Plant and Soil 263: 43–51, 2004.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

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Soil washing of Pb, Zn and Cd using biodegradable chelator andpermeable barriers and induced phytoextraction by Cannabis sativa

B. Kos1 & D. Leštan1,2

1Agronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana,Slovenia. 2Corresponding author∗

Received 5 August 2003. Accepted in revised form 2 November 2003

Key words: Cannabis sativa, heavy metals, induced phytoextraction, soil washing, [S,S]-EDDS

Abstract

The feasibility of combined phytoextraction and in situ washing of soil contaminated with Pb (1750 mg kg−1), Zn(1300 mg kg−1), and Cd (7.2 mg kg−1), induced by the addition of biodegradable chelator, [S,S] stereoisomereof ethylenediamine discuccinate ([S,S]-EDDS), was tested in soil columns with hemp (Cannabis sativa) as thephytoextracting plant. The addition of [S,S]-EDDS (10 mmol kg−1 dry soil) yielded concentrations of 1026 ± 442,330.3 ± 114.7 and 3.84 ± 1.55 mg kg−1 of Pb, Zn and Cd in the dry above-ground plant biomass, respectively.These concentrations were 1926, 7.5, and 11 times higher, respectively, compared to treatments with no chelatoraddition. Horizontal permeable barriers, composed of a 3 cm high layer of nutrient enriched sawdust and vermi-culite mixture, and a 3 cm layer of soil, vermiculite and apatite mixture, were positioned 20, 30 and 40 cm deepin the soil. In chelator treatments, barriers placed 30 cm deep reduced leaching of Pb, Zn and Cd by 435, 4 and53 times, respectively, compared to columns with no barrier, where 3.0, 4.3 and 3.3% of total initial Pb, Zn andCd, respectively, was leached during 6-weeks water irrigation after chelator addition. Lower positioned barrierswere almost equally effective in preventing leaching of Pb than barriers positioned closer to the soil surface, lesseffective for Cd, and did not prevent leaching of Zn. 2.53% of total initial Pb and 2.83% of Cd was washed fromthe contaminated soil and accumulated into the barrier. Combined method was less effective than simulated ex situsoil washing, where 14.2, 5.5 and 24.5% of Pb, Zn and Cd, respectively, were removed after 1-h extraction, butcomparable effective to 48-h extraction.

Abbreviations: BCF – bioconcentration factor; EDTA – ethylene diaminetetraacetate; HM – heavy metal; PP –phytoextraction potential; [S,S]-EDDS – [S,S]-ethylenediamine disuccinate.

Introduction

Heavy metals (HMs) make a significant contributionto soil contamination. Soil contamination is seldommonometallic, and several elements are usually sim-ultaneously present in elevated concentrations in soil.Concentrations of Pb, Zn and Cd have increasedthroughout the 20th century because of mining andatmospheric deposition from Pb and Zn smelting, andsoil applications of sludges, mineral fertilizers andpesticides.

∗FAX No: +386-61-123-1088.E-mail: domen.lestan@bf.uni-lj.si

Some HMs are considered to be essential micronu-trients for at least some forms of life. Zn deficiency insoil is one of the most important trace element defi-ciencies in crops. Pb and Cd have no known biologicalfunction. Pb, Cd and excessive concentrations of Zn insoil are phytotoxic and have adverse effects on crops,livestock and man (Oliver, 1997).

Of the current methods for remediation of HM con-taminated soil most physico-chemical treatments (i.e.,chelator extraction of HMs from the soil slurry in anex situ washing technique) inhibit soil fertility, whilephytoextraction is considered to preserves the qualityof the remediated soil. Plants can extract only HMs

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that are phytoavailable in the soil. The phytoavailabil-ity of Cd is generally high compared to other metals,as a result of the predominance of low-energy bondsto the soil solid phase (Alloway, 1995). There are sev-eral hyperaccumulating plants for Zn and Cd, withexceptional tolerance and accumulating capacity toextract both HMs from the soil (Zhao et al., 2003).There are no reliable reports on hyperaccumulatorspecies for Pb under natural conditions, presumablysince the phytoavailability of Pb is restricted by thestrong complexation of Pb within solid soil fractions.To overcome the problem and make plants take upPb, chelators have been used to artificially enhancePb solubility in soil solution. The main drawback ofchelator induced Pb phytoextraction is that most syn-thetic chelators form chemically and microbiologic-ally stable complexes with heavy metals, which posea threat of groundwater contamination (Grcman et al.,2001; Wenzel et al., 2003). Environmental concernstherefore limit the use of induced phytoextraction tohydraulically isolated treatment sites.

