Unveiling Molecular Constraints on the Formation and Fate of Organoiodine Compounds under Aquifer Conditions

The interaction between dissolved organic matter (DOM) and iodine, as well as the complexation of organoiodine compounds (OICs) with Fe (hydr)oxides, is significantly influenced by their respective molecular characteristics. However, molecular constraints governing the formation and mobility of OICs within aquifer systems remain inadequately understood. Here, we employed ultrahigh-resolution mass spectrometry to investigate the molecular fractionation of organic compounds induced by DOM (humic acid (HA) and fulvic acid (FA)) iodination and by the subsequent formation of OICs complexation with Fe oxides under aquifer conditions. The findings demonstrated that FA served as a major source of unsaturated and aromatic DOM, resulting in the generation of 1390 OICs, which markedly exceeded the 240 iodinated products from dissolved HA. The formation of OICs was primarily dominated by the addition reaction, accounting for 47.0% of the total, followed by the substitution reaction at 33.8%. Unlike the substitution of hydrogen atoms with iodine at the C-H bonds, the addition of reactive iodine to C═C bonds in organic compounds could generate new oxygen-containing structures. These newly generated oxygen-containing single-bond functional groups significantly enhanced the complexation of the aqueous OICs with Fe oxides. These newly acquired molecular insights enhance the current understanding of the hydrogeochemical behavior of OICs in aquifer environments.