Synthesis and Characterization of Thiolated Chitosan Beadsfor Removal of Cu(II) and Cd(II) from Wastewater

Soon Kong Yong & Nanthi Bolan & Enzo Lombi &William Skinner

Received: 25 June 2012 /Accepted: 22 March 2013 /Published online: 20 November 2013# Springer Science+Business Media Dordrecht 2013

Abstract Removal of Cu(II) and Cd(II) fromwastewaterusing porous chitosan beads is likely to be enhanced bythe introduction of thiol groups (−SH). This is because, inaccordance with the Hard Soft Acid Base concept, the softLewis base of –SH forms a strong bond with soft Lewisacid of Cd(II) or with borderline Lewis acids such asCu(II). Possible formation of thiourea and disulfidecrosslinks (−S–S–) may also confer increased bead stabil-ity in acidic solution. Thiolated chitosan beads (ETB)prepared and investigated in this study had a total sulfurcontent of 7.9 %. The thiolation process slightly increasedthe Brunauer-Emmett-Teller surface area of the chitosan

beads from 39.5 to 46.3 m2/g. This ETB was categorisedas a microporous material (pore aperture: 1.8 nm) withmultiple and uniform porous layers. Analysis by X-rayphotoelectron spectroscopy indicated the presence of threesulfur species, S(−I), S(−II) and S(V) attributed to –S–S–,–SH and sulfonate (−SO3

−) groups. The Langmuir sorp-tion capacity, qmax, for Cd(II) was improved by 18 timesby thiolation of chitosan. However, the qmax for Cu(II) byETB was seven times lower than that of pristine chitosanbeads, possibly due to exhaustion of amine groups(−NH2). The batch sorption data was generally fittedwell by a linearised Freundlich isotherm model and aHo’s pseudo-second-order kinetic model, indicatingmetal interaction with the heterogeneous surface ofETB and chemical adsorption as the possible rate-limiting step, respectively. Themetal uptake has resultedin the oxidation of –SH to –SO3

− group in ETB, therebydecreasing the stability of metal-sulfide bonds as well astheir metal uptake.

Keywords Chitosan . Heavymetal . Thiourea .

Crosslink .Wastewater

1 Introduction

Pollution by toxic heavy metals such as cadmium (Cd(II))in water poses a threat to human health. Exposure toCd(II) can occur when using contaminated water but alsovia ingestion of food products tainted with Cd(II). On theother hand, copper (Cu(II)) is an essential trace metal vitalfor life. However, elevated level of Cu(II) may causeenvironmental problems and, in extreme cases, poisoning

Water Air Soil Pollut (2013) 224:1720DOI 10.1007/s11270-013-1720-0

Guest Editors: R Naidu, Euan Smith, MH Wong, MegharajMallavarapu, Nanthi Bolan, Albert Juhasz, and Enzo Lombi

This article is part of the Topical Collection on Remediation ofSite Contamination

S. K. Yong (*) :N. Bolan : E. LombiCentre for Environmental Risk Assessmentand Remediation, University of South Australia,Mawson Lakes 5095, Australiae-mail: yonsy015@mymail.unisa.edu.au

N. BolanCooperative Research Centre for ContaminantsAssessment and Remediation of the Environment,University of South Australia,Mawson Lakes 5095, Australia

W. SkinnerIan Wark Research Institute, University of South Australia,Mawson Lakes 5095, Australia

S. K. YongUniversiti Teknologi MARA,Shah Alam 40450, Malaysia

in humans (e.g. Wilson disease, etc.). The major sourcesof Cd(II) and Cu(II) contamination are due to extensiveuse of these metals in the electroplating industryand electronic manufacturing as well as poor miningpractices. According to guidelines set by Australian andNew Zealand Environment and Conservation Council(ANZECC) and Agriculture and Resources ManagementCouncil of Australia and New Zealand (ARMCANZ),the safe levels of Cu(II) and Cd(II) in aquatic ecosystemsare 1.4 and 0.2 μg/L, respectively (Kumar et al. 2009).Excessive uptake of both Cu(II) and Cd(II) could causepoisoning in microorganisms, earthworms or aquatic life(Van Zwieten et al. 2007). Toxic metals may also enterthe food chain which could ultimately harm humanhealth. Thus, wastewater remediation of these metals isof high priority in order to safeguard the wellbeing ofboth the environment and human health.

Chitosan has been used in wastewater treatment as acoagulant and adsorbent for organic and inorganicpollutants (Renault et al. 2009). The metal removalability of chitosan is due to an extensive network ofamine (−NH2) and hydroxyl (−OH) functional groups,which work either by chemical or physical adsorption(Juang and Shao 2002). Treatment of wastewater incontinuous flow reactors can be enhanced by usingporous beads instead of fine particles. In fact, the highsurface area of porous beads not only offers greatersorption capacity but also higher permeability. However,the practical use of chitosan beads is affected by their lowdurability at low solution pH and poor sorption for heavymetals. Chitosan beads may rupture or solubilise inmonoprotic acidic solution, thus increasing the risk ofpore clogging in flow reactors. Furthermore, sorption forCd(II) using chitosan is less selective as compared toCu(II) (Vold et al. 2003). This is because hard Lewisbases of –NH2 and –OH groups, characteristic of chito-san, may not form strong interaction with soft Lewisacids such as Cd(II), causing poor sorption capacityand selectivity (Horzum et al. 2010).

