Behaviour and mechanism of Zn(II) adsorption on Chinese loess at dilute slurry concentrations

Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 83:673–682 (2008)

Behaviour and mechanism of Zn(II)adsorption on Chinese loess at diluteslurry concentrationsXiaowu Tang,∗ Zhenze Li and Yunmin ChenKey Laboratory of Soft Soils and Geoenvironmental Engineering, Ministry of Education (Zhejiang University), Department of CivilEngineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, China

Abstract

BACKGROUND: Zn(II) is commonly present in mining drainage in developing countries. Since loess is abundantand always located near mining sites in China, it would be useful to investigate the possibility and efficiency ofusing loess to remove Zn(II) from aqueous solution.

RESULTS: The Zn(II) adsorption capacity of Chinese loess was determined as 215.9 mg g−1. The adsorptionfollowed pseudo-second-order kinetics and took place mainly by surface diffusion. Generally, higher initial pHand solute concentration resulted in higher % Zn(II) removal, while higher temperature and slurry concentrationled to lower % Zn(II) removal. A thermodynamic study revealed that the adsorption process was exothermic, withthe predicted enthalpy change ranging from −20.87 to −4.06 kJ mol−1. With the assistance of X-ray photoelectronspectroscopy and X-ray diffraction, the high adsorption capacity was ascribed to the growth of micro-organismsand mineral constituents such as kaolinite and goethite.

CONCLUSION: Chinese loess proved effective for Zn(II) adsorption in this study. The optimal adsorptionconditions included pH >3.0, temperature ≤15 ◦C and contact time ≈400 min. As an abundant natural soil in aridareas with very low population density, it would be appropriate to develop this material into a wastewater-purifyingagent. 2008 Society of Chemical Industry

Keywords: Zn(II) adsorption; Chinese loess; kinetics; thermodynamics; micro-organism

INTRODUCTIONZinc is an essential trace nutrient required by mostliving organisms for healthy growth and enzymefunction.1,2 However, it becomes toxic to plants andanimals at elevated concentrations. Therefore themaximum concentration limits (MCLs) in surfacewater, ground water and potable water are strictlyregulated.2 However, high concentrations of Zn(II)are often observed in industrial wastewater dischargedby metal-processing companies, battery manufacturersand ore-mining corporations. Uncontrolled emissionof such wastewater is common in developing countriesand thus results in severe environmental problems.

Many methods are available to remove Zn(II)from aqueous solution. Chemical precipitation withCa(OH)2, Fe2(SO4)3 or NaOH is a traditionallyaccepted process. In spite of their superior effects, thelarge consumption of chemicals implies a relativelyhigh operating cost, voluminous by-products andsludge.3 As a result, some other methods have beendeveloped recently. Adsorbents such as plant residue,4

natural soil,5 industrial waste6 and activated carbon7

have been widely studied and proven as potentialor alternative means to decrease the concentration ofheavy metal ions in water. Various kinds of methods ofmodifying sorbents to improve their sorption capacityfor Zn(II) from aqueous solution have been developedand recently become practical.8,9

Lead/zinc ore mining is one of the most importantindustries in some areas of western China. Non-ferrousmetal metallurgy in Silver City, Gansu Province has ahistory of over 2000 years. The average concentrationof heavy metals in the surface water of the EasternDitch, which runs across the city, has been found to be13.0–73.0 times higher than the legislated maximumallowable concentration (MAC) in China.10 Zn(II) isthe most abundant pollutant in the wastewater, with amaximum concentration of 51.04 mg g−1.10 Recently,the local administration put forward a huge plan topurify the industrial wastewater. Although there aremany ways to achieve this aim, cost and efficiency

∗ Correspondence to: Xiaowu Tang, Key Laboratory of Soft Soils and Geoenvironmental Engineering, Ministry of Education (Zhejiang University), Departmentof Civil Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, ChinaE-mail: [email protected]/grant sponsor: Key Project of National Natural Science Foundation of China (NSFC); contract/grant number: 50538080Contract/grant sponsor: National Science Fund for Distinguished Young Scholars; contract/grant number: 50425825(Received 6 September 2007; revised version received 25 October 2007; accepted 31 October 2007)Published online 12 March 2008; DOI: 10.1002/jctb.1848

2008 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2008/$30.00

X Tang, Z Li, Y Chen

are two key points for local administrators and relatedcorporations. Similar situations also exist elsewhere inChina and other developing countries.

Loess is a widely distributed soil in northwesternChina, middle Asia, Russia, the Middle East andNorth America. The Chinese loess covers about640 000 km2 to an average depth of several hundredmetres.11,12 This soil is in the size range of silt clay withplenty of calcite, which is an effective acid-bufferingmaterial. The feasibility of utilising loess as a potentialadsorbent for heavy metals remains unclear. It wouldthus be useful to investigate the heavy metal adsorptionbehaviour of loess with a view to decreasing the costof industrial wastewater treatment.

