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Pb, Zn and Cd mobility, availability and fractionation in aged soil remediated by EDTA leaching
Chemosphere 74 (2009) 1367–1373
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Pb, Zn and Cd mobility, availability and fractionation in aged soil remediated by EDTA leaching Metka Udovic, Domen Lestan * Agronomy Department, Biotechnical Faculty, University of Ljubljana, Center for Soil and Environmental Science, Jamnikarjeva 101, 1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 18 June 2008 Received in revised form 10 November 2008 Accepted 10 November 2008 Available online 24 December 2008 Keywords: Remediation Residual fraction Heavy metals Mobility Availability Soil ageing
a b s t r a c t Soil washing remediation techniques usually remove only the labile heavy metal (HM) species from the soil, leaving the residual ones in less available/mobile forms, thus disturbing the chemical equilibrium among different species of HM in the soil. Re-establishing such equilibrium and shifting HM back to more available/mobile chemical forms could occur after exposing the remediated soil to environmental abiotic (ageing) factors. Contaminated soil from a smelter site (Pb 4600 mg kg 1, Zn 1800 mg kg 1, Cd 30 mg kg 1) was leached with increasing EDTA concentrations (2.5, 5.0, 10.0, 20.0, 40.0 and 4-consecutive steps of 40.0 mmol EDTA kg 1 of soil). A gradient of removed HM was reached: from 6% to 73% of initial Pb, from 3% to 23% of initial Zn and from 17% to 74% of initial Cd were removed. Repetitive temperature changes (105 °C and 20 °C) were used to mimic abiotic factors acting on residual HM after EDTA soil leaching in saturated soil at 10% and 90% of soil water holding capacity. Fractionation using sequential extractions, mobility, and phytoavailability of Pb, Zn and Cd and Pb oral bioavailability were determined for aged and non-aged soil. The ageing treatment consistently lowered HM phytoavailability in the original (non-leached) and all treated (chelant-leached) soils. However, Pb, Zn and Cd behaved differently from each other; Pb mobility increased, Cd mobility decreased, while Zn mobility did not change. The results indicate that abiotic (ageing) processes change the availability/mobility of residual HM in all leaching treatments and should thus be considered in final remediation effectivity evaluation. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Heavy metals in soils are a serious environmental and human risk and should be remediated. Among the remediation techniques in use, chelant washing/leaching with ethylenediaminetetraaceticacid (EDTA) has potential as an effective and soil-friendly method (Finzgar and Lestan, 2006). However, chelant-assisted soil washing/leaching is still in the development phase (Lestan et al., 2008), and cannot remove heavy metals from soil entirely, even at high chelant-to-metal ratios (Nowack et al., 2006). Such a technique is able to remove only a part of the heavy metals present in soil, i.e. the labile (mobile and available) and weakly bound metals, leaving the residual heavy metals in non-labile and thus non-toxic forms (Lestan and Finzgar, 2007). The fate of the residual fraction is still unclear. Removing labile heavy metal species from the soil could disturb the chemical equilibrium among different species of heavy metals. By re-introducing the remediated soil into the environment, we expose it to abiotic and biotic environmental factors (soil ageing factors), which could eventually initiate the transition of the residual heavy metals back to more labile forms to re-establish the disturbed equilibrium. * Corresponding author. Tel.: +386 01 423 1161; fax: +386 01 423 1088. E-mail address: [email protected] (D. Lestan). 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.11.013
Such a shift would increase the toxicity of the residual heavy metals and, consequently, decrease the final efficiency of soil remediation. Our previous studies on the impact of biotic environmental soil ageing factors using different earthworm species as models showed that they do in fact cause the residual heavy metals to increase their mobility and availability (Udovic and Lestan, 2007; Udovic et al., 2007). A logical question followed: do abiotic environmental ageing factors also affect the residual heavy metal fraction? The aim of our study was to evaluate the effect of ageing on the fractionation and mobility/availability of Pb, Zn and Cd in soil after remediation with EDTA leaching. To the authors’ knowledge, sparse literature is available on the effects of soil ageing on remediated soil. In a similar study on ageing effect, which simulated high temperatures, Lacal et al. (2003) showed that simulations can to a certain extent predict the long or medium term toxicity of heavy metals in pyritic sludge. Lock and Janssen (2002) exposed artificial soil spiked with Zn to sequences of different ageing treatments: storage at 20 °C, percolation with deionized water, heating at 60 °C, and freezing at –20 °C. No effect of ageing on Zn speciation and ecotoxicity for enchytraeid worm Enchytraeus albidus was detected in their study. It has been recognized that soil contaminants are not 100% available to organisms and that heavy metal bioavailability in soils
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is largely dependent on the partition of the metals (Rieuwerts et al., 1998; Rodriguez et al., 1999; Mulligan et al., 2001). Pb, Zn and Cd fractionation and availability to plants (hereinafter: phytoavailability) and Pb oral-bioavailability are therefore assessed to detect possible changes due to simulated ageing processes.
2. Materials and methods 2.1. Soil and soil analysis Lead smelting and mining activity in the Upper Mezˇica Valley in Slovenia (x = 489 300 m and y = 152 300 m, Gaub-Krüger coordinate system) ceased in 1990, after more than three centuries of uninterrupted activity. Soil was collected from the upper 30 cm layer of a regularly managed vegetable garden near an abandoned lead smelter in this valley. For standard pedological analysis, the pH in soils was measured in a 1/2.5 (w/v) ratio of soil and 0.01 M CaCl2 water solution suspension. Soil samples were analyzed for organic matter by Walkley-Black titrations, cation exchange capacity by the ammonium acetate method, soil texture by the pipette method, easily extractable P was determined colorimetrically according to the Egner-Doming method, and carbonates manometrically after soil reaction with HCl (Kalra and Maynard, 1991). Pedological analysis of non-leached soil was performed in triplicate. The following soil properties were determined: pH 6.6–7.0, organic matter 11.3 ± 0.4%, total N 0.48 ± 0.01%, carbonate 20.2 ± 1.5%, sand 42 ± 1%, silt 47.5 ± 1.4%, clay 10.5 ± 0.4%, P2O5 133 ± 1.8 mg 100 g 1 and K2O 28.2 ± 1.8 mg 100 g 1. 2.2. Soil leaching Air-dried soil (4.6 kg per column) was sieved through a 5-mm mesh sieve and placed in 15-cm diameter soil columns 27 cm high. Plastic mesh (D = 0.2 mm) at the bottom of the column retained the soil. The soil was first leached with different EDTA concentrations; 2.5, 5, 10, 20, 40, and 4-consecutive steps of 40 mmol kg 1 EDTA in 3.1 L of tap water. The leaching EDTA solution was circulated through the soil columns in a closed loop for 24 h using a peristaltic pump (flow rate 15 mL min 1) to mobilize and remove Pb, Zn and Cd as water-soluble heavy metals–EDTA complexes. Each column was then rinsed with 80 L of pure tap water in an open loop (flow rate 15 mL min 1) to remove mobilized heavy metal species and EDTA. EDTA in the rinsing solutions was determined spectrophotometrically according to the procedure of Hamano et al. (1993). The most intensively treated soil was leached in four consecutive steps (4 40 mmol kg 1 EDTA). In each leaching step, the EDTA solution was circulated through the soil column in a closed loop for 24 h, a new leaching step with a new 40 mmol kg 1 EDTA dosage. Rinsing with tap water was performed as described above. 2.3. Laboratory simulation of the ageing processes This ageing treatment was designed to simulate the ageing process that could be induced in soil by long exposures to high temperatures in summer and low temperatures in winter. The soil samples were air dried and sieved (2 mm). Ageing processes were simulated for two moisture conditions, 10 and at 90% of soil water holding capacity. Equal amounts of air dried remediated and nonremediated soil (75 g) were placed in Teflon sealed pressure-proof vessels commonly used for microwave digestion, moistened with tap water and the vessels carefully sealed. Ageing was simulated with five repetitive cycles consisting of 5-d exposure to 105 °C in an oven followed by 5-d exposure to 20 °C in a freezer. A separate vessel was equipped with a manometer and air pressure measured during the ageing treatment. Total pressure in the vessel at 105 °C