Chitosan modification has been proposed as a way toovercome such problems in order to enhance its usage asa sorbent for heavy metals. These modifications usuallyinvolve the introduction of sulfur-containing functionalgroups acting as soft Lewis acids. For example, dithio-carbamate (−HN–CS2−) (Muzzarelli et al. 1982), xan-thate (−O–CS2−) (Sankararamakrishnan and Sanghi2006) and thiourea groups (Gavilan et al. 2009) wereintroduced into the structure of chitosan which resultedin an increase in the uptake and selectivity of heavy

metals. The introduction of sulfur also contributes togreater stability in acidic solution due to the formationof crosslinks (Yong et al. 2012). The decomposition ofprimary dithiocarbamate to form thiourea has beenreported (Erve et al. 1998). This together with –S–S– groups form crosslinks that improves the durability ofchitosan at low solution pH values. However, there hasbeen no comprehensive study on the uptake mechanismof Cu(II) and Cd(II) by thiolated chitosan. Moreover, thesynthesis of chitosan dithiocarbamate hydrogel beadshas not been conducted. In this study, porous chitosanbeads were functionalized with dithiocarbamate groupswith partial decomposition to simultaneously improvemetal uptake values and bead stability under acidic con-ditions. The synthesis and metal uptake mechanisms forthiolated chitosan beads (ETB) were elucidated to inves-tigate their potential use as a Cu(II) and Cd(II) sorbentfor wastewater treatment in a flow reactor.

2 Methods

2.1 Preparation of Thiolated Chitosan Beads (ETB)

Chitosan (degree of deacetylation=88 %) and carbondisulfide (CS2) were purchased from Qingdao YuanrunChemical Company Limited and Sharlau, respectively;sodium hydroxide (NaOH) was supplied by Labserv. Allchemicals were used without purification. Chitosan ace-tate solution was prepared by dissolving 2 g of chitosanin 50 mL 0.2 M acetic acid. Chitosan gel beads wereprepared by adding chitosan acetate dropwise into a0.5 M NaOH bath. They were then thoroughly rinsedwithMilliQ water and soaked in a mixture of ethanol andCS2 (1:1 mol ratio) for 7 days to produce the thiolatedchitosan beads (ETB). All ETB were rinsed thoroughlywith ethanol and stored in distilled water at 4 °C.

The pHZPC of chitosan beads and ETB were deter-mined by immersing and shaking two bead samples in10 mL of 0.01 M NaNO3 with initial pH of 2, 3, 4, 5, 6,7, 8 and 9 for 48 h at 25 °C. The final solution pH wasalso recorded. The difference between final and initialpH (ΔpH) was calculated and plotted against initial pH(pHo). The (pHZPC) was taken as the intersection withthe x axis (Wan Ngah et al. 2010a). Determination ofcationic exchange capacity (CEC) on ground chitosanbeads and ETB began by dispersing 0.1 g of dry samplesin 40 mL of 0.02 M sodium acetate solution. Thecontents were shaken for 34 h, centrifuged and then

1720, Page 2 of 12 Water Air Soil Pollut (2013) 224:1720

decanted at room temperature. This step was repeatedtwice with two aliquots of 80 mL of isopropyl alcohol toeliminate excess NaNO3. Finally, the Na(I) ions adsorbedon ETB were desorbed with two aliquots of 0.02 mMammonium acetate (pH 6.5–7.0) to make 100 mL using avolumetric flask. The Na(I) concentration was determinedusing inductively coupled plasma optical emission spec-trometry (ICP-OES). The CEC value is represented bysodium ion concentration (Popuri et al. 2009). The sulfurcontent was determined using a LECO sulfur elemen-tal analyser on ground ETB and energy-dispersivespectroscopy (EDS) on freeze-dried ETB.

