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Chemical stabilization of metals and arsenic in contaminated soils using oxides – A review

Review

Chemical stabilization of metals and arsenic in contaminated soils using oxides e A review

Michael Komárek a ,*,Ale s Van e

k b ,Vojt e ch Ettler c a

Department of Environmental Geosciences,Faculty of Environmental Sciences,Czech University of Life Sciences Prague,Kamycká129,Prague 6e Suchdol 16521,Czech Republic

b Department of Soil Science and Soil Protection,Faculty of Agrobiology,Food and Natural Resources,Czech University of Life Sciences Prague,Kamycká129,Prague 6e Suchdol 16521,Czech Republi

c c

Institute of Geochemistry,Mineralogy and Mineral Resources,Faculty of Science,Charles University in Prague,Albertov 6,Prague 2,12843,Czech Republic

a r t i c l e i n f o

Article history:

Received 16March 2012Received in revised form 20July 2012

Accepted 25July 2012Keywords:

Immobilization Remediation

Soil contamination Metal Arsenic

a b s t r a c t

Oxides and their precursors have been extensively studied,either singly or in combination with other amendments promoting sorption,for in situ stabilization of metals and As in contaminated soils.This remediation option aims at reducing the available fraction of metal(loid)s,notably in the root zone,and thus lowering the risks associated with their leaching,ecotoxicity,plant uptake and human exposure.This review summarizes literature data on mechanisms involved in the immobilization process and presents results from laboratory and ?eld experiments,including the subsequent in ?uence on higher plants and aided phytostabilization.Despite the partial successes in the ?eld,recent knowledge high-lights the importance of long-term and large-scale ?eld studies evaluating the stability of the oxide-based amendments in the treated soils and their ef ?ciency in the long-term.

1.Introduction

The widespread pollution of soils with metals and metalloids is a high environmental and toxicological concern.Metal(loid)s enter various environmental compartments (soils,ground and surface water,air,microbial,plant and animal communities)and can have adverse effects on individual biological recipients and populations.Therefore,cost-ef ?cient,safe and the least destructive remediation techniques suitable for such contaminated soils are wanted and their ef ?ciency in the long-term needs to be evaluated.Stabilization techniques,including chemical stabilization (the application of various stabilizing amendments,which by chemical means reduces contaminant mobility,bioavailability and bioaccessibility),phy-tostabilization (the use of higher plants and associated microor-ganisms for the immobilization of contaminants located in the root zone)and their combination (so-called aided phytostabilization)have shown to be possible less destructive alternatives to conven-tional remediation options,such as soil excavation and dumping,ex situ and in situ soil washing/?ushing,electrokinetics,vitri ?cation and asphalt capping,ground freezing etc.(Mench et al.,2003,

2006a ,b ,2009;2010;Adriano et al.,2004;Marques et al.,2009;

Vangronsveld et al.,1995,2009).Most stabilization techniques aim at rendering less available the metal(loid)fractions that can pose signi ?cant environmental and/or toxicological risks and protecting the functionality of the soil environment.The targeted metal(loid)fractions include the mobile,soluble,bioavailable (for biota)or bioaccessible (for humans),and refer to the portion of the contaminant that can be leached to surface and ground water or taken up by living organisms (Kumpiene et al.,2008).These frac-tions are either operationally de ?ned (e.g.,using various chemical extractions)or directly determined using various ecotoxicological bioassays and in vivo experiments.

Soil oxides (thereafter in the text including hydroxides,hydrous oxides,oxyhydroxides etc.)are ubiquitous natural soil components present virtually in all soil types.These weathering products occur in soils as discrete crystals,coatings on other particles and as mixed gels and although they are not usually present in large quantities (tens to thousands mg kg à1),they play a major role in soil chem-istry (Sparks,2003).Soil oxides are usually characterized by their small particle size (tens nm to thousands m m)and low solubility in the range of common pH values found in soils (Sposito,2008).Due to their important sorption properties,metal (especially Fe)oxides have been extensively studied as potential stabilization amend-ments in soils contaminated with metals and As.Their application,either direct or indirect through the application of their precursors

*Corresponding author.

E-mail addresses:komarek@fzp.czu.cz

(M.Komárek),vaneka@af.czu.cz

(A.Van e

k),ettler@natur.cuni.cz (V.

Ettler).Contents lists available at SciVerse ScienceDirect

Environmental Pollution

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Environmental Pollution 172(2013)9e 22

(e.g.,iron grit or Fe sulfates)is supposed to decrease mobile, bioavailable and bioaccessible fractions of the metal(loid)s and minimize thus possible risks of environmental contamination, leaching and uptake by soil organisms,plants,crops and humans.

This review aims at summarizing works published on the use of oxides as stabilizing amendments.The paper describes the mech-anisms and models of metal and As retention involved and the ef?ciency and suitability of various oxides for different elements. Despite the fact that these amendments are promising for the remediation of metal(loid)-contaminated soils,there are only a few reviews published in the last decade dealing with their use,where the authors discuss the potentials and drawbacks of various stabilizing amendments and phytostabilization in general (Puschenreiter et al.,2006;Mench et al.,2006b;Kumpiene et al., 2008;Vangronsveld et al.,2009).Metal oxides and their precur-sors have been widely tested and used as stabilizing amendments in laboratory/greenhouse and?eld conditions with various successes and this review focuses solely on these studies.

1.1.Metal(loid)sorption on oxides

The physico-chemical properties of oxide surfaces containing various reaction sites allow chemical reactions to occur,allowing thus sorption of various chemical inorganic and organic species from the aqueous solution.In general,adsorption mechanisms can be described as(i)speci?c(or chemical)adsorption e more selective and less reversible reactions,such as the formation of inner-sphere complexes and(ii)nonspeci?c(or physical)adsorp-tion e weak and less selective outer-sphere complexation.While ions forming outer-sphere complexes maintain their complete hydration sphere,ions in the inner-sphere lose part or their entire hydration sphere(Koretsky,2000;Bradl,2004;Appelo and Postma, 2005).As a rule,inner-sphere complexes are due to their coordinate-covalent bonding more stable than outer-sphere complexes with prevailing electrostatic bonding.The mechanisms of speci?c adsorption include surface complexation processes e reactions between metal ions in solutions and surface functional groups.These reactions take place at e OH groups on the mineral surfaces that are usually negatively charged(deprotonated)at high pH values and positively charged(protonated)at low pH values (McBride,1994).The surface charge of a metal(X)oxide is thus the sum of positively and negatively charged sites resulting from its amphoteric character(Dzombak and Morel,1990):

b XOHt2/b XOH0tHt(1)

h XOH0/h XOàtHt(2)

Additionally,the sorption of various cationic and anionic species in?uences the surface charge as well.The surface complexation reactions of a divalent metal(Me2t)and As on an uncharged oxide surface can be expressed by:

h XOH0tMe2t/h XOMettHt(3)

h XOH0tAsO3à4/h XOHAsO3à4(4)

The distance between the OH e OH groups in the Fe,Mn,Al oxides matches with the coordination polyhedra of many metal(loid)cations and anions,which are adsorbed on different surface sites,the former share edges and the latter double corners with Fe(O,OH)6octahedra(Manceau et al.,1992).

Speciation is an important factor in?uencing metal(loid)sorp-tion onto oxides.The presence of organic and inorganic ligands (e.g.,low molecular weight organic acids e LMWOA,humic and fulvic acids,phosphates,sulfates)in soils and their complexation with the metal(loid)s signi?cantly in?uences their adsorption onto oxides and thus the stabilization ef?ciency(Violante et al.,2003; Zaman et al.,2009;Zhu et al.,2011a,b).Dissolved organic matter (DOM,humic and fulvic acids)in soil solution signi?cantly in?u-ences the sorption behavior of metals onto the soil sorption complex by forming stable complexes(e.g.,with As,Cu,Pb), increasing thus their solubility,especially at alkaline pH(Martínez-Villegas and Martínez,2008).Organic ligands can either promote metal(loid)adsorption by forming stable surface-metal(loid)e ligand complexes or hinder it through the formation of soluble complexes,depending on the nature of the ligand,pH,surface properties of the oxide and ligand/metal(loid)ratio(Violante et al., 2003).For example,higher oxalate concentrations signi?cantly reduce Cu sorption onto Fe oxides at pH4.5e5.5;on the other hand, low oxalate concentrations and a lower pH value pH(3.5)inhibit the sorption(Zhu et al.,2011a).Arsenic sorption onto Fe oxides is also hindered in the presence of LMWOA(oxalate,citrate,malate, and tartarate)and inorganic ligands,especially phosphates(Zhu et al.,2011b;Kumpiene et al.,2012).