was 135 kPa. The aged soil samples were air dried, homogenized and sieved for further analysis. 2.4. Six-step sequential extraction A modified Tessier’s sequential extraction procedure (Lestan et al., 2003) was used to determine the fractionation of Pb, Zn and Cd in soil and in aged soil. The water soluble fraction in the soil solution was extracted from 1 g of air dried non-aged and aged soil sieved to 2 mm, with 10 mL of deionized water for 1 h. The exchangeable fraction from soil colloids to the soil solution was extracted from the residual soil sample with 10 mL of 1 M MgNO3 for 2 h. The fraction bound to carbonates was extracted with 10 mL of 1 M NH4OAc (pH 5) for 5 h. The fraction bound to Fe and Mn oxides was extracted with 20 mL of 0.1 M NH2OH HCl (pH 2) for 12 h. The fraction bound to organic matter was obtained after heating the soil suspension in 3 mL of 0.02 M HNO3 and 5 mL of 30% H2O2 for 3 h at 85 °C, followed by extraction with 15 mL of 1 M NH4OAc for 3 h. The last, residual fraction was obtained after digestion of the residual samples with aqua regia. Three determinations of Pb, Zn and Cd concentration were done for each fractionation sequence. The final fractional recovery of Pb, Zn and Cd was calculated after summing the concentrations of all six steps of sequential extractions and comparing them with the total amount of Pb, Zn and Cd in the relevant soil. 2.5. Toxicity characteristic leaching procedure The mobility and leachability of Pb, Zn and Cd in soil and in aged soil was determined using TCLP analysis (US EPA, 1995). Analyses were conducted in triplicate. The procedure involves shaking a 1 g soil sample, sieved to 2 mm, in 20 mL of 0.0992 M acetic acid and 0.0643 M NaOH extraction solution (1:20 ratio) with a pH of 4.93 ± 0.05, for 18 h on a rotary shaker at about 300 rpm. After the reaction period, the contents were filtered (Sartorius filter paper, pore size 2–3 lm), the filtrate acidified with 65% HNO3 to pH < 2 and stored at 5 °C for heavy metal determination. The extractions were conducted in triplicate. 2.6. Heavy metal phytoavailability Heavy metal phytoavailability was assessed after diethylenetriaminepentaacetic acid (DTPA) extraction, designed to extract simultaneously plant available Zn, Fe, Mn and Cu in near-neutral and calcareous soil (Lindsay and Norvell, 1978), as well as to determine potential metal ecotoxicity (Conder et al., 2001; Cheng and Wong, 2002; Ma et al., 2002). The DTPA extraction solution was prepared to contain 0.005 M DTPA, 0.01 M CaCl2, and 0.1 M triethanolamine (TEA) and was adjusted to pH 7.30 ± 0.05. The procedure involves shaking a 5 g soil sample, sieved to 2 mm, in 10 mL of DTPA extracting solution for 2 h on a horizontal shaker at about 120 cycles min 1. After the extraction period, the contents were filtered (Whatman No. 42 filter paper) and the filtrates analyzed for Pb, Zn and Cd. The extractions were conducted in triplicate. 2.7. Lead oral-bioavailability Children ingest more soil and dust particles than adults, due to their mouthing behavior, i.e. 0.135 g d 1 as estimated by the US Environmental Protection Agency (Ruby et al., 1996), and are thus more exposed to soil pollutants (Davis and Mirick, 2006). The physiologically based extraction test (PBET) used here is designed around pediatric gastrointestinal tract parameters for a child 2–3 years old, as described in Ruby et al. (1996). The PBET includes two phases. Firstly, 0.4 g of sieved soil sample (250 lm) is digested in a reaction flask for 1 h at constant temperature (37 °C) in simu-
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lated gastric fluid (40 mL) prepared by adjusting 1 L of deionized water to pH 2.50 ± 0.05 with diluted HCl and adding 1.25 g of pepsin (porcine, Sigma), 0.50 g of citrate, 0.50 g of malate, 420 lL of lactic acid and 500 lL of acetic acid. The pH of the reaction mixture was measured every 10 min and adjusted with HCl as necessary to keep it at a value of 2.50 ± 0.05. Samples (3 mL each) were collected after 1 h, and centrifuged at 2500 rpm for 25 min. The liquid fraction was decanted for further analysis. The 3 mL sample volume was replaced with gastric solution to maintain a constant volume in the reaction flask. After 1 h, the reaction was titrated to pH 7 by adding a 10 cm long dialysis bag (8000 MWCO, Spectra/Por cellulose ester tubing) containing approximately 1 g of NaHCO3 and 3 mL of deionized water. When the reaction vessel reached
Table 1 Total concentration of Pb, Zn and Cd in soil before and after remediation with leaching with different EDTA concentrations. Results are presented as means of three replicates ±SD. Soil Before leaching After leaching 2.5 mmol kg 1 EDTA 5 mmol kg 1 EDTA 10 mmol kg 1 EDTA 20 mmol kg 1 EDTA 40 mmol kg 1 EDTA 40 (4) mmol kg 1 EDTA
Pb (mg kg
1
)
Zn (mg kg
1
)
Cd (mg kg
1
)
4603 ± 94
1826 ± 61
30.4 ± 0.3
4323 ± 169 3998 ± 138 2712 ± 207 2522 ± 85 2112 ± 122 1239 ± 9
1774 ± 102 1756 ± 36 1599 ± 118 1560 ± 64 1464 ± 120 1402 ± 94
25.3 ± 1.9 20.4 ± 0.6 14.1 ± 0.7 12.0 ± 0.5 10.2 ± 1.5 7.8 ± 0.3
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equilibrium at pH 7, 70 mg of bile salts (porcine, Sigma) and 20 mg of pancreatin (porcine, Sigma) were added, thus simulating small intestine conditions. After 1 h, the reaction solutions were centrifuged at 2500 rpm for 25 min. The liquid fraction was decanted and analyzed as the small intestine fraction. During both phases, a constant moistened argon flow (1 L min 1) at 37 °C was conducted through the reaction mixture in order to simulate peristalsis. The PBET was conducted in triplicate. 2.8. Heavy metal determination Air-dried samples of soil and aged soil (3 g) were ground in an agate mill, digested in boiling aqua regia (28 mL), diluted with deionized water up to 100 mL, and Pb, Zn and Cd analyzed by flame (acetylene/air) (Perkin–Elmer 1100-B, Norwalk, USA). Pb, Zn and Cd in extracts (leaching solutions, rinsing water, solutions from sequential extractions, PBET, TCLP and DTPA solutions) were determined by AAS directly (acetylene/air) (Varian AA240FS). A standard reference material used in inter-laboratory comparisons (WEPAL 2003.1.1), from HBLFA Raumberg-Gumpenstein, Irdning, Austria, was used in the digestion and analysis as part of the QA/ QC protocol. The recovery percentages were 106 ± 6, 108 ± 13 and 105 ± 30. The limits of quantification (LOQ) were set at the lowest limit of the optimum working range of the apparatus as specified in the Varian’s Manual of AnalyticalMethods (1989). They were 0.1, 0.01 and 0.02 mg L 1 for Pb, Zn and Cd, respectively. Reagent blank and analytical duplicates were also used to ensure accuracy and precision in the analysis.
Fig. 1. Fractionation of Pb before and after soil leaching, in the soil and in the soil aged at 10% and 90% water holding capacity (WHC). Results are presented as means of three replicates ±SD. The superscript letters (a, b, ab, c) denote statistically different fractionations within soil and soil aged at 10% and 90% WHC, respectively. LOQ, below limit of quantification.
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2.9. Statistical analysis A Student’s t-test (P < 0.05) for paired data sets and Duncan’s multiple range test (P < 0.05) (Statgraphics 4.0 for Windows) were used to determine differences between soil and respective aged soil properties. 3. Results 3.1. Soil: leaching with EDTA The purpose of soil leaching was to remove the mobile and available (toxic and health threatening) heavy metals fraction, leaving the less mobile and non-accessible (i.e. non-toxic) forms in the soil. By using leaching solutions that contained five different EDTA concentrations ranging from 2.5 to 4-consecutive steps of 40.0 mmol kg 1 (total 160 mmol kg 1) EDTA, we reached a gradient in removed heavy metals: 6%, 13%, 41%, 45%, 54% and 73% of initial Pb, 3%, 4%, 13%, 15%, 20% and 23% of initial Zn and 17%, 33%, 54%, 61%, 66% and 74% of initial Cd (Table 1). After the leached soil was rinsed with excessive amounts of clean tap water, no Pb, Zn, Cd or EDTA were present in the final solution. The increasing removal of Pb, Zn and Cd from the soil (gained by the increasing EDTA concentration in the leaching solutions) was reflected in decreasing fractionation, mobility, phytoavailability and oral-bioavailability of soil residual heavy metals. Six-step sequential extraction showed that most of the Pb was removed from the fraction bound to carbonates, Mn and Fe-oxides and organic matter (Fig. 1). Zn was mostly present in the residual fraction (Fig. 2), thus explaining the lower effectiveness of EDTA,
compared to Pb and Cd. Zn was however consistently removed from the exchangeable fraction and from the fraction bound to carbonates, Fe and Mn-oxides and organic matter. Cd was considerably removed from all fractions, but mostly from the fraction bound to carbonates, by factors up to 8.3 (Fig. 3). The mobility of Pb, Zn and Cd decreased, up to factor 8.6 for Zn and even below the LOQ in the case of Pb and Cd (Tables 2 and 3). The initial values, however, were lower than the limits set by US EPA, i.e. 5 mg L 1 for Pb, 250 mg L 1 for Zn and 1 mg L 1 for Cd (US EPA, 1995). The phytoavailability of Pb, Zn and Cd determined by DTPA extraction decreased up to factors 12.2, 10.5 and 20.6, respectively, with the increasing EDTA concentration in the leaching solution (Tables 2 and 3). Since the authors of the in vitro PBET used validated the test with in vivo animal models only for Pb and As (Ruby et al., 1996), an assessment of Zn and Cd oral-bioavailability was not performed. Pb oral-bioavailability decreased after leaching, up to factor 8 in the stomach phase and up to factor 6.1 in the small intestine phase in the soil, with altogether 73% of total Pb removed (Table 2). 3.2. Effect of ageing on soil pH and Pb, Zn and Cd fractionation After ageing simulation treatments had been applied to the leached and non-leached soil, the soil pH significantly (P < 0.05) decreased up to factor 1.1 in both moisture treatments (soil aged at 10 and at 90% soil water holding capacity) (data not shown). Significant changes in Pb, Zn and Cd fractionation were observed after the ageing. However, we observed only random differences between the two ageing treatments, indicating that the difference in soil moisture did not significantly affect the ageing
Fig. 2. Fractionation of Zn before and after soil leaching, in the soil and in the soil aged at 10% and 90% water holding capacity (WHC). Results are presented as means of three replicates ±SD. The superscript letters (a, b, ab, c) denote statistically different fractionations within soil and soil aged at 10% and 90% WHC, respectively. LOQ, below limit of quantification.