The surface and core morphology was studied usingscanning electron microscopy (SEM) on freeze-dried,cryo-fractured ETB bead using Phillips XL30 SEM.ETB bead was immobilised on a sample stub usingdouble-sided adhesive and sputter-coated with carbon.SEM images were obtained by scanning coated samplesat low energy intensity (10 kV) with a secondary electron(SE) detector. The core morphology of ETB bead wasinvestigated with X-ray micro, computed tomography(μCT) using Xradia Micro-CT 400 instrument. Onewet ETB bead was filled into a Kapton capillary tube(internal diameter=3.95 mm) and rotated over a full 360˚during the X-ray imaging resulting in the production of2D images which were further treated with the recon-struction software (Avizo7.1 ) to produce 3D densitymap of the sample. The Brunauer-Emmett-Teller (BET)surface area of freeze-dried ETB was determined using aMicrometrics Gemini system. The Fourier transforminfrared (FTIR) spectra were recorded using Agilent660-IR spectrometer. FTIR spectra of chitosan, ETBand metal-sorbed ETB were obtained from 1w/w%KBr disc (2 mg dry weight to 200 mg KBr). A total of64 scans were conducted on background and sampleswith wavenumber range of 4,500 to 400 cm−1. The X-ray photoelectron spectroscopy (XPS) analysis of wetsamples of ETB and metal-sorbed ETB was conductedon a Kratos Axis-Ultra spectrometer, using a monochro-matic Al Kα source (1487 eV) operating at 15 kV and14 mA, 10−8 Pa vacuum in the analyser chamber and ananalysis spot size of 300×700 μm. Spectrometer passenergy of 10 eV was used for all elemental spectralregions, whilst 160 eV pass energy was used for thesurvey spectra. The spectrometer was calibrated usingthe metallic Cu 2p3/2 and Cu 3p3/2 lines and Au FermiEdge. All measurements were performed at a take-offangle of 90°. The CasaXPS (version 2.3.5) software wasused to fit photoelectron spectra.

2.2 Batch Sorption Experiment

Cu(II) nitrate and Cd(II) nitrate salts were purchased fromSigma Aldrich Australia and Sharlau Chemie S.A.,respectively. Metal nitrate solutions (1 mM Cu(II) orCd(II)) were prepared by dissolving 0.233 g Cu(II) nitratepentahydrate (Cu(NO3)2·5H2O) or 0.308 g Cd(II) nitratetetrahydrate (Cd(NO3)2·4H2O) in 1 L MilliQ water with10 mM NaNO3 as background electrolyte. The sorptionpH and sorbent dosage of ETB was optimised prior tokinetics and isotherm studies. To determine the optimumsolid-solution ratio for adsorption measurements, variousnumbers of wet ETB and chitosan (2, 5, 10, 20 and 50beads) were added to 30 mL of 1.0 mMCu(II) and Cd(II)nitrate solution and were shaken using an end/end shakerfor 5,000 min at pH 5 and 25 °C. The optimum sorptionpH was determined on five pH values (ca. 3, 4, 5, 6 and7), adjusted using 0.1 M HNO3 or 0.1 NaOH at constantnumber of 20wet beads. Time-based sorption experimentwas conducted using 20 ETB or chitosan beads in 30 mLof 1.0 mM Cu(II) and Cd(II) nitrate solution at pH 5.4and 4.4, respectively. The sorption isotherms experimentswere conducted at a constant number of 20 wet beads,between 0.02 and 1.0 mM of initial metal concentration.All batch sorption experiments were conducted in dupli-cate and the equilibrium concentrations of Cu(II) andCd(II) were analysed using ICP-OES. The average up-take values of Cu(II) and Cd(II), qe in unit millimoles pergram and percent were calculated based on Eqs. 1 and 2,respectively.

qe mmol=gð Þ ¼ Co−Ce

mSrb � NM� V ð1Þ

qe %ð Þ ¼ Co−Ce

Co� 100 ð2Þ

Where Co and Ce are metal concentrations (milli-grams per liter) before and after treatment with ETB; Vis the volume of wastewater (liter);mSrb is the dry massof ETB and chitosan bead sorbent (grams) and NM isthe atomic mass of elemental Cu or Cd (grams permole). All sorption data were analysed using MicrosoftOffice Excel 2007.

2.3 Sorption Kinetics

Sorption kinetics was analysed using linearised modelsof pseudo-first-order, pseudo-second order, as well as

Water Air Soil Pollut (2013) 224:1720 Page 3 of 12, 1720

Elovich kinetic models. All kinetic model equationsare shown in Table 1 where qe (milligrams per gram)and qt (milligrams per gram) are the metal uptakevalues (milligrams per gram) at equilibrium and t time(minutes); respectively; k1 (per minute) and k2 (gramsper milligram per minute) are the pseudo-first andpseudo-second rate constants; qt is the amount of sol-ute adsorbed at time, t. α is the initial adsorption rate(milligrams per gram per minute) and β is the desorp-tion constant (grams per milligram).

2.4 Sorption Isotherms

Sorption of Cu(II) and Cd(II) by ETB were analysedusing linearised Langmuir and Freundlich models, aswell as Temkin isotherm models (Table 2) where qe isthe equilibrium concentration of metal on the sorbent(milligrams per gram); Co and Ce are the initial andequilibrium metal concentration (milligrams per liter);mix is the Langmuir sorption capacity (milligrams pergram) while KL is the Langmuir sorption constants(liters per milligram); n is the empirical constant (litersper milligram) while KF is the Freundlich sorptionconstant (milligrams per gram); A and B are Temkinconstants for sorption potential (liters per gram) andheat of adsorption (Joules per mole), respectively.

