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Journal of Hazardous Materials 261 (2013) 181– 187

Contents lists available at ScienceDirect

Journal of Hazardous Materials

jou rn al hom epage: www.elsev ier .com/ locate / jhazmat

mmobilization of heavy metals on pillared montmorillonite with grafted chelate ligand

oren Brown, Kenneth Seaton, Ray Mohseni, Aleksey Vasiliev ∗

epartment of Chemistry, East Tennessee State University, PO Box 70695, Johnson City, TN 37614, United States

i g h l i g h t s

Mesoporous organoclay for immobilization of heavy metal cations was obtained.The material has a porous structure with high contents of surface adsorption sites.Leaching of heavy metals from soil reduced in the presence of this adsorbent.The adsorbent demonstrated high effectiveness in neutral and acidic media.

r t i c l e i n f o

rticle history:eceived 26 March 2013eceived in revised form 11 July 2013ccepted 12 July 2013vailable online xxx

a b s t r a c t

The objective of this work was the development of an efficient adsorbent for irreversible immobilizationof heavy metals in contaminated soils. The adsorbent was prepared by pillaring of montmorillonite withsilica followed by grafting of a chelate ligand on its surface. Obtained adsorbent was mesoporous with highcontent of adsorption sites. Its structure was studied by BET adsorption of N2, dynamic light scattering,and scanning electron microscopy. The adsorption capacity of the organoclay was measured by its mixing

eywords:eavy metalsrganoclayhelate ligand

mmobilization

with contaminated kaolin and soil samples and by analysis of heavy metal contents in leachate. Deionizedwater and 50% acetic acid were used for leaching of metals from the samples. As it was demonstratedby the experiments, the adsorbent was efficient in immobilization of heavy metals not only in neutralaqueous media but also in the presence of weak acid. As a result, the adsorbent can be used for reductionof heavy metal leaching from contaminated sites.

eaching

. Introduction

The main sources of land contamination from heavy metalsnclude metal refining industry sites, mine tailings, fertilizers, andewage sludge [1]. In particular, wastewater treatment plants usu-lly generate millions of tons of residual sludge worldwide everyear. Sewage sludge is the treated residuals from wastewater treat-ent that can be used beneficially. For example, application of

ludge on the soils at the rate of 40 t/ha increases crop yields by twoold [2]. However, in accordance with federal regulations, sewageludge cannot be applied to the soil if the concentration of any pol-utant in the sewage sludge exceeds the ceiling concentration forhe pollutant [3].

The management of the contaminated soils is a major part

f waste treatment, involving substantial cost and efforts. Sludgeanagement may include wet air oxidation, land filling, incin-

ration, ocean disposal, and land-based soil treatment (e.g.,

∗ Corresponding author. Tel.: +1 423 4394368.E-mail address: vasiliev@etsu.edu (A. Vasiliev).

304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2013.07.024

© 2013 Elsevier B.V. All rights reserved.

agricultural, forest). In land-based soil treatment, sludge is usednot only to improve the quality of the soil but also serves as a fertil-izer. Until a few years ago, sewage sludge could be re-used directlyin agriculture as fertilizer. Recently, however, there has been anincreased concern because of the legal criteria for the concentra-tion of heavy metals in sewage sludge [4,5]. It was reported that thetotal heavy metal content was about 0.5–2.0% on a dry weight basisand in some cases may rise up to 4% on a wet weight basis, espe-cially for metals such as cadmium (Cd), chromium (Cr), copper (Cu),nickel (Ni), lead (Pb) and zinc (Zn) [6,7]. In the United States, sludgefrom 50% to 60% of the municipal wastewater treatment plants can-not be applied on agricultural land because the Cd content exceedsthe standard [8].

There are two major strategies to improve the quality of the soil:removal of heavy metals and their irreversible immobilization. Themost cost-effective strategy of remediation of contaminated soilsis based on the irreversible immobilization of heavy metals. In this

method, the risks related to the presence of soluble or availableheavy metal ions are reduced, although the metals are still present.However, they are converted into an insoluble state that reducesthe risk of leaching from the ground and makes them available for

1 dous Materials 261 (2013) 181– 187

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urther bioremediation [9,10]. No waste processing or disposal isecessary in this case, which has a significant economic advantage.