The main purpose of the present work was toexamine the possibility and efficiency of using Chineseloess to remove Zn(II) from aqueous solution, aswell as the related factors affecting this process.The mechanisms of interaction between Zn(II) andthe sorbent are also discussed with the assistance ofmethods of direct observation.

MATERIALS AND METHODSPreparationThe sorbent used in this study, Chinese loess, wascollected from the suburban area of Xi’an, China. Thesoil sample was taken at a level 5 m below the surfacein the unsaturated zone. It has been reported that thesoil in this location is a typical Quaternary loess.11 Thesampled soil was sealed in plastic bags for storage. Noplant residues were found in the soil. Before furthertesting, the soil was oven dried at 105 ◦C for 24 h toremove bulk water.

Conical flasks (250 mL) were cleaned by rinsingwith 1 mol L−1 HCl solution for 24 h and then washedwith clean water and deionised water (DW) three timeseach. DW was bubbled with N2 to eliminate dissolvedCO2. Zinc sulfate (analytical grade) was dissolved inDW to prepare solutions with Zn(II) concentrationsranging from 10 to 700 mg L−1.

Characterisation of sorbentThe chemical constituents of loess were determinedby Inductively Coupled Plasma mass spectrometry(ICP-MS PQ3, Thermo Electron Corp., Waltham,USA.). The content of each element in the sorbentwas transformed into the wt% of its related oxide.The organic content was determined by ignitionat 650 ◦C for 8 h in an oven in the presence ofO2 gas. The loss of weight was assigned to theorganic content. The surface area of the sorbent wasdetermined by the N2 adsorption method, and theresults were analysed by Brunauer–Emmett–Teller(BET) adsorption theory.13,14 The cation exchangecapacity (CEC) of the sorbent was evaluated byammonium exchange following the method of Tan.13

The natural pH (pHna) of loess was determined bymixing 0.1 g of loess with 50 mL of DW, and thepoint of zero charge pHpzc of loess was determined

by acid–alkaline titration of the sorbent. The solutionpH was measured with a glass electrode potentiometer(Shanghai Precision & Scientific Instrument Co.,Shanghai, China).

MethodsIsothermal adsorption testA sample of loess was divided into several parts,each weighing 0.100 ± 0.002 g. The sorbent wastransferred to pretreated conical flasks containingZn(II) solutions (50 mL) of various concentrations.The flasks were covered and placed in a shaker(180 rpm) at 25 ± 0.2 ◦C for 24 h. The slurry wasthen centrifuged at 1400 A – g for 5 min and theequilibrium Zn(II) concentration in the supernatantwas determined by atomic adsorption spectroscopy(AAS). Controls were conducted to evaluate theadsorption of Zn(II) on the inner surface of the flasks.Each test was duplicated to obtain an average result.The net adsorption amount of Zn(II) on the sorbentwas calculated by subtracting the amount adsorbedon the flask (obtained from the control test) from thedifference between the initial and equilibrium Zn(II)concentrations.

Effect of slurry concentrationSlurry concentrations were varied from 0.1 to 0.5 gloess per 50 mL in increments of 0.1 g loess per50 mL for the determination of sorbent dosage. Inthis set of tests the solution pH was unadjusted,the reaction duration was 24 h, the initial Zn(II)concentration was 300 mg L−1 and the temperaturewas 25 ± 0.2 ◦C. Zn(II) removal efficiency (R) wascalculated by dividing the difference between theinitial (Ci) and final (Cf ) Zn(II) concentrations (mgL−1) by the initial concentration (mg L−1): R (%) =[(Ci − Cf )/Ci] × 100.

Effect of pHThe sorbent (0.100 ± 0.002 g) was placed in apretreated flask. Zn(II) solution (50 mL) of con-centration 200 mg L−1 was first adjusted with10−1 –10−3 mol L−1 H2SO4 or NaOH solutions to therequired pH between 1.0 and 7.2 and then poured intothe flask with the sorbent. The addition of acid or alkaliwas not allowed to exceed 10% of the total volume ofthe solution; in fact, the volume of the additive wasstrictly controlled and did not even reach 0.5%. Otherconditions included a temperature of 25 ± 0.2 ◦C, ashaking time of 24 h and a shaking speed 180 rpm.The final solution pH values were also measured.

Adsorption kineticsIn this series of tests the dosage of the sorbentwas 0.100 ± 0.002 g per 50 mL, the temperature was25 ◦C, the Zn(II) concentrations were 50, 100 and200 mg L−1 and the solution pH was not adjusted.The shaking durations were increased from 10 minto 24 h. At the end of the specific duration, shakingwas stopped, the relevant flasks were removed for

674 J Chem Technol Biotechnol 83:673–682 (2008)DOI: 10.1002/jctb

Zn(II) adsorption on Chinese loess

sampling and testing, and shaking was resumed tocontinue equilibrating the other samples.

