SOILS, SEC 3 • REMEDIATION AND MANAGEMENT OF CONTAMINATED OR DEGRADED LANDS • RESEARCH ARTICLE

Are soil amendments able to restore arsenic-contaminatedalkaline soils?

Mariano Simón & Verónica González & Sergio de Haro &

Inés García

Received: 30 January 2014 /Accepted: 26 July 2014# Springer-Verlag Berlin Heidelberg 2014

AbstractPurpose This study aims to assess the effectiveness of amend-ments in reducing the mobility and bioavailability of arsenic(As) in alkaline soils compared with acidic soils.Materials and methods An alkaline soil highly polluted withAs was amended with marble sludge (limestone), compost(organic matter) and iron oxides in six different combinations.The soils were watered to field capacity and placed in plasticpots, and lettuce (Lactuca sativa L.) bioassays were subse-quently carried out. Leachates and pore water were collected,and the pH, Eh, electrical conductivity and As concentrationsof both solutions were measured. The As concentration in theleaves and roots of the lettuce was measured, the root and leafdry weight indices were estimated and sequential As extrac-tion was performed. The results obtained with the unamendedand amended soils were compared with those obtained withunpolluted soil.Results and discussion Iron oxide amendments were the mosteffective in reducing the mobility and bioavailability of Asbecause these amendments significantly increased the Asbound to hydrous oxides (non-bioavailable), decreased theAs concentration in pore water and decreased the non-specifically and specifically sorbed As (bioavailable). Com-post was less effective because it increased the concentrationsof As in pore water and non-specifically sorbed As. Marble

sludge, although it decreased the concentration of As in porewater and non-specifically sorbed As, was not effective be-cause As bound to CaCO3 (specifically sorbed) was taken upby the lettuce. However, the amendments had an additiveeffect, and the use of a mixture of the three amendmentsresulted in the best lettuce development.Conclusions Although the amendments were effective in re-ducing the concentration of mobile and bioavailable As, noneof the amendment combinations were able to decrease theshoot As concentration to a level below the maximum con-centration observed in lettuce growing in unpolluted soils.Thus, the restoration goals for these highly polluted soilsshould not include future food production.

Keywords Alkalinesoil .Arsenic .Leachates .Lettuce .Porewater

1 Introduction

At the present time, the areas surrounding the main miningdistricts of the province of Almería (in southeastern Spain) arecultivated, with lettuce being one of the most widespreadcrops. Lettuce is an arsenic-tolerant plant that is able to miti-gate the toxic effects of arsenic (As) through the activation ofdefence mechanisms (Gusman et al. 2013). Arsenic can thusaccumulate in lettuce leaves, thereby entering the food chain.Stabilisation techniques using amendments, which are costeffective and are not disruptive to the environment, have beenwidely used in the restoration of polluted soils. In the provinceof Almería, abundant waste from agricultural greenhouse(high organic matter content) and from marble mining (highCaCO3 content) are periodically generated, which could bereused as amendments given that As adsorption occurs mainlyin clay minerals, hydrous metals oxide, carbonates and organ-ic matter (Magalhães 2002).

Responsible editor: Ravi Naidu

M. Simón (*) :V. González : S. de Haro : I. GarcíaDepartamento de Agronomía. Área de Edafología y QuímicaAgrícola, Universidad de Almería, Campus de ExcelenciaInternacional Agroalimentario ceiA3. Carretera de Sacramento s/n.04120, Almería, Spaine-mail: msimon@ual.es

V. GonzálezLaboratoire Interdisciplinaire des Environnments (LIEC),CNRS-UMR 7360, Université de Lorraine, rue du GeneralDelestraint, 57070 Metz, France

