Tributyltin

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Tributyltin (cation)
tributyltin hydride model

Tributyltin (TBT) are organotin compounds that contain the (C4H9)3Sn moiety, such as tributyltin hydride or tributyltin oxide.[1] For 40 years TBT was used as a biocide in anti-fouling paint, commonly known as bottom paint. Antifouling paint improves ship performance and durability, because its use prevents and slows the growth of organisms that attach to the hull. Although TBT anti-fouling paint is the most effective, TBT leaches from the ships hulls and has toxic effects on organisms at all points of the food chain, including mammals. It is particularly dangerous because it impacts development, which has led to collapse of whole populations of organisms.[2]

Currently, TBT compounds are banned and are included in the Rotterdam Convention[3] and have been banned by the International Convention on the Control of Harmful Anti-fouling Systems on Ships of the International Maritime Organization.[4] Even though TBT is banned, it still presents a danger to our environment. Because TBT anti-fouling paint is still the most effective and cost efficient, it is being used in countries with poor regulation enforcement. This is particularly an issue in the Caribbean.[5] Furthermore, TBT has a long half-life and remains in the ecosystem as a toxin for up to 30 years. This means that even if TBT use was completely eliminated, without a clean up effort, it would continue to have toxic effects on the environment.[6]

Chemical properties[edit]

TBT compounds are organic derivatives of tin (Sn4+), where 3 carbon atoms covalently bonds the tin. The general formula for these compounds is (n-C4H9)3Sn-X. The X could be either an anion or a group linked covalently through an atom other than carbon and hydrogen. 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 can vary with the pH of the water. Studies have shown that 95% of TBT can be released from the sediments, and back into the aquatic environment. This makes it difficult to accurately quantify the amount of TBT in an environment, since its concentration in the water is not representative of the total amount.[1]

In the chemical laboratory, tributyltin hydride generated by the action of lithium aluminium hydride on tributyltin chloride is used to replace halogens in organic compounds for hydrogen.

Uses[edit]

TBT compounds were discovered in the 1950s in the Netherlands by a research group, van der Kerk, while trying to synthesize a powerful biocide for anti-fouling paint. 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.[7] It was found to that TBT is an effective biocide, but wrongly deemed safe enough to not cause damage to the environment. By the mid 1960s it became the most popular anti-fouling paint worldwide.[2] 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. However, the TBT used on the ship hulls was found to leach into, and severely damage, the marine, brackish and freshwater environment.[8]

While its use has been banned worldwide, it is still being used illegally because of its efficiency as an anti-fouling paint. It is especially persistent in the Caribbean on cruise ships, where recorded tributyltin levels are unsafe for invertebrate organisms.[5]

Toxicity[edit]

After thirty years of research on the effects of TBT it has been concluded that TBT compounds leaching into the marine and aquatic environment are toxic and have deleterious effects on the development and survival of the animals. The effects of antifouling paint go beyond the organisms that it is meant to kill. By poisoning barnacles, algae, and other organisms at the bottom of the food chain TBT is biomagnified up the marine predators' food net. It has been shown to have a harmful effects on many layers of the ecosystem, affecting invertebrates and vertebrates, including humans. Toxic effects in some species occur at 1 nano-gram per liter of water.[9]

Even with its ban, TBT still presents a danger to the environment. One of the most problematic aspects of TBT is its accumulation in sediments and its long half life of about 2 years. TBT often bonds to suspended material and sediments to the bottom, where it can remain and be released, for up to 30 years.[6] Additionally, TBT can be introduced into non-aquatic or marine ecosystems because dissolved TBT can evaporate into the air and be dispersed by rain.[8]

Invertebrates[edit]

TBT has been shown to impact invertebrate development. One of the most studied organisms are the stenoglossan gastropods, a marine shelled snail. TBT disrupts their endocrine system by inhibiting Cytochrome P450 molecule. P450 normally converts androgen, which have male hormone properties to oestrogen, which have female hormone properties. This inhibition leads to masculinization in females, because the androgen levels are higher than normal. Since less fertile females are available for mating, the population begins to decline and seriously impacts the balance of the ecosystem.[9]

Chironomus riparius has been used as a model invertebrate to test the effects of TBT on development and reproduction at sublethal concentrations found in marine environments. It was found that only 0.05 ng ml− 1 range is enough to have developmental effects on their larvae, and 10-100 ng l−1 was enough to seriously offset the female to male ratio in the population. At 10 ng l−1 females were at 55.6% of the population and 85.7% at 100 ng l− 1. These results are interesting because unlike the masculinization of the stengoglassan gastropods, this experiment shows feminization.[8]

Vertebrates[edit]

Vertebrates become effected by TBT by being in waters contamined with TBT, and by consuming organisms that have already been poisoned. Oryzias latipes, commonly called Japanese rice fish, has been used as a model vertebrate organism to test for effects of TBT at developmental stages of the embryo. It was observed that developmental rate was slowed by TBT in a concentration-related manner and that tail abnormalities occurred.[9]

To see the extent to which TBT has infiltrated the food chain, Skipjack Tuna was globally tested for presence of TBT. Practically all samples were positive. The tuna had particularly high levels of TBT in waters around developing Asian nations, where regulation of TBT is not enforced as rigorously as in Europe or US.[10]

Studies have shown that TBT is detrimental to the immune system. Research shows that TBT reduces resistance to infection in fish which live on the seabed and are exposed to high levels of TBT. These areas tend to have silty sediment like harbours and estuaries.[4]

TBT compounds have been described to interfere with glucocorticoid metabolism in the liver, by inhibiting the activity of the enzyme 11beta-hydroxysteroiddehydrogenase type 2, which converts cortisol to cortisone.[8]

Mammals[edit]

The main way that mammals are exposed to TBT, especially humans, is through the diet. Its effects are more difficult to study, but some correlations have been established.

