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Journal of Hazardous Materials 168 (2009) 1512–1520

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

ssociation of individual soil mineral constituents and heavy metals as studied byorption experiments and analytical electron microscopy analyses

éter Siposa,∗, Tibor Németha, Viktória Kovács Kisb, Ilona Mohaic

Institute for Geochemical Research, Hungarian Academy of Sciences, H-1112 Budapest, Budaörsi út 45, HungaryResearch Institute for Technical Physics and Materials Sciences, Hungarian Academy of Sciences, H-1121 Budapest, Konkoly Thege Miklós út 29-33, HungaryInstitute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1025 Budapest, Pusztaszeri út 59-67, Hungary

r t i c l e i n f o

rticle history:eceived 26 November 2008eceived in revised form 4 February 2009ccepted 9 March 2009

a b s t r a c t

Sorption characteristics of bulk soil samples and discrete soil mineral constituents were studied by Cu,Zn and Pb batch sorption experiments and analytical electron microscopy analyses. Copper and zincsorbed mostly on soil mineral constituents, while lead was associated mainly to soil organic matter.Additionally, the competitive situation resulted in increase of the role of iron oxides in Pb sorption. Close

vailable online 18 March 2009

eywords:oxic metalsorption capacityron oxideslay minerals

association of iron oxides and silicates resulted in significant change in their sorption capacities for allthe studied metals. The alkaline conditions due to the calcite content in one of the studied soil samplesresulted in both increased role of precipitation for Pb and Cu and elevated sorption capacity for Cu bydiscrete mineral particles. Using the analytical electron microscopy analyses the sorption characteristicsof metals were supported by particular data. When the methods used in this study are combined, theybecome an extremely powerful means of getting a deeper insight into the soil–metal interaction.

oil carbonate

. Introduction

The ability of soils to adsorb metal ions from aqueous solutions of special interest and has consequences both for agriculturalssues and environmental questions. As adsorption is a major pro-ess for accumulation of potentially toxic metals in soils, its studys of utmost importance for the understanding of how metalsre transferred from a liquid mobile phase to the surface of aolid phase [1]. The exact retention mechanism of metal ions atoil surfaces is often unknown, so the term sorption is preferred,hich in general involves the loss of a metal ion from an aque-

us to a contiguous solid phase and consist of three importantrocesses: adsorption, surface precipitation and fixation [2]. Theserocesses are dependent on soil properties which are strongly influ-nced by the soil constituents. The organic matter, clay minerals,s well as Fe and Mn oxides are the most important componentsetermining the sorption of metals in soils [3]. The organic com-onents form stable metal–organic complexes with a variety of

etals, while clay minerals and oxides concentrate heavy metal

ons through surface ion exchange and metal-complex surfacedsorption.

Batch equilibrium techniques are widely used to study the reten-ion of metals in soils and the sorption data are described by

∗ Corresponding author. Fax: +36 1 3193137.E-mail address: sipos@geochem.hu (P. Sipos).

304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2009.03.033

© 2009 Elsevier B.V. All rights reserved.

using isotherms. The analysis of isotherms may provide informationabout the retention capacity and the sorption strength by whichthe sorbate is held onto the soil. However, the information gainedthrough adsorption isotherms is limited because the interaction ofmetals with solid phases cannot be determined and the actual par-titioning of metals in various chemical phases cannot be identified[4]. Additionally, when extending the number of the studied soils,including soils of contrasting characteristics, it is difficult to drawclear conclusions on the soil phases responsible for the sorption ofthe target metals, since sorption is a process that depends on varietyof factors [5]. Additionally, despite the intense study of metal sorp-tion capacity of separated soil constituents, little is known about themetal retention capacity of discrete mineral constituents within thesoil.

To overcome the limitations of sorption studies mentionedabove, it is worth combining sorption isotherm analyses with directinstrumental analytical techniques to study the characteristics ofmetal sorption onto soil components. Besides the application of X-ray absorption spectroscopy methods [6,7], this kind of approachwas successfully used also by Sipos et al. [8] who studied the sorp-tion properties of lead onto soil mineral constituents by analyticalelectron microscopy. However, metal sorption in soils is a com-

petitive process among different ions [9], so the direct analysis ofsorption of a given metal in the presence of others is of primaryimportance. In this study, the integrated use of analytical electronmicroscopy analyses and competitive sorption experiments wereperformed on soil samples with different composition. Samples for

ous M

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P. Sipos et al. / Journal of Hazard

u, Zn and Pb sorption experiments were selected according to theirignificant organic matter, carbonate mineral and iron oxide con-ent, as well as to their different clay mineralogy as these phaseslay an important role in metal immobilization in soils. The aimf this study was to investigate the metal sorption capacity of dis-rete soil mineral constituents (mainly clay minerals, iron oxidesnd carbonates). Special attention was directed to study the effectf presence of organic matter as well as carbonate and iron oxidehases on the sorption capacity of discrete soil clay mineral phases.

