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Journal of Colloid and Interface Science 277 (2004) 1–18www.elsevier.com/locate/jcis

Feature article

Adsorption of heavy metal ionson soils and soils constituents

Heike B. Bradl∗

Department of Environmental Engineering, Umwelt-Campus Birkenfeld, University of Applied Sciences Trier,P.O. Box 301380, 55761 Birkenfeld, Germany

Received 16 December 2003; accepted 1 April 2004

Available online 24 April 2004

Abstract

The article focuses on adsorption of heavy metal ions on soils and soils constituents such as clay minerals, metal (hydr)oxides, and soil organic matter. Empirical and mechanistic model approaches for heavy metal adsorption and parameter determination in such modelreviewed. Sorption mechanisms in soils, the influence of surface functional groups and surface complexation as well as parameteing adsorption are discussed. The individual adsorption behavior of Cd, Cr, Pb, Cu, Mn, Zn and Co on soils and soil constituents is 2004 Elsevier Inc. All rights reserved.

Keywords: Adsorption; Soil; Heavy metals; Clay minerals; Metal (hydr)oxides; Soil organic matter; Cd; Cr; Pb; Cu; Mn; Zn; Co

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1. Introduction

Soil is one of the key elements for all terrestric ecostems. It provides the nutrient-bearing environment for pllife and is of essential importance for degradation atransfer of biomass. Soil is a very complex heterogenemedium, which consists of solid phases (the soil matcontaining minerals and organic matter and fluid phasessoil water and the soil air), which interact with each othand ions entering the soil system[1]. The ability of soils toadsorb metal ions from aqueous solution is of special inest and has consequences for both agricultural issues susoil fertility and environmental questions such as remetion of polluted soils and waste deposition.

Heavy metal ions are the most toxic inorganic pollutawhich occur in soils and can be of natural or of anthpogenic origin[2]. Some of them are toxic even if their cocentration is very low and theirtoxicity increases with accumulation in water and soils. Adsorption is a major procresponsible for accumulation of heavy metals. Therefore thstudy of adsorption processes is of utmost importancethe understanding of how heavy metals are transferreda liquid mobile phase to the surface of a solid phase.

The most important interfaces involved in heavy meadsorption in soils are predominantly inorganic colloids s

* Fax: +49-6782-171317.E-mail address: [email protected].

0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2004.04.005

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as clays[3], metal oxides and hydroxides[4], metal carbon-ates and phosphates. Also organic colloidal matter of detorigin and living organisms such as algae and bacteria prvide interfaces for heavy metal adsorption[5–8]. Adsorptionof heavy metals onto these surfaces regulates their solutioconcentration, which is also influenced by inorganic andganic ligands. Those ligands can be of biological origin sas humic and fulvic acids[9–11]and of anthropogenic origisuch as NTA, EDTA, polyphosphates, and others[12–15],which can be found frequently in contaminated soils awastewater.

The most important parameters controlling heavy madsorption and their distribution between soil and water asoil type, metal speciation, metal concentration, soil psolid: solution mass ratio, and contact time[16–20]. In gen-eral, greater metal retention and lower solubility occurshigh soil pH[21–25].

To predict fate and transport of heavy metals in soils bconceptual and quantitative model approaches have beeveloped. These models include the determination of theture of the binding forces, the description of the chemand physical mechanisms involved in heavy metal–surreactions and the study of the influence on variationsparameters such as pH, Eh, ionic strength and otheradsorption. The scope of this article covers the theorebackground on adsorption mechanisms, empirical and manistic models, description of surface functional groupsof basic parameters influencing adsorption of heavy me

http://www.elsevier.com/locate/jcis

2 H.B. Bradl / Journal of Colloid and Interface Science 277 (2004) 1–18

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by soils and soil constituents such as clay minerals, m(hydr)oxides, and humic acid. Also the quantitative desction of adsorption processes through adsorption isotheand the individual adsorption behavior of selected hemetals (Pb, Zn, Cd, etc.) in soils will be taken into accou

2. Adsorption of heavy metal ions: background

First theoretical models for adsorption of metal ionsoxides surfaces appeared approximately 30 years agonected with experimental studies of oxide surfaces suchtitration[26–28]. Theoretical models have been increasinapplied to adsorption data and since the 1990s experimconfirmation of surface stoichiometries is possible bying surface spectroscopic techniques such as TRLFS (tresolved laser-induced fluorescence spectroscopy), EX(extended X-ray adsorption fine structure) or XANES (X-adsorption near edge structure). These techniques pra deeper inside into the nature and the environment oadsorbed species and lead to a sharper description osurfaces involved. Thus, the fit of theoretical models toperimental data is improved[29–34].

3. Adsorption of heavy metal ions: model approaches

There are two different approaches to adsorption melling of heavy metal adsorption. The empirical modelproach aims at empiric description of experimental adsorption data while the semiempirical or mechanistic moapproach tries to give comprehension and description of basic mechanisms[35,36]. In the empirical model, the modform is chosen a posteriori form the observed adsorpdata. To enable a satisfying fitting of experimental datamathematical form is chosen to be as simple as possiblethe number of adjustable parameters is kept as low assible. Parameters are adjusted according to only a limnumber of variables such as equilibrium metal concention in the liquid phase and are therefore of only limitvalue. Nevertheless, empirical models can be very useone only aims at the empirical description of experimedata.

In the mechanistic or semiempirical model, the matmatical form is chosen a priori by setting up equilibriureactions linked by mass balances of the different comnents and surface charge effects. As the number of adjusparameters is higher the mathematical form of mechatic models is more complex than that of empirical modDue to the variety of components taken into account a highnumber of experimental variables are required, which mamechanistic models in general more valid than empirmodels. Yet the difference between empirical and mecnistic models is often not very distinct. Simple empirimodels may be extended by considering additional menisms such as competition for sorption sites or heterogen

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of solid phase. One of the main differences between themodel approaches is that mechanistic models include electrostatic terms, whereas empirical models do not.

4. Empirical models

Empirical models are usually based upon simple mematical relationships between concentration of the hemetal in the liquid phase and the solid phase at equilibrand at constant temperature. This equilibrium can befined by the equality of the chemical potentials of the tphases[37]. These relationships are called isotherms. Molayer adsorption phenomena of gases on homogeneounar surfaces were first explained mathematically and pically by Langmuir in 1916[38]. Langmuir‘s theory wasbased upon the idea that, at equilibrium, the number ofsorbed and desorbed molecules in unit time on unit surare equal. The lateral interactions and horizontal mobof the adsorbed ions were neglected. Later, statisticalmodynamics were incorporated and new isotherms formogeneous surfaces were derived[39]. The classical thermodynamic interpretation of adsorption is given by Gib[40] who introduced the idea of a dividing surface (thecalled Gibbs surface). He also proved that, in any casadsorption, the excess adsorbed amount is the solely apble and acceptable definition which should be considereevery calculation and measurement. An isotherm of mlayer gas–solid adsorption has been developed by BrunEmmett, and Teller[41], the so called BET equation. Thisotherms most commonly used for empirical descriptionheavy metal adsorption on soils are referred to as generpurpose adsorption isotherms (GPAI).

4.1. Adsorption isotherms

The most commonly used isotherm is the Langmisotherm, which has been originally derived for adsorpof gases on plane surfaces such as glass, mica, andinum [42]. It is applied for adsorption of heavy metal ioonto soils and soil components in the form

(1)qi = b

(Kci

1+ Kci

),

where the quantityqi of an adsorbatei adsorbed is relateto the equilibrium solution concentration of the adsorbatci

by the parametersK andb. The steepness of the isotheis determined byK. K can be looked upon as a measof the affinity of the adsorbate for the surface. The vaof b is the upper limit forqi and represents the maximuadsorption ofi determined by the number of reactive surfaadsorption sites. The parametersb andK can be calculatefrom adsorption data by convertingEq. (1) into the linearform:

(2)qi = bK − Kqi.
ci

H.B. Bradl / Journal of Colloid and Interface Science 277 (2004) 1–18 3

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Then the ratioqi/ci (the so-called distribution coefficienKd ) can be plotted againstqi . If the Langmuir equation cabe applied, the measured data should fall on a straightwith slope of−K andx intercept ofbK.

