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Journal of Environmental Management 92 (2011) 2675e2682

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Binding of heavy metal contaminants onto chitosans e An evaluationfor remediation of metal contaminated soil and water

A. Kamari 1, I.D. Pulford*, J.S.J. HargreavesWestCHEM, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK

a r t i c l e i n f o

Article history:Received 20 December 2010Received in revised form12 May 2011Accepted 3 June 2011Available online 25 June 2011

Keywords:Contaminated soilContaminated waterChitosanCross-linked chitosansBinding efficiency

* Corresponding author. Tel.: þ44 0141 330 5950; fE-mail address: Ian.Pulford@glasgow.ac.uk (I.D. Pu

1 Present address: Department of Chemistry, FacultyUniversiti Pendidikan Sultan Idris, 35900 Perak, Mala

0301-4797/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.jenvman.2011.06.005

a b s t r a c t

The binding efficiency of chitosan samples for Agþ, Cd2þ, Cu2þ, Pb2þ and Zn2þ has been evaluated inorder to consider their application to remediate metal contaminated soil and water. The sorptionbehaviour of metal ions was assessed using a batch technique at different contact time and initial metalconcentration with different background electrolytes. The kinetics followed a pseudo-second-ordermodel, while the equilibrium data correlated well with the Freundlich and Langmuir isotherm models.For example, the maximum sorption capacity (Q) for chitosan was estimated as 1.93 mmol/g for Agþ,1.61 mmol/g for Cu2þ, 0.94 mmol/g for Zn2þ, 0.72 mmol/g for Cd2þ and 0.64 mmol/g for Pb2þ. Covalentinteraction between metal ions and functional groups (amino and hydroxyl) of the chitosans was themain binding mechanism. Ion exchange is not an important process. Chitosan and cross-linked chitosanswere able to bind metal ions in the presence of Kþ, Cl� and NO3

�. The nature of Cl� and NO3� ions did not

affect Zn2þ binding by the chitosans. Even at 11x dilution, the chitosans were able to retain metal ions ontheir surfaces.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Toxic heavy metals are of concern due to their harmful effectsand long-term persistence in the environment. They enter theenvironment through a variety of anthropogenic activities and posea serious threat to human health. Heavy metals in soil may enterthe food chain through plants (Liphadzi and Kirkham, 2006). Angand Ng (2000) studied metal contamination in mango, guava andpapaya grown on ex-mining land in Malaysia and found that theconcentrations of lead and zinc in the fruits exceeded theMalaysianFood Act permissible limits. In fact, the lead content in guava wasfound to be 17 fold greater than that permitted.

Several techniques are available to remediate water and soilcontaminated by heavy metals, but many are costly. Therefore,effective and economical techniques are needed to remediatemetalcontaminated soil and water. Recently, the use of waste-derivedmaterials in this context has received much attention due to theirlow cost and ready availability in large quantities. The attraction ofthis remediation technique is that it reduces the cost of wastedisposal and converts wastes into useful materials for soil andwater decontamination.

ax: þ44 0141 330 4888.lford).of Science and Mathematics,ysia.

All rights reserved.

Metal immobilising soil amendments induce various processessuch as adsorption, surface precipitation, formation of stablecomplexes with organic ligands and ion exchange, resulting inreduced metal mobility and bioavailability (Kumpiene et al., 2008).According to Basta et al. (2005), amendmentswith functional groupssuch as amino, hydroxyl and carbonyl groups are particularly effec-tive at immobilising metal due to their ability to bind or complexmetal. Forexample, theabilityof spentmushroomcompost to reducecadmium, lead and zinc accumulation in perennial ryegrass estab-lished on lead-zinc tailings was related to the presence of hydroxyl,phosphoryl and phenolic groups (Jordan et al., 2009). Meanwhile,iron oxides in red-mud and furnace slag played an important role inreducing cadmium, lead and zinc uptake by lettuce (Lee et al., 2009).

Chitosan, poly-b(1/4)-2-amino-2-deoxy-D-glucose, is derivedfrom chitin - a natural biopolymer found in the outer shell ofcrustaceans such as crabs, shrimps and prawns. It comprises aminoand hydroxyl functional groups that can serve as binding sites formetals. Previous studies have shown that chemical modification onchitosan improves metal uptake. In the literature, much attentionhas been paid to the use of composites of chitosan with materialssuch as clays, polymers and minerals (Wan Ngah et al., 2011), withsome studies on cross-linked chitosans (Crini and Badot, 2008).Many of these studies report binding behaviour of single metal ionsonto chitosan and/or its derivatives or composites. This study wasundertaken to investigate the behaviour of metals with differentbinding properties onto chitosan and cross-linked chitosans.

A. Kamari et al. / Journal of Environmental Management 92 (2011) 2675e26822676

The ultimate aim of this work is to use chitosans as immobilisingmaterials to remediate heavy metal contaminated soil and water.Previously (Kamari et al., 2011), we examined the physical andsurface properties of chitosan as affected by cross-linking treat-ment. Changes in the physical and surface properties of chitosansfollowing metal interactionwere also investigated. The second partof thework, evaluation ofmetal binding capacity of these chitosans,is described here.

2. Materials and methods

2.1. Chitosan and cross-linking treatment

Chitosan, prepared from crab shells, was purchased from Sig-maeAldrich. The deacetylation degree of chitosan was 87%, asdescribed elsewhere (Kamari et al., 2011). ANALAR grade reagentswere used throughout unless otherwise stated.

Chitosan was cross-linked using glutaraldehyde (GLA), epich-lorohydrin (ECH) and ethylene glycol diglycidyl ether (EGDE) ata mole ratio of 1:1 (NH2 group of chitosan:CHO or CH2O group ofcross-linking reagents), and the physical and surface properties ofthese chitosans were reported in our earlier work (Kamari et al.,2011).