. Materials and methods

.1. Studied soil samples

Four natural soil samples were used in the laboratory experi-ents. The samples were collected from a Calcic (P9) and a Haplic

uvisol (P13) profiles, with silt loam and clay loam textures, respec-ively [10]. The air-dried samples were passed through a 2 mm sieve.ome physico-chemical properties of the studied samples are pre-ented in Table 1. The sample from the A horizon of the profile9 (P9A) is characterized by relatively high total organic carbonontent (67.4 g kg−1) and intermediate cation exchange capacityCEC = 24.2 cmol kg−1) as compared to the other studied samples.wo samples are from the Bt horizons of the two profiles. One ofhem (P9B) is characterized by relatively low clay mineral content15%) and clay mineralogy dominated by vermiculite, as well asy intermediate CEC (22.8 cmol kg−1), while the other one (P13B)y high clay mineral content (45%) and clay mineralogy domi-ated by montmorillonite [11]. This latter sample has the highestEC (38.7 cmol kg−1) among the studied samples due to its veryigh montmorillonite content (45%). The fourth sample (P9C) is

rom the Ck horizon of the profile P9, which is characterized by0% carbonate mineral (calcite) content and relatively low CEC12.4 cmol kg−1).

.2. Adsorption experiments

Batch adsorption experiments were carried out in polypropy-ene centrifuge tubes of 50 ml by mixing 200 mg sample with0 ml solution containing various concentrations of the studiedetals. Metals were added in form of nitrates (Cu(NO3)2·2.5H2O,

n(NO3)2·6H2O and Pb(NO3)2). The highest added metal con-entrations in solution used in the experiment were as follows:8.351 mol L−1 for Zn, 1.207 mol L−1 for Pb and 3.148 mol L−1 for Cu.dditionally, the following dilution rates of this solution with dis-

illed water were used: 0.5, 0.25, 0.125, 0.075, 0.05, 0.04, 0.025 and.01. The pH values of initial solutions were acidified to pH 4.5 with

ilute HNO3. The chemistry of this solution is similar to those found

n composts of urban wastes [12]. The samples mixed with the nineolutions of different metal concentrations were shaken lengthwiseor 48 h at 25 ◦C. Then they were separated by centrifugation at000 rpm for 20 min.

able 1ome physico-chemical properties of the studied soil samples.

ample Horizon Depth (cm) Color pH (H2O) CEC (cmol kg−1) TOC (g k

9A A 0–5 10YR3/4 6.39 24.2 67.49B Bt 15–45 10YR5/8 6.85 22.8 3.99C Ck 55–75 2.5YR6/4 8.08 12.4 <113B Bt 60–120 7.5YR5/2 4.84 38.7 1.5

arb = carbonate, Fered = reducible iron content (extracted by 0.04 M NH2·OH·HCl in 25% (v/vixed structures, Ill = illite.* Clay mineral and carbonate contents of the samples were estimated using semi-quan

aterials 168 (2009) 1512–1520 1513

The amount of metals adsorbed by the studied soil samples werecalculated using the equation:

Q = (Ci − Ce)VW

,

where Q is the adsorbed metal amount per unit weight of solid(mmol kg−1), Ce is the equilibrium metal concentration in the solu-tion (mmol L−1), Ci is the initial metal concentration in the solution(mmol L−1), V is the volume of the solution (mL) and W is the weightof the air-dried soil (g).

The linear and Langmuir isotherm equations were used todescribe the sorption of the metals from the solution onto the stud-ied soil samples. The simple linear relationship between sorbed andsolution phases is expressed as

Q = KdC,

where Kd (L kg−1) is the distribution coefficient which describes theequilibrium partitioning of a metal between solid and liquid phases,and thus can be used as an index of metal mobility in the soil.For a linear adsorption isotherm Kd is constant and independentof metal solution concentration. However, for a nonlinear isothermKd depends on the concentration and increases with decreasingmetal concentration in the solution. This means that it is difficultto have one Kd value that adequately represents the sorption, if thesorption isotherm is not linear over the range of interest of metalconcentration [13]. Nevertheless, according to Kaplan et al. [14]the joint distribution coefficient Kd� could be useful to study themetal partition between the solid and liquid phases also for nonlin-ear isotherms and can be calculated by summarizing the Kd valuescalculated for each initial metal concentration used. The Langmuirisotherm is expressed as

Ce

Q= Ce

Qmax+ 1

Qmaxb,

where Qmax is the maximum adsorption capacity of the solid(mmol kg−1) and b represents the Langmuir bonding term relatedto the adsorption energy (L kg−1).

The relative percentage change or “sorption intensity” (SI�) wasalso used to compare the metal sorption capacity of the samples[15]. This parameter is calculated as follows:

SI∑ =∑

(Ci − Ce)∑

C i100.

2.3. Analytical techniques

The studied soil samples were characterized for mineralogi-cal composition using a Philips 1710 X-ray diffractometer (XRD).Mineral composition of the bulk soil was determined on random-powdered samples by semi-quantitative phase analysis after themethod by Bárdossy et al. [16]. Clay minerals were identified from

g−1) Clay* (%) Carb* (%) Fered (mg kg−1) Relative amount of clayminerals (%)

Ver Mm Chlo/Verm Ill

8 – 2863 ± 163 60 – – 4015 – 4137 ± 151 80 – – 207 20 2747 ± 124 – – 75 25

45 – 2670 ± 22 – 90 – 10

) HOAc), Ver = vermiculite, Mm = montmorillonite, Chlo/Verm = chlorite/vermiculite

titative phase XRD analyses.

1514 P. Sipos et al. / Journal of Hazardous Materials 168 (2009) 1512–1520

Table 2Results of the sorption study.

