Optimizing humic acid breakdown and methane production via semiconductor-assisted photo-anaerobic digestion

Humic acids (HAs), whether naturally present in anaerobic digestion (AD) substrates or HAs newly formed during fermentation, can become inhibitory to methanogenesis as their concentrations reach certain levels. This study delved extensively into the mechanism underlying the alleviation of HAs inhibition in photo-AD by N-doped carbon quantum dots (NCQDs). These NCQDs were efficiently synthesized from straw using an environmentally friendly pretreatment method. Our proposed method harnessed the combined effect of light exposure and NCQDs, resulting in synergistic enhancement of the cumulative CH4 yield within the HA-inhibited AD system, achieving a remarkable yield of 293.7 ± 17.7 mL/g VS. In-depth analyses were conducted on the remaining dissolved organic matter (DOM) within digesters using 3D-EEM and ESI FT-ICR MS. Simultaneously, the remaining HAs were extracted and subjected to FT-IR analysis. The findings revealed that NCQDs effectively degraded humic acid-like components in DOM into smaller, more manageable micromolecules characterized by lower carbon numbers and reduced double bond equivalent. Additionally, under the influence of light, NCQDs promoted the degradation of aromatic components within HAs. These resulting micromolecules were made readily available for utilization by microorganisms, further contributing to methanogenesis. Furthermore, photoelectrochemical analysis and specific gene qPCR analysis revealed that the photoelectrons from NCQDs and HAs were received and transferred by Ech (electron acceptors) for methanogenesis. Remarkably, this methanogenesis pathway, akin to photosynthesis, played a pivotal and transformative role in the photo-AD system. This work comprehensively revealed the remarkable potential mechanism of semiconductors within the photo-AD system, offering profound insights that can catalyze the development of innovative AD reactors and semiconductor accelerators.