Metka Udovic

Immobilization of potentially toxic metals using different soil amendments.

The in situ stabilization of potentially toxic metals (PTMs), using various easily available amendments, is a cost-effective remediation method for contaminated soils. In the present study, we investigated the effectiveness of apatite and a commercial mixture of dolomite, diatomite, smectite basaltic tuff, bentonite, alginite and zeolite (Slovakite) on Pb, Zn, Cu and Cd stabilization by means of decreasing their bioavailability in contaminated soil from an old lead and zinc smelter site in Arnoldstein, Austria. We also investigated the impact of 5% (w/w) apatite and Slovakite applications on soil functionality and quality, as assessed by glucose-induced soil respiration, dehydrogenase, acid and alkaline phosphatase and β-glucosidase activity. Both amendments resulted in increased soil pH and decreased PTM potential bioavailability assessed by diethylenetriamine pentaacetic acid extraction and by sequential extractions in the water-soluble and exchangeable fractions. The efficiency of stabilization was reflected in the soil respiration rate and in enzymatic activity. The β-glucosidase activity assay was the most responsive of them.

Pb, Zn and Cd mobility, availability and fractionation in aged soil remediated by EDTA leaching.

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 degrees C and -20 degrees 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.

EDTA and HCl leaching of calcareous and acidic soils polluted with potentially toxic metals: remediation efficiency and soil impact.

The environmental risk of potentially toxic metals (PTMs) in soil can be diminished by their removal. Among the available remediation techniques, soil leaching with various solutions is one of the most effective but data about the impact on soil chemical and biological properties are still scarce. We studied the effect of two common leaching agents, hydrochloric acid (HCl) and a chelating agent (EDTA) on Pb, Zn, Cd removal and accessibility and on physico-chemical and biological properties in one calcareous, pH neutral soil and one non-calcareous acidic soil. EDTA was a more efficient leachant compared to HCl: up to 133-times lower chelant concentration was needed for the same percentage (35%) of Pb removal. EDTA and HCl concentrations with similar PTM removal efficiency decreased PTM accessibility in both soils but had different impacts on soil properties. As expected, HCl significantly dissolved carbonates from calcareous soil, while EDTA leaching increased the pH of the acidic soil. Enzyme activity assays showed that leaching with HCl had a distinctly negative impact on soil microbial and enzyme activity, while leaching with EDTA had less impact. Our results emphasize the importance of considering the ecological impact of remediation processes on soil in addition to the capacity for PTM removal.

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.

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.

Redistribution of residual Pb, Zn, and Cd in soil remediated with EDTA leaching and exposed to earthworms (Eisenia fetida)

After soil remediation with ethylenediamine tetraacetate (EDTA) leaching/washing, the residual Pb, Zn, and Cd species are left in the soil in non‐labile forms. The effect of earthworms as main soil biotic factors on the residual Pb, Zn, and Cd fraction lability (mobility, bioavailability to plants, and oral‐availability) was investigated. Contaminated soil from a smelter site was treated with increasing EDTA concentrations (2.5 to 4‐times 40 mmol kg−1) to gradually reduce the heavy metal content and lability. Leached soils were processed by Eisenia fetida and heavy metal lability and fractionation determined in casts and earthworm‐processed soils. In general E. fetida increased heavy metal accessibility/mobility, but the induced changes diminished with the intensity of soil EDTA treatment. Fractionation results indicate the possibility of a time‐dependent complexation of heavy metals to carbonates favoured by earthworms’ gut processes. The transition of residual heavy metals in time (ageing) should be considered.

Long-term efficiency of soil stabilization with apatite and Slovakite: The impact of two earthworm species (Lumbricus terrestris and Dendrobaena veneta) on lead bioaccessibility and soil functioning

Remediation soil is exposed to various environmental factors over time that can affect the final success of the operation. In the present study, we assessed Pb bioaccessibility and microbial activity in industrially polluted soil (Arnoldstein, Austria) stabilized with 5% (w/w) of Slovakite and 5% (w/w) of apatite soil after exposure to two earthworm species, Lumbricus terrestris and Dendrobaena veneta, used as model environmental biotic soil factors. Stabilization resulted in reduced Pb bioaccessibility, as assessed with one-step extraction tests and six-step sequential extraction, and improved soil functioning, mirrored in reduced β-glucosidase activity in soil. Both earthworm species increased Pb bioaccessibility, thus decreasing the initial stabilization efficacy and indicating the importance of considering the long-term fate of remediated soil. The earthworm species had different effects on soil enzyme activity, which can be attributed to species-specific microbial populations in earthworm gut acting on the ingested soil.

An in vivo invertebrate bioassay of Pb, Zn and Cd stabilization in contaminated soil.

UNLABELLED The terrestrial isopod (Porcellio scaber) was used to assess the remediation efficiency of limestone and a mixture of gravel sludge and red mud as stabilizing agents of Pb, Zn and Cd in industrially polluted soil, which contains 800, 540 and 7mgkg(-1) of Pb, Zn and Cd, respectively. The aim of our study was to compare and evaluate the results of the biological and non-biological assessment of metal bioavailability after soil remediation. Results of a 14d bioaccumulation test with P. scaber showed that that Pb and Zn stabilization were more successful with gravel sludge and red mud, while Cd was better stabilized and thus less bioavailable following limestone treatment. In vivo bioaccumulation tests confirmed the results of chemical bioaccessibility, however it was more sensitive. Biotesting with isopods is a relevant approach for fast screening of bioavailability of metals in soils which includes temporal and spatial components. CAPSULE Bioavailability assessed by P. scaber is a more relevant and sensitive measure of metal bioavailability than chemical bioaccessibility testing in remediated industrially polluted soil.

Mobility and Availability of Toxic Metals After Soil Washing with Chelating Agents

Remediation techniques for soils polluted with toxic metals can be divided into two main groups: immobilization and soil washing. Immobilization technologies leave metals in soil, but minimize their availability, while soil washing with chelating agents removes metals from soil. Metals in soil are not entirely accessible to chelating agents and, hence, not entirely removed. Residual metals left in the soil after remediation remain in chemically stable species bound to non-labile soil fractions and are considered nonmobile and non-bioavailable and thus nontoxic. However, with the reintroduction of remediated soil into the environment, the soil is exposed to various environmental factors, which could eventually promote or initiate the transition of the residual metals back to more labile forms to re-establish the disturbed equilibrium. Such a shift is likely to increase the toxicity of the residual metals and, consequently, decrease the final efficiency of soil remediation. Different extraction techniques are used to assess metals bioavailability and the efficiency of soil remediation. Reduced bioavailability of contaminants for organisms is most often assessed by established chemical extraction tests. However, do the chemical extraction tests really provide (include) reliable information on the availability of metals for soil fauna? In the present chapter, the effect of biotic and abiotic environmental factors on the mobility and availability of residual metals in soil after remediation is discussed. Furthermore, the benefits of in vivo assessment of soil remediation efficiency by terrestrial organisms is highlighted.