Metal Soil sample Kd� (L kg−1) SI� (%) Langmuir

Isotherm type Qmax (mmol kg−1) Qmax (mg kg−1) b (L mmol−1) R2

Pb P9A 222 69 H2 63.3 13 114 39.5 0.9950P9B 71 42 H2 25.1 5 193 66.5 0.9992P9C* 1 741 95 C1 – – – 0.0034P13B 76 43 H2 28.5 5 903 39.0 0.9978

Cu P9A 137 58 H2 116.3 7 389 28.7 0.9941P9B 89 47 H1 82.0 5 209 40.7 0.9939P9C 1 651 94P13B 39 28 H2 33.3 2 118 60.0 0.9994

Zn P9A 23 19 H2 104 6 811 16.0 0.9995P9B 20 16 H4 101 6 605 5.2 0.9992P9C 54 35 H2 244 15 949 205 0.9997

elatio

tpwa9Siw[rolw

PTlfcesdpeLat2yZmtafimc

3

3

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P13B 17 15 H2

* The sorption of Pb in sample P9C can be described satisfactory with the linear r

he clay fractions of bulk samples by XRD diagrams obtained fromarallel-oriented specimens. The following diagnostic treatmentsere carried out for all of the samples: ethylene glycol solvation

t 60 ◦C overnight, Mg saturation followed by glycerol solvation at5 ◦C overnight, K saturation, heating on 350 and 550 ◦C for 2 h.eparation of the clay fractions were performed by sedimentationn aqueous suspension. Total organic carbon content of the samplesas determined by the Rock-Eval method (Delsi Oil Show Analyzer)

17]. Soil pH (H2O) was measured by a Radelkis OP 211 analyzer. Theeducible iron content of the samples was analyzed by extractionf 1.0 g of soil sample by 0.04 M NH2·OH·HCl in 25% (v/v) HOAc fol-owing a 1 M NaOAc extraction [18]. Soil cation exchange capacities

ere determined by the sodium acetate method [19].Metal concentrations in the solutions were analyzed by a

erkinElmer AAnalyst 300 atomic absorption spectrometer (AAS).he relative standard deviations for the studied metals were as fol-ows in the duplicate samples: 1.6% for Pb, 2.8% for Cu and 3.0%or Zn. Analytical transmission electron microscopy analyses werearried out to characterize the sorption capacity of discrete min-ral phases in the spiked soil samples. The studied samples werelightly grounded under ethanol and the resulted suspensions wereropped on an Au grid for the analyses. The measurements wereerformed on a Philips CM 20 instrument equipped with a Norannergy-dispersive spectrometer (EDS) operating at 200 kV with aaB6 filament. For the chemical analyses a 5 nm beam parameternd counting times of 100 s were used. The chemical composi-ion was calculated on the basis of 100 nm sample thickness and.5 g cm−3 density. The relative standard deviations of the EDS anal-ses are between 1% and 5% above 10 atomic percent (at.%) of Cu,n and Pb concentration, while 6–15% between 1 and 10 at.% ofetal concentration, and 17–30% below 1 at.% of metal concen-

ration depending of the element and phase studied. We tend tonalyze only one discrete particle in each case, which could be con-rmed from its diffraction pattern. The identification of the studiedineral phases was performed from their diffraction pattern and

hemical composition.

. Results and discussion

.1. Sorption studies

Copper, Zn and Pb sorption experiments were carried out to

tudy the immobilization of metals by the soil samples with dif-erent composition. As the affinity of metals to soil may be stronglynfluenced by the initial metal concentrations in solutions usedwhich is six times higher for Zn than for Cu in our case), there-ore, the sorption intensity values will be used to compare the

80.6 5 273 10.3 0.9994

n with R2 = 0.929 and Kd = 1 635 L kg−1.

sorption capacity of the studied samples. According to Xiong et al.[15] sorption intensity is a useful measure to compare and contrastsorption between different metal ions present in varying initial con-centrations. The results show that significant metal amounts wereimmobilized by all of the studied samples (Table 2): the sorptionintensity of the studied metals on the samples were between 28%and 94% for Cu, between 15% and 35% for Zn and between 42% and95% for Pb.

By far the highest metal amounts were immobilized by the sam-ple P9C containing carbonate (up to 2–3 times more than by theothers) despite the fact that this sample can be characterized bythe lowest CEC (12.4 cmol kg−1). Copper and Pb showed almostcomplete immobilization by this sample (their sorption intensityis 94% and 95%, respectively), while more than the half (54%) ofadded zinc was sorbed on this sample. However, in this case theprecipitation is the most important immobilizing process for Cuand Pb. This is suggested by the shape of sorption isotherms ofthese metals as the amount of sorbed Cu and Pb to the solid phaseincreased continuously with increasing initial metals concentra-tions (Fig. 1). Copper showed complete retention by this samplebetween 2 and 100 mg L−1 initial Cu concentration, while 88% ofthis metal was immobilized from the solution with highest addedCu (3.148 mol L−1). In this case, adsorption isotherm equations nor-mally used in the literature (such as linear, Langmuir, Freundlichetc.) can not be fitted to the sorption data of Cu. Contrarily, sorptionintensity of Pb on sample P9C was constant (94 ± 2%) in the wholerange of initial concentrations applied (from 0.012 to 1.207 mol L−1).The sorption data for Pb showed the best fit to the linear sorptionisotherm equation (R2 = 0.929) in this case. These isotherm shapessuggest that sample containing carbonate did not reach saturationfor Cu and Pb, even for the solution with highest added metal con-centrations. Fontes et al. [12] found similar sorption behaviour forCu and Pb in limed soil samples. Their results indicated that thepresence of carbonates in the soil created new sorption sites andalso favoured the precipitation of these metals. The precipitationof Cu and Pb in the sample containing carbonate (P9C) was alsosupported by the XRD analyses in our case. Characteristic peaksbelonging to cerussite (PbCO3; JCPDS No. 85-1088) at 0.359, 0.350and 0.252 nm have appeared in the X-ray pattern of the sampletreated with the solution containing the highest added metal con-centrations (Fig. 2). The presence of copper carbonates (JCPDS No.70-2053) is also probably based on the small discrete reflections

at 0.299 and 0.265 nm, but its amount is less than that of lead-carbonate. However, there is no X-ray evidence for the presenceof zinc-carbonate (smithsonite) in this sample. Metal precipitationin soils can occur at alkaline conditions, relatively high metal con-centrations, and low metal solubility or also in the case of small

