The continuing loss of fertile arable land increases the importance of cleaning contaminated soils. The remediation of polluted sites received increasing attention over the last decades. Today, dig and dump approaches cannot live up to the increasing need for sustainable solutions; so different in- and ex-situ remediation technologies have been intensively investigated, also in combination with complimentary soil amendments. In this context, soil washing techniques comprise an enormous potential for the efficient and sustainable remediation of trace metal (TM) contaminated soils. In pot experiments and an outdoor raised-bed setup, we investigate the effect of a new ex-situ washing technique using Ethylenediaminetetraacetic acid (EDTA) which efficiently extracts TMs (Zn, Cd, Pb) from a contaminated Cambisol located at a former lead mining and smelting region in Austria. Our research goal is the reduction of TM toxicity in these soils while restoring the losses in soil functionality by the application of organic and inorganic amendments.

Our results show that compared to the untreated soil, the washing procedure successfully reduced the total content of the main contaminants Pb, Cd and Cd by 80 %, 40 % and 70 %, resulting in a significantly lower concentration of Cd and Zn in the aerial tissue of radish (Raphanus sativus L.) cultivated in these soils. The significant increase in biomass production, observed in the EDTA-washed treatments, could have been caused by reduced toxicity levels. The Pb concentrations on the other hand, tended to increase in the plants after the washing with EDTA. Despite extensive soil rinsing after the washing procedure, these findings suggest the presence of mobile metal chelate residuals. To counter this effect, the application of Fe⁰ was investigated for its ability to adsorb and immobilize EDTA, eventually reducing Pb in vegetable plants to concentrations below food security levels. Water extractable ETDA-TM complexes will be measured spectrophotometrically after induced ferroin formation in the extract.

In parallel, the application of organic amendments (vermicompost and biochar) after the washing procedure is studied to analyze the potential for the restoration of plant nutrient availability, water holding capacity and the microbial community. The water retention curves of different amendment mixtures and soil microbial phospholipid fatty acids will be determined with respect to the efficiency of the treatments in revitalizing the washed soils. These findings will be used to select appropriate amendment treatments for the raised-bed experiment.