3 Results and Discussion

3.1 Characterisation of ETB

The physicochemical properties of ETB and chitosanbead are shown in Table 3. The chitosan beads andETB had 95 and 92 % water content, respectively. Theaverage dry weight of ETB was slightly heavier thanthe pristine chitosan bead. The diameter of ETB (data

not reported) was slightly decreased after 7 days ofcontact time with CS2. The thiolation process alsoslightly increased the BET surface area from 39.5 to46.3 m2/g. The mean pore aperture for both ETB andchitosan bead were 1.8 nm, which is categorised asmicroporous material (Wan Ngah et al. 2010a). Theinternal structure of ETB consisted of multiple porouslayers with most cavities located near the surface of thebead. As shown in Fig. 1, the surface and core porestructure of ETB were also similar. The surface chargeof ETB was more negative compared with chitosanbeads. The pH of point of zero charge (pHZPC) ofETB was 6.3, slightly lower than chitosan (6.5). Theacidic behaviour of ETBmay be caused by depletion of–NH2 group during thiolation as well as presence of –SO3

− group due to –SH oxidation. The CEC of ETB atpH 6.5 was 31.9±2.3 cmol/kg, which was higher thanpristine chitosan bead (7.6±0.37 cmol/kg), wherethiolation process has introduced negative-charge sul-fur functional groups to ETB. The total sulfur in theETB, as determined using LECO analyser, was 7.9 %,which was slightly higher than the sulfur content mea-sured by EDS analysis (6.9 %). The proposed structur-al formula based on their elemental composition isshown in Fig. 2.

Table 1 Equations of linearised pseudo-first order, pseudo-second-order and Elovich kinetic models

Linearised kinetic model Equation

Pseudo-first-order log qe−qtð Þ ¼ logqe−k1 t

2:303

Pseudo-second-order tqt¼ 1

k2q2eþ 1

qe

� �t

Elovich qt ¼ 1β ln aβð Þ þ 1

β lnt

Table 2 Linearised equations of Langmuir, Freundlich andTemkin isotherm models

Isotherm model Equation

Langmuir 1qe¼ 1

KLqmax

� �1Ceþ 1

qmax

Freundlich lnqe ¼ 1n lnCeð Þ þ lnK F

Temkin qe ¼ RTB lnAþ RT

B lnCe

Table 3 Physicochemical properties of ETB and chitosan bead

Physical properties Chitosan bead ETB

Total sulfur (% w/w) 0.1 7.9

Wet weight (mg/bead) 35±0.1 35±0.1

Dry weight (mg/bead) 1.9±0.1 2.8±0.2

BET surface area (m2/g) 39.5 46.3

Pore diameter (nm) 1.8 1.8

CEC (cmol/kg) 7.6±0.4 31.9±2.3

pHPZC 6.5 6.3

1720, Page 4 of 12 Water Air Soil Pollut (2013) 224:1720

3.2 FTIR Spectrometry

The stacked FTIR spectra of freeze-dried samples ofchitosan beads and ETB are shown in Fig. 3. The loweramount of protonated amine (−NH3

+) and acetate wasindicated by a decrease in their respective band inten-sities (Nunthanid et al. 2004). The thioureide band ofdithiocarbamate (>N=CS2

−) at 1,480 cm−1 (Muzzarelliet al. 1982; Humeres et al. 2002), dithioester band ofxanthate (−O–CS2−) between 1,215 and 1,175 cm−1

(Sankararamakrishnan et al. 2006) and thiol group(−SH) at 2,550 cm−1 (Sousa et al. 2009) were notvisible in all ETB spectra, thus these functional groupswere not present in ETB samples. The appearance of astrong band at 1,553 cm−1 and a weak shoulder at around1,378 cm−1 were assigned to the thioamide I and II bands(Ozturk et al. 2009), possibly from the formation of newthiourea group.

Amide and thioamide bands shifted to higher wave-number when ETB was treated with Cu(II) and Cd(II)wastewaters. As shown in Table 4, the medium amideI band at around 1,553 cm−1 shifted to 1,558 and1,556 cm−1 in Cu-ETB and Cd-ETB, respectively. Aweak ν(C-N) band at 1,239 cm−1 also shifted to1,248 cm−1 in Cu-ETB but remained unchanged inCd-ETB. Another thioamide II band at around1,378 cm−1 also shifted to 1,384 cm−1 in Cu-ETB and

1,382 cm−1 in Cd-ETB. The ν(CH–OH) and ν(CH2–OH) bands which are attributed to C3 and C6 hydroxylgroups in ETB has shifted to lower wavenumber. ν(CH–OH) and ν(CH2–OH) bands at 1,072 and 1,034 cm−1

has shifted to 1,068 and 1,032 cm−1 in Cu-ETB, and to1,065 and 1,030 cm−1 in Cd-ETB. The band shifts haveindicated interaction between their corresponding func-tional groups and metals, which is discussed in thesection on “Metal Uptake Mechanisms.”