P. Sipos et al. / Journal of Hazardous Materials 168 (2009) 1512–1520 1515

Fig. 1. Sorption isotherms and the linear forms of Langmuir

Fig. 2. XRD pattern of the sample containing carbonate (P9C) before andafter the treatment with the solution of highest added metal concentrations.Pb = Pb-carbonate (cerussite), Cu = Cu-carbonate, Qtz = quartz, Plag = plagioclase,Calc = calcite.

isotherm of Pb, Cu and Zn on the studied soil samples.

number of specific adsorption sites [20]. In this sample the pres-ence of carbonates led to elevated pH level (soil pH 8.08) which mayhave encouraged metal carbonate precipitation reactions. Yong andMacDonald [21] found that since Pb and Cu carbonates precipitateat lower pH values than calcite and dolomite, it is possible for thesemetals to precipitate in this form because of the dissolution of Mgand Ca carbonates. The equilibrium pH of the solution containingthe highest added metal concentrations was 5.82. At this pH valueboth copper and lead carbonates may form at natural soil condi-tions, while zinc is stable in form of Zn2+ [22]. These data suggeststhat only the precipitation of Pb and Cu was possible as carbonatephases in the sample P9C due to the calcite dissolution. In contrastto Cu and Pb, the amount of Zn sorbed to this sample increasedrapidly up to a certain point and then this increase reached a plateau

showing a H2 type isotherm according to the classification of Gileset al. [23]. The almost perfect fit for the linearized Langmuir equa-tion (R2 = 0.9997) shows that precipitation has not played role inthe immobilization of Zn in the sample containing carbonate (P9C)at the equilibrium pH.

1 ous M

m5(sscctpfpwghtoaacbsattptsem(uP4m

(osalwtpCfttcrnmimiwisiwotfpot

516 P. Sipos et al. / Journal of Hazard

Similarly to the sample P9C, samples free of carbonates showeduch higher sorption intensity for Cu and Pb (between 28% and

8%, as well as between 42% and 69%, respectively) than for Znbetween 15% and 19%). The shapes of sorption isotherms for eachtudied metal and for each sample free of carbonates is similar andhow a plateau after rapid increase with increasing initial metaloncentrations. All of these isotherms are H type according to thelassification of Giles et al. [23], and they show a very good fit tohe linearized Langmuir equation in each case (R2 > 0.99). The sam-le P9A with the highest TOC content exhibits the highest affinityor all three metals compared to the other carbonate-free sam-les. The sorption intensities of the studied metals for this sampleere 69% for Pb, 58% for Cu and 19% for Zn. These results are in

ood agreement with those on metal affinity sequence towardsumic substances published by several authors [e.g. 24, 25]. Thisrend follows the Irving-Williams series which describes the orderf stability of organic–metal complexes. The order of this seriesrises in part from a decrease in ionic size and in part from lig-nd field effects [26]. This kind of behavior of metals during theirompetitive sorption onto humic substances can be justified on theasis of the hard–soft acid base concept [27]. Zinc ions are con-idered as a soft Lewis acid, while Pb2+ and Cu2+ are borderlinecids that can behave both as hard and as soft acids depending onhe environment solution. The data suggest that Pb showed rela-ively much hardness than Cu at the studied conditions (equilibriumH of the solution containing the highest added metal concentra-ions was 4.30 for the sample P9A rich in TOC) resulting in higherorption intensity on sample rich in organic matter. The cationxchange capacity of this sample (P9A CEC = 24.2 cmol kg−1) wasuch lower than that of the sample containing montmorillonite

P13B CEC = 38.7 cmol kg−1). However, both soil pH and the pH val-es of the equilibruim solutions were much higher for the sample9A (pHsoil 6.39; pHeq 4.30) compared to the sample P13B (pHsoil.84; pHeq 4.00). This differences may also affect the sorption ofetals on the studied samples.Samples containing vermiculite (P9B) and montmorillonite

P13B) as the dominant clay minerals immobilized lower amountsf metals than the sample rich in organic matter (P9A). Many studieshowed the higher sorption capacity of humic substances for metalss compared to inorganic soil components [28]. Lead was immobi-ized on these samples in similar amounts: its sorption intensityas 42% for sample P9B and 43% for the sample P13B. This was

he case also for Zn with 16% of sorption intensity for the sam-le P9B and 15% for the sample P13B. The sorption intensities foru were more varying since this value was almost twice as much

or the sample P9B (48%) as for the sample P13B (28%) showinghe higher affinity of Cu to vermiculite (and free Fe oxides) thano montmorillonite. Additionally, the much higher pH of the soilontaining vermiculite (pH 6.85) than that one containing montmo-illonite (pH 4. 84) may also result this difference. It is interesting toote that the sample characterized by montmorillonite (P13B) hasuch higher CEC (38.7 cmol kg−1) compared to the one character-