We recently proposed a novel technique for re-mediation of Pb contaminated soil, based on combinedchelator induced phytoextraction and environmentallysafe in situ soil washing of HMs (Kos and Leštan,2003a). By using biodegradable [S,S]-stereoisomer ofethylenediamine-disuccinate ([S,S]-EDDS) as a che-lator, and placing a horizontal permeable barrier belowthe layer of treated soil, we demonstrated that leachingof Pb through the soil profile was efficiently preven-ted by the retention of Pb in the barrier. When thetargeted level of soil cleansing is reached after one orseveral cycles of [S,S]-EDDS addition, the barrier ma-terial can be excavated and the contamination removedfrom the soil. The barrier was composed of substrate,which facilitates microbiological degradation of [S,S]-EDDS-Pb complexes and apatite, which retains thereleased Pb.

Plants with high phytoextraction potential (definedas the total amount of HM extracted per ha of soil in asingle phytoextraction cycle) are needed for significantcontribution of phytoextraction to the combined re-mediation technique. In a former study, we examined14 different plants for their phytoextraction poten-tial for Pb, Zn and Cd in ethylenediamine tetraacet-ate (EDTA) and [S,S]-EDDS induced phytoextraction.Hemp (Cannabis sativa) in [S,S]-EDDS treatmentswas found to be the most effective phytoextractor (Koset al., 2003).

In this study, the feasibility of a novel techniqueof combined [S,S]-EDDS induced phytoextraction and

in situ soil washing was evaluated for a soil multi-contaminated with Pb, Zn and Cd, in a column ex-periment with Cannabis sativa as the phytoextractingplant. The efficiency of the combined method wascompared to results obtained by extraction of HMsfrom soil slurry in a bench-scale simulation of an exsitu soil washing technique.

Materials and methods

Soil properties

Soil samples were collected from the 0–30 cm sur-face layer at an industrial site of a former Pb and Znsmelter in the Mežica Valley in Slovenia. Sequentialextractions (Leštan et al., 2003) were used to deter-mine fractionation of heavy metals into six soil frac-tions. Selected soil characteristics and heavy metalcontents are summarized in Table 1.

Experimental set up

Air-dried soil was sieved through a 5 mm sieve. Com-bined chelator induced phytoextraction and in situ soilwashing of Pb, Zn and Cd was tested in 25 cm dia-meter soil columns set in a greenhouse. Columns wereequipped with trapping devices for leachate collection.Temperature and relative humidity in the greenhouseranged from 15 to 10 ◦C, and from 85 to 75% dur-ing the day and at night, respectively. Four replicatesof each treatment were made. Horizontal permeablebarriers was positioned 20, 30 and 40 cm deep inthe soil (soil density was 1.25 g cm−3) except for30 cm high control columns with no barrier. Barri-ers were composed of a 3 cm wide layer consistingof sawdust (181.17 g), soya meal (181.17 g) and ver-miculite (40.26 g), followed by a 2 cm wide layerof soil, and a 3 cm wide layer composed of apatite(Ca5(PO4)3OH, Riedel-de Haen, Seelze) (35.4 g), ver-miculite (113.5 g) and soil (659 g). The apatite layerwas followed by a 2 cm soil layer in the bottom ofthe column. Plastic meshes (D = 4 mm) were placedbetween the layers to separate them and additionally(D = 0.2 mm) at the bottom of the columns to retainthe soil.