The Freundlich equation has the form

(3)qi = acni ,

wherea andn are adjustable positive valued parameters wn ranging only between 0 and 1. Forn = 1 the linear C-typeisotherm would be produced. The parameters are estimby plotting logqi against logci with the resulting straighline having a y intercept of loga and a slope ofn. The Freu-ndlich equation will fit data generated from the Langmequation. Converting the Freundlich equation(3) to the log-arithmic form, the equation becomes

(4)logqi = loga + n logci.

Considering the adsorption of heavy metals by soils,qi isequated to the total adsorbed metal concentration (MT inmg kg−1) andci is equated to the dissolved metal conctration (MS in mg l−1) in the batch solution at equilibriumwith the solid. Defining loga as a constant, the equation bcomes

(5)logMT = C + n logMS.

This form of the equation can be used to relate the amof heavy metal adsorbed on specific soils to the dissoconcentration of free metal ions. A generalized LangmuFreundlich isotherm can also be used as a model base fointerpretation of competitive adsorption isotherms.

The Langmuir equation for adsorption of heavy meions in soils and clays has been derived and applied by mauthors[43–48]. Also deviations between experimental daand calculated behavior have been observed, which has beexplained by the presence of competition of different adbates for the adsorption sites on the surface. Consequethe original Langmuir equation(1) had to be modified toinclude competitive effects and can be expressed as thcalled competitive Langmuir equation:

(6)q1 = bK1c1

1+ K1c1 + K2c2.

A well known situation for competitive behavior is the inflence of pH on heavy metal adsorption. As it can be shin Fig. 1, pH and ionic strength effects on As(III) adsorption a Wyoming montmorillonite can be interpreted as a copetition between protons and heavy metal for the adsorpsites[49].

Another source of deviations observed between expmental data and calculated behavior according to single-sitisotherms is the heterogeneity of adsorption sites, whmeans that the interaction between metal and surfacecannot be described by a single affinity parameter. This pnomenon is frequently encountered when dealing with cdue to imperfections in the crystal lattice and the differnature and position of charges on the surface. There are

e

,

Fig. 1. Adsorption of As(III) on Wyoming bentonite as a function of pand ionic strength. Reaction conditions: 25 g/l clay, [As(III)] 0 = 0,4 µM,reaction time= 16 h (redrawn after[49]).

different ways, by which heterogeneity effects can becluded into modified single-site Langmuir-type isothermFirst, a discrete number of different types of sites, whare characterized by different concentration and affinity fothe adsorbate, can be taken into account. Adsorption ispressed as the sum of the adsorption onZ types of sites, eacone following the Langmuir isotherm[35,49]resulting in themultisite Langmuir isotherm

(7)qi =Z∑

j=1

biKic

1+ Kic

with 2Z adjustable parameters andj referring to each adsorption site. Second, a single type of site with a continudistribution of the affinity parameter can be considered.do this, it is assumed that the affinity parameter in the sinsite isotherm is continuously distributed according to aaffinity distribution function (SADF). An overall isothermcan then be derived by integrating the single-site or loisotherm along SADF. IfΦt(c) is the overall isotherm anΨ (K,c) the local isotherm, the overall isotherm can be baccording to

(8)Φt(c) =∫

Ψ (K,c)f (k) dk,

wheref (k) is the SADF andf (k) dk is the fraction of siteswith K comprised amongk andk + dk. By takingEq. (1),which is the single-site Langmuir as the local isotherm,alytical solutions ofEq. (8)have been calculated for thretypes of distribution functionf (K), which are of the forms[50].

Langmuir–Freundlich:

(9)Φt(c) = (Kc)β

1+ (Kc)β,

Generalized–Freundlich:

(10)Φt(c) =(

Kc)β

,
1+ c


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These equations are characterized by the three adjustable prametersb, K, andβ . β is a heterogeneity index ranginfrom 0 to 1 (corresponding to very flat to very sharp dtribution). Forβ = 1 all composite isotherms will revert tthe single-site Langmuir isotherm. While modifications csidering influence of competition and surface heterogeity have extended the original Langmuir isotherm onone hand, the number of adjustable parameters hasincreased. Often, this model is too flexible in respect toperimental data. This is also of importance when discusmechanistic models.

5. Mechanistic (semiempirical) models

General purpose adsorption isotherms do not take intocount the electrostatic interactions between ions in soluand a charged solid surface as it is the case in most surencountered when dealing with soils such as clay mials, metal (hydr)oxides, and others. Adsorption as a funcof pH and ionic strength is described as a competitionadsorption sites only. The effects of modifying the electriproperties of the surface due to the adsorption of chaions and its effect on affinity parameters cannot be takenaccount in using GPAI.

The term “mechanistic models” therefore refers tomodels, which describe adsorption by accounting fordescription of reactions occurring between ions in sotion and the charged surface. Models available may varthe description of the nature of surface charge, the nber and position of potential planes, and the positionthe adsorbed species. The two main reactions occuare ion exchange, which is mainly of electrostaticture, and surface complexation, which is mainly of cheical nature. Surface complexation models allow thescription of macroscopic adsorption behavior of solutemineral–aqueous solution interfaces[51]. Combined withan electric double layer model, this is a powerful approto predict ion adsorption on charged surfaces predonant in soils such as clays and metal (hydr)oxides[52].There are different electrostatic models available, whiccan be distinguished by the way the double layer atsolid/solution interface is described. The three modused most are the constant capacitance model, the dlayer model and the triple layer model, which describedouble layer by two, three and four potential adsorpplanes[53].

5.1. Constant capacitance model

This model was developed by Stumm, Schindler anders[54–56]and considers the double layer as consisting

n

s

e

Fig. 2. Schematic illustration of the interface according to the constanpacitance model (CCM) (redrawn after[35]).

two parallel planes (Fig. 2). The surface chargeσ0 is associ-ated to the one plane and the counter chargeσ 1 is associatedto the other plane. The model contains the following assutions: first, all surface complexes are inner-sphere complformed through specific adsorption; second, the consionic medium reference state determines the activity coeficients of the aqueous species in the equilibrium constand no surface complexes are formed with ions frombackground electrolyte; third, surface complexes existchargeless environment in the standard state; and fourthface charge drops linearly with distance x from the surfand is proportional to the surface potentialΨ through a con-stant capacitanceG:

(12)σ0 = GΨ.

The surface chargeσ0 is simply calculated by summationall specifically adsorbed ions while all nonspecifically asorbed ions are excluded from plane 0. In this simple mothe only adjustable parameter is the capacitanceG, whichhas to be optimized by regression of the experimentalsorption data. As for the application of the constant captance model (CCM) to adsorption of heavy metal ions oclays and metal (hydr)oxides a combined ion exchansurface complexation model with two kinds of binding siwas proposed[57]. One kind of site consists of a weakacidic site (≡XH) which can undergo ion exchange wiboth Me2+ and Na+ ions, while the other kind of site iformed by amphoteric surface hydroxyl groups (≡SOH)which form surface complexes≡SOMe2 + and (≡SO)2Meand bind Na+ as outer sphere complexes. The CCMlooked upon as a limiting case of the basic Stern model[58]for high ionic strengths whereI � 0.1 mol l−1 although it ismore often applied to lower ionic strengths in the literat[35]. The CCM is the simplest of the surface complexatmodels with the least number of adjustable parametercan only be used for the description of specifically adsorbeions and is unable to describe changes in adsorption oring with changes in solution ionic strength.


