Microbial selenium sulfide reduction for selenium recovery from wastewater

Microbial reduction of selenium sulfide (SeS₂) is a key step in a new treatment process to recover selenium from selenate and selenite streams. In this process, selenate is first reduced to selenite, and subsequently selenite is reduced by sulfide and precipitates from the solution as SeS₂. The latter is bio-reduced to elemental selenium and sulfide. Two anaerobic granular sludges (Eerbeek and Emmtec) were tested for their efficiency to reduce commercial crystalline SeS₂. Emmtec sludge had the highest reducing capacity with commercial SeS₂ and was therefore also used for the bioreduction of laboratory synthesized amorphous SeS₂. Synthesized SeS₂ was formed mixing a sulfide solution and effluent containing selenite. With both SeS₂ solids (commercial and synthesized SeS₂), Emmtec sludge produced sulfide and a solid consisting of hexagonal elemental selenium. The crystalline hexagonal structure suggests the absence of biomolecules, which stabilize amorphous selenium bio-particles under comparable process conditions (T=30°C and a pH between 6 and 7). Selenium particles were not attached to the biomass, suggesting an extracellular formation. The results support the feasibility of the bio-reduction process using sulfur for recovering selenium from water.