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Removal of cobalt(II) ion from aqueous solutionby chitosan–montmorillonite

Hailin Wang1, Haoqing Tang2,3, Zhaotie Liu2,3, Xin Zhang1,Zhengping Hao1, Zhongwen Liu2,3,⁎

1. Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China. E-mail: wanghailin@cee.cn2. Key Laboratory of Applied Surface and Colloid Chemistry, Shanxi Normal University, Xi'an 710062, China3. School of Chemistry & Chemical Engineering, Shanxi Normal University, Xi'an 710062, China

A R T I C L E I N F O

⁎ Corresponding author. E-mail: zwliu@snnu.e

http://dx.doi.org/10.1016/j.jes.2014.06.0211001-0742/© 2014 The Research Center for Ec

A B S T R A C T

Article history:Received 25 October 2013Revised 19 March 2014Accepted 3 April 2014Available online 2 July 2014

Montmorillonite (MMT) modified with chitosan (CTS, molecular weight = 5 × 104) wasapplied to remove heavy metal cations by using Co2+ as a model ion. An increase in MMTinterlayer distance observed from X-ray diffraction indicates the intercalation of CTS intoMMT. Together with the results of scanning electron microscopy and Fourier transforminfrared spectroscopy, it was concluded that the composite material of CTS and MMT (CTS–MMT) was prepared successfully. The mass ratio of CTS to MMT had a strong influence onthe adsorption performance of CTS–MMT. The highest adsorption value of 150 mg/g wasobtained over the composite material with CTS to MMT mass ratio of 0.25, which is muchhigher than those reported in other studies. The adsorption isotherms and kinetic resultsindicated that Co2+ was adsorbed over CTS–MMT in a multilayer model, and the chemicalsorption of Co2+ was determined to be the rate-limiting step.© 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.

Published by Elsevier B.V.

Keywords:ChitosanMontmorilloniteCo2+

IsothermKinetics

Introduction

Treatment of industrial wastewater with heavy metal ions is amajor issue and the development of effective adsorbents forremoval of those metal ions is of great importance. Activatedcarbon has been widely studied and used for the removal ofmercury, cadmium, lead, cobalt, copper and so on in terms ofcost, simplicity of design and operation (Netzer and Hughes,1984; Corapcioglu and Huang, 1987; Gomez-Serrano et al.,1998). However, high capital and regeneration cost limit itslarge scale application. Recent studies have focused onrelatively low cost adsorbents such as zeolite, montmorillon-ite, kaolinitic clay, mineral silicates, sepiolite and others (Aliand El-Bishtawi, 1997; Orumwense, 1996; Sanchez et al., 1999;Brigatti et al., 2000; Green-Pederson et al., 1997). Chitosan

du.cn (Zhongwen Liu).

o-Environmental Science

(poly-β-(1,4)-2-amino-2-deoxy-glucopyranose, CTS) is a prod-uct of the deacetylation of chitin (Septhum et al., 2007), whichis the second most abundant natural biopolymer on earth aftercellulose. CTS is an excellent natural adsorbent in that itsamine (\NH2) and hydroxyl (\OH) groups may serve ascoordination sites to form complexes with various heavymetal ions. It can remove metal ions effectively by adsorptiveenrichment. Compared with activated carbon, CTS is selfdegradative, non-toxic, low cost, and environmentally benign.Thus, it is widely used in many fields such as textiles, leather,food, bioengineering, agriculture, sewage treatment and so on.However, the poor selective adsorption, small surface area andlow porosity of chitosan limit its application in pollutantadsorption (Ahmaruzzaman, 2008). Montmorillonite (MMT) isa 2:1 layered aluminosilicate of smectite-type clay, which is

s, Chinese Academy of Sciences. Published by Elsevier B.V.

1880 J O U R N A L O F E N V I R O N M E N T A L S C I E N C E S 2 6 ( 2 0 1 4 ) 1 8 7 9 – 1 8 8 4

composed of two tetrahedral sheets with Si in the cationic sitessandwiching an octahedral Al sheet. The partial substitutionof Al3+ for the tetrahedral Si4+ and Mg2+ makes the layersnegatively charged, which is neutralized by exchangeablecations in the interlayer space. Commonly, alkali or alkalineearth metal cations like Na+, Ca2+, etc., are in the interlayer ofnatural MMT. The porous structure, high surface area, capa-bility of exchanging interlayer cations, and environmentalcompatibility make MMT one of the most promising candidatesfor decontamination and disposal of wastewater with high-level heavy metal cation content (Bhattacharyya and Gupta,2008).