Thermodynamic testA dosage of 0.100 ± 0.002 g per 50mL without pHadjustment was used to investigate the effect of varyingthe equilibration temperature. For each set of tests theduration was 24 h and the initial Zn(II) concentrationswere 200, 500 and 700 mg L−1. The temperature wasincreased from 15 to 45 ◦C in 10 ◦C increments.

Direct observationsX-ray photoelectron spectroscopy (XPS), X-raydiffraction (XRD) and transmission electronmicroscopy (TEM) were performed on both crudeand Zn(II)-loaded loess samples to investigate theinherent uptake mechanism.

XPS data were acquired with an AXIS-Ultrainstrument (Kratos Analytical, Manchester, UK)located in the Analytical and Analysis Centreof the Chemistry College of Peking University.Monochromatic Al Kα radiation (225 W, 15 mA,15 kV) and low-energy electron flooding were utilisedfor charge compensation. To compensate for surfacecharge effects, binding energies were calibrated usingthe C-1s hydrocarbon peak at 284.80 eV. The datawere converted into VAMAS file format and importedinto the CasaXPS software package (Casa SoftwareLtd, Cheshire, UK) for curve fitting.

XRD patterns were obtained using a D/MAX-RAinstrument (Rigaku Corp., Tokyo, Japan) equippedwith a Cu Kα tube and an Ni filter.

TEM was conducted on a JEOL 200CX instrument(JEOL, Tokyo, Japan). These tests were carried outwith the assistance of engineers in the Analytical andTesting Centre of Zhejiang University. The Zn(II)-loaded loess was obtained by equilibrating the sorbent(0.1 g per 50 mL) in 200 mg L−1 Zn(II) solution for24 h, filtered through a 0.45 µm filtration membraneand then dried under IR radiation. In TEM tests thesample was dispersed in DW with turbulent agitationand then settled for 24 h. Finally, the supernatantliquid was sampled for observation.

RESULTS AND DISCUSSIONCharacterisation of sorbentThe basic elemental constituents of the sorbentare listed in Table 1. The contents of silicateand aluminium oxide were 63.68 and 12.77 wt%respectively, corresponding to clay minerals such asillite and kaolinite.11 Calcium was the third mostabundant element (9.56 wt% in oxide form). Asthe pHna of loess is 8.24, it may act as an effectivebuffer for acidic solutions. However, the zero point ofsurface charge was observed at pH 2.82, indicating thenegative surface charge of Chinese loess. The surfacearea and CEC were determined as 24.1 m2 g−1 loessdry weight and 11.2 cmol(+) kg−1 respectively.

Table 1. Constituents and basic parameters of sampled Chinese

loess

Constituent Amount (wt%)

SiO2 63.68Al2O3 12.77CaO 9.56MgO 3.14K2O 3.01Fe2O3 2.74Na2O 2.35FeO 0.89TiO2 0.78MnO 0.09

Parameter Value

Loss on ignition (g/100 g) 1.64Specific gravity 2.75Surface area (m2 g−1) 24.1CEC (cmol(+) kg−1) 11.2pHna 8.24pHpzc 2.82

IsothermsFigure 1 shows the Zn(II) adsorption isotherm. Threeisothermal equations were applied to evaluate the testresults.

The Langmuir isotherm can be written as

1/qe = 1/Q + 1/bQCe (1)

where qe is the unit adsorption amount (mg g−1), Ce

is the equilibrium solute concentration (mg L−1), Q isthe maximum surface density of the sorbent and b isthe Langmuir constant (L mg−1).

The Freundlich isotherm can be expressed as

qe = kfC1/ne (2)

where kf is the Freundlich constant (mg g−1)indicating the adsorption capacity and n is theheterogeneity factor.

Figure 1. Isothermal adsorption of Zn(II) on Chinese loess (25 ◦C,0.1 g per 50 mL).

J Chem Technol Biotechnol 83:673–682 (2008) 675DOI: 10.1002/jctb


The Dubinin–Rasdushkevich (D–R) isotherm isgiven by14

ln qe = ln qm − kε2 (3)

where qm is the maximum adsorption capacity (mgg−1), k is a constant related to the adsorption energyand

ε = RT ln(1 + 1/Ce) (4)

is the Polanyi potential. The mean free energy ofadsorption is

E = 1/(2k)1/2 (5)

Basically, when E lies between 8 and 16 kJ mol−1,the adsorption is in the form of ion exchange.14 AnE value between 1 and 8 kJ mol−1 implies physicaladsorption.15,16

The test results were fitted with Eqn (1) and theLangmuir constants are presented in Table 2. Theadsorption capacity is predicted as 215.9 mg g−1 witha correlation coefficient as high as 0.995. Followingthe definition of Giles and Smith,17 the isothermshown in Fig. 1 can be designated as type ‘H’. Theseresults confirm the high affinity of loess for Zn(II).Other researchers have conducted similar studies onvarious sorbents,1,18,19 but few have been reported tohave a Zn(II) adsorption capacity as high as that ofloess. This may indicate the applicability of loess forindustrial wastewater treatment. Given the low costand wide distribution of loess, it is important thatdeveloping countries pursue the study and utilisationof this natural soil as a purifying material.

