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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.


In what way and in what form does iodine react with water?

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).
When iodine is added to water, the following reaction results:

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.
Iodine may also occur as I3-(aq), HIO(aq), IO-(aq) en HIO3(aq). Iodine can bind to many different substances, for example other halogens. The compounds that are form behave differently when they come in contact with water.


Solubility of iodine and iodine compounds

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]-

When iodine ends up in surface waters, it may escape as iodine gas. Most iodine compounds are readily soluble in water or alcohol. Inorganic iodines are highly water soluble.

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.
Anthropogenic activities add iodine to the environment. Iodine is applied for different purposes, such as iodine x-rays, which are applied to patients in large doses (up to 200 g), and are than excreted through urine. This and other medicinal applicants may end up in groundwater through wastewater discharge. Iodine is applied for cleansing and disinfecting wounds, and is added to facial soap and band aids. Iodine disinfection mechanisms are ascribed to oxygen release from water molecules.
Chemical industries produce paint and chemicals for photography, batteries, lubricants and other purposes from iodine. Radioactive iodine is applied in medicine, for example in thyroid cancer treatment. It can be released during nuclear accidents. Iodine often ends up in surface water from waste water treatment plants, including radioactive isotopes. Iodine is retained by sludge for 2-25%. The residue remains in water, causing effluents to contain between 1 and 16 μg/L. High iodine concentrations may be present near chemical waste dumps.


What are the environmental effects of iodine in water?

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.
The LD50 value for rats (oral uptake) is about 14,000 mg/kg.
The most commonly known environmental impact of radioactive iodine was the nuclear reactor explosion at Chernobyl in 1986. The iodine isotope 131I was released in large amounts. The isotope has a half life of 8 days, and it can be taken up by cows and other cattle, thereby moving into human milk and water supplies. In the Ukraine radioactive iodine in flowing water still causes problems. Most rivers flow south. High tide erases radioactivity of the mainland. Normally, humans hardly ever come into contact with radioactive iodine, unless there is a work-related reason or some kind of medical treatment.

For more information on Chernobyl, see environmental disasters page


What are the health effects of iodine in water?

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.
Worldwide iodine uptake differs greatly between different peoples, because people that eat large amounts of fish take up higher amounts of iodine. This is between 50 μg and 10 mg per day.
Iodine may also negatively influence the thyroid gland. This is usually associated with hyperactivity. Sensitive people may develop an illness involving symptoms such as inches, bronchitis, sleeping disorders, rashes, etc, after long-term exposure to iodine. Skin contact with concentrated iodine may lead to pigmentation. Iodine fumes cause eye and lung irritation.
As a pure element, iodine is toxic, even amounts as small as 2 g may be lethal. In an alcoholic solution an amount of 30 ml may be lethal. Iodines are relatively harmless in comparison. However, rat experiments point out these may influence female fertility.
Children react fiercely to increased or decreased iodine uptake, because the thyroid gland is developing.


Which water purification technologies can be applied to remove iodine from water?

Iodine may be removed from water by means of active carbon.
Iodine is often applied to purify water. Its primary application is drinking water disinfection. Iodine tablets that may be added directly to water can be obtained.
Iodine can be regenerated, which is often applied for economic purposes.

Literature and the other elements and their interaction with water

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