Insights into selenate removal mechanism of hydrogen-based membrane biofilm reactor for nitrate-polluted groundwater treatment based on anaerobic biofilm analysis

The sclenate removal mechanism of hydrogen-based membrane biofilm reactor (MBfR) for nitrate-polluted groundwater treatment was studied based on anaerobic biofilm analysis. A laboratory-scale MBfR was operated for over 60 days with electron balance, structural analysis, and bacterial community identification. Results showed that anaerobic biofilm had an excellent removal of both selenate (95%) and nitrate (100%). Reduction of Selenate -> Selenite -> Se-0 with hydrogen was the main pathway of anaerobic biofilm for selenate removal with amorphous Se precipitate accumulating in the biofilm. The element selenium was observed to be evenly distributed along the cross-sectional thin biofilm. A part of selenate (3%) was also reduced into methyl-selenide by heterotrophic bacteria. Additionally, Hydrogenophaga bacteria of beta-Proteobacteria, capable of both nitrate and selenate removal, worked as the dominant species (over 85%) in the biofilm and contributed to the stable removal of both nitrate and selenate. With the selenate input, bacteria with a capacity for both selenate and nitrate removal were also developed in the anaerobic biofilm community.