2.2. Preparation of metal solutions

Metal solutions were prepared using AgNO3 (Sigma),CdCl2.2.5H2O (Hopkin & Williams), CuCl2.2H2O (Fluka), Pb(NO3)2(Riedel-deHaën) and ZnCl2 (Sigma) salts. 0.1 mol/L KCl and KNO3were used as electrolytes to mimic the soil solution and to controlthe ionic strength (Kamari, 2011). KCl and KNO3 (Fischer Scientific)solutions were prepared in deionised water. The Agþ and Pb2þ

solutions were made up with 0.1 mol/L KNO3, while 0.1 mol/L KClwas used to prepare Cd2þ, Cu2þ and Zn2þ solutions.

2.3. Metal binding capacity

Sorption studies were conducted by shaking 0.1 g of each sorbentwith 50 mL of each of the metal ion solutions (5e500 mg/L) ina 60 mL screw top glass jar. The suspension was agitated on an end-over-end shaker (30 rpm) at room temperature for 90min (chitosan)and 180min (cross-linked chitosans). For the kinetics study, a singlemetal concentration (500 mg/L) was used and the mixture wasshaken for 15, 30, 45, 60, 90, 120, 150, 180, 300, 600 and 1440 min.

After shaking, the sample was filtered through Whatman No. 2filter paper (125 mm). The filtrate was collected in a plastic bottleand stored at 276e277 K until analysis, while the sorbent loadedwith metal was left to dry on the filter paper. Dried spent sorbentwas collected for analysis. Metal solution without sorbent was alsoshaken for 180 min, filtered and analysed to give an accuratemeasure of the initial metal concentration. The initial and finalmetal concentrations were determined by a PerkineElmer AAna-lyst 400 Atomic Absorption Spectrometer (AAS) using an air-acetylene flame. The sorption capacity, q (mg/g) was calculatedaccording to Eq. (1):

q ¼�Co � Cf

�V

W(1)

where, Co and Cf are the initial and final metal concentrations,respectively. V is the volume of metal solution, and W is the weightof sorbent.

The pHof the solutionwasmeasured using aMettler Delta 320 pHmeter,fittedwith a combinedglass-reference electrode. ThepHof themetal solutions was in the range of 4.7e5.7, which did not change

muchwithdilution. Previous studieshave reported that the optimumsorption of Agþ, Cd2þ, Cu2þ, Pb2þ and Zn2þ from aqueous solutionstook place at pH range 3.0e6.0 (Matos and Arruda, 2003; Ajmal et al.,2006; Chen and Wang, 2008). In addition, the removal of metal ionsby precipitation as the hydroxide occurs under alkaline conditions(Amarasinghe andWilliams, 2007). Therefore, sorption experimentswere carried out without adjusting the pH of the solution.

Triplicate measurements were carried out and metal free blankswere used as controls. Statistical analyses were carried out usingMinitab 15 Statistical Software (Minitab Inc., PA, USA). The datawere analysed using the general linear model of one-way analysisof variance (ANOVA), followed by Tukey’s test at a significance levelof p ¼ 0.05 to determine least significant difference (LSD) for thecomparison of means.

2.4. Effect of background electrolytes

The influence of different background electrolytes on metalbinding by the samples was investigated. Stock solutions of 100mg/L Zn2þwere prepared using ZnCl2 and Zn(NO3)2.6H2O (BDH) salts ineither 0.1 mol/L KCl or KNO3 electrolyte solution. The Zn2þ sorptiontrials were set up as: Set A (ZnCl2 þ KCl), Set B (Zn(NO3)2.6H2O þKNO3), Set C (ZnCl2 þ KNO3) and Set D (Zn(NO3)2.6H2Oþ KCl). Zn2þ

solutionwas added to 0.1 g of sorbent in a glass jar. Themixturewasshaken and filtered as described in Section 2.3.

2.5. Sorption-desorption study

Replicate samples of 0.02 g of each sorbent were treated sepa-ratelywith10mLof100and500mg/Lmetal solutions in60mLscrewtop glass jars and shaken for 24h on an end-over-end shaker at roomtemperature. After 24 h, samples were diluted with either 0.1 mol/LKCl or KNO3 at different dilution factors, i.e., 0, x1.5, x2.0, x3.5, x6.0,x8.5 and x11.0. Samples were then shaken for a further 24 h fordesorption to occur. The suspensions were filtered through What-manNo. 2 filter paper into plastic bottles and analysed by AAS. Theseexperiments were conducted in triplicate and controls were run.

2.6. Characterisation

2.6.1. FTIR analysisIR analysis was conducted on a Jasco 4100 Fourier Transform

Infrared (FTIR) Spectrometer using the KBr disc technique, asdescribed elsewhere (Kamari et al., 2011).

2.6.2. SEM analysisThe surface morphology of the sorbents was recorded at

different magnifications using a Philips XL 30 EnvironmentalScanning Electron Microscope (ESEM), as described elsewhere(Kamari et al., 2011).

3. Results and discussion

3.1. Effect of contact time

The time profiles of metal ions sorption by chitosan andchitosan-EGDE, as an example of a cross-linked chitosan, are pre-sented in Fig. 1. The amount of metal ions sorbed increased withcontact time before plateauing, implying equilibrium has beenreached.With an initial concentration of 500mg/L, it was estimatedthat 43% of Agþ, 24% of Pb2þ, 15% of Cu2þ, 13% of Cd2þ and 8% of Zn2þ

was bound to chitosan within 15 min (Fig. 1a). At the same time,20e27% of Agþ, 14e26% of Pb2þ, 9e20% of Cu2þ, 7e15% of Cd2þ and0.5e9% of Zn2þ were sorbed by the three cross-linked chitosans.

Table 1Effect of initial metal concentration on amount of metal ions sorbed by chitosan andcross-linked chitosans.

