Study of partitioning and dynamics of metals in contaminated soil using modified four-step BCR sequential extraction procedure

The modified four-step BCR sequential extraction procedure (exchangeable and weak acid available species, reducible, oxidisable and residual fractions) was used to examine the distribution of As, Cd, Cr, Cu, Pb, and Zn with soil depth in an area (Baia Mare — Bozanta, Romania) with both high natural level of elements considered as toxic and historical pollution resulting from nonferrous metallurgy. The BCR approach proved a high metal input of anthropogenic origin down to 40 cm, while at lower depths the naturally elevated metal content must be considered. Results of the partitioning study and XRD analysis of solid matrix showed the greatest potential for chemical remobilisation of Cd, Zn, and Cu in weak acidic medium as well as their affinity for the oxidisable fraction (organic matter/sulphide). The tendency of Cr, Pb, and As to be immobilised as residual or reducible species on Fe-Mn oxides was evident. Although the partitioning of As in chemically inactive forms such as scorodite (FeAsO4 · 2H2O) soluble under reducible conditions and beudantite (PbFe3(AsO4)(SO4)(OH)2)), a residual species soluble in acid media, chemical mobilisation from soil in groundwater was confirmed. Dynamic processes of metal retention in soil under different conditions, namely acidic, reducing or oxidisable, were predicted from the Pearsonșs correlation analysis of element species with soil characteristics and components such as Fe, Mn, organic matter content, pH, and total element content, respectively. At the moment of the study, soil and groundwater in the area were found to be polluted with As, Cd, Cu, Pb and Zn.