3.3 XPS Analysis

Analysis of the XPS spectra, for as-prepared and Cu(II)and Cd(II) adsorbed ETB, is shown in Fig. 4. The mainpeak observed at 161.5 eV for the as-prepared ETB inthe S 2p XPS spectrum (A) is assigned to S(−II) in –SHgroup, with disulfide S(−I) in –S–S– crosslink andS(V) of –SO3

− group at 163.6 and 168.0 eV bindingenergy, respectively (Castner et al. 1996; Rieley et al.1998). The –SH and thiolate (−S–) groups were presentalong with thiourea group due to tautomerism of ETB(Atia 2005). The –S–S– crosslinks were formed viaionisation and oxidation of –SH groups. The presenceof –SO3

− groups was most likely caused by oxidationof sulfur from the prolonged heating of ETB (Goubert-Renaudin et al. 2009). Partial conversion on ETB is

(i) (ii) (iii)

Fig. 1 Slice image of a ETB granule obtained by X-ray micro, computed tomography (μCT) (1) surface SEM, (2) cross-sectioned SEMand (3) micrographs of ETB bead

O

ONH2

OHO

OH

O

NH

OHO

OH

S NH

OHO

OH

CH3

O

0.52 0.120.45

Fig. 2 Proposed structural formula of ETB

Water Air Soil Pollut (2013) 224:1720 Page 5 of 12, 1720

indicated by the presence of –NH2 group at 399.9 eVand –NH3

+ group at 402.0 eV (Dambies et al. 2000).The comparison of the XPS spectral analysis between

ETB, Cu-ETB and Cd-ETB is shown in Table 5. Afteradsorption of Cu(II) or Cd(II), the relative proportions ofsulfur and nitrogen species changes. The appearance ofnew Cu 2p3/2 peak at 933.0 eV showed a predominantpresence of Cu(I) (Kim et al. 2004) with no indication ofthe characteristic satellite contributions from Cu(II) (Skin-ner et al. 1996). The adsorption of Cu(II) coincided with asignificant increase in S(V) peak area as well as a decreasein S(−II) peak area. The peak proportions of Cd-ETBwere not as dramatic. However, the peak areas of S(−II)and –NH2 groups have decreased by 0.7 and 0.6 %,respectively. Similarly to Cu-ETB, S(V) area in Cd-ETB

has also increased by 0.3 %. The presence of Cd(II) ion inETB is indicated by a peak at 405.4 eV. The peak area ofS(−I) in ETB and metal-treated ETB remained a constantproportion of the S 2p peak area, while an increase of –NH3

+ peak area was observed in the N 1 s for both Cu andCd-ETB.

3.4 Batch Sorption Experiments

3.4.1 Effects of Bead Number/Solution Ratio

The effect of sorbent/solution ratio on metal uptakevalues was monitored by varying the number of beadsin the equilibrium solution. The Cu(II) and Cd(II)uptake values (percent) by 2 to 50 pieces of chitosan

898

10341072

1151

34101317

14131378

Chitosan beads - - - - -ETB

897

1030

1073

1380

1419

1157

34331653

1602

1553

1320

1262

1239

4400 4200 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 20002000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400

70

65

60

55

50

45

40

35

30

25

20

15

10

5

0

Wavenumber (cm-1)

%T

Fig. 3 FTIR spectra of ETB and chitosan beads

Table 4 Wavenumbers of FTIR spectra of chitosan bead, ETB and metal-treated ETB (Cu-ETB and Cd-ETB)

Sample Wavenumber (cm-1)

δ(N-H)/ν(C-N)

δ(N-H)/ν(HN-CS)

δ(C-H)/ν(HN-CS)

δ(N-H)/ν(C-N)

ν(C-N) νas(C-O-C)/ν(HNC=S)

ν(CH-OH) ν(CH2-OH)

Chitosan 1,653 m 1,559 m 1,380 m 1,320w – 1,157 s 1,072vs 1,030vs

ETB 1,645 m 1,553 m 1,378 m 1,317w 1,239vw 1,151 s 1,072vs 1,034vs

Cu-ETB 1,649 m 1,558 m 1,384 m 1,325w 1,248vw 1,153 s 1,068vs 1,032vs

Cd-ETB 1,648 m 1,556 m 1,382 m 1,321w 1,240vw 1,153 s 1,065vs 1,030vs

Peak intensity (with respect to the strongest peak in the spectrum): vsb (very strong, broad) 80–100 %, vs (very strong) 80–100 %, s(strong) 60–80 %, m (medium) 40–60 %, w (weak) 20–40 %, vw (very weak) 0–20 %

Bond vibration mode: ν=stretching, δ=bending, as=symmetric

1720, Page 6 of 12 Water Air Soil Pollut (2013) 224:1720

(a) (b) (c)

(d) (e) (f)

159164169174

BE (eV)

ETB

159164169174

BE (eV)

Cu-ETB

159164169174

BE (eV)

