The effect of earthworms on the fractionation, mobility and bioavailability of Pb, Zn and Cd before and after soil leaching with EDTA

The effect of two ecologically contrasting earthworm species Eisenia fetida (epigeic) and Octolasion tyrtaeum (endogeic) on the fractionation (accessed using sequential extractions), mobility (toxicity characteristic leaching procedure, TCLP) and oral bioavailability (Ruby's physiologically based extraction test, PBET) of Pb, Zn and Cd was studied before and after soil remediation with soil leaching. Twenty-step leaching, with 2.5 mmol kg(-1) EDTA used in each step, removed 58.4%, 25.0% and 68.0% of initial soil Pb, Zn and Cd, respectively, shifted the fractionation of residual heavy metals toward less labile forms, and decreased their mobility by 83.7%, 80.3%, and 90.9%. Pb oral bioavailability was reduced by 3.1-times (in each stomach and intestinal phase). After soil leaching, both earthworm species enriched the carbonate soil fraction in their casts with residual Pb, and increased the Pb bioavailability in the simulated intestinal phase by a factor of 2.4 (E. fetida) and 2.8 (O. tyrtaeum). The concentration of Pb in TCLP leachate from E. fetida casts was 6.2-times higher than in the bulk of the remediated soil. These results indicate that the effect of biotic factors on the availability of heavy metals residual in soil after soil leaching requires consideration.     

A comparison of physiologically based extraction test (PBET) and single-extraction methods for release of Cu, Zn, and Pb from mildly acidic and alkali soils

In vitro digestion test can be applied to evaluate the bioaccessibility of soil metals by measuring the solubility of the metals in synthetic human digestive tract. Physiologically based extraction test (PBET), composed of sequential digestion of gastric and intestinal phase, is one of the frequently used in vitro digestion tests. In this study, the PBET was chosen to determine the bioaccessibility of Cu, Zn, and Pb in 14 mildly acidic and alkali (pH 5.87–8.30) soils. The phytoavailability of Cu, Zn, and Pb in the same soils was also measured using six single-extraction methods (0.1 M HNO₃, 0.4 M HOAc, 0.1 M NaNO₃, 0.01 M CaCl₂, 0.05 M EDTA, and 0.5 M DTPA). The extraction efficiencies of the methods were compared. The PBET had a strong ability to extract metals from soil, which was much greater than neutral salt extraction and close to dilute acid and complex extraction in spite of the last 2 h neutral intestinal digestion. The amounts of bioaccessible Cu, Zn, and Pb in the gastric phase and in the gastrointestinal phase were both largely determined by the total content of soil Cu, Zn, and Pb. But the results of gastrointestinal digestion reflected more differences resulting from element and soil types than those of gastric digestion did. It was noticed that most of variations in the amounts of soil Cu, Zn, and Pb extracted by EDTA were well explained by the total soil Cu, Zn, and Pb, as same as the PBET. Moreover, the solubility of Cu, Zn, and Pb in the gastric phase and gastrointestinal phase were all positively linearly correlated with the results of EDTA. It was suggested that EDTA extraction can be used to predict the bioaccessibility of Cu, Zn, and Pb in mildly acidic and alkali (pH?>?5.8) soils, and the PBET and EDTA could be applied to measure, in a certain extent, the bioaccessibility and phytoavailability of Cu, Zn, and Pb in mildly acidic and alkali (pH?>?5.8) soils at the same time.     