Recent studies illustrated that some materials can be effec-ive for immobilizing heavy metals from soil. For example, Ciccut al. showed that addition of fly ash and red muds (bauxite orerocessing waste) significantly reduced heavy metal ion (Pb, Zn,d, Cu, As) contents in ground water [11]. Querol et al. successfully

mmobilized Zn, Pb, As, Cu, Sb, Co, Tl and Cd on zeolite materialsynthesized from coal fly ash [12]. Raicevic et al. used apatite min-rals to adsorb Pb, Cd, as well as other toxic metals from pollutedoils [13]. Clay materials (Na-bentonite, Ca-bentonite, and zeolite)ere used for immobilization of heavy metals (Zn, Cd, Cu, and Ni)

n a sewage sludge-contaminated soil [14,15]. The extractability ofhese metals was significantly decreased.

Heavy metals can have different mobilities when they areresent in their various forms. Simple and complex cations are theost mobile, exchangeable cations in organic and inorganic com-

lexes are of medium mobility, while chelated complexes are lessobile. Ravishankar et al. evaluated several contaminated sludge

amples to study bioleaching processes [16]. They concluded thathe most stable sludge contained higher contents of organicallyound metals. Consequently, the degree of immobilization on inor-anic materials was not sufficiently high.

However, up to now immobilized organic substances were notsed for soil treatment. The goal of this research was to develop aew hybrid organic–inorganic material (containing organic ligandrafted on insoluble inorganic support) based on natural miner-ls that are capable of irreversible immobilization of heavy metalations in the contaminated soil. The proposed approach to decon-amination of lands is quite suitable for wide-scale use.

. Experimental

.1. Materials

The following clay materials and salts of heavy metalsere used for this study: kaolinite (Kt), Ni(CH3COO)2·4H2O,

nd Cd(CH3COO)2·2H2O were purchased from Sigma–Aldrich (St.ouis, MO); FeCl3·6H2O, CuSO4·5H2O and Zn(CH3COO)2 were pur-hased from Fisher Scientific (Pittsburgh, PA). Montmorillonite-10 (Mt) was purchased from Acros Organic (Geel, Belgium).n adsorbent containing immobilized chelating ligand, N-[3-

trimethoxysilyl)propyl]ethylenediamine triacetic acid trisodiumalt (TMS-EDTA), (Mt(EDTA)) was synthesized in accordance withhe method described earlier [17].

The samples of contaminated soils 1-8 were supplied by Tes-America (Nashville, TN). The samples 1-6 contained mostly iron%): 1 (0.36), 2 (1.07), 3 (0.44), 4 (1.53), 5 (1.30), 6 (1.63). The sam-les 7 and 8 were polycontaminated and contained the followingetals (%): sample 7, Fe (11.56), Zn (8.4 × 10−2), Cu (1.6 × 10−2), Ni

1.3 × 10−3); sample 8, Fe (1.66), Zn (3.4 × 10−2), Cu (3.5 × 10−3), Ni5 × 10−4).

.2. Preparation of contaminated samples

Artificially contaminated samples of Kt containing 20 ppm ofeavy metals were prepared by mixing 30 g of Kt with 30 mL of aolution of a heavy metal salt. A concentration of the salt in eacholution was calculated to provide a 20 ppm concentration of theetal. Then the mixture was dried in an oven at 120 ◦C overnight,

nd thoroughly grinded. Additionally, samples containing 10 ppm

f the metals were prepared by using a portion of each sampleixed with equal amounts of pure Kt and thoroughly grinded. A

ample with metal mixture containing 4 ppm of each metal wasrepared by mixing equal amounts of contaminated Kt.

Fig. 1. Immobilization of heavy metal ions on modified pillared clay.

2.3. Study of immobilizing ability of Mt(EDTA)

A sample of each contaminated Kt or soil (8 g) was divided intotwo equal portions. One part was mixed with 40 mg of the adsor-bent and thoroughly grinded. A control part was used as it wasprepared. Each sample was placed in a funnel with filter paper and3 mL of deionized water was added. When the volume of leachatereached 2 mL, it was collected and analyzed. A general scheme ofmetal immobilizations is shown in Fig. 1.

The same experiments were conducted with 50% acetic acid(pH = 1.92) instead of water.

For determination of the effect of the adsorbent amount on ironleaching, the adsorbent in different amounts was mixed with thesamples of contaminated soil (4 g each) and the treatment wasconducted as described above.

2.4. Characterization

The specific surface areas and porosities of Mt and Mt(EDTA) werestudied on a Quantachrome Nova porosimeter (Boynton Beach,FL). The BET surface areas were determined by adsorption of N2at −196 ◦C. Prior to measurements, the samples were degassed invacuum at 120 ◦C for 3 h. Total pore volumes and pore size distribu-tions were calculated using the Barrett, Joyner and Halenda (BJH)method. The micropore volumes were determined by the Dubininand Radushkevic (DR) method.

