Pollutants Chemistry Tributyltin compounds Joel Emmanuel Bortoni-‐González June 11th, 2013
Introduction Tributyltin (TBT), (C4H9)3Sn, is a chemical used mostly in biocides in the form of other compounds like tributyltin hydride, (C4H9)3SnH, and tributyltin oxide, C24H54OSn2, due to the fact that TBT by itself is unstable and will break down unless combined. It is part of the aromatic hydrocarbon chemical family. These types of hydrocarbons have alternating double and single bonds between carbon atoms forming rings, like benzene in Figure 1. The term aromatic has nothing to do with the physical property of aromaticity, a chemical property that describes the way in which a conjugated ring of unsaturated bonds, lone pairs or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone, but rather to the fact that these chemical compounds have a scent, sweet in most cases. Figure 1. Benzene model where the circle denotes the alternating double and single bonds between carbon atoms. The most common TBT compound is the tributyltin oxide (TBTO) and has been the subject of most TBT tests. This compound and other eight TBT compounds are referred to as organotins. Characteristics TBT compounds are liquids often colorless, unlike TBTO that tends to have a slightly yellow color, and have an odor similar to gasoline. TBTO is insoluble in water due to the fact that they react, but is soluble in hexane and most organic solvents (ethanol, ether, halogenated hydrocarbons, etcetera) and is flammable but does not form explosive mixtures with air. Its melting point is located around 53ºC and its boiling point around 193ºC and it has a density of 1.103 g/ml at 20ºC. It breaks down slowly in the presence of oxygen, light or heat. Prominent uses TBTO is an effective biocidal preservative for wood, cotton textiles, paper and paints and stains for residential homes. Mainly, it is added as an antifouling agent in numerous formulations of marine paints and boat hulls, docks, fishnets and buoys to discourage the growth of marine organisms such as barnacles, bacteria, tubeworms, mussels and algae.
Quality regulations Most of the international regulations on TBTO derived from previous cases of imposex on several snail species in France and Great Britain. Nowadays regulations tend to decrease the use of TBT based antifoulings. In 1987 a Europe-‐wide ban of its use on boats of under 25 meters long was established. In the United Kingdom the use of TBT based paints continues on larger vessels and it remains at present the most effective means of controlling fouling. In November 1998 the International Maritime Organization made the decision to introduce a worldwide ban of the use of TBT in antifouling paints for most ships from January 2003, a ban that has been in place for several years mainly in countries with a lot of maritime activity, such as Japan. Pressure for a complete ban of TBT in antifouling paints has been increasing due to evidence that it is bio-‐accumulating in food chains, with particularly high levels being found in marine mammals. Toxicity Since TBTO is used mostly on products that will be in touch with water studies of the effects of TBTO products on living organisms have been primarily performed on aquatic life. Effects of TBT products on humans are not clear, but several incidents of human exposure to the biocide have been reported: underwear treated with TBT has caused severe skin irritation to its wearer, shipyard workers exposed to TBT dust and vapors developed breathing problems, irritated skin, headaches, colds, flu, fatigue dizziness and stomach ache. TBT exposure can also irritate the eye and mucous membranes and prolonged exposure may cause liver and kidney damage. M & T Chemicals, one of the main producers of TBTO, claim that workers exposed to this substance metabolize it within 3 days. Single exposure of TBTO on rats demonstrated a transient increase in adrenal weight shortly after exposure and a transient effect on thyroid follicles; these effects are reversible. Inhalation studies of a single 4 hours exposure of rats to aerosols of TBTO showed irritation and enteritis. Studies where 10 male and 10 female rats were exposed to saturated gases of TBTO didnt show any death occurring during exposure for 7 hours or in the following 14 days observation period. Short term exposure of TBTO on 10 male and 10 female rats involving repeated inhalation in "nose only" chambers during 4 hours 5 days per week produced sever toxic effects, inflammatory reactions in the total respiratory tract and histological changes in the lymphatic organs were observed; 5 males and 6 females died during this study. In mammals, high levels of TBTO can affect the endocrine glands, upsetting the hormone levels in the pituitary, gonad and thyroid glands. Large doses of TBT have been shown to damage the reproductive and central nervous systems, bone structure and the gastrointestinal track of mammals. A large number of studies have been conducted showing that TBTO causes depression of immune functions dependant on the thymus. These effects occur at doses lower than those that cause other toxicity; the critical effect for TBTO is immunotoxicity.