J Soils SedimentsDOI 10.1007/s11368-014-0953-x

The effectiveness of organic matter (OM) and CaCO3 toreduce As mobility presents some controversy. Cao and Ma(2004) observed As adsorption by OM, while Fitz andWenzel(2002) found no evidence that OM contributes to the adsorp-tion of significant amounts of As in soils. Moreno-Jiménezet al. (2013) indicate that the addition of compost to thecontaminated soils increases the dissolved organic carbonand As mobilization. These contradictory results might beexplained by differences in the pH, redox potential (Eh) andtype of OM, which play important roles in As mobility(Shiralipour et al. 2002; Grafe et al. 2002). In general, theaddition of CaCO3 increases As mobility and leaching intosoils because of the higher mobility of As at higher pH levels(Mench et al. 2003; Seaman et al. 2003). Wenzel et al. (2001)concluded that calcium (Ca) has no significant effect on Asbinding, while other researchers have observed that Ca re-duces Asmobility and leaching in response to the formation ofAs-Ca complexes (Hartley et al. 2004; Porter et al. 2004).Moon et al. (2004) indicated that As immobilisation increasesas the Ca/As molar ratio increases. Alexandratos et al. (2007)showed that corner-sharing coordination occurs betweenAsO4 tetrahedra and surface CaO6 octahedra, forming aninner-sphere complex, this suggesting that calcite should bean effective sorbent for As. González et al. (2012a) indicatesthat CaCO3 decreases the As concentration in the pore waterof metallic-As-polluted soils.

Large amounts of waste from both agricultural greenhouseand marble industry are generated in the Almería Province(SE Spain), which should be reused. The study described inthe present paper was focused on assessing the effectivenessof compost from agricultural greenhouse wastes and thesludge from cutting and polishing marble in reducing themobility and bioavailability of As, trying to add value to thesewastes through a better understanding of its properties. Sinceit is well known that iron oxides control As mobility throughadsorption and co-precipitation processes (Kim et al. 2003;Bradl 2004; Drahota and Filippi 2009) and minimising therisk of environmental contamination and uptake by organisms(Hartley et al. 2004; Mench et al. 2006; Kumpiene et al. 2008;Hartley and Lepp 2008; Komárek et al. 2013), the waste frommarble mining and agricultural greenhouse were also mixedwith iron oxides in order to try to improve the properties ofthese wastes so that, either alone or in combination with iron,it could be reused as amendments to restore As-contaminatedsoils.

In a previous study, we assessed the effectiveness ofthese same amendments in restoring As-contaminatedacidic soils (González et al. 2013), so that the presentpaper has been focused on assessing the effectiveness ofthese amendments in reducing the mobility and bioavail-ability of As in alkaline soils. The results will be com-pared with those obtained for acidic soils to assess thebehaviour of the amendments in both soil types and to

advance the state of knowledge about the restoration ofAs-contaminated soils.

2 Materials and methods

2.1 Contaminated soil and amendments

The top layer (20 cm) of a contaminated soil from theRodalquilar gold mine (located in the province of Almería insoutheastern Spain), which is approximately 60 % coveredwith Mediterranean scrub vegetation, was collected, air-driedat 25 °C, passed through a 2-mm sieve and thoroughly mixedto ensure homogeneity. Three different amendments, specifi-cally compost from agricultural greenhouse wastes (C), mar-ble sludge from cutting and polishing marble (M) and acommercial product composed of synthetic iron oxides(Bayferrox) that is mainly composed of goethite (O), wereapplied to the contaminated soil. These amendments weresieved through a 2-mm mesh, and the amended soils werehomogenised by hand for 15 min. Only the treatments thatwere found to be the most effective in reducing As bioavail-ability to Lactuca sativa L. plants in previous studies(González et al. 2012a; Melgar-Ramírez et al. 2012) wereselected for use in this study. The amendments were labelledwith a number and a letter: the letter refers to the amendmentand the number refers to the amount of amendment supple-mented (%, w/w). The treatments were labelled according tothe combination of amendments used for each one. The un-amended soil was labelled ‘UA’. A total of seven treatmentswere studied: UA, 6C, 6M, 6M6C, 6C3O, 6M3O and6M6C3O.

2.2 Soil and amendment analysis

The particle size distributions of the soils were determinedusing the pipette method (Loveland and Whalley 1991). Thecalcium carbonate equivalent content (CaCO3) was estimatedmanometrically (Bascomb 1961). The pH was measured in a1:2.5 soil/water suspension. The total carbon content wasanalysed by dry combustion in a LECO SC-144DR analyser.The organic carbon (OC) content was determined as thedifference between the amounts of total carbon and inorganiccarbon (from CaCO3). The polluted soil and amendmentswere finely ground (<0.05mm) and digested with strong acidsin two steps. In the first step, 0.5 g of the finely groundmaterial was treated with 8 mL of concentrated HNO3, 5 mLof 35%HCl and 1.5 mL of 40%HF. In the second step, 1 mLof 40%HF and 5 mL of H3BO3 (5% in solution) were added.Following the digestion, the As concentration was measuredby inductively coupled plasma mass spectrometry (ICP-MS)using a Hewlett-Packard 4500 STS spectrometer (detectionlimit 0.1 μg As L−1). The accuracy of the method was

