Selenium treatment via integrating flow electrode capacitive deionization (FCDI) and bio-electrochemical systems (BES)

Selenium pollution in aquatic environments poses a major global challenge, with a significant gap in effective treatment technologies. In this study, we explored a novel approach integrating flow-electrode capacitive deionization (FCDI) with bio-electrochemical systems (BES) for the removal and reduction of selenate and selenite ions in one compact reactor. Our integrated system was electricity-driven, eliminating chemical usage. Up to 76 % selenium removal from the waste streams was achieved, followed by up to 66 % and 54 % reduction of selenate and selenite to elemental selenium respectively. The addition of acetate, a carbon source, enhanced selenate reduction by 14 % but lowered selenite reduction by 21 %, suggesting the substrate-dependent and bioelectrochemical-driven nature of selenate and selenite reduction respectively. Metagenomic sequencing revealed that Geobacter sulfurreducens and Pseudomonas stutzeri two known Se-reducing species, likely contributed to both selenite and selenate reduction through up-regulating functional genes related to sulfide reductase, fumarate reductase, and multi-heme c-type cytochromes. Thauera spp. and Alishewanella spp., two species not previously associated with selenium reduction, were likely involved in selenite reduction via the up-regulation of genes related to sulfite reductase and selenium reductase.