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Environmental Pollution 164 (2012) 204e210

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Environmental Pollution

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Metal leaching along soil profiles after the EDDS application e A field study

Aiguo Wang a, Chunling Luo b,c,*, Renxiu Yang a, Yahua Chen a, Zhenguo Shen a,*, Xiangdong Li c

aCollege of Life Sciences, Nanjing Agricultural University, Nanjing 210095, ChinabGuangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, ChinacDepartment of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

a r t i c l e i n f o

Article history:Received 26 October 2011Received in revised form10 January 2012Accepted 20 January 2012

Keywords:EDDSMaizeMetal leachingPhytoextractionField study

* Corresponding authors.E-mail addresses: clluo@gig.ac.cn (C. Luo), zgshen

0269-7491/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.envpol.2012.01.031

a b s t r a c t

One concern about the chelant-enhanced phytoextraction is the potential metal leaching associated withchelant application. A field study was carried out and the metal leaching along the 60-cm depth soilprofiles were evaluated within 36 days after the biodegradable chelant EDDS was applied. Resultsshowed EDDS significantly increased soluble Cu in the top 5 cm soil layer 1 day after the application, andthe increase of soluble metals was generally limited in the top 20 cm soil. Metal speciation analysisindicated all Cu and Zn were in forms of Cu-EDDS and Zn-EDDS complexes in soil solution, and Ca wasthe major competitor with trace metals to EDDS. The soluble metals decreased quickly with time, and nosignificant difference was observed in the extractable Cu between EDDS treatments and the controls 22days after the EDDS addition. The potential leaching associated with biodegradable EDDS addition maybe controlled under field conditions.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Metal contamination of soils is one of major environmentalproblems in the world. The in situ phytoremediation, using plantsto restore the deteriorated soils, is a promising technology in clean-up of polluted sites due to the characters of less destructive, lowcost and environmentally friendly nature. Among various types ofphytoremediation, phytoextraction e by removing metals fromsoils through harvesting the metal accumulating plant biomasse isregarded as a complete metal clean-up strategy, and becomes moreand more appealing in recent decades (Arthur et al., 2005; Garbisuand Alkorta, 2001; Salt et al., 1998; Zhao and McGrath, 2009).

Limited by the generally low bioavailability of metal contami-nants and the low biomass and slow growth of known hyper-accumulators, chelant-assisted phytoextraction by applying chelantand using high biomass plants to enhancemetal removal is proposedas an alternative in metal phytoextraction (Blaylock et al., 1997;Huang et al., 1997; Le�stan et al., 2008; Luo et al., 2005; Shen et al.,2002; Wu et al., 1999). EDTA (ethylenediaminetetraacetic acid) isthe most studied chelant due to its high effectiveness in increasingtrace metal solubility in soils (Blaylock et al., 1997; Shen et al., 2002;Wu et al.,1999). However, EDTA-heavymetal complexes are toxic forboth plants and soil microorganisms and persistent in the

@njau.edu.cn (Z. Shen).

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environment due to its low biodegradability (Bucheli-Witschel andEgli, 2001; Gr�cman et al., 2003). The long residual time of highconcentrations of chelants and metal-chelant complexes wouldincrease potential off-site migration of metals, either in surfacerunoff or by leaching of metals into ground waters (Gr�cman et al.,2001; Nowack, 2002). Therefore, in addition to selecting appro-priate plants andminimizing the amount of chelant applications, thebiodegradability of chelating agents is of great importance in soilphytoremediation processes for environmental concern.

Inorder to reduce thepotentialmetal leaching associatedwith theapplication of chelants, the use of easily biodegradable chelatingagent S, S-ethylenediaminedisuccinic acid (EDDS) has been investi-gated as a substitute of EDTA to enhance plantmetal uptake (Gr�cmanet al., 2003; Kos and Le�stan, 2003a,b; Luo et al., 2005; Meers et al.,2005). EDDS is a structural isomer of EDTA produced naturally bya number of microorganisms. The degradation rate of EDDS variedlargely, and most studies showed the half-life of EDDS was in therange of 2e8 days in soils (Jaworska et al., 1999; Meers et al., 2005;Schowanek et al., 1997; Tandy et al., 2006a). Factors influencing thedegradation of EDDS mainly include the applied dosage, metal typesand soil properties (Luo et al., 2005; Meers et al., 2005, 2008;Vandevivere et al., 2001). Epelde et al. (2008) observed that theconcentration of Pb in the solution of EDDS-treated soil wasapproximately decreased by 50%, in the presence of 2500mg Pb kg�1,24 h after chelant addition. Hauser et al. (2005), however, showedabout 18e42% of the applied EDDS (20mmol kg�1) was lost through

