Light-driven Methanosarcina barkeri-g-C3N4 biohybrid for conversion of carbon dioxide in anaerobic biogas to methane

Conversion of organic pollutants to biogas through anaerobic digestion has been considered as an effective way to recover energy from wastewater. However, the biogas produced typically contains over 30% of CO2, which significantly reduces its energy density. In this work, a biohybrid composite consisting of Methanosarcina barkeri and g-C3N4 (M. barkeri-g-C3N4) was synthesized to selectively convert CO2 in the biogas into CH4 under the light condition. The physicochemical and structural properties of the M. barkeri-g-C3N4 biohybrid were thoroughly characterized. Key factors influencing the biohybrid's performance on light-driven CH4 production were investigated, including g-C3N4 dosage, NaHCO3 concentration, pH, and cysteine concentration. A g-C3N4 concentration of 0.033 g/L, a pH of 7.02, a NaHCO3 concentration of 10 mM, and a cysteine concentration of 0.05 wt% were suggested as the optimal conditions. It was indicated that light-driven electrons can be used by M. barkeri for the conversion of CO2 to CH4. Additionally, the generated electrons can also be stored in the g-C3N4 semiconductor during light exposure and subsequently released in the dark to M. barkeri for CO2-to-CH4 conversion.