Multi-step leaching of Pb and Zn contaminated soils with EDTA

The efficiency of multi-step leaching of heavy metal contaminated soils was evaluated in a laboratory scale study. Four different soils contaminated with Pb (1136+/-16-4424+/-313mgkg(-1)) and Zn (288+/-5-5489+/-471mgkg(-1)) were obtained from industrial sites in the Mezica Valley, Slovenia and Príbram district, Czech Republic. Different dosages (2.5-40mmolkg(-1)) of ethylenediamine tetraacetate (EDTA) were used to treat soils in 1-10 leaching steps. Higher EDTA dosages did not result in a proportional gain in Pb and Zn removal. EDTA extracted Pb more efficiently than Zn from three of four tested soils. The percentage of removed Zn did not exceed 75% regardless of the soil, EDTA dosage and leaching steps. Significantly more Pb (in three of four soils) and Zn were removed from soils when the same amount of EDTA was applied in several leaching steps. The interference of major soil cations Fe and Ca with EDTA complexation as a possible factor affecting Pb and Zn removal efficiency with multi-step heap leaching was examined and is discussed. The results of our study indicate that, for some soils, using multi-step leaching instead of the more traditionally used single dose EDTA treatment could improve heavy metal removal efficiency and thus the economics of soil remediation.     

The effect of earthworms on the fractionation and bioavailability of heavy metals before and after soil remediation

The effect of two earthworm species, Lumbricus rubellus and Eisenia fetida, on the fractionation/bioavailability of Pb and Zn before and after soil leaching with EDTA was studied. Four leaching steps with total 12.5 mmol kg(-1) EDTA removed 39.8% and 6.1% of Pb and Zn, respectively. EDTA removed Pb from all soil fractions fairly uniformly (assessed using sequential extractions). Zn was mostly present in the chemically inert residual soil fraction, which explains its poor removal. Analysis of earthworm casts and the remainder of the soil indicated that L. rubellus and E. fetida actively regulated soil pH, but did not significantly change Pb and Zn fractionation in non-remediated and remediated soil. However, the bioavailability of Pb (assessed using Ruby's physiologically based extraction test) in E. fetida casts was significantly higher than in the bulk of the soil. In remediated soil the Pb bioavailability in the simulated stomach phase increased by 5.1 times.     

Heap leaching of Pb and Zn contaminated soil using ozone/UV treatment of EDTA extractants

The feasibility of a novel EDTA-based soil heap leaching method with treatment and reuse of extractants in a closed process loop was evaluated on a laboratory scale. Ozone and UV irradiation were used for oxidative decomposition of EDTA-metal complexes in extractants from Pb (1243 mg kg(-1)) and Zn (1190 mg kg(-1)) contaminated soil. Released metals were absorbed in a commercial metal absorbent Slovakite. Six-consecutive additions of 2.5 mmol kg(-1) EDTA (total 15 mmol kg(-1) EDTA) removed 49.6 +/- 0.6% and 19.7 +/- 1.7% of initial total Pb and Zn from soil (4.6 kg) packed in 22 cm high columns. The efficiency of extraction was similar to small-scale simulations of heap leaching (15 0 g of soil), where EDTA used in the same manner removed 49.7 +/- 1.0% and 13.7 +/- 0.4% of Pb and Zn. The new heap leaching method produced discharge extractant with fairly low final concentrations of Pb, Zn and EDTA (1.98 +/- 2.17 mg l(-1), 4.55 +/- 2.36 mg l(-1), and 0.05 +/- 0.04 mM, respectively), which could presumably be reduced even further with continuation of treatment. The results of our study indicate that for soils contaminated primarily with Pb, treating the EDTA extractants with ozone/UV and reuse of extractants enables efficient soil heap leaching with very little or no wastewater generation, easy control over emissions, and lowers the requirements for process water.     

Metal associations in soils before and after EDTA extractive decontamination: implications for the effectiveness of further clean-up procedures

The distribution of Pb, Ni and Zn in two contaminated soils was determined before and after treating the soils with an EDTA solution. After the EDTA extraction, the proportion of Pb accumulated in the acid-extractable fraction considerably increased, which was related to the greater degree of metal extraction from the other fractions. EDTA was also able to extract certain amounts of Pb, Zn and Ni from the silicate matrix, which implied that these extractable amounts were not so strongly fixed to the residual fraction as previously supposed. As a consequence, after EDTA application, metal content (especially Pb) remained more weakly adsorbed to soil components (more easily leachable), potentially favouring the application of phytoremediation technologies. The extraction recoveries (for only one application) were generally low for the three metals (33-37% for Pb, 5-11% for Ni and 14-19% for Zn), although this fact is an advantage as plants would not be able to assimilate very high mobilised contents of metals.     