Dynamic light scattering (DLS) analyses were performed usingthe Beckman Coulter N4 Plus particle analyzer (Brea, CA). The tem-perature of the sample holder was 20 ◦C and the intensity of thescattered light was measured at an angle of 11◦. Moreover, stocksuspensions were made by dispersing the material to a concentra-tion of 10 mg/mL followed by sonication for 2 min. A 20 �L aliquot ofthis dispersion was diluted with 3 mL deionized water and 100 �Lof this dilution was further diluted with 3 mL of deionized water.The DLS measurements were made on this final dilution.

Scanning electron microscopy (SEM) images were obtained

on a Zeiss DSM 940 scanning electron microscope (Oberkochen,Germany) at 20 kV. The samples were coated with gold before imag-ing.

L. Brown et al. / Journal of Hazardous Materials 261 (2013) 181– 187 183

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in leachates demonstrated a significant reduction of the metal

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Fig. 2. BET surface areas and pore size distributions of Mt and Mt(EDTA).

Contents of heavy metals in leachates were determined on ahimadzu AA-6300 atomic absorption spectrophotometer (Kyoto,apan).

. Results

.1. Structural characteristics of montmorillonites

Pillaring of the clay and grafting of the chelate ligand on the

urface affected porosity of Mt in high degree (Fig. 2). BET surfacerea reduced more than twice (Table 1). Total pore volume andicropore volume was also reduced. However, the average pore

able 1tructural characteristics of the montmorillonites.

Material BET surfacearea (m2/g)

Total porevolume (cm3/g)

Microporevolume (cm3/g)

Mt 163.7 0.29 0.18Mt(EDTA) 71.9 0.18 0.07

Fig. 4. SEM images of

Fig. 3. Particle size distributions of Mt and Mt(EDTA).

size slightly changed. In the mesopore region of both samples, amaximum at r = 30 A is present.

Weight average particle size of Mt increased significantly aftermodification (Table 1 and Fig. 3). Most of the Mt particles rangedin size between 250 and 700 nm while all Mt(EDTA) particles werebigger than 3500 nm.

SEM images of raw and modified clays showed that the modifi-cation changed morphology of clay particles. Raw montmorillonitemostly consisted of small platelets, which surfaces are smooth(Fig. 4). Mt(EDTA) particles were grainy with rough surfaces.

3.2. Immobilization of heavy metals on contaminated kaolinite

Study of the effect of the adsorbent on heavy metals contents

concentrations. Water leaching from Kt containing the adsorbentverified this trend. As it is shown in Fig. 5, the difference in metalconcentrations at use of Kt with and without adsorbent was up

Average porediameter (nm)

Average particlediameter (nm)

Polydispersity index

3.30 384 0.9193.28 4573 0.820

Mt and Mt(EDTA).

184 L. Brown et al. / Journal of Hazardous Materials 261 (2013) 181– 187

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Fig. 6. Concentrations of heavy metals in acidic leachates from Kt contaminated bymetals (20 ppm and 10 ppm): � – without adsorbent, – with adsorbent.

Fig. 7. Effect of Mt(EDTA) on concentrations of heavy metals in aqueous (1) and acidic

ig. 5. Concentrations of heavy metals in aqueous leachates from Kt contaminatedy metals (20 ppm and 10 ppm): � – without adsorbent, – with adsorbent.

o 0.8 ppm for Cd2+. The most notable reduction of metal concen-rations in leachate was observed on samples containing Cu2+ ande3+, which remained adsorbed on Kt strongly even in absence ofhe chelate adsorbent.

At leaching with acidic solution, concentrations of heavy metalsn leachates were much higher; however, the adsorbent was alsoffective in acidic media (Fig. 6). Especially notable effects werebserved for clay highly contaminated by Ni2+ and Cd2+. Thus, con-entration of Cd2+ in its presence reduced on 0.5 ppm.

In the case of a metal mixture, it was found that the watereachate contained notable concentrations of Ni2+ and Zn2+ (Fig. 7).pplication of the adsorbent significantly reduced nickel concen-

ration and completely stopped leaching of zinc. In acidic media,dsorption of all metals was much lower and the leachate containedll metals with the highest content of copper. In these conditionshe adsorbent did not affect leaching of nickel but reduced contentsf other metals in the leachate.

