KOH activation increased biochar's capacity to regulate electron transfer and promote methanogenesis

Accelerating direct interspecies electron transfer (DIET) between syntrophic partners can boost methanogenesis. Although biochar can promote DIET, low specific surface area (SSA) and electron transfer capacity (ETC) limit its application. KOH activation can improve SSA and ETC of biochar and its effectiveness in methanogenesis, yet the related methanogenic mechanism remains unclear, particularly for association between the KOH-activated biochar's properties and microbes. Herein, three biochars produced at 300, 500 and 800 °C were used to generate the corresponding KOH-activated biochars via a sequential carbonization and KOH activation procedure (CKAP) at 800 °C, which were labeled as CKAP300, CKAP500 and CKAP800, respectively. The effectiveness of KOH activation relied on the biochar type, exhibiting an increase of 92–1532 times in SSA and 11–22 % in graphitization degree. The KOH-activated biochar outperformed the pristine biochar in promoting methanogenesis, with a maximum increase of 75 % in methane yield and 3.1 times in maximum methane production rate (Rm). CKAP800 enhanced methane yield by 12 % and Rm by 29 % compared to CKAP300 and CKAP500, possibly due to the facts that high SSA facilitated the attachment and interaction of DIET partners ( Treponema and Methanosaeta ), and that high graphitization degree, c-type cytochrome and C-O groups promoted the DIET-based syntrophic metabolism. • KOH activation raised biochar's SSA and graphitic structure and introduced C-O group. • The effectiveness of KOH activation relied on the biochar type. • The pristine biochar was inferior to the KOH-activated biochar in methanogenesis. • CKAP800 had better performance in promoting methanogenesis than CKAP300 and CKAP500. • CKAP800 helped promote DIET between Treponema and Methanosaeta.