The Freundlich isotherm constants are also listed inTable 2. The predicted parameter n (4.98) indicatesthe high degree of surface heterogeneity, which willbe further verified by the TEM results. Therefore thisresult suggests that multilayer adsorption could beconsidered as the mode.

The predicted adsorption capacity with the D–Risotherm was 351 mg g−1, much higher than thatobtained with the Langmuir model. This may bedue to the inherent presumption of the D–R modelthat all micro/macropores are filled with solute. Sincesuch a situation is difficult to achieve in practice, the

Table 2. Predicted isothermal adsorption constants

Parameter Value

Langmuir isothermQ (mg g−1) 215.9b (L mg−1) 0.0443R 0.995Freundlich isothermkf (L g−1) 65.39n 4.98R 0.990D–R isothermXm

′ (mol g−1) 0.00544K′ (mol2 kJ2) 0.003E (kJ mol−1) 12.91R 0.999

predicted adsorption capacity was much higher thanthe test data. The calculated average free energy (E)was 12.91 kJ mol−1, indicating the occurrence of ionexchange. This was consistent with the results of otherresearchers.15,16

Effect of slurry concentrationFigure 2(a) shows the plots of % Zn(II) removaland unit adsorption amount of loess versus slurryconcentration. As the proportion of sorbent increasedfrom 0.002 to 0.010 g mL−1, the Zn(II) removalexhibited a concomitant rise from 86.0 to 99.1%, whilethe unit adsorption amount decreased from 107.3 to20.1 mg g−1. It was observed that a fivefold increase insorbent addition led to only a 13.1% increase in Zn(II)removal. A similar phenomenon has been reported inother studies. In dilute slurry the degree of hydrationof single sorbent particles is high, thus providing moreactive sites for heavy metals.20,21 Accordingly, thetreatment process of Zn(II)-contaminated wastewatercan be designed to include several sequential additionsand separations of loess at low slurry concentration tomaintain a high removal efficiency.

Effect of pHFigure 2(b) shows the variation in final equilibriumZn(II) concentration (Cf) and % Zn(II) removal withinitial pH (pHi). It is seen that Cf decreased linearlyfrom 17.5 to 0.5 mg L−1 as pHi increased from 1.0to 5.0. A further increase in pHi did not give riseto a further decrease in Cf . The removal efficiencyincreased from 91 to 99.75% as pHi rose from 1.0to 5.0. Thus nearly complete removal of Zn(II) wasobtained at pHi values higher than 5.0.

Figure 2(b) also shows that the pH at equilibrium(pHe), which was close to pHi at pHi ≤ 3.0, reacheda plateau near 7.0 at pHi ≥ 4.0. It was observed thatall the final pH values of solutions lay above or nearthe diagonal, indicating the buffering effect of thecarbonate in loess.

Adsorption kineticsFigure 2(c) shows the results of kinetic tests, whichwere further analysed with pseudo-first-order kinetics,pseudo-second-order kinetics as well as intraparticlediffusion kinetics.

The pseudo-first-order kinetic equation is14

log(qe − qt) = log(qe) − (k1/2.303)t (6)

where qe and qt are the amounts of solute adsorbed perunit adsorbent at equilibrium and time t respectively(mg g−1) and k1 is the pseudo-first-order rate constantfor the adsorption process (min−1).

The pseudo-second-order kinetic equation is14

t/qt = 1/k2q2e + (1/qe)t (7)

where k2 is the pseudo-second-order rate constant (gmg−1 min−1).



Figure 2. Effects of various factors on Zn(II) adsorption on Chinese loess. (a) Effect of slurry concentration on Zn(II) removal and amount ofadsorption on loess (Ci = 300 mg L−1, 25 ◦C, unadjusted pH). (b) Effect of initial pH on amount of Zn(II) adsorption and equilibrium pH of solution(25 ◦C, 0.1 g per 50 mL, Ci = 200 mg L−1). (c) Effect of reaction time on amount of Zn(II) adsorption (25 ◦C, 0.1 g per 50 mL, unadjusted pH). (d) Fit ofkinetic results with intraparticle diffusion model. (e) Effect of temperature on amount of Zn(II) adsorption at various solute concentrations.

The equation for the intraparticle diffusion modelis14

qt = kintt1/2 + C (8)

where kint is the relevant rate constant (mgg−1 min−1/2) and C is the intercept.

The test data were analysed with the above threekinetic equations. Table 3 gives the predicted param-eters. It was shown that higher initial concentrationresulted in higher qe. By comparison of the correla-tion coefficients obtained by model simulations, thepseudo-second-order kinetics was the best model tofit the results.

