread use in wastewatertreatment is sometimes restricted due to its higher cost. It is nowrecognized that adsorption using low-cost adsorbents is an effective andeconomic method for water decontamination. A large variety of non-conventional adsorbents have been examined for their ability to removevarious types of pollutants from water and wastewater. Biosorbentsgain wide attention as these are available in large quantities worldwideand are eco-friendly. The use of adsorbents containing natural polymershas received reorganization, in particular polysaccharides such as chitinand its derivate chitosan.

Chitin was first discovered in mushrooms by the French Professor,Henrni Braconnot, in 1811. In 1820s chitin was also isolated frominsects. Chitin contains 2-acetamido-2-deoxy-β-D-glucose through a β(1→4) linkage. Chitin is the most abundant natural fiber next to thecellulose and is similar to cellulose inmany respects. Themost abundantsource of chitin is the shell of crab and shrimp. Chitosanwas discoveredin 1859byProfessor C. Rouget. Chitosan contains2-acetamido-2-deoxy-β-D-glucopyranose and 2-amino-2-deoxy-β-D-glucopyranose residues.

Chitosanhas drawnparticular attention as effective biosorbent due toits low cost compared to activated carbon and its high contents of aminoand hydroxyl functional groups showing high adsorption potential forvarious aquaticpollutants [4–8]. This biopolymer represents anattractivealternative to other biomaterials because of its physico-chemicalcharacteristics, chemical stability, high reactivity, excellent chelationbehavior and high selectivity toward pollutants [4–8]. Natural chitosanhas been modified by several methods (either physically or chemically)in order to enhance the adsorption capacity for various types of pollut-ants. Different shapes of chitosan, e.g. membranes, microspheres, gelbeads and films have been prepared and examined for the removal ofvarious pollutants from water and wastewater. The detailed descriptionabout structures of chitin and chitosan and their physical and chemicalmodification by various techniques has been documented in detail inearlier reviews [4–10].

Previous reviewarticles discussed thepotential of chitin and chitosanformetal ions [5–7,9] or dyes [8]. However, oneof theaimsof this reviewis to compile and present the adsorption potential (adsorptioncapacities) of chitin- and chitosan-derivatives for various other aquaticpollutants e.g. phenol and substituted phenols; different anions andmiscellaneous pollutants such as pesticides, fungicides, humic sub-stances etc. This review provides, besides the critical discussions, therecent literature of about the past 10–15 years to demonstrate theusefulness of chitin- and chitosan-derivatives in water and wastewaterapplications. A summary of relevant published data (in terms ofadsorption capacities of chitin and chitosan-derivatives for the removalof various different pollutants) with some of the latest importantfindings and giving a source of up-to-date literature on the adsorptionproperties of chitin and chitosan for diverse types of pollutants ispresented and some of the results have been discussed here.

2. Chitin- and chitosan-derivatives for detoxification of waterand wastewater

2.1. Chitin- and chitosan-derivatives for metals (cations and anions) andradionuclides removal

Metal ions are one of the important categories of water pollutants,which are toxic for humans through the food-chain pyramid. Varioustoxic heavy metal ions discharged into the environment throughdifferent industrial activities, constituting one of the major causes ofenvironmental pollution. Chitin- and chitosan-derivatives have beenextensively investigated as adsorbents for the removal of metal ionsfromwater and wastewater. The high adsorption potential of chitosanfor heavy metals can be attributed to (1) high hydrophilicity due tolarge number of hydroxyl groups of glucose units, (2) presence of a

large number of functional groups, (3) high chemical reactivity ofthese groups, and (4) flexible structure of the polymer chain [10].

Cadmium and its compounds are extremely toxic even in lowconcentrations, and bioaccumulate in organisms and ecosystems. Adisease known as “Itai Itai” in Japan is associated with cadmiumpoisoning, which results in multiple fractures in the body. Chitin hasbeen explored as adsorbent for the removal of cadmium (Cd) ionsfrom aqueous solutions by batch experiments [11]. An adsorptioncapacity of 14 mg/g of chitin for Cd(II) ions was observed. Scanningelectron microscopy coupled with X-ray energy dispersed analysiswas used to demonstrate that cadmium-containing nodules existedon the chitin surface for cadmium-equilibrated chitin. They alsoexamined the effect of co-ions (Cu2+ and Zn2+) on the cadmiumbiosorption at free solution pH and 25 °C, in static conditions [12].Biosorption of individual metals followed the sequence: Cu>Cd>Zn.Zn2+ ions had little affect on cadmium uptake capacity of chitin underthe conditions examined, whereas, the presence of Cu2+ ionsdecreased the affinity of chitin for cadmium.

The efficiency of cadmium removal using chitosan has also beeninvestigated by Jha et al. [13]. An adsorption capacity of 5.93 mg ofCd(II)/g of chitosan at a pH range of 4.0–8.3 was reported and thepresence of ethylenediaminetetraacetic acid (EDTA) was found tosignificantly decrease cadmium removal. This is expectable, becauseEDTA and other aminopolycarboxylic acids are known to formextremely stable complexes with heavy metals [14,15].

Mercury (Hg), copper (Cu), nickel (Ni), zinc (Zn), lead (Pb) andmanganese (Mn) are some of the othermetals ions, which are ofmajorconcern when present at higher concentrations in the environment.Their removal has been investigated fromwater by several researchersby chitin and chitosan-derivatives. McKay et al. investigated theadsorption of some metal ions by chitosan [16]. It was found that theadsorption capacity of chitosan forHg(II), Cu(II), Ni(II) and Zn(II)were815, 222, 164 and 75 mg/g, respectively. The ability of chitin orchitosan to sorb Cu(II) ions fromaqueous solutionswas studied, takinginto account kinetic, equilibrium, and mass transfer aspects [17].Equilibrium isotherm studies revealed that the Cu(II) sorption ontochitin and chitosan was best described by the Langmuir and Redlich-Peterson models. The sorption capacity of chitosan for Cu(II) ions wasfour to five times higher than that of chitin.

The use of chitosan flakes for the removal of Cu2+ from water hasbeen examined [18]. Chitosan showed excellent ability for Cu2+

adsorption with a capacity of 1.8–2.2 mmol/g dry mass. However, thecapacity sharply rises when the solutions contain a high concentrationof chloride ions. The variation of solution pH leads to competitionbetween the coordination of Cu2+ with chitosan. The optimal pHrange for copper adsorption onto chitosan flakes was 5.4–6.0.

The performance of commercially available sources of chitin,chitosan and chitosan cross-linked with benzoquinone for variousmetal ions has also been explored [19]. Initial pH of the metal solutionsignificantly influenced metal uptake capacity. The highest metaluptake values (137, 108, 58, and 124 mg/g for copper, zinc, arsenic,and chromium, respectively) were achieved with chitosan (1 g/L, atpH 4) with initial metal concentrations of 400 mg/L.

Simultaneous removal of various metal ions (zinc, copper,cadmium, and lead) using commercially available chitosan flakesfrom aqueous solutions under variable physico-chemical conditionshas also been reported [20]. The results obtained from the adsorptionstudies showed that there was significant uptake of these metal ionsby chitosan. Chitosan flakes exhibited maximum uptake capacity forcopper ions. The order of metal ion adsorption by chitosan decreasedfrom Cu2+ to Zn2+ as follows: copper>lead>cadmium>zinc. Theheavy metal uptake by chitosan was found to be pH dependent. Theuptake of metal ions by chitosan was enhanced by increasing the pHfrom 4 to 7. This was attributed to the greater availability of aminogroups at higher pH. On the other hand, the reduced adsorption ofmetal ions at acidic pH values could be attributed to the fact that at

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lower pH, the metal ions that would coordinate with the lone pair ofnitrogen would have to compete with H3O+ for an active site.

Several chemical modifications have been carried out to increasethe uptake capacity of cross-linked chitosan beads [21]. Among them,aminated chitosan beads formed by the chemical reaction ofethylenediamine and carbodiimide showed the highest uptakecapacity for mercury (Hg2+) ions. The uptake capacity of aminatedchitosan beadswas ca. 2.26 mmol Hg2+/g drymass at pH 7. This valuewas thought to be one of the highest uptake capacities among variousbiosorbents. Beads also showed the characteristic of competitivesorption between mercury and hydrogen ions and finally it wassuccessfully modelled by an equilibrium model based on isothermalsorption data. Recently, Hg(II) removal from water by chitosan andits derivatives has extensively been reviewed by Miretzky and Cirelli[9].

To enhance the sorption capacity of chitosan for Cd removal,Rorrer et al. performed experiments to increase the porosity of thechitosan beads by adding acidic chitosan solution into a sodiumhydroxide solution precipitation bath [22]. The gelled chitosan beadswere cross-linkedwith glutaraldehyde and then freeze dried. Beads of1- and 3-mm diameter were prepared. Beads of 1-mm diameterpossessed surface areas exceeding 150 m2/g and mean pore sizes of560 Å and were insoluble in acid media at pH 2. Adsorption isothermsat 25 °C and pH 6.5 over the concentration range 1–1690 mg Cd2+/Lpossessed a stepped shape and maximum adsorption capacities forthe 1- and 3-mm beads were found to be 518 and 188 mg of Cd/g ofbeads, respectively. The stepped shape of the isotherm was explainedby a pore-blockage mechanism. The modified chitosan showed manyadvantages over flaked or powdered chitosan e.g. higher internalsurface area, and cross-linkage of beads making them insoluble inlow-pH solutions, thus proving their suitability over a broad pH range.

A new composite chitosan biosorbent was prepared by coatingchitosan on to perlite ore and investigated for Cu(II) and Ni(II)removal [23]. Maximum removal of Cu(II) and Ni(II) on chitosancoated on perlite was at pH 5.0. The maximummonolayer adsorptioncapacity of chitosan coated on perlite was 196.07 mg/g for Cu(II) and114.94 mg/g for Ni(II).

Chitosan obtained from silkworm chrysalides (ChSC) was exam-ined for the removal of Pb2+ and Cu2+ from battery manufacturewastewater [24]. The best pure ChSC deacetylation degrees (DDs)obtained for the removal of Pb2+ and Cu2+ from battery wastewaterwere 80% (90 min ChSC deacetylation) and 92% (180 min ChSCdeacetylation), respectively. The maximum adsorption capacities forPb2+ and Cu2+ using pure ChSC with 80% and 92% DD were 72 mg/gand 87 mg/g, respectively, under the experimental conditions: pH 5.0,particle size from 300 to 425 µm, temperature 20.0±0.1 °C, and250 rpm stirring.

Three cross-linked chitosan-derivativeswereused as sorbents for theremoval of Cu(II) from aqueous solutions [25]: (i) Ch, without grafting;(ii) Ch-g-Aam, grafted with acrylamide; and (iii) Ch-g-Aa, grafted withacrylic acid.Ch-g-Aamaterial presented the highest sorption capacity forCu(II) removal (318 mg/g at pH 6) among the studied and earlier citedchitosan materials. The interaction between Cu(II) and the preparedsorbents was confirmed by FTIR spectroscopy. The peaks of aminogroupsof Cu(II)-loadedchitosan sorbentspresented shiftswith respect tonon-loaded ones (Ch, 1665–1660 cm−1; Ch-g-Aam, 1672–1674 cm−1;Ch-g-Aa, 1674–1670 cm−1), suggesting a chelated complex.

