A functional transformation of humic acids in bioelectrochemical anaerobic digestion: From inhibitors to electron shuttles and methane precursors

Anaerobic digestion of hydrothermally liquefied sludge (HLS) is often constrained by the presence of recalcitrant organics, particularly humic acids (HAs), which hinder biodegradability and electron transfer. This study investigated the effect of electric field-assisted anaerobic digestion on the structural transformation of HAs and its impact on methane production. A moderate electric field (2 V/cm) significantly reduced the molecular weight and aromaticity of HAs, as evidenced by UV absorbance ratios (A4/A6, A2/A6) and FTIR analysis of functional groups. These changes enhanced the redox activity of HAs, enabling them to act as both electron shuttles and direct precursors for methanogenesis. As a result, methane production increased 15.6-fold (325.5 mL) compared to the control. Cyclic voltammetry further confirmed greater electrochemical activity at 2 V/cm, while functional gene prediction revealed upregulation of key genes such as fabG, mtr, and nqr, which are associated with the degradation of HAs and electron transport processes. Overall, these findings demonstrate that bioelectrochemical stimulation improves electron transfer efficiency, promotes syntrophic microbial interactions, and significantly enhances methane production in anaerobic digestion systems treating substrates rich in HAs.