Metal and pH stress regulate selenite reduction in anaerobic membrane bioreactor: Mechanistic insights into microbial interactions

The anaerobic membrane bioreactor (AnMBR) has emerged as a promising technology for the microbial reduction of selenium (Se)-laden wastewaters. Nonetheless, it remains understudied how the removal performance and mechanism of the AnMBR for Se oxyanions respond to the joint impacts of coexisting pollutants and influent pH. In this study, a lab-scale AnMBR was employed to treat synthetic SeO32-wastewater amended with competing electron acceptors (i.e., SO42-) and coexisting metal ions (i.e., Cd2+ and Ni2+) at different influent pH. Nearly complete SeO32-removal could be attained at moderate pH (5.0-7.0), benefiting from the stable metabolic activity of functional microbial communities. In contrast, a low pH of 3.0 markedly decreased removal efficiencies for all contaminants, although it mitigated membrane fouling to a certain extent. The microbial reduction kinetic rates of SeO32-and SO42-oxyanions decreased by one order of magnitude as pH declined from 7.0 to 3.0, indicating the strong pH sensitivity of microbial respiration. At moderate pH, metal ions were detoxified primarily through precipitation as metal hydroxides, selenides and sulfides, and partially via complexation by functional groups of microbial products (including proteins, polysaccharides and humic acid). Seven representative functional genera were identified by metagenomic analysis based on known functional roles, including selenium-reducing bacteria (SeRB) and sulfate-reducing bacteria (SRB) with relative abundances greater than 1 %. The dominant and minor SeRB drove the reduction of SeO32-toward Se0 and Se(-II), respectively, while SRB contributed through enzymatic transformation and abiotic reaction involving biogenic S (-II). At pH 3.0, SRB became nearly undetectable, whereas SeRB persisted with reduced activity, suggesting differential acid sensitivity between the two groups. The findings provided insights into the microbial interaction mechanism of AnMBR for removing multiple oxyanions under metal and proton stress.