New chitosan-derivative has been synthesized by cross-linking ametal complexing agent, [6,6′-piperazine-1,4-diyldimethylenebis(4-methyl-2-formyl) phenol] (L), with chitosan (CTS) by Krishna-priya and Kandaswamy [26]. Adsorption experiments (pH depen-dency, kinetics, and equilibrium) of the cross-linked chitosan ligand(CCTSL) of various metal ions such as Mn(II), Fe(II), Co(II), Cu(II), Ni(II), Cd(II) and Pb(II) were carried out at 25 °C. The results showedthat the adsorption was dependent on pH of the solution, withmaximum capacity between pH 6.5 and 8.5. The order of adsorption

capacities for the metal ions studied was found to be Cu(II)>Ni(II)>Cd(II)≥Co(II)≥Mn(II)≥Fe(II)≥Pb(II). Chemical sorption was sug-gested as the rate-limiting step of adsorption mechanism.

A new biosorbent was developed by coating chitosan on topolyvinyl chloride (PVC) beads [27]. Equilibrium and column flowadsorption characteristics of copper(II) and nickel(II) ions on thebiosorbent were studied. The maximum monolayer adsorptioncapacity of chitosan-coated PVC sorbent as obtained from Langmuiradsorption isotherm was found to be 87.9 mg/g for Cu(II) and120.5 mg/g for Ni(II) ions, respectively. The results indicated thatthe maximum uptake of Cu(II) ions took place at pH 4.0 while themaximumuptake of Ni(II) ions occurred at an initial pH of 5.0. Columnexperiments exhibited that it was possible to remove the metal ionsfrom aqueous medium by biosorption on to chitosan-coated PVCbeads.

The magnetic chitosan nanocomposites were synthesized on thebasis of amine-functionalized magnetite nanoparticles [28]. Thesenanocomposites provide a very efficient, fast, and convenient tool forremoving Pb2+, Cu2+, and Cd2+ from water. It was suggested thatsynthesized magnetic chitosan nanocomposites can be used as arecyclable tool for heavy metal ion removal.

The adsorption of Al(III) from aqueous solutions onto chitosanwasfound to be 45.45 mg/g at 30 °C. [29]. The adsorption of Al(III)increased with the rise in adjusted pH of the solution from 3.0 to 4.0and decreased after pH 4.0. The pseudo-second-order kinetic modelwas used to describe the kinetics which is based on the assumptionthat the rate-limiting step may be chemisorption involving valencyforces through sharing or exchange of electrons between the –NH2

groups in chitosan and Al(III). It was observed by the authors thatbefore equilibrium was reached, an increase in temperature led to anincrease in adsorption rate which indicated a kinetically controlledprocess, while the adsorption of Al(III) on chitosan was controlled byan exothermic process.

Metal anions are another class of aquatic pollutants which poseserious threat to human health even at low concentrations. The removalof metal anions by chitin and chitosan-derivatives has been examinedbymany researchers. Chitosan was found very efficient for the removalof vanadate anions fromdilute solutions through electrostatic attractionand anion exchange in acidic solutions [30]. The optimumpHwas foundto be in the range 3.0 to 3.5. Sorption capacity under optimumexperimental conditions reached 8–9 mmol/g (ca. 400–450 mg/g),which corresponds to a molar ratio of ca. 1.3–1.5 between vanadateand –NH3

+ groups. This stoichiometric ratio, together with the shape ofthe sorption isotherm curves, which was correlated to the predomi-nance of anionic species, indicated that sorption occurred through thereaction of protonated amine sites with decavanadate species (prefer-entially to other polynuclear anionic species of lower vanadium unitnumber). In acidic solutions, the predominance of decavanadatesolutions lead to almost rectangular sorption isotherm curves. In nearneutral solutions, the predominance of nonadsorbable vanadate speciesresulted in negligible adsorption up to a concentration, dependent onpH, at which decavanadate species appeared in solution. At this limitresidual concentration, sorption capacity steeply increased. Above pH7,the protonation of amine groups was significantly reduced and thepolymerwas not able to exchange counteranionswith vanadate speciesor to attract anionic species. Thus, the protonation of the polymerrevealed a key parameter, as does the distribution of metal species, onthe efficiency of vanadium sorption on chitosan. Sorption kinetics wasalso strongly controlled by the pH and the metal concentration. Thepredominance of decavanadate species had been correlated to fastestkinetics. Desorption was optimum in alkaline solutions, in whichvanadium took the form of vanadate species in solution.

Cross-linked chitosan gel beads were used for molybdate sorptionand were found to show enhanced sorption performance in batchsystems [31]. The authors reported that, in continuous systems,sorption capacities could reach 700 mg/g, a level close to that

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obtained in batch studies. They also suggested that the sorption ofmolybdate (MoO4

−) on chitosan flakes depends on the cross-linking. Itdecreased from the range 300–800 to 150–400 mg Mo/g when thechitosan flakes were cross-linked with glutaraldehyde.

Cross-linking of chitosan particles, in glutaraldehyde, epichlorhy-drine, or EGDE (ethylene glycol glycidyl ether) enhances theresistance of sorbent beads against acids, alkali or chemicals. Thisproperty of glutaraldehyde cross-linked chitosan beads was exploredfor vanadate and molybdate sorption [32]. Uptake capacities of402.5 mg/g for vanadate and 763 mg/g for molybdate were obtained.The sorption into beads (763 mg molybdate/g) was better than intoflakes (329.7 mg molybdate/g). Authors reported that both anions(molybdate and vanadate) showed similar sorption behavior whichwas likely due to the similarities in the chemistry of these ions. A pH ofapproximately 3 was favorable for the sorption of these metal anions,which was consistent with (i) the electrostatic attraction between theprotonated amine sites and the strongly anionicmetal species and, (ii)the appearance of polynuclear hydrolyzedmetal species in the 3.0–3.5pH range. Sorption kinetics were mainly controlled by intraparticlediffusion for beads, while for flakes the controlling mechanisms wereboth external and intraparticle diffusions.

The adsorption of sulfate (SO42−) and molybdate (MoO4

−) ions hasalso been evaluated by Moret and Rubio [33] on chitin-based materialwith different deacetylation degrees (DD) at pH 4.5. The capacitieswerehigh, 150 mg SO4

2−/g with DD=25%. Chitin also proved to adsorbmolybdate ions in the presence of sulfate ions, but reaction requiredlonger equilibration time (60 min for 92% removal). Practical examplesof removal of these anions were studied in actual mining effluents,attaining values of the order of 71% SO4

2− and 85%MoO4− from a Cu–Mo

flotation mill effluent and 80% sulfate removal from a coal AMD-acidmine drainage (Mo free). The regeneration of the adsorbent materialwas possible through the anions desorption in alkaline medium.

Arsenic in natural waters is a worldwide problem. Arsenic toxicity,its health hazards, and the treatment techniques are well known andhave been widely reported. Extensive research is being conducted tocontrol/minimize the arsenic contamination in drinking water.Dambies et al. [34] investigated the sorption of As(V) on molybdate-impregnated chitosan gel beads. The sorption capacity of raw chitosanfor As(V) was increased by impregnation with molybdate. Theoptimum pH for arsenic uptake was ca. pH 3. The As(V) sorptioncapacity, over molybdenum loading, was ca. 160 mg/g. The exhaustedsorbent was regenerated by phosphoric acid desorption. Threesorption/desorption cycles were conductedwith only a small decreasein sorption capacity.

Chitosan powder derived from shrimp shells, was converted intobead form and used to remove As(III) and As(V) from water in bothbatch and continuous operations [35]. The optimal pH value for As(III)and As(V) removal was ca. 5. Adsorption capacities of 1.83 and1.94 mg As/g bead for As(III) and As(V), respectively, were reported.

The sorption of As(V) onto chitosan flakes has been studied byKwok et al. [36]. Authors selected 96 h contact time to ensure thatequilibrium has been achieved over the whole concentrationspectrum. The rate of desorption of arsenate from chitosan increasedwith the increase of the initial pH from 3.50 to 4.50. The capacity ofthe arsenate ion on chitosan was governed by the protonationreaction of chitosan and an increase of pH led to a decrease of theprotonated groups on chitosan which were available for sorption ofarsenate ion and influenced the speciation of arsenate ions in theaqueous phase. The dominant arsenate ions were in the form ofH2AsO4

−. It was believed that the removal mechanism of arsenate ionsfrom the aqueous phase might be due to the adsorption of arsenateions to the protonated amine group on chitosan. A novel pseudo-firstorder reversiblemodel, incorporating the effect of changing pH profilethroughout the adsorption and desorption cycle, was newly devel-oped to describe the sorption and the desorption in the batch kineticsystems of As(V) and chitosan simultaneously. The pH value of the

adsorption and desorption reaction of arsenate and chitosan could bepredicted from the pH against time profile. The adsorption capacitieswere reported from 1331 µg arsenate/g chitosan at initial pH 5.5 to14,160 µg arsenate/g chitosan at initial pH 3.5.

A study on the removal of arsenic from groundwater using iron–chitosan composites was conducted [37]. Removal of As(III) and arsenic(V) was studied through adsorption at pH 7.0 under equilibrium anddynamic conditions. The monolayer adsorption capacity from theLangmuir model for iron chitosan flakes (ICF) (22.47±0.56 mg/g forAs(V) and 16.15±0.32 mg/g for As(III)) was found to be considerablyhigher than that obtained for iron chitosan granules (ICB) (2.24±0.04 mg/g for As(V); 2.32±0.05 mg/g for As(III)). Anions includingsulfate, phosphate and silicate at the levels present in groundwater didnot cause serious interference in the adsorption behavior of arsenate/arsenite. The column regeneration studies were carried out for twosorption–desorption cycles for both As(III) and As(V) using ICF and ICBas sorbents. One hundred and forty-seven bed volumes of As(III) and112 bed volumes of As(V) spiked groundwater were treated in columnexperiments using ICB, reducing arsenic concentration from 500 to<10 μg/L. The eluent used for the regeneration of the spent sorbentwas0.1 M NaOH. The adsorbent was also successfully applied for theremoval of total inorganic arsenic down to <10 μg/L from arseniccontaminated groundwater samples.

Chromium, another important metal, has many industrial applica-tions such as in textile, electroplating, leather tanning and metallurgyindustries, and therefore, the wastes generated by these industries arerich in hexavalent Cr(VI) or trivalent Cr(III) forms of chromium. Cr(VI)is more toxic than Cr(III) and has therefore, lead greater environ-mental concern. Chromium is potentially toxic to humans as it isconsidered a carcinogen. The adsorption of Cr(VI) on chitosan flakeswas investigated against the process parameters such as pH,adsorbent dose and initial Cr(VI) concentration by Aydın and Aksoy[38]. The effects of these factors were studied in the ranges 1.5–9.5,1.8–24.2 g/L and 15–95 mg/L, respectively. Maximum removal wasattained from a solution as concentrated as 30 mg/L at pH 3 with anadsorbent dosage of 13 g/L. The adsorption capacity of chitosan flakeswas determined as 22.09 mg/g at these specified conditions. However,the adsorption capacity was recorded as high as 102 mg/g for 100 mg/L initial Cr(VI) concentration. Pseudo-second-order kinetic modelexhibited the highest correlation with data. The results showed thatboth monolayer adsorption and intraparticle diffusion mechanismslimited the rate of Cr(VI) adsorption.