TBT has been shown to lead to immunosuppression in mammals such as sea-otters and dolphins. Studies have shown that wild, dead sea otters (Enhydra lutris) and stranded bottlenose dolphins can have extremely high levels of tributyltin in their livers.[11] In addition, it was found that otters dying of infectious causes tend to have higher levels of tissue butyltins than those dying of trauma or other causes.[12] TBT has also been blamed by hearing experts for causing hearing loss in mammalian top predators such as toothed whales. Because hearing is important for mating and predetation in these animals, long term consequences could be a drastic.[13][14]

In humans, organotin compounds have been detected both in blood and the liver.[1] Studies have linked Tributyltin to developmental abnormalities and obesity. There are studies indicating that it stimulates the endocrine system by activating retinoid X receptor. (RXRS).[15]

Regulation[edit]

Because TBT is the most effective anti-fouling agent discovered, it was frequently used in anti-fouling paint throughout the globe. Its inexpensive cost has been a major factor when resisting complete prohibition, compared to previous copper formulas.[4]

Bans on TBT on boats less than 25 metres long first started in the 1980s. In 1990, the Marine Environment Protection Committee adopted Resolution MEPC 46(30), which recommended that the Government eliminate the use of TBT-containing antifouling paints on smaller vessels. This resolution was intended to be a temporary restriction until the International Maritime Organization could implement a complete ban of TBT anti-fouling agents for ships. Several countries followed with a ban of use, and in 1997 Japan banned the production of TBT-based anti-fouling paints.[4]

The use of organotin compounds acting as biocide in anti-fouling paint was completely banned in 2008 by the International Convention on the Control of Harmful Anti-fouling Systems on Ships of the International Maritime Organization.[4] It states that ships cannot bear organotin compounds on their hulls or external parts or surfaces unless there is a coating that forms a barrier so that organotin compounds cannot leach out. This measure helps reduce exposure by allowing recovery to occur. Despite the ban, TBT will most likely be present in the water column and sediment for up to twenty years because of its long half-life.[1]

See also[edit]

References[edit]

  1. ^ a b c d Antizar-Ladislao, Blanca (Feb 2008). "Environmental levels, toxicity and human exposure to tributyltin (TBT)-contaminated marine environment. A review". Environmental International 34 (2): 292–308. PMID 17959247. Retrieved Mar 12, 2014. 
  2. ^ a b Konstantinou, Ioannis (Feb 22, 2006). Antifouling Paint Biocides. Springer. p. 1. 
  3. ^ Secretariat for the Rotterdam Convention on the Prior Informed Consent Procedure for Certain Hazardous Chemicals and Pesticides in International Trade (1 February 2009). "Decision Guidance Document for Tributyltin Compounds". United Nations Environment Programme. Retrieved 2013-01-04. 
  4. ^ a b c d e "Focus on IMO - Anti-fouling systems". International Maritime Organisation. 
  5. ^ a b "Persistent Organic Pollutants (POPs) and Pesticides". The Caribbean Environment Programme. Retrieved 4/1/2014. 
  6. ^ a b Champ, Michael (Sep 30, 1996). Organotin: Environmental Fate and Effects. Springer. p. 469. 
  7. ^ Walmsley, Simon. "Tributyltin pollution on a global scale. An overview of relevant and recent research: impacts and issues.". WWF. Retrieved 4/2/14. 
  8. ^ a b c d Mora, ed. by Stephen J. De (1996). Tributyltin : case study of an environmental contaminant (1. publ. ed.). Cambridge [u.a.]: Cambridge Univ. Press. ISBN 0521470463. 
  9. ^ a b c Walmsley, Simon. "Tributyltin pollution on a global scale. An overview of relevant and recent research: impacts and issues.". WWF UK. 
  10. ^ Down, Steve. "Tuna is attuned to tin". Ezine. Retrieved 4/28/14. 
  11. ^ Murata S, Takahashi S, Agusa T, Thomas NJ, Kannan K, Tanabe S (April 2008). "Contamination status and accumulation profiles of organotins in sea otters (Enhydra lutris) found dead along the coasts of California, Washington, Alaska (USA), and Kamchatka (Russia)". Marine pollution bulletin 56 (4): 641–9. doi:10.1016/j.marpolbul.2008.01.019. PMID 18304586. 
  12. ^ Kannan et al. 1998. Butyltin residues in Southern sea otters (Enhydra lutris nereis) found dead along California coastal waters. Environ. Sci. Technol. 32:1169-1175
  13. ^ Matt Apuzzo (2005-01-28). "Whale Deafness Linked To Chemical". Associated Press via CBS News. Retrieved 2008-07-30. 
  14. ^ Santos-Sacchi Joseph, Song Lei, Zheng Jiefu, Nuttall Alfred L (2006-04-12). "Control of Mammalian Cochlear Amplification by Chloride Anions". Journal of Neuroscience 26 (15): 3992–3998. doi:10.1523/JNEUROSCI.4548-05.2006. PMID 16611815. 
  15. ^ Nakanishi, T (Oct 19, 2005). "Trialkyltin compounds bind retinoid X receptor to alter human placental endocrine functions.". Mol Endocrinol. PMID 15941851. Retrieved 4/28/14. 

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