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Fig. 3. Fractionation of Cd before and after soil leaching, in the soil and in the soil aged at 10% and 90% water holding capacity (WHC). The first sequence is not presented, since all the values are below the limit of quantification (LOQ). Results are presented as means of three replicates ±SD. The superscript letters (a, b, ab, c) denote statistically different fractionations within soil and soil aged at 10% and 90% WHC, respectively.
processes (Figs. 1–3). Pb concentration in the water soluble and exchangeable fractions significantly increased after the ageing treatment, as well as concentrations in the fraction bound to organic matter (P < 0.05). On the other hand, Pb concentration decreased up to factor 1.6 in the fraction bound to carbonates and below the limit of quantification in the fraction bound to Fe and Mn-oxides (Fig. 1). Compared to the non-aged, Zn concentration in the aged soils significantly (P < 0.05) decreased in the carbonate, Fe and Mn-oxides and organic matter fractions, but increased in the water soluble and residual fractions (Fig. 2). After ageing, the Zn concentration in the exchangeable fraction increased (P < 0.05) only in the nonremediated soil and in the soil with the least amount of Zn removed (17%). On the other hand, in soils treated with high EDTA concentrations, Zn concentration decreased in soils after ageing (P < 0.05). In general, Cd fractionation was similar to Pb. However, differences (P < 0.05) in fractionation between the two ageing treatments were observed in Fe and Mn-oxides and in the residual fraction. The transbound of Cd in the residual fraction was significantly higher (P < 0.05) in the soil moistened to 10% of water holding capacity than in that moistened to 90% of water holding capacity (Fig. 3).
The mobility of Pb increased (P < 0.05) up to factor 1.8 in the soil with 61% Pb removed (Table 2). The mobility of Zn remained unaltered in non-leached soil and in soils leached with 2.5, 5.0 and 10.0 mmol EDTA kg 1. In soils leached with higher EDTA concentrations, Zn mobility increased after ageing up to factor 1.6 (Table 3). The Pb, Zn and Cd phytoavailability in aged soils decreased up to factors 3.8, 4.3 and 5.4, respectively (Tables 2 and 3). Differences (P < 0.05) in Pb, Zn and Cd mobility between the two ageing treatments were observed only in non-leached and in soil leached with the lowest EDTA concentration. Differences in Pb oral-bioavailability between non-aged and aged soils fluctuated (Table 2). A trend of decreasing Pb oral-bioavailability in the intestinal phase was observed after ageing. However, due to large standard deviations, the results are not statistically significant. The two applied ageing treatments (soils aged at 10 and 90% water holding capacity) generally did not affect heavy metal behavior. Differences between the two ageing treatments were observed only for Zn: for soils leached with low EDTA concentrations, ageing at 90% water holding capacity increased Zn mobility and oral-bioavailability. The trend was opposite for soils leached with high EDTA concentrations (Table 3).
3.3. Effect of ageing on Pb, Zn and Cd mobility, phytoavailability and Pb oral-bioavailability
4. Discussion
The three investigated heavy metals in aged soils behaved differently in terms of their mobility. The mobility of Cd decreased below the limit of quantification in all leached soils (Table 3).
The lower percentage of Zn removed from soil in all EDTA leaching treatments compared to the removal of Pb and Cd (Table 1) could be explained by the different lability of complexed and bound metals in soil (Sun et al., 2001) and by differences in the
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Table 2 Pb oral-bioavailability, mobility and phytoavailability in aged and non-aged soil, before and after leaching with different EDTA concentrations, assessed with a PBET test, by TCLP procedure and by DTPA extraction. Results are presented as means of three replicates ±SD, different superscript letters denote significant differences (Duncan, P < 0.05). Comparison between non-aged soil and soil aged at two moistures (10% and 90% water holding capacity, WHC) is shown for each treatment. LOQ, below limit of quantification. PBET Pb (mg kg
DTPA
After leaching 2.5 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC) 5.0 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC)
Small intestine phase
a
ab a
761 ± 48 550 ± 91 b 509 ± 151
a
914 ± 351 459 ± 71 ab 601 ± 104 b
a
DTPA (mg kg
)
b
306 ± 36 385 ± 14 a 451 ± 122 a
328 ± 64 368 ± 27 b 211 ± 22
Pb (mg kg
1
a
952 ± 3 551 ± 62 b 425 ± 112 b
)
Pb (mg L
1.1 ± 0.0 1.7 ± 0.1 b 1.5 ± 0.1
a
a
b
b
a
215 ± 98 241 ± 27 ab 218 ± 35 b
a
901 ± 19 483 ± 55 b 397 ± 89 b
20.0 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC)
a
a
ab
a
276 ± 28 378 ± 17 b 493 ± 86
127 ± 60 163 ± 43 a 160 ± 26
b
1.0 ± 0.0 1.4 ± 0.1 1.4 ± 0.1
a
0.8 ± 0.1 1.3 ± 0.1 b 1.4 ± 0.1 b
a
a
b
b
646 ± 46 428 ± 17 b 358 ± 105
a
a
ab
a
a
a
292 ± 77 214 ± 19 a 217 ± 69
133 ± 63 116 ± 34 a 153 ± 57
277 ± 13 322 ± 75 b 193 ± 78
0.7 ± 0.0 1.1 ± 0.1 1.0 ± 0.1
b
a
0.4 ± 0.0 0.6 ± 0.0 b 0.7 ± 0.1 b
1
40.0 mmol kg EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC)
a
238 ± 90 192 ± 16 a 177 ± 55 a
4 40.0 mmol kg 1 EDTA a 95 ± 4 Soil a 108 ± 13 Aged soil (10% WHC) a 144 ± 31 Aged soil (90% WHC)
a
a
b
a
40 ± 24 93 ± 4 b 91 ± 21 a
54 ± 27 50 ± 13 a 53 ± 4 a
212 ± 1 239 ± 8 a 213 ± 26 a 78 ± 5 158 ± 22 a 115 ± 38
b
1
)
1
)
DTPA (mg L
1
)
TCLP (mg L
1
)
Before leaching Soil Aged soil (10% WHC) Aged soil (90% WHC)
a
230 ± 16 b 99 ± 15 c 59 ± 15
a
189 ± 4 b 95 ± 21 c 45 ± 5
a
2±0 2±0 a 2±0 a
a
12 ± 1 b 3±1 b 2±1
0.1 ± 0.0 LOQ LOQ
a
a
987 ± 5 546 ± 102 c 260 ± 42
)
b
a
170 ± 78 232 ± 24 a 218 ± 33
Cd TCLP (mg kg
1
1
10.0 mmol kg EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC)
Zn
TCLP
1
Stomach phase Before leaching Soil Aged soil (10% WHC) Aged soil (90% WHC)
Table 3 Zn and Cd mobility and phytoavailability in aged and non-aged soil, before and after leaching with different EDTA concentrations assessed by TCLP procedure and by DTPA extraction. Results are presented as means of three replicates ±SD, different superscript letters denote significant differences (Duncan, P < 0.05). Comparison between non-aged soil and soil aged at two moistures (10% and 90% water holding capacity, WHC) is shown for each treatment. LOQ, below limit of quantification.