Metalion Initial metalconcentration(mg/L)

Amount of metal ions sorbed

Chitosan Chitosan-GLA

Chitosan-ECH

Chitosan-EGDE

Agþ 5 2.44 (0.02) 2.41 (0.02) 2.38 (0.02) 2.42 (0.02)10 4.85 (0.04) 4.83 (0.04) 4.80 (0.04) 4.76 (0.04)50 24.3 (0.23) 23.9 (0.22) 24.2 (0.22) 24.4 (0.23)

100 46.6 (0.43) 41.9 (0.39) 41.3 (0.38) 42.5 (0.39)300 141 (1.31) 93.7 (0.87) 111 (1.03) 123 (1.14)500 205 (1.90) 119 (1.10) 162 (1.50) 147 (1.36)

Pb2þ 5 2.30 (0.01) 2.06 (0.01) 1.95 (0.01) 2.26 (0.01)10 4.43 (0.02) 4.57 (0.02) 4.13 (0.02) 4.51 (0.02)50 21.6 (0.10) 23.6 (0.11) 17.2 (0.08) 23.7 (0.11)

100 32.8 (0.16) 46.5 (0.22) 32.4 (0.16) 44.9 (0.22)300 92.9 (0.45) 79.9 (0.39) 75.0 (0.36) 116 (0.56)500 126 (0.61) 105 (0.51) 83.5 (0.40) 153 (0.74)

Cu2þ 5 2.39 (0.04) 2.24 (0.04) 2.14 (0.03) 2.44 (0.04)10 4.78 (0.08) 4.75 (0.07) 4.11 (0.06) 4.77 (0.08)50 23.7 (0.37) 20.1 (0.32) 13.9 (0.22) 20.8 (0.33)

100 42.7 (0.67) 28.3 (0.45) 20.2 (0.32) 31.7 (0.50)

0

30

60

90

120

150

180

210

0 200 400 600 800 1000 1200 1400 1600

Ag(I) Pb(II) Cu(II) Cd(II) Zn(II)

0

20

40

60

80

100

120

140

160

0 200 400 600 800 1000 1200 1400 1600

Ag(I) Pb(II) Cu(II) Cd(II) Zn(II)

a

b

Fig. 1. Effect of contact time on the sorption of metal ions by (a) chitosan and (b)chitosan-EGDE (Error bars are � standard deviation of three replicates).

A. Kamari et al. / Journal of Environmental Management 92 (2011) 2675e2682 2677

The initial high amount of metal ions sorbed indicates instan-taneous sorption, which can be attributed to the availability ofbinding sites of the sorbents. However, as these sites progressivelyreact, the sorption of metal ions slowed before attaining equilib-rium at 90 min for chitosan and 180 min for cross-linked chitosans.Cross-linked chitosans reached equilibrium later than natural chi-tosan due to their rigid structure, which may have made somebinding sites less accessible. The ability of chitosans to bind a largeamount of metal ions within 15 min suggests that they are effectivesorbents. Furthermore, the rapid kinetics has significant practicalimportance as it will facilitate the application to smaller reactorvolumes ensuring efficiency and economy (Aksu, 2002).

300 82.6 (1.30) 46.7 (0.74) 38.9 (0.61) 68.5 (1.08)500 107 (1.69) 60.0 (0.94) 52.9 (0.83) 90.3 (1.42)

Cd2þ 5 1.97 (0.02) 2.35 (0.02) 1.26 (0.01) 1.49 (0.01)10 4.40 (0.04) 4.12 (0.04) 3.37 (0.03) 3.80 (0.03)50 22.3 (0.20) 14.3 (0.13) 15.3 (0.14) 16.8 (0.15)

100 36.0 (0.32) 17.6 (0.16) 23.3 (0.21) 25.3 (0.23)300 57.9 (0.52) 36.9 (0.34) 31.2 (0.28) 48.0 (0.43)500 81.0 (0.72) 49.7 (0.44) 38.7 (0.34) 66.4 (0.59)

Zn2þ 5 2.18 (0.03) 1.99 (0.03) 1.83 (0.03) 1.97 (0.03)10 4.00 (0.06) 3.66 (0.06) 3.39 (0.05) 3.74 (0.06)50 15.5 (0.24) 11.5 (0.18) 8.20 (0.13) 10.6 (0.16)

100 22.1 (0.34) 16.6 (0.25) 10.4 (0.16) 15.5 (0.24)300 44.2 (0.68) 27.0 (0.41) 13.3 (0.20) 36.3 (0.56)500 58.2 (0.89) 36.7 (0.56) 16.3 (0.25) 45.8 (0.70)

Values represent mean of three replicates, units of mg/g. Values within parenthesesare in mmol/g.

3.2. Effect of initial metal concentration

Table 1 presents the effect of initial metal concentration on themetal uptake by the chitosans at equilibrium. It is apparent thatthey have similar capacity to bind metal ions at initial metalconcentrations of 5 and 10 mg/L. However, when the initial metalconcentration was increased from 100 to 500 mg/L, a significantdifference was observed. The binding of metal ions onto the chi-tosans was in the order of Agþ > Pb2þ > Cu2þ > Cd2þ > Zn2þ.

From Table 1, it can be seen that the amount of metal ions boundper unit weight of sorbent is higher at high initial metal concen-trations. However, the sorption percentage decreased with

increasing initial metal concentration (Fig. S1, Supplementarymaterial). This can be explained by the fact that at low concentra-tions the ratio of available binding sites to the total metal ions beinghigh and all metal ions may be bound to the sites. At highconcentrations, however, this ratio is lower and consequently thebinding of metal depends on the initial concentrations (Li et al.,2008). For example, when the Agþ concentration was increasedfrom 5 to 50 mg/L, the Agþ sorption by chitosan-ECH (Fig. S1)remained at its maximum (99%e100%), and decreased to 95% with100 and 200 mg/L Agþ solutions. A further reduction was observed(78%), when 300 mg/L was used as the initial concentration. Theresults demonstrated that the chitosans are able to bind metal ionsover a wide range of initial metal concentration.

3.3. Effect of background electrolytes

Soil and industrial wastewater normally contain a number ofcations and anions. For example, Kþ, Naþ, Ca2þ, Cl� and NO3

� arecommonly found in soil solution. Therefore, it is important toexamine the ability of sorbents to bind one specific metal ion frommixed ion solutions.