The ability of cross-linked and non-cross-linked chitosan as anadsorbent for Cr(VI) removal from aqueous solution has beendemonstrated [39]. Cr(VI) adsorption behavior could be describedusing the Langmuir isothermmodel over thewhole concentration rangeof 10 to 1000 mg/L Cr(VI). The maximum adsorption capacity for bothtypes of chitosan was found to be 78mg/g for the non-cross-linkedchitosan and 50 mg/g for the cross-linked chitosan for the Cr(VI)removal. The optimum pH for maximum Cr(VI) removal was 5.

To achieve enhanced removal of Cr(VI), perlite beads were coatedwith chitosan by drop-wise addition of a liquid slurry containingchitosan and perlite to an alkaline bath [40]. The adsorption capacity ofchitosan-coated perlite was found to be 104 mg/g of adsorbent from asolution containing 5000 mg/L of Cr(VI). On the basis of chitosan, thecapacity was 452 mg/g of chitosan. It was reported by the authors thatthis capacity was considerably higher than that of chitosan in its naturaland modified forms, which was in the range of 11.3 to 78 mg/g ofchitosan. The beads loaded with chromium were regenerated withsodium hydroxide solution of different concentrations.

Boddu et al. [41] prepared a composite chitosan biosorbent bycoating chitosan, onto nonporous ceramic alumina. Batch andcontinuous column sorption experiments with this sorbent werecarried out at 25 °C to evaluate Cr(VI) adsorption from synthetic andactual chrome plating wastewater samples. Experimental equilibriumdata were fitted to Langmuir and Freundlich models. The maximum



capacity for Cr(VI) obtained from the Langmuir model was 154 mg/g.The adsorption capacities reported in this study on the twice-coatedbiosorbent was considerably higher than those previously reportedvalues in literature, indicating that the chitosan in the compositebiosorbent has greater adsorption capacity than unsupported chit-osan and the coating process significantly improved the adsorptioncapacity of chitosan. This improvement was attributed to theincreased surface area and facilitation of transport of chromium ionsto the binding sites on chitosan.

Quaternary ammonium salt of chitosan (QCS) was synthesized viareaction of a quaternary trimethyl ammonium, glycidyl chloride [42]and examined for Cr(VI) removal. The sorption capacity was found tobe pH dependent. The optimum pH range for adsorption was found tobe 3.5–4.5, at which Cr(VI) existed frequently as the most easilyadsorbed species, HCrO4

−. The Cr(VI) adsorption capacity at pH 9.0was 30.2 mg/g, while at pH 4.5 the capacity was 68.3 mg/g. Thekinetics of Cr(VI) on the cross-linked quaternary chitosan salt (QCS)were found to follow second-order rate mechanism. Cr(VI) ions wereeluted from cross-linked QCS by treatment with a 1 mol/L solution ofNaCl/NaOH to give a chromium efficiency of more than 95%.

Chromium removal onto cross-linked chitosan was studied by Rojaset al. [43] as a function of pH, particle size, adsorbent dose, chromiumconcentration and chromium oxidation state. The optimum adsorptionpH was 4.0, while chromium(VI) was partially reduced in the rangepH≤3.0. The chitosan potential for chromium(III) removal was foundlower (6 mg/g), while higher uptake was observed for chromium(VI)(215 mg/g). This high capacity was explained due to the largestoichiometry of protonated amine sites in the acidic range of pH.

Three cross-linked chitosan-derivatives were used as sorbents forthe removal of Cr(VI) from aqueous solutions [25]: (i) Ch, withoutgrafting; (ii) Ch-g-Aam, grafted with acrylamide; and (iii) Ch-g-Aa,grafted with acrylic acid. Ch-g-Aam material presented the highestsorption capacity for Cr(VI) removal (935 mg/g at pH 4) among thestudied and cited chitosan materials. The FTIR spectra of Cr(VI)-loadedand non-loaded sorbents were also shown to elucidate the mechanismof Cr(VI) sorption onto the prepared chitosan sorbents. The peaks ofamino groups of Cr(VI)-loaded chitosan sorbents presented shifts withrespect to non-loaded ones (Ch, 1665–1669 cm−1; Ch-g-Aam, 1672–1679 cm−1; Ch-g-Aa, 1726–1731 cm−1), suggesting the electrostaticinteraction between HCrO4

− and amino groups of chitosan. Furthermore,two new peaks were observed in FTIR spectra of Cr(VI)-loaded sorbentswhich attributed to Cr–O and Cr=O bonds of chromate anions,confirming the sorption of Cr(VI) onto the chitosan-derivatives at 789and 910 cm−1, 781 and924 cm−1,779 and 907 cm−1 for νCr–O and ν

Cr=O,respectively, in the case of Ch, Ch-g-Aam and Ch-g-Aa. The regenerationof sorbentswas affirmed in four sequential cycles of sorption–desorptionexperiments, without significant loss in sorption capacity.

Chitosan–Fe(0) nanoparticles (chitosan–Fe(0)) were prepared usingnon-toxic and biodegradable chitosan as a stabilizer and batchexperiments were conducted to evaluate the influences of initial Cr(VI)concentration and other factors on Cr(VI) reduction on the surface of thechitosan–Fe(0) [44]. The authors suggested that the overall disappear-ance of Cr(VI) might include both physical adsorption of Cr(VI) onto thechitosan–Fe(0) surface and subsequent reduction of Cr(VI) to Cr(III).Characterization with high-resolution X-ray photoelectron spectrosco-py revealed that after the reaction, relative toCr(VI) and Fe(0), Cr(III) andFe(III) were the predominant species on the surface of chitosan–Fe(0).Chitosan also inhibited the formation of Fe(III)–Cr(III) precipitation dueto its highefficiency in chelating the Fe(III) ions. This studydemonstratedthat chitosan–Fe(0) has the potential to become an effective agent for insitu subsurface environmental remediation.

A novel aminated chitosan adsorbent with higher adsorptionability for metal cations and metal anions was prepared [45]. Throughcross-linking amination reaction, the content of amidocyanogen ofaminated chitosan adsorbent was enhanced four times than that ofchitosan cross-linked adsorbent. The adsorption ability of the novel

aminated chitosan adsorbent for nickel citrate and Cr(VI) wasenhanced remarkably. When the initial concentration of metallic ionwas 1000 mg/L, the adsorption capacity of the novel aminatedchitosan adsorbent for nickel citrate and Cr(VI) was up to 30.2 mg/gand 28.7 mg/g, respectively.

Rhenate sorption on chitin and chitosan has been studied by Kimet al. [46] as an analogue of radioactive pertechnate anion. Themechanism was described by ion-pair formation in the diffuse doublelayer between NH3

+ surface group and rhenate anions in solution. Theamount of sorbed rhenate decreased with the increase in pH from 3 to6.5, indicating a lowering of the sorption capacity of chitosan. At pH 3, alarge number of amine groups were in protonated form and couldinteract with the negatively charged perrhenate anion via electrostaticinteractions. At higher pH, the amount of protonated amine groupsdecreased drastically, limiting the sorption of rhenate.With the increasein the ionic strength from 0.01 to 0.1 M, rhenate sorption on chitosandecreased by 90%. The sorption capacity of chitin and chitosan forrhenate adsorption was also compared. pH 3 was found favorable foradsorption on chitin, however, pH 4.1 was found more favorable in caseof chitosan. Even under these conditions, the percentage of rhenatesorbed on chitin was much smaller than the percentage of rhenatesorbed on the chitosan. The main difference between the two organicpolymers was that, for chitin, the majority of NH2 groups wereacetylated, so that the amount of protonated amine groups was smallerby far, than in chitosan for the similar experimental conditions.Consequently, chitin had a much smaller surface charge. Thus, a smalleramount of perrhenate anionswere located in thediffuse double layer viaelectrostatic interactions. Theweak sorptionof rhenate on chitin showedthat the sorption of perrhenate occurred only when a large number ofprotonated amine groups were present on the organic molecule.

Acid-washed Ucides shells (AWUS) showed good adsorptionpotential for anionic gold-cyanide (Au(CN)2−), selenate (SeO4

2−),chromate (CrO4

2−) and vanadate (VO43−) at low pH [47]. Equilibrium

biosorption uptakes by AWUS were up to 0.17 mmol Au/g AWUS (pH3.4), 0.15 mmol Se/g (pH 3.0), 0.54 mmol Cr/g (pH 2.0) and0.79 mmol V/g (pH 2.5). An increased ionic strength (IS) suppressedthe primary anion uptake as chloride ions competed for biosorbentprotonated sites andhigher IS reduced the activity of ions in solution. Thebiosorptionmechanismwas suggested to involve electrostatic attraction.

Chitosan-derivatives have also been explored for the removal andrecovery of precious metals. The adsorption of gold (Au(III)) ions ontochitosan and N-carboxymethyl chitosan (NCMC) has been investigat-ed [48]. The percentage adsorption increased first with the rise in pHand then reached amaximumat about pH 4.0 and 6.0, for chitosan andNCMC, respectively. It decreased sharply with a further increase inequilibrium pH. The uptake of Au3+ on chitosan and NCMC were30.95 mg/g of chitosan and 33.90 mg/g of NCMC. The Au3+ ions werereadily removed from chitosan and NCMC by treatment with anaqueous EDTA solution. Based on the desorption studies, it wasconcluded that ion exchange was the controlling mechanism.

Chitosan-derivatives were found very efficient for removing goldfrom dilute acidic solutions [49]. Maximum uptake capacity reached600 mg/g (ca. 3 mmol/g). The speciation of gold (under chloride andhydroxide–chloride forms) appeared to be a predominant parameterinfluencing uptake. In acidic solutions, chitosan was protonated andprotonated amine groups were available for the sorption of anionic goldspecies. The optimum pH range was pH 2–3 for glutaraldehyde cross-linked chitosan, but the sorption capacity strongly decreased withincreasing pH. Rubeanic acid grafting decreased the influence of pH ongold sorption. Sulfur grafting increased the polymer chelating sites thatwere less influencedbypH than ion-exchange sites (protonated amines)involved in ion-pair formation. Sorption kinetics was influenced by pHand the type of chitosan-derivatives. The grafting of sulfur compoundsand the hexamethylene diisocyanate cross-linking enhanced sorptionkinetics probably due to the presence of highly reactive chelating sitesand better diffusion properties.



