Toward sulfur retention and H2S mitigation in composting: elucidating and validating the molecular pathway of Fe(ii)/Fe(iii)-driven sulfur incorporation into humic acid

Abstract

This study elucidated the molecular mechanisms by which iron of different valence states (Fe(II)/Fe(III)) regulates the migration and stabilization of sulfur in humic acid (HA) during composting, as well as its key role in efficiently suppressing hydrogen sulfide (H₂S) emissions. The results demonstrated that the iron valence state dictates the final pathway and stability of sulfur fixation: Fe(III) acted as an oxidative catalytic center, driving sulfur through an oxidation pathway, ultimately leading to its covalent fixation within the complex HA framework as stable functional groups such as sulfonyl and sulfonamide (e.g., C₁₉H₃₀N₄O₄S), achieving the “long-term sequestration” of sulfur. This pathway resulted in a 58.04% increase in sulfur-containing humic acid content and synergistically achieved a 94.68% reduction in H₂S emissions. In contrast, Fe(II) directed sulfur through a reductive pathway, forming unstable intermediates like thioethers (e.g., C₁₀H₁₆N₄S₂), which accumulated rapidly in the early stages but proved difficult to maintain over the long term. Abiotic synthesis experiments further confirmed the direct catalytic role of Fe(III) in the formation of stable organic sulfur. The study also found that iron additives significantly suppressed the expression of functional genes (e.g., dsrA) in sulfate-reducing bacteria, thereby reducing H₂S generation at the source. Based on these findings, this study ultimately establishes a dual-safeguard mechanism for iron-mediated H₂S emission reduction, namely “source inhibition of microbial reduction” and “end-of-pipe catalytic oxidative fixation”, providing a theoretical basis and technical pathway for the precise control and resource recovery of sulfur pollution during composting.