As shown in Fig. 2(d), the fitted curves with theintraparticle diffusion model appeared multilinear,with double linearity for solutions with Ci = 50 and100 mg L−1 and triple linearity at Ci = 200 mg L−1.The predicted model constants are listed in Table 3.The slopes of the first linear portions (ki1) increasedfrom 2.80 to 5.86 and 6.22 mg g−1 min−1/2 withincreasing Ci from 50 to 100 and 200 mg L−1

respectively, indicating faster surface adsorption athigher solute concentrations. The absence of thesecond linear portion at Ci = 50 and 100 mg L−1

indicated that diffusion through micropores into soilparticles did not occur, because the initial surface



Table 3. Predicted constants for Zn(II) adsorption kinetics

Ci (mg L−1)

Adsorption kinetics 50 100 200

Pseudo-first-order kineticsqe 24.56 53.91 95.83k1 0.014 0.018 0.007R2 0.9948 0.9899 0.9535Pseudo-second-order kineticsqe (mg g−1) 24.73 48.73 97.85k2 (10−4 g mg−1 min−1) 9.1 5.1 1.5R2 0.9992 0.9990 0.9964Intraparticle diffusion kineticski1 (mg g−1 min−1/2) 2.80 5.86 6.22ki2 (mg g−1 min−1/2) – – 2.43ki3 (mg g−1 min−1/2) 0.12 0.20 0.21C1 −7.56 −17.24 −18.37C2 – – 43.38C3 19.79 39.86 85.42R2

1 0.9936 0.9929 0.9777R2

2 – – 1.0000R2

3 0.9872 0.9858 0.9559

adsorption had removed nearly all the Zn(II) insolution. Otherwise, the slopes of the third linearportions in Fig. 2(d) increased from 0.12 to 0.20 and0.21 mg g−1 min−1/2 with increasing Ci from 50 to100 and 200 mg L−1 respectively. The adsorptionon loess progressed rapidly at the beginning and thenslowed down. Micropore diffusion, although occurringat Ci = 200 mg L−1, did not dominate the Zn(II)adsorption process. Instead, boundary layer diffusionof Zn(II) (the first linear portion) played a major role.

Effect of temperature and thermodynamicsFigure 2(e) shows the effect of temperature on theequilibrium amount of Zn(II) adsorption on loess.It can be seen that qe decreased linearly from 96.9,174.5 and 205.5 to 91.25, 163.5 and 188 mg g−1

for solutions with Ci = 200, 500 and 700 mg L−1

respectively as the temperature increased from 15.0to 45.0 ◦C. The associated thermodynamic behaviourwas evaluated with the following equations:

�G0 = −RT ln Kc (9)

�G0 = �H0 − T�S0 (10)

where Kc is the distribution coefficient (ratio of theequilibrium concentration in the solid phase to that insolution), R is the gas constant, T is the temperature(K), �H0 is the change in enthalpy, �S0 is the changein entropy and �G0 is the standard Gibb’s free energy.Equations (9) and (10) can be combined to give alinear relationship between Kc and 1/T :

ln Kc = �S/R − �H/RT (11)

The thermodynamic parameters, i.e. enthalpychange and entropy change, can be determinedaccordingly.

The model parameters are listed in Table 4and show that both the initial solute concentra-tion and the temperature had impacts upon thethermodynamic parameters. Here the initial con-centration appeared comparatively more influential.Thus, as Ci increases from 200 to 500 and 700 mgL−1, the predicted enthalpy changes increase from−20.87 to −3.66 and −4.06 kJ mol−1 respectively,while the predicted entropy changes increase from−12.70 to 23.60 and 18.10 J mol−1 K−1 respec-tively. Negative enthalpy change denotes the exother-mic nature of the reaction. However, the unusuallysmall enthalpy change (−20.87 kJ mol−1) and neg-ative entropy change (−12.70 J mol−1 K−1) at Ci =200 mg L−1 show a different mode of adsorption pro-cess.

The negative entropy change indicated a decreasein freedom of ions in solution. Thus the totalconcentration of ionic species could be considered tobe decreased in solutions at Ci = 200 mg L−1, whichmay result from the effects of bacteria. Otherwise,solutions with higher Zn(II) concentrations might leadto lysis of bacteria. Once the growth of organisms wassuppressed, the complexing effect of the sorbent mightbe weakened, resulting in positive entropy changes.This will be discussed further in the next subsection.