Chitosanwas also examined for the removal of soluble silver fromindustrial waste streams [50]. Stirred-batch and column methodswere used to remove free (hydrated) silver ion as well as theammonia, thiocyanate, thiosulfate, and cyanide complexes of silverin simulated wastewater at an initial concentration of 50 mg/L and ina pH range of 2–10. In the case of the free silver ion, Ag+, and inparticular for Ag(NH3)2+, the applicable pH rangewas quite broad (4–8).However, the anions, Ag(SCN)32− and Ag(S2O3)23−, were bound only atpH 2, and Ag(CN)2− was not bound to any significant extent at any pHexamined in these experiments. The calculated silver ion capacity in aflow-through column at pH 6 was reported ca. 42 mg/g.

The chitosan microparticles were prepared using the inverse phaseemulsion dispersionmethod andmodifiedwith thiourea (TCS) [51]. Theresults showed that the maximum adsorption capacity was found at pH2.0 for bothPt(IV) andPd(II). TCS can selectively adsorbPt(IV) andPd(II)frombinarymixtureswith Cu(II), Pb(II), Cd(II), Zn(II), Ca(II), andMg(II).The adsorption reaction followed the pseudo-second-order kinetics,indicating themain adsorption mechanism of chemical adsorption. Theisotherm adsorption equilibrium was well described by Langmuir iso-thermswith themaximumadsorption capacity of 129.9 mg/g for Pt(IV)and112.4 mg/g for Pd(II). The adsorption capacity of both Pt(IV) andPd(II) decreasedwith increase in temperature. The adsorbentwas stablewithout loss of the adsorption capacity up to at least 5 cycles and thedesorption efficiencies were above 95% when 0.5 M EDTA–0.5 M H2SO4

eluent was used. The results also showed that the pre-concentrationfactor for Pt(IV) and Pd(II) was 196 and 172, respectively, and therecovery was found to be more than 97% for both precious metal ions.

Chitosan-derivatives have also been explored for the removal ofsome radionuclides from water. Chitosan benzoyl thiourea derivativehas been synthesized and used successfully for the removal of thehazardous 60Co and 152+154Eu radionuclides from aqueous solutionsby Metwally et al. [52]. The maximum adsorption capacities for Co(II)and Eu(III) were 29.47 (mg/g) and 34.54 (mg/g), respectively. It wasfound that uptake% of Eu(III) and Co(II) reached its maximum valuesof 75.0% and 98.0% at pH 3.5 and 8.0, respectively, at 10−5 M ionconcentration.

Adsorption of uranium(VI) from aqueous solution onto cross-linkedchitosan (CCTS) was investigated in a batch system by Wang et al. [53].Themaximum Langmuirmonolayer capacity was found to be 72.6 mg/g.Theuranium(VI) adsorption capacity byCCTSwas strongly dependent oncontact time, pH, and initial uranium(VI) concentration. Kinetic studiesshowed that the adsorption followed a pseudo-second-order kineticmodel, indicating that the chemical adsorptionwas the rate-limiting step.

Readers interested in a detailed discussion of the interaction ofmetalions with chitosan should refer to the excellent comprehensive reviewspublished elsewhere [5–7]. A summary of adsorption capacities ofchitin- and chitosan-derivatives for different metal ions and radio-nuclides has been presented in Table 1.

It is evident from the vast literature survey that chitin-, chitosan-and its derivatives have been proven very promising biosorbents forthe removal of metal ions from water and wastewater. The mechanismof metal adsorption on chitin- and chitosan-derivatives has beenproposed to occur via electrostatic interactions in acidic media (ionexchange), metal chelation (coordination) and due to the formation ofion pairs [5–7]. Several parameters influence this reaction such as, ioniccharge of the adsorbent, solution pH and the chemistry of themetal ion(ionic charge, ability to be hydrolyzed and to form polynuclear species)[5–8].

2.2. Chitin- and chitosan-derivatives for dyes removal

Dyes are important water pollutants which are generally presentin the effluents of textile, leather, paper and dye manufacturingindustries. The worldwide high level of production and extensive useof dyes generates colored wastewaters which cause water pollution.The colored dye effluents are generally considered to be highly toxic

to the aquatic biota and affect the symbiotic process by disturbing thenatural equilibrium through reduced photosynthetic activity due tothe coloration of water in streams. Some dyes are reported to causeallergy, dermatitis, skin irritation, and cancer in humans. Thus, theremoval of dyes from effluents before they are mixed up withunpolluted natural water bodies is important. Various studies onchitin and chitosan for dyes removal fromwater and wastewater havebeen conducted in recent years [53–66]. These studies demonstratedthat chitosan-based biosorbents are efficient materials and have anextremely high affinity for many classes of dyes.

Table 1Adsorption capacities of chitin and chitosan-derivatives for various metal ions andradionuclides removal from water.

S. no. Adsorbent Adsorbate Adsorptioncapacity

Reference

1. Chitin Cd(II) 14.0 mg/g [11]2. Chitosan Cd(II) 5.93 mg/g [13]3. Chitosan Hg(II) 815.0 mg/g [16]4. Chitosan Cu(II) 222.0 mg/g [16]5. Chitosan Ni(II) 164.0 mg/g [16]6. Chitosan Zn(II) 75.0 mg/g [16]7. Chitosan flakes Cu2+ 1.8–2.2 mmol/g [18]8. Chitosan Cu, Zn, As,

and Cr137, 108, 58,and 124 mg/g

[19]

9. Aminated chitosan beads Hg 2.26 mmol/g [21]10. Porous-Magnetic Chitosan beads Cd(II) 188–518 mg/g [22]11. Chitosan coated on perlite Cu(II) 196.07 mg/g [23]12. Chitosan coated on perlite Ni(II) 114.94 mg/g [23]13. Chitosan obtained from

silkworm chrysalides (ChSC)Pb(II) 72.0 mg/g [24]

14. Chitosan obtained fromsilkworm chrysalides (ChSC)

Cu(II) 87.0 mg/g [24]

15. Carboxylgrafted chitosan Cu(II) 318.0 mg/g [25]16. Amido-grafted chitosan Cr(VI) 935.0 mg/g [25]17. Chitosan coated on to polyvinyl

chloride (PVC) beadsNi(II) 120.5 mg/g [27]

18. Chitosan coated on to polyvinylchloride (PVC) beads

Cu(II) 87.9 mg/g [27]

19. Chitosan Al(III) 45.45 mg/g [29]20. Chitosan VO4

3− ca. 400–450 mg/g

[30]

21. Glutaraldehyde cross-linkedchitosan beads

VO43− 402.5 mg/g [32]

22. Glutaraldehyde cross-linkedchitosan beads

MoO4− 763.0 mg/g [32]

23. Molybdate-impregnatedchitosan gel beads

As(V) ca. 160.0 mg/g [34]

24. Chitosan beads As(III) 1.83 mg/g [35]25. Chitosan beads As(V) 1.94 mg/g [35]26. Chitosan flakes As(V) 1331–

14,160 µg/g[36]

27. Chitosan flakes Cr(VI) 22.09–102.0 mg/g

[38]

28. Non-cross-linked chitosan Cr(VI) 78.0 mg/g [39]29. Cross-linked chitosan Cr(VI) 50.0 mg/g [39]30. Chitosan coated onto nonporous

ceramic aluminaCr(VI) 154.0 mg/g [41]

31. Quaternary ammonium salt ofchitosan (QCS)

Cr(VI) 30.2–68.3 mg/g [42]

32. Cross-linked chitosan Cr(III) 6.0 mg/g [43]33. Cross-linked chitosan Cr(VI) 215.0 mg/g [43]34. Aminated chitosan Ni(II) 30.2 mg/g [45]35. Aminated chitosan Cr(VI) 28.7 mg/g [45]36. Chitosan particles VO4

3− 90 mg/g [47]37. Chitosan Au(III) 30.95 mg/g [48]38. N-carboxymethyl chitosan

(NCMC)Au(III) 33.90 mg/g [48]

39. Chitosan Ag 42.0 mg/g [50]40. Chitosan benzoyl thiourea

derivativeCo(II) 29.47 mg/g [52]

41. Chitosan benzoyl thioureaderivative

Eu(III) 34.54 mg/g [52]

42. Cross-linked chitosan U(VI) 72.6 mg/g [53]



Wu et al. [58] examined the suitability of chitosan for the removalof reactive dyes. A comparison of the maximum adsorption capacityfor reactive red 222 (RR222) by chitosan flakes and beads showeduptake capacities of 293 mg/g for flakes and 1103 mg/g for beadswhich was explained by the fact that the beads possessed a greatersurface area than the flakes.

The abilities of removal of three reactive dyes (RR222, RB222, andRY145) and of immobilization of tyrosinase using flake-type andhighly swollen bead-type of chitosans at 30 °C have also beeninvestigated [59]. The capacity of dye adsorption using swollenchitosan beads was about five times, up to 1653 g/kg, as compared tochitosan flakes. The adsorption process could be best described by thepseudo-second-order equation, indicating the controlling nature ofchemisorption (chemical reaction). The highly swollen chitosan beadsapplied in this work, showed promising potential for enzymeimmobilization and color removal.

The adsorption of reactive dye (reactive red 189) from aqueoussolutions on cross-linked chitosan beads was studied in a batchsystem [60]. The equilibrium isotherms at different particle sizes (2.3–2.5, 2.5–2.7 and 3.5–3.8 mm) and the kinetics of adsorption withrespect to the initial dye concentration (4,320, 5,760 and 7,286 g/m3),temperature (30, 40 and 50 °C), pH (1.0, 3.0, 6.0 and 9.0), and cross-linking ratio (cross-linking agent/chitosan weight ratio: 0.2, 0.5, 0.7and 1.0) were investigated. The maximum monolayer adsorptioncapacities obtained from the Langmuir model were very large, (1936,1686 and 1642 mg/g) for small, medium and large particle sizes,respectively, at pH 3.0 and 30 °C, for the cross-linking ratio of 0.2. Theinitial dye concentration and the pH of aqueous solutions significantlyaffected the adsorption capacity of dye RR189 on the cross-linkedchitosan. However, the adsorption of the dye on chitosan was slightlyinfluenced by the temperature and the epichlorohydrin (ECH)/chitosan weight ratio. It was suggested that the rate-limiting stepmay be the chemical adsorption but not the mass transport.

A batch systemwas also applied to study the adsorption of reactivedye (reactive red 189) from aqueous solutions by cross-linkedchitosan beads [61]. The ionic cross-linking reagent sodium tripoly-phosphate was used to obtain rigid chitosan beads. To stabilizechitosan in acid solutions, chemical cross-linking reagent epichloro-hydrin (ECH), glutaraldehyde and ethylene glycol diglycidyl etherwere used and ECH showed the highest adsorptive removal of the dye.The Langmuir model agreed well with experimental data and itscalculated maximum monolayer adsorption capacity was found to be1802–1840 g/kg at pH 3.0, and 30 °C. The electrostatic interactionsbetween the dye and chitosan beads governed the adsorptionmechanism. The desorption data showed that the removal percentof dye from the cross-linked chitosan beads was 63% in NaOHsolutions at pH 10.0 and 30 °C. It was observed that the desorbedchitosan beads could be reused to adsorb the dye and to reach thesame capacity as that before desorption.