It is hypothesized that the combination of CTS with MMTmay create a more efficient adsorbent. Thus, in this work, wedeveloped a simple method to prepare a composite of MMT andCTS, and Co2+ was selected as a typical heavy ion to test theadsorption with the aim of testing the removal efficiency of thisnew material.

1. Materials and methods

1.1. Preparation of CTS–MMT

CTS of 0.5 g with molecular weight 5 × 104 (>90% degree ofdeacetylation, Golden-Shell Biochemical Co., Ltd., China) wasdissolved in 50 mL, 5% (V/V) acetic acid (China PharmaceuticalGroup Co., Ltd., China) to form a sol, and the sol was stirred at60°C for 3 hr. After cooling to 30°C, a certain content of MMT(Zhejiang Sanding Technology Co., Ltd., China) was added intothe sol according to the designed mass ratio (CTS:MMT = 1,0.5, 0.25, 0.1, 0.05) and stirred for 24 hr. After drying the solidin a vacuum oven at 70°C, it was ground and sieved to100 mesh particles that were then collected for adsorptionstudies.

1.2. SEM, XRD and FT-IR characterization

Scanning electron microscope (SEM) observations were car-ried out (Quanta 200 Philips, USA). X-Ray diffraction patterns(XRD) were recorded in the range of 2–40° (2θ) with adiffractometer (D/max 2550 VB+/PC Rigaku, Japan) using CuKα radiation (λ = 0.154 nm). The applied voltage and currentwere 30 kV and 40 mA, respectively. Fourier transforminfrared spectroscopy (FT-IR) was also performed using aspectrometer (Avatar 360 Nicolet, USA). FT-IR spectra in thetransmittance mode were recorded in the range of 500–4000 cm−1 at a resolution of 4 cm−1.

1.3. Co2+ adsorption tests

Co2+ adsorption experiments were carried out by adding 0.1 gCTS–MMT into 25 mL, 8.25 × 10−4 g/mL Co(NO3)2 (China Phar-maceutical Group Co., Ltd., China) solution at 30°C for 1, 2, 4, 8,12 and 24 hr, respectively. The slurry after adsorption wasfiltered and centrifuged. The concentration of Co2+ in thesolution was measured with an atomic adsorption spectro-photometer (AAS, Beijing Purkinje, China), and the interactionof Co2+ with the adsorbent was studied using a UV–Vis diffusereflectance spectrophotometer (UV-3000, Hitachi, Japan), andthermodynamic and kinetic studies of Co2+ adsorption werealso carried out.

2. Results and discussion

2.1. Properties of CTS, MMT and CTS–MMT

SEM micrographs of CTS, MMT and CTS–MMT are shownin Fig. 1. CTS displays an irregular particulate morphologyhaving smooth surfaces. The MMT is composed of flakes oragglomerated flakes with varied sizes. In the case of CTS–MMT, larger seemingly gelled particles with small flakes andrough surfaces were observed.

The XRD patterns of CTS, MMT and CTS–MMT are given inFig. 2. From the characteristic (001) diffraction, structuralchanges of the MMT are clearly observed. The (001) diffractionpeak of MMT occurred at 2θ of 7.545°, corresponding to a d001spacing of 1.1707 nm. However, the d001 spacing of CTS–MMTwas slightly increased to 1.3497 nm (2θ = 6.543°), indicatingthat the MMT interlayer expanded, which can be reasonablyascribed to the intercalation of CTS into the MMT interlayerregion, leading to the formation of CTS–MMT.