Studies have shown that the presence of cations such as Kþ, Naþ

and Ca2þ inhibited metal ion binding by sorbents. For example,Huang et al. (2009) reported that Cu2þ sorption onto aspen woodfibres was influenced by Naþ, Ca2þ and Al3þ. Moreira et al. (2008)noted that Ca2þ caused a greater inhibition effect than Naþ onNi2þ sorption by two Oxisols and an Alfisol.

The influence of Cl� and NO3� ions on Zn2þ binding by chitosan

and cross-linked chitosans was investigated using KCl and KNO3(0.1 mol/L) background electrolytes in four sorption systems, asdescribed in Section 2.4. Kinetic parameters for Zn2þ sorption for

A. Kamari et al. / Journal of Environmental Management 92 (2011) 2675e26822678

each system are given in Table 2. Based on the R2 values, thesorption data were best fitted with the pseudo-second-orderkinetic model, while the pseudo-first-order and intraparticlediffusion models fitted the data poorly, as discussed in Section 3.4.Therefore, the amount of Zn2þ sorbed onto chitosans at equilib-rium, qe, was obtained from the pseudo-second-order equation.From Table 2, the estimated qe values for each system showed nosignificant difference at p < 0.05 (Tukey’s Least Significant Differ-ence). This suggests that the nature of Cl� and NO3

� ions did notaffect Zn2þ binding by the chitosans.

FTIR analysis was carried out to confirm the binding mechanismof Zn2þ in different background electrolytes. The FTIR spectra ofchitosan-GLA before and after Zn2þ uptake are shown in Fig. S2(Supplementary material) as an example. The FTIR spectra ofchitosan-GLA after Zn2þ sorption showed similar major changes forall systems studied implying similar binding mechanism. FromFig. S2, the wide band at 3433 cm�1 assigned to the stretchingvibration of eOH, the extension vibration of NeH and intermo-lecular hydrogen bonds of polysaccharides shifted to 3426 cm�1.The prominent band at 1662 cm�1, corresponding to the iminebond (eC¼Ne), shifted to a position in the range 1670 - 1658 cm�1.A change in the intensity of the CeN stretches at 1419, 1379 and1320 cm�1 was observed, while the intensity of the second primaryamine band at 1595 cm�1 decreased significantly.

The similarity in binding mechanism of Zn2þ sorption indifferent electrolytes was further evident by SEM analysis. The SEMimages at 2500� magnification of chitosan before and after Zn2þ

sorption, presented in Fig. S3 (Supplementary material) as anexample, revealed that the irregular surface of chitosan becamegrooved in all systems tested. Therefore, it can be postulated thatZn2þ sorption in different background electrolytes involves a similarbinding mechanism, producing similar morphological effects.

3.4. Sorption kinetics

An ideal sorbent for metal decontamination should not onlyhave a large sorbate capacity but also a fast sorption rate (Crini andBadot, 2008). According to Sud et al. (2008), predicting the rate atwhich sorption takes place and the binding mechanism are bothvital to determine the efficiency of a sorption process.

Table 2Kinetic parameters for Zn2þ sorption in different background electrolytes.

Sorbent Set qe (mg/g) Pseudo-first-order

K1 (1/min) R2

Chitosan A 23.2 2.93 � 10�3 0.3429B 28.4 2.56 � 10�3 0.4301C 28.6 2.52 � 10�3 0.7543D 25.1 2.76 � 10�3 0.8194

LSD 8.27Chitosan-GLA A 15.8 2.04 � 10�3 0.2824

B 19.9 2.12 � 10�3 0.4331C 19.6 2.30 � 10�3 0.5399D 16.1 2.07 � 10�3 0.3748

LSD 7.84Chitosan-ECH A 8.69 1.92 � 10�3 0.1530

B 8.85 1.84 � 10�3 0.1634C 7.60 2.78 � 10�3 0.6846D 6.53 2.53 � 10�3 0.7800

LSD 5.35Chitosan-EGDE A 17.4 2.61 � 10�3 0.5098

B 20.8 2.38 � 10�3 0.5877C 20.2 2.34 � 10�3 0.5316D 18.5 2.21 � 10�3 0.6768

LSD 4.91

Set A (ZnCl2 þ KCl), Set B (Zn(NO3)2.6H2O þ KNO3), Set C (ZnCl2 þ KNO3) and Set D (Zn

Unlike the extent of sorption, which depends only on the initialand final equilibrium state, the rate of sorption depends on thewhole process (Lodeiro et al., 2006). There are four independentprocesses that may control the sorption kinetics in a solideliquidsystem (Lodeiro et al., 2006; Ho et al., 1996): (i) solute transfer fromthe bulk solution to the boundary film that surrounds the sorbent’ssurface, (ii) solute transport from the boundary film to the sorbent’ssurface, (iii) solute transfer from the sorbent’s surface to the activeintraparticle sites, and (iv) interaction(s) between solute andbinding sites of the sorbent. The first and second steps are generallyconsidered as rapid processes (Miretzky et al., 2010). Therefore,intraparticle diffusion or interaction(s) between solute and bindingsites (chemical binding reaction) may potentially control thesorption kinetics.

The pseudo-first-order (Lagergren, 1898), pseudo-second-order(Ho and McKay, 2000) and intraparticle diffusion (Weber andMorris, 1963) kinetic models were employed to determine therate constant and the controlling mechanism of sorption process.The linear form of pseudo-first-order equation is expressed as Eq.(2) (Lagergren, 1898):

logðqe � qtÞ ¼ log qe � k12:303

t (2)

where, qe and qt (mg/g) are the amount of metal ions sorbed atequilibrium and at time t (min), respectively and k1 (1/min) is therate constant of pseudo-first-order equation.

The linear form of pseudo-second-order equation is rendered asEq. (3) (Ho and McKay, 2000):

tqt

¼ 1k2q2e

þ tqe

(3)

where, k2 (g/mg/min) is the rate constant of pseudo-second-orderequation.