a a
0.4 ± 0.0 0.4 ± 0.0 0.5 ± 0.1
b
LOQ LOQ LOQ
kinetics of metal desorption/dissolution (Finzgar and Lestan, 2006). The majority of Zn was present in the residual fraction (Fig. 2), which also explains why EDTA was not particularly effective in Zn removal (Udovic et al., 2007). The mobility and hence the availability of metals in soil depends on their concentration in the soil solution and on the characteristics and ability of the soil to release them from the solid phase to compensate for those removed from the solution (Backes et al., 1995). Soil ageing processes alter the soil characteristics and thus potentially affect the residual heavy metal fraction left in the soil after its remediation. Simulations of soil ageing processes are a useful tool for indicating possible changes in the time of metal mobility and availability in soil (Lacal et al., 2003), which are important in assessing the long-term effect of soil remediation. Soil pH plays a crucial role in the adsorption-desorption behavior of heavy metals in soil (Basta and Tabatabai, 1992), which is inversely proportional to the soil pH (Rieuwerts et al., 1998; Adriano, 2001; Cao et al., 2001). The decrease in soil pH after ageing, probably due to high temperature enhanced oxidation processes (Lacal et al., 2003), could to some extent explain the substantial increase in Pb, Zn and Cd concentrations in water-soluble and exchangeable fractions (Figs. 1–3). This fractionation pattern could also be attributed to the effect of the temperature used in the ageing simulation process (105 °C). The rates of metal desorption from Fe and Mnoxides at ambient temperature are slower than the absorption rates (McBride, 1991). Using repetitive high temperature treat-
After leaching 2.5 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC) 5.0 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC) 10.0 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC) 20.0 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC) 40.0 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC) 4 40.0 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC)
a
a
2±0 2±0 b 1±0
162 ± 2 b 55 ± 20 b 58 ± 12
a
0.1 ± 0.0 LOQ LOQ
a
a
b
b
0.1 ± 0.0 LOQ LOQ
1±0 2±0 a 1±0 a
108 ± 7 b 56 ± 1 b 59 ± 15
a
9±0 3±1 b 2±1
a
1±0 1±0 a 1±0
b
b
7±0 2±1 2±1
a
a
b
4±0 2±0 b 2±0
LOQ LOQ LOQ
a
a
1±0 1±0 a 1±0
a
a
a
a
2±0 1±0 1±1
LOQ LOQ LOQ
0.4 ± 0.0 b 1±0 b 1±0
a
1±0 1±0 a 1±0
LOQ LOQ LOQ
a
0.3 ± 0.0 0.4 ± 0.0 b 0.4 ± 0.1
a
b
a
LOQ LOQ LOQ
51 ± 4 50 ± 12 a 40 ± 21
ab
41 ± 2 45 ± 2 b 37 ± 3
a
a
a
22 ± 2 33 ± 6 ab 24 ± 8 b
a
a
a
1±0 1±0 1±0
ments, the desorption rate apparently increased and led to the mobilization of Pb, Zn and Cd into more available chemical forms (fractions) ( Figs. 1–3). The decrease in carbonate-bound Pb, Zn and Cd can be explained by the behavior of the organic matter soil constituents (e.g. humic acids), which enhance heavy metal mobility and solubility by complexation (Weng et al., 2002). Reacting with the heavy metals bound to carbonates, they can form more soluble carboxylates, which can consequently already be extracted in the first two fractions (Udovic, personal communication). In the process of hydrolysis, heavy metal complexes and heavy metal hydroxy-complexes are formed, especially at neutral and higher pH values (Basta and Tabatabai, 1992; Rieuwerts et al., 1998), which are usually poorly soluble and precipitate in the soil solution (Basta and Tabatabai, 1992; Bourg, 1995). Reaction rates are known to be exponentially enhanced by temperature increase. However, they depend also on the characteristics of the extraction solution. The TEA in the extracting solution used in the assessment of heavy metal phytoavailability (DTPA extraction has a close to neutral pH (7.30), which is higher than the pH of the aged soil (ranging from 6.10 to 6.27), and thus also enhances the formation of heavy metal-hydroxides (Rieuwerts et al., 1998). This could explain the large decrease in Pb, Zn and Cd phytoavailability (Table 2 and 3) contrasting the increased concentration in the water soluble and exchanging fractions. The results of TCLP extraction (Tables 2 and 3) indicate that the ageing processes differently affect Pb, Zn and Cd forms present in soil. Apparently, a lower pH value of the TCLP extracting solution (pH 4.93 ± 0.05; US EPA, 1995) and a high temperature cycle de-
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crease the solubility of Cd, but increase the solubility of Pb. However, additional studies are needed to make more considerable conclusions. 5. Conclusions The behavior of the residual heavy metal forms left in the soil after its remediation is largely unknown. Reintroduction of such soil to the environment involves exposing it to natural ageing processes that could affect the residual, non-labile and non-toxic metallic forms, transforming them into more labile forms. To the authors’ knowledge, no studies have been made on the effect of soil ageing on soils remediated by leaching with chelants. The results obtained in this study indicate that abiotic ageing processes do in fact affect the residual heavy metal fractions by changing their lability. The results also show that Pb, Zn and Cd behave differently. Further studies are needed better to understand the underlying mechanisms and fully to evaluate the final results of remediation. A laboratory ageing simulation tool such as that we used in our study or similar would simplify such future evaluations. Acknowledgements The authors are grateful to Dr. Boris Udovic for constructive discussions. This work was supported by the Slovenian Ministry for Education, Science and Sport, Grant J4 9277. References Adriano, D.C., 2001. Trace Elements in Terrestrial Environments; Biogeochemistry. Bioavailability and Risks of Metals. Springer-Verlag, New York. Backes, C.A., McLaren, R.G., Rate, A.W., Swift, R.S., 1995. Kinetics of cadmium and cobalt desorption from iron and manganese oxides. Soil Sci. Soc. Am. J. 59, 778– 785. Basta, N.T., Tabatabai, M.A., 1992. Effect of cropping systems on adsorption of metals by soils: II. Effect of pH. Soil Sci. 153, 195–204. Bourg, A.C.M., 1995. Speciation of heavy metals in soils and groundwater and implications for their natural and provoked mobility. In: Salomon, W., Förstner, U., Mader, P. (Eds.), Heavy Metals. Problems and Solutions. Springer-Verlag, Germany, pp. 19–31. Cao, X., Chen, Y., Wang, X., Deng, X., 2001. Effects of redox potential and pH value on the release of rare elements from soil. Chemosphere 44, 655–661. Cheng, J., Wong, M.H., 2002. Effect of earthworms of Zn fractionation in soils. Biol. Fert. Soils 36, 72–78. Conder, J.M., Lanno, R.P., Basta, N.T., 2001. Assessment of metal availability in smelter soil using earthworms and chemical extractions. J. Environ. Qual. 30, 1231–1237. Davis, S., Mirick, D.K., 2006. Soil ingestion in children and adults in the same family. J. Expo. Sci. Env. Epid. 16, 63–75. Finzgar, N., Lestan, D., 2006. Heap leaching of Pb and Zn contaminated soil using ozone/UV treatment of EDTA extractants. Chemosphere 63, 1736–1743.
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Hamano, T., Mitsuhashi, Y., Kojima, N., Aoki, N., 1993. Sensitive spectrophotometric method for the determination of ethylenediaminetetraacetic acid in foods. Analyst 118, 909–912. Kalra, Y.P., Maynard, DG., 1991. Methods Manual for Forest Soil and Plant Analysis. Canadian Forest Service. Northern Forestry Centre, Edmonton. Lacal, J., da Silva, M.P., García, R., Sevilla, M.T., Procopio, J.R., Hernández, L., 2003. Study of fractionation and potential mobility of metal in sludge from pyrite mining and affected river sediments: changes in mobility over time and use of artificial ageing as a tool in environmental impact assessment. Environ. Pollut. 124, 291–309. Lestan, D., Finzgar, N., 2007. Leaching of Pb contaminated soil using ozone/UV treatment of EDTA extractants. Separ. Sci. Technol. 42, 1575–1584. Lestan, D., Grcman, H., Zupan, M., Bacac, N., 2003. Relationship of soil properties to fractionation of Pb and Zn in soil and their uptake into Plantago lanceolata. Soil Sediment Contam. 12, 507–522. Lestan, D., Luo, C.-L., Li, X.-D., 2008. The use of chelating agents in the remediation of metal-contaminated soils: a review. Environ. Pollut. 153, 3–13. Lindsay, W.L., Norvell, W.A., 1978. Development of a DTPA test for zinc, iron, manganese and copper. Soil Sci. Soc. Am. J. 42, 421–428. Lock, K., Janssen, C.R., 2002. The effect of ageing on the toxicity of zinc for the potworm Enchytraeus albidus. Environ. Pollut. 116, 289–292. Ma, Y., Dickinson, N.M., Wong, M.H., 2002. Toxicity of Pb/Zn mine tailings to the earthworm Pheretima and the effects of burrowing on metal availability. Biol. Fert. Soils 36, 79–86. McBride, M.B., 1991. Processes of heavy and transitional metal sorption by soil minerals. In: Bolt, G.H., de Boodt, M.F., Hayes, M.H.B., McBride, M.B., de Strooper, E.B.A. (Eds.), Interactions at the Soil Colloid–Soil Solution Interface. Kluwer Academic Publ., Netherlands, pp. 149–175. Mulligan, C.N., Yong, R.N., Gibbs, B.F., 2001. An evaluation of technologies for the heavy metal remediation of dredged sediments. J. Hazard. Mater. 85, 145–163. Nowack, B., Schulin, R., Robinson, B.H., 2006. Critical asessment of chelant metal phytoextraction. Environ. Sci. Technol. 40, 5225–5232. Rieuwerts, J.S., Thornton, M.E., Farago, M.E., Ashmore, M.R., 1998. Factors influencing metal bioavailability in soils: preliminary investigations for the development of a critical loads approach for metals. Chem. Spec. Bioavailab. 10, 61–75. Rodriguez, R.R., Basta, N.T., Casteel, S.W., Pace, L.W., 1999. An in vitro gastrointestinal method to estimate bioavailable arsenic in contaminated soil and solid media. Environ. Sci. Technol. 33, 642–649. Ruby, M.V., Davis, A., Schoof, R., Eberle, S., Sellstone, C.M., 1996. Estimation of lead and arsenic bioavailability using a physiologically based extraction test. Environ. Sci. Technol. 30, 422–430. Sun, B., Zhao, F.J., Lombi, E., McGrath, S.P., 2001. Leaching of heavy metals from contaminated soils using EDTA. Environ. Pollut. 113, 111–120. Udovic, B., personal communication. Udovic, M., Lestan, D., 2007. The effect of Eisenia fetida on the redistribution of residual Pb, Zn and Cd in soil after remediation with EDTA leaching. Environ. Pollut. 148, 663–668. Udovic, M., Plavc, Z., Lestan, D., 2007. The effect of earthworms on the fractionation, mobility and bioaccessibility of Pb, Zn and Cd metals before and after soil leaching with EDTA. Chemosphere 70, 126–134. US EPA, 1995. Test Methods for Evaluation of Solid Waste, Vol. IA. Laboratory Manual Physical/Chemical Methods, SW 86, 40 CFR Parts 403 and 503. US Government Printing Office, Washington, DC. Varian, 1989. Analytical Methods. Varian Australia Pty Ltd., Mulgrave, Victoria, Australia. Weng, L., Temminghoff, E.J.M., Lofts, S., Tipping, E., Van Riemsdijk, W.H., 2002. Complexation with dissolved organic matter and solubility control of heavy metals in a sandy soil. Environ. Sci. Technol. 36, 4804–4810.
