Changes in organic matter composition caused by EDTA washing of two soils contaminated with toxic metals.

Two soils contaminated with potentially toxic metals (PTMs) contrasting in pH and mineralogy were remediated with CaEDTA, and changes in soil organic matter (SOM) composition were investigated. Previous studies showed no significant loss of SOM from CaEDTA-treated soils, but the results of our study reflected significant decreases (from 46 to 49%) in the free fraction of humic acids (HAs). Remediation affected the composition of the free HA fraction via disturbance of intermolecular bonds - an increase in phenolic and aromatic groups with a simultaneous decrease in carbohydrates - which was confirmed by FTIR spectroscopy in both soils. Because non-radical molecules such as carbohydrates were selectively removed, the concentration of free radicals in the free HA fraction increased in acidic soil. The bound fraction of HAs and fulvic acids (FAs) in SOM, which are important due to their stability and the permanent effects they have on the soil's physical properties, remained unchanged in both remediated soils. The effect of soil recultivation was observed only in the excitation emission matrix (EEM) fluorescence spectra of HAs. In terms of SOM, CaEDTA soil washing can be considered moderately conservative; however, the restoration of free humic fractions is likely to be a long-term process.

The use of zero-valent Fe for curbing toxic emissions after EDTA-based washing of Pb, Zn and Cd contaminated calcareous and acidic soil.

The use of EDTA-based soil washing is prevented by chelant environmental persistence and the hazard of toxic post-remedial emissions. Calcareous and acidic soils with 828 and 673 mg Pb kg-1, respectively, and co-contaminated with Zn and Cd, were washed with 90 and 60 mM EDTA, respectively, to remove 67 and 80% of Pb. Washed soils were rinsed until 6.5 and 5.1 mM EDTA, respectively, was measured in the final rinsing solutions. Emissions of residual EDTA and chelated metals from remediated soils were mitigated by adsorption on zero-valent Fe (ZVI), which was added (0.5-1.5%, w/w) to the slurry of washed soil immediately before rinsing. ZVI addition prevented the initial post-remedial surge of toxic metals leachability and minimised toxic emissions from calcareous and acidic soil as soon as 6 and 7 days after remediation, respectively. The extractability/leachability of EDTA and toxic metals from remediated and ZVI amended soils diminished to close to emissions from the original soils, frequently below the limit of quantification by flame-AAS, and was not affected by the pH of the leaching solutions. Efficient curbing of toxic post-remediation emissions as demonstrated herein is of paramount importance for recognition of EDTA-based remediation as environmentally safe.

EDTA retention and emissions from remediated soil.

EDTA-based remediation is reaching maturity but little information is available on the state of chelant in remediated soil. EDTA soil retention was examined after extracting 20 soil samples from Pb contaminated areas in Slovenia, Austria, Czech Republic and USA with 120 mM kg(-1) Na2H2EDTA, CaNa2EDTA and H4EDTA for 2 and 24 h. On average, 73% of Pb was removed from acidic and 71% from calcareous soils (24 h extractions). On average, 15% and up to 64% of applied EDTA was after remediation retained in acidic soils. Much less; in average 1% and up to the 22% of EDTA was retained in calcareous soils. The secondary emissions of EDTA retained in selected remediated soil increased with the acidity of the media: the TCLP (Toxicity Characteristic Leaching Procedure) solution (average pH end point 3.6) released up to 36% of EDTA applied in the soil (28.1 mmol kg(-1)). Extraction with deionised water (pH > 6.0) did not produce measurable EDTA emissions. Exposing soil to model abiotic (thawing/freezing cycles) and biotic (ingestion by earthworms Lumbricus rubellus) ageing factors did not induce additional secondary emissions of EDTA retained in remediated soil.

Prediction of blood lead levels in children before and after remediation of soil samples in the upper Meza Valley, Slovenia.

The Meza Valley, Slovenia, has been contaminated by Pb smelting, resulting in an epidemic of lead poisoning in childhood. The potential of remediation with EDTA soil washing to mitigate the risk from Pb poisoning was investigated by applying the Integrated Exposure Uptake Bio-kinetic (IEUBK) model. Soils from 79 locations were collected and the total and bio-accessible Pb concentrations were determined before and after extraction with 60 mmol kg(-1) EDTA. Extraction reduced the soil Pb concentration in towns of Mezica, Zerjav and Crna by 53, 67 and 62%, respectively, and the concentration of in vitro bio-accessible Pb in the simulated human gastric phase by 2.6-, 3.2- and 2.9-times, respectively. The predictions of the IEUBK model based on Pb contamination data were verified with data on blood Pb levels in children. The IEUBK model predicted that, after soil remediation, the number of locations at which the expected blood Pb level in children was higher than the stipulated 10 µg d L(-1) would decrease by 90, 38 and 91% in the towns of Mezica, Zerjav and Crna, respectively. The results confirmed the feasibility of soil washing with EDTA as an efficient remediation measure in Mezica and Crna and advice for soil capping/removal for the most polluted town of Zerjav.