The intraparticle diffusion equation is described as Eq. (4)(Weber and Morris, 1963):

qt ¼ kidt0:5 (4)

where, kid (mg/g/min0.5) is the rate constant of intraparticle diffu-sion equation.

Pseudo-second-order Intraparticle diffusion

k2 (g/mg/min) R2 kid (mg/g/min0.5) R2

3.93 � 10�3 0.9988 3.00 � 10�1 0.23666.99 � 10�3 0.9999 4.48 � 10�1 0.32104.37 � 10�3 0.9999 4.34 � 10�1 0.33323.82 � 10�3 0.9999 3.91 � 10�1 0.3557

1.60 � 10�3 0.9996 2.03 � 10�1 0.36592.97 � 10�3 0.9992 4.33 � 10�1 0.48402.63 � 10�3 0.9995 3.99 � 10�1 0.55188.77 � 10�3 0.9998 2.64 � 10�1 0.3720

8.04 � 10�3 0.9989 1.81 � 10�1 0.44246.33 � 10�3 0.9982 2.05 � 10�1 0.50199.49 � 10�3 0.9998 1.46 � 10�1 0.47391.02 � 10�3 0.9998 1.22 � 10�1 0.4764

9.19 � 10�3 0.9999 2.80 � 10�1 0.34615.68 � 10�3 0.9999 3.58 � 10�1 0.39233.12 � 10�3 0.9996 4.01 � 10�1 0.50896.78 � 10�3 0.9999 2.94 � 10�1 0.3539

(NO3)2.6H2O þ KCl).

A. Kamari et al. / Journal of Environmental Management 92 (2011) 2675e2682 2679

The kinetic parameters obtained for the sorption along withtheir corresponding R2 values are given in Table 3. The experi-mental equilibrium sorption capacities (qe experimental) determinedfrom the contact time study were in good agreement with thetheoretical equilibrium sorption capacities (qe theoretical) calculatedusing the pseudo-second-order kinetic model. Moreover, experi-mental sorption data correlated well to the pseudo-second-orderkinetic model (R2 values range from 0.9842 to 0.9999).

Several other metal sorption studies using sorbents of biologicalorigin, such as swine bone char (Pan et al., 2009), Saccharomycescerevisiae (budding yeast) (Chen and Wang, 2008), tobacco stems(Li et al., 2008) and tea leaves (Amarasinghe and Williams, 2007),showed that sorption kinetics followed the pseudo-second-orderkinetic model. It has been reported that the pseudo-first-orderand intraparticle diffusion equations did not fit well to the wholerange of contact time and were generally applicable only over theinitial stage of the process. For example, Mohan et al. (2006) notedthat the plots of Lagergren’s pseudo-first-order model for Cu2þ andCd2þ sorption by Eucalyptus black liquor lignin deviated consider-ably after 40 min. Meanwhile, Miretzky et al. (2010) reported thatthe binding of Cd2þ onto Eleocharis acicularis biomass followed theintraparticle diffusion kinetic model for the first 5 min only.However, Liu et al. (2009) reported that the pseudo-first-orderequation fitted Zn2þ, Cu2þ, Ni2þ and Cd2þ binding to Laminariajaponica very well.

A possible explanation for different kinetic behaviour relates tothe different physical and chemical properties of the sorbents.Results suggest that the binding of metal ions studied onto chito-sans was best described by the pseudo-second-order kinetic modeland that the chemical binding reaction was the rate-limiting step,as discussed by Ho and McKay (2000).

3.5. Sorption isotherms

A sorption isotherm correlates solute concentration in bulksolution and in the sorbed state at a given temperature underequilibrium conditions (Li et al., 2008; Febrianto et al., 2009).Several isotherms originally used for gas phase sorption are avail-able and readily adapted to explain solution equilibria (Febriantoet al., 2009), the most widely used being the Freundlich (1906)

Table 3Kinetic parameters for metal ions sorption onto chitosan and cross-linked chitosans.

Metalion Sorbent Pseudo-first-order Pseudo-secon

k1 (1/min) R2 k2 (g/mg/min

Agþ Chitosan 1.15 � 10�3 0.1199 0.87 � 10�3

Chitosan-GLA 2.53 � 10�3 0.4369 0.67 � 10�3

Chitosan-ECH 1.61 � 10�3 0.2102 0.55 � 10�3

Chitosan-EGDE 2.07 � 10�3 0.2904 0.50 � 10�3

Pb2þ Chitosan 1.84 � 10�3 0.2072 0.90 � 10�3

Chitosan-GLA 1.72 � 10�3 0.2235 0.73 � 10�3

Chitosan-ECH 2.26 � 10�3 0.2364 1.04 � 10�3

Chitosan-EGDE 2.30 � 10�3 0.3595 0.43 � 10�3

Cu2þ Chitosan 1.67 � 10�3 0.2448 0.73 � 10�3

Chitosan-GLA 2.39 � 10�3 0.5036 1.42 � 10�3

Chitosan-ECH 2.18 � 10�3 0.4046 1.18 � 10�3

Chitosan-EGDE 2.05 � 10�3 0.3390 1.10 � 10�3

Cd2þ Chitosan 1.62 � 10�3 0.1842 1.35 � 10�3

Chitosan-GLA 2.36 � 10�3 0.4051 4.17 � 10�3

Chitosan-ECH 2.11 � 10�3 0.3228 3.79 � 10�3

Chitosan-EGDE 2.28 � 10�3 0.4582 1.22 � 10�3

Zn2þ Chitosan 2.29 � 10�3 0.4303 1.17 � 10�3

Chitosan-GLA 2.53 � 10�3 0.5484 2.07 � 10�3

Chitosan-ECH 1.21 � 10�3 0.2934 1.05 � 10�3

Chitosan-EGDE 2.47 � 10�3 0.4448 1.53 � 10�3

and Langmuir (1916) isotherms. These models were applied todescribe the isotherms and determine the isotherm constants ofmetal ion sorption by the chitosans.