Aug 8 Aug 15 Aug 22 Aug 29 Sep 5 Sep 120

20

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Rai

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Rainfall EDDS application plant harvest soil sampling

Aug 14 Aug 24

Fig. 1. The precipitations recorded during the experiment period. EDDS solution wasapplied on 14 August, and the maize plants were collected on 24 August. The soilprofiles (up to 60 cm depth) were obtained 0, 1, 8, 15, 22, 29 and 36 days after theapplication of EDDS.

Table 1Selected physical and chemical characteristics of the field soil.

Physico-chemical properties

pH 6.79 � 0.23Clay (%) < 0.002 mm 10.3 � 0.35Silt (%) 0.002e0.005 mm 24.6 � 6.38Sand (%) > 0.05 mm 65.2 � 10.2Texture Sandy loamCEC (cmolc/kg) 9.3 � 1.41Organic matter (%) 3.08 � 0.95Total metals (mg kg�1, 0e60 cm soil profile)Cu (0e10 cm) 566 � 332Cu (10e20 cm) 303 � 109Cu (20e30 cm) 225 � 87.1Cu (30e40 cm) 202 � 73.8Cu (40e50 cm) 198 � 36.4Cu (50e60 cm) 190 � 19.2Pb 37.7 � 6.34Zn 183 � 24.6

Cu in different operationally defined fractions (mg kg�1 and % of total)Exchangeable 0.35 (0.08%)Carbonate/specifically adsorbed 6.25 (1.44%)MneFe oxides 10.1 (2.32%)Organic/sulphides 7.23 (1.66%)Residual 409 (94.4%)

Values are means � S.D. (n ¼ 3).

A. Wang et al. / Environmental Pollution 164 (2012) 204e210 205

biodegradation after 7 weeks. In addition, Komárek et al. (2011)observed that the addition of 6 mmol kg�1 EDDS into a coppercontaminated soil led to a significant increase of water-extractablemetals even after 2 years of the experiment. Meers et al. (2005,2008) found that EDDS could be fully degraded within a period of54 d. Mobilized metal concentrations in contaminated soil weredecreased to control values within 50 d when EDDS was applied atthe rate of 4 mmol kg�1 soil. Nevertheless, soluble Zn concentrationwas still significantly higher than the untreated control soil after 50 dwhen 8 mmol kg�1 EDDSwas applied. Tandy et al. (2006a) observedan initial delay of 7e11 d before EDDS started to be degraded. Meerset al. (2008) reported that no significant decrease inmobilizedmetalsduring the first 25e32 d after EDDS was applied, followed bya distinct breakdown with a half-life of 3.4e5.8 d. It was suggestedthat the adaptation of the microbiological communities in the soil tometabolize the chelant canbeassociatedwith the soil typeandextentof pollution. Study of the residual effects showed that the EDDS-treated soil did not have any effect in enhancing the concentrationof metals in the shoots of corn 6 months after the chelant treatment,unlike that was observed in the EDTA treatment (Luo et al., 2006c).

Although EDDS is biodegradable, and may be a promising agentto improve metal solubility, the possible metal leaching risk to thesurrounding environment is still of concern. Prior to the applicationof this technology under the real field conditions, the extent of thepossible effects on the surrounding ecosystem should be criticallyevaluated. However, very few studies assessed the movement ofmetals along the soil profile downward (Hu et al., 2007). Thepresent study was conducted at a Cu contaminated site wheremetal contaminants resulted from previous Cu mining activities,and existingmining tailings. The leaching of heavymetals along soilprofiles, the degradation of EDDS and the potential risk of metals tothe groundwater after EDDS application were investigated underfield conditions. Useful information about the potential anddrawbacks of this technology would be drawn from the presentstudy for possible future application in the field.