Cd-ETB

395400405410415

BE (eV)

ETB

395400405410415

BE (eV)

Cd-ETB

925935945955

BE (eV)

Cu 2p 3/2

Cu-ETB

Fig. 4 Core level S 2p XPS spectra of ETB (a), Cu-ETB (b), Cd-ETB (c); N 1 s of ETB (d), Cd 3d and N 1 s of Cd-ETB (e) and Cu 2p ofCu-ETB (f)

Table 5 XPS binding energies of ETB, Cu(II) and Cd(II)-treated ETB (Cu-ETB and Cd-ETB)

Sample Binding energy (eV)

S 2p3/2 N 1 s Cu 2p3/2 Cd 3d5/2

(−SO3−) (−SH) (−S–S–) –NH2 –NH3

+ Cu(I) Cd(II)

ETB 168.0 161.9 163.6 399.9 402.0 – –0.3 1.6 0.5 6.2 0.6

Cu-ETB 168.2 162.1 163.5 400.0 402.0 933.0 –1.0 0.6 0.5 5.4 1.1 0.2

Cd-ETB 168.1 161.8 163.6 399.9 401.9 – 405.6

0.6 0.9 0.4 5.6 0.7 0.1

Italic figures are atomic percentages of elemental species

Water Air Soil Pollut (2013) 224:1720 Page 7 of 12, 1720

beads and ETB are shown in Fig. 5. The uptake valuesof Cu(II) by chitosan and ETB increased linearly andreached the plateau when 20 beads were used. Theuptake values of Cu(II) and Cd(II) by ETB were 97and 94 %, respectively. The Cu(II) uptake value byETB beads was slightly lower than that of chitosanbeads, probably due to exhaustion of –NH2 groups bythiolation. However, removal of Cd(II) was enhancedto 79 %, almost ten times that of chitosan beads. Theincrease in Cd(II) uptake value was attributed to greaterchemical adsorption of Cd(II) ion by –SH groups in theETB.

3.4.2 Effects of Initial pH

The Cd(II) uptake value by ETB was higher thanchitosan beads, but Cu(II) uptake value by chitosanbead was greater than ETB except at around pH 2.5.As shown in Fig. 6, the uptake values of Cu(II) andCd(II) by ETB increased from ca pH 2.5 and reachedplateau at around pH 4, where it remained constantuntil pH 6.5. The removal of Cd(II) was less affectedby pH as compared with that Cu(II), possibly due toformation of strong bonds between Cd(II) ion with –SH groups in ETB. The high pKa values of –SH and –NH2 groups in ETB suggest limited deprotonation atlow pH value. The elevated concentration of protonshas created competitive adsorption with Cu(II) andCd(II) ions. In fact, chitosan beads were partiallydissolved when the pH value was 2.5. Although chito-san beads removed more Cu(II) than ETB at pH 3.5–6.5, the blue-coloured Cu(II)-chitosan beads were

ruptured due to excessive swelling. This would causethe release of sorbed Cu(II) as well as pore clogging ina flow-reactor. The low stability of chitosan in acidicsolution is probably due to high proton concentrationthat enhances protonation of –NH2 and subsequentlypromotes repulsion between chitosan (Wan Ngah et al.2010b). ETB remained physically stable in all solutionpH values. Greater stability of ETB in acidic solution ismost likely due to the presence of thiourea or –S–S–crosslinks in ETB. This may minimise swelling to ETBstructure due to repulsion of –NH3

+ groups.

3.4.3 Sorption Kinetics

Overall, sorption equilibriums of both chitosan andETB beads were achieved at the 1,000th and 2,000thminute, respectively. For the linear regression ofpseudo-first-order kinetic model, the qe and k1 valueswere calculated from the log qe intercept and slope,respectively. As for pseudo-second-order kinetic mod-el, the qe and k2 values were calculated from the slopeand t/qe intercept, respectively. The Elovich constant βvalues were calculated from the slope of qt versus ln tplot while a constant values from the intercept. Theadsorption kinetics parameters of both chitosan andETB are shown in Table 6.

The pseudo-first-order model was only suitable todescribe sorption of Cu(II). In contrast, the linearisedElovich kinetic model was suitable for all sorption dataexcept for Cd(II) uptake value by chitosan beads. Allsorption data were well described by Ho’s linearisedpseudo-second-order kinetic model with high correlation

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

2 3 4 5 6 7

Met

al u

pta

ke (

mm

ol/g

)

Initial pH

Cd by ETB

Cu by ETBCd by chitosan

Cu by chitosan

Fig. 6 Cu(II) and Cd(II) uptake values (millimoles per gram) by20 chitosan beads and ETB as a function of initial pH

-100

102030405060708090

100

0 10 20 30 40 50 60

Met

al u

pta

ke (

%)

Number of beads

Cd by ETB

Cu by ETBCd by chitosan

Cu by chitosan

Fig. 5 Cu(II) and Cd(II) uptake values (%) by chitosan beadsand ETB as a function of number of beads

1720, Page 8 of 12 Water Air Soil Pollut (2013) 224:1720

coefficients (R2=0.9842 to 0.9991). The calculated qevalues were close to their respective experimental qevalues. The pseudo-first-order kinetic model is suitablefor reversible reaction of liquid and solid phases atequilibrium whereas the pseudo-second-order kineticmodel assumes rate-limiting step by adsorption mecha-nism but not mass transport (Azlan et al. 2009). The highgoodness of fit of the sorption kinetic data to pseudo-second-order model indicates that the rate-limiting stepfor the metal adsorption is likely controlled by a chemicalprocess.

