Optimisation of selenite reduction using Enterococcus spp. under anaerobic conditions

Abstract

With the drastic increase in selenium-releasing anthropogenic activities, the bioaccumulation of toxic selenium oxyanion in the environment has increased significantly, posing deleterious effects to living organisms. The bio-removal of selenite by Enterococcus spp. under both aerobic and anaerobic batch conditions was examined using batch reactors containing Tryptone Soy Broth (TSB) laced with sodium selenite. The optimal conditions for bacterial growth and selenite reduction were determined under anoxic conditions. Enterococcus spp. reduced selenite more effectively under anaerobic conditions, with 77 % selenite reduction after 20 h. The bacteria also reduced selenite under aerobic conditions, but the process was much slower, with approximately 68 % reduction after 72 h. This correlated with the more rapid bacterial growth seen in the anaerobic batch reactor. Optimal growth conditions for Enterococcus spp. were obtained at a temperature of 25°C, pH of 6 and initial selenite concentration of 30 mg/L. Optimal selenite removal was at temperature of 35°C, pH of 7, and initial selenite concentrations of 30 mg/L. A red precipitate was formed during the selenite removal experiments, a probable sign that the selenite was being reduced to form elemental selenium. The results of this study demonstrated the capability of Enterococcus spp. to bio-transform toxic selenite oxyanions under both oxic and anoxic conditions.