Soils in columns were fertilized in all treatmentswith 100 mg kg−1 N and K as (NH4)2SO4 and K2SO4,respectively. 2 week old seedlings of Cannabis sativawere transplanted into the columns. [S,S]-EDDS(Octel, Cheshire) was applied in 500 mL deionizedwater in a single dose of 10 mmol kg−1 soil, 21 days

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Table 1. Physical and chemical characteristics and fractionation of Pb, Zn and Cdin the soil used in this study

Soil properties

pH (CaCl2) 7.1

Organic matter (%) 9.3

N (%) 0.27

P (mg kg−1) 310.3

K (mg kg−1) 50.6

CO−3 (g kg−1) 153.6

CEC (mmol C+ 100 g−1) 23.3

Sand (%) 56.3

Silt (%) 32.6

Clay (%) 11.1

Texture Sandy loam

Total Pb (mg kg−1) 1750

Total Zn (mg kg−1) 1300

Total Cd (mg kg−1) 7.2

Fractionation Pb (%) Zn (%) Cd (%)

In soil solution NDa 0.03 ± 0.01 NDa

Exchangeable 0.12 ± 0.01 0.48 ± 0.00 NDa

Bound to carbonate 11.84 ± 5.75 4.48 ± 2.03 25.65 ± 14.44

Bound to Fe and Mn oxides 0.50 ± 0.01 4.40 ± 0.24 25.16 ± 4.73

Bound to organic matter 75.24 ± 5.33 18.36 ± 0.73 40.03 ± 4.38

Residual fraction 12.30 ± 0.77 72.25 ± 4.24 9.16 ± 1.83

Recovery 82.0 91.5 77.5

aND, not detected.

after plants were transplanted. Columns were irrigatedwith 50 mL kg−1 soil of tap water twice a week for6 weeks after [S,S]-EDDS addition. Leachates werefiltered through white ribbon filter paper (Nr. 595) andstored in cold storage (5 ◦C) for further analysis. Theabove-ground tissues of Cannabis sativa were harves-ted 5 days after chelator addition by cutting the stem1 cm above the soil surface. Biomass was determinedafter plant tissues were dried at 60 ◦C and had reacheda constant weight.

Water retention capacity

The soil and mixtures of reactive materials used forthe construction of permeable barriers were put intoplastic jars with perforated bottoms and soaked withexcess deionized water. After 24 h incubation at 25 ◦Cthe material was removed from the jars and weighed.The material was then dried at 60 ◦C until a constantweight was reached, and weighed again. The waterretention capacity of the soil and mixtures of materialsfor the barriers was expressed as a percentage of watersorbed.

Microbial activity in the horizontal barriers

Microbial activity in the horizontal barriers was de-termined as the metabolic heat generated in the soilcolumns. Temperature probes (10 cm long) were in-serted horizontally into the horizontal barriers and intothe soil of the control columns, and difference inT recorded.

Analysis of metallic soil deposits

Low vacuum scanning electron microscopy coupledwith energy dispersive X-ray spectroscopy (SEM-EDS) was used to analyze the chemical composition ofmetal-looking soil deposits, observed in [S,S]-EDDStreated soil columns. Fresh, untreated soil sampleswith inclusions of deposits (measuring several mm2

across) were taken from the layer of soil above the sub-strate layer of the permeable barrier and the depositswere immediately analyzed by SEM-EDS (magni-fication 30–1600 times, accelerated voltage 20 kV,pressure 15–24 Pa).

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Heavy metal determination

For the analysis of metal concentrations, soil sampleswere ground in an agate mill for 10 min and thenpassed through a 150 µm sieve. After digestion inaqua regia, Atomic Absorption Spectroscopy (AAS)was used for the determination of HM concentrations.

Shoot tissues were collected and thoroughlywashed with deionized water. They were dried toa constant weight and ground in a titanium cen-trifugal mill. Metal concentrations in plant tissuesamples (250–300 mg dry weight) were determinedusing an acid (70% HNO3) dissolution techniquewith microwave heating (Kalra and Maynard, 1991)and analysed by Flame-AAS. HMs concentrationsin leachates were determined by Flame-AAS. Con-trols of the analytical procedure were performed us-ing blanks and reference materials (BCR 60 andBCR 141R, Community Bureau of Reference, forplant and soil) that were treated identically to exper-imental samples. Two measurements of HMs wereperformed for each sample.

