Microbial electrolysis cell produced biogas as sustainable electron donor for microbial chromate reduction

Microbial electrolysis cell (MEC) is an attractive technology to produce biogas containing hydrogen (H2) and methane (CH4). Both gaseous products are effective electron donors for chromate [Cr(VI)] bioreduction when either is applied individually. However, microbial Cr(VI) reduction driven by H2 and CH4 mixture remains poorly understood. This study investigates Cr(VI) biotransformation supported by fluctuant MEC-produced biogas. Synergy between H2 and CH4 promoted Cr(VI) detoxification. Maximum Cr(VI) removal rate reached 1.81 ± 0.17 mg/L·d with 200.5 ± 13.6 μmol H2 and 203.3 ± 9.0 μmol CH4 produced by MEC at 0.6 V in a 4-d cycle. Biogas yield and H2/CH4 mole ratio were mainly regulated by Geobacter and Methanobacterium. The process in which Cr(VI) reduced to insoluble Cr(III) was proposed in two major pathways. A single autohydrogenotrophic genus (e.g., Hydrogenophaga, Thiobacillus) oxidized H2 independently coupled with Cr(VI) reduction. Cr(VI) reduction was also achieved through CH4-metabolizing microorganisms (e.g., Methanobacterium, Methanosaeta), which convert CH4 into organic metabolites to donate electrons for heterotrophic Cr(VI) reducers (e.g., Geobacter, Anaerolineaceae). Genes relevant to Cr(VI) reduction (yieF) and CH4 oxidation (mcrA) increased significantly. Electrons were effectively transferred through extracellular cytochrome c and intracellular nicotinamide adenine dinucleotide. This study offers an innovative route to sustainable bioremediation of Cr(VI)-polluted groundwater and promising direct utilization of mixed biogas from MEC.