Hidden Organic Iodine Dynamics: How Coupled C–Fe–S Biogeochemical Cycling Influences Iodine Behavior in Groundwater Systems

Coupled sulfur and methane cycling regulates iodine behavior in reducing groundwater systems, yet their influence on iodine bound to iron oxide-organic matter (Fe-OM) complexes, particularly the formation and transformation of organic iodine (OI), remains poorly understood. Here, we conducted controlled incubation experiments using a ferrihydrite-fulvic acid-iodine system with sulfur-disproportionating and methanogenic microbial consortia enriched from high iodine sediments in the Jianghan Plain aquifer to simulate in situ conditions, aiming to elucidate iodine mobilization and transformation regulated by coupled carbon–iron–sulfur (C–Fe–S) biogeochemical processes. Compared with the control systems, functional microorganisms selectively utilized organic matter, promoting reductive dissolution of iron oxides and increasing total iodine release by 13–40%, with rates rising from 1.37 to 1.55–1.93 mg·L–1·day–1. During the sulfur disproportionation-dominated stage, rapid iodate reduction coupled with iron oxide dissolution accounted for about 70% of total iodine release, while dissolved organic matter evolved toward higher aromaticity and molecular weight to facilitate OI formation. In the subsequent sulfate reduction-methanogenesis stage, highly aromatic OI underwent biotic/abiotic dehalogenation, releasing approximately 30% of total iodine as iodide. These findings demonstrate that OI is a highly dynamic and previously underappreciated source sustaining iodine groundwater enrichment. This study provides new mechanistic insights into the genesis of high-iodine groundwater and offers a theoretical basis for iodine risk assessment and groundwater management.