Enhanced electron transfer mediated by surface functional groups of targetedly modified sludge-based biochar for sustainable microbial chain elongation.

While unmodified sludge-based biochar (SBBC) exhibits limited electron transfer capacity in chain elongation (CE), its surface functional groups are recognized as critical factor influencing direct interspecies electron transfer (DIET). This study employed three modification methods, i.e., H2O2, HNO3, and anthraquinone sulfonate (AQS), to explore the effects of modified SBBC on CE process. Results indicated that the AQS-modified SBBC (SBBC-AQS) exhibited optimal electron transfer properties and electrochemical activity. Thus, SBBC-AQS greatly promoted DIET-mediated CE, leading to 62% higher caproate production compared to no biochar. Further recycling experiments confirmed the reusability of SBBC-AQS, along with the biofilm formation. Lastly, metagenomic analysis revealed that modified SBBC improved the abundance of CE bacteria such as Clostridium kluyveri, and introduced electron-transferring bacteria such as Petrimonas, so as to reinforce ethanol oxidation and medium-chain fatty acids production. This study offers novel insights into the sustainable material and microbes synergistic regulation to achieve an efficient CE system.