Cancer assessment has been conducted in rats and mice following oral exposure. Increases in benign pituitary tumors, in pheochromocytomas and in parathyroid tumors at the highest dose tested were shown. It is unclear if TBTO is responsible for these tumors since the strain of rats used they normally occur with variable incidence. The mice didnt show any sign of a tumor due to TBTO. Eco toxicity Much of the concern of the use of tributyltin stems from its use as a marine antifoulent in paints. This compound is slowly released from the paint on the hull of the boat into the adjoining water hindering the growth and attachment of a variety of organisms to the boat. Consequently, tributyltin concentrations in harbors and bays in Great Britain, France and the United States were high enough to significantly affect oyster and mussel production. Imposex, the development of male characteristics in females, has been initiated by TBT exposure in several snail species. In laboratory tests, reproduction was inhibited when female snails exposed to 50 ppt of TBT developed male characteristics, such as male genitalia. Imposex was also noted in the mud snail at less that 3 ppt of TBT. TBT is extremely toxic to crustaceans. Lobster larvae show a nearly complete decrease in growth at just 1 ppb of TBT. Molluscs, used as indicators of TBT pollution because of their high sensitivity to those chemicals, react adversely to very low levels of TBT (0.06-‐2.3 ppb of TBT). They release TBT very slowly from their bodies after it has been absorbed. TBT toxicity in the field may be substantially underestimated in laboratory studies. TBT binds to the sides of containers and plankton, which contributes to this underestimation of its potential toxicity. Generally, the larvae of any tested species are more sensitive of tributyltin exposure than the adults. Some fish can degrade TBT due to special enzymes that these fishes contain. In the Chinook salmon, once absorbed it breaks down into di-‐n-‐butyltin (DBT). Rainbow trout eggs are killed between 10-‐12 days of TBT exposure at 5 ppb. At lower levels no deaths occurred, but blood and liver metabolism changes were noticed. Growth reduction and liver changes also occurred in young trout exposed to lower levels of tributyltin chloride. Also, after seven days of low level TBTO, the corneal membranes of the rainbow trouts eyes were destroyed. TBTO has been shown to inhibit cell survival of marine unicellular algae at very low concentrations. Pollution in Arcachon Bay and beginning of the ban on its use Until the mid 1970s, Arcachon Bay had been an important area for oyster culture, with production of 10,000-‐15,000 tons per year, covering substantial areas of the tidal mud flats. The bay was also popular with leisure craft, with vessels numbers increasing from 7,500 in the mid 1970s to 15,000 at the start of the 1980s. Estimated inputs of TBT to the bay peaked at around 8 kg per day. Imposex was first observed in the bay in 1970, affecting the predator oyster drill, leading rapidly to its near extirpation from the bay. TBT was identified as the responsible agent only in the early 1980s.
Had the adverse effects been limited to the loss of this species, considered a pest within the shellfish industry for its damage of oyster stocks, little if any action may have followed. However, this early warning was followed by failure of the oyster stocks themselves. Despite a normal spawning event in the summer of 1976, few of the larvae survived. Larval settlement largely failed through the late 1970s and into the 1980s, resulting in massive financial losses by the shellfish industry. By 1981, oyster production had fallen to only 3,000 tons. In addition to reproductive failure, adult oysters were rendered unsaleable by shell deformation leading, in sever cases, to ball shaped specimens. Such observations predated analytical techniques sensitive enough to describe in detail environmental distributions of TBT. In 1986 the first survey of organotins in the waters of Archon Bay was provided, while sediments data were not available until the 1990s. Nevertheless, the severity of impacts on the ecology of Archon Bay, manifest in heavy financial losses, was sufficient to stimulate relatively swift action by the French government. Acting on the best information available linking the oyster collapse to the presence of TBT paints to small vessels (less than 25 meters long) in 1982, beginning the ban on said boats. These controls undoubtedly markedly reduced TBT inputs to marinas throughout France. In the case of Arcachon, implementation was probably aided by the local provenance of many boat owners and their interest in preserving a local industry. Sources Concise International Chemical Assessment Document 14: Tributyltin Oxide. -‐ Dr. Robert Benson; World Health Organization. Tributyltin. -‐ Extension Toxicology Network. Retrieved from: http://pmep.cce.cornell.edu/profiles/extoxnet/pyrethrins-‐ziram/tributyltin-‐ext.html Tributyltin (TBT) antifoulants: a tale of ships, snails and imposex. -‐ David Santillo, Paul Johnston and William J. Langston. Tributyltin pollution on a global scale. An overview of relevant and recent search: impacts and issues. -‐ Dr. Simon Walmsley; World Wildlife Fund.