Sulfate Reduction-Driven Inorganic–Organic Iodine Interconversion: Molecular Insights into Geogenic Iodine Mobilization in Aquifers

Sulfate reduction is recognized as a key process driving the transformation of weakly mobile iodate to highly mobile iodide in groundwater. However, its role in mediating the interconversion between inorganic and organic iodine in aquifers remains elusive, where iodate and organic iodine (OI) constitute the primary sedimentary reservoirs. Herein, controlled incubation experiments were conducted using sulfate-reducing microbial consortia enriched from iodine-rich sediments to examine iodine mobilization from ferrihydrite–iodine associations under aquifer conditions coupled with ultrahigh-resolution mass spectrometry to resolve molecular-level transformations. Sulfate reduction led to significant increases in dissolved iodide concentrations in both iodate and iodate–OI systems, while similar iodide generation was also observed in abiotic sulfide treatments. Molecular evidence demonstrated that iodine mobilization proceeded through iodate reduction coupled with organic matter (OM) iodination, together with OI dehalogenation. Specifically, OM iodination occurred primarily via addition (37.9%) and substitution (35.8%) reactions, with oxygen-poor compounds contributing over 50% of the newly formed OI. In contrast, OI dehalogenation exhibited minimal dependence on molecular categories and was associated with subsequent incorporation of S–H and S–OH functional groups, leading to iodide enrichment and CHO + S molecule formation. Collectively, these findings elucidate molecular pathways linking sulfate reduction to inorganic–organic iodine interconversion and advance the mechanistic understanding of geogenic iodine cycling.