Mechanism discussionFigure 3(a) shows the XPS spectra of crude andZn(II)-loaded soil. The observed mass percentage of

Table 4. Predicted constants for Zn(II) adsorption thermodynamics

Ci (mg L−1) T (K) Gibbs free energy (kJ mol−1) Enthalpy change (kJ mol−1) Entropy change (J mol−1 K−1) R

200 288 −17.21 −20.87 −12.70 0.8655303 −17.02318 −16.83333 −16.64

500 288 −10.46 −3.66 23.60 0.8467303 −10.81318 −11.17333 −11.52

700 288 −9.27 −4.06 18.10 0.9124303 −9.54318 −9.81333 −10.08



Figure 3. Spectroscopic analyses of crude loess and Zn(II)-loaded loess. (a) XPS scans of sorbent: a, crude loess; b, Zn(II)-loaded loess. (b) XPSspectrum of C-1s groups and simulated basic organic bands for crude loess. (c) XPS spectrum of C-1s groups and simulated basic organic bandsfor Zn(II)-loaded loess. (d) XRD scans of sorbent: a, crude loess; b, Zn(II)-loaded loess. Minerals: A, albite; Al, aluminium oxide; C, calcite; G,goethite; I, illite; K, kaolinite; M, muscovite; Q, quartz.

doubly charged zinc (2.2%) deviated from the testedadsorption amount in the batch test. Since XPS givesinformation only about the surface portion of thesorbent, those Zn(II) species existing in the innerpart of soil particles cannot be detected. This is inagreement with the intraparticle diffusion mechanismsuggested by the adsorption kinetic studies.

In Fig. 3(a) the bands related to carboxyl organicsare seen to increase significantly. By exact calculationof C-1s peaks the mass percentage of elemental Cwas identified to increase from 29.34% (crude loess)to 41.25% (Zn(II)-loaded loess), and the C–H andC–O groups increased from 9.20 and 15.09 to 11.92and 23.37% respectively. The XPS spectrum of C-1s groups and the simulated basic organic bands forcrude and Zn(II)-loaded loess are shown in Figs 3(b)and 3(c) respectively. The bands corresponding toC–H groups represents the existence of organics,especially considering the high percentage of C–Ogroups, carbohydrates. Such groups possess very activebonding sites for heavy metals with high affinity.Many studies have revealed the high efficiency ofbiosorption where carboxyl groups act as the majoradsorption sites.1,22–25 Accordingly, the growth ofbacteria can be regarded as one main reason for the

high Zn(II) adsorption capacity of loess, as shownbelow:

R–COOH + Zn2+ −−−→ R–COOZn+ + H+ (12)

This explanation was also supported by theresults of the thermodynamic study. In Table 4 thecalculated enthalpy change for Zn(II) adsorption atCi = 200 mg L−1 was particularly low, indicating theextra high quantity of energy released. This canonly be due to the effect of micro-organisms, whosegrowth and digestion exhausted and transformedthe nutrients originally contained in the natural soilinto energy. Some stimulatory effects on micro-organism populations in soil samples were noted inZn(II)-spiked soil.26 Murray et al.27 also reportedthat microbial activity was not affected by the levelof Zn(II) within certain ranges in contaminatedsoils, confirming the current results. Otherwise, theenthalpy changes at higher Ci were about 20% ofthat at Ci = 200 mg L−1, indicating that the growthof micro-organisms had been inhibited. This wasin agreement with the fact that large quantitiesof Zn(II) will become toxic and act as a growthinhibitor. In order to further verify this assumption,



the Zn(II) uptake amount on loess, treated by H2O2

solution to remove the contained organics (includingbacteria and spores), was measured as 70–75 mg g−1.Compared with the maximum adsorption capacityof crude loess in this study (215.9 mg g−1), thelarge reduction in sorption capacity for treated loesscould inevitably be attributed to the effect of micro-organisms.

The negative entropy change at Ci = 200 mg L−1

indicates decreased ionic freedom (or concentration)in solution, which was most likely due to thecomplexation with micro-organisms rather than ionexchange with the mineral constituents. This mayonly be likely at lower Zn(II) concentrations. Suchexplanations seem to conflict with the conclusionof ion exchange adsorption obtained from D–Risothermal analysis. Nevertheless, it should be realisedthat the results of isothermal analysis were averagedand represent the whole range of solute concentrationsfrom 0 to 700 mg L−1 in this study. With this inmind, the mechanism of Zn(II) uptake at low Zn(II)concentrations as complexation with micro-organismsis understandable.

Figure 3(d) shows the XRD spectra of crude andZn(II)-loaded loess. The bands assigned to specificminerals are noted in this figure. It was observed thatthe patterns of goethite (at 17.7 and 35.9◦), kaolinite(12.5 and 45.3◦) and calcite (at 29.4, 47.6 and 48.6◦)decreased after equilibration with Zn(II) solution.

Clay minerals could provide active adsorption sitesfor heavy metals. Goethite has been widely reported asan active compound for Zn(II) adsorption.18,28–30 Theother main component of loess, kaolinite, was reportedto react with Zn(II) by surface complexation.31,32 Thereduction of the corresponding bands of these mineralsafter contact with Zn(II) indicated possible adsorptionmechanisms such as

Fe–OH + Zn2+ −−−→ Fe–OZn+ + H+ (13)

≡SOH + Zn+ −−−→ ≡SOZn+ + H+ (14)

where ≡S represents silicate contained in the clayminerals.

