Adsorbent materials for high-iodine groundwater remediation: a critical review.

High-iodine groundwater is a widespread hydrogeochemical anomaly and an important environmental and health concern worldwide. It is commonly distributed in arid and semi-arid inland basins, alluvial plains, and coastal zones influenced by seawater intrusion. Its formation is governed by coupled hydrogeochemical and biogeochemical processes. Long-term exposure to high-iodine groundwater is associated with adverse health outcomes, particularly thyroid dysfunction and increased risks of goiter and thyroid cancer. Because in situ remediation alone is often insufficient to ensure sustained reduction of iodine to levels suitable for safe drinking water, engineered water treatment remains the most direct and reliable option. Among them adsorption is widely studied and practically relevant treatment approach. This review critically evaluates the adsorbent materials relevant to high-iodine groundwater remediation, including metal-based materials, carbonaceous adsorbents and functionalized organic materials. These materials target the dominant groundwater iodine species through distinct mechanisms such as ion exchange, strong metal-iodine interactions, moderate oxidation-adsorption coupling, and multi-site binding enabled by tailored functional groups or pore environments. Current evidence shows that adsorbent performance depends not only on adsorption capacity, but also on species selectivity, matrix tolerance, and practical applicability under trace-level and chemically complex groundwater conditions. However, most existing adsorbents remain limited by narrow iodine-species selectivity, interference from competing anions and natural organic matter, and insufficient evidence for regeneration and field applicability. This review highlights the key material classes, mechanistic foundations, critical limitations, and design principles that should guide the development of selective, robust, and practically usable adsorbents for high-iodine groundwater remediation.