Molecular Interaction of Aqueous Iodine Species with Humic Acid Studied by I and C K-Edge X-ray Absorption Spectroscopy

Hidden Role of Organic Matter in the Immobilization and Transformation of Iodine on Fe-OM Associations

The biogeochemical processes of iodine are typically coupled with organic matter (OM) and the dynamic transformation of iron (Fe) minerals in aquifer systems, which are further regulated by the association of OM with Fe minerals. However, the roles of OM in the mobility of iodine on Fe-OM associations remain poorly understood. Based on batch adsorption experiments and subsequent solid-phase characterization, we delved into the immobilization and transformation of iodate and iodide on Fe-OM associations with different C/Fe ratios under anaerobic conditions. The results indicated that the Fe-OM associations with a higher C/Fe ratio (=1) exhibited greater capacity for immobilizing iodine (∼60-80% for iodate), which was attributed to the higher affinity of iodine to OM and the significantly decreased extent of Fe(II)-catalyzed transformation caused by associated OM. The organic compounds abundant in oxygen with high unsaturation were more preferentially associated with ferrihydrite than those with poor oxygen and low unsaturation; thus, the associated OM was capable of binding with 28.1-45.4% of reactive iodine. At comparable C/Fe ratios, the mobilization of iodine and aromatic organic compounds was more susceptible in the adsorption complexes compared to the coprecipitates. These new findings contribute to a deeper understanding of iodine cycling that is controlled by Fe-OM associations in anaerobic environments.

Enrichment of Geogenic Organoiodine Compounds in Alluvial-Lacustrine Aquifers: Molecular Constraints by Organic Matter

Organoiodine compounds (OICs) are the dominant iodine species in groundwater systems. However, molecular mechanisms underlying the geochemical formation of geogenic OICs-contaminated groundwater remain unclear. Based upon multitarget field monitoring in combination with ultrahigh-resolution molecular characterization of organic components for alluvial-lacustrine aquifers, we identified a total of 939 OICs in groundwater under reducing and circumneutral pH conditions. In comparison to those in water-soluble organic matter (WSOM) in sediments, the OICs in dissolved organic matter (DOM) in groundwater typically contain fewer polycyclic aromatics and polyphenol compounds but more highly unsaturated compounds. Consequently, there were two major sources of geogenic OICs in groundwater: the migration of the OICs from aquifer sediments and abiotic reduction of iodate coupled with DOM iodination under reducing conditions. DOM iodination occurs primarily through the incorporation of reactive iodine that is generated by iodate reduction into highly unsaturated compounds, preferably containing hydrophilic functional groups as binding sites. It leads to elevation of the concentration of the OICs up to 183 μg/L in groundwater. This research provides new insights into the constraints of DOM molecular composition on the mobilization and enrichment of OICs in alluvial-lacustrine aquifers and thus improves our understanding of the genesis of geogenic iodine-contaminated groundwater systems.

Molecular characteristics of natural organic matter in the groundwater system with geogenic iodine contamination in the Datong Basin, Northern China

Natural organic matter (NOM) plays an important role in the iodine mobilization in the groundwater system. In this study, the groundwater and sediments from iodine affected aquifers in the Datong Basin were collected to perform chemistry analysis and molecular characteristics of NOM by Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS). Total iodine concentrations in groundwater and sediments ranged from 1.97 to 926.1 μg/L and 0.001-2.86 μg/g, respectively. A positive correlation was observed between DOC/NOM and groundwater/sediment iodine. FT-ICR-MS results showed that the DOM in the high-iodine groundwater system is characterized by less aliphatic and more aromatic compounds with higher NOSC, indicating the features of more unsaturated larger molecule structures and more bioavailability. Aromatic compounds could be the main carriers of sediment iodine and were easily absorbed on amorphous iron oxides to form the NOM-Fe-I complex. More aliphatic compounds, especially those containing N/S, experienced a higher degree of biodegradation, which further mediated the reductive dissolution of amorphous iron oxides and the transformation of iodine species, thereby causing the release of iodine into groundwater. The findings of this study provide some new insights into the mechanisms of high-iodine groundwater.

1H-13C heteronuclear single quantum coherence NMR evidence for iodination of natural organic matter influencing organo-iodine mobility in the environment

The complex biogeochemical behavior of iodine (I) isotopes and their interaction with natural organic matter (NOM) pose a challenge for transport models. Here, we present results from iodination experiments with humic acid (HA) and fulvic acid (FA) using ¹H-¹³C heteronuclear single quantum coherence (HSQC) nuclear magnetic resonance (NMR) spectroscopy. Even though not a quantitative approach, ¹H-¹³C HSQC NMR corroborated that iodination of NOM occurs primarily through aromatic electrophilic substitution of proton by I, and also revealed how iodination chemically alters HA and FA in a manner that potentially affects the mobility of iodinated NOM in the environment. Three types of iodination experiments were conducted with HA and FA: a) non-enzymatic iodination by IO₃^- (pH 3) and I^- (pH 4 and 7), b) addition of lactoperoxidase to promote I^--iodination in the presence of the co-substrate, H₂O₂ (pH 7), and c) addition of laccase for facilitating I^--iodination in the presence of O₂, with or without a mediator (pH 4). When mediators or H₂O₂ were present, extracellular oxidases and peroxidases enhanced I^- incorporation into NOM by between 54% and 3400%. Iodination of HA, which was less than that of FA, enhanced HA's stability (inferred from increases in aliphatic compounds, decreases in carbohydrate moieties, and thus increased molecular hydrophobicity) yet reduced HA's tendency to incorporate more iodine. As such, HA is expected to act more as a sink for iodine in the environment. In contrast, iodination of FA appeared to generate additional iodine binding sites, which resulted in greater iodine uptake capability and enhanced mobility (inferred from decreases in aliphatic compounds, increases in carbohydrates, and thus decreases in molecular hydrophobicity). These results indicate that certain NOM moieties may enhance while others may inhibit radioiodine mobility in the aqueous environment.