The FT-IR spectra of CTS, MMT and CTS–MMT are shownin Fig. 3. The IR spectra of CTS exhibit an \OH stretchingvibration at around 3400 cm−1, \CH stretching vibration at2932 cm−1, and \NH bending vibration at 1650 cm−1, respec-tively. The\CH2 bending vibration observed at 1451 cm−1 andthe peaks at 1018 and 1110 cm−1 belong to the \CO stretchingvibration in the \COH group. The shoulder IR adsorption at3625 cm−1 originates from the stretching of hydroxyl groupscoordinated to octahedral cations. The 3442 and 1639 cm−1

bands for MMT are assigned to stretching and bendingvibrations of hydroxyl groups in molecular water present inMMT. The most intense peak at 1040 cm−1 derives from thestretching vibration of Si\O in the tetrahedral sheet. Incomparison with the spectra of CTS and MMT, two character-istics of CTS–MMT are observed, one is that an intensenew peak and a small new peak appear at 2355 cm−1 and1554 cm−1, the other is that \OH, \NH, and \CH grouppositions clearly shifted. These changes indicate that stronginteractions are present in CTS–MMT.

2.2. Adsorption behavior

Fig. 4 shows the adsorption results of Co2+ on CTS–MMT fordifferent times (1, 2, 4, 8, 12 and 24 hr). It was found that theadsorption amounts for these CTS–MMT materials increasedquickly in 4 hr and then became slow and reached themaximum value at 12 hr. CTS–MMT with a mass ratio of 0.25showed the highest adsorption amount (150 mg/g) at 12 hr.The adsorption potential of CTS–MMT for cobalt in this studywas compared with that of other reported adsorbents assummarized in Table 1 (Manohar et al., 2006; Yavuz et al.,2003; Parab et al., 2006; Anoop Krishnan and Anirudhan, 2008;Ahmadpour et al., 2009; Suhasini et al., 1999; Erdem et al.,2004; Smiciklas et al., 2006; Repo et al., 2009; Bhatnagar et al.,2010). It can be seen that the prepared adsorbent in this studyshows good performance for cobalt removal, which might berelated to the combination of the advantages of both CTS andMMT.

As mentioned above, CTS is known as a good adsorbentdue to its abundant amine (\NH2) and hydroxyl (\OH)

Fig. 1 – SEM images of MMT, CTS, and CTS–MMT.

1881J O U R N A L O F E N V I R O N M E N T A L S C I E N C E S 2 6 ( 2 0 1 4 ) 1 8 7 9 – 1 8 8 4

groups, which can form complexes with various heavy metalions. Here, the prepared CTS–MMT maintains the adsorptioncharacteristics of CTS, and \OH and \NH groups are alsoobserved via FT-IR, which might serve as coordination sites toform complexes with Co2+. The UV–Vis diffuse reflectancespectra of Co2+-absorbed CST–MMT are shown in Fig. 5. It canbe observed clearly that the spectrum is dominated by twopeaks occurring at 250 nm and 300–600 nm, respectively. Thenarrow peak at 250 nm can be attributed to the absorption ofCST–MMT.Meanwhile, the broad peak occurring at 300–600 nmis characteristic of Co2+ (Zhang et al., 2011). Furthermore, nopeak attributable to Co3+ is detected. Thus, the removal of Co2+

is carried out by the formation of Co2+ complexes, and theadsorption is a chemical adsorption process.

Langmuir, Freundlich and Temkin isotherms are widelyused to describe the adsorption characteristics of adsorbents.Important information about adsorbent surface propertiescan be known from the adsorption isotherms (Liu et al., 2011;Juang and Shao, 2002), and the linear form of Langmuir,Freundlich and Temkin isotherms can be written as Eqs. (1)–(3), respectively.

Ce

Qe¼ 1

BQmþ Ce

Qmð1Þ

5 10 15 20 25 30 35 40

Inte

nsity

(a.u

.)

2θ (°)

CTS-MMT

CTS

MMT

Fig. 2 – XRD patterns of MMT, CTS, and CTS–MMT.

logQe ¼ logK þ 1nlogCe ð2Þ

Qe ¼ Aþ B logCe ð3Þ

where, Ce (g/L) is the equilibrium concentration of Co2+ insolution, Qe (mmol/g) is the amount adsorbed per unitmass of absorbent, Qm (mmol/g) is the maximum sorptioncapacity, and B, K, n, and A are adsorption constants. Thecorrelation coefficients based on the three isotherms aresummarized in Table 2. Independent of the adsorbents, theTemkin equation gave the best fit for all the results,indicating multilayer adsorption of Co2+ on adsorption siteshaving different energies.

