Magnetite-assisted in situ microbial oxidation of H2S to S0 during anaerobic digestion: a new potential for sulfide control

Abstract

Direct interspecies electron transfer (DIET) between exoelectrogenic fatty acid-oxidizing bacteria and electrotrophic methanogens has recently been discovered, and studies have suggested that promoting DIET by adding electrically conductive material can effectively enhance the methanogenic performance and stability of anaerobic digestion (AD). This study investigated the effect of conductive magnetite (Fe3O4) addition on the AD of a sulfur-rich organic waste mixture, with an emphasis on the fate of sulfur and on H2S production. In contrast to previous findings, methanogenic performance under magnetite-added conditions was not significantly enhanced within the tested dose range of up to 20mM Fe. When magnetite was added, H2S production decreased remarkably along with the extracellular accumulation of S-0. Moreover, the H2S content in biogas was below 100 ppmv at magnetite doses of 8mM Fe or higher (> 6000 ppmv under control conditions, i.e., without magnetite). The reduced H2S production appears to be due to the anaerobic oxidation of sulfide to S-0 because sulfate reduction remained active, whereas FeS was not produced under magnetite-added conditions. Based on microbial community analysis results and thermodynamic calculations, the electric syntrophy via DIET between exoelectrogenic anaerobic sulfide-oxidizing bacteria and electrotrophic methanogens is suggested to have been promoted by magnetite. To the best of our knowledge, this study is the first to propose an electro-syntrophic association that couples the oxidation of sulfide to S-0 with the reduction of CO2 to CH4 in methanogenic environments. The present findings open a new possibility for in situ H2S control and sulfur recovery in AD processes for sulfur-rich waste treatment.