Al-Degsa et al.5 have reported that calcite, analkaline component of loess, can contribute to theadsorption of Zn(II). In order to understand the effectof calcite on Zn(II) uptake, the distribution of ionicspecies in solution with calcite slurry as a function ofpH was obtained by Visual MINTEQ software (EPA,USA) (25 ◦C, 200 mg L−1 ZnSO4 and 0.002 mol L−1

calcite, following the test conditions). The results areshown in Fig. 4. At pHe ≤ 7.0 more than 98.5% ofthe calcite dissolved, while at pHe ≤ 4.0 nearly all ofthe carbonate was transformed into CO2. Thus theapproximately 10 wt% of calcite contained in loesswould hardly impose any great influence on Zn(II)adsorption on loess in this study owing to the lowequilibrium pH values, in contrast to the results of Al-Degsa et al.5 Although ZnCO3 begins to form at pH>6.5, the amount of the precipitate was only 4.26%

Figure 4. Distributions of species for Zn(II) at various pH values inpresence of calcite (25 ◦C, calcite concentration 2 × 10−3 mol L−1,calculated by Visual MINTEQ).

Figure 5. TEM image of dispersible crude loess particles in water.

at pH 7.0. Thus in this study the immobilisation ofZn(II) on loess could be considered predominantly asan adsorption process.

Figure 5 shows the TEM image of loess particles.It was observed that the diameters of most particleswere less than 0.3 µm. Such particles can be specifiedas clay colloids and may inevitably contribute to thelarge surface area of loess (24.1 m2 g−1) and its highadsorption capacity for Zn(II). The morphology ofthe soil particles showed that their surface was notslippy or regular; instead, it was uneven with hollowsand bulges. Some particles had inherent microporeswith orifices on their surface, consistent with theintraparticle diffusion of cations from the liquid tothe solid phase. The grey spots on the soil particlesindicate that the spatially distributed microporesmight be perpendicular to the horizontal plane. Allthese features support the surface heterogeneity ofthe adsorbent implied by the Freundlich isotherm.Since the dimension (diameter) of the microporeswas so large (compared with the diameter of the Znatom), diffusion of solute ions inwards may not bethe rate-limiting stage as revealed by intraparticlediffusion kinetics. Instead, surface diffusion andsurface complexation will be preferential.



CONCLUSIONS1. The Zn(II) adsorption capacity of Chinese loess

was determined as 215.9 mg g−1. The adsorptionwas shown by the Freundlich equation to beheterogeneous, and the D–R model indicated thatthe mode of adsorption was ion exchange.

2. The kinetic study showed that the Zn(II) adsorp-tion followed pseudo-second-order kinetics andwas mainly surface adsorption, while microporediffusion was not the rate-limiting step.

3. The factors affecting the adsorption capacity ofloess included initial pH, initial solute concentra-tion, temperature and slurry concentration. Positiverelationships were observed for the first two, butnegative relationships for the last two.

4. The thermodynamic study showed that the adsorp-tion was exothermic. Both the enthalpy change andentropy change at low Ci were much lower thanthose at higher Ci, suggesting a different kind ofadsorption mechanism ascribable to the effect ofmicro-organisms.

5. The XPS spectra proved that the growth oforganisms was related to the high adsorptioncapacity. The XRD patterns suggested that kaoli-nite and goethite were the major clay mineralsresponsible for Zn(II) removal. The TEM imageshowed the existence of many micropores, indi-cating pore filling as a possible adsorption pro-cess.

ACKNOWLEDGEMENTSThe authors would like to express their sincere grati-tude to the Key Project of National Natural ScienceFoundation of China (NSFC) (grant 50538080) andthe National Science Fund for Distinguished YoungScholars (grant 50425825) for their financial supportof this study.

REFERENCES1 Bhattacharya AK, Mandal SN and Das SK, Adsorption of

Zn(II) from aqueous solution by using different adsorbents.Chem Eng J 123:43–51 (2006).

2 WHO, Guidelines for Drinking-water Quality: Incorporating FirstAddendum. Vol. 1, Recommendations (3rd edn). World HealthOrganisation, Geneva (2006).

3 Meunier N, Drogui P, Montane C, Hausler R, Mercier Gand Blais JF, Comparison between electrocoagulation andchemical precipitation for metals removal from acidic soilleachate. J Hazard Mater 137:581–590 (2006).

4 Srivastava SK, Tyagi R and Pant N, Adsorption of heavy metalions on carbonaceous material developed from the waste slurrygenerated in local fertilizer plants. Water Res 23:1161–1165(1989).

5 Al-Degsa YS, El-Barghouthi MI, Issa AA, Khraisheh MA andWalker GM, Sorption of Zn(II), Pb(II), and Co(II) usingnatural sorbents: equilibrium and kinetic studies. Water Res40:2645–2658 (2006).

6 Cetin S and Pehlivan E, The use of fly ash as a low cost,environmentally friendly alternative to activated carbon forthe removal of heavy metals from aqueous solutions. ColloidsSurf A: Physicochem Eng Aspects 298:83–87 (2007).