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Pb, Zn and Cd mobility, availability and fractionation in aged soil remediated by EDTA leaching Metka Udovic, Domen Lestan * Agronomy Department, Biotechnical Faculty, University of Ljubljana, Center for Soil and Environmental Science, Jamnikarjeva 101, 1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 18 June 2008 Received in revised form 10 November 2008 Accepted 10 November 2008 Available online 24 December 2008 Keywords: Remediation Residual fraction Heavy metals Mobility Availability Soil ageing
a b s t r a c t Soil washing remediation techniques usually remove only the labile heavy metal (HM) species from the soil, leaving the residual ones in less available/mobile forms, thus disturbing the chemical equilibrium among different species of HM in the soil. Re-establishing such equilibrium and shifting HM back to more available/mobile chemical forms could occur after exposing the remediated soil to environmental abiotic (ageing) factors. Contaminated soil from a smelter site (Pb 4600 mg kg 1, Zn 1800 mg kg 1, Cd 30 mg kg 1) was leached with increasing EDTA concentrations (2.5, 5.0, 10.0, 20.0, 40.0 and 4-consecutive steps of 40.0 mmol EDTA kg 1 of soil). A gradient of removed HM was reached: from 6% to 73% of initial Pb, from 3% to 23% of initial Zn and from 17% to 74% of initial Cd were removed. Repetitive temperature changes (105 °C and 20 °C) were used to mimic abiotic factors acting on residual HM after EDTA soil leaching in saturated soil at 10% and 90% of soil water holding capacity. Fractionation using sequential extractions, mobility, and phytoavailability of Pb, Zn and Cd and Pb oral bioavailability were determined for aged and non-aged soil. The ageing treatment consistently lowered HM phytoavailability in the original (non-leached) and all treated (chelant-leached) soils. However, Pb, Zn and Cd behaved differently from each other; Pb mobility increased, Cd mobility decreased, while Zn mobility did not change. The results indicate that abiotic (ageing) processes change the availability/mobility of residual HM in all leaching treatments and should thus be considered in final remediation effectivity evaluation. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Heavy metals in soils are a serious environmental and human risk and should be remediated. Among the remediation techniques in use, chelant washing/leaching with ethylenediaminetetraaceticacid (EDTA) has potential as an effective and soil-friendly method (Finzgar and Lestan, 2006). However, chelant-assisted soil washing/leaching is still in the development phase (Lestan et al., 2008), and cannot remove heavy metals from soil entirely, even at high chelant-to-metal ratios (Nowack et al., 2006). Such a technique is able to remove only a part of the heavy metals present in soil, i.e. the labile (mobile and available) and weakly bound metals, leaving the residual heavy metals in non-labile and thus non-toxic forms (Lestan and Finzgar, 2007). The fate of the residual fraction is still unclear. Removing labile heavy metal species from the soil could disturb the chemical equilibrium among different species of heavy metals. By re-introducing the remediated soil into the environment, we expose it to abiotic and biotic environmental factors (soil ageing factors), which could eventually initiate the transition of the residual heavy metals back to more labile forms to re-establish the disturbed equilibrium. * Corresponding author. Tel.: +386 01 423 1161; fax: +386 01 423 1088. E-mail address: [email protected] (D. Lestan). 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.11.013
Such a shift would increase the toxicity of the residual heavy metals and, consequently, decrease the final efficiency of soil remediation. Our previous studies on the impact of biotic environmental soil ageing factors using different earthworm species as models showed that they do in fact cause the residual heavy metals to increase their mobility and availability (Udovic and Lestan, 2007; Udovic et al., 2007). A logical question followed: do abiotic environmental ageing factors also affect the residual heavy metal fraction? The aim of our study was to evaluate the effect of ageing on the fractionation and mobility/availability of Pb, Zn and Cd in soil after remediation with EDTA leaching. To the authors’ knowledge, sparse literature is available on the effects of soil ageing on remediated soil. In a similar study on ageing effect, which simulated high temperatures, Lacal et al. (2003) showed that simulations can to a certain extent predict the long or medium term toxicity of heavy metals in pyritic sludge. Lock and Janssen (2002) exposed artificial soil spiked with Zn to sequences of different ageing treatments: storage at 20 °C, percolation with deionized water, heating at 60 °C, and freezing at –20 °C. No effect of ageing on Zn speciation and ecotoxicity for enchytraeid worm Enchytraeus albidus was detected in their study. It has been recognized that soil contaminants are not 100% available to organisms and that heavy metal bioavailability in soils
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is largely dependent on the partition of the metals (Rieuwerts et al., 1998; Rodriguez et al., 1999; Mulligan et al., 2001). Pb, Zn and Cd fractionation and availability to plants (hereinafter: phytoavailability) and Pb oral-bioavailability are therefore assessed to detect possible changes due to simulated ageing processes.
