Prediction of trace element mobility in contaminated soils by sequential extraction

The modified three-step sequential extraction procedure proposed by the Community Bureau of Reference (or Bureau Communautaire de Reference, BCR) was used to predict trace element mobility in soils affected by an accidental spill comprising arsenopyrite- and heavy metal-enriched sludge particles and acid waste waters. The procedure was used to obtain the distribution of both the major (Al, Ca, Fe, Mg, and Mn) and trace elements (As, Bi, Cd, Cu, Pb, Tl, and Zn) in 13 soils of contrasting properties with various levels of contamination and in the sludge itself. The distributions of the major elements enabled us to confirm the main soil fractions solubilized in each of the three steps, and, in turn, to detect the presence of pyritic sludge particles by the high Fe extractability obtained in the third step. Cadmium was identified as being the most mobile of the elements, having the highest extractability in the first step, followed by Zn and Cu. Lead, Tl, Bi, and As were shown to be poorly mobile or nonmobile. In the case of some of the trace elements, the residual fractions decreased at higher levels of contamination, which was attributed to the anthropogenic contributions to the polluted samples. Comparison with soil-plant transfer factors, calculated in plants growing in the affected area, indicated that a relative sequence of trace element mobility was well predicted from data of the first step.