Chiou et al. [62] examined the adsorption of four reactive dyes,three acid dyes and one direct dye onto cross-linked chitosan beads.The adsorption capacity values ranged from 1911 to 2488 mg/g at pH3–4. The authors reported that the adsorption conforms to Langmuirisotherm model and pseudo-second-order kinetic model. In addition,adsorption capacity appeared to increase with the decreasing pH ofsolution. The adsorption capacities of the cross-linked chitosan beadsweremuch higher than those of chitin for anionic dyes. It showed thatthe major adsorption site of chitosan is an amine group, –NH2, whichis easily protonated to form –NH3

+ in acidic solutions. The strongelectrostatic interaction between the –NH3

+ of chitosan and dyeanions was used to explain the high adsorption capacity of anionicdyes onto chemically cross-linked chitosan beads.

Kinetics of the adsorption of reactive dyes by chitin was studied byAkkaya et al. [63]. The effect of initial concentration, temperature,shaking rate and pH on the adsorption of reactive yellow 2 (RY2) andreactive black 5 (RB5) using chitin was investigated. The authors

reported that RY2 and RB5 are significantly adsorbed on chitin. Theadsorption capacities were found ca. 38 mg/g for RY2 at 293 K, while65 mg/g for RY5 at 333 K. It was suggested by the authors that theadsorption kinetics was controlled by surface diffusion as the BETsurface area of chitin used in this study was very low. At particularlylower temperatures, surface diffusion was more dominant.

The treatment of synthetic reactive dye wastewater (SRDW) byadsorption process was studied using chitin modified by sodiumhypochlorite and original chitin in batch experiments [64]. Maximumdye adsorption by chitin increased from 133 mg/g to 167 mg/g withrise in temperatures from 30 to 60 °C. For modified chitin, the capacitydecreased from 124 mg/g to 59 mg/gwhen the temperature increasedfrom 30 °C to 60 °C, respectively. The authors suggested that althoughmodified chitin had lower adsorption capacity as compared to chitin,elution of the dye from modified chitin was easier than chitin.Therefore, modified chitin could be suitable in a column system fordye pre-concentration as well as wastewater minimization. Inaddition, the column study showed that modified chitin could beused for more than four cycles of adsorption and eluted by distilledwater. The main mechanism of dye adsorption onto modified chitinwas physical adsorption, while the chemical adsorption was respon-sible in chitin.

Adsorption of reactive orange 16 by quaternary chitosan salt (QCS)was used as amodel to demonstrate the removal of reactive dyes fromtextile effluents by Rosa et al. [65]. The adsorption experiments wereconducted at different pH values and initial dye concentrations. Themaximum adsorption capacity determined was 1060 mg of reactivedye/g of adsorbent, corresponding to 75% occupation of theadsorption sites. The results indicated that the adsorption processwas not dependent on solution pH, since the most probablemechanism for adsorption was the interaction of the polymerquaternary ammonium groups with the dye sulfonate groups.

The adsorption of Remazol black 13 dye onto chitosan in aqueoussolutions was investigated [66]. Experiments were carried out as afunction of contact time, initial dye concentration (100–300 mg/L),particle size (0.177, 0.384, 1.651 mm), pH (6.7–9.0), and temperature(30–60 °C). The maximum adsorption capacity (qm) was found to be91.47–130.0 mg/g. The amino group nature of the chitosan providedreasonable dye removal capability. The kinetics of reactive dyeadsorption followed the pseudo-first and second-order rate expres-sion which demonstrates that intraparticle diffusion plays a signifi-cant role in the adsorption mechanism. The authors also providedscanning electron microscopy (SEM) images to show that the dye wasdensely and homogeneously adhered to the surface of the carrier, as aresult of either natural entrapment in to the porous chitosan material,due to physical adsorption by electrostatic forces or covalent bindingbetween the cellular chitosan and the carrier.

Chitosan was cross-linked using glutaraldehyde in the presence ofmagnetite by Elwakeel [67]. The resinobtainedwas chemicallymodifiedthrough the reactionwith tetraethylenepentamine followed by glycidyltrimethylammonium chloride, to produce chitosan/amino resin (R1)and chitosan bearing both amine and quaternary ammonium chloridemoieties (R2), respectively. The uptake of reactive black 5 (RB5) fromaqueous solutions using R1 and R2 resins was studied using batch andcolumn methods. The resins showed high affinity for the adsorption ofRB5 and an uptake value of 0.63 and 0.78 mmol/g was reported forresins R1 and R2, respectively at 25 °C. The resin was regeneratedeffectively using NH4OH/NH4Cl buffer (pH 10).

Chitosan was modified to possess the ability to adsorb cationicdyes from water by Chao et al. [68]. Four kinds of phenol derivatives:4-hydroxybenzoic acid (BA), 3,4-dihydroxybenzoic acid (DBA), 3,4-dihydroxyphenyl-acetic acid (PA), and hydrocaffeic acid (CA), wereused individually as substrates of tyrosinase to graft onto chitosan.These modified chitosans were used in experiments for uptake of thecationic dyes, crystal violet (CV) and bismarck brown Y (BB). Theoptimum adsorptive uptake for CV and BB occurred at pH 7 and 9,



respectively at 30 °C. Langmuir type adsorption was found to occurand the maximum adsorption capacities for both dyes were increasedwith the following order: CTS–CA>CTS–PA>CTS–DBA>CTS–BA.

Chitosan-derivatives, by grafting poly (acrylic acid) and poly(acrylamide) through persulfate induced free radical initiatedpolymerization processes and covalent cross-linking of the preparedmaterials, were prepared and evaluated as biosorbents for RemacrylRed TGL (a basic dye) removal [69]. It was found that the graftingmodifications greatly enhanced the adsorption performance of thebiosorbents. Kinetic studies also revealed a significant improvementof sorption rates by the modifications. The amount of adsorbed valuesderived from the Langmuir model at 298 K were 0.479, 0.727, and1.068 mmol/g (204.22, 309.82, and 510.74 mg/g) for three forms ofchitosan, respectively.

Chitosan beads were synthesized for the removal of a cationic dye,malachite green (MG), from aqueous solution [70]. The monolayeradsorption capacities were found to be 93.55 mg/g at 303 K;74.83 mg/g at 313 K; 82.17 mg/g at 323 K, respectively. The kineticexperiments confirm that sorption process obeys the pseudo-second-order kinetic model. The temperature strongly influenced theadsorption process. The adsorption of malachite green increasedfrom 18.80% to 99.38% with an increase in pH of the solution from 2.1to 11.0. Beyond pH 8, the dye adsorption remained almost constant.

Batch adsorption experiments were carried out for the removal ofmethylene blue (MB), a cationic dye, from its aqueous solution usingchitosan-g-poly(acrylic acid)/montmorillonite (CTS-g-PAA/MMT) nano-composites as adsorbent [71]. The adsorption behaviors of the nano-composite showed that the adsorption kinetics and isotherms were ingood agreement with pseudo-second-order equation and the Langmuirequation, respectively, and the maximum adsorption capacity was1859 mg/g for CTS-g-PAA/MMT with wt.% of 30% and weight ratio of7.2:1. The desorption studies revealed that the nanocomposite providedthe potential for regeneration and reuse after MB dye adsorption.

The adsorption using chitosan-based adsorbent for the removal ofbasic blue 3 (BB 3) from aqueous solutions was studied [72]. Thisadsorbent exhibited interesting sorption properties toward cationic dye(themaximum adsorption capacity was 166.5 mg/g), depending on thepresence of sulfonate groups. The sorptionmechanismwas amulti-stepprocess, involving adsorption on the external surface, diffusion into thebulk and electrostatic interactions. It was explained by the authors thatsulfonate groups contributed to the sorption mechanism throughelectrostatic interactions between SO3

− groups of the sorbent (whichare known as strong cation exchangers) and the cationic sites of BB 3.Addition of NaCl enhanced the performance of the adsorbent.

N-benzyl mono- and disulfonate derivatives of chitosan were usedfor the removal of dyes from aqueous solution [73]. The effectivenessof these materials in adsorbing basic blue 9 (BB 9) has been studied asa function of agitation time, initial concentration and solution salinity.Experimental results confirmed the strong cation exchanger characterof the sulfonated derivatives and showed that disulfonate derivativesof chitosan exhibited higher sorption capacities toward cationic dyethan the monosulfonic one. Sorption of BB 9 reached equilibriumwithin 20–40 min and the maximum adsorption onto disulfonatederivative was 121.9 mg/g at pH=3.

Chitosan intercalated montmorillonite (Chi-MMT) was preparedby dispersing sodium montmorillonite (Na+-MMT) into chitosansolution at 60 °C for 24 h and was examined for three basic dyes viz.basic blue 9 (BB9), basic blue 66 (BB66) and basic yellow 1 (BY1) [74].The Chi-MMT showed the highest adsorption capacity in the range of46–49 mg/g, when the initial dye concentration was 500 mg/L, beingequivalent to 92–99 wt.% of dye removal. The adsorption capacities ofChi-MMT for all basic dyes increased with an increase of initial dyeconcentration. An increase of adsorption capability of Chi-MMT wasattributed to the existence of intercalate-chitosan. It could enlarge thepore structure of Chi-MMT, facilitating the penetration of macromo-lecular dyes, and also electrostatically interact with the applied dyes.

The ability of chitosan as an adsorbent for the removal of aciddyestuff, namely, acid green 25, acid orange 10, acid orange 12, acidred 18, and acid red 73, from aqueous solution has been studied [75].The monolayer adsorption capacities were determined to be 645.1,922.9, 973.3, 693.2, and 728.2 mg of dye/g chitosan for acid green 25,acid orange 10, acid orange 12, acid red 18, and acid red 73,respectively. The differences in adsorption capacities might be due tothe effect of molecular size and the number of sulfonate groups ofeach dye. The results demonstrated that monovalent and/or smallerdyemolecules had superior adsorption capacities due to an increase inthe dye/chitosan ratio in the system. The smaller dye molecules wereable to penetrate deeper into the internal pore structure of thechitosan particles. Electrostatic attractions governed the possible aciddye adsorption mechanism onto chitosan.

The efficacy of chitosan in the form of hydrobeads to remove congored, an anionic dye, from water has been examined [76]. It wasreported that chitosan beads is a good adsorbent for the removal ofcongo red from its aqueous solution and 1 g of chitosan in the form ofhydrobeads can remove ca. 93 mg of the dye at pH 6.0. The reason forhigher adsorption of congo red on prepared chitosan in this study wasexplained due to the fact that chitosan being polycationic in natureattracted congo red, an anionic dye, and thus increased dyeadsorption. Sodium chloride and sodium dodecyl sulfate (SDS) werefound to inhibit the adsorption process. Authors reported thatphysical forces as well as ionic interaction were responsible forbinding of congo red with chitosan.

The adsorption performance of chitosan (CS) beads impregnatedwith triton X-100 (TX-100) as a nonionic surfactant and sodiumdodecyl sulfate (SDS) as an anionic surfactant was investigated for theremoval of anionic dye, congo red (CR), from aqueous solution byChatterjee et al. [77]. The Sips maximum adsorption capacity in dryweight of the CS/TX-100 beads was 378.79 mg/g and 318.47 mg/g forthe CS/SDS beads, higher than the 223.25 mg/g of the CS beads.Modification of CS beads by impregnationwith nonionic surfactant, oreven anionic surfactant, at low concentrations was found to enhanceadsorption of anionic dye. It was suggested by the authors thatadsorption of CR onto impregnated beads involved some hydrophobicinteractions between CR and surfactant molecules (TX-100 and SDS)impregnated in the beads.