3.4.4 Sorption Isotherm

The isotherm parameters of chitosan beads and ETB areshown in Table 7. In general, the Cd(II) sorption data ofboth ETB and chitosan beads were better fitted by theLangmuir isothermsmodel, while Cu(II) sorption data bythe Freundlich isotherm model. Other isotherm modelssuch as Temkin also has high goodness of fit for metalsorption by ETB. The Langmuir adsorption isothermassumes that adsorption takes place at specific homoge-neous sites within the adsorbents (Dahiya et al. 2008).The Langmuir qmax values were calculated from theintercept of the 1/qe versus 1/Ce plot. The thiolation tochitosan resulted in an increase in Langmuir qmax valuefor sorption of Cd(II) by up to 18 times. However, theLangmuirqmax value of Cu(II) decreased by almost seventimes. The Langmuir sorption constants, KL (liters permilligram) which reflects the affinity of metal adsorbateto sorbent were obtained from subsequent substitution ofqmax to the slope (1/KLqmax). The degree of suitability ofETB beads to adsorb Cu(II) and Cd(II) was estimated by

the separation factor constant (RL) calculated using Eq. 3.The RL values ranged between 0 and 1. This indicatedthat metal sorption to ETB and chitosan is a favourableprocess (Wang et al. 2010).

RL ¼ 1

1þ KLCoð3Þ

The Freundlich isotherm is an empirical equationemployed to describe heterogeneous absorption system(Benguella and Benaissa 2002). The n values were calcu-lated from the reciprocal of the slope of the plot of ln qeversus ln Ce, while KF values were derived from theintercept (ln KF). The n and KF values describe the sorp-tion intensity and capacity, respectively (Chatterjee et al.2010). All n values were greater than 1 except for Cd(II)uptake value by chitosan bead. n values of ETB was alsohigher than chitosan beads. This indicates that the bondsbetween metals and ETB are stronger than chitosan bead(Wan Ngah et al. 2010a). The Temkin isotherm modeldescribes adsorption on heterogenous surface. It assumesa linear decline of heat of adsorption with surface cover-age. The Temkin constants of heat of adsorption (B) canbe calculated from the slope of qe when plotted versus lnCe, whereas the sorption potential constant (A) can bederived from the intercept. The A values of ETB for bothCu(II) and Cd(II) were greater than chitosan beads. Thisindicates that thiolation has improved interactions of ad-sorbate (Cu(II) or Cd(II)) in comparison to the chitosanadsorbent (Rengaraj et al. 2007).

Table 6 Sorption kineticparameters of ETB andchitosan beads for Cu(II)and Cd(II)

Model Parameters ETB Chitosan

Cd(II) Cu(II) Cd(II) Cu(II)

Pseudo-first-order R2 0.8341 0.9874 0.5466 0.958

k1 (×10−4) 6.909 6.909 13.82 6.909

qe (mg/g) 22.77 25.57 1.823 37.18

Pseudo-second-order R2 0.9991 0.9842 0.9953 0.9846

k2 (×10−4) 1.911 1.068 23.46 0.7613

Calculated qe (mg/g) 42.74 34.01 6.337 48.08

Experimental qe (mg/g) 41.88 28.51 6.280 47.06

Elovich R2 0.9888 0.9294 0.5365 0.9302

α (mg g−1 min−1) 1.572 0.7481 203.9 1.063

β (g/mg) 0.1616 0.2330 2.341 0.1648

Water Air Soil Pollut (2013) 224:1720 Page 9 of 12, 1720

3.4.5 Metal Uptake Mechanisms

In general, metal uptake by ETB caused solution to bemore acidic. Both –SH and thiourea coexist as tauto-mers in ETB, where –SH binds with metal ions. Thedecreasing pH value was probably due to liberation ofproton from –SH group upon metal coordination(Donia et al. 2008). The proposed metal uptake mech-anism of ETB is shown in Fig. 7.

The band shifts in the FTIR spectra indicated metalcoordination with the corresponding functional groups(Cárdenas et al. 2009). The increasing wavenumber ofthioamide bandswas possibly due to an increase of doublebond character of ν(C-N), indicating metal coordinationwith S of the in the thioamidemoiety (Kagaya et al. 2010).Metal ions were fixed to amide and/or thiomide groups inETB. Cu(II) interaction with amide group in ETB wasmore evident than Cd(II). The stretching band of ν(CH–OH) also shifted to lower wavenumber in all metal-treated

ETB (Table 4), which indicated that metal–oxygeninteraction may have taken place.