J Soils Sediments

confirmed by analysing Standard Reference MaterialSRM2711 Montana Soil (US NIST 2003), six replicates. ForAs, the average recoveries ranged between 91 and 105 % ofthe certified reference values. Pills made of fine material andlithium tetraborate at a ratio of 0.6:5.5 (w/w) were prepared,and the total iron (Fe) and phosphorus (P) contents weremeasured by X-ray fluorescence (XRF) in a Philips PW-1404 spectrometer.

2.3 Greenhouse experiment

The unamended polluted soil samples, amended polluted soilsamples and unpolluted soil samples were watered to fieldcapacity with distilled water (≈200 cm3 kg−1 dry soil) andallowed to air-dry in cycles of 5 days (eight repetitions) toreact and restore the soil microbiological communities (Mar-tínez andMotto 2000). Later, plastic (PVC) pots with drainagesystems designed to allow for the extraction of leachates werefilled with 400 g of each soil (three replicates) and five lettuceseeds were sown in each pot. The pots were placed in agreenhouse with a natural day/night regime for 10 weeks.Distilled water (25 cm3) was added to each pot three timesper week at 50 cm3 h−1. To prevent nutrient deficiency in theseedlings during their development (Smical et al. 2008),25 cm3 of a nutrient solution prepared with analytical-gradereagents [4 mmol L−1 Ca(NO3)2, 2 mmol L−1 KNO3,2.5 mmol L−1 K2HPO4 and 2 mmol L−1 MgSO4] were sup-plied once per week. In the fourth week after sowing, thebetter-developed lettuce seedlings were left in the pots; theothers were carefully uprooted. Leachates (L) and pore water(PW, extracted using a RhizonTM soil moisture sampler) werecollected every 2 weeks, and the extraction amounts, pH, Ehand electrical conductivity (EC) of both solutions were mea-sured. Immediately after collection, the leachates and porewater were filtered through cellulose filters (0.45 μm poresize) by vacuum suction into an acid-washed PyrexTM flask,acidified with HNO3 and stored at <4 °C. The As concentra-tions in both solutions were measured by ICP-MS.

2.4 Plant analysis

At the end of the experiment, the lettuce plants were carefullyremoved and washed with distilled water, and the length of theroots and shoots were measured. The roots and shoots werethen oven dried at 65 °C for 72 h and weighed. The dry weightindices of the leaves (the percentage of the leaf dry weight inrelation to the dry weight of leaves grown in unpolluted soil orDWL) and of the roots (the percentage of the root dry weightin relation to the dry weight of roots grown in unpolluted soilor DWR) were calculated. The organic material from the rootsand leaves was microwave-digested in strong acid (HNO3)and H2O2 using closed digestion vessels (Kingston and Jassie1986; Sah and Miller 1992). The As concentration in each

digested sample was measured by ICP-MS. The accuracy ofthe method was confirmed by an analysis (six replicates) ofNIST Standard Reference Material 1572 (citrus leaves). ForAs, the average recoveries ranged between 90 and 106 % ofthe certified reference values. The As bioconcentration factor(BF) in L. sativa was determined from the PAs/TAs ratio,where PAs is the As concentration in the plant (mg kg−1)and TAs is the total As concentration in the soil (mg kg−1).The As translocation factor from the root to the leaf (TF) wasestimated from the LAs/RAs ratio, where LAs and RAs arethe As concentrations (mg kg−1) in the leaf and root,respectively.

2.5 Sequential extraction procedure

A sequential, five-step As extraction procedure was per-formed for all soils according to Wenzel et al. (2001): step 1,extraction with (NH4)2SO4, non-specifically sorbed As; step2, extraction with NH4H2PO4, specifically sorbed As; step 3,extraction with NH4

+-oxalate, As bound to amorphous andpoorly crystalline hydrous oxides; step 4, extraction withNH4

+-oxalate and ascorbic acid, As bound to well-crystallised hydrous oxides; step 5, determination of the re-sidual fraction. The As concentrations of all the extracts weremeasured by ICP-MS.

