Deciphering the effects of engineered biochar on methane production and the mechanisms during anaerobic digestion: Surface functional groups and electron exchange capacity

Along with the growth in carbon-based strategies for methanogenic improvement, there is increasing concern over correlations between biochar properties and stimulatory effects upon anaerobic digestion. In this study, the related characteristics and potential mechanisms of two modified biochars on anaerobic digestion were explored. Results showed the accumulative methane yield of H2O2-oxidized biochar group was 197.8 ± 4.4 mL/g COD, which increased by 58.7% compared to the blank group. Moreover, both methane yield and methane production rate were further enhanced by H2O2-oxidized biochar compared with raw algal biochar. On the contrary, methanogenesis ceased after the addition of HNO3-oxidized biochar. The H2O2-oxidized biochar enhanced acetoclastic methanogenesis, hydrogenotrophic methanogenesis could also benefit from the improved electron transfer activity. Compared with raw biochar, specific functional groups such as phenolic and lactonic groups on H2O2-oxidized biochar were increased by 1.2 and 5.1 times, respectively. The redox capacity of biochar, especially the electron-donating capacity was increased by 64.9% after H2O2 oxidization, thereby leading to the improved electron transfer activity and the enhanced methanogenesis. However, both acidogenesis and methanogenesis were inhibited by the HNO3-oxidized biochar, which could be due to the generation and/or dissolution of inhibitory compounds and the pH disturbance by nitrate/nitro groups after HNO3 oxidization. These findings provided fundamental evidence and knowledge for pre-selecting and even engineering effective biochar to improve methane production, which would be valuable for waste reduction and energy recovery via enhanced anaerobic digestion.