2. Materials and methods

2.1. Site description

The field experiment was carried out during June 1 e September 19 of 2009 ona farmland in the vicinity of a past copper mine (Wu et al., 2011), located at theTangshan Town of Nanjing City, Jiangsu Province, Eastern China (N 32�04040.1600 E119�05015.0500). The air temperature during this experiment period was in the rangeof 19e37 �C with an average of 27.3 �C, and the precipitation was recorded withdetailed rainfall events shown in Fig. 1. The soil is sandy loam soil (U.S. TextureTriangle), and the sites were previously planted by crops, such as maize (Zea maysL.), Kang Kong (Pomoea aquatica Forsskal), lettuce (Ctuca sativa), and amaranth(Maranthus mangostanus L.). Selected physical and chemical properties of the soil arepresented in Table 1.

2.2. Experiment set-up

Maize (Zea mays L. cv. Denghai 3622) was chosen in this study as it isa commonly cultivated crop in this region, and it has high biomass yield (Chen et al.,2008). An experimental plot of 9 m � 3 m was delineated in the center of the testfield. Maize seeds were directly planted in a hole-seeding way on June 1. On June 18,when themaize seedlings were about 15 cm tall, seedlings were thinned to a densityof 3.3 � 105/ha.

On August 14, during the flowering period of maize plants, 6 plots(1.5 m � 1.5 m) with uniform growth conditions were selected, and a subplot witha size of 60 cm � 80 cm was chosen in the center of each plot. Two differenttreatments were conducted on the subplots: the control, and the treatment withEDDS followed by irrigation with hot water (90 �C) 2 days later (Chen et al., 2008).EDDS was applied with 4.8 L of 78 mmol L�1 Na3EDDS solution (Fluka ChemieGmbH, UK) in a single application to the surface of subplots which equaled toa dosage of 3 mmol per kg soil (calculated based on a soil depth of 20 cm and a soilbulk density of 1.3 g/cm3). To make the EDDS solution uniformly irrigated to the soilsurface, each subplot was divided into 48 grids with a size of 10 cm � 10 cm, andeach grid was irrigated with 100 ml EDDS solution or water (the control group). Twodays later, EDDS-treated subplots were irrigatedwith 4.8 L 90 �C hot water using the

same irrigating method, by which treatment could facilitate metal uptake by rootsand transport upward to the shoots (Chen et al., 2008). For the control group, thesame dosage of water at normal ambient temperature was irrigated.

On August 24, 84 days after the maize was sowed (10 d after the EDDS appli-cation), the above ground parts of maize plants were harvested. These plantmaterials were washed with tap water followed by rinsing with deionized water,and dried at 60 �C to consistent weight. The dried biomass was recorded, and metalsin plant tissues were analyzed after acid digestion.

Soil samples along a 60 cm depth soil profile were collected in each subplot witha stainless soil probe on 1, 8, 15, 22, 29, 36 d after the application of EDDS. The top 6samples were collected at every 5 cm along 0e30 cm depth, and the other 3 sampleswere collected in every 10 cm along 30e60 cm depth. Five equally sized, and discretesubsamples were taken per subplot for every layer from evenly spaced, adjacentsampling points to form composite samples. Samples were immediately taken tolaboratory and stored in refrigerator. Total metals in the soil samples were deter-mined with ICP-AES (PerkineElmer Optima 3300 DV) after strong acid digestion(Luo et al., 2005). In addition, the soil samples were extracted with DIW (pH ¼ 6.8),and the soil solution was used for the analysis of major elements (Al, Ca, Mn and Fe)and trace metals (Cu, Zn and Pb), TOC and pH. Metal speciation in soil solutions wasalso modeled with ECOSAT (Version 4.8).

A. Wang et al. / Environmental Pollution 164 (2012) 204e210206

2.3. Plant and soil analysis

Soil samples were freeze dried before passing a 2 mm sieve. The soil sampleswere ground with an agate postal for further chemical analysis. Oven-dried plantshoots were ground with a stainless steel disintegrator before metal analysis.

