Influence of iron-rich water treatment residues and compost on the mobility of metal(loid)s in mine soils

Two different amendments, an iron-rich water treatment residue (Fe-WTR), a municipal solid waste compost (MSW-C) and their combination (Fe-WTR+MSW-C) were added at different rates (from 2 to 4% w/w) to three mining soils (S1, S2, S3) mainly polluted with As (from 371 to 22661 mg•kg−1d.w.) and different co-occurring trace metals (i.e. Pb, Zn and Cu) to evaluate their effectiveness as metal(loid)s-immobilizing agents. After four months of soil-amendment contact, sequential extractions revealed that MSW-C and Fe-WTR induced an increase of the residual As (non extractable) fraction. Compost was the most effective amendment at increasing the residual As in treated soils (e.g. +16% in S1-MSW-C with respect to untreated S1), although its addition increased at the same time the exchangeable and water-soluble As fraction and the extractability of Pb, Zn and Cu, especially in S1-MSW-C. Leaching experiments highlighted a similar trend, with the highest cumulative fraction of As leached recorded in S1 and S2 soils amended with MSW-C (3.8 and 1.4-fold higher than respective controls), and the lowest recorded in S1 and S2 soils amended with Fe-WTR (1.2 and 1.8-fold lower than respective controls). On the other hand, Fe-WTR and Fe-WTR+MSW-C were the most effective at reducing the total cumulative concentration of metal (Pb, Zn and Cu) in soil leachate. The results of this study show that the amendments considered influenced with a different extent metal(loid)s mobility, and this was depending on soil and amendment characteristics, as well as the type and amount of contamination.