.3. Immobilization of heavy metals in soils

Immobilization of iron in soil samples reduced the leaching ofen+ by water for most of the soils (Fig. 8). No clear dependencef Fen+ concentration in leachate on its contents in the soil sam-les was noted. In particular, leaching from the least contaminatedample 1 was the highest. Moreover, the most heavily contami-ated sample 6 showed the lowest leaching. In contrast, leachingas high in acidic media, especially from sample 6. Mt(EDTA) signif-

cantly reduced contamination of the leachate by Fen+ cations fromll samples.

Concentration of Fen+ in the leachate from sample 6 contain-ng different amounts of the adsorbent reduced with increase of

t(EDTA) in soil (Fig. 9).

Immobilization of various metals in polycontaminated soil sam-

les 7 and 8 is shown in Figs. 10 and 11. Leaching of metals fromample 7 was reduced by the adsorbent in neutral and acidic mediay 1.4 and 4.5 ppm, respectively. In the case of sample 8, leaching in

(2) leachates from Kt containing metal mixture without (a) and with (b) adsorbent(reduction of total square area after leaching with Mt(EDTA) is proportional to thereduction of total contamination).

neutral aqueous media was relatively low even with no adsorbent.In acidic media, concentrations of all metals in leachate decreasedby 1.8 ppm in total. As a result, the adsorbent decreased concentra-tions of all metals present in the samples.

L. Brown et al. / Journal of Hazardous Materials 261 (2013) 181– 187 185

Fig. 8. Concentrations of iron in aqueous (a) and acidic (b) leachates from soil sam-ples contaminated by iron: � – without adsorbent, – with adsorbent.

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Fig. 10. Effect of Mt(EDTA) on concentrations of heavy metals in aqueous (1) andacidic (2) leachates from soil sample 7 containing metal mixture without (a) andwith (b) adsorbent.

complexes with EDTA are: Fe (25.1), Cu (18.78), Ni (18.4),

ig. 9. Concentrations of Fen+ in leachates from sample 6 with different contents oft(EDTA).

. Discussion

.1. Structure and composition of the adsorbent

The choice of the ligand grafted on the pillared clay was basedn previous successful results of its use for immobilization of heavyetals on silica [18] and clay materials [17]. In contrast to tradition-

lly used grafted amines, this ligand can form stable complexes ofhelate structure with metals like ethylenediaminetetraacetic acid.owever, due to large size of TMS-EDTA molecule, it cannot enter

nterlayer space of non-modified clays. The distance between layersn Mt is 8.4 A [19] while the volume of TMS-EDTA molecule calcu-ated with the use of Spartan ‘08 software is 385 Å3 that corresponds

o a diameter of about 9 A. Pillaring of Mt with silica resulted in sig-ificant increase of TMS-EDTA ability to reach adsorption sites onhe Mt surface and to graft there. Thus, a content of organic phase

Fig. 11. Effect of Mt(EDTA) on concentrations of heavy metals in aqueous (1) andacidic (2) leachates from soil sample 8 containing metal mixture without (a) andwith (b) adsorbent.

on pillared Mt was 1.25 mmol/g while it was only 0.05 mmol/g onraw Mt.

Clay particles average size increased after pillaring that wasprobably caused by formation of interparticle silica pillars.

Another notable feature of chelate complexes is their stability inlow pH medium. Usually in the environment, heavy metal leachingis often combined with increased water acidity, e.g., at acid minedrainage contamination. Thus, reducing leaching of heavy metalsin acidic media requires their stable fixation in soil or sludge.

Non-modified montmorillonite is a good adsorbent for heavymetals itself due to its ability to exchange sodium cations on heavymetals [20]. However, this type of binding is pH-dependent andeffectively works in neutral media only. Chelate complexes of heavymetals are much more stable. Thus, log(Kf) of some heavy metal

3+ 2+ 2+

Cd2+ (16.5), and Zn2+ (16.5). For modified adsorbents we expectedtwo types of metal adsorption: reversible by cation exchange, andirreversible by chelate complex formation.

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.2. Immobilizing ability

For the study of heavy metals leaching from soil, 1% of Mt(EDTA)as added to contaminated kaolin samples. The adsorbent used in

his work for immobilization of heavy metals demonstrated earlierigh effectiveness in cleanup of flowing water from heavy metals17].

