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

Hematite could induce the formation ofa biological capacitorwith HA and manipulate the membrane-bound electron transport chainfor superior energy recovery efficiency. Humic acid (HA) has attracted much attention for itselectron-competitiveeffect of quinone groups on anaerobic methanogenesis. This study analyzedthe biological "capacitor" to determine how it mighteffectively reduce electron competition. As biological capacitor-producingadditives, three semiconductive materials, including magnetite, hematite,and goethite, were selected. The results showed that hematite andmagnetite could significantly alleviate the inhibited methanogenesiscaused 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 productionrate (18.97%) compared with sole-AQDS. Electrochemical investigationshowed that AQDS might have its oxidation potential reduced by adsorbingon hematite, which results in an energy band bending for hematiteand the formation of a biological capacitor. The biological capacitor'sintegrated electric field helps with the transfer of electrons fromreduced AQDS to anaerobic consortia via bulk hematite. Metagenomicand metaproteomic sequencing analyses revealed that the ferredoxinand Mph-reducing hydrogenase in hematite addition increased by 7.16and 21.91%, respectively, compared to sole-AQDS addition. Accordingly,this research suggested that AH(2)QDS may re-transfer electronsto methanogens via the biological capacitor and the membrane'sMph-reducing hydrogenase, thus lowering the HA electron competition.