Redox-based electron exchange capacity of biowaste-derived biochar accelerates syntrophic phenol oxidation for methanogenesis via direct interspecies electron transfer

[Display omitted] •Effects of six types of biochar on anaerobic degradation of phenol was investigated.•Biochar addition triggered rapid methanogenic phenol degradation.•Electro-active Geobacter and Methanosaeta were enriched after biochar added.•Potential DIET was enhanced with biochar as electron transfer mediator.•Redox-active groups rather than electrical conductivity of biochar likely stimulated DIET. In this study, six different types of biochar (based on two feedstocks and three pyrolytic temperatures) were prepared as individual additives for both syntrophic phenol degradation and methanogenesis promotion. The results showed that for phenol degradation, the addition of biochar (15 g/L) shortened the methanogenic lag time from 15.0 days to 1.1–3.2 days and accelerated the maximum CH4 production rate from 4.0 mL/d to 10.4–13.9 mL/d. Microbial community analysis revealed that the electro-active Geobacter was enriched (from 3.8–7.7% to 11.1–23.1%), depending on the type of biochar that was added. This indicates a potential shift of syntrophic phenol metabolism from a thermodynamically unfavorable pathway with H2 as the interspecies electron transfer mediator to direct interspecies electron transfer (DIET). Integrated analysis of methanogenesis dynamics and the electrochemical properties of biochar showed that compared with electrical conductivity, the electron exchange capacity of biochar was more likely to dominate the DIET process, which was due to the presence of redox-active organic functional groups in biochar. The removal of biochar from the anaerobic system generally prolonged the lag time, revealing the importance of adsorption capacity of biochar to mitigate bio-toxicity of phenol to microbial activity.