High-iodine groundwater has become a global public health concern due to its widespread distribution and the significant health risks associated with long-term consumption. However, systematic understanding of the migration, transformation, and enrichment mechanisms of iodide in groundwater remains limited. This study uses the western Xilingol League, China, as a case area to clarify the dominant forms, mobilization behavior, and main hydrogeochemical controls of iodine in shallow groundwater. The results show that iodide concentrations are negatively correlated with redox potential (Eh), and their spatial distribution is generally consistent with that of chemical oxygen demand (COD), indicating that reducing conditions and organic matter are important factors promoting iodine mobilization. Influenced by regional hydrodynamic conditions, water-rock interactions, and evaporative concentration, iodide is enriched mainly in groundwater discharge zones and low-lying closed depressions. Thermodynamic modeling confirms that iodide is the most stable iodine species under the observed hydrogeochemical conditions of the study area. Surface complexation modeling reveals that iodide is primarily adsorbed onto hydrous ferric oxide (HFO) surfaces via an outer-sphere mechanism. Its adsorption capacity decreases with increasing pH and ionic strength, while bicarbonate (HCO 3-) and sulfate (SO42-) enhance iodide mobility through competitive adsorption.