TRANSFORMATIONS OF SELENIUM AS AFFECTED BY SEDIMENT OXIDATION REDUCTION POTENTIAL AND PH

The influence of Eh and pH on selenium solubility, speciation, and volatilization was studied. Kesterson Reservoir sediments contaminated with selenium were incubated under controlled redox (−200, 0, 200, and 450 mV) and pH (6.5, natural, 8.5, and 9) conditions. Under reduced conditions selenium solubility was low and controlled by an iron selenide phase. Se(−II,0) comprised 80–100% of the total soluble selenium. Upon oxidation dissolved selenium concentrations increased. The oxidation of Se(−II,0) to selenite was rapid and occurred immediately after the oxidation of iron. Above 200 mV selenite slowly oxidized to selenate. Under oxidized conditions (450 mV) selenium solubility reached a maximum. Selenate was the predominant dissolved species present, constituting 95% at higher pH's (8.9, 9) to 75% at lower pH's (7.5, 6.5) of the total soluble selenium at 450 mV. Biomethylation of selenium occurred only under oxidized conditions. Redox potential and pH are key factors in the biogeochemistry of selenium. © 1990, American Chemical Society. All rights reserved.