The adsorption of eosin Y, as a model anionic dye, from aqueoussolution using chitosan nanoparticles prepared by the ionic gelationbetween chitosan and tripolyphosphate was examined by Du et al.[78]. The adsorption capacity was found to be 3.333 g/g. Theadsorption process was endothermic in nature with an enthalpychange (ΔH) of 16.7 kJ/mol at 20–50 °C. The optimum pH value foreosin Y removal was found to be 2–6. The dye was desorbed from thechitosan nanoparticles by increasing the pH of the solution. Thedesorption percentage was about 60% within 60 min at pH 11.0,whereas 98.5% of the dye could be eluted at pH 12 in 150 min.

The capabilities of chitosan and chitosan–EGDE (ethylene glycoldiglycidyl ether) beads for removing acid red 37 (AR 37) and acid blue25 (AB 25) from aqueous solution have also been investigated [79].Chitosan beads were cross-linked with EGDE to enhance its chemicalresistance and mechanical strength. It was shown that the adsorptioncapacities of chitosan for both acid dyes were comparatively higherthan those of chitosan–EGDE. The desorption study revealed that afterthree cycles of adsorption and desorption by NaOH and HCl, bothadsorbents retained their promising adsorption abilities. FTIR analysisproved that the adsorption of acid dyes onto chitosan-basedadsorbents was physical adsorption.

The performance of nanochitosan (with particle size range from0.0663 μm to 1.763 μm) as an adsorbent to remove acid dyes fromaqueous solution has been explored by the researchers [80]. Themonolayer adsorption capacities were determined to be 1.77, 4.33,1.37 and 2.13 mmol/g nanochitosan for acid orange 10, acid orange12, acid red 18 and acid red 73, respectively. The differences in



capacities might be due to the differences in the particle size of dyemolecules and the number of sulfonate groups on each dye molecule.The results have demonstrated that monovalent and smaller dyemolecular sizes have superior capacities due to the increase in dye/chitosan surface ratio in the system and deeper penetration of dyemolecules into the internal pore structure of nanochitosan. Themechanism of the adsorption process of acid dye on nanochitosanwasproposed to be the ionic interactions of the colored dye ions with theamino groups on the chitosan.

Poly(methylmethacrylate) grafted chitosan was found to be anefficient adsorbent for the removal of three anionic azo dyes (ProcionYellow MX, Remazol Brilliant Violet and Reactive Blue H5G) over awide pH range of 4–10 and maximum at pH 7 [81]. The adsorptioncapacity for yellow, violet and blue dyes was 250, 357 and 178 mg/g,respectively. The kinetic study results suggested that adsorptionkinetics of the dye molecules were not diffusion controlled, butchemisorption was the main sorption mechanism. The anionic dye,bearing sulfonic groups, is electrostatically attracted by protonatedamine groups of the chitosan, thus, neutralized the anionic charges ofdyes that can bind together. The removal of the dye reached amaximum at pH 4 with the complete neutralization of the anioniccharges. However, with increasing pH, deprotonation at amino grouptook place that resulted in poor interaction between the dye and thebiopolymer and therefore, the decrease in adsorption. On the otherhand, at much acidic pH (pH<4), protonation took place at thenitrogen and carbonyl groups present in the dyeswhich decreased theadsorption due to electrostatic repulsion between the same charges.However, in chitosan-graftpoly(methylmethacrylate) (Ch-g-PMMA),the dangling ester groups at the grafted chitosan appeared to bemainly responsible for the adsorption and not the amino groups aswas also evident by IR spectrum of dye loaded adsorbent. Further,other than electrostatic interaction some interaction in the form ofconformational affects might also be operative. The sensitivity to pHafter pH 4 was not seen in the graft copolymer because the majorbinding took place at PMMA grafts and only a very small amount ofNH2 were available for binding after grafting.

Several other workers also examined the potential of chitosan andits derivatives for different types of dyes removal fromwater [82–88].A summary of adsorption capacities of chitin- and chitosan-deriva-tives for different dyes has been presented in Table 2.

Readers interested in a detailed discussion of the interaction ofdyes with chitosan and its derivatives should refer to an excellentcomprehensive review by Crini and Badot [8]. It is evident from therecent literature survey that chitin- and especially chitosan and itsderivatives have been found promising adsorbents for dyes removal.Various classes of dyes have been studied so far and all these studiessuggest that chitosan shows higher potential for various dyes.However, it is necessary to continue the identification of the mostpromising types of chitosan to achieve higher sorption capacities. Themechanism of dye adsorption on chitin- and chitosan-derivativesneeds further detailed investigation as different mechanisms havebeen proposed by different researchers [8], such as surface adsorption,chemisorption, diffusion and adsorption–complexation. Further re-search is needed to gain a better understanding of adsorptionmechanism of dye adsorption on chitin- and chitosan-derivatives.Such studies would provide a better understanding of adsorptionphenomenon involve in the uptake of a given dye [8].

2.3. Chitin- and chitosan-derivatives for phenols removal

Among the various aqueous pollutants generally present inwastewaters, phenol and substituted phenols are considered aspriority pollutants. Phenols cause unpleasant taste and odor ofdrinking water and can exert negative effects on different biologicalprocesses. The ubiquitous nature of phenols, their toxicity even intrace amounts and the stricter environmental regulations make it

necessary to develop processes for the removal of phenols fromwastewaters. Chitin- and chitosan-derivatives have also been inves-tigated for phenol and substituted phenols removal from water and

Table 2Adsorption capacities of chitin and chitosan-derivatives for various dyes removal fromwater.


Reference

1. Chitosan flakes Reactive red 222 293.0 mg/g [58]2. Chitosan beads Reactive red 222 1103.0 mg/g [58]3. Cross-linked

chitosan beadsReactive red 189 1642–

1936mg/g[60]

4. Chitosan-ECH Reactive red 189 1802–1840 g/kg

[61]

5. Cross-linkedchitosan beads

Four reactive dyes,three acid dyesand one direct dye

1911 –

2488 mg/g[62]

6. Chitin Reactive yellow 2 38.0 mg/g [63]7. Chitin Reactive black 5 65.0 mg/g [63]8. Chitin Synthetic reactive

dye wastewater133–167mg/g [64]

9. Chitin modifiedby sodium hypochlorite

Synthetic reactivedye wastewater

59–124 mg/g [64]

10. Quaternary chitosansalt (QCS)

Reactive orange 16 1060.0 mg/g [65]

11. Chitosan Remazol black 13 91.47–130.0 mg/g

[66]

12. Chitosan/amino resin Reactive black 5 0.63 mmol/g [67]13. Chitosan bearing both

amine and quaternaryammonium chloridemoieties

Reactive black 5 0.78 mmol/g [67]

14. Powdered cross-linkedchitosan grafted withacrylic acid

Remacryl Red TGL 204.2–510.7 mg/g

[69]

15. Chitosan bead Malachite green 74.83–93.55 mg/g

[70]

16. Chitosan-g-poly(acrylic acid)/montmorillonite(CTS-g-PAA/MMT)nanocomposites

Methylene blue 1859.0 mg/g [71]

17. Chitosan-basedadsorbent

Basic blue 3 166.5 mg/g [72]

18. Disulfonate derivativesof chitosan

Basic blue 9 121.9 mg/g [73]

19. Chitosan Acid green 25 645.1 mg/g [75]20. Chitosan Acid orange 10 922.9 mg/g [75]21. Chitosan Acid orange 12 973.3 mg/g [75]22. Chitosan Acid red 18 693.2 mg/g [75]23. Chitosan Acid red 73 728.2 mg/g [75]24. Chitosan Congo red 93.0 mg/g [76]25. Chitosan

nanoparticlesEosin Y 3.33 g/g [78]

26. Nanochitosan Acid orange 10 1.77 mmol/g [80]27. Nanochitosan Acid orange 12 4.33 mmol/g [80]28. Nanochitosan Acid red 18 1.37 mmol/g [80]29. Nanochitosan Acid red 73 2.13 mmol/g [80]30. Poly(methylmethacrylate)

grafted chitosanProcion Yellow MX 250.0 mg/g [81]

31. Poly(methylmethacrylate)grafted chitosan

Remazol BrilliantViolet

357.0 mg/g [81]

32. Poly(methylmethacrylate)grafted chitosan

Reactive BlueH5G

178.0 mg/g [81]

33. Chitin Indigo carmine 1.24±0.16×10−5mol/g

[85]

34. Chitosan Indigo carmine 1.54±0.03×10−4mol/g

[85]

35. Cross-linked chitosanbeads

Metanil yellow 1334.0 mg/g [87]

36. Cross-linked chitosanbeads

Reactive blue 15 722.0 mg/g [87]

37. Chitosan grafted withamide groups

Remazol YellowGelb 3RS

1211.0 mg/g [88]

38. Chitosan grafted withcarboxyl groups

Basic yellow 37 595.0 mg/g [88]



wastewater. The equilibrium and kinetics of chlorophenols sorptionby chitosan, polyD-glucosamine, were studied under simulatedgroundwater conditions [89]. Comparison between two types ofchitosan, flakes and highly swollen beads, demonstrated that themaximum pentachlorophenol (PCP) uptake capacities depend on thespecific surface area of the particles. Out of the five kinds ofchlorophenols, i.e. 2,4,6-trichlorophenol (2,4,6-TCP), 3,4-dichloro-phenol (3,4-DCP), 2,3-dichlorophenol (2,3-DCP), 2,6-dichlorophenol(2,6-DCP) and 3-monochlorophenol (3-MCP), flake-type chitosanexhibitedmaximum sorption efficiency for TCP, followed by DCPs, andfinally MCP (the three kinds of DCP, with the same elementalcompositions, were sorbed equally).

The adsorption of phenol onto chitin was studied as a function ofinitial pH, temperature and initial phenol concentration [90]. The pHprimarily affected the degree of ionization of the phenol and thesurface properties of chitin. The functional groups of chitin will beprotonated at low-pH values and resulted in a stronger attraction fornegatively charged ions in the adsorption medium. Phenol beingweakly acidic would be partially ionized in solution. These ions wouldbe negatively charged and would be directly attracted due toelectrostatic forces by the protonated amino groups of the chitin. Asthe pH increased, the overall surface charge of the chitin becamenegative and adsorption decreased. The equilibrium uptake of phenolby chitin was also affected by temperature which was attributed tothe enlargement of pore size or creation of some new active sites onthe adsorbent surface due to bond rupture. The highest phenoladsorption capacity was determined as 21.5 mg/g for 300 mg/L initialphenol concentration at pH 1.0 and 40 °C.