Phytoextraction potential and bioconcentration factor

The phytoextraction potential (PP) was calculatedfrom the soil and plant HM concentrations and drybiomass plant yield, as the total amount of heavy metalextracted per ha of soil in a single phytoextractioncycle, and expressed as kg ha−1. A biomass yield ofCannabis sativa of 25 tons of dry biomass ha−1 wasused in the calculation (Jevtic, 1986).

The bioconcentration factor (BCF) provides anindex of the ability of the plant to accumulate a par-ticular metal with respect to its concentration in thesoil (Zayed et al., 1998). BCF was calculated as a ratiobetween HM concentration in the plant tissue and HMconcentration in the soil.

Extraction of HMs from soil slurry

Bench-scale soil washing tests, imitating the tech-nology of ex situ soil washing in a reactor, wereconducted in 50 mL plastic tubes filled with soil slurryconsisting of 15 g of dry HM contaminated soil and15 mL of 10 mmol kg−1 [S,S]-EDDS solution. Thesoil slurry was mixed vigorously at 250 rpm and 25 ◦Cusing a laboratory shaker for 1, 3, 6, 12, 24 and 24 h.The soil-washing extractans were obtained after cent-rifugation of the soil slurry at 4000 g for 30 min andstored in the cold (5 ◦C) for further analysis. Threereplicates of each treatment were made.

Results

Phytoextraction of Pb, Zn and Cd

The addition of [S,S]-EDDS increased the Pb, Znand Cd uptake into the test plant, Cannabis sativa,by 1926-, 7.5- and 11-times over the control treat-ments, respectively (Table 2). The greater accumu-lation power of Cannabis sativa for Pb against Znand Cd during chelator induced phytoextraction wasalso reflected in higher BCF. In the control treatmentwith no chelator-mobilized HMs, BCFs were reversed:Cd>Zn�Pb. This was expected since Cd is knownto be highly interchangeable between solid and liquidsoil fractions and easily taken up by plants, while Pbis strongly bound to the soil solid phase.

The addition of chelator resulted in visual symp-toms of toxicity (necrotic lesions on the leaves) andrapid senescence and drying of the Cannabis sativa.Cannabis sativa is a high biomass producing crop.This is important, since the effectiveness of phytoex-traction depends on the total metal extraction indicatedby the PP (Table 2).

Functioning of permeable horizontal barriers

Barriers were constructed of a layer of substrate (soyameal enriched sawdust mixed with vermiculite) withextensive surfaces to support the development of mi-crobial films, and a layer of apatite mixed with soiland vermiculite for precipitation of HMs. The purposeof the substrate was enhanced microbial degradationof the HMs-[S,S]-EDDS complex formed in contam-inated soil after chelator addition, and subsequentbinding of released HM ions with immobilization(adsorption) materials in the barrier.

We assumed that a high water retention capacity ofthe barrier materials was important to retain the soilsolution with HMs-chelator complexes in the barrierand thus to prolong the time available for microbialdegradation of HMs-[S,S]-EDDS and for HM bind-ing and adsorption. We used vermiculite to retain thesoil solution. In a separate set of experiments (datanot shown) the content of vermiculite in the barrierwas optimized to give a water retention capacity of3.14 g g−1 for substrate, and 0.98 g g−1 for the apat-ite layer. The water retention capacity of the soil was0.66 g g−1.

As shown in Figure 1, the T in the substrate layer ofpermeable barriers remained higher than in the controlcolumn during the remediation process. The higher T

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Table 2. Concentration of Pb, Zn and Cd accumulated in the above ground tissues ofCannabis sativa in response to a addition of 0 (control) and 10 mmol kg−1 [S,S]-EDDS.The phytoextraction potential (PP) and the bioconcentration factor (BCF) were calculatedon the basis of plant dry biomass yield and initial heavy metals concentration in the soil,respectively. Means and standard deviations (n = 4) are presented

Biomass concentration PP BCF

(mg kg−1) (kg ha−1)

Control

Pb 0.53 ± 1.07 0.013 ± 0.027 0.000 ± 0.001

Zn 44.29 ± 8.36 1.107 ± 0.209 0.034 ± 0.006

Cd 0.35 ± 0.30 0.009 ± 0.007 0.049 ± 0.041

[S,S]-EDDS treatment

Pb 1026.49 ± 442.89 25.662 ± 11.072 0.587 ± 0.253

Zn 330.28 ± 114.70 8.257 ± 2.868 0.254 ± 0.088

Cd 3.84 ± 1.55 0.096 ± 0.039 0.534 ± 0.215

Figure 1. Difference in temperature between the substrate layer ofa column with barrier positioned 30 cm deep and the soil of thecontrol column with no horizontal permeable barrier.

