Enhancement of short-chain fatty acids production by biochar-supported zero-valent iron in anaerobic fermentation: focusing on metabolic reactions and electron transport.

Carbon-based and iron-based materials have been widely reported as effective promoters in biogas fermentation due to the promotion of electron transport. However, the effect of these materials, especially in combination, on short-chain fatty acids (SCFAs) production has been scarcely reported. In this study, the production of short-chain fatty acids (SCFAs) from green cabbage waste was promoted by adding biochar (BC) and biochar-supported zero-valent iron (BC@ZVI). The underlying mechanisms, focusing on metabolic pathways and electron transport, were subsequently investigated through metagenomic analysis. The optimal SCFAs yields were achieved with BC (5 g·L⁻1) and BC@ZVI (15 g·L⁻1). While BC notably enhanced n-butyrate production (89.4-fold), BC@ZVI balancedly promoted acetate and n-butyrate. Metagenomics revealed that BC@ZVI's superiority stemmed from its enhanced ability to enrich functional microbes and facilitate electron transfer. Metagenomic analysis revealed that BC@ZVI enriched Sphaerochaeta and Herbinix, which could participate in the direct interspecies electron transfer process. The abundance of almost all functional enzymes involved in carbohydrate hydrolysis and the synthesis of acetate and n-butyrate were remarkably increased by BC@ZVI. BC and BC@ZVI lead to a notable enrichment of conductive pili genes, including pilB, pilC, and pilM. BC@ZVI enriched both conductive pili and c-type cytochromes, which could be considered a more effective selection than BC. Notably, BC@ZVI was more effective than BC in stimulating n-butyrate-type fermentation, significantly shortening the lag phase and the overall fermentation cycle, thereby exhibiting better comprehensive performance, enhancing pH buffering capacity, and strengthening electron transfer and substrate hydrolysis. The results proved the potential of BC@ZVI in SCFAs fermentation and deciphered the underlying mechanisms, which provided a new perspective to promote resource recovery of organic waste by anaerobic system.