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Fig. 3. Schematic illustration of the interface according to the diffuse lamodel (DLM) (redrawn after[35]).

5.2. Diffuse layer model

The generalized diffuse layer model was introducby Stumm et al.[59] and developed by Dzombak anMorel [60]. The model contains the following assumtions: first, all surface complexes are inner-sphere complformed through specific adsorption; second, no surface cplexes are formed with ions from the background electrolthe infinite dilution reference state is used for the solutand a reference state of zero charge and potential is usethe surface. Three different planes are introduced (Fig. 3).First there is the surface plane 0 where ions are adsoas inner sphere complexes, second the plane d, whichresents the distance of closest approach of the counterand third a plane, after which surface potential is conered to drop to zero. The surface chargeσ0 is determinedas the sum of all specifically adsorbed ions like it is callated in the CCM. Yet the capacitanceG is calculated by theGouy–Chapman theory and the ionic strength is takenaccount. For a z:z electrolyte the relationσ0 = f (Ψ ) can becalculated as:

(13)σ0 = −σd =√

8εε0RT I103 sinh

(zFΨ0

2RT

),

whereε is the dielectric constant,ε0 the permittivity of freespace, andI the medium ionic strength. The DLM has bepresented as a limiting case of the Stern model for low iostrengthI � 0.1 mol l−1. The advantage of the DLM is thait is able to describe adsorption as a function of changsolution ionic strength and has only a small number ofjustable parameters.

5.3. Triple layer model

The CCM and the DLM have both been developed asiting cases for high and low ionic strength. The triple laymodel (TLM), however, can be applied to the whole ranof ionic strengths and is a version of the extended Smodel [61,62]. This model comprises four planes (Fig. 4),

r

-,

Fig. 4. Schematic illustration of the interface according to the triple lamodel (TLM) (redrawn after[35]).

and electrolyte and metal ions can be adsorbed as innouter-sphere complexes depending on where the diffeions are located. The adsorption of ions on the additioplaneβ creates a chargeσβ and electroneutrality can be epressed as:

(14)σ0 + σβ + σd = 0.

Considered that the regions between planes 0 andβ and be-tweenβ and d are plane condensers with capacitanceG1 andG2, respectively, the relation between charge and potentialgiven by:

(15)Ψ0 − Ψβ = σ0

G1

and

(16)Ψβ − Ψd = σ0 + σd

G2= − σd

G2.

The relation between charge and potential on the diffplane d can be calculated by the Gouy–Chapman theofollows:

(17)σd =√

8εε0RT I103 sinh

(zFΨd

2RT

).

In a more general approach, the adsorption of metal ionsoccur either at the 0 plane or theβ plane[63]. If the TLM isto be applied the determination of the two capacitancesG1andG2 is necessary. The TLM is more complex and cotains more adjustable parameters the other models descabove. It offers the advantage of being more realistic becausboth inner- and outer-sphere surface complexation reaccan be taken into account.

There are other model approaches such as the ONEK

model and the TWO-pK model[64–66]. These models arspecial cases of a more generalized model called the MtiSIte Complexation model (MUSIC) which considers eqlibrium constants for the various types of surface groupsthe various crystal planes of oxide minerals[67,68]. Thesemodels are very complex and involve a large number ofjustable parameters.


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5.4. Parameter determination in mechanistic models

Once the set of equilibrium reactions and the relatedterial balances have been defined the model can bethe experimental data by adjustment of unknown paramesuch as site concentration andspecies formation constanThere are two critical points when defining the model strture. First, often the set of equilibrium reactions is moreless hypothesized, and second, the model has too manjustable parameters with respect to experimental constrai.e., the model structure becomes too flexible. Definingmodel structure follows in fact a trial-and-error approawhere the model definition is also a part of the overall fittprocedure to the experimental data. As a result, the menistic model approach is reduced to a semiempirical onit was discussed earlier. If the model is too flexible differsets of adjustable parameters may result in similar destion of experimental data[69,70].

Also the mathematical form of the model and the quaof the experimental data may cause poor parameter idenbility. Therefore, it is often difficult to choose from differemodels and little information can be derived about the phyical reality. In order to overcome these difficulties it is bto introduce as many constraints as possible for both mform and parameter values and to determine as manyables experimentally as possible[35]. For example, concentration or adsorption of all species in chemical equilibriawell as surface charges and potentials should be calcuand initial and final concentrations of all soluble componentshould be measured in order to obtain the numerical stion of the model. Often, only a simplified approach is usi.e., the acid–base properties of the absorbent in absenthe heavy metal of interest are determined by titration. Thheavy metal adsorption is determined as a function of pHionic strength[71].

Alternatively, it is possible to use all experimental vaables available simultaneously[72]. In this modelling ap-proach, three dependent variables (heavy metal adsorpacid–base titration, and surface charge) were expressefunction of three independent variables (pH, ionic strenand heavy metal concentration in the solution at equilibriuby using a multivariate nonlinear least squares procedurfitting. It was shown that all models used were able to scessfully simulate heavy metal adsorption on clays as a ftion of pH and heavy metal concentration at equilibriuHowever, most adjustable parameters (e.g., the formaconstants) are estimated with large uncertainty. The bestto overcome the problem of pooridentifiability is the furtherincrease of calculated variables, which can be determexperimentally.

As for surface potentials, good agreement betweenmeasured zeta potential and the calculated diffuse layetential in a TLM for the sphalerite/water interface has breported[73], but for other oxide/water and clay/water intefaces such correspondences have not been observed[74–76].As for the determination of adsorbed species at the interfac

-,

-

-

l-

d

f

,a

-

several spectroscopic methods can be used for the detnation of surface reactions and species which are impofor the adsorption process[33,77,78].

6. Sorption mechanisms in soils

As the retention mechanism of metal ions at soil sfaces is often unknown, the term “sorption” is preferred[79],which in general involves the loss of a metal ion fromaqueous to a contiguous solid phase and consists ofimportant processes: adsorption, surface precipitation,fixation [4].

Adsorption is a two-dimensional accumulation of matter at the solid/water interface and is understood primily in terms of intermolecular interactions between soland solid phases[80]. These interactions comprise of diffeent interactions: first, surface complexation reactions whare basically inner-sphere surface complexes of the mion and the respective surface functional groups; secelectrostatic interactions where the metal ions form ousphere complexes at a certain distance from the surthird, hydrophobic expulsion of metal complexes containg highly nonpolar organic solutes, and fourth, surfacadsorption of metal–polyelectrolyte complexes due toduced surface tension. Often, heavy metal adsorption isdescribed in the scientific literature in terms of two bsic mechanisms: specific adsorption, which is characterizeby more selective and less reversible reactions incluchemisorbed inner-sphere complexes, and nonspecificsorption (or ion exchange), which involves rather weakless selective outer-sphere complexes[81]. Specific adsorption brings about strong and irreversible binding of hemetal ions with organic matter and variable charge minals while nonspecific adsorption is an electrostatic phenenon in which cations from the pore water are exchanfor cations near the surface. Cation exchange is a formouter-sphere complexation with only weak covalent boing between metals and charged soil surfaces. It is reverin nature and occurs rather quickly as it is typical for re-actions which are diffusion-controlled and of electrostanature[82].

Specific adsorption can be described by a surface cplexation model which defines surface complexation fortion as a reaction between functional surface groups anion in a surrounding solution, which form a stable unit[83].Functional surface groups can be silanol groups, inorghydroxyl groups, or organic functional groups. Specificsorption is based upon adsorption reactions at OH-groat the soil surfaces and edges, which are negatively chaat high pH. The adsorbing cation bonds directly by anner sphere mechanism to atoms at the surface. As asequence, the properties of the surface and the natuthe metal constituting the adsorption site influence thedency for adsorption. These reactions depend largely onare equivalent to heavy metal ion hydrolysis and can be


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idg-ur

the

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scribed as follows for a metal cation Me and a surface S

(18)S–OH+ Me2+ + H2O↔ S–O–MeOH+2 + H+.

