Understanding the positive effects of low pH and limited aeration on selenate removal from water by elemental iron

Abstract

Selenate (SeO42-) is a toxic soluble compound and its presence in the environment should be minimized. Reduction of selenate ions by elemental iron is a promising approach for treating contaminated waters. However, uncertainty exists regarding both the favorable conditions required and the associated mechanisms. In this study, the kinetics and mechanisms of selenate removal using Fe0 at different pH and aeration rates was investigated using quantitative measurement of reactants and products. Under acidic anaerobic conditions, selenate was reduced directly by Fe0. Limited aeration increased removal rates by up to four times due to formation of reactive Fe(II)-bearing solid intermediates on the iron surface. Further increase in aeration rate decreased removal rates but also decreased iron release and acid consumption. Under basic conditions, selenate by Fe0 was negligible under anaerobic conditions and low under limited aeration. Selenate removal rates were shown to be dependent on Se concentration, on available reducing solid surface and on competition with oxygen over electrons.