zed by vermiculite (55.8 cmol kg−1). However, the pH conditionsay have modified the potential sorption capacity of the samples

n our case. That is why lower sorption on sample with higher CECas found. Generally, these samples show the following sorption

ntensity sequence for the studied metals: Pb > Cu > Zn. The resultshow that Pb and Zn have a similar affinity to the samples contain-ng vermiculite (P9B) and montmorillonite (P13B), while more Cuas immobilized on the sample containing vermiculite than thene containing montmorillonite. According to Abollino et al. [29]

he order of affinity of the studied metals both for vermiculite andor montmorillonite are similar to that found in this study in a wideH range (between 2.5 and 8.0). They showed that the adsorptionf divalent metal ions on these phases increases with decreasing ofhe ability of cations to hydrolyze, and with increasing of the atomic

aterials 168 (2009) 1512–1520

weight of the metal ion. At pH values higher than 5.5, the adsorp-tion of these metals were complete on motmorillonite, whereasit reached 100% only at pH 8.0 on vermiculite. The equilibriumpH values were below 5.5 both for sample characterized by ver-miculite (P9B, pHeq 4.50) and montmorillonite (P13B, pHeq 4.00)in our case. The much higher sorption capacity of vermiculite ascompared to montmorillonite, as well as the differences in the equi-librium pH values could result in differences in sorption capacitiesof the samples P9B and P13B for Cu. In the case of Pb, however,the association of clay minerals and iron oxide phases in the sam-ple containing montmorillonite may have led to similar sorptioncapacities in these samples. Many studies showed that the pres-ence of iron oxide phases in soils may significantly contribute to theadsorption of these metals, especially for Pb and Zn [e.g. 30, 31]. Asit will be shown by the AEM analyses, the close association of ironoxide and clay minerals in the sample containing montmorillonitesignificantly contributed to the enhanced adsorption capacity ofclay mineral particles despite the fact that much higher reducibleiron could be dissolved from the sample characterized by vermi-culite (P9B) than that one containing montmorillonite (P13B) (seeTable 1).

Contrary to Cu and Pb, Zn remained mostly in solution for allthe studied samples while Cu and Pb showed much higher sorptionintensities. This phenomenon was observed by several authors ata wide range of soils and can be explained by the fact that sorp-tion of Zn depends rather on electrostatic forces while that of Cuand Pb on covalent bonding with mineral surfaces [32]. That iswhy the sorption intensity of Zn was lower than that of Cu andPb in each samples studied. Moreover, metal sorption generallydecreases with increasing acidity and the chemical conditions arenot favorable for Zn sorption below pH 5.5 as Cu and Pb may havestill high sorption intensity [33,34]. This observation is also con-firmed by our results which showed almost complete retention forCu and Pb in the sample containing carbonates with equilibrium pHof 5.82 in contradiction to Zn with sorption intensity of 54% in thesame sample. At lower equilibrium pH values (between 4.00 and4.50) in the samples free of carbonate, considerably lower sorptionintensities were found for all the studied metals than in case of sam-ple containing carbonate (Table 2). Sorption intensities, however,were always higher for Pb and Cu than for Zn. Our data also indi-cated that Zn was displaced by Cu and Pb ions out from the sorptionsites in the studied samples despite the considerably higher initialZn concentration (which is six times higher for Zn than for Cu inour case). This observation is in accord with that of Miller et al. [35]who found that the preferential sorption of Cu and Pb in soils is notinfluenced by the presence of Zn in wide concentration range.

3.2. Analytical electron microscopic analyses

Analytical electron microscopic (AEM) studies were carried outin order to study the metal sorption capacity of discrete mineralphases composing the studied soil samples. This method is partic-ularly efficient to investigate metal speciation in the fine fractionsof the contaminated soils and sediments [36]. It is, however, notsuitable for specifying the exact location of metals within the min-eral structures [37]. Our measurements were mainly focused on themineral phases mostly influencing the metal sorption in soils, thuson clay minerals (montmorillonite, vermiculite, illite and mixedstructures), iron oxides and carbonate particles.

The results of the AEM analyses are presented in Table 3. Theseanalyses confirmed the higher sorption of Cu (up to 3.32 at.%) as

compared to Zn (up to 1.33 at.%) (Fig. 3). Contrarily, generallysmaller amount of Pb (up to 0.64 at.%) was sorbed on the stud-ied mineral particles than Zn, which fact indirectly refers to theimportant role of organic matter in the adsorption of lead. This is inaccordance with the previous studies by Sipos et al. [8] who showed

P. Sipos et al. / Journal of Hazardous Materials 168 (2009) 1512–1520 1517

Table 3Composition of the studied soil mineral particles (at.%).