2.6 Statistical analysis

The data distribution across the different treatments andamendments was established by calculating the mean valuesand standard deviations. The differences between the individ-ual means were compared by one-way analysis of variance(ANOVA), and the significance of differences in the meanvalues was assessed using Tukey’s test (p<0.05). Pearson’scorrelation coefficients were calculated between the amountof each amendment added, the As concentration in pore water,DWR, DWL, LAs, RAs, BF, TF and the concentration of Asextracted with different reagents. The SPSS software package(PASW Statistics 18) was used for all statistical analyses.

3 Results and discussion

3.1 Soils and amendments

The uncontaminated soil was alkaline (Table 1), loamy sand(textural class), with a relatively low salt content, an OCcontent typical of soil in the province of Almería(11.0 mg kg−1) and trace element concentrations lower thanthe baselines proposed by Sierra et al. (2007) for this province(54.6 mg As kg−1; 0.3 mg Cd kg−1; 145.1 mg Zn kg−1; and120.7 mg Pb kg−1). The contaminated soil was a saline (a soil

J Soils Sediments

is considered saline when EC >4 dS m−1), alkaline, loamysand, with relatively low OC content and Zn, Cd and Pbconcentrations that were similar to the baselines described inprevious studies. However, the As concentration was 16 timeshigher than the baseline reported in previous studies, indicat-ing that the soil was polluted primarily with As. The marblesludge had a very high CaCO3 content, an alkaline pH (8.5), alow EC value and very low trace element concentrations. Thecompost from the greenhouse crops had a very high OCcontent, an alkaline pH (8.7), a high EC value, a relativelyhigh P content and low trace element concentrations. The ironoxides had a basic pH and a low EC value, with very high Feand relatively high Zn contents.

3.2 Leachates and pore water

The pH and Eh values (Fig. 1a, b) were similar for theleachates (L) and the pore water (PW). In comparison to theunamended soil (UA), the addition of compost (6C, 6M6C,6C3O and 6M6C3O) significantly increased the pH values inleachates and pore water (Fig. 1a), while the addition ofmarble sludge and iron oxides (6M, 6M3O) did not signifi-cantly alter the pH value in either solution. The Eh values inleachates and pore water were relatively high in all soils(>400 mV, oxidising conditions) and were not significantlydifferent across the different treatments (Fig. 1b). In a previousstudy, the addition of these same amendments to acidic soilswas found to greatly increase the pH values and significantlydecrease the Eh values in lixiviates and pore water (Gonzálezet al. 2012a). In the current study, the EC values exhibited

larger differences (Fig. 1c); specially, the compost and marblesludge treatments (6C, 6M, 6M6C and 6M6C3O) significant-ly decreased the EC values in leachates, although the combi-nation of iron oxides with compost (6C3O) or marble sludge(6M3O) did not significantly change the EC values. Theaddition of marble sludge (6M) and marble sludge and com-post (6M6C) significantly decreased the EC values in porewater, whereas the other amendment combinations did notsignificantly affect (6C, 6C3O and 6M3O) or increase(6M6C3O) the EC values.

The largest differences were observed in the As concentra-tion (Fig. 1d). Compared with the unamended soil (UA), theaddition of compost (6C) and marble sludge and compost(6M6C) significantly increased the As concentrations inleachates and pore water. The increase in soluble As couldbe attributed to the following three mechanisms: (a) the or-ganic matter changes the As speciation by reducing As(V) tothe more mobile As(III) (Balosoiu et al. 2001), which isunlikely due to the aerate conditions (loamy sand texture)and high redox potential (Table 1, Fig. 1b) of the studied soil;(b) the mobility of As increases with increasing pH (Seamanet al. 2003; Mench et al. 2006); and (c) the soluble organiccompounds (Redman et al. 2002; Moreno-Jiménez et al.2013) and phosphorous (Moreno-Jiménez et al. 2012; Beesleyet al. 2013) present in the compost compete with As forbinding sites on soil particles and enhance As mobilisation.In contrast, the addition of marble sludge (6M) and especiallyiron oxides (6C3O, 6M3O and 6M6C3O) greatly decreasedthe As concentration in both solutions (Fig. 1d). In the case ofthe iron oxides, the decreases were most likely the result of the

Table 1 Mean values and standard deviations (sd) of the main properties and trace element concentrations (n=3) in uncontaminated soil (UCS),contaminated soil (CS), compost (C), marble sludge (M) and iron oxides (O)

