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.