Several empirical(isotherms)and mechanistic(surface complexation models,SCM)models have been proposed for describing metal(loid)adsorption onto soils and oxides(Dzombak and Morel,1990;Goldberg,1992;Bradl,2004;Limousin et al., 2007;Barrow,2008).Although they do not provide information about the actual retention mechanism,empirical models are easy to use and usually suf?cient for evaluating the sorption capacities of the selected amendments and soils.There are no studies to date using SCM for predicting the suitability and ef?ciency of oxides as stabilizing amendments in contaminated soils and only few studies have used SCM for predicting metal sorption and behavior in contaminated soils with various success(e.g.,Dijkstra et al.,2004; Bonten et al.,2008).Nevertheless,coupled equilibrium and kinetic modeling together with transport and geochemical models would present a powerful tool for predicting metal(loid)transport in contaminated and remediated soils(Parkhurst and Appelo,1999; Jacques et al.,2008;Zhang and Selim,2008).

1.2.Metal(loid)co-precipitation with oxides

In addition to sorption reactions,the formation of newly formed secondary oxides and their coprecipitation with the target metal(loid)s(e.g.,FeAsO4$2H2O,MnPbO3,metal(loid)-ferrihydrite) is another important stabilization mechanism occurring in the treated soils(Martínez and McBride,1998;Tournassat et al.,2002; Davranche et al.,2003;Kim et al.,2003;Kumpiene et al.,2008).The coprecipitation of metal(loid)s with secondary metal oxides results into their lower solubility in soils when compared to other processes(e.g.,sorption reactions)(Martínez and McBride,1998).In the case of As,a number of studies have shown that the formation of Fe(II/III)-As minerals(e.g.,relatively insoluble FeAsO4$H2O, FeAsO4$2H2O,and Fe3(AsO4)2)in?uences As immobilization and (bio)availability in soils(Kim et al.,2003;Kumpiene et al.,2008; Drahota and Filippi,2009).

1.3.Methods of oxide characterization and quanti?cation

There are a number of methods applicable to oxide character-ization and quanti?cation(see,e.g.,compilations in Cornell and Schwertmann,1996;Sparks,2003;Pansu and Gautheyrou,2006). In the context of soil remediation,these methods should provide necessary information about the stability of the phases in soils and their possible alteration.The most commonly used is the X-ray diffraction analysis(XRD),which is able to detect phases based on their crystallinity,with detection limit of approx.2%in a mixed

M.Komárek et al./Environmental Pollution172(2013)9e22 10

sample.This method is routinely used for the basic characterization of Fe,Mn and Al oxides(e.g.,O’Reilly and Hochella,2003;Filip et al., 2007),but it is always supplemented with other methods to investigate in more detail the redox state of major elements(Fe, Mn)or binding mechanism of metal(loid)s.Numerous microbeam techniques can be used to visualize the oxide particles,including those from contaminated soils.These samples often require a separation of the heavy mineral fraction where the Fe and Mn oxides are concentrated.Scanning Electron Microscopy(SEM)is used to visualize objects larger than1m m(?eld emission SEM,FE-SEM,has a higher resolution)and allows obtaining images depicting the topography of the sample(Secondary Electrons,SE) or to assess the differences in chemical compositions(Back-Scat-tered Electrons,BSE)(e.g.,O’Reilly and Hochella,2003;Van e k et al., 2008).The semiquantitative chemical composition of the objects can be obtained by coupling SEM with Energy Dispersion Spec-trometry(EDS).For the qualitative composition of oxides,Electron Probe Microanalysis(EPMA)equipped with Wavelength Dispersion Spectrometers(WDS)can be used,but generally requires a polished specimen or a thin section and only limited applications of this method for soil systems are available in the literature.Because Fe, Mn and Al oxides commonly occur as nano-sized materials, Transmission Electron Microscopy(TEM)is commonly used for their visualization(Scheinost et al.,2001;Quantin et al.,2002;Filip et al.,2007).This method can further provide crystallographic information using Selected Area Electron Diffraction(SAED)and also chemical data when coupled to EDS(Lee,2010).Using Electron Energy Loss Spectroscopy(EELS)technique coupled to TEM,the redox state of the elements(e.g.,Fe)can also be measured in the specimen(Lee,2010).High-resolution TEM(HRTEM)is often used in oxide characterization and is able to provide atomic-scale imaging(point-to-point resolution is<0.1nm)(Filip et al.,2007; Lee,2010).Another key method used for characterization of pure and pedogenic Fe oxides is57Fe M?ssbauer Spectroscopy(Pansu and Gautheyrou,2006;Filip et al.,2007).The X-ray Photoelectron Spectroscopy(XPS),also called Electron Spectroscopy for Chemical Analysis(ESCA),has been adopted for studies of chemical compo-sitions of the surface,including the surface binding of metal(loid)s on oxides,although its applications to oxides are relatively scarce in the literature(Ding et al.,2000).In contrast,synchrotron-based methods such as X-ray absorption spectroscopy(XAS,represented by EXAFS[Extended X-ray Absorption Fine Structure]and XANES [X-ray Absorption Near Edge Structure])have been often used in the last decade for understanding the chemical and local structure states of a particular element in solids,but also in a solution in contact with a solid,colloidal precipitates or sorbates on surfaces (Wogelius and Vaughan,2000;Sparks,2003).The potential of these techniques in characterization of oxides and elements bound to their surfaces have been successfully demonstrated in a number of publications(e.g.,Scheinost et al.,2001;O’Reilly and Hochella, 2003).Kumpiene et al.(2011,2012)used the XANES and EXAFS techniques to identify phases(e.g.,poorly crystalline vs.crystalline Fe oxides)responsible for metal(loid)s retention in contaminated soils treated with Fe oxides.There are also several techniques having a supporting character to those above-mentioned,which are sensitive to the homogeneity of an analyzed sample and are used to a lesser extent,e.g.,DRIFT(Diffuse Re?ectance Infrared Fourier Transform Spectroscopy),Raman Spectroscopy,VMP(Vol-tammetry of Microparticles),TGA(Thermogravimetric Analysis), etc.(for references see Eggleton and Fitzpatrick,1988;Potts et al., 1995;Wogelius and Vaughan,2000;Grygar and van Oorschot, 2002;Sparks,2003;Pansu and Gautheyrou,2006;Müller et al., 2010).

For decades,selective chemical extractions have been used for targeting Fe,Mn and Al oxides in soils,although their selectivity for a given phases is often questionable(Gleyzes et al.,2002;Pansu and Gautheyrou,2006;Bacon and Davidson,2008).Single extractions based on the application of reducing agents at different pH and L/S (liquid/solid)ratios(e.g.,oxalate,dithionite e citrate e bicarbonate, and hydroxylamine hydrochloride)are routinely applied in soil sciences followed by the analysis of Fe,Mn and Al in the obtained extracts(e.g.,Kostka and Luther,1994;McCarty et al.,1998; Neaman et al.,2004;Pansu and Gautheyrou,2006).The“reducing”step targeting Fe and Mn oxides is generally included in sequential extraction procedures(Gleyzes et al.,2002;Bacon and Davidson, 2008)with numerous modi?cations and improvements to distin-guish between Fe and Mn oxides(e.g.,Leleyter and Probst,1999). Various sequential extraction schemes have been also used for evaluating changes in metal(loid)fractionation in contaminated soils treated with oxides and their precursors(e.g.,Lombi et al., 2002a;Friesl et al.,2003;Hartley and Lepp,2008a;Kumpiene et al.,2011,2012;Van e k et al.,2011).As expected,the results usually revealed decreases of the most labile metal(loid)fractions (i.e.,exchangeable)and increases of the reducible(i.e.,oxide-bound)fraction.

2.Iron oxides

2.1.Occurrence and properties of iron oxides

Major types of Fe oxides relevant for soil chemical stabilization techniques including their basic properties are given in Table1. Goethite(a-FeOOH)is the most common stable Fe oxide in soils;it exists and is stable in almost all soil types with higher abundances in cool and wet climates.Hematite(a-Fe2O3)and goethite’s poly-morph,lepidocrocite(g-FeOOH),are usually found in association with goethite.Hematite is formed mainly in warm climatic regions (e.g.,tropics and subtropics).Substantial Al e Fe substitution is possible in both minerals,which alters their solubility(Sposito, 2008).Ferrihydrite(5Fe2O3$9H2O or Fe5HO8$4H2O,in geochem-ical literature often written in a simpli?ed way as Fe(OH)3)is due to its high speci?c surface an important component of the soil sorp-tion complex(Cornell and Schwertmann,1996;Sauvéet al.,2000; Sparks,2003;Essington,2004).It is often found in association with goethite and can be formed in soils when other organic ligands (e.g.,humic substances)or soluble silica are present and the crys-tallization of goethite or hematite is inhibited(Sposito,2008).

