Unveiling iodine mobilization pathways in high-iodine aquifer: A biogeochemical perspective

High-iodine groundwater poses global risks to public health, but the biogeochemical mechanisms governing the mobilization of different iodine speciation in heterogeneous hydrogeological environments remain unclear. This study investigates sediment samples from two boreholes in the Jianghan Plain (a typical high-iodine area in central China) integrating geochemical analysis, organic matter fluorescence spectroscopy analysis and microbial functional prediction to unravel universal iodine mobilization pathways. The results identify two distinct iodine speciation-dominated systems within the same high-iodine aquifer: an iron oxide-bound iodine (I_Feox)-dominated system (B1) and an organic matter-bound iodine (I_resi)-dominated system (B2). A key finding is that assimilatory sulfate reduction (ASR)-coupled iron reduction is strongly associated with I_Feox mobilization, ASR synthesizes labile sulfur-containing organic matter that may serve as electron donors for iron-reducing bacteria, thereby promoting reductive dissolution of iron oxides and subsequent release of I_Feox. In contrast, the mobilization of I_resi is correlated with a process involving anaerobic oxidation of methane (AOM)-coupled sulfur reduction. AOM may mediate the degradation of iodine-containing organic matter through microbial and chemical processes, and the released iodine furtherly complexes with carbonate to form carbonate-bound iodine (I_carb). This study challenges the traditional paradigm that dissimilatory sulfate reduction (DSR) dominates I_Feox release, filling the gap in the role of ASR in iodine biogeochemical cycling. It also highlights the regulatory role of AOM in linking methane cycle to iodine mobilization, providing a novel biogeochemical perspective for understanding the potential migration risk of organic iodine in high-iodine aquifers.