In contrast to adsorption, surface precipitation is characized by the growth of a new solid phase, which repeats itin three dimensions and forms a 3-D network[80]. Metalsmay precipitate as oxides, hydroxides, carbonates, sulfior phosphates onto soils. Surface precipitation is mainfunction of pH and the relative quantities of metals andions present. It has been reported that surface precipitaof hydrous oxide-type soil constituents occurs at pH vallower than those required for metal hydroxide precipitatin pure aqueous solutions without soil suspension[84].

The surface complexation model is able to describeadsorption behavior at low cation concentrations veryactly but it is not able to describe the adsorption curobtained at higher concentrations. In the first case, the cucan be described approximately by a Langmuir isothwhere a saturation of the adsorption capacityis reached.In the second case a continuous increase without sation at the surface is observed, which is fitted better bFreundlich isotherm. To explain this behavior the so-casurface precipitation model has been developed, which tinto account precipitation reactions in addition to adsotion reactions at the surface[85,86]. This model postulatea multilayer sorption process along a newly formed hydroxide surface, which is caused by the metal adsorption asurface and includes the formation of a surface phaseso-called solid solution. The surface precipitation modelbe described by two reactions: first a surface complexmation of a metal cation Me and a surface S as descrby Eq. (16)and second the precipitation of Me at the sface S:

S–O–MeOH+2 + Me2+ + H2O

(19)↔ S–O–MeOH+2 + Me(OH)2(s) + 2H+.

This model results in a Langmuir type isotherm at lmetal concentration and in a Freundlich type isothermincreasing metal concentrations. If the metal concentratioincreases further solid solution precipitation predomina(Fig. 5). There is often a continuum between surface coplexation and surface precipitation[80].

The third principal mechanism of sorption is fixationabsorption, which involves the diffusion of an aqueous mspecies into the solid phase[87]. Like surface precipitationor coprecipitation, absorption is three-dimensional in natHeavy metals that are specifically adsorbed onto clay mials and metal oxides may diffuse into the lattice structurethese minerals. The metals become fixed into the pore spacof the mineral structure (solid-state diffusion). In order tomove the heavy metals, the total dissolution of the partiin which they are incorporated may be required.

,

-

Fig. 5. Classification of adsorption isotherms by shape (redrawn after[3]).

7. Surface functional groups

The existence of surface functional groups is vital forsorption. Surface complexation theory describes adsorptioin terms of complex formation reactions between thesolved solutes and surface functional groups. In genersurface functional group is defined as a chemically reacmolecular unit bound into the structure of a solid phase aperiphery such that the reactive components of this unit arin contact with the solution phase[80]. The nature of the surface functional groups controls stoichiometry, i.e., whetmetal binding is monodentate or bidentate and also inences the electrical properties of the interface. Adsorptiocapacity is a function of their density.

Soil contains a variety of hydrous oxide minerals aorganic matter. Those substances possess surface hydgroups whose protons can be donated to the surrounsolution and can take up metal ions in return. Therefore,sorption of metal ions onto these sites is a function ofAnother important group of minerals in soils are alumoscates (clay minerals, micas, zeolites, and most Mn oxidwhich are characterized by a permanent structural chaThese minerals possess exchangeable ion-bearing sitesthe surface in addition to surface protons[88]. Soil surfacesdisplay a variety of hydroxyl groups having different reactivities. Alumina surfaces, for example, possess terminal –groups which are more likely to accept an additional pton in acidic solution compared to a bridging –OH grouThe terminal –OH group (being a weaker acid) will formpositively charged≡Al–OH+

2 site as it resists dissociationthe anionic≡Al–H− form. Once deprotonated, the termin–OH group bonds more strongly to metals than the bring –OH group[81]. Goethite (α-FeOOH) possesses fotypes of surface hydroxyls, whose reactivities depend oncoordination environment of the oxygen atom in the≡Fe–OH group. Alumosilicates display both aluminol (≡Al–OH)and silanol (≡Si–OH) edge-surface groups. The depronated aluminol group (i.e.,≡Al–O−) binds metals in theform of more stable surface complexes. The different tyof hydroxyl groups can be distinguished by IR spectrosccombined by isotopic exchange, thermogravimetric an


tieside

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sis, or reaction with methylating agents. Typical densiof surface functional groups on oxide and hydrous oxtype minerals are in the range between 2–12 sites/nm2 ofsurface area. For general adsorption modelling of bulk cposite materials, a typical value of 2.31 sites/nm2 is recom-mended[89].

The most significant surface functional groups of sorganic matter are the carboxyl (–COOH), carbonylphenolic groups. Natural environments are often chaterized by low metal concentrations and intermediate plevels (pH 4–7). Under these conditions, the sorption byboxylic groups is more important than the sorption by pnolic groups due to the wide difference between their aciconstants[90]. Also, soil colloidal particles provide large interfaces and specific surface areas, which play an imporole in regulating the concentrations of many trace elemenand heavy metals in natural soils and water systems. Pchemical weathering, which includes biologically mediatenatural chemical transformations may determine the surchemistry of soils. Weathering may produce interlayerdroxypolymers, interstratification, external-surface orgaand inorganic coatings on smectite, and organic andoxide coatings on kaolinite.

8. Surface complexes

In aqueous solutions, metals can act as a Lewis(i.e., an electron acceptor). Anelectron-pair donating suface functional group (such as –OH, –SH, and –COOH)an electron-pair acceptor metal ion (such as Me2+) formLewis salt-type compounds. For an oxide (e.g., ferric oxthe functional surface hydroxo groups≡Fe–OH may act aLewis basis in deprotonated form (≡Fe–O−) to bind a Lewisacid metal ion Me2+:

(20)≡Fe–OH+ Me2+ ↔ ≡Fe–OMe2+ + H+.

Metal oxianions (e.g., HAsO2−4 ) may release OH− ions from

the surface upon complexation:

(21)≡S–OH+ HAsO2−4 ↔ ≡S–OAsO3H− + OH−,

where ≡S–OH represents a surface functional group.there are no molecules of the aqueous solvent (i.e., wateterposed between the surface functional group and the mion bound to it these surface complexes are called “insphere complexes”. If there are water molecules interpbetween the surface functional group and the bound ionthe resulting type of surface complex is called “outer-sphcomplex”:

≡S–OH+ Me(OH2)2+n

(22)↔ ≡S–O(H2O)Me+ + (n − 1)H2O+ H+.

Inner-sphere complexes are in general more stableouter-sphere complexes as the primary bonding forcinner-sphere complexes is coordinate-covalent bondin

t

-

-l

contrast to electrostatic bonding in outer-sphere compleSpectroscopic studies of surface complexes showed thaspectra of these complexes are often reminiscent to those oanalogous aqueous complexes[91]. Inner-sphere complexewhich form with 1:1 stoichiometry are called monodentcomplexes (e.g.,≡S–OCu+ or ≡S–OAsO3H−) while thosewith 1:2 stoichiometry are called bidentate complexes

(23)2≡S–OH+ Cu2+ ↔ (≡S–O)2Cu+ 2H+,

(24)2≡S–OH+ CrO2−4 ↔ (≡S–)2CrO4 + 2OH−.