Mineral phases O Si Al Fe Mg Ca Na K Ti Mn C Cu Zn Pb

Vermiculite (P9A) 60.92 14.51 10.12 4.57 8.68 0.00 0.00 0.14 0.06 0.00 – 0.60 0.34 0.00Vermiculite (P9B) 61.73 14.48 10.64 7.75 3.88 0.00 0.00 0.47 0.00 0.00 – 0.44 0.21 0.27Smectite (P9A) 61.99 18.37 11.41 1.62 3.65 0.00 0.00 0.85 0.00 0.10 – 0.72 0.18 0.00Smectite (P9B) 61.80 17.37 11.46 2.41 4.34 0.00 1.01 0.37 0.00 0.00 – 0.57 0.43 0.00Fe-smectite (P13B) 62.81 18.08 5.50 9.75 0.66 0.27 0.23 0.30 0.17 0.00 – 0.74 0.89 0.53Fe-smectite (P13B) 63.37 19.69 10.24 4.34 0.00 0.00 0.00 0.56 0.16 0.00 – 0.44 0.67 0.26Fe-smectite (P13B) 61.49 19.76 11.59 3.38 1.99 0.00 0.00 0.55 0.10 0.00 – 0.28 0.56 0.18Illite (P9A) 62.20 18.94 13.19 0.89 0.63 0.26 1.36 1.91 0.07 0.00 – 0.46 0.13 0.00Illite (P9B) 62.46 18.30 14.12 0.61 2.09 0.00 0.00 1.59 0.00 0.00 – 0.28 0.34 0.00Illite (P9B) 62.02 18.25 14.57 0.46 0.96 0.05 0.00 3.51 0.03 0.00 – 0.10 0.00 0.00Illite (P9B) 62.65 18.68 13.51 1.18 1.75 0.07 0.00 1.53 0.05 0.00 – 0.29 0.18 0.08Fe-illite (P13B) 61.20 15.28 9.17 7.36 1.01 0.00 0.34 2.60 0.32 0.08 – 1.74 1.28 0.64Fe-illite (P13B) 61.89 17.49 12.12 3.01 0.78 0.00 0.49 2.62 0.29 0.00 – 0.34 0.74 0.19Mg-chlorite (P9C) 60.03 14.69 8.45 2.28 13.52 0.09 0.00 0.00 0.00 0.00 – 0.69 0.19 0.05Mica (P9A) 62.11 17.74 15.37 0.61 0.57 0.00 0.00 2.59 0.05 0.00 – 0.23 0.10 0.07Mica (P9C) 61.84 16.86 16.49 0.24 0.82 0.10 0.00 3.25 0.07 0.00 – 0.21 0.09 0.00Illite/smectite (P9A)* 61.47 16.52 14.49 1.05 2.08 0.05 0.93 1.89 0.06 0.00 – 0.52 0.44 0.48Illite/smectite (P9A)* 62.40 17.30 14.93 1.13 2.04 0.09 0.00 1.20 0.06 0.00 – 0.41 0.22 0.18Illite/smectite (P9A)* 62.61 17.49 14.89 1.63 1.25 0.00 0.00 1.24 0.08 0.00 – 0.42 0.18 0.00Illite/smectite (P9A)* 62.37 18.36 13.86 0.74 1.80 0.09 0.00 1.37 0.00 0.00 – 0.56 0.21 0.13Illite/vermiculite/smectite (P9B)* 61.73 17.25 12.01 2.12 4.14 0.00 0.00 1.14 0.00 0.00 – 0.52 0.28 0.12Illite/vermiculite/smectite (P9B)* 61.65 16.89 13.27 1.04 4.51 0.05 0.00 1.02 0.00 0.05 – 0.51 0.32 0.28Chlorite/vermiculite (P9C)* 60.19 14.86 9.51 1.80 10.66 0.72 0.00 0.26 0.00 0.00 – 1.45 0.41 0.09Chlorite/vermiculite (P9C)* 60.22 12.81 8.68 6.67 8.19 0.23 0.00 0.11 0.00 0.00 – 1.97 0.73 0.29Chlorite/vermiculite (P9C)* 59.45 11.95 9.64 4.47 11.77 0.49 0.00 0.21 0.00 0.00 – 1.39 0.48 0.09Chlorite/vermiculite (P9C)* 59.59 12.97 9.14 4.30 10.82 0.11 0.00 0.00 0.06 0.00 – 1.23 0.55 0.11Chlorite/vermiculite (P9C)* 56.13 10.24 3.10 5.51 16.11 0.00 4.57 0.00 0.00 0.00 – 3.23 0.93 0.00Fe-oxide (P9B) 60.00 4.90 8.48 21.54 3.09 0.00 0.00 0.00 0.25 0.00 – 0.81 0.45 0.15Fe-oxide (P9B) 60.22 2.69 1.08 34.16 0.00 0.00 0.00 0.22 0.24 0.00 – 0.52 0.29 0.49Fe-oxide (P9B) 60.83 7.06 5.86 23.36 0.96 0.00 0.00 0.25 0.11 0.00 – 0.73 0.33 0.39Fe-oxide (P9B) 60.93 6.39 7.15 23.74 0.91 0.00 0.00 0.06 0.06 0.00 – 0.32 0.16 0.19Calcite (P9C) 50.24 1.11 0.64 0.17 0.40 9.41 0.00 0.02 0.00 0.00 37.70 0.18 0.05 0.05Calcite (P9C) 48.26 1.42 0.91 0.20 0.34 8.05 0.00 0.00 0.00 0.00 40.31 0.31 0.12 0.08Quartz (P9A) 76.04 23.91 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 – 0.02 0.02 0.00Silica (P9A) 76.23 17.69 5.98 0.02 0.00 0.01 0.00 0.00 0.00 0.00 – 0.05 0.01 0.00Albite (P9B) 61.01 21.34 8.36 0.67 0.63 0.05 7.45 0.24 0.00 0.02 – 0.15 0.06 0.00Hematite (P13B) 63.36 2.62 0.60 31.34 0.34 0.00 0.42 0.17 0.93 0.18 – 0.18 0.11 0.00Rutile (P9C) 63.44 3.18 2.87 1.05 3.88 0.45 1.51 0.12 22.56 0.00 – 0.53 0.25 0.16C 7.04S 1.67