7 Sekar M, Sakthi V and Rengaraj S, Kinetics and equilibriumadsorption study of lead(II) onto activated carbon preparedfrom coconut shell. J Colloid Interface Sci 279:307–313 (2004).

8 Girgis BS, Attia AA and Fathy NA, Modification in adsorptioncharacteristics of activated carbon produced by H3PO4under flowing gases. Colloids Surf A: Physicochem Eng Aspects299:79–87 (2007).

9 Jeon C, Nah IW and Hwang KY, Adsorption of heavy metalsusing magnetically modified alginic acid. Hydrometallurgy86:140–146 (2007).

10 Nan ZR, Assessment on the soil environmental quality in thecentral district of Baiyin City, Gansu Province. J NorthwestNormal Univ (Nat Sci) 30:83–88 (1994).

11 Assallay AM, Rogers CDF, Smalley IJ and Jefferson IF, Silt:2–62 µm, 9–4ϕ. Earth Sci Rev 45:61–88 (1998).

12 Sun JM and Ding ZL, Deposits and soils of the past130,000 years at the desert–loess transition in northern China.Quaternary Res 50:148–156 (1998).

13 Tan KH, Soil Sampling, Preparation, and Analysis. MarcelDekker, New York, NY (1995).

14 Do DD, Adsorption Analysis: Equilibrium and Kinetics. ImperialCollege Press, London (1998).

15 Weng CH and Huang CP, Adsorption characteristics of Zn(II)from dilute aqueous solution by fly ash. Colloids Surf A:Physicochem Eng Aspects 247:137–143 (2004).

16 Cho H, Oh D and Kim K, A study on removal characteristicsof heavy metals from aqueous solution by fly ash. J HazardMater 127:187–195 (2005).

17 Giles CH and Smith D, A general treatment and classificationof the solute adsorption isotherm: I. Theoretical. J ColloidInterface Sci 47:755–765 (1974).

18 Trivedi P and Axe L, Ni and Zn sorption to amorphous versuscrystalline iron oxides: macroscopic studies. J Colloid InterfaceSci 244:221–229 (2001).

19 Alvarez MT, Crespo C and Mattiasson B, Precipitation ofZn(II), Cu(II) and Pb(II) at bench-scale using biogenichydrogen sulfide from the utilization of volatile fatty acids.Chemosphere 66:1677–1683 (2007).

20 Bordas F and Bourg A, Effect of solid/liquid ratio on theremobilization of Cu, Pb, Cd and Zn from polluted riversediment. Water Air Soil Pollut 128:391–400 (2001).

21 Kaya A and Oren AH, Adsorption of zinc from aqueoussolutions to bentonite. J Hazard Mater 125:183–189 (2005).

22 Sharma A and Bhattacharyya KG, Adsorption of chromium(VI)on Azadirachta indica (Neem) leaf powder. Adsorption10:327–338 (2004).

23 Narita H, Zavala MAL and Iwai K, Transformation andcharacterisation of dissolved organic matter during thethermophilic aerobic biodegradation of faeces. Water Res39:4693–4704 (2005).

24 Choi SB and Yun YS, Biosorption of cadmium by various typesof dried sludge: an equilibrium study and investigation ofmechanisms. J Hazard Mater 138:378–383 (2006).

25 Shashidhar T, Philip L and Bhallamudi SM, Bench-scale col-umn experiments to study the containment of Cr(VI) inconfined aquifers by bio-transformation. J Hazard Mater131:200–209 (2006).

26 Hemida SK, Omar SA and Abdel-Mallek AY, Microbial popu-lations and enzyme activity in soil treated with heavy metals.Water Air Soil Pollut 95:13–22 (1997).

27 Murray P, Ge Y and Hendershot WH, Evaluating three tracemetal contaminated sites: a field and laboratory investigation.Environ Pollut 107:127–135 (2000).

28 Schwertmann U, Cambier P and Murad E, Properties ofgoethites of varying crystallinity. Clays Clay Miner 33:369–378(1985).

29 Boudeulle M and Muller JP, Structural characteristics ofhematite and goethite and their relationships with kaolinitein a laterite from Cameroon: a TEM study. Bull Mineral111:149–166 (1988).

30 Trivedi P and Axe L, An analysis of zinc sorption to amorphousversus crystalline iron oxides using XAS. J Colloid Interface Sci244:230–238 (2001).



31 Ikhsan J, Johnson BB and Wells JD, A comparative study of theadsorption of transition metals on kaolinite. J Colloid InterfaceSci 217:403–410 (1999).

32 Srivastava P, Singh B and Angove M, Competitive adsorptionbehavior of heavy metals on kaolinite. J Colloid Interface Sci290:28–38 (2005).


Behaviour and mechanism of Zn(II) adsorption on Chinese loess at dilute slurry concentrations

Documents

Transcript of Behaviour and mechanism of Zn(II) adsorption on Chinese loess at dilute slurry concentrations