Semi-continuous anolyte circulation to strengthen CO₂ bioelectromethanosynthesis with complex organic matters as the e^-/H⁺ donor for simultaneous biowaste refinery

CO₂ bioelectromethanosynthesis represents a promising strategy for the capture and utilization of CO₂. In such process, the continuous generation of electron (e^-) and proton (H⁺) in anodic oxidation are of prime importance for the efficient cathodic CO₂ electroreduction and process stability. Proton transfer, however, is very easy to be hindered due to the fouling of proton exchange membrane (PEM). In this study, an artificial channel in microbial electrolysis cell (MEC) was proposed to strengthen the transport of protons from anodic to cathodic compartment, and H⁺-rich anolyte was semi-continuously circulated to the cathodic chamber to provide protons for CO₂ electroreduction. The results indicated that the daily CH₄ yield in cathode with anolyte circulation (18.5 mL/d·L-reactor) was 5.4-fold higher than that without circulation (2.9 mL/d·L-reactor). Meanwhile, efficient anodic biodegradation of organic components was observed with COD, proteins and polysaccharides removal of up to 95.6 ± 1.9%, 96.3 ± 3.7% and 99.1 ± 0.2% respectively, which supplied a continuous e^-/H⁺ donor for CO₂ electroconversion. 16S rRNA gene pyrosequencing analysis identified a large number of proteins-utilizing Bacteroidetes (14.11%) and polysaccharides-consuming Thermotogae (18.49%) in anodic biofilm, conductive to the biodegradation of organic components. Moreover, a high abundance of Methanobacterium (81.07%) was detected to prevail in cathodic biofilm, demonstrating the occurrence of highly enhanced CH₄ bioelectrosysthesis. The syntrophic and symbiotic relationship was established in the dual-bioelectrode system, creating a beneficial environment and an energy-efficient approach for biowaste refinery and CO₂ electromethanosynthesis.