Surface spectroscopic techniques are a useful tool to dguish between inner- and outer-sphere surface complX-ray absorption fine structure spectroscopy (XAFS)been used to determine bond distances of surface O–Pions at high and low ionic strengths to reveal outer-inner-sphere lead adsorption complexes on montmorillo[92]. Inner-sphere complexes of strongly binding aqumetal ions are characterized by high adsorption equilibrconstants. In general, adsorption edge pH is below the ppzcof pure oxides (e.g., iron and aluminium oxides) and adstion increases with pH. The adsorbed metal ions showpoor desorbability, and metal adsorption is independent frinert electrolytes.

Heavy metals are usually complexed with natural ligasuch as humic or fulvic acids or anthropogenic complexsuch as EDTA or NTA. Complexation will alter metal reativity, affecting properties such as catalytic activity, toxiciand mobility[93]. The adsorption of a heavy metal onto tsurface of a hydrous oxide is also represented as the fotion of a metal complex. As hydrous oxide surfaces dispamphoteric properties, they are able to coordinate withands as well. These three components—metal, ligand,reactive surface—afford the formation of a ternary compThis ternary complex can be exceedingly stable andpossess properties, which are very different from thosthe individual component species. The formation of a ternsurface complex can be explained by two different meanisms. First, bonding of the complex occurs throughmetal to the surface:

S–OH+ Men+ + HmLig

(25)↔ S–OMe–Lig(n−m−1)+ + (m + 1)H+,

where Lig represents the ligand and S–OH represents a hydroxyl functional group on the oxide surface. The surfcomplex is designated as “metal-like” or “type A”[94,95].This mechanism is usually characterized by increasing asorption with increasing pH (Fig. 6A). Second, the liganmay form a bridge between the surface and the metal, wis only possible when it is multidentate so it can coordinwith both species:

S–OH+ Men+ + HmLig

(26)↔ S–Lig–Me(n−m−1)+ + (m + 1)H+ + H2O.

Adsorption via a ligand bridge is classified as “ligand-likor “type B” and occurs preferably at low pH (Fig. 6B). A va-


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Fig. 6. Schematic representation of metal-like (A) and ligand-like adstion (B).

riety of studies have been conducted on metal complexsorption. Only a few studies have examined the adsorpof metal–inorganic complexes. The majority of studiesternary complexes have focused on the adsorption of mcomplexed with EDTA and related chelates. The presenof SO2−

4 has been reported to increase Cd(II) adsorponto goethite over that in the presence of the more icoion NO−

3 [96]. This behavior was explained by metal-likternary surface complex formation:

(27)S–OH+ Cd2+ + SO2−4 ↔ S–OCd+ − SO2−

4 + H+.

Similar reactions have been suggested the formation oCu:P2O7 surface complexes on iron oxyhydroxide[97] andAg+:S2O2−

3 complexes on amorphous iron oxide[98]. Thismechanism has been doubted by the results of sometroscopic examinations[99]. EXAFS has been used to evauate several ligands that have shown enhancement of Cadsorption onto oxides on goethite. No local coordinabetween S and Cd and between P and Cd could be foIt was suggested that Cd sorption enhancement due to

-

.-

Fig. 7. Adsorption of Co(II)–, Cu–, Ni–, Pb–, and Zn–EDTA onto goeth(redrawn after[105]).

fate and phosphate resulted from the reduction of oxideface charge caused by anion adsorption and could noattributed to the formation of ternary complexes.

Ternary complex formation can both enhance and dimish heavy metal adsorption by soils depending on pH cotions and complexing agents involved. As for humic acidis known that under acidic to neutral pH conditions, signifi-cant amounts can be adsorbed to positively charged soileral surfaces (such as Fe- and Al-oxides and oxyhydroxidwhich may lead to charge reversal[100]. Humic-coated min-eral surfaces strongly adsorb heavy metal ions, whichlead to diminished heavy metal mobility in groundwa[101,102]. At higher pH values, the relative abundancesanionic forms of humic acid increase in aqueous solutAqueous complexation between these ligands and mcan significantly enhance heavy metal mobility[7,102]. Sta-ble anionic complexes (e.g., those with EDTA) are notstrongly adsorbed as the sole metal ions at higher pH, anegatively charged surface repulses such complexes[103].

Various studies have been conducted on metal–EDcomplex adsorption as EDTA has strong complexing aities and is widespread in the environment due to itsmerous commercial and industrial uses. The adsorptiometals on various oxides of iron, aluminium, titanium, asilicon has been studied and has always been found to bandlike, as described inFig. 6A with significant adsorptionoccurring at low pH decreasing to almost zero at pH nneutral. At very low pH (2–3) the complex becomes unsble so divergence of metal ad EDTA adsorption occurs.

Only very little difference occurs between adsorptiondifferent divalent metal types–EDTA complexes onto thsame surface[104–106]. Studies of adsorption of Co(II)–Cu–, Ni–, Pb–, and Zn–EDTA onto goethite showed ovlapping adsorption (Fig. 7). The only exception was PdEDTA, which has a much larger aqueous stability constThe formation of adsorbed Cd–EDTA has been implicain inhibiting the desorption of Cd(II) from goethite[107].Co(II)–EDTA adsorption onto goethite[108] and a poorly-


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Table 1Surface complexation constants for adsorption of metal–EDTA ontoides using constant capacitance model, 0 ionic strength (S–OH + Me–EDTA2− + H+↔ S–EDTA–Me2− + H2O)

Metal Goethite HFO δ-Al2O3 γ -Al2O3

Ca 12.26 – 11.09 –Cd – – 11.54 –Co(II) 11.05 – – 11.97Cu 11.44 – 11.08 –Ni 11.26 9.36 11.55 10.44Pb 11.18 – 11.03 –Pd 15.26 11.32 – –Zn 10.85 – 11.54 –

crystalline iron-oxide coated sand[109] exhibited ligand-like behavior. The adsorption of Co(II)–EDTA onto sevesubsurface sediments was similar to that onto commoand Al oxides[108].

The adsorption of metal–EDTA complexes onto sevhydrous oxides was modelled using the surface compltion reaction analogous toEq. (24) [110]:

(28)

S–OH+ Me–EDTA2− + H+ ↔ S–EDTA–Me2− + H2O.

A constant capacitance electrical double layer expreswas employed. The surface stability constants for thisaction are provided inTable 1. The surface complexatioconstants were found to be similar for all metals for eoxide (except for Pd). All these metals form quinquedencomplexes with EDTA. For trivalent metals such as Co(and Cr(III), hexadentate complexes are formed[105]. Al-though the modelling studies assume a direct, inner-sphebonding where the interactions with the surface are donated by the chelating abilities of EDTA, FTIR spectroscoand EXAFS showed no indications of inner-sphere compation between Pb–EDTA and goethite[111]. Spectra confirmed hexadentate coordination between the EDTA andbut exhibited no evidence of EDTA–Fe-specific interactioIt was suggested that the mechanism of Pb–EDTA adstion was through hydrogen bonding between the comand goethite surface sites, which might explain the very silar behavior of metal–EDTA for Cu, Zn, Pb, Ni, Cd, ewhich could be attributed to the nonspecific, hydrogen boing mechanism.

NTA is a triprotic acid with four possible coordinatiosites, which forms strong complexes with metals, butas strong as EDTA. Therefore, adsorption characteriof metal–NTA complexes are different as compared wEDTA. Studies of adsorption of Co–NTA onto gibbsite[112]and Pb–NTA onto TiO2 [113] showed that chelation of thmetal had only small effects on the adsorption of the monto the surface. Obviously, the oxide surface competethe individual metal and the ligand, respectively andCo(II)–NTA complex is broken in favor of individual ioadsorption. Spectroscopic evidence suggested the formof weak mono- and binuclear metal-like outer-sphere cplexes.

n

9. Parameters influencing adsorption

Adsorption of heavy metal ions on soils and soil costituents is influenced by a variety of parameters, the mimportant ones being pH, type and speciation of metalinvolved, heavy metal competition, soil composition aaging[5]. The influence of these factors is discussed seprately.