2.3.1. Soil characterizationThe soil pH (top 20 cm) was measured by 0.01 M CaCl2 at 1:5 ratio (w/v) using

a pHmeter. The cation exchangeable capacity (CEC) of the soil was determined usingthe ammonium acetate saturation method. The soil organic matter content wasmeasured by the procedures described by Avery and Bascomb (1982). The grain sizedistribution of soil samples of the top 20 cmwas analyzed with laser diffractometry.Metal fractionation in the soil was performed with a sequential leaching proceduremodified from Wong et al. (2002) including the following operationally definedphases: (1) the exchangeable fraction: readily soluble and exchangeable phase;(2) the carbonate bound and specifically adsorbed fraction: carbonate bound,specifically adsorbed, and weak organic and inorganic complexes; (3) the FeeMnoxide fraction: bound to iron and manganese oxides (FeeMn oxide); (4) theorganic/sulphide fraction: bound to stable organic and/or sulphide (organic)complexes; and (5) the residual fraction: held in primary and secondary mineralswithin their crystal structure. The overall recovery rates of Cu, Pb, and Zn (the sum ofthe five fractions compared with the total metal concentrations) were 92, 90, and89%, respectively, in the soil samples.

2.3.2. Metal quantificationThe soil and plant samples were digested using strong acid (4:1 concentrated

HNO3 and HClO4 (v/v)) (Li et al., 2001; Luo et al., 2005). Certified standard referencematerials of NIST1515 and NIST2709 (National Institute of Standards and Tech-nology, USA) were used in the analysis as part of the QA/QC protocol. Reagent blankand analytical duplicates were also used where appropriate to ensure accuracy andprecision in the analysis. The recovery rates were around 90� 7% for all of themetalsin the plant and soil reference material. Metals were determined by ICP-AES(PerkineElmer Optima 3300 DV).

2.3.3. Metal extraction and speciation modelingThree grams of soils (dry weight after passing 2 mm sieve) were placed in

a 50-mL polypropylene centrifuge tube. Fifteen ml DIW was added to the soil andthe suspension were shaken on a tabletop orbital shaker for 1 h at 200 rpm underroom temperature. After centrifugation (4000 rpm for 10 min), the supernatant wasfiltered through a 0.45 mm filter paper (Whatman [Maidstone, UK]), acidified withHNO3 and analyzed for different metals by ICP-AES.

Subsamples of the supernatant were used for the DOC analysis by ShimadzuTOC-5000A analyzer. DOC concentration of the soil solutions with no EDDS additionwas taken as the background concentration of DOC in soils, and the difference ofDOC from the EDDS-treated soil and the control soil was used to estimate theconcentration of EDDS in soil (Komárek et al., 2010).

Metal speciation in the soil solution was modeled by ECOSAT (Version 4.8). Inputparameters include the calculated EDDS concentrations (estimated from DOC data),concentrations of dissolved metals (Cu, Zn, Pb, Al, Ca, Mn and Fe), solution pH, anddissolved organicmatters (DOM), whichwere assumed to be 50% from fulvic acid andanother 50% from humic acid (Tandy et al., 2006a; Yip et al., 2009). The stabilityconstants of metal-EDDS complexes were from Yip et al. (2009).

2.4. Statistical analysis

Statistical analysis was performed using the SPSS statistical package (Version11.0). All the values reported in this work were the means of at least three inde-pendent replications. Analysis of variance (ANOVA) with subsequent Duncan testwas performed at the significance level of p < 0.05.

3. Results and discussion

3.1. Soil property and metal concentration

Basic physico-chemical property of field soils, total concentra-tions of trace metals at different soil depths, and metal

Table 2Maize shoot biomass, concentrations and total uptake of Cu, Pb, and Zn on the 10th day

Treatment Shoot biomass (g plant�1) Metal concentration(mg kg�1 DW

Cu Pb

Control 56.7 � 17.9a* 9.07 � 0.63a 5.68 � 0.18aEDDS 53.5 � 4.7a 24.1 � 12.3b 5.76 � 0.34a