We used kaolin as a model natural material with low porositynd low stability of its surface metal complexes. These complexesre especially unstable at low pH. After addition of the metal saltsolutions, the metal ions were partially adsorbed on the kaolin sur-ace. In aqueous media, the adsorbed metal ions are in equilibriumith the ions in the solution [21]. If the adsorbent was added, they

ormed more stable complexes with the organic ligand in its pores.Particularly good results were obtained for Fe3+ and Cu2+ ions.

n our experiments with immobilization, these two metals werelmost completely retained in kaolin as well. Though other metalsere present, their concentrations were very low in the leachate.

Content of adsorption sites on the surface of Mt(EDTA) is.25 mmol/g. The samples of kaolin containing 10 ppm of singleeavy metals had the following metal concentrations in mmol/g:e (0.18), Zn (0.15), Cu (0.16), Ni (0.17), and Cd (0.09). In the sam-les with 20 ppm contents, these numbers were doubled. Most ofhe metals were adsorbed under these conditions.

Significant retention of heavy metals by the adsorbent wasbserved in acidic media. Concentrations of all metals except ironere lowered with the addition of Mt(EDTA). Competitive adsorption

f all five metal cations from their mixture showed higher selectiv-ty on Cu2+ while it was the lowest for Ni2+. It was shown earlierhat copper-exchanged montmorillonite is the least stable in acidic

edia [20] and decomposes at pH above 5.5. Our results of acideaching are in a good agreement with this data: Cu2+ is a prevalentation in leachate from contaminated kaolin. However, addition oft(EDTA) significantly reduced its contents, which is also in agree-ent with the data on log(Kf) of Cu2+ chelate complexes with EDTA.e attribute this reduction of copper contamination in leachate to

he effect of the grafted ligand. This effect was observed for all metalations studied except for Ni2+.

The results of experiments demonstrated high effectiveness oft(EDTA) in adsorption of highly toxic water contaminants, which

re Zn2+, Cu2+ and Cd2+. In accordance with EPA National Drink-ng Water Regulations, their maximum contaminant levels (MCL)n drinking water are 5, 1.3 and 0.005 ppm, respectively. Iron isot a hazardous substance; however, it can produce brown stainsn plumbing fixtures and laundry. Therefore, its MCL is limited to.3 ppm.

In the case of iron-containing soil samples 1-8, the total contentsf iron significantly exceeded the adsorption capacity of the adsor-ent. However, as it was found, leaching from iron-contaminatedamples did not correlate with the degree of their contamination.t was concluded that it probably depended on the chemical formf iron and presence of soil components capable of retaining Fen+

ations in soil (i.e., clays). It is known that iron can exist in two oxi-ation states: compounds with Fe2+ cations are well soluble whilehe solubility of Fe3+ compounds is low. Increasing contents of thedsorbent reduces contamination of leachate that is an evidencef relatively low contents of soluble iron compounds in the soilamples.

The adsorbents can be used for cost-effective treatment of con-aminated soils instead of its expensive disposal. In the Unitedtates, over 7 × 106 t of dry sewage sludge were produced in 1990,owever, only 33.5% were applied to the lands such as agricultural

ands, pasture and range lands, forests, disturbed lands, construc-ional sites, gravel pits, parks and gardens [22]. Development ofovel efficient adsorbents for immobilization of heavy metals willllow increasing the part of the sludge used in land applications.

[

aterials 261 (2013) 181– 187

Thus, the valuable resources of plant nutrients will be used for thebenefits of agriculture and sustainable environment.

5. Conclusion

An efficient adsorbent on the basis of montmorillonite wassynthesized by its pillaring with silica and grafting of a chelateligand. In addition, the material obtained was highly mesoporous.This adsorbent demonstrated high adsorption capacity for variousheavy metals contained in clay and soil samples. Moreover, it wasstill efficient even at low pH of the media. Therefore, excellent char-acteristics of this material make it prospective in immobilization ofheavy metals on contaminated sites.

Acknowledgments

This project was supported by RDC ETSU grant 13-007sm. Wethank Prof. F. Hossler for his assistance in taking SEM images andProf. K.P. Vercruysse (Tennessee State University) for measure-ments of particle size distributions.

References

[1] R.A. Wuana, F.E. Okieimen, Heavy metals in contaminated soils: a review ofsources, chemistry, risks and best available strategies for remediation, ISRNEcology (2011) (Article ID 402647).

[2] M. Jamil, M. Qasim, M. Umar, Utilization of sewage sludge as organicfertilizer in sustainable agriculture, Journal of Applied Science 6 (2006)531–535.

[3] The standards for the use or disposal of sewage sludge (Title 40 of theCode of Federal Regulations, part 503), Federal Register (58 FR 9248-9404),1993.

