Leaching potential of metallic elements from contaminated soils under anoxia

Understanding metallic element (ME) behaviour in soils subjected to alternating redox conditions is of significant environmental importance, particularly for contaminated soils. Although variations in the hydrological status of soils may lead to the release of ME, redox-driven changes in ME dynamics are still not sufficiently understood. We studied the effects of alternating redox cycles on the release, leaching and redistribution of Zn, Cu and Pb in metal mine-contaminated and non-contaminated soils by means of a column experiment. Although the release of Zn was promoted by the onset of reductive conditions, successive redox cycles favoured metal partitioning in less labile fractions limiting its further mobilization. The release of Cu in soil pore waters and redistribution in the solid phase towards more labile pools were strongly dependent on the alternation between oxidizing and reducing conditions. In contaminated soils, the presence of chalcopyrite could have determined the release of Cu under oxic conditions and its relative immobilization under subsequent anoxic conditions. The behaviour of Pb did not seem to be influenced by the redox status, although higher concentrations in the column leachates with respect to soil pore waters suggested that alternating redox conditions could nonetheless result in substantial mobilization. This study provides evidence that the alternation of soil redox conditions may play a more important role in determining the release and leaching of ME from soils with respect to reducing or oxidizing conditions considered separately.