Iodine (I) and water
Iodine and water: reaction mechanisms, environmental impact and health effects
| Iodine is naturally present in water. The average concentration in seawater is about 60 ppb, but varies from place to place. Rivers usually contain about 5 ppb of iodine, and in mineral sources some ppm can even be found. Brown algae accumulate up to 0.45% (dry mass) of iodine. Corals, sea sponges, shells and fishes accumulate high iodine concentrations, usually in the shape of thyroxin or tri-iodine thyroxin. |
Iodine is strongly reactive, even though it is less extreme for iodine than for other halogens. Iodine cannot be found as an element, but rather as I2 molecules, as I- ions, or as iodate (a salt of iodinic acid with IO3- anion).
I2(l) + H2O(l) -> OI-(aq) + 2H+(aq) + I-(aq)
I2 molecules and water molecules react to substances such as hypoiodite (OI-). The reaction can move both ways of the equilibrium, depending on the pH of the solution.
Water solubility of iodine is determined by temperature (20oC) and pressure (1 bar), and is relatively low. Iodine is better soluble in iodine solutions. The following reaction mechanism occurs:
I2 + I- -> [I3]-
Why is iodine present in water?
Iodine ends up in surface waters naturally through rains and water evaporation. Eventually, it also ends up in groundwater. Other options include weathering of iodine-containing rocks, and volcanic activity (including under-water volcanoes). In nature iodine can be found in reasonably large amounts, but only in compounds.
Iodine is attributed to water hazard class 1. This means it is only slightly harmful when dissolved in water. However, reactions with alkali metals, aluminum, mercury, fluorine or terpentine may increase the risk. Environmental risks may differ between iodine substances.
For more information on Chernobyl, see environmental disasters page
Iodine plays an important role in the production of thyroid hormones. The human body usually contains about 10-15 mg of this substance. The larger part can be found in the thyroid gland. The recommended daily intake for iodine is between 150 and 200 μg. Generally this dose is achieved by eating kitchen salt, to which iodine is added.
Iodine may be removed from water by means of active carbon.