was generated due to metabolic heat and indicated in-creased microbial activity. In the soil layer a few cmabove the barrier, metal-looking deposits of severalmm across were observed (Figure 2). These depositswere not present in the soil previous to the experiment.The stechiometry of chemical composition of depos-its, with equimolar amount of Pb and S determined bySEM-EDS, indicated the presence of galenite, PbS.

Leaching and soil washing of Pb, Zn and Cd

As shown in Table 3, leaching of Pb, Zn and Cdfrom columns with barriers placed 20 cm deep in thesoil was very low during the 6-week water irriga-tion period after chelator addition. Barriers positioned30 cm deep reduced the leaching of Pb, Zn and Cd

Figure 2. PbS deposits in the soil. Ten times enlarged.

by 435-, 4- and 53-times, respectively, compared tocolumns with the same soil height but with no barrier.Barriers positioned 40 cm deep in the soil were effect-ive in reducing the leaching of Pb, less effective forCd, and did not prevent the leaching of Zn (Table 3).The dynamics of Pb, Zn and Cd leaching from chelatortreated columns is presented in Figure 3. The leachingof Pb and Cd in columns with barriers was completed6 weeks after the chelator addition. The leaching ofZn was delayed compared to control treatments with

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Table 3. Percentage of the total initial Pb, Zn and Cd removed from the contaminated soil above the permeable barrier andleached from the soil columns after 10 mmol kg−1 [S,S]-EDDS amendment followed by 42 days of irrigation. Barrierswere positioned 20, 30 and 40 cm deep in the soil. Means and standard deviations (n = 4) are presented

Treatment HMs removed from soil (%) HMs leached from columns (%)

Pb Zn Cd Pb Zn Cd

No barriera 3.043 ± 0.877 4.31 ± 0.77 3.268 ± 1.356

20 cm barrier 2.07 ± 1.64 4.15 ± 2.65 1.39 ± 1.18 0.003 ± 0.001 0.18 ± 0.06 0.025 ± 0.023

30 cm barrier 2.53 ± 0.24 7.27 ± 2.59 2.83 ± 2.60 0.007 ± 0.002 1.07 ± 0.17 0.062 ± 0.044

40 cm barrier 3.33 ± 2.69 11.69 ± 2.19 3.66 ± 1.30 0.007 ± 0.003 3.85 ± 0.99 0.102 ± 0.069

aColumns filled with a 30 cm height of soil.

no barriers for 2–4 weeks. In treatments with barri-ers positioned 20 and 30 cm deep, the Zn leachingseemed not to reach a maximum in the 6 weeks of ourmeasurements.

The concentrations of Pb, Zn and Cd in columnswith barriers positioned 40 cm deep in the soil areshown in Figure 4. The addition of [S,S]-EDDS to thesoil acted as a chemical plough for Pb and Cd, redis-tributing these HMs from the upper to the soil layerclose to the barrier. Concentrations of Pb and Cd inthe organic, substrate layer of the barrier increased, in-dicating accumulation, as well as Pb concentration inthe apatite layer. As shown in Table 3, 2.53 and 2.83%of Pb and Cd was washed from the contaminated soiland then accumulated in the barrier. The addition ofchelator reduced Zn fairly uniformly through the soilbarrier, with some sorption of Zn in the substrate layerof the barrier.

To evaluate the efficiency of the combined che-lator induced phytoextraction and in situ soil washingmethod, a bench-scale simulation of ex situ soil wash-ing in a reactor was performed. In ex situ soil washing,the same (10 mmol kg−1) concentration of [S,S]-EDDS after 1 h of intensive mixing removed 14.2,5.5 and 24.5% of the initial total Pb, Zn and Cd, re-spectively. After 48 h of mixing, the removal of Pband Cd decreased to 3.5 and 7%, while removal of Znincreased to 7.3% (Figure 5).