*Values are means � standard deviations. (n ¼ 3); the different small letters stand for s

fractionation patterns are presented in Table 1. The soil was sandyloam soil. The concentrations of Cu in the arable layer (top 20 cm)were 2.5e5 times above the maximum level permitted of agricul-tural purpose in China (GB15618-1995), which could be mainlyattributed to the past Cu mining operation. For Zn, the concentra-tion was close to the soil limit in China (200 mg kg�1). No signifi-cant contamination of Pb was observed at this site (Table 1). Metalconcentrations along the soil profiles indicated the high hetero-geneity of heavy metals in soil at this location. However, there wasan obvious decreasing trend of metal concentrations withincreasing soil depth. For instance, in the top 10 cm, the concen-tration of Cu was about 1.87 times of the 10e20 cm layer, and 2.5times of 20e30 cm depth. From 30 to 60 cm of the profile,concentrations of Cu varied in a narrow range in comparison withthe top 30 cm layers.

The heterogeneity of metals was typical for contaminated soilsin old mining areas although the soil properties that could influ-ence metal distributions, such as soil texture, CEC and organicmatter, were not quantified along the soil depth in the presentstudy. Not far from this site, mine tailings were not well managed,and this could facilitate the transport of metal rich soil particles bywind, and the surface runoff of leachate with rainwater (Wu et al.,2011). Sequential chemical extraction results revealed that Cu, Pb,and Zn were predominantly associated with the residual fraction;followed by the carbonate/specifically adsorbed phases. Onlya small portion of metal contaminants was presented in theexchangeable fraction, implying the low availability of contami-nants under normal conditions.

3.2. Effects of EDDS on plant growth

Before the application of EDDS, all maize plants had beengrowing for 75 days with good conditions, and there were novisual symptoms of toxicity although the site was contaminatedwith a few metals. In chelant-assisted phytoextraction process,the application of chelant is normally carried out when theplants achieved certain biomass to match the soluble metalpools from chelant application (Shen et al., 2002; Wu et al., 1999).In the present study, the maize already reached about 2 m aboveground when EDDS was applied. Symptoms of toxicity wereobserved 7 days after the EDDS application, with some filemotnecrotic spots appearing on the young leaves. However, no signif-icant reduction of biomass was recorded in themazie plants treatedwith EDDS for 10 d in comparison with the control group (Table 2).The results were probably attributed to the low dose of the appliedchelant and the comparatively long growth period relative to theEDDS treatment before harvesting.

3.3. Effects of EDDS on metal accumulation in maize

The application of EDDS significantly increased the accumula-tion of Cu in maize, about 2.7 and 2.5 fold in the Cu concentrationand total Cu phytoextraction by the shoots over the control group(Table 2), respectively. The efficiency of EDDS-enhancing Cu accu-mulation in shoots was rather lower compared with values ob-tained in pot experiments (Luo et al., 2005). Similar results were

after the EDDS application.

) Total extraction (mg kg�1 DW)

Zn Cu Pb Zn

96.2 � 3.20a 0.52 � 0.19a 0.32 � 0.11a 5.46 � 1.75a93.9 � 2.67b 1.28 � 0.75b 0.31 � 0.04a 5.02 � 0.55b

tatistical significance at the p ¼ 0.05 level.

A. Wang et al. / Environmental Pollution 164 (2012) 204e210 207

reported in a field study by Chen et al. (2008). For Zn and Pb, nosignificant enhancement of plant uptake was obtained. EDDS hada high stability constant with Cu (log K ¼ 18.4), and it was prefer-ential complexed with and solubilize Cu from the soil matrix whenseveral metals co-exist (Luo et al., 2005; Tandy et al., 2006b). Theimproved Cu uptake by maize was primarily attributed to theenhancement of soluble Cu in the soil matrix, about 293 foldincrease of soluble metal attained at the top 10 cm soil after theEDDS addition for 1 d (see Fig. 2). Apart from the high solubility ofsoil metals, the transport of metals from roots to shoots is anotherdetermining factor for metal uptake.

