Novel Role of Hematite in Anaerobic Digestion: Manipulating Membrane-Bound Electron Transport Chain by the Construction of Biological Capacitors with Humic Acid

Humic acid (HA) has attracted much attention for its electron-competitive effect of quinone groups on anaerobic methanogenesis. This study analyzed the biological "capacitor" to determine how it might effectively reduce electron competition. As biological capacitor-producing additives, three semiconductive materials, including magnetite, hematite, and goethite, were selected. The results showed that hematite and magnetite could significantly alleviate the inhibited methanogenesis caused by the HA model compound anthraquinone-2,6-disulfonate (AQDS). The electrons flowing to methane in hematite-AQDS, magnetite-AQDS, control, sole-AQDS, and goethite-AQDS groups accounted for 81.24, 77.12, 75.42, 70.55, and 56.32% of the total produced electrons, respectively. Hematite addition significantly accelerated the methane production rate (18.97%) compared with sole-AQDS. Electrochemical investigation showed that AQDS might have its oxidation potential reduced by adsorbing on hematite, which results in an energy band bending for hematite and the formation of a biological capacitor. The biological capacitor's integrated electric field helps with the transfer of electrons from reduced AQDS to anaerobic consortia via bulk hematite. Metagenomic and metaproteomic sequencing analyses revealed that the ferredoxin and Mph-reducing hydrogenase in hematite addition increased by 7.16 and 21.91%, respectively, compared to sole-AQDS addition. Accordingly, this research suggested that AH₂QDS may re-transfer electrons to methanogens via the biological capacitor and the membrane's Mph-reducing hydrogenase, thus lowering the HA electron competition.