2. Materials and methods 2.1. Soil and soil analysis Lead smelting and mining activity in the Upper Mezˇica Valley in Slovenia (x = 489 300 m and y = 152 300 m, Gaub-Krüger coordinate system) ceased in 1990, after more than three centuries of uninterrupted activity. Soil was collected from the upper 30 cm layer of a regularly managed vegetable garden near an abandoned lead smelter in this valley. For standard pedological analysis, the pH in soils was measured in a 1/2.5 (w/v) ratio of soil and 0.01 M CaCl2 water solution suspension. Soil samples were analyzed for organic matter by Walkley-Black titrations, cation exchange capacity by the ammonium acetate method, soil texture by the pipette method, easily extractable P was determined colorimetrically according to the Egner-Doming method, and carbonates manometrically after soil reaction with HCl (Kalra and Maynard, 1991). Pedological analysis of non-leached soil was performed in triplicate. The following soil properties were determined: pH 6.6–7.0, organic matter 11.3 ± 0.4%, total N 0.48 ± 0.01%, carbonate 20.2 ± 1.5%, sand 42 ± 1%, silt 47.5 ± 1.4%, clay 10.5 ± 0.4%, P2O5 133 ± 1.8 mg 100 g 1 and K2O 28.2 ± 1.8 mg 100 g 1. 2.2. Soil leaching Air-dried soil (4.6 kg per column) was sieved through a 5-mm mesh sieve and placed in 15-cm diameter soil columns 27 cm high. Plastic mesh (D = 0.2 mm) at the bottom of the column retained the soil. The soil was first leached with different EDTA concentrations; 2.5, 5, 10, 20, 40, and 4-consecutive steps of 40 mmol kg 1 EDTA in 3.1 L of tap water. The leaching EDTA solution was circulated through the soil columns in a closed loop for 24 h using a peristaltic pump (flow rate 15 mL min 1) to mobilize and remove Pb, Zn and Cd as water-soluble heavy metals–EDTA complexes. Each column was then rinsed with 80 L of pure tap water in an open loop (flow rate 15 mL min 1) to remove mobilized heavy metal species and EDTA. EDTA in the rinsing solutions was determined spectrophotometrically according to the procedure of Hamano et al. (1993). The most intensively treated soil was leached in four consecutive steps (4 40 mmol kg 1 EDTA). In each leaching step, the EDTA solution was circulated through the soil column in a closed loop for 24 h, a new leaching step with a new 40 mmol kg 1 EDTA dosage. Rinsing with tap water was performed as described above. 2.3. Laboratory simulation of the ageing processes This ageing treatment was designed to simulate the ageing process that could be induced in soil by long exposures to high temperatures in summer and low temperatures in winter. The soil samples were air dried and sieved (2 mm). Ageing processes were simulated for two moisture conditions, 10 and at 90% of soil water holding capacity. Equal amounts of air dried remediated and nonremediated soil (75 g) were placed in Teflon sealed pressure-proof vessels commonly used for microwave digestion, moistened with tap water and the vessels carefully sealed. Ageing was simulated with five repetitive cycles consisting of 5-d exposure to 105 °C in an oven followed by 5-d exposure to 20 °C in a freezer. A separate vessel was equipped with a manometer and air pressure measured during the ageing treatment. Total pressure in the vessel at 105 °C
was 135 kPa. The aged soil samples were air dried, homogenized and sieved for further analysis. 2.4. Six-step sequential extraction A modified Tessier’s sequential extraction procedure (Lestan et al., 2003) was used to determine the fractionation of Pb, Zn and Cd in soil and in aged soil. The water soluble fraction in the soil solution was extracted from 1 g of air dried non-aged and aged soil sieved to 2 mm, with 10 mL of deionized water for 1 h. The exchangeable fraction from soil colloids to the soil solution was extracted from the residual soil sample with 10 mL of 1 M MgNO3 for 2 h. The fraction bound to carbonates was extracted with 10 mL of 1 M NH4OAc (pH 5) for 5 h. The fraction bound to Fe and Mn oxides was extracted with 20 mL of 0.1 M NH2OH HCl (pH 2) for 12 h. The fraction bound to organic matter was obtained after heating the soil suspension in 3 mL of 0.02 M HNO3 and 5 mL of 30% H2O2 for 3 h at 85 °C, followed by extraction with 15 mL of 1 M NH4OAc for 3 h. The last, residual fraction was obtained after digestion of the residual samples with aqua regia. Three determinations of Pb, Zn and Cd concentration were done for each fractionation sequence. The final fractional recovery of Pb, Zn and Cd was calculated after summing the concentrations of all six steps of sequential extractions and comparing them with the total amount of Pb, Zn and Cd in the relevant soil. 2.5. Toxicity characteristic leaching procedure The mobility and leachability of Pb, Zn and Cd in soil and in aged soil was determined using TCLP analysis (US EPA, 1995). Analyses were conducted in triplicate. The procedure involves shaking a 1 g soil sample, sieved to 2 mm, in 20 mL of 0.0992 M acetic acid and 0.0643 M NaOH extraction solution (1:20 ratio) with a pH of 4.93 ± 0.05, for 18 h on a rotary shaker at about 300 rpm. After the reaction period, the contents were filtered (Sartorius filter paper, pore size 2–3 lm), the filtrate acidified with 65% HNO3 to pH < 2 and stored at 5 °C for heavy metal determination. The extractions were conducted in triplicate. 2.6. Heavy metal phytoavailability Heavy metal phytoavailability was assessed after diethylenetriaminepentaacetic acid (DTPA) extraction, designed to extract simultaneously plant available Zn, Fe, Mn and Cu in near-neutral and calcareous soil (Lindsay and Norvell, 1978), as well as to determine potential metal ecotoxicity (Conder et al., 2001; Cheng and Wong, 2002; Ma et al., 2002). The DTPA extraction solution was prepared to contain 0.005 M DTPA, 0.01 M CaCl2, and 0.1 M triethanolamine (TEA) and was adjusted to pH 7.30 ± 0.05. The procedure involves shaking a 5 g soil sample, sieved to 2 mm, in 10 mL of DTPA extracting solution for 2 h on a horizontal shaker at about 120 cycles min 1. After the extraction period, the contents were filtered (Whatman No. 42 filter paper) and the filtrates analyzed for Pb, Zn and Cd. The extractions were conducted in triplicate. 2.7. Lead oral-bioavailability Children ingest more soil and dust particles than adults, due to their mouthing behavior, i.e. 0.135 g d 1 as estimated by the US Environmental Protection Agency (Ruby et al., 1996), and are thus more exposed to soil pollutants (Davis and Mirick, 2006). The physiologically based extraction test (PBET) used here is designed around pediatric gastrointestinal tract parameters for a child 2–3 years old, as described in Ruby et al. (1996). The PBET includes two phases. Firstly, 0.4 g of sieved soil sample (250 lm) is digested in a reaction flask for 1 h at constant temperature (37 °C) in simu-
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lated gastric fluid (40 mL) prepared by adjusting 1 L of deionized water to pH 2.50 ± 0.05 with diluted HCl and adding 1.25 g of pepsin (porcine, Sigma), 0.50 g of citrate, 0.50 g of malate, 420 lL of lactic acid and 500 lL of acetic acid. The pH of the reaction mixture was measured every 10 min and adjusted with HCl as necessary to keep it at a value of 2.50 ± 0.05. Samples (3 mL each) were collected after 1 h, and centrifuged at 2500 rpm for 25 min. The liquid fraction was decanted for further analysis. The 3 mL sample volume was replaced with gastric solution to maintain a constant volume in the reaction flask. After 1 h, the reaction was titrated to pH 7 by adding a 10 cm long dialysis bag (8000 MWCO, Spectra/Por cellulose ester tubing) containing approximately 1 g of NaHCO3 and 3 mL of deionized water. When the reaction vessel reached
Table 1 Total concentration of Pb, Zn and Cd in soil before and after remediation with leaching with different EDTA concentrations. Results are presented as means of three replicates ±SD. Soil Before leaching After leaching 2.5 mmol kg 1 EDTA 5 mmol kg 1 EDTA 10 mmol kg 1 EDTA 20 mmol kg 1 EDTA 40 mmol kg 1 EDTA 40 (4) mmol kg 1 EDTA
Pb (mg kg
1
)
Zn (mg kg
1
)
Cd (mg kg
1
)
4603 ± 94
1826 ± 61
30.4 ± 0.3
4323 ± 169 3998 ± 138 2712 ± 207 2522 ± 85 2112 ± 122 1239 ± 9
1774 ± 102 1756 ± 36 1599 ± 118 1560 ± 64 1464 ± 120 1402 ± 94
25.3 ± 1.9 20.4 ± 0.6 14.1 ± 0.7 12.0 ± 0.5 10.2 ± 1.5 7.8 ± 0.3
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equilibrium at pH 7, 70 mg of bile salts (porcine, Sigma) and 20 mg of pancreatin (porcine, Sigma) were added, thus simulating small intestine conditions. After 1 h, the reaction solutions were centrifuged at 2500 rpm for 25 min. The liquid fraction was decanted and analyzed as the small intestine fraction. During both phases, a constant moistened argon flow (1 L min 1) at 37 °C was conducted through the reaction mixture in order to simulate peristalsis. The PBET was conducted in triplicate. 2.8. Heavy metal determination Air-dried samples of soil and aged soil (3 g) were ground in an agate mill, digested in boiling aqua regia (28 mL), diluted with deionized water up to 100 mL, and Pb, Zn and Cd analyzed by flame (acetylene/air) (Perkin–Elmer 1100-B, Norwalk, USA). Pb, Zn and Cd in extracts (leaching solutions, rinsing water, solutions from sequential extractions, PBET, TCLP and DTPA solutions) were determined by AAS directly (acetylene/air) (Varian AA240FS). A standard reference material used in inter-laboratory comparisons (WEPAL 2003.1.1), from HBLFA Raumberg-Gumpenstein, Irdning, Austria, was used in the digestion and analysis as part of the QA/ QC protocol. The recovery percentages were 106 ± 6, 108 ± 13 and 105 ± 30. The limits of quantification (LOQ) were set at the lowest limit of the optimum working range of the apparatus as specified in the Varian’s Manual of AnalyticalMethods (1989). They were 0.1, 0.01 and 0.02 mg L 1 for Pb, Zn and Cd, respectively. Reagent blank and analytical duplicates were also used to ensure accuracy and precision in the analysis.
Fig. 1. Fractionation of Pb before and after soil leaching, in the soil and in the soil aged at 10% and 90% water holding capacity (WHC). Results are presented as means of three replicates ±SD. The superscript letters (a, b, ab, c) denote statistically different fractionations within soil and soil aged at 10% and 90% WHC, respectively. LOQ, below limit of quantification.