9.1. Role of pH

Soil pH is the most important parameter influencmetal-solution and soil-surface chemistry. The dependof heavy metal adsorption on, e.g., clays on solution pHbeen noticed early[114]. The number of negatively chargesurface sites increases with pH. In general, heavy metasorption is small at low pH values. Adsorption then increaat intermediate pH from nearzero to near complete adsorption over a relatively small pH range; this pH rangereferred to as the pH-adsorption edge. At high pH valuthe metal ions are completely removed.Fig. 8 shows thepH dependence of Cd, Cu, and Zn adsorption onto aiment composite, which consists basically of Al-, Fe-, aSi-oxides. 50% of the copper is adsorbed at pH 4.1, andslope of the Cu adsorption curve is steeper than the Cd oslopes.Fig. 9shows the adsorption of different heavy metonto soil humic acid[5]. 50% of the Cd or Zn is adsorbebetween pH 4.8–4.9. In general, adsorption of heavy meonto oxide and humic constituents of soil follows the batrend of metal-like adsorption,which is characterized by increased adsorption with pH[115,116]. The pH is a primaryvariable, which determines cation and anion adsorptionoxide minerals.

9.2. Role of metal ion

Universally consistent rules of metal selectivity canbe given as it depends on a number of factors such achemical nature of the reactive surface groups, the lof adsorption (i.e., adsorbate/adsorbent ratio), the pHwhich adsorption is measured, the ionic strength of thelution in which adsorption is measured, which determinethe intensity of competition by other cations for the bonding sites, and the presence of soluble ligands that ccomplex the free metal. All these variables may chathe metal adsorption isotherms. Competition from mono-valent metal in background electrolytes has relatively lieffect on adsorption on heavy metals, although preseof Ca ions does suppress adsorption on Fe oxide[117].Preference or affinity is measured by a selectivity ortribution coefficientKd [118]. The reduction of this selectivity with increased adsorption is observed for meadsorption on both clays as soil components and pureerals[119,120].


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Fig. 8. Cd, Cu, and Zn adsorption onto sediment composite in 10-3 MNaNO3 (redrawn after[4]).

Fig. 9. Adsorption of Pb, Cu, Cr, Cd, Zn, Ni, Co, and Mn onto humic aas a function of pH (redrawn after[5]).

9.3. Role of soil type

The soil type and composition plays an important rfor heavy metal retention. In general, coarse-grained sexhibit lower tendency for heavy metal adsorption than figrained soils. The fine-grained soil fraction contents soil pticles with large surface reactivities and large surface asuch as clay minerals, iron and manganese oxyhydroxhumic acids, and others and displays enhanced adsorproperties. Clays are known for their ability to effectiveremove heavy metals by specific adsorption and cationchange as well as metal oxyhydroxides[121]. Soil organicmatter exhibits a large number and variety of functiogroups and high CEC values, which results in enhanheavy metal retention ability mostly by surface complation, ion exchange, and surface precipitation[122,123].X-ray absorption spectroscopy and ESR studies suggesPb, Cu, and Zn form inner-sphere complexes with soil

,

t

mic acid[124]. Also aging may play an important role foheavy metal retention as stable surface coatings are foas a function of time and heavy metal retention onto asoils acquires a more irreversible character[4].

10. Individual adsorption behavior of selected heavymetals

10.1. Cadmium

The occurrence of cadmium in natural soils is larginfluenced by the amount of cadmium in the parent rock.erage cadmium concentration in soils derived from ignerocks is reported to be in the range from<0.10–0.30 ppmwhile soils derived from sedimentary rocks contain 0.311 ppm Cd[125]. Adsorption is the main operating mechnism of the reaction of Cd at low concentrations with soMost studies conducted found that adsorption behavioCd in soils can be described by either the Langmuir orFreundlich isotherm. Adsorption of Cd by hydrous iron oide was found to conform to the Langmuir isotherm[126].Cd adsorption was demonstrated to be a fast process w>95% of the adsorption took place within the first 10 mand equilibrium was attained within 1 h[127]. Fig. 10showsCd adsorption isotherms for two soils, a loamy sand ansandy loam, as a function of pH. The sorption capacity ofsoil increases approximately three times per unit increaspH. In addition to adsorption, precipitation can play an iportant role in controlling Cd levels in soils. In general,solubility in soils decreased as pH increased[128] with thelowest values for calcareous soils (pH 8.4). The precipitaof CdCO3 occurs in sandy soils with low CEC, low contein organic matter, and alkaline pH and controls Cd solubiat high Cd concentrations[129].

Fig. 10. Cadmium adsorption isotherms for two soils as influenced bytexture and pH (redrawn after[136]).


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Precipitation occurs in general at higher Cd2+ activi-ties while ion exchange predominates at lower Cd2+ ac-tivities. Studies of behavior of Cd2+ in the presence oCaCO3 showed that initial chemisorption of Cd2+ on CaCO3was very rapid, while CdCO3 precipitation at higher Cdconcentrations was slow[130]. Chemisorption may regulate Cd2+ activity in calcareous soils by producing mulower solubilities than predicted by the solubility produfor CdCO3. Cd adsorption is influenced by variable pameters, the most important being pH, ionic strength, andchangeable cations[131]. In the presence of Cl−, uncharged(CdCl02) and negatively charged complexes of Cd with C−ligands (e.g., CdCl−

3 , CdCl2−4 , etc.) will form. The chloro

species of Cd are less strongly adsorbed than the Cd2+. Cdadsorption is also influenced by the presence of organicands such as EDTA, NTA, or others[132]. The presenceof dissolved organic C or chelates could prevent metalprecipitation with CdCO3 or minimize adsorption of metaonto solid phases[133]. Cd adsorption is also strongly influenced by the presence of competing cations such as divCa and Zn. These cations compete with Cd for sorption sin soils or are able to desorb Cd from the soils[127,134]. Ex-periments with pure clays showed that Cd2+ competes withCa2+ for clay adsorption sites while with field soils, Cd2+was preferably adsorbed over Ca2+ [135]. Obviously, soilcolloids carry various specific adsorption sites with higbonding energy for Cd than pure clays. Nevertheless, atical environmental concentrations, the presence of alkalinearth elements has only small effect on the adsorption of Con amorphous iron oxyhydroxides[136].

10.2. Chromium

Adsorption and precipitation behavior of Cr in soilscontrolled by a variety of factors such as redox potenoxidation state, pH, soil minerals, competing ions, comping agents, and others. These factors control most ofpartitioning processes of Cr between the solid and the aous media in soils. The most important among these arehydrolysis of Cr(III) and Cr(VI), redox reactions of Cr(IIand Cr(VI), and adsorption/desorption and precipitationCr(VI). Fig. 11Ashows the distribution of Cr(III) speciesa function of pH whileFig. 11Bpresents the predicted EhpH stability field for chromium species in aqueous syste

Hexavalent Cr species are adsorbed by a varietysoil phases with hydroxyl groups on their surfaces sas Fe, Mn, and Al oxides, kaolinite and montmorillon[137–141]. Fig. 12 shows the adsorption of hexavalentonto various adsorbents as a function of pH[138]. The ad-sorption increases with decreasing pH due to the protonaof the hydroxyl groups. Obviously, Cr(VI) adsorption is fvored if the surfaces are positively charged and displaypHpzc values at low to neutral pH. This reaction candescribed as a surface complexation reaction betweeCr(VI) species and the surface hydroxyl sites. Fe oxideshibit the strongest affinity for Cr(VI) followed by Al2O3,

t

-

kaolinite and montmorillonite. Cr(VI) adsorption was fouto be greatest in lower pH materials enriched with kaolinand crystalline Fe oxides[141].

