Reactive transport modeling to quantify the transformation of iron oxyhydroxides on the enrichment of iodine in groundwater of central Datong Basin

The enrichment of groundwater iodine is posing health risk for residents relying on groundwater for drinking. The transformation of iron oxyhydroxides plays the vital role in mobilizing sediment iodine into groundwater. However, few studies have provided sufficient knowledge of iodine adsorption on iron oxyhydroxides. In this study, we obtained thermodynamic parameters of iodate adsorption on magnetite and aluminum silicates through batch experiments. The extended three-layer model (ETLM) was developed to simulate the adsorption behavior under varying pH, ionic strength, and solid concentration. Using thermodynamic constants, a reactive transport model involving several scenarios was developed to explore the transformation of iron oxyhydroxides on the enrichment of iodine based on sediment profiles in the Datong Basin. Results indicate that the transformation from ferrihydrite to goethite within shallow aquifers reduces groundwater iodine concentrations by increasing the thermodynamic stability of the mineralogical system constituting the sediment, making iron oxyhydroxides less prone to reductive dissolution and release adsorbed iodine. In deep aquifers, newly formed secondary hematite and magnetite exhibit complementary surface properties, enhancing adsorption capacities of iodine compared to single-mineral systems. Overall, this study emphasizes the importance of minerals compositions and transformation on the mobilization of iodine in the groundwater system.