The linear form of the Freundlich isotherm is expressed as Eq.(5) (Freundlich, 1906):

log qe ¼ logKF þ1nlogCe (5)

where, Ce (mg/L) is the equilibrium concentration of metal ions, KF(mg/g) is the relative sorption capacity constant and n is theFreundlich constant indicating sorption intensity.

The linear form of the Langmuir equation is given by Eq. (6)(Langmuir, 1916):

Ceqe

¼ CeQ

þ 1Qb

(6)

where, b (L/mg) is the Langmuir constant related to the affinity ofbinding sites, and Q (mg/g) is the maximum sorption capacity.

The Freundlich and Langmuir constants and R2 values are givenin Table 4. The Freundlich and Langmuir models fitted the isothermequilibrium data reasonably well with R2 values of �0.9604. Thevalues of maximum sorption capacity (Q) obtained from theLangmuir equation were close to the qe theoretical and qe experimentalvalues determined from the kinetic study (Table 3). Therefore, theequilibrium data were more satisfactorily fitted to the Langmuirisotherm than the Freundlich isotherm model.

As presented in Table 4, the 1/n values from application of theFreundlich model lie between 0 and 1 indicating the metal ions arefavourably sorbed by the chitosans. The 1/n value can be used topredict binding affinity of the sorbents towardmetal ions; a smallervalue of 1/n implies stronger interaction between sorbent andmetal (Freundlich, 1906). From Table 4, the 1/n values for Agþ werefound to be lower than for Pb2þ suggesting stronger bindinginteraction between Agþ and active sites of the chitosans. This wasfurther evident by the greater values of b for Agþ obtained from theLangmuir model.

Meanwhile, the higher value of b (from the Langmuir model) forchitosan-EGDE (0.0849) than chitosan (0.0333) suggests that it hasbetter affinity to sorb Pb2þ ions. This might be attributed to greatersurface area, less crystalline structure and better surface

d-order Intraparticle diffusion

) qe (mg/g) R2 kid (mg/g/min0.5) R2

Exp. Theo.

206 208 0.9998 3.61 0.3485119 119 0.9997 2.35 0.4458160 164 0.9997 3.22 0.4456143 141 0.9995 2.94 0.4754126 128 0.9996 2.43 0.3970106 108 0.9995 2.21 0.451882.4 83.3 0.9991 1.75 0.4498

151 154 0.9996 3.20 0.4938107 109 0.9993 2.23 0.420860.2 61.7 0.9999 1.14 0.421653.4 54.1 0.9996 1.10 0.486888.5 89.3 0.9998 1.64 0.430979.9 80.7 0.9994 1.60 0.397550.3 51.3 0.9999 0.90 0.378238.6 38.0 0.9995 0.73 0.412166.1 66.7 0.9999 1.17 0.414158.6 59.5 0.9996 1.22 0.454036.4 38.2 0.9998 0.68 0.439916.8 17.8 0.9842 0.54 0.485743.9 45.7 0.9996 0.93 0.4974

Table 4Freundlich and Langmuir isotherm constants for metal ions binding to chitosan and cross-linked chitosans.

Metalion Sorbent Freundlich Langmuir

KF (mg/g) 1/n R2 Q (mg/g) Q (mmol/g) b (L/mg) R2

Agþ Chitosan 53.9 0.3656 0.9789 208 1.93 0.5581 0.9945Chitosan-GLA 24.8 0.2773 0.9889 112 1.04 0.1147 0.9787Chitosan-ECH 41.8 0.2628 0.9851 154 1.43 0.1934 0.9780Chitosan-EGDE 29.5 0.3335 0.9746 147 1.36 0.2092 0.9933

Pb2þ Chitosan 6.89 0.5525 0.9864 133 0.64 0.0333 0.9656Chitosan-GLA 10.9 0.4192 0.9659 104 0.50 0.0505 0.9816Chitosan-ECH 3.54 0.6010 0.9619 93.5 0.45 0.0249 0.9875Chitosan-EGDE 12.7 0.5452 0.9803 156 0.75 0.0849 0.9903

Cu2þ Chitosan 15.6 0.3509 0.9774 103 1.61 0.0865 0.9800Chitosan-GLA 11.6 0.2597 0.9818 58.1 0.91 0.0483 0.9753Chitosan-ECH 2.95 0.4718 0.9950 52.6 0.83 0.0176 0.9911Chitosan-EGDE 7.95 0.4111 0.9942 89.3 1.39 0.0312 0.9788

Cd2þ Chitosan 6.73 0.4400 0.9899 81.3 0.72 0.0297 0.9639Chitosan-GLA 5.42 0.3450 0.9673 50.3 0.45 0.0206 0.9824Chitosan-ECH 2.78 0.4600 0.9706 41.2 0.37 0.0200 0.9846Chitosan-EGDE 3.67 0.4916 0.9604 70.9 0.63 0.0147 0.9978

Zn2þ Chitosan 3.07 0.5117 0.9948 61.4 0.94 0.0205 0.9657Chitosan-GLA 2.54 0.4463 0.9981 37.7 0.58 0.0199 0.9982Chitosan-ECH 2.17 0.3412 0.9751 16.4 0.25 0.0344 0.9856Chitosan-EGDE 2.27 0.4996 0.9921 51.0 0.78 0.0131 0.9895

A. Kamari et al. / Journal of Environmental Management 92 (2011) 2675e26822680

morphology, which favour metal binding on chitosan-EGDE(Kamari et al., 2011). Except for Pb2þ sorption by chitosan-EGDE,cross-linked chitosans had lower binding capacity than chitosanfor the metal ions studied. A possible reason for this is that thecross-linking treatment restricts the diffusion of metal ion to theinternal sites of the sorbents by establishing insoluble cross-linkednetworks. As a consequence, fewer metal ions are bound to cross-linked chitosans. However, the maximum binding capacity (Q) ofcross-linked chitosans for Agþ, Cd2þ, Cu2þ, Pb2þ and Zn2þ is higherthan other low-cost sorbents (Ajmal et al., 2006; Chen and Wang,2008; Amarasinghe and Williams, 2007; Kogej and Pavko, 2001;Bulut and Tez, 2007). In fact, the uptake values of Cu2þ and Zn2þ

by chitosan-GLA obtained from this work are comparable withbrown alga-GLA, reported by Liu et al. (2009).

