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Tin (Sn) and water

Tin and water: reaction mechanisms, environmental impact and health effects

On average seawater contains 1-10 ppt of tin, and river water contains 6-40 ppt. The Lacuta seaweed generally contains 12 ppb of tin, and mussles contain up to 160 ppb (dry mass). Dissolved in water tin generally occurs as SnO(OH)3-, and in both seawater and freshwater mono-, di- and trimethyltin compounds can be found. This are partially decomposed to volatile compounds.


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

Under normal circumstances tin is stable in water. When it comes in contact with hot water vapour a reaction results, forming tinoxide and hydrogen:

Sn + 2 H2O -> SnO2 + 2 H2

Some tin compounds hydrolyse in water. Examples include tin (IV) chloride, which forms tinoxide when heated.


Solubility of tin and tin compounds

Elementary tin does not dissolve in water under normal conditions (T = 20oC and pressure = 1 bar). Most tin compounds do not dissolve in water either, examples include tin (IV) oxide, tin (II) hydroxide, tin (IV) sulphide and tributyltin (TBT). Other tin compounds such as tin (II) chloride are water soluble. Organotin compounds are relatively badly water soluble, but may adsorb to sediments.


Why is tin present in water?

A number of tin minerals occur naturally, of which only cassiterite plays a role in commercial processes. Its main constituent tin (IV) oxide is water soluble and thereby weathering is prevented. The naturally occurring amount of tin in soils and water is relatively small. The release of tin from anthropogenic processes exceeds tin release from geological processes 110 times.
Tin is applied as a protective layer on iron cans to prevent corrosion. This property is mainly applied in conserve can production, and relatively large amounts of tin may enter the food chain through weathering from unvarnished cans. Conserve cans are partially recycled. Tin is present in various alloys, such as soldering tin, bronze and amalgam for dentistry. However, most tin is applied in cans.
Tin compounds such as TBT are applied as polymer additives in antifouling paint as a ship hull preservative. In a thin layer these may serve as a protective coating on glass and ceramics. Tin (II) chloride is a reductive agent, and tin (IV) sulphide serves as tin bronze for paints. Tin (II) fluoride is added to toothpaste.
Organotin compounds are applied as fungicides, insecticides, acaricides and bactericides. Additionally, these are applied as PVC and PCB heat stabilizers.
Superconductor magnets, cylinder blocks in engines and drum brakes contain this metal. TBT oxide is applied for wall paper soaking, as a wood preservative, and for sludge removal from industrial wastewater. It is a constituent of fire extinguishers, and of electro-industrial wastewater. When security measures are inadequate, tin and tin compounds may end up in the environment.
Radioactive tin isotopes are applied in nuclear physics.


What are the environmental effects of tin in water?

Tin is possibly a dietary requirement for a number of organisms. This may be caused by its presence in gastrine, a stomach and intestinal hormone.
In normal air-dried soil approximately 1-20 ppm of tin is present, and plants contain 0.8-7 ppm (dry mass). Plants readily absorb tin, but do not apply it. It usually remains within plant roots. Extreme soil tin concentrations are above 30 ppm. In some contaminated soils tin concentrations up to 2000 ppm were found. Approximately 1-4 ppm of tin (IV) oxide naturally occurs in most soils, but in some soils this concentration is below 0.1 ppm. Peat is an exeption, this may contain up to 300 ppm of tin (IV) oxide.
Upon oral intake tin compounds may be toxic. It is unknown exactly how much tin contributes to toxicity. Inorganic tin compounds are generally non-toxic. Toxicity of other tin compounds varies strongly. Organic tin compounds are toxic to bacteria and fungi, whereas these are non-toxic to animals and humans. The tolerable soil concentration of this compound is 50 ppm. Tri organic tin compounds are bound to mitochondria, and may disturb oxidative phosphorilation. Fortunately, these may be microbially decomposed in soils. LD50 values for tri organo derivates for rats are between 4 and 50,000 mg/kg, depending on the actual compound.
TBT may cause major environmental problems. This compound is extremely toxic, but nevertheless was applied in protective paint for ship hulls, starting 1960. In 1980 one came to the conclusion that TBT causes sexe alterations in water snails, which may lead to reproductive failure. This occurs at concentrations above 1 ppb in water, and often affects up to 90% of the population. The species in question threatened to become extinct. Marine organism mortality increases when TBT is present. The most extreme TBT concentrations were found in coastal areas and hunting areas.
Examples of tin toxicity are: LD50 = 700 mg/kg for tin (II) chloride, LD50 = 2300 mg/kg for tin (IV) chloride for rats upon oral intake.
Tin occurs as 10 stable and 19 instable isotopes. 113Sn is strongly radiotoxic.


What are the health effects of tin in water?

The human body contains approximately 0.2 ppm of tin. There is no concrete evidence that suggests a dietary requirement of tin, except for the fact that it is a constituent of the gastrine hormone (see earlier). A number of literature sources mention that tin deficits may lead to hair loss, anorexia and acne.
Our daily intake of tin is approximately 0.3 mg, of which 0.2 mg occurs in food and the remainder is added by conserve can corrosion. In food tin concentrations of 200-300 ppm are tolerated, and in the past concentrations up to 700 ppm occurred. Consequences of toxic tin levels are vomiting and diarrhoea. Of the 3% of tin (II) and 1% of tin (IV) that is absorbed by the body, most is excreted. Small amounts end up in the liver and the bones.
Metallic tin is generally non-toxic to humans, even upon uptake of small concentrations for a long period of time. Naturally occurring tin compound rarely affect human health. Some tin compounds do cause adverse health effects. This mainly concerns organic tin compounds. For example, TBT oxide causes skin irritations, breathing problems, vomiting, headaches and facial disorder. Tin tetra ethyl is absorbed rapidly by skin and mucous membranes, was found a nerve gas in animal tests, and may therefore be toxic to humans. Tin (IV) hydride is another toxic nerve gas. Trimethyltin and triethyltin are mildly toxic to humans, whereas tripentyltin is generally non-toxic and is therefore applied as a pesticide.
The above-mentioned TBT may cause hormonal disorder in humans, which may ultimately result in infertility.


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

Like any other metal tin may be removed from water by ion exchange.

Literature and the other elements and their interaction with water

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