Amendments

UCS CS C M O

Mean sd Mean sd Mean sd Mean sd Mean sd

CaCO3 (g kg−1) 5.3 0.3 nd nd 982 2 nd

pH 8.3 0.2 8.4 0.2 8.7 0.2 8.5 0.1 7.6 0.1

EC(dS m−1) 1.6 0.2 9.0 0.4 7.5 0.2 2.1 0.1 1.7 0.1

OC (g kg−1) 11.5 0.3 0.8 0.1 412 1 7.9 0.8 nd

Sand (g kg−1) 557 10 583 6 nm 38 4 nm

Silt (g kg−1) 216 7 279 5 nm 646 8 nm

Clay (g kg−1) 197 8 138 2 nm 323 7 nm

Fe (g kg−1) 46.1 2.7 40.3 2.1 4.8 3.1 1.1 0.1 707 15

P (g kg−1) 4.2 0.4 3.5 0.3 19.4 1.8 0.30 0.04 nm

Zn (mg kg−1) 68.7 1.2 142.1 5.7 71.3 3.1 7.1 0.2 499 16

As (mg kg−1) 11.6 0.3 874.1 25.4 1.3 0.3 3.8 0.1 28.2 1.6

Cd (mg kg−1) 0.12 0.00 0.59 0.02 0.65 0.03 0.21 0.02 nd

Pb (mg kg−1) 19.6 0.5 123.8 4.7 17.3 0.7 1.3 0.1 3.8 0.1

nd not detected, nm not measured

J Soils Sediments

formation of inner-sphere complexes (Sherman and Randall2003) or a ligand exchange of As species for OH2 and OH−

groups on the Fe oxide-hydroxide surface (Jain et al. 1999). Inthe case of the marble sludge, the decreases presumablyresulted from the formation of low-solubility calcium arse-nates (Bothe and Brown 1999; Rodríguez et al. 2008) or fromthe formation of inner-sphere complexes (Alexandratos et al.2007; Sø et al. 2008). The effects of the limestone and ironoxides were additive: the soil amended with 6M3O had thelowest concentration of dissolved As in leachates and porewater.

If these results are compared with those of González et al.(2012a), who added the same amendments to an acidic soil, itcan be concluded that the amendment effect was similar forboth soils (organic matter tends to increase the soluble Asconcentration, whereas iron oxides and limestone tend todecrease the soluble As concentration); however, the ratio ofsoluble As to total As (both in mg kg−1) was clearly higher forthe alkaline soil (mean=3.38×10−3) than for the acidic soil(mean=0.89×10−3). The high As solubility of these alkalinesoils was noted by González et al. (2012b). Therefore, al-though the effects of the amendments were similar for bothacidic and alkaline soils, the ability of the amendments to

decrease the concentration of soluble As to levels that aretolerable for plants was less apparent in the alkaline soils.

3.3 Bioassay

In the unamended soil, As significantly decreased the growthrate of the leaves (DWL) and the roots (DWR) of lettuce(Fig. 2), most likely because As can affect metabolic processessuch as photosynthesis, respiration, growth regulation andreproduction (Stoeva et al. 2004; Rahman and Naidu 2009).In the amended soils, the values of the dry weight indices ofthe roots and the leaves increased compared with those ofplants grown in unamended soil, with the DWL values clearlybeing higher than the DWR values (Fig. 2). These resultsindicate that the root development was more strongly affectedby As pollution than leaf development, which can be attribut-ed to the higher concentration of As in the roots (Fig. 3a).

The greatest lettuce growth (Fig. 2, higher DWL and DWRvalues) and the lowest bioaccumulation (lower BF values) andAs concentrations in the roots and leaves (Fig. 3a, b) werefound in the soils amended with iron oxides and compost(6C3O and 6M6C3O). These amendments are associated witha significant decrease in As concentration in the pore water

aa

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Fig. 1 Mean values and standard deviations of the pH (a), Eh (b), EC (c) and As concentrations (d) in leachates (open bars) and pore water (stripedbars). Means values followed by the same letter do not differ significantly (Tukey’s test: p<0.05)

