Influence of reducing conditions on metallic elements released from various contaminated soil samples.

The redox conditions of soil may have significant consequences for the mobility of metallic elements (ME), but unlike pH, very few studies have investigated this parameter. A procedure was established to study the solubilization of ME from soil samples in various reducing conditions using a batch method and sodium ascorbate solutions. The change in redox potential from +410 to +10 mV was studied from four contaminated soil samples (designated A-D) of different origins and compositions. The results showed that ME mobilization greatly increased with decreasing redox potential within a limited and very sensitive range. Depending on the soil sample studied, various sensitive ranges of potentials were obtained (A, 220-345 mV; B, 280-365 mV; C, 260-360 mV; and D, 240-380 mV), and the induced percentages of ME mobilization varied (i.e., maximal values for Zn: A, 45%; B, 59%; C, 53%; and D, 58%). The results could be explained by the combined effect of potential and pH decrease on ME-carrying phases; in particular, Fe and Mn (oxy)hydroxides.

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.