Cr(III) is rapidly and specifically adsorbed by Fe and Moxides and clay minerals, with about 90% of added beingsorbed within 24 h. Adsorption increases with increasingand content of soil organic matter while it decreases in thpresence of competing cations or dissolved organic ligain the solution. Both Freundlich and Langmuir isotherms

Fig. 11. (A) Distribution of Cr(III) species as a function of pH wheresolution is in equilibrium with Cr(OH)3(s). (B) Predicted Eh–pH-stabilitfield for chromium species in aqueous systems (redrawn after[164]).

Fig. 12. Sorption of Cr(VI) by various absorbents for a fixed adsorptionconcentration (redrawn after[141]).


olidly)ofgead-00byrac-

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be used to describe adsorption behavior of Cr(III) on sphases[142–144]. Trivalent Cr is known to be extensivehydrolyzed in acid solutions to species such as Cr(OH2+,Cr2(OH)2+

4 , or Cr6(OH12)6+. The increased adsorptionCr(III) with increasing pH is caused by cation exchanreactions of the hydrolyzed species. Cr(III) is preferablysorbed by clay minerals to Cr(VI) to an extent of 30–3times. The high affinity of Cr for Fe oxides was confirmedexperiments where Cr(III) was added to soil and a large ftion of the added Cr was extracted with the Fe oxides[145].

10.3. Lead

The chemistry of Pb in soils is affected by three main ftors: first, specific adsorption to various solid phases, preitation of sparingly soluble or highly stable compounds, athird, formation of relatively stable complexes or chelatthat result from interaction with soil organic matter.Fig. 13Ashows predicted aqueous monomeric chemical speciatiolead as a function of pH whileFig. 13B displays the predicted Eh–pH-stability field for Pb. Pb undergoes hydrolyat low pH values and displays multiple hydrolysis reactioAbove pH 9, the formation of Pb(OH)2 is important, whilePb(OH)+ is predominant between pH 6 and 10.

Adsorption of Pb onto soilsand clay minerals has beefound to conform to either the Langmuir or the Freundl

Fig. 13. (A) Predicted aqueous monomeric chemical speciation of leaa function of pH. (B) Predicted Eh–pH-stability field for lead; the assumactivities of dissolved species are: Pb= 10−6, S= 10−3, C = 10−3 (re-drawn after[164]).

f

isotherm over a wide range of concentrations[47,146]. Car-bonate content in soils plays an important role in contling Pb behavior. In noncalcareous soils, Pb solubilitycontrolled by different Pb hydroxides and phosphates sas Pb(OH)2, Pb3(PO4)2, Pb4O(PO4)2, or Pb5(PO4)3OH,depending on pH[128]. With increasing pH, the formation of Pb orthophosphate, Pb hydroxypyromorphite,tetraplumbite phosphate is possible as well as formatioPbCO3 in calcareous soils[147]. The presence of Mn anFe oxides may exert a predominant role on Pb adsorptiosoils. It was found that Pb adsorption onto synthetic Mnide was up to 40 times greater than that to Fe oxide, andPb was adsorbed more strongly than any other metal stu(Co, Cu, Mn, Ni, and Zn)[148]. Three possible mechanismmay account for the binding of Pb onto Mn oxides: firstrong specific adsorption, second, a special affinity foroxides as it has been found for Co[149,150], and third, theformation of some specific Pb–Mn minerals such as conadite. The presence of soil organic matter also playsimportant role in Pb adsorption. Soil organic matter mimmobilize Pb via specific adsorption reactions, while molization of Pb can also be facilitated by its complexion wdissolved organic matter or fulvic acids[151]. Fig. 14showsthe effect of ionic strength on Pb adsorption onto montmrillonite in the presence of humic acid as a function of iostrength[152]. An increase in ionic strength results in a dcrease in Pb adsorption.

Pb adsorption ontoα-Al2O3 has been found to involvseveral mechanisms. In general, adsorption kinetics of Pbexhibit a biphasic behavior. An initial fast reaction is folowed by a slower reaction. The slow adsorption reactionot caused by surface precipitation of Pb but may beto diffusion to internal sites, adsorption onto sites that hslower reaction rates due to low affinity, and probably fmation of additional adsorption sites due to the slow traformation ofα-Al2O3 into the less reactive solid phase. Tinitial fast reaction is most likely caused by chemical retions on readily accessible surface sites[153]. Pb has beenshown to exhibit the strongest affinity to clays, peat, Feides, and usual soils[154,155].

Fig. 14. Adsorption of Pb on montmorillonite as a function of ionic stren(redrawn after[152]).


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10.4. Copper

Copper in soils may occur in several forms that are ptitioned between the solution and the solid phases. Dibution of Cu between different soil constituents is mosinfluenced by the presence of soil organic matter, andand Fe oxides. Cu shows a strong affinity for soil orgamatter so that the organic-fraction Cu is high comparethe that for other metals even though the absolute amoare low[156]. The most important sinks for Cu in soils aFe and Mn oxides, soil organic matter, sulfides and carbates while clay minerals and phosphates are of lesseportance[157]. Adsorption maxima among soil constituendecrease in the order Mn oxide> organic matter> Fe ox-ide > clay mineral. Specific adsorption seems to plamore important role than nonspecific adsorption (i.e., caexchange). Sorption isotherms indicate preferential adstion of Cu onto soil organic matter associated with the cfraction of the soil[158]. Fig. 15 shows the adsorption oCu onto various soil constituents[159]. Mn oxide and soilorganic matter are the most likely to bind Cu in a nonchangeable form. Sorption of Cu has been shown to followeither the Langmuir or the Freundlich isotherms[160,161].

Cu in soil solution exists primarily in a form complexewith soluble organics[162]. Complexation by organic mattein the form of humic and fulvic acids is an effective mecanism of Cu retention in soils. It has been shown thatis most extensively complexed by humic materials[163] incomparison to other metals. The following preference seriefor divalent ions for humic acids and peat is indicated aslows: Cu> Pb> Fe> Ni = Co = Zn > Mn = Ca [164].Synthetic chelating agents such as ETDA, DTPA, anders combine with heavy metals to increase their levels insolution. The stability of metal-synthetic chelating agenta function of soil pH. CuDTPA is unstable in acidic soimoderately stable in slightly acidic soils, and stable inkaline and calcareous soils while CuEDTA is most stabl

Fig. 15. Adsorption of Cu by different soil constituents as a function of(redrawn after[164]).

slightly acidic to neutral soils (pH 6.1–7.3). In acidic sowith pH below 5.7 Cu–EDTA becomes unstable sincedisplaces Cu.

10.5. Manganese

The biogeochemistry of Mn in soils is very complex dto the following observations: Mn can exist in severalidation states, Mn oxides can exist in several crystalor pseudocrystalline states, the oxides can form copreciptates with Fe oxides, Fe and Mn oxides exhibit amphotbehavior and interact both with cations and with anioand oxidation–reduction reactions involving Mn are inflenced by a variety of physical, chemical, and microbiolocal processes. Therefore, Mn adsorption is more complicateas it forms insoluble oxides in response to Eh–pH cotions. Fig. 16 displays the predicted Eh–pH-stability fiefor Mn. In most acid and alkaline soils, Mn2+ is the pre-dominant solution species.

Adsorption of Mn has been shown to conform toLangmuir or Freundlich isotherm[165]. Fig. 17shows Mnadsorption by the Ao (14A) and A2 (14B) horizon ofhighly weathered sand. The adsorption conforms to the Fndlich model. Enhanced adsorption of the Ao horizon nthe surface (0–4 cm) is due to the higher CEC, higherorganic matter, and higher content in amorphous Fe oxAdsorption enhances with increasing pH, which can beplained by the increased hydrolysis of Mn2+, increased like-lihood of Mn precipitation, and increased negative chargthe exchange complex.