J Soils Sediments

(Fig. 1), significant decreases in non-specifically and specifi-cally sorbed As (Fig. 4a) and an increase in As bound tohydrous oxides (Fig. 4c, d), suggesting that the soluble As inpore water as well as the non-specifically and specificallysorbed As were bioavailable, while the As bound to ironhydrous oxides was non-bioavailable. These results indicatethat the iron oxides were the most effective amendments inreducing As bioavailability in alkaline soils, whereas in acidicsoils, the most effective amendment was OM (González et al.2013). The fraction of As bound to well-crystallised oxideswas the highest in these alkaline soils (Fig. 4), so that a lowerFe-oxyhydroxides crystallisation in acidic soils comparedwith alkaline soils (Schwertmann and Cornell 2000), togetherwith the ability of plants to take up As linked to poorlycrystalline iron (Kumpiene et al. 2012), would justify the

lower effectiveness of iron to reduce As bioavailability inacidic soils (González et al. 2013).

Amendments with compost (6C and 6M6C) caused in-creases in the As concentration in the pore water (Fig. 1)and in non-specifically sorbed As (Fig. 4a), probably due toan increase in dissolved organic carbon (Moreno-Jiménezet al. 2013). The increase of As in the pore water led to anincrease in the BF values and As concentrations in the rootsand leaves as well as a greater decrease in the values of DWLand DWR compared with the treatments containing iron ox-ides. These results indicate that the treatments with compostwere less effective in decreasing As bioavailability comparedwith those containing iron oxides.

A special circumstance was noted in the case of the soilamended only with marble sludge (6M): although the porewater As concentration was considerably lower than that ofthe unamended soil (Fig. 1d), the BF values and As concen-trations in the roots and leaves were very high (Fig. 3a, b) andthe DWL and DWR values (Fig. 2) were very low (similar tothose of the unamended soil), suggesting that the lettuce rootswere able to take up As sorbed by the limestone. The lower Asconcentration in the pore water (the lowest for any of thestudied soils) together with the lower DWL and DWR values,higher BF values and higher As concentration in the leavesand roots of plants exposed to the 6M3O treatment versusthose exposed to the 6C3O treatment (Figs. 2 and 3) alsoappear to indicate that plants were able to take up As sorbedby limestone. Similar results were obtained in a previous studyfor an acidic soil amended with marble sludge (González et al.(2013), which indicates that acidification of the rhizosphereby H+ extrusion could be the mechanism that mobilises the Assorbed by carbonates. The significantly higher As concentra-tion extracted with NH4H2PO4 in the 6M treatment (Fig. 4b)suggests that the As was specifically sorbed on the surface ofthe carbonates (Bothe and Brown 1999) which, together with

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Fig. 2 Mean values and standard deviations of the root dry weightindices (DWR, open bars) and leaf dry weight indices (DWL, stripedbars).Means values followed by the same letter do not differ significantly(Tukey’s test: p<0.05)

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Fig. 3 Mean values and standard deviations of the As concentrations (a) in lettuce roots (open bars) and leaves (striped bars) and (b) bioconcentration(BF, open bars) and translocation (TF, striped bars) factors.Means values followed by the same letter do not differ significantly (Tukey’s test: p<0.05)

J Soils Sediments

the above, would support the notion that specifically sorbedAs was bioavailable. In this case, the improved developmentof lettuce in the 6M6C and 6M3O treatments compared with

the 6M treatment could be related to the decrease in specifi-cally sorbed As in the soil treated with compost and ironoxides (Fig. 4b).

a a

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Fig. 4 Mean values and standard deviations of the As fractions extracted with a (NH4)2SO4 (AS-As); b NH4H2PO4 (AP-As); cNH4+-oxalate (AO-As)

and d NH4+-oxalate and ascorbic acid (AOA-As). Means values followed by the same letter do not differ significantly (Tukey’s test: p<0.05)

Table 2 Pearson’s correlation coefficients between the amount of eachamendment added, As concentration in pore water (PWAs), dry weightindices of the leaves (DWL) and roots (DWR), As concentrations inleaves (LAs) and roots (RAs), bioconcentration (BF) and translocation

(TF) factors and As concentrations extracted with (NH4)2SO4 (AS-As),NH4H2PO4 (AP-As), NH4

+-oxalate (AO-As) and NH4+-oxalate and

ascorbic acid (AOA-As)