The presence of LMWOA in soils(e.g.,as root exudates or released by microorganisms)can inhibit the crystallization of Fe oxides.The inhibition effect depends on their concentration and pH of the soil solution,i.e.,increasing LMWOA concentrations and decreasing pH generally decrease the crystallization process.The role of LMWOA during ferrihydrite formation depends on whether and how strongly the acid adsorbs on the oxide surface and how strongly it complexes with Fe3tin solution.Similar results were found during goethite precipitation(Cornell and Schwertmann, 1979).Another important factor suppressing Fe oxide precipita-tion is the presence of inorganic compounds(e.g.,sulfates)and soluble humic compounds competing for the Fe ion(Kodama and Schnitzer,1977).

2.2.Iron oxides as stabilizing amendments

For the purpose of chemical stabilization,increasing the concentration of Fe oxides in soils can be practically achieved through the application of their precursors,i.e.,Fe sulfates, elemental Fe(0)(hereafter Fe(0)refers also to other similar compounds,such as iron grit containing mainly Fe(0)and some impurities)etc.For example,the application of commercial grade FeSO4at1.89%(w/w)creates0.54%of Fe oxides in treated soils

M.Komárek et al./Environmental Pollution172(2013)9e2211

(Warren and Alloway,2003).Although the application of ferrous sulfates is ef ?cient for As immobilization in contaminated soils and sediments (Moore et al.,2000;Warren et al.,2003),their applica-tion results into acidi ?cation of soils (Eq.(5))and lime has to be applied to control soil pH,because the resulting acidi ?cation could easily remobilize metallic cations,such as Cd,Cu,Mn,Zn and create unfavorable conditions for plant growth (Moore et al.,2000;Warren and Alloway,2003;Hartley et al.,2004).

4FeSO 4tO 2t6H 2O /4FeOOH t4SO 2à4t8H

(5)

Natural CaCO 3is able,to some extent,buffer the pH changes;however,when the addition of FeSO 4exceeded the buffering level limited by the soil carbonate content,soil pH decreased signi ?-cantly (Moore et al.,2000).

The use of Fe(0)as a precursor of Fe oxides in soils has proved to be a potentially effective and cheap amendment for decreasing the mobility and potential bioavailability of various metal(loid)s in contaminated soils (Kumpiene et al.,2006).When Fe(0)is added to the soil,it oxidizes to form amorphous/poorly crystalline Fe oxides according to the following equations:

Fe à0át2H 2O tO 2/Fe àII á

t4OH à(6)Fe àII át2H 2O tO 2/Fe àIII á

t4OH à(7)Fe àIII á

t2H 2O /FeOOH t3H t

(8)

Amorphous and poorly crystalline Fe oxides (e.g.,ferrihydrite)are ?rstly precipitated during Fe(0)oxidation,but it is needed to

point out that the used iron grit is not a pure substance and small quantities of other oxides (e.g.,Mn)are formed during the process (Mench et al.,2006b ).Although the oxidation reactions of Fe(0)(e.g.,from iron grit)are not as fast as those when Fe is supplied as sulfate salts,the former contains more Fe in terms of mass and seems to be more suitable in the long-term perspective (Kumpiene et al.,2008).Additionally,the oxidation of Fe(0)is not associated with soil pH and mobilization of cationic metals in the contami-nated soils as in the case when Fe sulfates are applied (see Eqs.(5)e (8)for comparison)(Kumpiene et al.,2006).

When no inhibitors are present,the crystallization increases with time and the ?nal type of solid phase is dependent on temperature and pH.Elevated temperatures generally favor the preferential formation of hematite.Low and high pH values (2e 5and 10e 14)favor the formation of goethite,while the crystalliza-tion of a -Fe 2O 3is typical for neutral pH values.Both a -FeOOH and a -Fe 2O 3are formed either through a dissolution/crystallization process or in the solid state,through topotactic transformations combined with dehydration (Eq.(9))(Cudennec and Lecerf,2006).The ?rst mechanism is probably dominant for dynamic soil systems,as the dissolution/crystallization reactions of Fe oxides are controlled by many factors, e.g.,water saturation,organic and inorganic ligands (including humic substances),pH,Eh etc.

5Fe 2O 3$9H 2O /5a -Fe 2O 3t9H 2O

(9)

The crystallinity of the oxides plays a key role in the retention of metal(loid)s and the aging of Fe oxides in the treated soils,i.e.,their transformation from amorphous to more crystalline phases,together with the presence of root exudates (LMWOA,such as

Table 1

Oxides relevant for chemical stabilization of contaminated soils and their basic properties.Oxide Formula

Speci ?c surface (m 2g à1)pH zpc

Solubility product log K sp References

Iron oxides Ferrihydrite

Fe 5HO 8$4H 2O

(simpli ?ed as Fe(OH)3)

100e 700

7.8e 8.8

à37to à39a

Cornell and Schwertmann (1996)

Langmuir (1997)Essington (2004)

Goethite a -FeOOH 8e 2007.5e 9.4

à44a

Cornell and Schwertmann (1996)

Langmuir (1997)Sposito (2008)

Lepidocrocite g -FeOOH 15e 260 6.7e 7.5à38.7to à40.6a Cornell and Schwertmann (1996)Langmuir (1997)

Hematite

a -Fe 2O 3

2e 115

7.5e 9.5

à43.9?0.2a

Cornell and Schwertmann (1996)

Langmuir (1997)Sposito (2008)

Manganese oxides Birnessite (Na,Ca,K)x Mn 2O 4$1.5H 2O (simpli ?ed as d -MnO 2)35.4b 1.8e 2.2à85.5463c Tripathy et al.(2001)

O ’Reilly and Hochella (2003)Manganite g -MnOOH 8.9 5.4à0.1646c Weaver et al.(2002)

Pyrolusite

b -MnO 2

0.15

7.2

à17.6439c

Chung and Zasoski (2002)O ’Reilly and Hochella (2003)

Aluminum oxides Gibbsite g -Al(OH)31209.87.7560c Sverjensky and Sahai (1996)Langmuir (1997)Boehmite

g -AlOOH 224

8.6

7.5642c

Langmuir (1997)

Granados-Correa and Jiménez-Becerril (2009)

Diaspore

a -AlOOH

11 2.0e 7.5

7.1603c

Van Schuylenborgh and S?nger (1949)Smith and Yanina (2002)

Considering the dissolution reaction written as,e.g.,for goethite:FeOOH tH 2O ?Fe 3tt3OH à.

Higher speci ?c surface values were obtained by EGME method (ethylene glycol monoethyl ether):birnessite 375.8m 2g à1(O ’Reilly and Hochella,2003).c

Solubility products from the llnl.dat thermodynamic database available in the PHREEQC-2speciation-solubility code (Parkhurst and Appelo,1999).The dissolution reactions are written as follows:

Birnessite:Mn 8O 14$5H 2O t4H t?3MnO 42à

t5Mn

2tt7H 2O.Manganite:MnOOH t3H t?Mn 3t

t2H 2O.

Pyrolusite:MnO 2?0:5Mn 2tt0:5MnO 2à

Gibbsite:Al(OH)3t3H t?Al 3t

t3H 2O.

Boehmite and diaspore:AlOOH t3H t?Al 3tt2H 2O.

M.Komárek et al./Environmental Pollution 172(2013)9e 22

oxalic acid)promotes As desorption in contaminated soils (Kumpiene et al.,2012).On the other hand,the aging of ferrihydrite and its transformation to goethite results in lower extractability of coprecipitated Cd,Cu and Pb(Ford et al.,1997;Martínez and McBride,1998).The nature of transformation products depends on several factors,including pH,Eh,temperature and the copreci-pitated metal(loid)s(Martínez and McBride,1998).All these factors need to be taken into account and more studies dealing with these geochemical/mineralogical transformations of the newly formed oxides in the treated soils are needed for evaluating the long-term ef?ciency of the stabilization process.

Taking into account the solubility products(K sp)of main Fe(III) oxides reaching10à39for ferrihydrite,10à41for goethite and10à43 for hematite(Table1)(which are very low)it can be concluded that more crystalline phases tend to be more stable at circumneutral pH values and the potential rate of mobilized Fe in soil decreases with increasing oxides crystallinity.The thermodynamic stability of Fe oxides is a function of crystal structure and particle size,which ultimately controls the solubility of the phases(Hansel et al.,2004). The substitution of Fe by Al in the oxides structures is another factor in?uencing their solubility,i.e.,Al substitution leads to increased stability of hematite and especially goethite(Schwertmann,1991). Basically,there are three main mechanisms involved in the disso-lution of oxides:(i)protonation(i.e.,acid dissolution),(ii)reduction and(iii)complexation by various ligands(e.g.,oxalates,citrates, synthetic chelating agents,etc.)(Nowack and Sigg,1997;Martin, 2005;Komárek et al.,2009).These processes can occur after surface complexation of protons,electrons or the ligands takes place on the oxide surface and are thus closely related to the pH zpc and surface area of the oxide(Schwertmann,1991).In the case of acid leaching,e.g.,in the presence of acidic soil solutions and complex-ationwith natural chelating agents,oxides with high reactive surface areas(e.g.,ferrihydrite)will be potentially preferentially altered.

