Tributyltin compounds
Tributyltin (TBT) is an umbrella term for a class of organotin compounds which contain the (C4H9)3Sn group, with a prominent example being tributyltin oxide. For 40 years TBT was used as a biocide in anti-fouling paint, commonly known as bottom paint, applied to the hulls of ocean going vessels. Bottom paint improves ship performance and durability as it reduces the rate of biofouling (the growth of organisms on the ship's hull). Unfortunately, the TBT slowly leaches out into the marine environment where it is highly toxic toward nontarget organisms. After it led to collapse of local populations of organisms, TBT was banned.
TBT compounds are organotin compounds, with 3 butyl groups covalently bonded to a tin(IV) centre. A general formula for these compounds is (n-C4H9)3Sn-X. The X group is typically an electronegative, such as chloride, hydroxide, or carboxylate. When introduced into a marine or aquatic environment, TBT adheres to bed sediments because of its high specific gravity and low solubility. However, the adsorption of TBT to sediments is reversible and depends on pH. Studies have shown that 95% of TBT can be released from the sediments back into the aquatic environment. This absorption process can complicate quantification of TBT in an environment, since its concentration in the water is not representative of its availability.
Because TBT is the most effective anti-fouling agent discovered, it was frequently used in anti-fouling paint around the globe. It is also relatively inexpensive. Its use has been largely banned.
The antifouling properties of TBT compounds were discovered in the 1950s in the Netherlands by van der Kerk and coworkers. The function of the biocide in the anti-fouling paint is to prevent the settling of organisms on the hull and to poison the organisms that do. Although an effective biocide, tributyltin was wrongly deemed safe environmentally. By the mid 1960s it became the most popular anti-fouling paint worldwide. TBT was mixed into paints to extend the life of antifouling coatings, and ships were able to continue operations for a longer time frame. The paints ensured fuel efficiency and delayed costly ship repairs. It is also an ingredient in some disinfectants, for example in combination with quaternary ammonium compounds.
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