O C M PWAs DWL DWR LAs RAs BF TF AS-As AP-As AO-As

PWAs −0.77** 0.30 −0.46* 1

DWL 0.85** 0.56** 0.26 −0.57** 1

DWR 0.86** 0.59** 0.15 −0.56** 0.96** 1

LAs −0.61** −0.64** −0.16 0.26 −0.71** −0.78** 1

RAs −0.73** −0.70** −0.01 0.33 −0.86** −0.92** 0.90** 1

BF −0.67** −0.68** −0.10 0.30 −0.79** −0.86** 0.98** 0.96** 1

TF −0.46* −0.54* −0.16 0.26 −0.50* −0.78** 0.99** 0.91** 0.98** 1

AS-As −0.91** 0.14 −0.45* 0.86** −0.68** −0.65** 0.44* 0.47* 0.45* 0.44* 1

AP-As −0.55** −0.64** 0.54* 0.03 −0.59** −0.68** 0.56** 0.72** 0.63** 0.55* 0.21 1

AO-As 0.97** 0.09 0.20 −0.74** 0.81** 0.80** −0.53* −0.66** −0.59** −0.53* −0.92** −0.49* 1

AOA-As 0.97** 0.28 0.21 −0.70** 0.90** 0.90** −0.66** −0.79** −0.73** −0.66* −0.86** −0.59** 0.94**

* p<0.05; ** p<0.001

J Soils Sediments

Pearson’s correlation coefficients (Table 2) con-firmed that the iron oxides significantly enhanced thedevelopment of lettuce because they significantly in-creased the As bound to hydrous oxides and decreasedthe non-specifically and specifically sorbed As,resulting in significant decreases in bioaccumulation,translocation and the As concentration in the leavesand roots of lettuce. The compost also significantlyincreased the growth of lettuce, although in this case,the increase was mainly due to the reduction of spe-cifically sorbed As. In the treatments containing ironoxides, the decrease in specifically sorbed As would becaused by an increase in As bound to hydrous oxides;however, in the treatments containing compost, thisdecrease appeared to be linked to an increase in theconcentration of As in the leachates (Fig. 1c), stimu-lating the removal and dispersion of the As. The mar-ble sludge significantly decreased the concentration ofAs in pore water and increased specifically sorbed As,and although it did not significantly affect the DWRand DWL indices (most likely because these valueswere increased when M was combined with C and/orO), the sharp decline in both indices under treatmentwith 6M confirms that the As specifically sorbed onthe surface of the carbonates was bioavailable.

In both acidic and alkaline soils, the greatest lettuce growthoccurred when the three amendments were pooled. However,the leaf As concentrations were still between 4 and 20 timeshigher (Fig. 3a) than the maximum As concentration estimat-ed by Mench and Baize (2004) for healthy lettuce(1.5 mg kg−1).

4 Conclusions

Similar to what was observed in the acidic soils used in aprevious study, the addition of organic matter tends toincrease the soluble As concentration and the addition ofiron oxides and limestone tend to decrease the soluble Asconcentration; however, the ratio of soluble As to total Aswas nearly four times higher in the alkaline soils. The Astaken up by lettuce grown in the alkaline soil tended toaccumulate in the roots, and the development of thebelowground biomass was more affected by As pollutionthan the development of the aboveground biomass.

Iron oxides, which decreased the bioavailable Asand increased the non-bioavailable As, were the mosteffective amendments in reducing the toxic effects ofAs in the alkaline soils. Compost increased the con-centration of As in pore water and was less effectivethan the addition of iron oxides in reducing the toxiceffects of As; however, compost stimulated As removal

through leachates and significantly enhanced the devel-opment of lettuce. Limestone was not effective in re-ducing the toxic effects of As because although itdecreased the As concentration in the pore water, italso increased the specifically sorbed As that, accord-ing to our results, was taken up by the lettuce. Thus,the As concentrations in the leaves and roots wereincreased, which negatively affected plant development.

Amendments tend to have additive effects, and thecombination of the three amendments was the most effec-tive treatment for improving lettuce development. How-ever, the concentration of As in the leaves was still toohigh for human consumption, which suggests that thegoals for restoration of these highly As-polluted soilsshould not include future food production.

Acknowledgments This study was funded by grants CTM2009-07921(Science and Innovation Ministry of Spain and FEDER) and P07-RNM-03303 (Andalusian Government and FEDER). Thanks to the Rijk ZwaanCompany for providing lettuce seeds.

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