Due to the high af?nity of As for Fe oxides,these amendments have been extensively studied for the remediation of As-contaminated soils(see Table2for references).The predominant and thermodynamically stable forms of As in soils and waters are H2AsOà4,HAsO2à4and H3AsO03,depending on pH/Eh conditions,with H2AsO4àbeing the most abundant species in aerobic soils(Warren and Alloway,2003).Nevertheless,due to the relatively slow redox reactions,both redox states can be present in the soil environment (Masscheleyn et al.,1991).The As(III)species is signi?cantly more toxic and more mobile in soils than As(V)(Manning et al.,2002a).In natural soil environments,Fe oxides are important scavengers of As species reducing their mobility,bioavailability and bioaccessibility (e.g.,Livesay and Huang,1981;Jain et al.,1999;Nickson et al.,2000; Stüben et al.,2003;Mench et al.,2006a).Arsenic speciation plays a crucial role in its retention,while As(III)sorption onto Fe oxides increases with increasing pH(maximum at pH8e10),As(V) adsorption increases with decreasing pH(maximum at pH3e5) (Raven et al.,1998;Masue et al.,2007).The oxidation e reduction reactions taking place on the oxide surfaces in?uence As specia-tion in the soils.Most authors agree that As(V)is not reduced by metallic Fe surfaces or in the presence of dissolved Fe in shorter time periods(e.g.,days)(Leupin and Hug,2005).On the other hand, As(III)oxidation associated with reductive dissolution of the oxides can occur in the presence of the oxides under oxidizing conditions (e.g.,De Vitre et al.,1991;Manning et al.,2002a,b).

Based on spectroscopic,kinetic and titration measurements, As(V)predominantly adsorbs to Fe oxides(i.e.,goethite,ferrihy-drite,lepidocrocite,and hematite)as inner-sphere surface complexes through ligand exchange with e OH groups at the mineral surfaces resulting from bidentate corner-sharing of AsO4 and FeO6polyhedra(Foster et al.,2003;Sherman and Randall, 2003;for more information on As speciation and surface structure see Wang and Mulligan,2008).On the other hand,there has been evidence of a monodentate bonding mechanism espe-cially at a pH value greater than8(Jain et al.,1999);however,such pH values are not typical for common soils.Arsenite adsorption onto Fe oxide surfaces occurs basically as Fe e O e As with the O atom being partly protonated at low pH values(Jain et al.,1999).The “chemical purity”of the Fe oxide plays an important role as well;

a high Al/Fe ratio in binary oxide mixtures naturally formed in soils or as a result of“impure”amendments(e.g.,red mud,see below) reduces As(especially As(III))sorption(Masue et al.,2007).

Besides As,several studies investigated soils contaminated with metals as well;however,with varying success(Table2).For example, the authors do not agree whether such stabilizing amendments are suitable for soils with high Cu concentrations(McBride and Martínez,2000;Hartley et al.,2004;Kumpiene et al.,2006;Bes and Mench,2008),mainly due to the possible mobilization of Cu (and Pb)by complexation with dissolved organic matter when the pH of the soil is altered during the stabilization(McBride and Martínez,2000;Ruttens et al.,2006b).On the other hand, Kumpiene et al.(2011)reported a Cu redistribution from the exchangeable to the reducible soil fraction(i.e.,bound to oxides)after the application of Fe(0)as a result of the formation of binuclear inner-sphere complexes on the Fe oxide surfaces(data from EXAFS).The higher ef?ciency of Fe(0)as a stabilizing amendment for As (compared to other metals)can be also illustrated by bioaccessibility tests(e.g.,evaluated using the Physiologically Based Extraction Test, PBET).The application of Fe(0)reduces As bioaccessibility to a greater extent than in the case of Cd,for example(Mench et al.,2006a).

The vast majority of authors using Fe(0)as the stabilizing agent use it at concentrations of1e2wt.%(Table2)with1%being usually suf?cient for metal stabilization(Hanauer et al.,2011).Hartley et al. (2004)tested various Fe-based amendments for As immobilization in contaminated soils and summarized their ef?ciency as Fe(III) sulfate>Fe(II)sulfate>Fe(0)>goethite.The stabilization ef?-ciency was assessed using long-term column experiments.The lower ef?ciency of Fe(0)(iron grit)can be explained by the fact that it was only left to oxidize on the soil surface.Nevertheless,despite the highest ef?ciency of the Fe sulfates to form amorphous Fe oxides,their addition to soils must be combined with lime appli-cation for avoiding unwanted pH changes.

Red mud(an alkaline industrial byproduct from bauxite re?ning)represents another Fe/Al-oxide rich material tested for metal(loid)stabilization in soils(Lombi et al.,2002a,b;2003;Friesl et al.,2003,2004,2006;Brown et al.,2005;Gray et al.,2006).It typically contains25e40%of Fe oxides and15e20%Al oxides(Gray et al.,2006).The application of this stabilizing material results not only into a redistribution of the metal fractions in soils,i.e.,from the exchangeable to the reducible(oxide-bound)fraction,but a signif-icant pH increase occurs as well,promoting thus the sorption of metallic cations(Lombi et al.,2002a;Friesl et al.,2003;Gray et al., 2006).However,signi?cant pH increases can mobilize anionic metal(loid)species(As,Cr)and lead to dissolution of soil organic matter(Friesl et al.,2003,2006).Because the?xation usually occurs through speci?c chemisorption in the Fe and Al oxide lattice,metals are less prone to remobilization if soil pH decreases(Lombi et al., 2002a).Furthermore,due to the immobilization of metals and pH increase,the application of red mud is bene?cial for plant growth and microbial populations(Lombi et al.,2002b).

3.Manganese oxides

3.1.Occurrence and properties of manganese oxides

Manganese oxides are another important group of secondary minerals commonly found in soils representing important

M.Komárek et al./Environmental Pollution172(2013)9e2213

Table2

Examples of studies dealing with metal(loid)stabilization in contaminated soils using various oxides and their precursors.

Metal(loid)

concentrations

(mg kgà1)

Treatment(wt.%)Effect Reference

Cd(18) Pb(1112) Zn(1434)-Fe(0)(1%)

-hydrous Mn oxide(1%)

-decreased water-soluble Cd,Pb and Zn

-decreased Cd uptake by plants(ryegrass,tobacco,bean)

-decreased Cd,Pb and Zn uptake by ryegrass

-no signi?cant changes in biomass production

Mench et al.(1994a,b)

Sappin-Didier et al.(1997)

Pb(250)-hydrous Mn oxide(1%)-reduced water-soluble and acetic acid(0.43M)

extractable Pb

-decreased Pb uptake by ryegrass

-increased biomass yields of ryegrass

Mench et al.(1997)

Cd(7)-steel sludge(FetFeOtFe2O3

at3.5e10.5%)tCaO,SiO2,MgO -decreased Cd uptake by wetland rice,Chinese cabbage

and wheat

-positive in?uence on biomass production of Chinese cabbage

Chen et al.(2000)

Cd(26e48)

Pb(1521e3291) Zn(4463e8649)-synthetic cryptomelane(0.25e0.5%)-reduction of Pb bioavailability(PBET)

-reduced soluble Pb(TCLP)

-reduced phytoavailable Pb(Swiss chard)

-lower in?uence on Cd and Zn

Hettiarachchi et al.(2000)

Hettiarachchi and Pierzynski

(2002)

Cu(3430)-birnessite(10%)

-ferrihydrite(10%)

-Al(OH)3(5%)-birnessite lowered free Cu2tactivity in soil solution,

but phytotoxicity persisted due to elevated total soluble

Cu concentrations as a result of increased DOC

-birnessite slightly improved corn growth

-ferrihydrite and Al(OH)3reduced soluble Cu

McBride and Martínez(2000)

As(683e4814)-FeSO4tlime(Fe:As molar

ratio0e50)-signi?cantly decreased water soluble As

(e.g.,at Fe:As?2from3790to0.79m g Là1)

-mobilization of Cu and Zn in the non-limed variant as a

result of pH decreases

Moore et al.(2000)

As(1215e1327)-synthetic FeOOH(1e5%)

-limonite(1e10%)

-synthetic Al(OH)3(1e5%)-reduced water-soluble As(55e100%)

-higher ef?ciencies for the synthetic phases

García-Sanchez et al.(2002)