The effect of municipal sludge compost on the mobility and bioavailability of Cd in a sierozem-wheat system in an arid region northwest of China

The effect of sewage sludge on the mobility and the bioavailability of trace metals in plant-soil systems have aroused wide interested and been widely explored. Based on a wheat-cultivating experiment, the effect of municipal sludge compost (MSC) on the mobility and bioavailability of Cd in a soil-wheat system was studied. With the application of MSC, soil organic matter (SOM), total nitrogen (TN), and total phosphorus (TP) in the soil increased significantly, while concentrations of trace metals (Cu, Zn, Ni, Pb, Cd) were below the China’s minimum thresholds. The application of MSC could improve wheat growth. The application of MSC at the rate of 0.5 % had no significant effect on the chemical fraction distribution of Cd in soil. In two soil treatments, Cd mainly existed in the labile chemical fractions (exchangeable chemical fraction (EXCF) and carbonate chemical fraction (CABF)). However, the application of MSC could reduce accumulation of Cd by wheat. Cd contents in each part of the MSC-applied wheat were significantly less than that of non-MSC-applied wheat. In the tested soils, the extractable concentrations decreased in the order: EDTA > MgCl₂ ≈ NH₄OAc > DTPA. There were no significant differences between soil treatments in the amounts of extractable Cd when the extraction was done under neutral conditions, although significant differences were observed when the extraction was done under alkaline conditions. In this study, the DTPA extraction procedure provided a good indication of Cd bioavailability. Our results suggest that, in the short term at least, amending soils with MSC may benefit crop dry matter production while not increasing the risk of human exposure to Cd through consumption of wheat grown on MSC-amended soils.     

Bioaccumulation in Porcellio scaber (Crustacea, Isopoda) as a measure of the EDTA remediation efficiency of metal-polluted soil

Leaching using EDTA applied to a Pb, Zn and Cd polluted soil significantly reduced soil metal concentrations and the pool of metals in labile soil fractions. Metal mobility (Toxicity Characteristic Leaching Procedure), phytoavailability (diethylenetriaminepentaacetic acid extraction) and human oral-bioavailability (Physiologically Based Extraction Test) were reduced by 85-92%, 68-91% and 88-95%, respectively. The metal accumulation capacity of the terrestrial isopod Porcellio scaber (Crustacea) was used as in vivo assay of metal bioavailability, before and after soil remediation. After feeding on metal contaminated soil for two weeks, P. scaber accumulated Pb, Zn and Cd in a concentration dependent manner. The amounts of accumulated metals were, however, higher than expected on the basis of extraction (in vitro) tests. The combined results of chemical extractions and the in vivo test with P. scaber provide a more relevant picture of the availability stripping of metals after soil remediation.     

Eisenia fetida avoidance behavior as a tool for assessing the efficiency of remediation of Pb, Zn and Cd polluted soil

Remediation by means of soil leaching with ethylenediaminetetraacetic acid (EDTA) is capable of extracting the most labile soil fractions, leaving the residual metals in biologically non-available forms. We evaluated the feasibility of the standardized earthworm (Eisenia fetida) avoidance test for assessing the efficiency of soil remediation of Pb, Zn and Cd polluted soil. Chemical extraction tests (six-step sequential extraction, toxicity characteristic leaching procedure, physiologically based extraction test, diethylenediaminepentaacetic acid extraction) indicated that the mobility, oral bioaccessibility and phytoavailability of Pb, Zn and Cd were consistently reduced. However, the avoidance test showed no significant avoidance of polluted soil in favor of that which had been remediated. Pb, Zn and Cd accumulation in E. fetida mirrored the decreasing pattern of metal potential bioavailability gained by leaching the soil with increasing EDTA concentrations. The calculated bioaccumulation factors indicated the possibility of underestimating the metal bioavailability in soil using chemical extraction tests.     