The efficiency of an adsorbent for the removal of metalcations is essentially determined by adsorption kinetics. Inorder to determine the controlling steps such asmass transferand chemical reaction, different kinetic models are used to fitthe experimental data (Annadurai et al., 2008; Zhou et al.,2009). The linear form of the pseudo first-order kinetic model,pseudo second-order kinetic model, and intraparticle diffu-sion kinetic model can be written as follows, respectively.

1qt

¼ k1qet

þ 1qe

ð4Þ

4000 3200 2400 1600 800

Tran

smitt

ance

(%)

Wavenumbers (cm- 1)

MMT

CTS-MMT

CTS

Fig. 3 – FT-IR spectra of CTS, MMT, and CTS–MMT.

0 5 10 15 20 25

1

2

3

4

Ads

orb

amou

nt (m

mol

/g)

Time (hr)

1 0.50.25 0.10.05

Fig. 4 – Adsorption of Co2+ on the CTS–MMT with differentmass ratios.

1882 J O U R N A L O F E N V I R O N M E N T A L S C I E N C E S 2 6 ( 2 0 1 4 ) 1 8 7 9 – 1 8 8 4

tqt

¼ 1k2qe2

þ 1qe

� �t ð5Þ

qt ¼ kit12 þ c ð6Þ

where, qt (mg/g) is the amount of Co2+ at time t (min), qe (mg/g)is the equilibrium adsorption capacity, and k1 (min−1), k2(g/(mg·min)) and ki (mg/(g·min0.5)) are the rate constants of thepseudo first-order kinetic model, pseudo second-order kineticmodel and intraparticle diffusion kinetic model, respectively.According to the three kineticmodels, the correlation coefficientsare listed in Table 3. The adsorption of Co2+ on CTS–MMT bestfits the pseudo second-order equation, indicating that chemicalsorption is the rate-limiting step.

The calculated values of free energy change (ΔG), enthalpychange (ΔH), and entropy change (ΔS) are given in Table 4.They were calculated using the following equations.

Kc ¼ CA

Csð7Þ

Table 1 – Comparison of Co2+ adsorption over different adsorbe

Adsorbent Co2+

(mg/L)Adsorbent dose Reaction time

(hr)

Clay 25 2.0 g/L 24Kaolinite 2400 1.0 g/L 2Coir pith 50 2.0 g/L 2Activated carbon 1000 2.0 g/L 4Almond green hull 50 5.0 g/L 0.1Biosorbent 500 2.0 g/L 24Zeolites 400 20 g/L 5.5Hydroxyapatite 5–1000 – 24Silica gel 400 2.0 g/L 4Lemon peel 1000 10 g/L 10CTS–MMT 825 4 g/L 12

–: not mentioned.

ΔGθ ¼ −RT lnKc ð8Þ

lnKc ¼ ΔSθ

R−ΔHθ

RTð9Þ

where, Kc is the equilibrium constant, CA (g) is the amount ofCo2+ adsorbed in per unit mass of absorbent at equilibrium, Cs

(g/L) is the equilibrium concentration of the Co2+ in solution,ΔGθ (kJ/mol) is the adsorption free energy change, ΔHθ (kJ/mol)is the enthalpy change, ΔSθ (J/(K·mol)) is the entropy change, R(kJ/(K·mol)) is the gas constant, and T (K) is the absolutetemperature. The values of enthalpy (ΔHθ) and entropy (ΔSθ)were obtained from the slope and intercept of lnKc vs 1/Tplots.

The calculated values of free ΔG, ΔH and ΔS were obtainedaccording to the methods in other studies (Septhum et al.,2007; Choudhari and Kariduraganavar, 2009). The value of ΔHθ

is positive (14.00 kJ/mol), suggesting that the adsorption ofCo2+ is endothermic. The value of ΔGθ (−1.66 kJ/mol) isnegative, indicating that the reaction is spontaneous at 30°C.The positive value of ΔSθ (51.67 J/(mol·K)) shows that particlesin equilibrium solution decrease randomness compared tothe initial solution. Before adsorption, there were only metalions and water molecules, but metal ions were combined withwater molecules, forming aquo-metal ions. When adsorptionequilibrium was reached, some metal ions in the solutionwere absorbed by CTS, and water molecules were releasedfrom the aquo-metal ions, thus the total particles in equilib-rium solution increased.