Cd(19e42)

Cu(78e1245) Pb(230e842) Zn(1756e2920)-red mud(2%)-increased soil pH

-decreased metal?uxes from the soil solid phase to solution

-redistribution of metals from the exchangeable to the

reducible(oxide-bound)fraction

-reduced phytotoxicity of metals,increased biomass yields and

decreased metal concentrations

in plants(oilseed rape,pea,wheat,lettuce)

-increased soil microbial biomass

Lombi et al.(2002a,b)

As(1325) Pb(170)-Fe(0)(1%)tcompost(5%)t

beringite(5%)

-bene?cial for pine growth in the long-term(3years)

-decreased As in aboveground biomass

-decreased As leaching

-successful revegetation of the treated soils

Bleeker et al.(2002)

Mench et al.(2003)

Cd(7e120)

Zn(700e2713)-red mud(1%)-reduced Cd and Zn extractability(1M NH4NO3)by70%and

89%,respectively

-Cd and Zn plant uptake reduced by38e87%and50e81%,

respectively

-redistribution of metals from the exchangeable to the

reducible(oxide-bound)fraction

Friesl et al.(2003)

As(577)-FeSO4(0.2e1.1%)tlime-reduction of As concentrations in lettuce(e.g.,at1.1%

FeSO4from13.8to1.45m g kgà1)

-no signi?cant changes in biomass production

Warren and Alloway(2003)

As(748)-FeSO4tlime(0.2e2.0%of Fe oxides)

-Fe(0)(0.2%of Fe oxides)-FeSO4application(at0.5%and1.0%of Fe oxides)reduced

As uptake by several crops(by32%in average)

-Fe(0)application did not decrease As concentrations in

plants

Warren et al.(2003)

As(60e78) Cd(1e36)

Cu(69e118) Pb(127e360) Zn(33e508)-FeSO4(1%)tlime

-Fe2(SO4)3(1%)tlime

-goethite(1%)

-Fe(0)(1%)

-all treatments reduced As concentrations in leachates

-increased concentrations of Cd,Cu,Pb,Zn in leachates

after the addition of Fe sulfates

-Fe(III)sulfate>Fe(II)sulfate>Fe(0)>goethite

-long-term stability

Hartley et al.(2004)

As(115e14,200)-Fe(0)(1%)-large-scale project

-As concentrations decreased in soil solution by39e95%https://www.wendangku.net/doc/3d12630575.html, (2006)

As(30e68)

Cd(5e34)

Cu(58e95)

Pb(913e8306) Zn(500e2039)-ferrihydrite(2%)

-goethite(2%)

-red mud(0.25%,0.5%)tgravel

sludge(2%,2.5%)

-ferrihydrite amendment reduced Cd and Pb uptake by barley

-immobilization of most metals(Cd,Cu,Pb,Zn)

-red mud increased soil pH

-risks of As mobilization due to elevated pH after the

application of red mud

Friesl et al.(2006)

Cd(79) Cu(311) Pb(4210) Zn(3970)-red mud(3%,5%)-increased soil pH

-decreased metal concentrations in soil pore water

-promoted growth of Festuca rubra(metal tolerant grass)

-decreased metal concentrations in F.rubra

Gray et al.(2006)

As(5904) Cr(3829) Cu(1509)-Fe(0)(1%)-As and Cr concentration decreased in leachates(by98%

and45%,respectively),in soil pore water(by99%and

94%,respectively),in plant shoots(by84%and95%,

respectively)

-no positive in?uence on Cu

Kumpiene et al.(2006) M.Komárek et al./Environmental Pollution172(2013)9e22

scavengers of metal(loid)s(Manceau et al.,2002;Table1).They often occur as?ne-grained coatings of other soil particles and as nodules(Post,1999;Essington,2004).Most Mn oxides are amor-phous(Sparks,2003).The most common Mn oxide is birnessite ([Na,Ca,K]x Mn2O4$1.5H2O,often simpli?ed and denoted as d-MnO2 in geochemical literature),which precipitates in soils mainly as a result of Mn2toxidation promoted by bacteria and fungi,and similarly to bacteriogenic ferrihydrite,it forms highly reactive bio?lms(Sposito,2008;Sparks,2003;O’Reilly and Hochella,2003; Villalobos et al.,2006).Manganese oxides are much more ef?cient in adsorbing some metals(especially Pb)compared to Fe oxides(Dong et al.,2000),especially due to their large speci?c surface areas and low pH ZPC(1.8e4.5;Stumm and Morgan,1996;Feng et al.,2007), which in common soil environments results into their negative surface charge.The sorption properties of selected synthesized Mn oxides were investigated by Feng et al.(2007)and birnessite was

Table2(continued)

Metal(loid)

concentrations

(mg kgà1)

Treatment(wt.%)Effect Reference

As(169)-Fe(0)(1%)tberingite(5%)-6-yr experiment

-decreased exchangeable As concentrations

-promoted growth of lettuce,cabbage and dwarf bean

-reduced As bioaccessibility(PBET)and bioavailability for

earthworms

-increase in microbial biomass,but no increase in

microbial species richness Mench et al.(2006a) Ascher et al.(2009)

Cu(2600)-Fe(0)(2%)torganic matter

(compost,activated carbon)-increased soil pH

-decreased Cu concentrations in soil solution

-improved growth of dwarf bean when Fe(0)was applied

with compost

-the lowest Cu concentration in leaves of dwarf bean

when Fe(0)was applied with compost

Bes and Mench(2008)

As(60e78) Cu(33e508)-FeSO4(1%)tlime

-Fe2(SO4)3(1%)tlime

-goethite(1%)

-Fe(0)(1%)

-decreased pH after sulfate addition

-no signi?cant changes in Cu fractionation

-reduced exchangeable As fraction,switch to less labile

fractions(reducible,residual)

-goethite application promoted the growth of ryegrass,

tomato and spinach;however,higher biomass yields

were only observed on ryegrass

-reduced As uptake by ryegrass,tomato and spinach after

Fe treatments

Hartley and Lepp(2008a,b)

Cr(42) Cu(630)-basic slag containing

Fe2O3,Al203(1e4%)

-increased soil pH and conductivity

-improved growth of bean

-decreased Al,Cr,Cu uptake by bean

Negim et al.(2010,2012)

Cd(1e3)

Cu(309e1027) Zn(314e819)-Fe(0)(1%)-decreased NH4NO3-exchangeable Cd and Cu(Cu>Cd>Zn)

-no signi?cant effect on metal uptake by spinach under?eld

conditions

-no adverse effect on soil pH

Hanauer et al.(2011)

Cu(2080)-Fe(0)(2%)-reduced exchangeable Cu,switch to less labile fractions

(especially reducible)

-positive in?uence on the growth of Agrostis castellana

Boiss.&Reut.

-decreased shoot Cu concentrations in A.castellana

Kumpiene et al.(2011)

Cd(4) Pb(214) Zn(260)-Fe(0)(2%)-reduced TCLP-and Ca(NO3)2-extractable Cd and Zn

-reduced bioaccessible(PBET)Pb and Zn

-reduced exchangeable Cd,Pb and Zn fractions

-similar biomass yields of lettuce compared to the control

-reduced Cd,Pb and Zn uptake by lettuce

-no effect on Cd,Pb and Zn concentrations in earthworms

-increased dehydrogenase activity in soils

Lee et al.(2011)

As(1033) Cr(371)-water treatment residue

containing mainly ferrihydrite

(2.5%,5%)

-reduced As and Cr leaching by98%and91%,respectively

-decreased As and Cr concentrations in pore

waters in a3-yr?eld experiment

Nielsen et al.(2011)

Tl(5)-birnessite(0.5%)-reduced easily mobilizable Tl,switch to less labile

fractions(reducible)

-decreased Tl uptake by white mustard

-positive in?uence on biomass production of white

mustard grown on a sandy soil

Van e k et al.(2011)

As(179) Cd(6)

Pb(3564) Zn(3127)-not precisely de?ned Fe oxides

(1%,3%)

-signi?cant decrease of As in pore water;not for Cd,Cu,

Pb and Zn

-no positive in?uence on lettuce seeds germination and

root growth

González et al.(2012)

As(145)-Fe(OH)3(5%)

-mine sludge containing goethite(5%)-a30%and50%reduction of As leaching when using mine

sludge and Fe(OH)3,respectively

Ko et al.(2012)

As(1457)-Fe(0)(2%)tcompost(5%)t

coal?y ash(5%)

-10-yr experiment

-decreased total As concentrations through leaching in the

long-term

-decreased exchangeable As fraction and fraction

associated with poorly crystalline Fe oxides,increased

residual fraction

Kumpiene et al.(2012)

M.Komárek et al./Environmental Pollution172(2013)9e2215

identi ?ed as the most ef ?cient adsorbent for Pb,Co,Cu,Cd,Zn (other minerals included todorokite [(Na,Ca,K)2Mn 6O 12$3e 4.5H 2O],cryp-tomelane [K 2-x Mn 8O 16or generally a -MnO 2]and hausmannite [Mn 3O 4]).Furthermore,Pb adsorbs more ef ?ciently onto Mn oxides compared to Fe ones (approx.by a factor of 40;McKenzie,1980).Nevertheless,due to the fact that Fe oxides are naturally more abundant in soils compared to Mn ones,the former present a more important sink for metals in soils.

