Fe-OM-I Redox Coupling during Groundwater Irrigation: A Hidden Pathway for Groundwater Iodine Redistribution and Organo-Iodine Formation

Chronic exposure to geogenic iodine-enriched groundwater increases thyroid disorder risks for millions of people worldwide. However, the under-recognized pathway of irrigation-induced iodine transport from groundwater to topsoil and its redistribution mechanisms in agricultural systems remain critical knowledge gaps. This study investigated geogenic iodine dynamics in the Jianghan Plain agricultural system, where widespread high iodine groundwater (54.5-655 μg/L) drives irrigation practices. Field experiments and hydrogeochemical analyses revealed that 7 days of irrigation increased surface soil iodine by 27-99% (reaching 1.47-2.92 μg/g). Fe(II) oxidation during aeration drives iodine cycling via Fe(III) oxyhydroxide formation, immobilizing groundwater iodine and transforming iodide to organic iodine (16%) and iodate (11%). Organic matter (OM) played dual roles: as an iodine reservoir and precursor for organo-iodine compounds (OICs) formation. Using Fourier transform ion cyclotron resonance mass spectrometry, we identified 93 newly formed OICs in dissolved organic matter (DOM) of aerated groundwater and 54 additional OICs in water-soluble organic matter (WSOM) of postirrigation soil, with highly unsaturated groundwater DOM and aromatic soil WSOM facilitating iodination. This research highlights irrigation-induced Fe-OM-I interactions as critical for understanding geogenic iodine exposure risks, emphasizing overlooked irrigation pathways in agricultural environments and establishing irrigation as a key exposure vector requiring urgent policy integration.