The EDTA effect on phytoextraction of single and combined metals-contaminated soils using rainbow pink (Dianthus chinensis)

Rainbow pink (Dianthus chinensis), a potential phytoextraction plant, can accumulate high concentrations of Cd from metal-contaminated soils. The soils used in this study were artificially added with different metals including (1) CK: original soil, (2) Cd-treated soil: 10 mg Cd kg(-1), (3) Zn-treated soil: 100 mg Zn kg(-1), (4) Pb-treated soil: 1000 mg Pb kg(-1), (5) Cd-Zn-treated soil: 10 mg Cd kg(-1) and 100 mg Zn kg(-1), (6) Cd-Pb-treated soil: 10 mg Cd kg(-1) and 1000 mg Pb kg(-1), (7) Zn-Pb-treated soil: 100 mg Zn kg(-1) and 1000 mg Pb kg(-1), and (8) Cd-Zn-Pb-treated soil: 10 mg Cd kg(-1), 100 mg Zn kg(-1), and 1000 mg Pb kg(-1). Three concentrations of 2Na-EDTA solutions (0 (control), 2, and 5 mmol kg(-1) soil) were added to the different metals-treated soils to study the influence of applied EDTA on single and combined metals-contaminated soils phytoextraction using rainbow pink. The results showed that the Cd, Zn, Pb, Fe, or Mn concentrations in different metals-treated soil solutions significantly increased after applying 5 mmol EDTA kg(-1) (p<0.05). The metal concentrations in different metals-treated soils extracted by deionized water also significantly increased after applying 5 mmol EDTA kg(-1) (p<0.05). Because of the high extraction capacity of both 0.005 M DTPA (pH 5.3) and 0.05 M EDTA (pH 7.0), applying EDTA did not significantly increase the Cd, Zn, or Pb concentration in both extracts for most of the treatments. Applying EDTA solutions can significantly increase the Cd and Pb concentrations in the shoots of rainbow pink (p<0.05). However, this was not statistically significant for Zn because of the low Zn concentration added into the contaminated soils. The results from this study indicate that applying 5 mmol EDTA kg(-1) can significantly increase the Cd, Zn, or Pb concentrations both in the soil solution or extracted using deionized water in single or combined metals-contaminated soils, thus increasing the accumulated metals concentrations in rainbow pink shoots. The proposed method worked especially well for Pb (p<0.05). The application of 2 mmol EDTA kg(-1) might too low to enhance the phytoextraction effect when used in silty clay soils.     

The use of poplar during a two-year induced phytoextraction of metals from contaminated agricultural soils

The efficiency of poplar (Populus nigra L.xPopulus maximowiczii Henry.) was assessed during a two-year chemically enhanced phytoextraction of metals from contaminated soils. The tested metal mobilizing agents were EDTA (ethylenediaminetetraacetic acid) and NH4Cl. EDTA was more efficient than chlorides in solubilizing metals (especially Pb) from the soil matrix. The application of chlorides only increased the solubility of Cd and Zn. However, the increased uptake of metals after the application of higher concentrations of mobilizing agents was associated with low biomass yields of the poplar plants and the extraction efficiencies after the two vegetation periods were thus comparable to the untreated plants. Additionally, the application of mobilizing agents led to phytotoxicity effects and increased mobility of metals. Higher phytoextraction efficiencies were observed for Cd and Zn compared to Pb and Cu. Poplars are therefore not suitable for chemically enhanced phytoextraction of metals from severely contaminated agricultural soils.     