3.2.Manganese oxides as stabilizing amendments

Besides their important sorption characteristics,Mn oxides signi ?cantly alter the speciation of redox-sensitive elements (e.g.,As,Co,Cr)and thus in ?uence their dissolved concentrations in water and soil solution (Manning et al.,2002a ).While the oxidation of As(III)reduces its mobility and toxicity,the oxidation of Cr(III)leads to a formation of more toxic and mobile Cr(VI)species (Kumpiene et al.,2008).In general,Mn oxides can strongly oxidize Cr(III)at lower pH values when no Cr(III)precipitation is expected (Feng et al.,2006)and As(III)and Cr(III)are thus able to readily dissolve Mn oxides through these oxida-tion/reduction processes (Tournassat et al.,2002).These processes can be described as As(III)/Cr(III)oxidation coupled with reductive dissolution of the Mn oxide at low pH (Manning et al.,2002a ;Feng et al.,2006):

MnO 2tH 3AsO 3t2H t/Mn 2ttH 3AsO 4tH 2O

(10)3MnO 2t2Cr eOH Tt2t2H t/3Mn

t2HCrO à4t2H 2O (11)

First,the As(III)species can form inner-sphere complexes with

the birnessite surface and the formed As(V)can be released with the reduced Mn(II)into the solution.A similar pattern can be ex-pected during Cr(III)oxidation to Cr(VI)and the extent of oxidation varies for different Mn oxides;the oxides which are being easily reduced exhibit stronger oxidative properties (Tan et al.,2005;Feng et al.,2007).This process possibly alters the oxide surface,creating new reaction sites available for As(V)adsorption on the reactive hydroxyl groups of the Mn oxide surface (Mn-OH)(Manning et al.,2002a ):

2Mn-OH tH 3AsO 4/eMnO T2AsOOH t2H 2O

(12)

Based on EXAFS results,mechanisms of As(V)sorption onto birnessite can be described as either monodentate mononuclear (Mn(OH)5(HAsO 4)3à)or bidentate binuclear (Mn 2(OH)8(AsO 4)3à)surface complexation (Manning et al.,2002a ).Nevertheless,As(V)can co-precipitate with hydrous Mn oxides and/or dissolved Mn 2t(Masscheleyn et al.,1991;Moore et al.,2000;Tournassat et al.,2002)via the following reactions:

3MnOOH t2HAsO 2à4t7H t

/Mn 3eAsO 4

t6H 2O (13)

Mn 2ttH 2AsO 4àtH 2O /MnHAsO 4$H 2O àkrautite á

tH t

(14)

The adsorption of divalent metals (Me 2t

)onto amphoteric surface groups of a Mn oxide (b MnOH)can be summarized as (Zaman et al.,2009):

h MnOH tMe 2t/h MnOCd ttH t(15)h 2MnOH tMe 2t/h eMnO T2Cd t2H t

(16)

As mentioned before,the presence of inorganic ligands (i.e.,phosphates)can promote metal(loid)adsorption onto the oxides,

where two types of ternary surface complexes can be formed depending on the pH (Zaman et al.,2009):

h MnOH tMe 2ttH 2PO à4/h MnOCdH 2PO 4tH t

(17)h MnOH tH 2PO à4tMe 2t/h MnHPO à24Cd

2ttOH à(18)

Cadmium sorption onto a Mn oxide is thus enhanced by

complexation with phosphates through the formation of ligand-like metal complexes at lower pH values (4)and metal-like adsorption at higher pH values (5e 6).

Due to their low pH zpc ,Mn oxides carry a negative charge in soil environments and thus are not a good choice for the stabilization of anionic species,such as As(V)(Moore et al.,2000).Their application to soils may increase soil pH,which promotes sorption of metallic cations,but can lead to soil organic matter dissolution and subse-quent metal(loid)remobilization (McBride and Martínez,2000;Martínez-Villegas and Martínez,2008).There are still only a limited number of studies dealing with Mn oxides as stabilizing amend-ments in available literature.In the pioneer studies of Mench et al.(1994a ,b ,1997)and Sappin-Didier et al.(1997),a hydrous Mn oxide (not precisely described)was successfully tested as a possible stabilizing amendment for Cd,Pb and Zn in contaminated soils,even those with a low pH.Its addition signi ?cantly reduced metal mobility and uptake by plants grown on the contaminated soils.Application of cryptomelane signi ?cantly decreases soluble Pb in contaminated soils,bioaccessibility and uptake by Swiss chard (Beta vulgaris L.)(Hettiarachchi et al.,2000;Hettiarachchi and Pierzynski,2002).The authors further highlighted the importance of phosphates present in the soil solution,which enhanced metal adsorption onto the oxides via the formation of surface ternary complexes (oxide surface e metal e ligand)and formation of phosphate-modi ?ed Mn oxides with reduced solubility (Hettiarachchi et al.,2000).Similar results were observed in a study dealing with Fe-rich materials (not precisely speci ?ed byproducts)combined with H 3PO 4(Brown et al.,2004).

Birnessite is able to reduce free Cu 2tconcentrations in highly contaminated soil solutions;however,the concentration of total soluble Cu (including dissolved Cu complexes with DOM)increased after the application,which led to phytotoxic symptoms on the tested plants (McBride and Martínez,2000).After birnessite application (0.5wt.%)to soils contaminated with Tl,the Mn oxide was able to effectively redistribute Tl from the easily mobilizable to the reducible fraction (associated with oxides),lowering Tl bioavailability in the soils and subsequent uptake (a 50%decrease)

by white mustard (Sinapis alba L.)(Van e

k et al.,2011).This high-lights the importance of Mn oxides as important scavengers of Tl in soils,which was con ?rmed by Jacobson et al.(2005).

4.Aluminum oxides

4.1.Occurrence and properties of aluminum oxides

The most important Al oxides occurring in soils and potentially applicable for soil remediation procedures are listed in Table 1.Gibbsite (g -Al(OH)3)is the most common Al (hydr)oxide,especially in highly weathered and acidic soils.Other secondary Al oxides,such as boehmite (g -AlOOH),are less common.The surface area of Al(OH)3phases vary according to the crystallinity of the mineral from 20to 600m 2g à1.The surface area of gibbsite usually ranges from 20to 50m 2g à1(Essington,2004)and its pH zpc (pH at zero point of charge)is w 9;therefore,in common soil conditions,its surface bears a net positive charge attracting thus anionic species (Sposito,2008).

M.Komárek et al./Environmental Pollution 172(2013)9e 22

4.2.Aluminum oxides as stabilizing amendments

Despite the fact that Al oxides are important scavengers of several metal(loid)species,information about their use as stabi-lizing amendments is scarce.Aluminum-rich wastes from drinking water treatment have been proposed as sorbents for As and Hg, respectively,and their possible use in soil remediation(Sarkar et al., 2007;Hovsepyan and Bonzongo,2009).The use of red mud con-taining Al oxides has been mentioned before.The adsorption of As(V)on amorphous and crystalline(e.g.,gibbsite)Al oxides is most pronounced in the pH range of3e4with amorphous oxides being more ef?cient sorbents(Moore et al.,2000).The slightly higher speci?c area of the synthetic amorphous Al oxide would suggest that it could be a more ef?cient sorbent for As compared to some Fe oxides(ferrihydrite and goethite);however,when added to contaminated soils at1%and5%w/w,the Fe and Al oxides exhibits similar immobilization potentials(García-Sanchez et al.,2002).This highlights the importance of in situ?eld studies evaluating directly the ef?ciency of the amendments,because several site-speci?c environmental factors can alter the immobilization potential of the amendments(e.g.,soil type,pH,temperature,content of “native”soil oxides,content and quality of soil organic matter, presence of root exudates etc.).

5.In?uence of oxide amendments on plants and microbial populations

The remediation process can be thought as successful not only when the mobility of the contaminants in the soil has been reduced,but also when their ecotoxicity has been lowered(or preferably eliminated).Additionally,the stabilizing amendment should not be phytotoxic either,because higher plants should be grown afterwards on the contaminated sites,either through natural revegetation or planting.For example,the application of a hydrous Mn oxide or Fe(0)does not decrease biomass production or in?u-ence the enzyme capacity(i.e.,the potential activity measured in vitro under non-limiting reaction conditions)of several plants grown on contaminated soils(Mench et al.,1994a,2003).Changes in the activity of selected plant enzymes(inhibition or increased capacity)can be an indicator of a stressed metabolism resulting from increased concentrations of metal(loid)s originating either from the contaminated soil or from the amendment(Van Assche et al.,1988).