N contenc and s

ttaaio

hlorite/vermiculite + Fe-oxide (P9A)* 60.83 12.18 11.09 8.18mectie + Fe-oxide (P9B)* 62.57 17.36 8.74 7.55

otes: O was calculated by stoichiometry. Fe content is supposed to be Fe3+. Chlorite/vermiculite are interstratified phases, while chlorite/vermiculite + Fe-oxide

hat the soil organic matter is the most important sink for Pb in

he same soil samples as presented here. Additionally, Clementend Bernal [38] found more significant immobilization for Pb afterdding humic rich materials to soils as compared to Cu and Znndicating the outstandingly high affinity of this metal towards soilrganic matter. The high affinity of lead to soil organic matter can

Fig. 3. Adsorbed metal amounts by the studied min

0.00 0.00 0.30 0.00 0.08 – 0.18 0.00 0.000.00 0.00 0.70 0.00 0.00 – 0.57 0.20 0.38

t was in the form of carbonates. *Illite/smectite, Illite/vermiculite/smectite andmectite + Fe-oxide are multiple phases.

be due to the high stability of metal–organic complexes of Pb as it

was discussed previously in this paper.

The highest sorption capacities were found by the swellingclay mineral particles. Similar amounts of Cu (0.3–0.7 at.%) andZn (0.2–0.8 at.%) and slightly less amount of Pb (0–0.5%) wereimmobilized on the vermiculite, smectite and Fe-smectite particles.

eral particles in the different samples in at.%.

1 ous M

Tt(ptpiafattcpsdscpTpv(p

518 P. Sipos et al. / Journal of Hazard

he sorption capacity of illite and mica particles is slightly lowerhan that of swelling clay mineral particles. Lower amounts for Cu0.1–0.5 at.%) and Pb (0–0.2 at.%) was immobilized on illite/micaarticles, while adsorbed Zn amounts (0–0.8 at.%) were similar tohose of swelling clay minerals. Clay minerals in soils are oftenresent in mixed structures and phases. The illite/smectite and

llite/vermiculite/smectite phases adsorb similar amounts of met-ls to illites: 0.4–0.5 at.% for Cu, 0.2–0.4 at.% for Zn and 0–0.5 at.%or Pb. Among the studied phases chlorite/vermiculite particlesdsorbed the highest Cu amounts (1.2–3.2 at.%), but similar quan-ities of Zn (0.4–0.9 at.%) and Pb (0–0.3 at.%) was immobilized onhem as compared to swelling clay particles. The higher sorptionapacity of swelling clay minerals compared to illite and micahases due to their higher cation exchange capacity and specificurface area is a well known fact [39]. However, data resulted fromirect observations are sparse. The metal sorption by clay mineralsignificantly varies with pH [40]. This is why highest Cu sorptionapacity was found on chlorite/vermiculite particles from the sam-le P9C containing carbonates and not on swelling clay particles.

his phenomenon can be only partly due to the vermiculite com-onent in these phases, but rather to the highest equilibrium pHalue (pHeq 5.82) of this sample as compared to the other onespHeq between 4.00 and 4.50). At this pH value chlorite/vermiculitearticles immobilized the highest Cu amounts among the studied

Fig. 4. Relations between the immobilized copper lead and zinc amou

aterials 168 (2009) 1512–1520

particles. Contrarily other metals and particles did not show highersorption at this pH.

The role of iron oxides in metal sorption is the most importantfor Pb. The highest amounts of this metal was adsorbed on ironoxide phases (0.2–0.5 at.%) and clay mineral particles closely asso-ciated with iron. Earlier studies by Sipos et al. [8] on the same soilsamples showed that generally 50% lower Pb amounts were sorbedonto iron oxide particles as compared to clay minerals even withlow Fe content when single element sorption experiments are car-ried out. This suggests that the presence of other metal cations inthe solution has a strong influence on the selectivity of Pb adsorp-tion. Additionally, iron oxide particles are effective sorbents for Cu(0.3–0.8 at.%) and Zn (0.2–0.5 0at.%), as well, and their metal sorp-tion capacity can be comparable with clay minerals in our case. Thesignificant role of iron oxides and oxyhydroxides in metal retentionin soils is also evidence [41,42], which is supported by this study, aswell.

The results of sorption studies showed that the presence of car-bonate has a significant effect on metals retention capacity of soils.

Several studies mention heavy metal adsorption on the surface ofsoil carbonates, mainly on calcite [43,44]. Our data show that metalsorption capacity of calcite is significantly lower than that of otherphases studied here. It is the highest for Cu (0.2–0.3 at.%), and thehalf-third of it for Pb and Zn (up to 0.1 at.%).

nts as well as iron content of the studied clay mineral particles.

