Iodine (I) and water

Iodine occurs in water mainly as dissolved iodide and iodate, with the proportions depending on the water’s chemistry and redox conditions. Elemental iodine is only sparingly soluble. Natural levels are usually low, but brines, wastewater and iodine-containing products can create local exceptions.

Iodine species in water

In natural waters, iodine is present chiefly as the anions iodide (I⁻) and iodate (IO₃⁻), rather than as elemental iodine. Seawater contains about 0.06 mg/L total iodine in commonly cited measurements, with both iodide and iodate present. WHO reports iodine concentrations of roughly 0.5–20 µg/L in rivers and lakes; individual sites can differ substantially.^[1]^[2]

Iodine is essential for thyroid-hormone production, but this does not mean that higher water concentrations are beneficial. WHO has not established a health-based guideline value for iodine in drinking water because the available evidence is insufficient and long-term exposure from water disinfection is not expected under recommended use. Iodine may be used as a point-of-use disinfectant in limited situations; WHO does not recommend it for routine, long-term drinking-water disinfection.^[2]^[3]

How iodine enters water

Weathering, marine sediments and groundwater interaction contribute natural iodine. Iodinated contrast agents and other iodine-containing products can enter wastewater after use, while iodine can also be intentionally added during emergency or point-of-use disinfection. Radioactive iodine isotopes from medical or nuclear sources require separate radiological monitoring; they should not be inferred from a measurement of stable iodine.

Managing iodine in water

There is no single removal method that can be recommended for all iodine-bearing waters. Treatment depends on whether iodine is present as iodide, iodate, elemental iodine or an organic compound, and on competing salts and natural organic matter. For a specific water, measure the relevant species and conduct treatability testing before selecting adsorption, ion exchange or membrane treatment. If radioactive iodine is suspected, consult the responsible radiological or public-health authority and use isotope-specific analysis.

Last reviewed: 31 October 2026 by Lenntech.