Hydrogen-fed biofilm reactors reducing selenate and sulfate: Community structure and capture of elemental selenium within the biofilm

Remediation of selenate (SeO₄ ²⁻) contamination through microbial reduction is often challenging due to the presence of sulfate (SO₄ ²⁻), which can lead to competition for the electron donor and the co-production of toxic H₂S. Microbial reduction of SeO₄ ²⁻ in the presence of SO₄ ²⁻ was studied in two hydrogen-based membrane biofilm reactors (MBfRs). One MBfR was initiated with SO₄ ²⁻-reducing conditions and gradually shifted to SeO₄ ²⁻ reduction. The second MBfR was developed with a SeO₄ ²⁻-reducing biofilm, followed by SO₄ ²⁻ introduction. Biofilms within both MBfRs achieved greater than 90% SeO₄ ²⁻ reduction, even though the SeO₄ ²⁻ concentration ranged from 1,000–11,000 μg/L, more than 20–200 times the maximum contaminant level for drinking water (50 μg/L). Biofilm microbial community composition, assessed by 16S rRNA gene-based amplicon pyrosequencing, was distinct between the two MBfRs and was framed by alterations in SeO₄ ²⁻ loading. Specifically, high SeO₄ ²⁻ loading resulted in communities mainly composed of denitrifying bacteria (e.g., Denitratisoma and Dechloromonas). In contrast, low loading led to mostly sulfate-reducing bacteria (i.e., Desulfovibrio) and sulfur-oxidizing bacteria (i.e., Sulfuricurvum and Sulfurovum). SeO₄ ²⁻ was reduced to elemental selenium (Se°), which was visualized within the biofilm as crystalloid aggregates, with its fate corresponding to that of biofilm solids. In conclusion, microbial biofilm communities initiated under either SeO₄ ²⁻ or SO₄ ²⁻-reducing conditions attained high SeO₄ ²⁻ removal rates even though their microbial community composition was quite distinct. Biotechnol. Bioeng. 2016;113: 1736–1744.