The essential feature of the Langmuir isotherm can be expressedin terms of a dimensionless constant separation factor or equilib-rium parameter, RL, which was defined by Hall et al. (1966), and isgiven by Eq. (7):

RL ¼ 11þ bCo

(7)

where, Co is the initial concentration of metal ion. RL values indicatethe shape of the isotherm (Hall et al., 1966): (i) irreversible (RL ¼ 0),(ii) favourable (0< RL< 1), (iii) linear (RL¼ 1), and (iv) unfavourable(RL > 1). It was found that the RL values are in the range of 0 <RL < 1(Table S1, Supplementary material), which suggests that the sorp-tion of metal ions on the chitosans is favourable.

3.6. Sorption-desorption study

Sorption experiments have shown that the chitosans arecapable of binding Agþ, Pb2þ, Cu2þ, Cd2þ and Zn2þ in KCl and KNO3

solutions. However, such information is not sufficient to elucidatethe feasibility of the chitosans to immobilise metal ions in realcontaminated soil and water. Therefore, it is necessary to study thereversibility of the process. This was done by subjecting the metal-saturated chitosans to desorption using 0.1 mol/L KCl and KNO3 aseluents at different dilution factors. In order to understand thenature of bonding between sorbents and sorbates, desorptionstudies were carried out using low and high initial metal concen-trations (100 and 500 mg/L).

Table 5 presents the percentage of metal retained on the surfaceof the sorbents following desorption. Overall, Agþ ions were mosteffectively retained on the chitosans, and Zn2þ the least retained. Inall cases, more than 90% of the Agþ ions were held followingdilution, implying a strong bond between the silver and the func-tional groups on the chitosans. For zinc, shaking for an additional24 h with no dilution resulted in loss of between 20 and 35% of thesorbed Zn2þ, while an 11-fold dilution caused a loss of between 70and 97% at the lower concentration, and about 60e70% at thehigher.

The behaviour shown by Pb2þ, Cu2þ and Cd2þ on all of thechitosans lay between these two extremes, with the exception ofthat on chitosan-ECH at an initial concentration of 100 mg/L. ForPb2þ and Cu2þ over all chitosans, and for Cd2þ at the lower dilu-tions, there was an apparent enhancement of sorption above thatexpected due to experimental error, and despite apparent equilib-rium having been obtained during the sorption phase. For Pb2þ,there was an apparent increase in the amount sorbed by a factor of2.5. We currently have no explanation for this effect, which wasreproducible, other than that it may relate to a change in themorphology on prolonged shaking. Chitosan-ECH is the most rigidof the cross-linked chitosans, and it may be that the additionalshaking results in physical breakdown, or even chemical modifi-cation, allowing access to more binding sites. The ionic size andstrength of binding may also be important. Agþ is the smallest ofthe ions tested, and may be able to penetrate the structure, so noadditional sorption occurs during the desorption phase. Zn2þ

however is roughly the same size as Pb2þ, Cu2þ, and Cd2þ, but is themost weakly sorbed ion. In this case desorptionmay be a preferableprocess to further sorption.

The reversibility of the process is not only dependent upon theproperties of the metal ions and the background solutions, but alsophysical characteristics of the sorbents (Ip et al., 2009). Fromsorption studies, it was found that chitosan has greater affinity tobind metal ions than cross-linked chitosans. However, the resultsfrom the desorption study suggest that in some cases cross-linkedchitosans have better ability to hold metal ions than chitosan. Apossible reason for this observation might be due to the dissolutionof chitosan. Cross-linked chitosans are rigid and chemically stableas compared to chitosan (Kamari et al., 2011). Therefore, they areable to withstand such changes in the sorption system.

Table 5Percentage ofmetal ions retained on the surface of sorbents (Co¼ 100 and 500mg/L).

Metalion Dilutionfactor

Chitosan Chitosan-GLA

Chitosan-ECH

Chitosan-EGDE

Agþ 0 99 (102) 99 (106) 100 (103) 99 (102)1.5 99 (101) 99 (99) 99 (101) 99 (99)2.0 98 (100) 99 (97) 99 (99) 99 (99)3.5 98 (97) 98 (98) 99 (97) 98 (97)6.0 98 (97) 97 (96) 97 (97) 97 (95)8.5 97 (96) 95 (96) 96 (97) 96 (95)

11.0 97 (95) 95 (94) 94 (97) 96 (95)Pb2þ 0 98 (116) 97 (106) 249 (98) 98 (112)

1.5 94 (116) 97 (104) 216 (86) 97 (111)2.0 93 (114) 95 (102) 210 (85) 97 (107)3.5 88 (111) 95 (102) 200 (81) 97 (99)6.0 79 (107) 94 (91) 162 (63) 96 (93)8.5 73 (102) 94 (85) 155 (50) 96 (85)

11.0 65 (100) 94 (78) 119 (46) 95 (80)Cu2þ 0 117 (103) 104 (125) 158 (106) 104 (105)

1.5 114 (98) 100 (119) 146 (85) 102 (104)2.0 111 (98) 99 (118) 131 (69) 101 (102)3.5 108 (96) 97 (114) 130 (63) 99 (99)6.0 101 (94) 92 (111) 129 (60) 97 (98)8.5 99 (92) 91 (104) 125 (43) 93 (94)

11.0 99 (90) 87 (103) 115 (43) 92 (88)Cd2þ 0 90 (92) 87 (85) 163 (95) 92 (92)

1.5 86 (95) 77 (97) 150 (96) 86 (100)2.0 83 (95) 75 (94) 119 (93) 83 (94)3.5 77 (93) 68 (89) 114 (87) 69 (92)6.0 72 (91) 60 (79) 85 (80) 58 (86)8.5 66 (77) 54 (76) 81 (72) 56 (82)

11.0 60 (74) 51 (70) 60 (68) 47 (72)Zn2þ 0 72 (79) 65 (81) 70 (70) 65 (80)

1.5 70 (76) 63 (74) 69 (66) 60 (73)2.0 61 (65) 58 (69) 63 (64) 52 (68)3.5 51 (62) 52 (64) 54 (52) 47 (64)6.0 42 (56) 45 (57) 25 (46) 34 (43)8.5 36 (53) 50 (50) 17 (38) 31 (41)

11.0 20 (43) 31 (40) 3 (33) 27 (35)

Values represent mean of three replicates. Values within parentheses are forCo ¼ 500 mg/L.