3. Conclusions

A simple method was developed to prepare CTS–MMTcomposites, whose formation was confirmed by XRD andFT-IR results. Greatly enhanced adsorption of Co2+ wasachieved on CTS–MMT in comparison with CTS or MMTalone, and was dependent on the specific experimentalconditions and the molecular weight of CTS. Irrespective ofthe initial concentration of Co2+, CTS–MMT with mass ratio0.25 (CTS:MMT) showed better performance for Co2+

nts.

Adsorption capacity(mg/g)

Reference

38.6 Manohar et al. (2006)0.9 Yavuz et al. (2003)

12.8 Parab et al. (2006)153.6 Anoop Krishnan and Anirudhan (2008)45.5 Ahmadpour et al. (2009)

190.0 Suhasini et al. (1999)14.4 Erdem et al. (2004)20.9 Smiciklas et al. (2006)20 Repo et al. (2009)22 Bhatnagar et al. (2010)

150 This study

200 300 400 500 6000.0

0.1

0.2

0.3

Wavelength (nm)

Abs

orba

nce

Fig. 5 – UV–Vis pattern of Co2+ absorbed CTS–MMT.

Table 2 – Parameters of three kinds of adsorption models.

Mass ratio Langmuir Frenudlich Temkin

Qm B R2 1/n k R2 A B R2

1 1.91 × 10−3 −2.53 0.9945 −2.26 1.77 × 10−3 0.9972 −1.7 × 10−3 −0.033 0.99980.5 2.84 × 10−3 −3.08 0.9986 −2.91 1.90 × 10−3 0.7942 −8.5 × 10−4 −0.029 0.99990.25 8.64 × 10−3 −10.24 0.9946 −0.53 6.88 × 10−3 0.9905 4.5 × 10−3 −0.016 0.99620.1 4.00 × 10−3 −4.05 0.9842 −1.22 3.44 × 10−3 0.9870 3.14 × 10−4 −0.026 0.99740.05 3.30 × 10−3 −3.39 0.9958 −1.49 2.88 × 10−3 0.9972 −4.28 × 10−4 −0.028 0.9996

Table 3 – Correlation coefficients (R2) of Co2+ adsorption byCTS–MMT.

Massratio

Pseudofirst-order

Pseudosecond-order

Intraparticlediffusion

1 0.8172 0.9966 0.81560.5 0.6476 0.9997 0.77680.25 0.2867 0.9935 0.82460.1 0.6228 0.9918 0.8640.05 0.3912 0.9918 0.9226

1883J O U R N A L O F E N V I R O N M E N T A L S C I E N C E S 2 6 ( 2 0 1 4 ) 1 8 7 9 – 1 8 8 4

adsorption than other samples, and the maximum value was150 mg/g. The adsorption isotherms of Co2+ on the compos-ites were best fitted by the Temkin equation, indicatingmultilayer adsorption of Co2+. Kinetics study revealed thatthe adsorption of Co2+ on CTS–MMT followed the pseudosecond-order equation, indicating that chemical sorption isthe rate-limiting step. Thermodynamic parameters such asΔGθ, ΔHθ, and ΔSθ were also evaluated to understand thenature of sorption.

Table 4 – Thermodynamic parameters.

Metalions

T (°C) lnKc ΔGθ

(kJ/mol)ΔSθ

(J/(mol·K))ΔHθ

(kJ/mol)

Co2+ 30 0.68 −1.66 51.67 14.0060 1.12 −3.10 / /90 1.61 −4.86 / /

As discussed above, the adsorption of Co2+ over CTS–MMTis a chemical process due to the formation of Co2+ complexeswith amine (\NH2) and hydroxyl (\OH) groups and theparameters ΔGθ and ΔSθ also suggest that the process isspontaneous and irreversible. Thus, the regeneration of thissynthesized material is difficult using common methodssuch as washing, calcination and others. Considering the lowcost of CTS–MMT and the highly enriched Co2+ content ofspent adsorbent, centralized disposal with other treatmentsto recycle Co-related materials might be a more economicaloption than regeneration.

Acknowledgments

This work was supported by the Fundamental Research Fundsfor the Central Universities (No. GK201002043).

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