As shown in Cu-ETB XPS results, Cu(II) ions werepossibly bound to –SH with simultaneously liberationproton ions, where the Cu(II) ion oxidized –SH to–SO3

− group (Gong et al. 2003). The binding energy of–NH2 have increased by 0.14 eV, indicating coordina-tion of lone pair electrons with Cu(II) ion (Dambieset al. 2000). The binding energy shifts of S(−II) andS(V) species showed interactions of –SH and –SO3

−

groups with Cu(II) and Cu(I) ions (Atzei et al. 1995).As for Cd-ETB, the bindingmechanism is similar to Cu-ETB. The decrease of peak areas of S(−II) and –NH2

coupled with a slight increase of peak areas of S(V) and–NH3

+ indicate binding of Cd(II) ion to –SH and –NH2

groups with minor oxidation –SH to –SO3− group. The

insignificant changes of binding energy values andatomic percentages of S(−I) species showed that metalbinding did not occur at –S–S– group.

Table 7 Sorption isotherm parameters of ETB and chitosan for Cu(II) and Cd(II)

Model Parameters ETB Chitosan

Cd(II) Cu(II) Cd(II) Cu(II)

Langmuir R2 0.9767 0.9472 0.8456 0.9778

KL (L/mg) 0.09139 3.049 0.01143 0.03360

qmax (mg/g) 48.31 16.00 2.682 108.7

RL 0.4856–0.08627 0.07373–0.004753 0.8342–0.4301 0.8442–0.2043

Freundlich R2 0.9310 0.9738 0.8155 0.9867

KF (mg/g) 5.375 10.46 0.02046 3.306

n (L/mg) 1.720 2.136 1.039 0.9963

Temkin R2 0.9601 0.9719 0.7178 0.8595

B (J/mol) 267.3 508.5 3942 169.8

A (L/g) 1.396 18.94 0.1124 0.8821

O

NH

OHO

OH

NH

S

O OHO

OH

O

N

OHO

OH

NH

SH

O OHO

OH

O

N

OH O

OH

NH

S

O OHO

OH

O

N

OH

O

OH

NH

S

OOH

O

OH

MM: Cd(II) or Cu(II)Tautomerism/isomerism

- 2H+

Fig. 7 Tautomerism of ETB and sorption mechanism with Cd(II) and Cu(II)

1720, Page 10 of 12 Water Air Soil Pollut (2013) 224:1720

Cu(II) ions which is considered as a borderline Lewisacid has binding preference to both hard Lewis base (−OHand –NH2 groups) and soft Lewis base (−SH and thioureagroups) (Pearson 1963). This may have accounted for thehigh uptake values in pristine chitosan beads, which havehigher amounts of –OH and –NH2 groups than ETB.However, chitosan modification at the –NH2 group didnot increase Cu(II) uptake value, possibly due to exhaus-tion of the metal-binding –NH2 group (Osifo et al. 2008).Even though –SH group has high affinity to Cu(II) ion,two –NH2 groups are converted to one >C=S groupduring thiolation of chitosan. Thus, ETB may have de-creased total binding sites for sorption of Cu(II) as com-pared with pristine chitosan. Moreover, the sorption ofCu(II) ions also resulted in substantial oxidation of –SH to–SO3

− group in ETB, leading to lower uptake values.However, sorption of Cd(II) ion did not cause significant–SH oxidation in ETB. The introduction of –SH grouphas increased the uptake value of Cd(II), possibly due togreater chemical interaction (Alfarra et al. 2004). Thisresult agrees with other sulfur-modified chitosan basedsorbent (mercaptosuccinic acid and thiirane graftedchitosan) (Lasko et al. 1993).

4 Conclusions

Over all, thiolation of chitosan beads has improved themetal interaction due to the presence of –SH or thio-urea groups in ETB. Chitosan beads were successfullythiolated with improvement of Cd(II) uptake by 18times. However, Cu(II) uptake of ETB was seven timeslower than that of pristine chitosan beads, possibly dueto exhaustion of –NH2 groups as a result of thioureacrosslinking in ETB. The significant oxidation of –SHto –SO3

− groups in Cu-ETB may have decreased uptakeof Cu(II). Furthermore, the presence of thiourea and –S–S– crosslinks has improved the stability of ETB in acidicconditions. This enables the application of ETB in flow-reactors for remediation of acidic wastewaters, as well asthe regeneration of spent ETB using acid. The elucidationof chitosan thiolation and its improved resistance to acidsolubilisation has potentially eliminated the unnecessaryuse of crosslinking agents to specifically improve beadstability.

Acknowledgment The senior author would like to thank Univer-sity of South Australia for UniSA President Scholarship award andUniversiti Teknologi MARA for UiTM Staff Scholarship award.

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