Although there were significant differences of the uptake of Cubetween the EDDS treatment and the control group, the observedincreases in EDDS-induced Cu accumulation are not consideredsufficient for optimal enhanced phytoextraction (Table 2).Assuming the metal uptake by plants keeps unchanging forsubsequent cropping, approximately 2000 cycles will be requiredto reduce Cu concentration in the present level in soils to the targetconcentration of 100 mg kg�1 (P.R. China guidelines for agriculturalsoil), which was much longer than the estimation from potexperiments (Luo et al., 2005). In general, results from pot

Total Zn (mg kg-1 )0 50 100 150 200 250

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01d Control01d EDDS08d EDDS15d EDDS22d EDDS29d EDDS36d EDDS

A

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Fig. 2. Concentration of total and soluble Cu (A and B), P

experiments are always more optimistic than real field tests due tothe fact of well controlled conditions in pot experiments, and highheterogeneity of soils and plants in the field. Measures, such asscreening more efficient chelants and plant species, optimizingchelant application, and improve agronomicmethods are needed inchelant-enhanced phytoremediation research (Le�stan et al., 2008).It was reported that a split application of chelates is more effectivethan the application of single dosages in enhancing the phytoex-traction of metals from the contaminated soils (Shen et al., 2002).The combined application of EDTA and EDDS also dramaticallyimproved the uptake of Pb by maize (Luo et al., 2006b).

Results from previous studies demonstrated that the uptakeof metal by plants would be strongly dependent on both theconcentration of the metal-chelate complex and the breakdown ofthe root exclusion mechanisms in chelant-enhanced phytoex-traction of heavymetals from contaminated soils (Luo et al., 2006d;Wei et al., 2007). Artificial root damage facilitated the transport ofPb-EDTA, Cu-EDDS and Cu-NTA into the plant roots, and resulted ingreatly improved accumulation of metals in plant shoots froma hydroponic study. In the pot experiment (Luo et al., 2006a, 2008)and field study (Chen et al., 2008), EDDS addition in hot solution or

Soluble Cu (mg kg-1)0 20 40 60 80 100 120 140

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b (C and D) and Zn (E and F) along the soil profiles.

Fig. 3. The distribution of EDDS in the field soil profile.

A. Wang et al. / Environmental Pollution 164 (2012) 204e210208

after EDDS application followed by hot water irrigation to soil twodays later led to considerable enhancement of Cu in the shoots ofmaize. The lower efficiency of EDDS-enhancing Cu accumulation inshoots of maize in this study might be explained by a lower degreeof physiological damage to roots caused by EDDS. Wei et al. (2007)observed that up to 3000 mmol L�1 EDDS did not result in a rapidincrease in the relative electrolyte leakage of the root and Evan’sblue uptake in the root tip of Chrysanthemum coronarium in thepresence of 50 mmol L�1 Cu.

3.4. Metal leaching along soil profile

Increasing metal solubility is the major purpose of applyingchelants to soil, and a precondition for enhanced metal uptake byplants. On the other hand, potential metal leaching associated withthe application of chelants may be of concern for the chemical-assisted phytoextraction. In the present study, soluble metalswere analyzed during a period of 36 d after EDDS application alongthe 60 cm depth soil profiles under field conditions. The resultsshowed the application of EDDS to the soils could significantlysolubilize trace metals from the soil matrix, and the soluble Cu andZn reached 123 and 11 mg kg�1 at the 0e5 cm of the soil 1 d afterthe EDDS application, which was 412 and 32 times, respectively, ofthe control (see Fig. 2). The increase of soluble metals was mainlyconfined to the top 20 cm of the soil profiles, and a sharp decreasingtrend was observed from the soil surface to 25 cm depth. Below25 cm depth, no significant increase of soluble metals was found.Total Cu content in the soil varied widely at different depth with anaverage about 494 mg kg�1 in the top 15 cm, and decreased to235 mg kg�1 at 15e30 cm, and then drop to 197 mg kg�1 below30e60 cm. No significant difference (p< 0.05) in the soluble metalswas observed between various depths in the control soil plots (noEDDS treatment) although the total metals varied from 200 to600 mg kg�1. Therefore, the heterogeneity of soil was not the majorreason for the decreasing trend of soluble metals when EDDS wasapplied.