Besides reducing the bioavailable and bioaccessible fractions of the metal(loid)s,the stabilization process should also restore soil enzyme activities,reduce plant and microbial toxicity and thus positively in?uence the soil ecosystem(Kumpiene et al.,2006; Ruttens et al.,2006a).The application of Fe(0)in combination with beringite(an alkali-trachyte combustion residue)to an As-contaminated soil has a positive effect on microbial biomass; however,not on species richness(Ascher et al.,2009).A switch in composition of the eubacterial and fungal communities and changes in enzymatic activities can also be observed(decreased acid phosphomonoesterase activity and increased alkaline phos-phomonoesterase,phosphodiesterase and protease activity) (Ascher et al.,2009).

As mentioned before,soil properties and speciation of the contaminants strongly in?uences their toxicity.High contents of birnessite(10%w/w)lower the toxicity resulting from high concentrations of free Cu2tin a highly contaminated soil and improved thus the growth of corn plants(McBride and Martínez, 2000).Furthermore,the addition of birnessite promotes the growth of white mustard in a sandy soil with elevated Tl concen-trations but not in a carbonate-rich soil with an initial lower Tl availability(Van e k et al.,2011).While Mench et al.,1994a and Hettiarachchi and Pierzynski(2002)did not observe any signi?cant differences in responses of various plant species to amendment addition,Chen et al.(2000)described signi?cant biomass reduc-tions of rice and increases for Chinese cabbage and wheat after the application of steel sludge to a slightly acidic soil.This phenomenon can be explained by the increase of pH and decrease of Cd avail-ability after the sludge addition and that some plants are less tolerant to acidic soil conditions and elevated soluble metal concentrations.

It is usually not suf?cient to treat the contaminated soil only with the metal oxide or its precursors alone and combinations with other amendments,such as compost,fertilizers,beringite,cyclonic ashes or lime(when Fe sulfates are applied)are needed to enhance plant growth on the site(Boisson et al.,1999;Bleeker et al.,2002; Warren and Alloway,2003;Ruttens et al.,2006a;Bes and Mench, 2008;Vangronsveld et al.,2009).For example,the application of Fe(0)can lower the availability of some nutrients(e.g.,Mg,P)for plants and fertilization may be needed for successful revegetation and/or phytoremediation.In fact,the lower P availability for plants in Fe-treated soils due to phosphate sorption onto Fe oxides could be an important drawback of the method(Mench et al.,2006b; Hartley and Lepp,2008a;Marchand et al.,2011).

The application of the amendments reduces metal uptake by plants to a various degree.For example,Mench et al.,1994a re-ported decreased Cd,Pb and Zn uptake by ryegrass(Lolium multi-?orum Lam.),but the uptake of Pb and Zn by tobacco(Nicotiana tabacum L.)and bean(Phaseolus vulgaris L.)was not decreased after the application of Fe(0)to the soils.Additionally,plant species and associated microorganisms such as rhizosphere and endophytic bacteria and mycorrhiza can in?uence the mobilization and uptake of metal(loid)s to a different extent,via various mechanisms in the rhizosphere,the functioning of constitutive and induced membrane transporters and sub-cellular compartmentation(Jones, 1998;Verbruggen et al.,2009;Verkleij et al.,2009).

6.Results from the?eld

Phytostabilization,the use of green plants and associated microorganisms to immobilize metal(loid)s in the root zone of contaminated soils,is a possible enhancement of the chemical stabilization process(Mench et al.,2003,2009,2010;Adriano et al.,2004;Marques et al.,2009;Vangronsveld et al.,2009). It is usually used right after the soil has been treated with a stabilizing amendment reducing metal(loid)s phytotoxicity; although,successive applications of amendments may be necessary(Pérez-de-Mora et al.,2011).Plant cover signi?cantly reduces soil erosion and percolation of the contaminants in the soil environment and improves the aesthetical value of the remediated site(Ruttens et al.,2006a,b).Despite the previous chemical stabilization by oxides,the chosen species must cope with the contamination on the site.Furthermore,the suitability of the chosen plant species is likely to be site-speci?c.For example,Mench et al.(2003)presented several grass candidates for this purpose usable in SW Europe(Agrostis castellana Boiss.& Reut.,Agrostis delicatula Pouret ex Lapeyr,and Holcus lanatus Boiss.&Reut.)and highlighted the importance of the native species in the remediated area.In general,fast growing plant species producing high biomass yields(fast growing trees,e.g., Salix spp.,Populus spp.or agricultural crops,e.g.,Zea mays L., Helianthus annuus L.)are a popular choice for phytoremediation of contaminated sites(Mench et al.,2009;Vangronsveld et al., 2009).However,the choice of the appropriate plant species depends also on climatic conditions in the area.

M.Komárek et al./Environmental Pollution172(2013)9e2217

Despite some successful trials at the laboratory/greenhouse scale,several unsuccessful results have been presented from the ?eld due to different reasons,e.g.,temperature variations,nutrient availability,rooting depth,precipitation and moisture,plant path-ogens,heterogeneous distribution of contaminants,soil structure etc.(Friesl et al.,2006;Mench et al.,2006b,2010;Vangronsveld et al.,2009).Additionally,results obtained from laboratory and pot experiments cannot be directly extrapolated to?eld conditions (Friesl et al.,2006)and should only provide preliminary and comparative results.

Several short-and long-term?eld studies have been performed to evaluate whether the stabilizing amendments would promote revegetation of the treated sites.The use of various amendments, including oxides(or more precisely their precursors),in selected long-term?eld phytostabilization studies are summarized by Mench et al.(2006b)and Vangronsveld et al.(2009).The combined addition of Fe(0)with compost(and/or beringite)and phosphate fertilizers(also promoting Pb immobilization)provided promising results in the?eld(e.g.,Bleeker et al.,2002;Mench et al.,2003;Bes and Mench,2008).On the other hand,Warren et al.(2003) proposed the use of FeSO4together with lime for As-contaminated soils,which led to better results compared to Fe(0) in the?eld;however,the large particle size of the iron grit used (<2.5mm)could explain the lower ef?ciency of the material.

The project Difpolmine(https://www.wendangku.net/doc/3d12630575.html,),located in southern France approx.100km from Toulouse near the gold mines at Salsigne,is one of the largest remediation projects that have been conducted in Europe.The project aimed at a site near the former metal processing factory at La Combe du Saut with approx.two millions m3of wastes and soils contaminated especially with As. The concentrations of As in soils reached tens to thousands mg kgà1 (with exceptions up to100,000mg kgà1)and hundreds mg Là1in water extracts.The site has been successfully remediated in the years1999e2007,including the application of1%w/w Fe(0) (330tons in total)and phytostabilization(14ha in total).The application of Fe(0)to soils signi?cantly reduced As concentrations in soil solution by39e95%.The costs associated with all the remediation procedures(factory cleanup and demolition,waste and soil excavation,waste and water treatment,chemical stabili-zation,phytostabilization,monitoring etc.)exceeded23.5millions V.However,as mentioned before,the drawbacks associated with the geochemical transformation of the Fe oxides in the long-term and the presence of root exudates that could promote As desorp-tion in the contaminated soils need to be taken into account and the site has to be continuously monitored(Kumpiene et al.,2012).

Other?eld trials where Fe(0)was tested as a stabilizing agent were established at the former Au e Ag mining site of Jales(NE Portugal)(Bleeker et al.,2002;Mench et al.,2003).The site (14.4ha)was extensively exploited for60years and the mining activities generated320,000m3of mine residues resulting into high concentrations of As(17,000mg kgà1)and other metals(e.g., 1000mg Pb kgà1;850mg Zn kgà1)in the spoils(Bleeker et al., 2002).Iron grit containing97%Fe were applied to the spoils at1% w/w either alone or in combination with other amendments (beringite,organic matter,and phosphate fertilizers).The combined application of all the amendments and revegetation of the site with tolerant grasses(Agrostis castellana Boiss.et Reut., Holcus lanatus L.)was partially successful,but additional applica-tions of the amendments would be probably needed in the long-term(Bleeker et al.,2002).On the other hand,the application of Fe(0)in combination with beringite and compost to these soils promoted pine(Pinus pinaster Ait.)growth in the longer term(3 years);however,this option would not be sustainable without maintenance(Mench et al.,2003).Results from other?eld exper-iments where Fe(0)was applied(Louis Fargue,and Reppel)showed some promising results(Mench et al.,2006b),the uptake of Cd and Zn by corn and lettuce grown on the sites was reduced;however, other metals(Cu,Pb)were not affected.When wheat was sown on the sites in2012,i.e.,15years after the application of Fe(0)and beringite,the plants did not develop on untreated and beringite-treated soils,but their growth was partly developed in the Fe(0)-treated soil(Mench,personal communication).