石灰干化污泥对土壤重金属稳定化处理的效果

以采自湖南省嘉禾县重金属复合污染土壤为研究对象,采用城市污水处理石灰干化污泥作为稳定剂,对污染土壤进行稳定化处理,并采用TCLP和BCR连续提取法对稳定化效果进行分析和评价。研究结果表明,单独使用石灰干化污泥,TCLP浸出浓度随着干化污泥质量分数的增加而显著减少,干化污泥的质量分数为40%时,稳定化率最大为Zn-98.92%、Cd-99.06%、Pb-96.84%;但是干化污泥的高pH值导致稳定后土壤中As的浸出增加。为了恢复植物生长功能,经过亚铁盐和磷酸调节pH后,石灰干化污泥稳定过的土壤pH有效降低,同时亚铁盐和磷酸有利于促进Pb和Zn的稳定效果;但是对Cd的稳定有负面影响;另外,亚铁盐的加入同时可以减少As的浸出浓度。经处理后土壤中重金属形态由不稳定态转为稳定态,使重金属的浸出浓度明显降低,减少了土壤重金属的浸出毒性。该研究结果表明,石灰干化污泥可以作为资源回收利用,应用于重金属污染土壤的修复中,并能改善稳定后土壤适宜植物生长的理化性质。     

Leaching of heavy metals from contaminated soils using EDTA

Ethylenediaminetetraacetic acid (EDTA) extraction of Zn, Cd, Cu and Pb from four contaminated soils was studied using batch and column leaching experiments. In the batch experiment, the heavy metals extracted were virtually all as 1:1 metal-EDTA complexes. The ratios of Zn, Cd, Cu and Pb of the extracted were similar to those in the soils, suggesting that EDTA extracted the four heavy metals with similar efficiency. In contrast, different elution patterns were obtained for Zn, Cd, Cu and Pb in the column leaching experiment using 0.01 M EDTA. Cu was either the most mobile or among the most mobile of the four heavy metals, and its peak concentration corresponded with the arrival of full strength EDTA in the leachate. The mobility of Zn and Cd was usually slightly lower than that of Cu. Pb was the least mobile, and its elution increased after the peaks of Cu and Zn. Sequential fractionations of leached and un-leached soils showed that heavy metals in various operationally defined fractions contributed to the removal by EDTA. Considerable mobilisation of Fe occurred in two of the four soils during EDTA leaching. Decreases in the Fe and Mn oxide fraction of heavy metals after EDTA leaching occurred in both soils, as well as in a third soil that showed little Fe mobilisation. The results suggest that the lability of metals in soil, the kinetics of metal desorption/dissolution and the mode of EDTA addition were the main factors controlling the behaviour of metal leaching with EDTA.     

Metal fractionation in soils and assessment of environmental contamination in Vallecamonica, Italy

Metal contamination was investigated in soils of the Vallecamonica, an area in the northern part of the Brescia province (Italy), where ferroalloy industries were active for a century until 2001. The extent in which emissions from ferroalloy plants affected metal concentration in soils is not known in this area. In this study, the geogenic and/or anthropogenic origin of metals in soils were estimated. A modified Community Bureau of Reference sequential chemical extraction method followed by inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses were employed to evaluate the potential bioavailability of Al, Cd, Mn, Fe, Cr, Zn, and Pb in soils. Principal component analysis (PCA) was used to assess the relationships among metal sources in soil samples from different locations. This approach allowed distinguishing of different loadings and mobility of metals in soils collected in different areas. Results showed high concentrations and readily extractability of Mn in the Vallecamonica soils, which may suggest potential bioavailability for organisms and may create an environmental risk and potential health risk of human exposure.     

