Sequential heavy metals extraction from polluted solids: influence of sulfate overconcentration.

The effect of sulfate on the chemical partitioning of Cu, Cd, and Pb in solid phases was assessed in this study. Modified BCR sequential extraction, speeded up by focused ultrasound, was systematically applied to various mixtures of typical geochemical solid phases (an artificial goethite spiked with Cu, Cd, and Pb and natural clays), with or without the addition of calcium sulfate. Sulfate was added so that three different concentrations were found in sequential extracts: 0.5, 1, and 1.5 g/L of sulfate. First, the results suggested that the goethite-surface adsorption sites for sulfate are limited. Then, significant changes in Cd and Pb fractionation were observed in the presence of sulfate, whereas Cu remained strongly adsorbed on the solid phases. The main modifications observed in all the studied samples were a decrease in metal amounts in the first three fractions to the profit of an increase in the residual fraction. These results suggested that the adsorption of metals onto the studied solids was enhanced by the presence of sulfate. From these considerations, some hypotheses are advanced to describe the behavior of Cu, Cd, and Pb and their adsorption mechanisms on solid phases in sulfate-rich systems.