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2.9. Statistical analysis A Student’s t-test (P < 0.05) for paired data sets and Duncan’s multiple range test (P < 0.05) (Statgraphics 4.0 for Windows) were used to determine differences between soil and respective aged soil properties. 3. Results 3.1. Soil: leaching with EDTA The purpose of soil leaching was to remove the mobile and available (toxic and health threatening) heavy metals fraction, leaving the less mobile and non-accessible (i.e. non-toxic) forms in the soil. By using leaching solutions that contained five different EDTA concentrations ranging from 2.5 to 4-consecutive steps of 40.0 mmol kg 1 (total 160 mmol kg 1) EDTA, we reached a gradient in removed heavy metals: 6%, 13%, 41%, 45%, 54% and 73% of initial Pb, 3%, 4%, 13%, 15%, 20% and 23% of initial Zn and 17%, 33%, 54%, 61%, 66% and 74% of initial Cd (Table 1). After the leached soil was rinsed with excessive amounts of clean tap water, no Pb, Zn, Cd or EDTA were present in the final solution. The increasing removal of Pb, Zn and Cd from the soil (gained by the increasing EDTA concentration in the leaching solutions) was reflected in decreasing fractionation, mobility, phytoavailability and oral-bioavailability of soil residual heavy metals. Six-step sequential extraction showed that most of the Pb was removed from the fraction bound to carbonates, Mn and Fe-oxides and organic matter (Fig. 1). Zn was mostly present in the residual fraction (Fig. 2), thus explaining the lower effectiveness of EDTA,
compared to Pb and Cd. Zn was however consistently removed from the exchangeable fraction and from the fraction bound to carbonates, Fe and Mn-oxides and organic matter. Cd was considerably removed from all fractions, but mostly from the fraction bound to carbonates, by factors up to 8.3 (Fig. 3). The mobility of Pb, Zn and Cd decreased, up to factor 8.6 for Zn and even below the LOQ in the case of Pb and Cd (Tables 2 and 3). The initial values, however, were lower than the limits set by US EPA, i.e. 5 mg L 1 for Pb, 250 mg L 1 for Zn and 1 mg L 1 for Cd (US EPA, 1995). The phytoavailability of Pb, Zn and Cd determined by DTPA extraction decreased up to factors 12.2, 10.5 and 20.6, respectively, with the increasing EDTA concentration in the leaching solution (Tables 2 and 3). Since the authors of the in vitro PBET used validated the test with in vivo animal models only for Pb and As (Ruby et al., 1996), an assessment of Zn and Cd oral-bioavailability was not performed. Pb oral-bioavailability decreased after leaching, up to factor 8 in the stomach phase and up to factor 6.1 in the small intestine phase in the soil, with altogether 73% of total Pb removed (Table 2). 3.2. Effect of ageing on soil pH and Pb, Zn and Cd fractionation After ageing simulation treatments had been applied to the leached and non-leached soil, the soil pH significantly (P < 0.05) decreased up to factor 1.1 in both moisture treatments (soil aged at 10 and at 90% soil water holding capacity) (data not shown). Significant changes in Pb, Zn and Cd fractionation were observed after the ageing. However, we observed only random differences between the two ageing treatments, indicating that the difference in soil moisture did not significantly affect the ageing
Fig. 2. Fractionation of Zn before and after soil leaching, in the soil and in the soil aged at 10% and 90% water holding capacity (WHC). Results are presented as means of three replicates ±SD. The superscript letters (a, b, ab, c) denote statistically different fractionations within soil and soil aged at 10% and 90% WHC, respectively. LOQ, below limit of quantification.
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Fig. 3. Fractionation of Cd before and after soil leaching, in the soil and in the soil aged at 10% and 90% water holding capacity (WHC). The first sequence is not presented, since all the values are below the limit of quantification (LOQ). Results are presented as means of three replicates ±SD. The superscript letters (a, b, ab, c) denote statistically different fractionations within soil and soil aged at 10% and 90% WHC, respectively.
processes (Figs. 1–3). Pb concentration in the water soluble and exchangeable fractions significantly increased after the ageing treatment, as well as concentrations in the fraction bound to organic matter (P < 0.05). On the other hand, Pb concentration decreased up to factor 1.6 in the fraction bound to carbonates and below the limit of quantification in the fraction bound to Fe and Mn-oxides (Fig. 1). Compared to the non-aged, Zn concentration in the aged soils significantly (P < 0.05) decreased in the carbonate, Fe and Mn-oxides and organic matter fractions, but increased in the water soluble and residual fractions (Fig. 2). After ageing, the Zn concentration in the exchangeable fraction increased (P < 0.05) only in the nonremediated soil and in the soil with the least amount of Zn removed (17%). On the other hand, in soils treated with high EDTA concentrations, Zn concentration decreased in soils after ageing (P < 0.05). In general, Cd fractionation was similar to Pb. However, differences (P < 0.05) in fractionation between the two ageing treatments were observed in Fe and Mn-oxides and in the residual fraction. The transbound of Cd in the residual fraction was significantly higher (P < 0.05) in the soil moistened to 10% of water holding capacity than in that moistened to 90% of water holding capacity (Fig. 3).
The mobility of Pb increased (P < 0.05) up to factor 1.8 in the soil with 61% Pb removed (Table 2). The mobility of Zn remained unaltered in non-leached soil and in soils leached with 2.5, 5.0 and 10.0 mmol EDTA kg 1. In soils leached with higher EDTA concentrations, Zn mobility increased after ageing up to factor 1.6 (Table 3). The Pb, Zn and Cd phytoavailability in aged soils decreased up to factors 3.8, 4.3 and 5.4, respectively (Tables 2 and 3). Differences (P < 0.05) in Pb, Zn and Cd mobility between the two ageing treatments were observed only in non-leached and in soil leached with the lowest EDTA concentration. Differences in Pb oral-bioavailability between non-aged and aged soils fluctuated (Table 2). A trend of decreasing Pb oral-bioavailability in the intestinal phase was observed after ageing. However, due to large standard deviations, the results are not statistically significant. The two applied ageing treatments (soils aged at 10 and 90% water holding capacity) generally did not affect heavy metal behavior. Differences between the two ageing treatments were observed only for Zn: for soils leached with low EDTA concentrations, ageing at 90% water holding capacity increased Zn mobility and oral-bioavailability. The trend was opposite for soils leached with high EDTA concentrations (Table 3).
3.3. Effect of ageing on Pb, Zn and Cd mobility, phytoavailability and Pb oral-bioavailability
4. Discussion
The three investigated heavy metals in aged soils behaved differently in terms of their mobility. The mobility of Cd decreased below the limit of quantification in all leached soils (Table 3).
The lower percentage of Zn removed from soil in all EDTA leaching treatments compared to the removal of Pb and Cd (Table 1) could be explained by the different lability of complexed and bound metals in soil (Sun et al., 2001) and by differences in the
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Table 2 Pb oral-bioavailability, mobility and phytoavailability in aged and non-aged soil, before and after leaching with different EDTA concentrations, assessed with a PBET test, by TCLP procedure and by DTPA extraction. Results are presented as means of three replicates ±SD, different superscript letters denote significant differences (Duncan, P < 0.05). Comparison between non-aged soil and soil aged at two moistures (10% and 90% water holding capacity, WHC) is shown for each treatment. LOQ, below limit of quantification. PBET Pb (mg kg
DTPA
After leaching 2.5 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC) 5.0 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC)
Small intestine phase
a
ab a
761 ± 48 550 ± 91 b 509 ± 151
a
914 ± 351 459 ± 71 ab 601 ± 104 b
a
DTPA (mg kg
)
b
306 ± 36 385 ± 14 a 451 ± 122 a
328 ± 64 368 ± 27 b 211 ± 22
Pb (mg kg
1
a
952 ± 3 551 ± 62 b 425 ± 112 b
)
Pb (mg L
1.1 ± 0.0 1.7 ± 0.1 b 1.5 ± 0.1
a
a
b
b
a
215 ± 98 241 ± 27 ab 218 ± 35 b
a
901 ± 19 483 ± 55 b 397 ± 89 b
20.0 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC)
a
a
ab
a
276 ± 28 378 ± 17 b 493 ± 86
127 ± 60 163 ± 43 a 160 ± 26
b
1.0 ± 0.0 1.4 ± 0.1 1.4 ± 0.1
a
0.8 ± 0.1 1.3 ± 0.1 b 1.4 ± 0.1 b
a
a
b
b
646 ± 46 428 ± 17 b 358 ± 105
a
a
ab
a
a
a
292 ± 77 214 ± 19 a 217 ± 69
133 ± 63 116 ± 34 a 153 ± 57
277 ± 13 322 ± 75 b 193 ± 78
0.7 ± 0.0 1.1 ± 0.1 1.0 ± 0.1
b
a
0.4 ± 0.0 0.6 ± 0.0 b 0.7 ± 0.1 b
1
40.0 mmol kg EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC)
a
238 ± 90 192 ± 16 a 177 ± 55 a
4 40.0 mmol kg 1 EDTA a 95 ± 4 Soil a 108 ± 13 Aged soil (10% WHC) a 144 ± 31 Aged soil (90% WHC)
a
a
b
a
40 ± 24 93 ± 4 b 91 ± 21 a
54 ± 27 50 ± 13 a 53 ± 4 a
212 ± 1 239 ± 8 a 213 ± 26 a 78 ± 5 158 ± 22 a 115 ± 38
b
1
)
1
)
DTPA (mg L
1
)
TCLP (mg L
1
)
Before leaching Soil Aged soil (10% WHC) Aged soil (90% WHC)
a
230 ± 16 b 99 ± 15 c 59 ± 15
a
189 ± 4 b 95 ± 21 c 45 ± 5
a
2±0 2±0 a 2±0 a
a
12 ± 1 b 3±1 b 2±1
0.1 ± 0.0 LOQ LOQ
a
a
987 ± 5 546 ± 102 c 260 ± 42
)
b
a
170 ± 78 232 ± 24 a 218 ± 33
Cd TCLP (mg kg
1
1
10.0 mmol kg EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC)
Zn
TCLP
1
Stomach phase Before leaching Soil Aged soil (10% WHC) Aged soil (90% WHC)
Table 3 Zn and Cd mobility and phytoavailability in aged and non-aged soil, before and after leaching with different EDTA concentrations assessed by TCLP procedure and by DTPA extraction. Results are presented as means of three replicates ±SD, different superscript letters denote significant differences (Duncan, P < 0.05). Comparison between non-aged soil and soil aged at two moistures (10% and 90% water holding capacity, WHC) is shown for each treatment. LOQ, below limit of quantification.