The highest extractable metals were recorded 1 d after EDDSapplication (August 15), and soluble metals in the soils decreasedquickly with time (Fig. 2). Seven days later (August 22), theconcentrations of soluble Cu were decreased by 28%, 68%, 74% and72% at 0e5, 5e10, 10e15 and 15e20 cm of soil profile, respectively,compared with those values obtained 1 d after EDDS application.The concentrations of soluble Pb and Zn also dramaticallydecreased in the upper layers of soil. Below 25 cm depth, nosignificant increase of soluble Cu, Pb and Zn was observed. Duringa period of 8 d after EDDS application, no rainfall event occurred.Thus, the observed decreases of soluble metals were probably dueto the biodegradation of these metal-EDDS complexes. As shown inFig. 3, detectable EDDS was only limited to the top 20 cm, and theamount of EDDS decreased very quickly with time. The degradationof EDDS could be influenced by the property of the soil, such as pH,metal compositions and concentrations, and the dose of appliedEDDS. Vandevivere et al. (2001) observed that the Ca-, Mg-, Cd-,Fe(III)-, Al-, Pb-, and Cr(III)-EDDS complexes biodegraded readily atan average rate of 0.3 mmol d�1. The calculated half-life of EDDS insludge-amended soil was 2.5 days (Jaworska et al., 1999). Meerset al. (2005) estimated that the effective half-life of EDDS rangedbetween 3.8 and 7.5 days when the dose that was applied rangedfrom 0.8 to 4 mmol kg�1.

After 22 d of EDDS application (September 5), no significantdifference was observed in the concentrations of soluble Cu, Pb andZn between EDDS-treated soils and untreated control alongthe whole profile (Fig. 2). It is interesting to note that tworainfall events occurred (with no runoff forming) on the 8th days(August 22) and 16e18th days (August 30-September 1) after EDDS

application. However, no significant difference was observed in theconcentrations of soluble Cu, Pb and Zn at a 25e60 cm depthbetween soils collected before (August 22) and after these rainfallevents (August 29 and September 5). It indicated that the rainfalldid not result in an increase in the concentration of soluble metalsin deeper soil layers.

In the present study, EDDS was found to be fully degradedduring the first 22 d after EDDS was applied (see Fig. 3). The rapidbiodegradation of EDDS made it more attractive over EDTA forsignificantly low leaching of associated metals to the surroundingenvironment after chelant application. When EDTA was appliedto the soil, high soluble metals could remain several months evenin the deeper soil (Neugschwandtner et al., 2008). Meers et al.(2008) observed a full degradation period of 54 d after EDDSapplied. Shorter phase of EDDS degradation may be attributedto the lower application dose of EDDS to contaminated soil in ourstudy.

The speciation of EDDS in soil solution also changed with thedegradation of EDDS. As shown in the Table 3, EDDS in the soilsolution was dominated by forms of free EDDS, Ca-EDDS and Cu-EDDS, and these three forms accounted for more than 86% of thetotal EDDS. With increasing time, the contribution of Ca-EDDS tothe total EDDS increased, whereas the percentage of Cu-EDDS/EDDS decreased after 8 d. In complex soil systems, the dissolu-tion of some metals, such as Al and Fe, could also influence thespeciation of EDDS (Tsang et al., 2009). Ca was found to be a majorcompetitor for complexing trace elements with the decreasing pH(Tandy et al., 2004).

As for the percentage of metal-EDDS in respective dissolvedmetal concentrations, Cu and Zn were complexed with EDDScompletely. This could be attributed to the high ionic potential, andthe electron configuration of these two metals (Tsang et al., 2009).Different from Cu and Zn, some portions of Pb-EDDS (about 10%)were dissoluted after 22 d, which might be related to DOM (Yipet al., 2009). Ca-EDDS decreased gradually with time, and thedissociation of Al and Fe-EDDS was also very fast. No Al-EDDS andFe-EDDS were detected in soil solution 15 days after EDDS appli-cation (Table 3). It was shown that Al and Fe predominantly existedas colloidal precipitates, DOM-complexes, or hydrolyzed species insoil (Yip et al., 2009).With EDDS degraded in soil, metals dissolutedfrom metal-EDDS complexes may re-precipitate into the soilmatrix, and become insoluble. This could limit the metal leachingto some extent after the degradation of chelant.

Table 3The percentage (%) of metal-EDDS/total EDDS and metal-EDDS/total metal in the soil solution (0e5 cm soil) in different sampling time after EDDS application.

