Deciphering the role and mechanism of rare earth element-rich biochar during anaerobic digestion of Dicranopteris pedata biomass

High solid anaerobic digestion (HSAD) is a promising technology to manage Dicranopteris pedata biomass (DPB) that generated from the phytoremediation process of rare earth tailings area. Rare earth element (REE)-rich DPB can be converted to biochar via a direct pyrolysis approach, yet the efficacy of REE-biochar in the HSAD process remains unclear. Here, three types of REE-biochars (B300, B500 and B800) were produced at 300, 500 and 800 °C. The results showed that elevating pyrolysis temperature improved the REE-biochar properties such as specific surface area (SSA), electrical conductivity (EC), REE content, and graphitization degree, but reduced its electron exchange capacity (EEC). Compared to B300 and B500, the B800 addition clearly boosted methane yield (up to 60% and 29%) and maximum methane production rate (up to 21% and 15%). Microbial analysis showed that B800 not only enriched the acidogenic bacteria (e.g., Ruminofilibacter) and electroactive microbes (e.g., DMER64 and Syntrophus) but also enhanced microbial metabolisms and electron transfer ability. Further analysis showed that B800 facilitated methanogenesis probably by coordinating the interaction of key genera through quorum sensing system and modulating electron transfer through its intrinsic graphitic matrices and REE oxides. This study offered a new approach to recycling REE-rich biomass and a guidance to design functional biochar material for improving the HSAD performance.