A. Kamari et al. / Journal of Environmental Management 92 (2011) 2675e2682 2681

During desorption, metal ions can be replaced by Hþ or Kþ ionsfrom the eluents, which becomes significant if the interactionbetween the metal ions and the functional groups of the chitosansinvolves weak binding forces. With the exception of Zn2þ, additionof eluents yielded low desorption percentage. This implies thatbinding of metal ions onto chitosans involves chemical binding, asopposed to ion exchange. From the sorption-desorption studies, itis apparent that chitosan and cross-linked chitosans are able toretain metal ions on their surfaces. This highlights their potentialuse as immobilising agents for heavy metals in contaminated soiland water.

3.7. Binding mechanisms

Metal ions can be bound to the surface of a sorbent by severalmechanisms including complexation, ion exchange, chelation,adsorption and co-ordination (Sud et al., 2008; Febrianto et al.,2009). However, only complexation and ion exchange have beenrecognised asmajor mechanisms for metal ion sorption by sorbentscontaining functional groups such as -C¼O, eNH2 and eOH (Sudet al., 2008).

To investigate the contribution of ion exchange, the pH of thesolution was measured before and after the shaking process. Aslight increase (ca. 0.5 units) in the pH value was observed aftermetal ion binding onto the chitosans (data not shown). Thissuggests that ion exchange is not significantly involved in thebinding process, under these conditions. Based on the electron-

donating nature of the N- and O- containing groups in chitosansand the electron-accepting nature of the metal ions, covalentinteraction preferentially occurs. In our earlier work (Kamari et al.,2011), the involvement of N and O atoms in binding metal from soilsolution was evident by FTIR analysis.

As several factors or parameters may contribute to metal ionbinding, Nieboer and McBryde (1973) proposed the concept of thecovalent index for metal ions, which is a complex parameterexpressed as Xm

2r, where Xm is electronegativity and r is ionicradius. This has been used to describe metal uptake capacity ofsome sorbents. For example, Brady and Tobin (1995) studied thebinding of metal ions to Rhizopus arrhizus biomass and reportedthat the equilibrium metal uptake decreased in the order ofPb2þ > Cu2þ > Cd2þ > Zn2þ > Mn2þ > Sr2þ, consistent with thecovalent index values. Based on the equilibrium sorption capacity(qe) (Table 3) and maximum sorption capacity (Q) (Table 4), metalion affinity was in the order of Agþ> Pb2þ> Cu2þ> Cd2þ> Zn2þ, inagreement with the covalent index values of 4.25 (Agþ), 3.29(Pb2þ), 2.98 (Cu2þ), 2.71 (Cd2þ) and 2.13 (Zn2þ), as estimated in theliterature (Chen and Wang, 2008; Brady and Tobin, 1995; Nieboerand Richardson, 1980).

Desorption studies revealed that Agþ formed the most stablecomplex and strong bondingwith the chitosans, while Zn2þ had theleast potential to be retained on the sorbents’ surface. According tothe hard and soft metal ion principle (Nieboer and Richardson,1980), metal ions can be classified into three groups (Class A,Class B and borderline) based on the magnitude of the complexformation constants. Class A ions have a donor atom preferencesequence for metal-binding ligands of O>N> S, while Class B havethe opposite preference. Agþ is a Class B metal ion, while Cd2þ,Cu2þ, Pb2þ and Zn2þ are borderline. Since the chitosans containnitrogen atoms, Agþ preferentially binds to form a stable complex,while the uptake and stability of complexes of Cd2þ, Cu2þ, Pb2þ andZn2þ are expected to be lower.

4. Conclusion

There is a ready supply of chitosan from the waste products ofthe shellfish industry. For example, in Malaysia it is estimated that70,000 tonnes of shellfish waste is produced each year (Kamari,2011). Its use as a clean up material for remediation of contami-nated soil and water would provide a solution to its disposal.

The results of this study suggest that chitosans have potentialapplication to the remediation of contaminated soil andwater. Theirkey property is the presence of amino groups (eNH2), and to someextent alsohydroxylgroups (eOH). Themetals testedhave a rangeofbinding stabilitieswith respect to these groups. The order of bindingcapacity (mmol/g) varies with sorbent: Chitosan: Agþ > Cu2þ

> Zn2þ > Cd2þ > Pb2þ, Chitosan-GLA: Agþ > Cu2þ > Zn2þ

>Pb2þ>Cd2þ, Chitosan-ECH:Agþ>Cu2þ>Pb2þ>Cd2þ>Zn2þ, andChitosan-EGDE: Agþ z Cu2þ > Zn2þ z Pb2þ > Cd2þ.

Metal binding ability however is only one aspect of such uti-lisation. Information is also needed on the stability and persistenceof chitosans in the soil-water environment. It is also necessary toestablish the ability of chitosans to reduce metal availability anduptake by plants.

Acknowledgements

A. Kamari thanks Ministry of Higher Education Malaysia andUniversiti Pendidikan Sultan Idris Malaysia for providing a SLAI-PhD Scholarship Award. We thank M. Beglan, I. Freer, K. Wilsonand J. Gallagher for their assistance.

A. Kamari et al. / Journal of Environmental Management 92 (2011) 2675e26822682

Appendix. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jenvman.2011.06.005.

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