The application of magnetite-immobilized chitin for pentachlorophe-nol (PCP) removal was demonstrated by Pang et al. [91]. For chitinimmobilization, the optimized conditionswere:magnetite to chitin (m:c)ratio at 1:2, initial pH 6, 25 °C, 200 rpm and 60 min in batch system. Theimmobilization efficiency (IE) was 99.4% and immobilization capacity(IC) was 2.0 mg chitin/mg magnetite. High initial pH (pH>11) andtemperature (>30 °C) lowered the IE and IC. For PCP (10 mg/L) adsorp-tion, the optimized conditionswere: 1500 mg/L immobilized chitin, initialpH 6, 25 °C, 200 rpm and 60 min in batch system. The removal efficiency(RE) was 57.9% and removal capacity (RC) was 5.4 mg/g. The adsorptionability of immobilized chitin decreased with pH and increased withtemperature. However, increasing the amount of immobilized chitin(24,000 mg/L) increased the RE up to 92%.

Beads of chitosan–sodium alginate were prepared from chitosanand sodium alginate (an anionic polysaccharide) [92]. These beadswere treated with CaCl2 in order to improve the stability as well as thesorption capacity of the biosorbent. It was reported that the percentremoval of phenol and o-chlorophenol increased rapidly withincrease in the dose of chitosan–calcium (CS/Ca) alginate blendedbeads due to the greater availability of binding sites of the biosorbent.The maximum Langmuir monolayer capacity was reported as108.69 mg/g for phenol and 97.08 mg/g for o-chlorophenol. Basedon the thermodynamic values, the adsorption processes wasassociated with chemical ion-exchange mechanism. The fixed bedcolumns of CS/Ca alginate beads saturated with phenol or chlor-ophenol was regenerated by passing 0.1 MNaOH solution as an eluentat a fixed flow rate of 1 mL/min. Further, results from the limitednumber of column adsorption–desorption cycle indicated that theadsorption capacity of the CS/Ca alginate beads decreased in thesecond and third adsorption–desorption cycles.

Functional chitosan, chemicallymodified by salicylaldehyde (CS–SA),β-cyclodextrin (CS–CD), and a cross-linked β-cyclodextrin polymer(EPI–CD) were prepared as adsorbents to remove phenol, p-nitrophenoland p-chlorophenol from aqueous solution [93]. It was found that theadsorption capacity of phenols onto chitosan was very small (1.98–2.58 mg/g). However, functional chitosan chemically modified by β-CDor salicylaldehyde exhibits much better adsorption ability for phenols.CS–CD (20.56–179.73 mg/g) and EPI–CD (41.11–131.50 mg/g) exhibit

outstanding adsorption efficiency for these phenols. The adsorptionability of CS–CD was better than EPI–CD. CS–SA presented goodadsorption ability for phenols. The adsorption of phenols onto EPI–CD,CS–CD and CS–SA was predominated by hydrophobic interaction,hydrogen bonding and π–π interaction, respectively. The low temper-ature was found favorable for the adsorption of phenols. Ethanol wasused forphenols desorption fromadsorbent and80–94.2%adsorbatewasremoved from the adsorbent. The regenerated adsorbent could adsorb69.4–78.9% adsorbate comparedwith the first adsorption capacity, and itcould be reused six times.

Laccase from Coriolus versicolor was immobilized on chitosan usingglutaraldehyde as a cross-linking agent [94]. After immobilization,laccase retained 52.2% of its original activity and was used to study 2,4-dichlorophenol (2,4-DCP) removal from aqueous solutions. Favorablecross-linking of laccase with chitosan occurred using 5% glutaraldehydefor 8 h. The ratio of laccase to cross-linked chitosan was 20 mg/g after6 h of reaction. The recovery of immobilized laccase activity was 52.2%.The optimum pH for immobilized laccase was 4.5, which was less thanthe optimum pH for the free enzyme (5.0). The immobilized enzymealso demonstrated greater stability at room temperature over time.When used to remove 2,4-DCP from wastewater, the optimum condi-tions for the immobilized enzyme were a pH of 5.5 and a temperaturerange of 35–45 °C. The immobilized enzyme retained removal efficiencyabove 50% for up to six usages.

Chitosan–abrus precatorius (CS/Ab) blended beads were used asadsorbent for the removal of phenolic compounds from aqueoussolution [95]. The maximum monolayer adsorption capacity of phenol,2-chlorophenol and 4-chlorophenol on to the (CS/Ab) beads was foundto be 156 mg/g, 204 mg/g and 278 mg/g, respectively. A summary ofadsorption capacities of chitin- and chitosan-derivatives for differentphenols is presented in Table 3.

As compared to metal ions and dyes, there are comparatively lessreports available, demonstrating the usefulness of chitin- and chitosanand its derivatives for the removal of phenols. Most of these studies arelimited only to chloro- or nitro-phenols. Additionally, chitin-, chitosanand its derivatives show low affinity for phenols removal and there is astrong need to conduct extensive research to enhance the removalefficiencies/adsorption capacities of these biosorbents for differentphenols after appropriate treatment. Furthermore, the mechanism ofphenols adsorption on chitin- and chitosan-derivatives also needs to bestudied in detail as most of the articles focused only on the adsorptionpotential (adsorption capacity) of chitin- and chitosan-derivatives forphenols removal and little efforts have been made to elucidate thesorption mechanism.

2.4. Chitin- and chitosan-derivatives for anions removal

Inorganic anions are one of the important classes of aquatic pollutantsand various inorganic anions have been found in potentially harmfulconcentrations innumerous drinkingwater sources. The removal of thesepollutants from drinking water supplies is an emerging issue. In recentyears, chitin- and chitosan-derivatives have been successfully utilized forsome anions removal from water. The adsorption of nitrate by chitosanhydrobeads was examined by Chatterjee and Woo [96]. The maximumadsorption capacitywas 92.1 mg/g at 30 °C. Intraparticle diffusion playedsignificant role at the initial stage of the adsorption process. Nitrateadsorptionwas found to increasewith a decrease in the pHof the solutionwhich was explained due to the fact that a decrease in the pH of thesolution resulted in more protons being available to protonate thechitosan amine group. This resulted in an enhancement of nitrateadsorption by chitosan beads due to increased electrostatic interactionsbetween negatively charged nitrate group and positively charged aminegroup. Above pH 6.4, an appreciable amount of nitrate adsorption bychitosanbeads indicated the involvementofphysical forces.Desorptionofnitrate from the loaded beads was accomplished by increasing the pH of



the solution to the alkaline range, and a desorption ratio of 87% wasachieved around pH 12.0.

The adsorption of nitrate onto chitosan beads modified by cross-linking with epichlorohydrin (ECH) and surface conditioning withsodium bisulfate was investigated by Chatterjee et al. [97]. Themaximumadsorption capacitywas found at a cross-linking ratio of 0.4and conditioning concentration of 0.1 mM NaHSO4. The maximumadsorption capacity was 104.0 mg/g for the conditioned cross-linkedchitosan beads at pH 5, while it was 90.7 mg/g for normal chitosanbeads. The nitrate adsorption was found strongly pH dependent, andthe maximum nitrate removal was found at pH 3. The high adsorptioncapacities in acidic solutions (pH 3–5) were due to the strongelectrostatic interactions between its adsorption sites and nitrate.

The applicability of neodymium-modified chitosan as adsorbentsfor the removal of excess fluoride ions from water was studied by Yaoet al. [98]. The effect of various physico-chemical parameters such astemperature (283–323 K), pH (5–9), adsorbent dose (0.2–2.0 g/L),particle size (0.10–0.50 mm) and the presence of co-anions (NO3

−, Cl−

and SO42−) on removal of fluoride ions were studied. The maximum

equilibrium sorption was 11.4–22.3 mg/g. The sorption process wasfound to be complex, and both the boundary of liquid film andintraparticle diffusion contributed to the rate-determining step. Theused adsorbents could be regenerated in 24 h by 4 g/L of sodiumhydroxide.

A novel adsorbent namely magnesia/chitosan (MgOC) compositewas prepared to remove fluoride from drinking water by Sundaram etal. [99]. The maximum Langmuir monolayer capacity was found ca.11 mg/g. The mechanism of fluoride removal of both MgO and MgOCcomposite was mainly governed by adsorption. The higher defluor-idation capacity (DC) of MgOC composite might be attributed to thefact that chitosan also contributed in the enhancement of DC byremoving fluoride through hydrogen bonding. It was suggested thatMgOC composite being biocompatible, biodegradable, cost effective,shaped into any desired form which possesses higher defluoridationcapacity with minimum contact time, can be used as a promisingsorbent for fluoride removal.

The metal-binding property of chitosan was used to incorporatetitanium (Ti) metal and applied as an adsorbent for fluoride adsorptionby Jagtap et al. [100]. Ti loading was varied from 5 to 50 wt.%. It wasobserved that an increase in titanium loading from 5 to 15% improved

fluoride removal from 61 to 89%. A 15% Ti loading was optimum as itshowed the highest fluoride removal and reduces the fluoride level topermissible limits. Further increasing titanium loading showed anegative effect onfluoride removal and thismight be due to overlappingof active sites. ThemaximumLangmuirmonolayer capacitywas found tobe 7.2 mg/g. Thefluoride uptakewasmaximumat pH7 anddecreased inacidic and alkaline pH. The presence of co-existing anions has a negativeeffect on fluoride adsorption. The adsorbent was found to have very fastkinetics in thefirst 30 min and then the rate sloweddownas equilibriumwas approached. A comparison of fluoride removal in simulated andfield water shows a high adsorption capacity in simulated water.

Protonated chitosan beads (PCB) were examined for fluorideremoval by Viswanathan et al. [101]. Sorption process was found to beindependent of pH and altered in the presence of other co-existinganions. The maximum Langmuir monolayer capacity was found in therange of 4.72–7.32 mg/g. The sorption process followed pseudo-second-order and intraparticle diffusion kinetic models. The adsorp-tion mechanism was explained on the basis of FTIR analysis, where aslight widening of –NH2 stretching band in FTIR spectra of the fluoridesorbed PCB was observed which confirmed the presence of hydrogenbonding between protonated amine (NH3

+) and fluoride, suggestingthat the PCB removed fluoride by means of hydrogen bonding whichwas due to electrostatic interactions between positively chargedsurface and negatively charged fluoride ions. 0.1 M HCl was identifiedas the best eluent. Field trial results indicated that PCB could beemployed as a best sorbent for fluoride removal.

The same researchers also prepared multifunctional chitosan beadsafter chemical modification by introducingmultifunctional groups, viz.,NH3

+ and COOH groups by means of protonation and carboxylation inorder to utilize both amine and hydroxyl groups for fluoride removal[102]. The protonated cum carboxylated chitosan beads (PCCB) showeda maximum DC of 1800 mg F−/kg, whereas, raw chitosan beadsdisplayed only 52 mg F−/kg adsorption capacity. The mechanism offluoride removal by PCCB was explained by H-bonding.