ous M

ocepta0icAeboma

iauPmittneifstekcwwtsc

ssP(i0cscwcspam(awtepaaahs

P. Sipos et al. / Journal of Hazard

A given phase may immobilize metals in a relatively wide rangef amount. This may result in low sorption capacity for a parti-le which was expected to immobilize high amount of metals inxtreme cases. For example, an illite/smectite particle in the sam-le P9A had higher sorption capacity than a vermiculite particle inhe sample P9B: the former immobilized 0.52 at.% Cu, 0.44 at.% Znnd 0.48 at.% Pb, while the latter one 0.44 at.% Cu, 0.21 at.% Zn and.27 at.% Pb. The sorption capacity of a mineral particle in soil can be

nfluenced by several factors, such as pH of the solution, presence ofompeting ions, the exposure and saturation of the mineral surface.s there is no two structurally and compositionally identical min-ral particles formed in soil, identical sorption capacity must note expected for them, as well. In spite of these facts, the amountf data presented here enables us to observe obvious differences inetal sorption capacity of the discrete mineral phases as described

bove.Our observations indicate an intimate association of iron and/or

ron oxide with both the primary and secondary silicates, even therere no iron phases detected as separate particles in the samplesnder analysis. Their presence is mostly suspected in the sample13B containing montmorillonite. Citeau et al. [42] also found thatost of the phyllosilicates were associated with high amounts of

ron in the colloid fraction of forest soils, which may be included inheir crystal structure or present as iron coatings. Little is known onhe effect of metal oxide coatings on the intrinsic sorption mecha-ism of trace metals to clay mineral surfaces, although in the mostxtreme scenarios these coating could control the metal uptakenstead of the underlying clay mineral. Nachtegaal and Sparks [45]ound that Zn complex formed at goethite-coated kaolinite surfacehowed the combination of the sorption mechanisms observed forhe individual sorbents. According to Zhuang and Yu [46] the pres-nce of iron oxide coatings increased the specific surface area ofaolinite, illite and montmorillonite. They found that iron oxideoatings on the clay minerals studied increased the sorption of Pbhen its concentration was lower than approximately 400 mg L−1,hile no significant change was found above this initial concentra-

ion. This implies that Pb was sorbed at least two types of bindingites and after the saturation of the edges of Fe-oxide, outer-sphereomplexes were formed on clay surfaces.

Contrary to Cu and Zn, Pb sorption was detected only on Fe-mectites, Fe-illites and vermiculite with high iron content in thetudied samples. The correlation between the Fe content and theb amount immobilized on these phases was generally strongr = 0.82 at 0.05 significance level). The amount of sorbed Zn alsoncreased with increasing Fe content of the particles (r = 0.73 at.05 significance level). On the other hand, increasing of Cu sorptionapacity of clay mineral particles due to their iron content was lessignificant (r = 0.50 at 0.05 significance level). The enhanced metal-arrying capacities of clay minerals coated by Fe-oxyhydroxidesere observed also by Citeau et al. [42]. There is one Fe-illite parti-

le analyzed with very high iron content (7.36 at.%), which adsorbedignificantly higher amount of Cu, Zn and Pb than other studiedarticles. Fig. 4 shows the relation between sorbed metal amountsnd iron content in the studied particles. The zinc content of clayineral particles showed strong correlation with their iron content

r = 0.73). Additionally, clay particles immobilized relatively highermount of lead and copper with their increasing iron content, asell. This relation was most characteristic in illite particles for all

he studied elements, and also in smectites for zinc and lead. Buatiert al. [37] found that Pb and Zn mostly associated with iron richarticles in smelter-contaminated soils. According to Hochella et

l. [47] clay mineral particles (mainly smectites) carry significantmounts of Cu and Zn in polluted soils by mining and smeltingctivities. They found that these metals can reside in the octa-edral layer of clay minerals with Fe, but it is also possible thatome may reside in the interlayer of the smectites. There was, how-

aterials 168 (2009) 1512–1520 1519

ever, no correlation between the amounts of the studied metals andmajor constitutive cations (Si, Al, Mg, Ca, K) of silicate phases thatcontained Cu, Pb or Zn. Our results also show that not only clay min-erals, but also other mineral particles can be coated by iron oxidesresulting sorption ability for this phases. That is why low amounts ofCu and Zn were found on albite (0.15 and 0.06 at.%, respectively), sil-ica (0.05 and 0.01 at.%, respectively) and quartz (0.02 and 0.02 at.%,respectively) particles.

4. Conclusions

Higher sorption intensity for Cu and lower for Zn was observedby both the results of sorption experiments and analytical electronmicroscopy analyses. Despite its highest sorption intensity amongthe studied metals, the generally lowest Pb sorption capacity ofdiscrete mineral particles was found suggesting indirectly the sig-nificant role of soil organic matter in Pb immobilization which wasalso supported by the sorption experiments. Additionally, the com-petitive situation resulted in increase of the role of iron oxides inPb sorption.

The mineralogical character of soil samples strongly influencedthe metal sorption capacity of discrete mineral particles. Close asso-ciation of iron oxides and silicates resulted in significant change intheir sorption capacities. Due to the probable iron oxide coatings,even two times more metal amounts can be adsorbed on the sametype of clay mineral surfaces. Additionally, these coatings providedsorption capacity for phases, which otherwise do not immobilizemetals. The alkaline conditions due to the calcite content in one ofthe studied samples resulted in increased role of precipitation forPb and Cu in the immobilization process. The sorption capacity ofcertain mineral particles has also significantly increased for Cu inthis case.

Owing the direct observation of metal–mineral association, thesorption characteristics of metals can be supported by particu-lar data. Discrete mineral particles of a given phase showed highvariance in their metal sorption capacities. However, obvious dif-ferences could be observed in this property for the different phases.The use of AEM analyses in addition to sorption analyses helps usin better understanding the heavy metal–mineral interactions insoils. However, significantly different conclusions could be resultedby using these methods independently of each other.

Acknowledgements

This study was financially supported by the Hungarian ScientificResearch Fund (OTKA no. F 62760 and PF 63973).

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