[4] C. Lue-Hing, D.R. Zenz, P. Tata, R. Kuchenrither, J.F. Malina, B. Sawyer (Eds.),Municipal Sewage Sludge Management: A Reference Text On Processing, Uti-lization And Disposal, vol. 4, 2nd ed., Technomic Publ. Co. Inc., Lancaster, PA,1998.

[5] H.M.J. Scheltinga, Sludge in agriculture: the European approach, Water Scienceand Technology 19 (1987) 9–18.

[6] D.K. Jain, R.D. Tyagi, Leaching of heavy metals from anaerobic sewage sludgeby sulfur-oxidizing bacteria, Enzyme and Microbial Technology 14 (1992)376–383.

[7] L.E. Sommers, Chemical composition of sewage sludges and analysis of theirpotential use as fertilizers, Journal of Environment Quality 6 (1977) 225–232.

[8] D. Couillard, G. Mercier, Optimum residence time (in CSTR and airlift reactor)for bacterial leaching of metals from anaerobic sewage sludge, Water Research25 (1991) 211–218.

[9] J. Vangronsveld, J.V. Colpaert, K.K. van Tichelen, Reclamation of a bare industrialarea contaminated by non-ferrous metals: physico-chemical and biologicalevaluation of the durability of soil treatment and revegetation, EnvironmentalPollution 94 (1996) 131–140.

10] L. Diels, N. van der Lelie, L. Bastiaens, New developments in treatment of heavymetal contaminated soils, Reviews in Environmental Science and Biotechnol-ogy 1 (2002) 75–82.

11] R. Ciccu, M. Ghiani, A. Serci, S. Fadda, R. Peretti, A. Zucca, Heavy metal immo-bilization in the mining-contaminated soils using various industrial wastes,Minerals Engineering 16 (2003) 187–192.

12] X. Querol, A. Alastuey, N. Moreno, E. Alvarez-Ayuso, A. Garcia-Sanchez, J. Cama,C. Ayora, M. Simon, Immobilization of heavy metals in polluted soils by theaddition of zeolitic material synthesized from coal fly ash, Chemosphere 62(2006) 171–180.

13] S. Raicevic, T. Kaludjerovic-Radoicic, A.I. Zouboulis, In situ stabilization of toxicmetals in polluted soils using phosphates: theoretical prediction and experi-mental verification, Journal of Hazardous Materials B 117 (2005) 41–53.

14] A. Usman, Y. Kuzyakov, K. Stahr, Effect of clay minerals on immobilizationof heavy metals and microbial activity in a sewage sludge-contaminated soil,Journal of Soils & Sediments 5 (2005) 245–252.

15] A.R.A. Usman, Y. Kuzyakov, K. Lorenz, K. Stahr, Remediation of a soil con-taminated with heavy metals by immobilizing compounds, Journal of PlantNutrition and Soil Science 169 (2006) 205–212.

16] B.R. Ravishankar, J.C. Auclair, R.D. Tyagi, Partitioning of heavy metals in someQuebec municipal sludges, Water Pollution Research Journal of Canada 29(1994) 457–470.

17] M. Addy, B. Losey, R. Mohseni, E. Zlotnikov, A. Vasiliev, Adsorption of heavy

metal ions on mesoporous silica-modified montmorillonite containing agrafted chelate ligand, Applied Clay Science 59/60 (59) (2012) 115–120.

18] A.N. Vasiliev, L.V. Golovko, V.V. Trachevsky, G.S. Hall, J.G. Khinast, Adsorptionof heavy metal cations by organic ligands grafted on porous materials, Micro-porous and Mesoporous Materials 118 (2009) 251–257.

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L. Brown et al. / Journal of Hazar

19] K.S. Kim, M. Park, W.T. Lim, S. Komarneni, Massive intercalation of urea inmontmorillonite, Soil Science Society of America Journal 75 (2011) 2361–2366.

20] O. Abollino, M. Aceto, M. Malandrino, C. Sarzanini, E. Mentasti, Adsorptionof heavy metals on Na-montmorillonite. Effect of pH and organic substances,Water Research 37 (2003) 1619–1627.

[

[

aterials 261 (2013) 181– 187 187

21] M.P. Papini, M. Majone, Modeling of heavy metal adsorption at clay surfaces, in:A.T. Hubbard (Ed.), Encyclopedia of Surface and Colloid Science, vol. 3, MarcelDekker, Inc., New York, 2002, pp. 3483–3498.

22] M.J. McFarland, Biosolids Engineering, McGraw-Hill Professional, New York,NY, 2000.