Use of sequential extraction to assess metal partitioning in soils

The state of heavy metal pollution and the mobility of Cd, Cu, Ni, Cr, Pb and Zn were studied in three texturally different agricultural soil profiles near a Cu-Ni smelter in Harjavalta, Finland. The pseudo-total concentrations were determined by an aqua regia procedure. Metals were also determined after division into four fractions by sequential extraction with (1) acetic acid (exchangeable and specifically adsorbed metals), (2) a reducing agent (bound to Fe/Mn hydroxides), (3) an oxidizing agent (bound to soil organic matter) and (4) aqua regia (bound to mineral structures). Fallout from the smelter has increased the concentrations of Cd, Cu and Ni in the topsoil, where 75-90% of Cd, 49-72% of Cu and 22-52% of Ni occurred in the first two fractions. Slight Pb and Zn pollution was evident as well. High proportions of mobile Cd, Cu and Ni also deeper in the sandy soil, closest to the smelter, indicated some downward movement of metals. The hydroxide-bound fraction of Pb dominated in almost all soils and horizons, while Ni, Cr and Zn mostly occurred in mineral structures. Aqua regia extraction is usefully supplemented with sequential extraction, particularly in less polluted soils and in soils that exhibit substantial textural differences within the profiles.     

Mobility of Pb, Cu, and Zn in the phosphorus-amended contaminated soils under simulated landfill and rainfall conditions

Phosphorus-bearing materials have been widely applied in immobilization of heavy metals in contaminated soils. However, the study on the stability of the initially P-induced immobilized metals in the contaminated soils is far limited. This work was conducted to evaluate the mobility of Pb, Cu, and Zn in two contrasting contaminated soils amended with phosphate rock tailing (PR) and triple superphosphate fertilizer (TSP), and their combination (P?+?T) under simulated landfill and rainfall conditions. The main objective was to determine the stability of heavy metals in the P-treated contaminated soils in response to the changing environment conditions. The soils were amended with the P-bearing materials at a 2:1 molar ratio of P to metals. After equilibrated for 2 weeks, the soils were evaluated with the leaching procedures. The batch-based toxicity characteristic leaching procedure (TCLP) was conducted to determine the leachability of heavy metals from both untreated and P-treated soils under simulated landfill condition. The column-based synthetic precipitation leaching procedure (SPLP) were undertaken to measure the downward migration of metals from untreated and P-treated soils under simulated rainfall condition. Leachability of Pb, Cu, and Zn in the TCLP extract followed the order of Zn?>?Cu?>?Pb in both soils, with the organic-C- and clay-poor soil showing higher metal leachability than the organic-C- and clay-rich soil. All three P treatments reduced leachability of Pb, Cu, and Zn by up to 89.2, 24.4, and 34.3 %, respectively, compared to the untreated soil, and TSP revealed more effectiveness followed by P?+?T and then PR. The column experiments showed that Zn had the highest downward migration upon 10 pore volumes of SPLP leaching, followed by Pb and then Cu in both soils. However, migration of Pb and Zn to subsoil and leachate were inhibited in the P-treated soil, while Cu in the leachate was enhanced by P treatment in the organic-C-rich soil. More than 73 % P in the amendments remained in the upper 0–10 cm soil layers. However, leaching of P from soluble TSP was significant with 24.3 % of P migrated in the leachate in the organic-C-poor soil. The mobility of heavy metals in the P-treated soil varies with nature of P sources, heavy metals, and soils. Caution should be taken on the multi-metal stabilization since the P amendment may immobilize some metals while promoting others’ mobility. Also, attention should be paid to the high leaching of P from soluble P amendments since it may pose the risk of excessive P-induced eutrophication.     

Kinetic extractions to assess mobilization of Zn, Pb, Cu, and Cd in a metal-contaminated soil: EDTA vs. citrate

Kinetic EDTA and citrate extractions were used to mimic metal mobilization in a soil contaminated by metallurgical fallout. Modeling of metal removal rates vs. time distinguished two metal pools: readily labile (QM1) and less labile (QM2). In citrate extractions, total extractability (QM1+QM2) of Zn and Cd was proportionally higher than for Pb and Cu. Proportions of Pb and Cu extracted with EDTA were three times higher than when using citrate. We observed similar QM1/QM2 ratios for Zn and Cu regardless of the extractant, suggesting comparable binding energies to soil constituents. However, for Pb and Cd, more heterogeneous binding energies were hypothesized to explain different kinetic extraction behaviors. Proportions of citrate-labile metals were found consistent with their short-term, in-situ mobility assessed in the studied soil, i.e., metal amount released in the soil solution or extracted by cultivated plants. Kinetic EDTA extractions were hypothesized to be more predictive for long-term metal migration with depth.     