Manganese is strongly adsorbed by clay minerals.sorption has been found to increase with increasing pH[166].In general, sorption of Mn onto soils can be facilitatedseveral mechanisms: first, the oxidation of Mn to highvalence oxides and/or precipitation of insoluble compouin soils subjected to wetting and drying, second, absorpinto the crystal lattice of clay minerals, and adsorption on

Fig. 16. Predicted Eh–pH-stability field for manganese; the assumedities of dissolved species are: Mn= 10−6, C = 10−3, S= 10−3 (redrawnafter[164]).


e-

CO

ormti-

tra-asC,d-ur-h a

esbe

pH

tting-

nex-

tox-in-ndsen-oils

por-esfs,us

e ofm.

andn

h ise re-

ithMn

di-o inc-

areaat

Fig. 17. Adsorption of Mn by soils from Ao (A) and B horizons (B) (rdrawn after[165]).

change sites. In calcareous soils, chemisorption onto Ca3and following precipitation of MnCO3 may play an impor-tant role. Presence of chelating agents is not able to fstable Mn complexes in soils because Fe or Ca can substute for Mn[167].

10.6. Zinc

Sorption is an important factor governing Zn concention in soils and is influenced by several factors, suchpH, clay mineral content, CEC, soil organic matter, CEand soil type. Clay minerals show variations in their asorbing capacity due to their different CEC, specific sface area, and basic structural makeup. 2:1 clays sucmontmorillonite and illite exhibit greater fixing capacitifor Zn than 1:1 clays such as kaolinite. This fact canexplained by entrapment of Zn2+ in the interlattice wedge

s

Fig. 18. Sorption of Co(II) onto Fe and Mn oxides as a function of(redrawn after[178]).

zones of the clay when the zones expanded due to weand contracted upon drying[168]. Clay-bound Zn was characterized as dominantly reversible in association with claysurface groups, while the rest exists in an irreversible nochangeable form associated with lattice entrapment[169].In calcareous and alkaline soils, Zn unavailability is duesorption of Zn by carbonates, precipitation of Zn hydroide or carbonates, or formation of insoluble calcium zcate[164]. The surface charge on hydrous oxides depehighly on pH and increases with increasing pH. Zn rettion is partly due to the presence of oxide surfaces in swhose clay fractions are dominated by layer silicates[170].Chelating agents, either natural or synthetic, play an imtant role in Zn mobility in soils. Zn also forms complexwith Cl−, PO−

4 , NO−3 , and SO2−

4 [171]. As the presence oEDTA in soil suspension can decrease Zn sorption by soilZn is believed to form strong complexes with EDTA thdecreasing its affinity for sorption sites[172]. In contrast,complex formation of Zn with Cl−, NO−

3 , and SO2−4 did not

have significant effects on Zn sorption. Thus, the presencsynthetic chelates maintains most of the Zn in mobile for

10.7. Cobalt

Co was found to accumulate in hydrous oxides of FeMn in soils [173,174]. It was also found that Co adsorptioby certain soils was increased by removal of Fe, whicbelieved to expose clay mineral surfaces that were moractive than previously exposed Fe oxide surfaces[175]. Cosorption capacity of soils was found to highly correlate wCo content and surface area and to a lesser extent withand clay contents and pH[176]. Almost all of the Co in soilscould be accounted for by that present in Mn minerals, incating that these minerals can be an important sink for Csoil [177]. Sorption of Co by Fe and Mn oxides as a funtion of pH is shown inFig. 18. Cryptomelane (K2Mn8O16)has a point of zero charge below 3 and a high surfaceof 200 m2/g. It sorbed significant amounts of Co even


as

pH-

ounith

ndoc-

ys-contalcurin.-

avyol-alsoetaldif-iricapos-, the

edrfacesucpeddeltion

iffusvel-ilib-

thetiontionupsgn ofsi-

ndcedt Cailsre-tysoilers

be-

beenydner-ndres-s iner-r orons.trol-nicand. Cuthralseas-

n

le isox-ate.es,

avy

oid

oid

80)

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99)

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5)

ta 58

.23

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Cos-

n-

relatively low pH. On the other side, goethite, which ha relatively small surface area of 90 m2/g and a point ofzero charge of 8.7, shows significant Co sorption only atvalues above 6.0[178]. Two forms of bound Co in montmorillonite have been identified[179]. The first form, which ischaracterized as being slowly dissociable, seems to be bin a monolayer by chemisorption and would exchange wZn2+, Cu2+, or other Co2+ ions but not with a Ca2+, Mg2+,or NH+

4 ions. The second form of Co is not dissociable ais believed to either enter the crystal lattice or becomecluded in the precipitates of another phase.

11. Summary

Soil is one of the key elements for all terrestric ecostems and is a very complex heterogeneous mediumsisting of soil matrix, soil water, and soil air. Heavy meions are the most toxic inorganic pollutants which ocin soils and can be of natural or of anthropogenic origAdsorption is a major process responsible for their accumulation. The most important interfaces involved in hemetal adsorption in soils are predominantly inorganic cloids such as clays, metal oxides and hydroxides, butorganic colloidal matter provides interfaces for heavy madsorption. For modelling heavy metal adsorption, twoferent approaches have been developed: first, the empmodel approach, where the model form is chosen ateriori form the observed adsorption data, and secondmechanistic model approach, where the mathematical formis chosen a priori by setting up equilibrium reactions linkby mass balances of the different components and sucharge effects. General purpose adsorption isothermsas the Langmuir or Freundlich isotherm have been develofor empirical models. As for the mechanistic models, moapproaches describing the double layer at the solid/soluinterface such as the constant capacitance model, the dlayer model, and the triple layer model have been deoped. The multisite complexation model considers equrium constants for the various types of surface groups onvarious crystal planes of oxide minerals. The main retenprocesses of metal ions at soil surfaces include adsorpsurface precipitation, and fixation. Surface functional groare vital for adsorption. The main parameters influencinheavy metal adsorption are soil pH, type and speciatiometal ion involved, heavy metal competition, soil compotion and aging.

The individual behavior of Cd, Cr, Pb, Cu, Mn, Zn, aCo in soils is described. Cd adsorption is strongly influenby the presence of competing cations such as divalenand Zn, which compete with Cd for sorption sites in soor are able to desorb Cd from soils. Adsorption and pcipitation behavior of Cr in soils is controlled by a varieof factors such as redox potential, oxidation state, pH,minerals, competing ions, complexing agents, and othwhich control most of the partitioning processes of Cr

d

-

l

h

e

,

,

tween the solid and the aqueous media. Fe oxides havefound to exhibit the strongest affinity for Cr(VI) followed bAl2O3, kaolinite, and montmorillonite. Cr(III) is rapidly anspecifically adsorbed by Fe and Mn oxides and clay mials. Adsorption of Cr(III) increases with increasing pH acontent of soil organic matter while it decreases in the pence of competing cations or dissolved organic ligandthe solution. Adsorption of Pb onto soils and clay minals has been found to conform to either the Langmuithe Freundlich isotherm over a wide range of concentratiCarbonate content in soils plays an important role in conling Pb behavior. Cu shows a strong affinity for soil orgamatter. The most important sinks for Cu in soils are FeMn oxides, soil organic matter, sulfides and carbonatesin soil solution exists primarily in a form complexed wisoluble organics. Mn is strongly adsorbed by clay mineand Mn adsorption has been found to increase with incring pH. In calcareous soils, chemisorption onto CaCO3 andfollowing precipitation of MnCO3 is an important retentiomechanism. Zn is readily adsorbed by clay minerals, whilein calcareous and alkaline soils, Zn is mostly unavailabdue to sorption by carbonates, precipitation of Zn hydride or carbonates, or formation of insoluble calcium zincCo was found to accumulate in hydrous Fe and Mn oxidwhich seem to be an important sink for Co in soil.

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