Discussion

The observed greater ability of [S,S]-EDDS to en-hance Pb plant uptake as against Zn and Cd (Table 2)has already been reported (Grcman et al., 2003). Sincethe stability constant for the formation of Pb- (logKs = 12.7) is lower than for Zn-[S,S]-EDDS complex(log Ks = 13.5) (Bucheli-Witschel and Egli, 2001),this presumably indicates that Cannabis sativa uses

Figure 3. Pb, Zn and Cd concentration in leachates from con-trol columns without permeable horizontal barriers and columnsequipped with permeable barriers positioned 20, 30 and 40 cmdeep in the soil. Soil columns were treated with 10 mmol kg−1

[S,S]-EDDS. Error bars represent standard deviation of the meanvalue (n = 4).

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Figure 4. Pb, Zn and Cd concentrations through the soil profile in soil columns with horizontal permeable barrier positioned 40 cm deep in thesoil, before (dotted line) and after 10 mmol kg−1 [S,S]-EDDS amendment followed by 42 days of irrigation (solid line). Error bars representstandard deviation of the mean value (n = 4).

Figure 5. Efficacy of removal of Pb, Zn and Cd from the soil slurryduring 48-hour extraction with 10 mmol kg−1 [S,S]-EDDS. Errorbars represent standard deviation of the mean value (n = 3).

a Zn exclusion strategy to adapt to toxic concentra-tions of Zn in the soil. Zn is an essential elementfor plant nutrition and the essential elements contentof most plants tends to be internally rather than ex-ternally regulated, and relatively independent of thenature of the substrate (Baker and Brooks, 1989). Thispresumably also explains why Zn plant uptakes in con-trol treatments were much higher than those of Pb(Table 2). Nevertheless, many plants are even knownto hyperaccumulate Zn (Baker and Walker, 1990). The

accumulation of Cd in the biomass of Cannabis sativawas low in both control and [S,S]-EDDS treated soil(Table 2). This is partly due to the lower stabilityconstant of Cd-[S,S]-EDDS complex (log Ks = 10.8,Bucheli-Witschel and Egli, 2001), but mostly due tothe lower concentration of Cd in the soil compared toPb and Zn.

High biomass producing Cannabis sativa is a po-tential energy and fiber crop. Its use as a phytoex-traction plant could compensate for some of the soilremediation cost. However, BCF for Pb, Zn and Cdof Cannabis sativa (Table 2) are much too low to en-able effective soil remediation. PP shown in Table 2indicate only 0.31, 0.13, and 0.28% removal of totalinitial soil Pb, Zn and Cd in one cycle of chelatoraddition. In addition, PP calculations tend to be over-optimistic since fully-grown plants are likely to con-centrate less HMs than the younger ones used in ourstudy, and because the efficiency of phytoextractionprobably declines after more labile fractions of HMsare removed.

The addition of [S,S]-EDDS redistributed HMsdown the soil profile (Figure 4), while permeable hori-zontal barriers, especially barriers 20 and 30 cm deepin the soil, effectively prevented leaching of Pb andCd (Table 3). This indicates that the use of biode-

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gradable [S,S]-EDDS in combination with horizontalpermeable barriers could be potentially more import-ant for environmentally safe in situ soil washing ofPb and Cd, than for induced phytoextraction. Thepresence of galenite (Figure 2) indicates that barriersoperated under reducing soil conditions. Galenite wasprobably formed from Pb2+ released after degrada-tion of Pb-[S,S]-EDDS complex and H2S generatedin the microbially active (Figure 1) substrate layerof the barrier (Karnachuk et al., 2002). Metal sulph-ides are water insoluble and were not available forfurther leaching or plant uptake. They are, however,stable only under reducing soil conditions. Only afew minutes after galenite deposits were exposed tothe air, they lost their characteristic bright metalliclustre, probably due to the rapid oxidation of sulphideto sulphate (Bosecker, 1997).

