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

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 20 mM Fe. When magnetite was added, H₂S production decreased remarkably along with the extracellular accumulation of S⁰. Moreover, the H₂S content in biogas was below 100 ppmv at magnetite doses of 8 mM 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 S⁰ 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 S⁰ 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.