Synergistic enhancement of caproate production via biochar-iron composites: mechanistic insights into microbial community regulation and direct electron transfer

Caproate, a valuable medium-chain fatty acid for biofuel/antimicrobial applications, suffers from inefficient electron transfer in anaerobic chain elongation (CCE) systems. While biochar (BC) and zero-valent iron (Fe0) individually enhance CCE, scalability is limited by BC's variable conductivity and Fe0's aggregation/passivation. The effect of biochar-supported zero-valent iron (BC@Fe) in CCE is unexplored. In this study, batch reactors with acetate/butyrate/mix electron acceptors (EA) were amended with BC, Fe0, or BC@Fe (1-10 g/L) to systematically evaluate BC@Fe efficacy and mechanisms for caproate production versus BC or Fe0 alone. The results demonstrates the superior performance of BC@Fe in enhancing CCE across the three EA systems, achieving the highest and most stable caproate yields compared to standalone BC or Fe0. Compared with standalone BC or Fe0, BC@Fe achieves the highest caproate yields (11.02-12.72 g/L) across acetate, butyrate, and mixed EA systems. Mechanistic analyses show that BC prevents Fe0 aggregation and maintains optimal pH (6.3-6.65), while metagenomic sequencing reveals selective enrichment of electroactive caproate producers (e.g., Eubacterium aggregans and Rummeliibacillus suwonensis) and enhanced direct interspecies electron transfer (DIET) via conductive pili and c-type cytochromes. Key synergistic mechanisms include physical stabilization of Fe0, microbial community restructuring, and upregulation of reverse.