Function identification of nitrogen forms in nitrogen-doped biochar for promoting methane production in anaerobic...

Nitrogen-doped biochar material (NBM) is considered as one of encouraging materials for enhancing methane yield through anaerobic digestion (AD). However, the key roles of nitrogen forms in NBM remain unclear. This study prepared NBM under four pyrolysis temperatures, and nitrogen forms were characterized to reveal the mechanisms how they affect methane production. Rising pyrolysis temperature reduced the nitrogen content in the NBM and altered its functional groups and nitrogen forms. The highest cumulative methane yield was obtained with NBM prepared at 700 °C, with a 31.8 % increase ratio compared with that in the control. Mechanism analysis indicated that a high content of graphitic nitrogen (accounting for 29.6 % of the nitrogen forms) in NBM-700 enhanced degradation of polysaccharides and proteins and accelerated the transformation of short-chain fatty acids, thereby promoting hydrolysis and acidogenesis processes. Meanwhile, it promoted the accumulation of redox-active substances within extracellular polymeric substances, thus accelerating IET between syntrophic bacteria and methanogens. In terms of key enzyme activities and microbial communities, pyrrolic nitrogen and graphitic nitrogen in NBM exert positive effects. Taking NBM-700 as an example, it increased α-glucosidase, total protease activities, cytochrome C, and coenzyme F₄₂₀ by 14.8 %, 32.1 %, 54.3 %, and 29.7 %. Additionally, graphitic nitrogen enriched electroactive microorganisms like Clostridium_sensu_stricto_13, Methanosaeta, and Methanosarcina, positively influencing the establishment of DIET pathway for methanogenesis. Besides, NBM enhanced the contribution potential of acetotrophic methanogenesis pathway to the production of methane. These findings provided valuable insights for enhancing methane production of AD with NBM as the exogenous additives.