Chemical speciation and extractability of Zn,Cu and Cd in two contrasting biosolids-amended clay soils

An incubation experiment was conducted to study the chemical speciation and extractability of three heavy metals in two contrasting biosolids-amended clay soils. One was a paddy soil of pH 7.8 and the other was a red soil of pH 4.7 collected from a fallow field. Anaerobically digested biosolids were mixed with each of the two soils at three rates: 20, 40 and 60 g kg(-1) soil (DM basis), and unamended controls were also prepared. The biosolids-amended and control soils were incubated at 70% of water holding capacity at 25 degrees C for 50 days. Separate subsamples were extracted with three single extractants and a three-step sequential extraction procedure representing acetic acid (HOAc)-soluble, reducible and oxidisable fractions to investigate the extractability and speciation of the heavy metals. As would be expected, there were good relationships between biosolids application rate and metal concentrations in the biosolids-amended soils. The three heavy metals had different extractabilities and chemical speciation in the two biosolids-amended soils. Ethylene diamine tetraacetic acid extracted more Cu, Zn and Cd than did the other two single extractants. The oxidisable fraction was the major fraction for Cu in both biosolids-amended soils and the HOAc-soluble and reducible fractions accounted for most of the Zn. In contrast, Cd was present mainly in the reducible fraction. The results are discussed in relation to the mobility and bioavailability of the metals in polluted soils.     

Mobility of Zn, Cd and Pb in soils as affected by poultry litter extract--II. Redistribution among soil fractions

The application of poultry litter to metal-contaminated soils may influence metal leaching and distribution of metals among soil fractions. Soil columns (one uncontaminated control, one metal-amended, and two metal-contaminated soils) were leached with H2O, CaCl2, EDTA, and poultry litter extract (PLE) solutions. After leaching, the soil samples in the columns were sequentially extracted for water soluble (WS), exchangeable (EXC), organic matter (OM), Mn oxide (MNO), amorphous Fe oxide (AFEO), crystalline Fe oxide (CFEO) and residual (RES) fractions. The OM fraction showed high retention for Zn from the PLE. The EDTA redistributed Zn, Cd and Pb from the EXC, OM and MNO fractions to the WS fraction. The PLE usually solubilized more Zn and Cd from the EXC fraction than CaCl2. Neither PLE nor CaCl2 mobilized Pb. The application of poultry litter on metal-contaminated soils might cause Zn and Cd redistribution from the EXC to the WS fraction and enhance metal mobility.     

Mobility of Zn, Cd and Pb in soils as affected by poultry litter extract--I. Leaching in soil columns

Application of poultry litter to cropland may increase metal mobility, because the soluble organic ligands in poultry litter can form water-soluble complexes with metals. In this study, one uncontaminated soil and two metal-contaminated soils were sampled. A portion of the uncontaminated soil was amended with Zn, Pb, and Cd at rates of 400, 200, and 8 mg kg(-1), respectively. Packed soil columns were leached with H2O, EDTA, CaCl2, and poultry litter extract (PLE) solutions separately. No leaching of Zn, Cd, and Pb with the PLE was found in the uncontaminated soil. The retention of PLE-borne Zn indicated the potential for Zn accumulation in the soil. A large portion of the metals was leached from the metal-amended soil, and EDTA solubilized more Zn, Cd, and Pb than CaCl2 and PLE. In the metal-contaminated soils, the leaching of Zn and Cd with PLE was consistently larger than that for CaCl2, indicating that these metals were mobilized by organic ligands. The PLE did not mobilize Pb in these soils. The utilization of poultry litter in metal-contaminated soils might accelerate the movement of Zn and Cd in soil profiles.