Enhancing photosynthetic CO2 fixation in microbial electrolysis cell (MEC)-based anaerobic digestion for the in-situ biogas upgrading.

[Display omitted] • In-situ biogas upgrading was achieved in a AD/PSB-coupled MEC. • Photosynthetic CO 2 fixation under electrical stimulation increased by 83.3%. • Electrically-stimulated PSB fixed CO 2 more efficiently even under power off. • Electrical stimulation enhanced the electroactivity and photoactivity of PSB. • Electrical stimulation changed the structural of photosynthetic reaction center. Biogas from anaerobic digestion usually contains 30%–50% of CO 2 , which needs to be upgraded before industrial utilization. However, current upgrading techniques typically consume large amounts of chemicals and energy. This study designed a green and low-energy-consumption microbial electrolysis cell (MEC)-based anaerobic digester for in-situ biogas upgrading, in which the cathode with photosynthetic bacteria (PSB) was used to fix CO 2. Results showed that charged MEC-AD increased photosynthetic CO 2 fixation by 83.3% and CH 4 production by 62.8% compared with uncharged open-circuit MEC-AD. Adding an extracellular electron uptake inhibitor to the cathode decreased the CO 2 fixation, accompanied by the decline of current between the two electrodes, which suggested that the cathode electron uptake was the main reason for the facilitated CO 2 fixation of PSB. The electrically-stimulated PSB fixed CO 2 more efficiently than unstimulated PSB even under power off, likely related to their enhanced electroactivity and photoactivity that could acquire more extracellular electrons and light energy for CO 2 fixation. The carbonyl-related hydrogen bonds in the LH1 complex of photosynthetic reaction centers increased under electrical stimulation, resulting in a red shift of absorbance spectra to improve the energy utilization of PSB. This upgrading technique consumed about 0.37 kWh/Nm3 CO 2removed of electric energy, much lower than other MEC-based techniques, providing a green and feasible strategy for in-situ biogas upgrading. [ABSTRACT FROM AUTHOR]