The long-term stability of the amendments in the?eld is crucial for an ef?cient remediation procedure.The geochemical trans-formations/alterations of the amendments in the soils can result into metal(loid)remobilization and increased plant uptake, a drawback that cannot be neglected in the long-term(Gray et al., 2006;Kumpiene et al.,2012).Therefore,more long-term?eld experiments are still needed to evaluate the stability of the amendments,changes in metal(loid)mobility and plant uptake, revegetation intensity and phytostabilization ef?ciency.

7.Perspectives and conclusions

While the sorption properties of the oxides support the reten-tion of most metal(loid)s in the remediated soils,some amend-ments are more suitable for selected elements than others.For example,while Mn oxides proved to be highly ef?cient for the immobilization of soil Pb(Hettiarachchi et al.,2000),they are not desirable for Cr stabilization due to their strong oxidative proper-ties(Kumpiene et al.,2008).The addition of a Fe-rich material and P signi?cantly reduced the concentration of Pb in plants but not of Cd and Zn(Brown et al.,2004).Nevertheless,Pb immobilization in soils using phosphate-induced precipitation of low-soluble pyro-morphite is probably a more ef?cient and cheaper method(e.g.,Cao et al.,2003;Brown et al.,2005).

The stability and behavior of either the incorporated or the newly formed metal oxides in the soil is another relevant factor.The formation of stable phases,e.g.,insoluble FeAsO4(and hydrous species of this compound such as scorodite,FeAsO4$2H2O)is bene?cial for the stabilization procedure.However,data about the stability of the newly formed oxides in the treated soils are lacking. As mentioned before,the aging of the newly formed oxides in?u-ences the behavior of the coprecipitated and sorbed metal(loid)s. The nature of the transformations depends on several factors(e.g., pH,Eh,coprecipitated metal(loid)etc.)and this needs to be taken into account when evaluating the stabilization ef?ciency.Results from long-term experiments,describing the stability and trans-formations of the newly formed oxides in the treated soils are still lacking and should be the focus of upcoming research.Because the vast majority of polluted sites are contaminated with multiple metal(loid)s,the use of selective stabilizing amendments is a chal-lenging task and close attention has to be paid to the selection of the amendment according to the physico-chemical characteristics of the contaminated soils and metal(loid)contents.

The remediation ef?ciency is also in?uenced by the depth of the remediated soil(e.g.,if deep rooting trees are subsequently plan-ted),soil erosion,water cycling etc.(Friesl et al.,2006;Mench et al., 2006b).Thorough homogenization of the stabilizing agent with the contaminated soil at a suf?cient depth is also crucial.For example, when Fe(0)is applied only to the surface of the soil to oxidize,the stabilization ef?ciency is limited.Additionally,signi?cant release of Fe into soil pore water can occur before the oxides are formed (Mench et al.,2006b).The amendments should neither change the physical and structural properties of the soil nor hinder deep rooting of the plants grown on the remediated soils.Another limiting factor could be the spatial variability of the contamination (Hanauer et al.,2011),e.g.,contamination“hotspots”on the site should be treated separately using other remediation options(e.g., excavation).Additionally,the remediated sites have to be

M.Komárek et al./Environmental Pollution172(2013)9e22 18

monitored during the procedure(contaminant bio-and phytoa-vailability,bioaccessibility,leaching,oxide stability,activity of soil microorganisms etc.).

Beside the ef?ciency appraisal,the economic feasibility of in situ stabilization is in?uencing its implementation at large-scale (Vangronsveld et al.,2009).For example,Warren and Alloway (2003)calculated the costs associated with the stabilization of contaminated soils using FeSO4and lime to be4500$haà1(costs not corrected for in?ation).Hanauer et al.(2011)estimated the costs from experiments using Fe(0)to be24,000V haà1and 3000V haà1when a cheaper Fe material(steelwork cheroot) would be used.This is still relatively high and would limit the method for speci?c sites with governmental or industrial funding. The calculated costs are only predictive and will vary according to the length of the in situ stabilization process.On the other hand, when energy crops are grown on the remediated site,the plant biomass would provide additional economical value(Vangronsveld et al.,2009).

Nano-sized oxides and Fe(0)(particle size of1e100nm)are another possible enhancement of the stabilization method.Natural nano-particulate oxides are important scavengers of contaminants in soils(Waychunas et al.,2005)and due to their reactive and relatively large speci?c surface area(tens to hundreds m2gà1), engineered oxide nanoparticles are promising materials for the remediation of soils contaminated with inorganic pollutants (Zhang,2003;Nurmi et al.,2005;Mueller and Nowack,2010;Kim et al.,2012).The particles produced are reactive and small enough to be mixed well with contaminated soils and perspective results are already emerging(e.g.,Kim et al.,2012).The reactions taking place on the nanoparticles surfaces are signi?cantly faster compared to similar phases on the micro-and milli-scale(Li et al., 2006).Nano-sized Fe(0)is due to its high reactivity and reductive properties a promising agent for soils contaminated with Cr(VI);on the other hand,it would not be a good choice for soils contaminated predominantly with As(III).However,the high reactivity of nano Fe(0)can result into its low selectivity in multi-metal(loid) contaminated soils(Tratnyek and Johnson,2006).Although the use of nano-materials for soil remediation is still under develop-ment(Mueller and Nowack,2010)and their production costs are questionable,engineered nano-oxides and Fe(0)nanoparticles are promising amendments usable for purifying contaminated waters and to stabilize pollutants in soils and sediments(Nurmi et al., 2005;An and Zhao,2012;Shipley et al.,2011).Nevertheless,due to the limited information available,the impact of soil-applied nano-oxides and nano Fe(0)on higher plants,soil biota,humans and leachates from the root-zone would need to be closely evalu-ated.Indeed,their toxicity may be an important issue,because they are environmentally exposed after their direct application to the treated soil(Mueller and Nowack,2010).Additionally,due to its high reactivity(e.g.,spontaneous ignition on air),nano Fe(0)must always be handled as a slurry,which complicates its application (Mueller and Nowack,2010).As with other stabilizing amend-ments,nano-sized particles used for remediation should be precisely described,in terms of their stability in various soils (including the aging effects and transformations),potential toxicity (which may be higher due to their increased reactivity)and their sorption capacities for different elements(using isotherms,kinetics and surface complexation models).

In order to perform a successful remediation procedure on the contaminated soil,it is?rstly needed to stop all the contaminants inputs to the site.Subsequently,strategies in risk assessment and remediation activities should be based on accurate knowledge of relative bioavailability of metal(loid)s and other contaminants at a given site(Ehlers and Luthy,2003).It is also needed to take into account that some stabilizing amendments contain high concentrations of metal(loid)s(e.g.,red mud,steel shot)and their application can lead to unwanted concentration increases of these trace elements and in?uence their biogeochemical cycling(Mench et al.,2006b).

Despite some potential drawbacks,in situ chemical stabilization using oxides(as well as other amendments,such as phosphate for Pb-contaminated soils)associated with phytostabilization presents an interesting alternative to conventional soil remediation proce-dures.The ef?ciency of the stabilizing amendments and the phyto/ bioavailability and bioaccessibility of the contaminants has to be evaluated using different approaches,using chemical extractions (single and sequential),leaching tests,TCLP(Toxicity Characteris-tics Leaching Procedure),SPLP(Simulated Precipitation Leaching Procedure),PBET,DGT(Diffuse Gradient in Thin Films),ecotoxico-logical bioassays(phytotoxicity and zootoxicity tests),in vivo experiments and biodiversity monitoring(Lombi et al.,2002b; Geebelen et al.,2003;Adriano et al.,2004;Mench et al.,2006b; Vangronsveld et al.,2009;Bes et al.,2010;Lee et al.,2011).Devel-opment and enhancement of coupled transport and geochemical models(Parkhurst and Appelo,1999;Jacques et al.,2008)would help to predict contaminant behavior on the remediated site and the ef?ciency of the amendments.Furthermore,more long-term ?eld and economical studies are needed to closely evaluate their suitability for sites contaminated with metal(loid)s and whether the stabilizing agents would need to be re-applied in the long-term. Acknowledgment

The authors thank the Czech Science Foundation for?nancial support(projects GA CR P503/11/0840and GA CR P210/11/1597). The authors wish to thank four anonymous reviewers and Dr. Michel Mench for constructive and critical comments. References

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