Tributyl tin

Tributyl tin (TBT) is a synthetic organic tin compound that is primarily used as an additive to paints. In this application, TBT acts as a biocide or compound that kills plants such as algae, fungus, mildew, and mold that grow in or on the surface of the coating. These antifouling paints, as they are called, are applied to the hulls of ships to prevent sea life (e.g., barnacles and algae) from attaching to the hull. Growth of organisms on the hulls of ships causes ships to slow down and increases fuel consumption due to increased drag. Ships that are covered with these organisms can also transport non-native, or invasive, species around the world, which can disrupt an ecosystem and cause reductions in biological diversity. TBT is also used in lumber preservatives as an industrial biocide.

TBT-based paints were introduced more than 40 years ago. The first TBT anti-fouling paints, known as Free Association Paints (FAPs), where the TBT is mixed into the paint, prevented fouling by uncontrolled leaching of TBT into the environment . Starting 20 years ago, FAPS were gradually phased out and replaced with TBT Self Polishing Copolymer Paints, in which the TBT is bound in the paint matrix. This type of formulation allows for TBT to be released slowly and uniformly.

TBT, also known as organotin, is a widespread contaminant of water and sediments in harbors, shipyards, and waterways. TBT is persistent in marinas, estuaries, and other waters where circulation is poor and flushing irregular. In the natural aquatic environment, degradation of TBT through biological processes is the most important pathway for the removal of this toxic compound from the water column. At levels of TBT contamination typical of contaminated surface waters, the half-life is six days to four months. Half-life refers to the amount of time it takes for one-half of the TBT to be eliminated from the environment through natural means.

TBT has been shown to have significant environmental impact and is considered to be an endocrine disruptor that affects the immune and reproductive systems. In molluscs, TBT can be found in concentrations that are up to 250,000 times higher than in the surrounding sediment or seawater. Contaminated molluscs had deformed shells, slow growth rates, and poor reproduction, with eggs and larvae often dying. Exposure to TBT has resulted in sex change and infertility in at least 50 species of snails. In the dogwhelk, which is a small marine snail, TBT causes the female snail to grow a penis that blocks the oviduct and thus prevents reproduction. This abnormality is referred to as "imposex," and its development has been linked to the concentrations of TBT in tissue. Because of this association, imposex in the dogwhelk has been used as an indicator of TBT pollution . TBT has also been detected in large predators such as sharks , seals , and dolphins , indicating that TBT can accumulate in the food chain, as larger animals eat smaller animals contaminated with TBT. Currently, not enough data are available to help researchers determine whether TBT is a human carcinogen , so the U.S. Environmental Protection Agency (EPA) has assigned it to the "cannot be determined" category.

Environmental concerns over the effects of TBT-based paints has led to the regulatory control of TBT throughout the world. In 1988, the United States enacted the Organotin Antifouling Paint Control Act that restricts the use of TBT-based antifoulants to ships larger than 25 meters in length or those with aluminum hulls. The Act also restricted the release rate of TBT from the antifouling paints. In 1990, the International Maritime Organization (IMO) of the United Nations' Marine Environment Protection Committee adopted a resolution that recommended that countries adopt measures to restrict the use and release rate of TBT-based antifouling paints. Since that recommendation, many countries have prohibited the use of TBT antifouling paints on small craft. New Zealand and Japan banned the use of TBT anti-fouling paints on all ships. In 1998, the IMO agreed to ban the application of TBT-based paints by 2003 and their presence on ships by 2008. Due to the ensuing reductions in the use of TBT-based paints, both TBT contamination and its effects on organisms have been decreasing worldwide.

Less toxic alternatives to TBT are being developed quickly. In the animal and plant worlds, researchers have found over 50 naturally-occurring substances that have the same effects as TBT and other antifouling paints, keeping organisms such as algae and some invertebrates from becoming laden with other forms of algae and sea life. In addition, other alternatives to TBT have been developed, and include epoxy-copper flake paints, which may protect boats for up to 10 years, but cannot be used on ships with aluminum hulls. Ceramic-epoxy coats are also available, and can be used on fiberglass and metal hulls. Other alternativess to TBT are foulant-release coatings that contain silicone elastomers that last as long as the copolymers in TBT. Finally, researchers are studying eletrical current systems that can sterilize the hulls of ships for up to four years as another alternative.

[Judith L. Sims ]

RESOURCES

BOOKS

de Mora, S. J., ed.Tributyl Tin: Case Study of an Environmental Contaminant. Cambridge, UK: Cambridge University Press, 1997.

St. Louis, Richard.Tributyl Tin: The Case for Virtual Elimination in Canada.- Toronto, Canada: World Wildlife Fund Canada, 1999.

OTHER

"Antifoulants." Organotin Environmental Programme (ORTEP). Association, The Hague, The Netherlands. [cited June 21, 2002]. <http://ortepa.org/pages/antifoulants.htm>.