Speciation 1 d 8 d 15 d 22 d 29 d

Metal-EDDS in total EDDS (%) Cu-EDDS 30.2 � 1.32 45.1 � 1.98 31.3 � 1.51 12.4 � 0.43 15.6 � 0.58Pb-EDDS 0.095 � 0.004 0.055 � 0.003 0.047 � 0.002 0.158 � 0.007 0.069 � 0.003Zn-EDDS 2.61 � 0.122 3.54 � 0.168 0.653 � 0.021 1.91 � 0.012 2.80 � 0.09Al-EDDS 0.032 � 0.001 0.011 � 0.001 0.000 � 0.000 0.002 � 0.000 0.003 � 0.000Ca-EDDS 27.0 � 1.37 25.0 � 1.29 31.5 � 1.76 43.4 � 2.01 39.6 � 1.88Fe-EDDS 1.87 � 0.088 0.038 � 0.002 0.000 � 0.000 0.000 � 0.000 0.000 � 0.000Mn-EDDS 9.48 � 0.471 0.265 � 0.012 0.000 � 0.000 0.782 � 0.038 1.08 � 0.051Free EDDS 28.3 � 1.42 25.9 � 1.32 36.5 � 1.76 41.3 � 2.04 40.9 � 2.03Total 99.7 � 4.84 100 � 5.01 100 � 4.78 100 � 3.29 100 � 4.73

Metal-EDDS in total soluble fraction (%) Cu-EDDS 99.9 � 4.87 100 � 4.51 100 � 5.23 100 � 4.91 99.8 � 5.21Pb-EDDS 100 � 5.29 99.6 � 4.89 98.0 � 4.92 90.1 � 4.40 82.8 � 4.06Zn-EDDS 100 � 5.09 99.8 � 4.01 99.8 � 2.45 99.2 � 3.72 98.8 � 2.49Al-EDDS 3.61 � 0.164 1.49 � 0.059 0.10 � 0.003 0.08 � 0.003 0.06 � 0.004Ca-EDDS 8.05 � 0.386 3.73 � 0.162 3.15 � 0.134 0.50 � 0.021 0.35 � 0.014Fe-EDDS 77.5 � 3.758 3.75 � 1.87 0.000 � 0.000 0.000 � 0.000 0.000 � 0.000Mn-EDDS 99.8 � 4.92 99.0 � 5.04 94.0 � 4.81 93.7 � 4.12 91.7 � 4.03

A. Wang et al. / Environmental Pollution 164 (2012) 204e210 209

The key factor influencing the metal leaching in chelant-enhanced phytoextraction is the movement of soil water. When theinputs were larger than outputs, soil is saturated and leaching willoccur. In agricultural settings, there are two inputs: precipitationand irrigation. Avoiding the soil saturationwithmoisture before thechelant is broken down is a good solution to prevent metal andchelant leaching. The rainfalls during the experimental period weretypical in this study region, and the potential leaching associatedwith EDDS application seemed to be confined only in the top 25 cmsoil layers under current field conditions with no extreme heavyrainfall event.

4. Conclusions

The present study showed the solubilized metals resulted fromthe application of biodegradable chelant EDDS to the contaminatedsoils can be limited to the top 25 cm under field conditions.Calculated half-life of EDDS was in the range of 3.3e6.5 d, whichwas similar to laboratory experiments. EDDS could completelydegrade within 36 d after its application. The metal accumulated inplant shoots was not sufficient for an effective phytoremediationwithin an acceptable time frame. Measures to improve metaluptake by plant roots and translocation to shoots should be themajor tasks in the future research. Additional benefit could beachieved by selecting plant species which can be used as biofuel inorder to compensate partially the operation cost.

Acknowledgments

The work described here was supported by the Research GrantsCouncil of the Hong Kong SAR Government (PolyU 5231/08E), theInternational Copper Association (G-AS-10-25 (ENV-24853)), theNatural Science Foundation of Jiangsu Province, China (BK2010064),the Social Development Foundation of Jiangsu Province, China(BE2011781) and the Joint Funds of the National NaturalScience Foundation of China and the Natural Science Foundationof Guangdong Province, China (NSFC-GDNSF U1133004 andU0933002).

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