a a
0.4 ± 0.0 0.4 ± 0.0 0.5 ± 0.1
b
LOQ LOQ LOQ
kinetics of metal desorption/dissolution (Finzgar and Lestan, 2006). The majority of Zn was present in the residual fraction (Fig. 2), which also explains why EDTA was not particularly effective in Zn removal (Udovic et al., 2007). The mobility and hence the availability of metals in soil depends on their concentration in the soil solution and on the characteristics and ability of the soil to release them from the solid phase to compensate for those removed from the solution (Backes et al., 1995). Soil ageing processes alter the soil characteristics and thus potentially affect the residual heavy metal fraction left in the soil after its remediation. Simulations of soil ageing processes are a useful tool for indicating possible changes in the time of metal mobility and availability in soil (Lacal et al., 2003), which are important in assessing the long-term effect of soil remediation. Soil pH plays a crucial role in the adsorption-desorption behavior of heavy metals in soil (Basta and Tabatabai, 1992), which is inversely proportional to the soil pH (Rieuwerts et al., 1998; Adriano, 2001; Cao et al., 2001). The decrease in soil pH after ageing, probably due to high temperature enhanced oxidation processes (Lacal et al., 2003), could to some extent explain the substantial increase in Pb, Zn and Cd concentrations in water-soluble and exchangeable fractions (Figs. 1–3). This fractionation pattern could also be attributed to the effect of the temperature used in the ageing simulation process (105 °C). The rates of metal desorption from Fe and Mnoxides at ambient temperature are slower than the absorption rates (McBride, 1991). Using repetitive high temperature treat-
After leaching 2.5 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC) 5.0 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC) 10.0 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC) 20.0 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC) 40.0 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC) 4 40.0 mmol kg 1 EDTA Soil Aged soil (10% WHC) Aged soil (90% WHC)
a
a
2±0 2±0 b 1±0
162 ± 2 b 55 ± 20 b 58 ± 12
a
0.1 ± 0.0 LOQ LOQ
a
a
b
b
0.1 ± 0.0 LOQ LOQ
1±0 2±0 a 1±0 a
108 ± 7 b 56 ± 1 b 59 ± 15
a
9±0 3±1 b 2±1
a
1±0 1±0 a 1±0
b
b
7±0 2±1 2±1
a
a
b
4±0 2±0 b 2±0
LOQ LOQ LOQ
a
a
1±0 1±0 a 1±0
a
a
a
a
2±0 1±0 1±1
LOQ LOQ LOQ
0.4 ± 0.0 b 1±0 b 1±0
a
1±0 1±0 a 1±0
LOQ LOQ LOQ
a
0.3 ± 0.0 0.4 ± 0.0 b 0.4 ± 0.1
a
b
a
LOQ LOQ LOQ
51 ± 4 50 ± 12 a 40 ± 21
ab
41 ± 2 45 ± 2 b 37 ± 3
a
a
a
22 ± 2 33 ± 6 ab 24 ± 8 b
a
a
a
1±0 1±0 1±0
ments, the desorption rate apparently increased and led to the mobilization of Pb, Zn and Cd into more available chemical forms (fractions) ( Figs. 1–3). The decrease in carbonate-bound Pb, Zn and Cd can be explained by the behavior of the organic matter soil constituents (e.g. humic acids), which enhance heavy metal mobility and solubility by complexation (Weng et al., 2002). Reacting with the heavy metals bound to carbonates, they can form more soluble carboxylates, which can consequently already be extracted in the first two fractions (Udovic, personal communication). In the process of hydrolysis, heavy metal complexes and heavy metal hydroxy-complexes are formed, especially at neutral and higher pH values (Basta and Tabatabai, 1992; Rieuwerts et al., 1998), which are usually poorly soluble and precipitate in the soil solution (Basta and Tabatabai, 1992; Bourg, 1995). Reaction rates are known to be exponentially enhanced by temperature increase. However, they depend also on the characteristics of the extraction solution. The TEA in the extracting solution used in the assessment of heavy metal phytoavailability (DTPA extraction has a close to neutral pH (7.30), which is higher than the pH of the aged soil (ranging from 6.10 to 6.27), and thus also enhances the formation of heavy metal-hydroxides (Rieuwerts et al., 1998). This could explain the large decrease in Pb, Zn and Cd phytoavailability (Table 2 and 3) contrasting the increased concentration in the water soluble and exchanging fractions. The results of TCLP extraction (Tables 2 and 3) indicate that the ageing processes differently affect Pb, Zn and Cd forms present in soil. Apparently, a lower pH value of the TCLP extracting solution (pH 4.93 ± 0.05; US EPA, 1995) and a high temperature cycle de-
M. Udovic, D. Lestan / Chemosphere 74 (2009) 1367–1373
crease the solubility of Cd, but increase the solubility of Pb. However, additional studies are needed to make more considerable conclusions. 5. Conclusions The behavior of the residual heavy metal forms left in the soil after its remediation is largely unknown. Reintroduction of such soil to the environment involves exposing it to natural ageing processes that could affect the residual, non-labile and non-toxic metallic forms, transforming them into more labile forms. To the authors’ knowledge, no studies have been made on the effect of soil ageing on soils remediated by leaching with chelants. The results obtained in this study indicate that abiotic ageing processes do in fact affect the residual heavy metal fractions by changing their lability. The results also show that Pb, Zn and Cd behave differently. Further studies are needed better to understand the underlying mechanisms and fully to evaluate the final results of remediation. A laboratory ageing simulation tool such as that we used in our study or similar would simplify such future evaluations. Acknowledgements The authors are grateful to Dr. Boris Udovic for constructive discussions. This work was supported by the Slovenian Ministry for Education, Science and Sport, Grant J4 9277. References Adriano, D.C., 2001. Trace Elements in Terrestrial Environments; Biogeochemistry. Bioavailability and Risks of Metals. Springer-Verlag, New York. Backes, C.A., McLaren, R.G., Rate, A.W., Swift, R.S., 1995. Kinetics of cadmium and cobalt desorption from iron and manganese oxides. Soil Sci. Soc. Am. J. 59, 778– 785. Basta, N.T., Tabatabai, M.A., 1992. Effect of cropping systems on adsorption of metals by soils: II. Effect of pH. Soil Sci. 153, 195–204. Bourg, A.C.M., 1995. Speciation of heavy metals in soils and groundwater and implications for their natural and provoked mobility. In: Salomon, W., Förstner, U., Mader, P. (Eds.), Heavy Metals. Problems and Solutions. Springer-Verlag, Germany, pp. 19–31. Cao, X., Chen, Y., Wang, X., Deng, X., 2001. Effects of redox potential and pH value on the release of rare elements from soil. Chemosphere 44, 655–661. Cheng, J., Wong, M.H., 2002. Effect of earthworms of Zn fractionation in soils. Biol. Fert. Soils 36, 72–78. Conder, J.M., Lanno, R.P., Basta, N.T., 2001. Assessment of metal availability in smelter soil using earthworms and chemical extractions. J. Environ. Qual. 30, 1231–1237. Davis, S., Mirick, D.K., 2006. Soil ingestion in children and adults in the same family. J. Expo. Sci. Env. Epid. 16, 63–75. Finzgar, N., Lestan, D., 2006. Heap leaching of Pb and Zn contaminated soil using ozone/UV treatment of EDTA extractants. Chemosphere 63, 1736–1743.
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