Sulfur modification enhances promotion of carbon-iron composites on carbon chain elongation.

Efficient synthesis of caproate is crucial for the recovery of organic wastes. However, the yield and selectivity of caproate are limited by the efficiency of carbon chain elongation (CCE). This study presents sulfur-modified carbon-iron composites (BC[S-Fe]) as an innovative approach to enhance CCE for efficient caproate biosynthesis. Unlike conventional carbon-iron materials (BC[Fe]), sulfur modification stabilized nano-zero-valent iron (Fe) against oxidation and amplified redox activity, achieving an 83 % higher caproate yield (6914 mg·L) and 58 % carbon conversion efficiency. BC[S-Fe ] (S:Fe = 1:5) outperformed BC, Fe and BC[Fe] by enriching electroactive bacteria (e.g., Rummeliibacillus suwonensis, Seramator thermalis, and Rubeoparvulum massiliense) that upregulated electron transfer genes and CCE metabolic genes. Additionally, BC[S-Fe ] enhanced biofilm formation and quorum sensing, fostering microbial synergy to optimize electron flux. This work pioneers the integration of sulfur-modified composites into microbial CCE systems, offering a novel strategy to optimize caproate synthesis through enhanced electron transfer and functional microbiome engineering.