The fluoride adsorption potential of novel nano-hydroxyapatite/chitin (n-HApCh) composite was explored by Sundaram et al. [103].The effect of pH, interfering anions and contact time were examined.n-HApCh composite possesses higher defluoridation capacity (DC) of2840 mg F−/kg than nano-hydroxyapatite (n-HAp) which showed aDC of 1296 mg F−/kg. The enhancement in DC of n-HApCh compositeover n-HAp was explained due to biosorption by chitin, adsorption byphysical forces and fluoride ion entrapped in fibrilliar capillaries andspaces of polysaccharide network of the chitinmoiety. Recently, Xie etal. [104] investigated the removal of perchlorate from aqueoussolution by protonated cross-linked chitosan. The maximum

Table 3Adsorption capacities of chitin and chitosan-derivatives for various phenols removalfrom water.

S.no.

Adsorbent Adsorbate Adsorptioncapacity

Reference

1. Chitin Phenol 21.5 mg/g [90]2. Magnetite-immobilized chitin Pentachlorophenol 5.4 mg/g [91]3. Beads of chitosan–

sodium alginatePhenol 108.6 mg/g [92]

4. Beads of chitosan–sodium alginate

o-chlorophenol 97.0 mg/g [92]

5. Chitosan Phenol,p-nitrophenol andp-chlorophenol

1.9–2.5 mg/g

[93]

6. Functional chitosan, chemicallymodified by β-cyclodextrin(CS–CD)

Phenol,p-nitrophenol andp-chlorophenol

20.5–179.7 mg/g

[93]

7. Functional chitosan, chemicallymodified by cross-linkedβ-cyclodextrin polymer(EPI–CD)

Phenol,p-nitrophenol andp-chlorophenol

41.1–131.5 mg/g

[93]

8. Chitosan–abrus precatorius(CS/Ab)

Phenol 156.0 mg/g [95]


2-chlorophenol 204.0 mg/g [95]


4-chlorophenol 278.0 mg/g

[95]

Table 4Adsorption capacities of chitosan-derivatives for different anions removal.


Reference

1. Chitosan hydrobeads Nitrate 92.1 mg/g [96]2. Chitosan beads modified

by cross-linking withepichlorohydrin (ECH)and surface conditioningwith sodium bisulfate

Nitrate 104.0 mg/g [97]

3. Neodymium-modifiedchitosan

Fluoride 11.4–22.3 mg/g [98]

4. Magnesia/chitosan(MgOC) composite

Fluoride 11.0 mg/g [99]

5. Titanium-incorporatedchitosan

Fluoride 7.2 mg/g [100]

6. Protonated chitosanbeads (PCB)

Fluoride 4.7–7.3 mg/g [101]

7. Nano-hydroxyapatite/chitin (n-HApCh)

Fluoride 2840.0 mg/kg [103]

8. Protonated cross-linked chitosan Perchlorate 45.45 mg/g [104]



monolayer adsorption capacity was found to be 45.45 mg/g. Thepresence of other anions inhibited the perchlorate adsorption. Theadsorbent was well regenerated by sodium hydroxide solution withpH 12 and reused for about 15 cycles. Electrostatic attraction aswell asphysical forces was suggested as the main driven forces forperchlorate adsorption. A summary of adsorption capacities of chitin-and chitosan-derivatives for different anions is provided in Table 4.

Recent studies have shown that chitosan and its derivatives can besuccessfully applied for the removal of anions (especially fluoride andnitrate). However, as mentioned previously, it is necessary tocontinue identification of the most promising types of chitosan foranions removal. Additionally, mechanistic studies with anions need tobe performed in detail to propose the binding mechanism. There is, asyet, little information in the literature on this topic, which needs to beexplored more in detail.

2.5. Chitin- and chitosan-derivatives for miscellaneous pollutants removal

Two types of chitosan microparticles (CMs) and their silver-complex CMs (SCMs) were prepared using different cross-linkingagents, i.e. glutaraldehyde and epichlorohydrin, in order to investigatethe adsorption and release behaviors of a typical pesticide, methylparathion (MP) [105]. Themaximum adsorption capacities of Ag(I) onglutaraldehyde-cross-linked CM (CM # 1) and epichlorohydrin-cross-linked CM (CM # 2) were found to be 140 mmol/g and 290 mmol/g,and those of MP on SCM # 1 and SCM # 2 were 180 mmol/g and60 mmol/g, respectively. It was also confirmed that SCM# 1 has betterabsorptivity for MP than SCM # 2, in spite of its lower adsorptioncapacity of Ag(I). The ratios of MP/Ag adsorbed on the SCMs are 1.3and 0.2 for SCM # 1 and SCM # 2, respectively.

The removal of two fungicides, 4,4′-iso-Propylidene diphenol(BPA), and diphenylolpropane 4,4′-dioxyaceticacid (BPAc) has beeninvestigated by Şişmanoğlu, [106]. The Langmuir maximum mono-layer coverage was reported to be 4.30×10−3 mol/g for BPA and2.62×10−3mol/g for BPAc. The pseudo-first-order model andintraparticle diffusion were used to describe the adsorption behaviorof BPA and BPAc onto chitin.

The ability of chitin and chitosan to adsorb humic acid has beenstudied [107]. The adsorption capacities of humic acid at roomtemperature (27 °C) were 27.30 mg/g for chitin and 28.88 mg/g forchitosan. It was suggested that chitin and chitosan can be utilized toremove humic acid.

Chitosan was successfully coated on PET granules through a dipand phase inversion process and the coated granules were examinedfor the performance and mechanism of humic acid removal through aseries of batch adsorption tests [108]. Chitosan-coated granules werefound to have positive zeta potential values at pH<6.6, mainly due tothe protonation of the amino groups in chitosan. Adsorption of humicacid onto chitosan-coated granules was pH dependent and significantamounts of humic acid could be adsorbed under acidic and neutral pHconditions. The adsorption process involved a two-step process:protonation of the amino groups in chitosan, followed by attachmentof humic acid onto the protonated amino sites on the surface. Underlow-pH conditions, the adsorption process was transport-controlled,but under high-pH conditions, both transport and attachment playedan important role in humic acid adsorption onto chitosan-coatedgranules. While chitosan has been widely studied for metal removalfrom aqueous solution, it is also possible to extend the application ofchitosan as an adsorbent to remove charged organic compound suchas humic acid from water and wastewater.

The effects of pH and ionic strength on the adsorption capacity forfulvic acid (FA) by chitosan hydrogel beads were examined [109]. Themaximum adsorption capacity of FA on chitosan hydrogel beads was1.67 mg/g at pH 6.0. FTIR along with XPS analyses revealed that theamine groups on the beadswere involved in the sorption of FA and theorganic complex between the protonated amino groups and FA was

formed after FA uptake. Electrostatic interaction and surface com-plexation were found to be involved in the complex sorption of FA onthe chitosan hydrogel beads.

Chitin and chitosan have also been examined for the removal ofendocrine disrupting chemicals, estrone (E1) and 17β-estradiol (E2)fromwater [110]. It was reported that it took about 1 and 2 days for E1to achieve the equilibration onto chitin and chitosan, respectively. Thelimited adsorption capacity was observed by chitin and chitosan forestrogenic compounds. The authors reported that chitin was slightlymore effective than chitosan in adsorbing E1, suggesting that theprocess is primarily adsorption rather than ion exchange in nature.

The feasibility of chitosan, with different molecular weights, tosimultaneously remove various pollutants from the discharge of an eelculture pond was evaluated [111]. Experimental results indicated thatchitosan with high molecular weight was best in removing turbidity,suspended solids, and biological and chemical oxygen demand (BODand COD). In contrast, chitosan of low molecular weight excelled atremoving NH3 and PO4

3− from wastewater. Additionally, chitosan withhigh molecular weight did well at eliminating suspended solids ofvarious particle sizes relative to chitosan with low molecular weight.The best removal percentage achieved by chitosan for removingturbidity, suspended solids, BOD, COD, NH3, PO4

3−, and bacteria was87.7%, 62.6%, 52.3%, 62.8%, 91.8%, 99.1%, and 99.998% removal,respectively. When chitosan with a high molecular weight was addedat 12 mg/L, the quality of treated wastewater successfully compliedwith government discharge standards.

3. Conclusions and future perspectives

This review focuses on the recent developments related todetoxification of water and wastewater using chitin- and chitosan-derivatives and reports the main advances published over the last 10–15 years. It should be noted that the maximum adsorption capacitiesreported in this paper provide some idea of sorbent effectiveness foreach type of pollutant, andmainly depends on experimental conditions.The use of chitin-, chitosan and its derivatives for removing variouspollutants from water and wastewater presents many attractivefeatures such as the outstanding adsorption capacity, especially formetal ions and dyes, and the fact that thesematerials are low cost, non-toxic and biocompatible. However, their potential for other pollutants,e.g. phenols, anions, pesticides, humic substances needs extensiveresearch. Although the amount of available literature data for chitin/chitosan application inwater andwastewater treatment is increasing ata tremendous pace, there are still several gaps which need to be filled.Some of the important issues can be summarized below:

(1) Selection and identification of an appropriate form of chitin/chitosan is one of the key issues to achieve the maximumremoval/adsorption of specific type of pollutant dependingupon the adsorbent–adsorbate characteristics.

(2) The conditions for the production of chitosan loaded with highamino groups on its surface need to be optimized, which conse-quently would increase the maximum removal of pollutants.

(3) Cost factor should not be ignored. Low production cost withhigher removal efficiency would make the process economicaland efficient.

(4) Mechanistic studies with organic pollutants (phenols and dyes)and inorganic anions need to be performed in detail to proposea correct binding mechanism of these pollutants with chitin-and chitosan-derivatives.

(5) Regeneration studies need to be performed in detail with thepollutants-laden adsorbent (chitin- and chitosan-derivatives)to recover the metals as well as adsorbent. It will enhance theeconomic feasibility of the process.

(6) The potential of chitin/chitosan-derivatives under multi-com-ponent pollutants needs to be assessed. This would make a



significant impact on the potential commercial application ofchitosan to industrial systems.

(7) There is scarce data available for the adsorption ofmetal ions in thepresence of phenols, dye and other contaminants and vice-versa.Therefore, more research should be conducted in this direction.

(8) It is further suggested that the research should not limit to onlylab scale batch studies, but pilot-plant studies should also beconducted utilizing chitin- and chitosan-derivatives to checktheir feasibility on commercial scale.

(9) The effectiveness of the treatment depends not only on theproperties of the adsorbent and adsorbate, but also on variousenvironmental conditions and variables used for the adsorptionprocess e.g., pH, ionic strength, temperature, existence ofcompeting organic or inorganic compounds in solution, initialadsorbent concentration, contact time and speed of rotation etc.These parameters should be taken into account while examiningthe potential of chitin/chitosan-derivatives.

(10) The development in the field of adsorption process using chitin/chitosan-derivatives essentially requires further investigation oftesting these materials with real industrial effluents.

If it is possible to develop such adsorbents having all the above-mentioned characteristics, then these adsorbentsmay offer significantadvantages over currently available commercially expensive activatedcarbons and, in addition contribute to an overall waste minimizationstrategy.

Acknowledgments

The authors are grateful to the European Commission (Brussels)for the Marie Curie Research Fellowship for Transfer of Knowledge(No. MTKD-CT-2006-042637).

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Applications of chitin- and chitosan-derivatives for the detoxification of water and wastewater —...

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