Barriers were relatively ineffective in preventingleaching of Zn. The biodegradability of HMs-chelatorcomplex and binding capacity of adsorptive materialsin the barrier for released HM ions are essential forthis type of barrier to function. Van Devivere et al.(2001b) reported biodegradability of Pb-, Cd- and Zn-[S,S]-EDDS complexes in activated sludge. In ourstudy, extraction of Pb and Cd from soil slurry with[S,S]-EDDS (bench-scale simulation of ex situ soilwashing) decreased with time (Figure 5), indicatingpossible degradation of Pb- and Cd-[S,S]-EDDS com-plexes and binding of released metal ions back intothe solid soil phase. The efficiency of Zn extractionincreased with time. This presumably indicates eitherlower biodegradability of Zn-[S,S]-EDDS complex orpoor adsorption of released Zn ions. In columns witha barrier, leaching of Zn was probably only delayedcompared to columns without a barrier (Figure 3).

Combined phytoextraction and in situ soil washingcan be considered to be a gentle remediation techniquethat preserves soil quality. This is important, as soilis considered to be a non-renewable natural resource.However, the total removal of Pb, Zn and Cd by com-bined method (phytoextraction, accumulation in thebarrier, leaching) was only 2.85, 8.47, and 3.17%, re-spectively (calculated from the data in Tables 2 and3, for columns with barriers 30 cm deep in the soil).Removal of Zn was comparable to extraction of Znfrom the soil slurry in ex situ soil washing simulation(Figure 5). In contrast to 1-h extraction, the efficiencyof 48-h extraction of Pb and Cd was not very muchhigher than in the combined method. Van Devivereet al. (2001a) also investigated [S,S]-EDDS for itsapplicability for ex situ washing of Pb, Zn, Cu, and

Cd from soil, sewage sludge, and harbor sediments. Incontrast to our results they reported that it was feasibleto achieve high, 70–90%, extraction of HMs from thesolids tested. Extraction efficiencies were equal or su-perior to those obtained with the benchmark chelatorEDTA.

As discussed above, our results seem to be theconsequence of rapid biodegradation of Pb-and Cd-[S,S]-EDDS, while Zn-[S,S]-EDDS complex appearsto be more persistent (at least when [S,S]-EDDS wasused in a 10 mmol kg−1 concentration). Our furtherresearch will therefore focus on (temporary) inhibitingthe biodegradation of HMs-[S,S]-EDDS complex inthe contaminated soil and, at the same time, increasingthe biodegradability of the complex in the substratelayer of the barrier. We recently reported that [S,S]-EDDS (10 mmol kg−1) induced leaching of Pb in soilsamended with 0.2% agricultural acrylamide hydrogelwas 2 times higher than in non-amended soil (Kos andLeštan, 2003b). Hydrogel allowed retention and con-centration of EDDS. EDDS in higher concentrationswas both more effective and (unpublished results) lessbiodegradable. For the adsorption layer of the barrier,we are testing commercial sorbent mixtures (Fran-colite, Slovacite) with affinity for more than one HM.Apatite was an effective sorption material for Pb (Fig-ure 4), possibly by conversion into pyromorphite, apoorly soluble Pb phosphate mineral (Laperche et al.,1996). However, apatite seemed to be less effective forZn and Cd (Figure 4).

Conclusions

The results of our study indicate that under the condi-tions tested, the phytoextraction potential of Cannabissativa for Pb, Zn and Cd is limited, and phytoex-traction is therefore excluded as a viable remediationoption. Use of horizontal permeable barriers dur-ing in situ soil washing with biodegradable chelator[S,S]-EDDS and induced phytoextraction preventedextensive leaching and thus the potential for subsoilcontamination by Pb and Cd, but it did not preventleaching of Zn. In situ soil washing of Pb, Zn andCd was generally less effective than simulated ex situsoil washing in the reactor. Nevertheless, in situ soilwashing of heavy metals using biodegradable chelatorand permeable barriers is a new method and morework with different soils, heavy metals, barrier ma-terials and operational conditions is needed to fully

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evaluate its feasibility as a soil-friendly remediationtechnology.

Acknowledgements

This work was supported by the Slovenian Ministryfor Education, Science and Sport, Grant J4-3084-0486-01. The authors are grateful to Dr A. Mladenovicfor SEM-EDS analyses and to the Octel company forthe gift of [S,S]-EDDS. D. Leštan is grateful to hisstudents for their help in experimental work.

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