3. Exposure

• 3.1 Environmental Fate
• 3.1.1 Environmental Partitioning
• 3.1.2 Environmental Fate and Persistence
• 3.1.3 Bioaccumulation
• 3.2 Environmental Concentrations
• 3.2.1 Estimated Concentrations¹
• 3.2.2 Measured Concentrations²

Environmental releases of organotins are expected to occur mostly to water. Organotins with a moderate to high adsorption coefficient would tend to partition to bottom sediments and to suspended particulate matter in the water column. Generally, partitioning of organotins to air is expected to be negligible. Some organotin substances that have a low water solubility and a high vapour pressure would partition to air to a greater, but still limited, extent.

In water, dissolution of many organotins yields the organotin cation, which is either hydrated or combined with the most prevalent anion (e.g., chloride ion in seawater). The rate of hydrolysis is uncertain. There is uncertainty regarding the hydrolytic stability of the tin-sulphur bond under environmentally relevant conditions. Studies have demonstrated the susceptibility of tin thiolates to hydrolysis when dissolved in organic solvents; however, there is still uncertainty concerning potential hydrolysis of these chemicals under natural environmental conditions and across a full spectrum of environmental pH (Environment Canada, 2006).

In general, the hydrolyzed organotins (i.e., RxSn^(4−x)+ moieties) are not believed to be persistent in water, with estimated half-lives of less than a few months at 20°C and longer at lower temperatures (Government of Canada, 1993).

Tributyltins do not appear to be persistent in water, but Maguire (2000) presented evidence that these substances have half-lives in sediments ranging from 0.9 to 15 years. Viglino et al. (2004) estimated half-lives for tributyltin of 8 ± 5 years in the surface oxic layer of sediment of the Saguenay Fjord in Québec and 87 ± 17 years in the deep anoxic layer.

Triphenyltins have also been reported to have a relatively long half-life (3.1 years) in sediment (Shim et al., 1999). Furthermore, high concentrations of triphenyltins in deep-sea organisms have been interpreted by Borghi and Porte (2002) to indicate that triphenyltins can persist for relatively long times in deep-sea environments.

Because organotins generally do not partition significantly to air, potential for long-range transport via air is expected to be limited.

Degradation of organotins occurs via sequential dealkylation of entire chain groups with hydroxylated intermediates, rather than systematic demethylation of the alkyl chains. For example, tributyltin degrades biologically and abiotically by sequential debutylation, yielding dibutyltin, monobutyltin and inorganic tin in water/sediment mixtures or water alone (Maguire, 1992). Tetrabutyltin and tetraphenyltin are expected to degrade in a similar stepwise manner, with tributyltin and triphenyltin compounds, respectively, being the first products of degradation.

Biological methylation of inorganic tin to mono-, di-, tri- or tetramethyltin can occur in the environment. Methylation rates are highest in sediments under anaerobic conditions. Reported yields of methyltin species are generally much less than 1%, but a total yield of 3.2% (0.03% monomethyltin, 0.08% dimethyltin, 2.86% trimethyltin and 0.19% tetramethyltin) has been reported (Rapsomanikis et al., 1987). Methylated butyltin compounds have been detected in the environment infrequently and at concentrations that are low relative to concentration of the organotin substance originally released into the environment (Maguire, 1992). Butyl-, octyl and phenyltins are not formed from inorganic tin by natural processes.

Based on this information, tributyltin and triphenyltin compounds meet the criterion for persistence in sediments (half-life ³ 365 days in sediment) as specified in the Persistence and Bioaccumulation Regulations of CEPA 1999 (Government of Canada, 2000).

Short-chain organotin chlorides are not expected to bioaccumulate appreciably in aquatic biota. The log octanol/water partition coefficient (Kow) values for monobutyltin trichloride and dibutyltin dichloride are 0.09 and 0.05, respectively (Government of Canada, 1993). The high molecular weights of some organotin substances would hinder their bioaccumulation.

However, some organotins do have the capacity to bioaccumulate. Maguire (2000) reported bioconcentration factors (BCFs) for tributyltin ranging up to 46 000 for freshwater fish, 330 000 for freshwater algae and 900 000 for freshwater mussels and up to 10 000 for marine periwinkles and oysters, 12 000 for eelgrass, 40 000 for crabs, 50 000 for marine fish, 100 000 for dogwhelks and 500 000 for marine clams. The log Kow for tributyltins is less than 5, so Maguire (2000) concluded that this high level of bioaccumulation resulted from a mechanism such as binding to metal-binding proteins in the liver and kidney, rather than simple lipophilic partitioning to fatty tissues. Hunziker et al. (2001) also concluded that triorganotin compounds interact with biological material by hydrophobic partitioning and complexation reactions. Maguire (2000) noted that the relatively high concentrations of tributyltin in tissue samples (up to 4 µg/g wet weight) from a range of top predators collected worldwide are further evidence of elevated potential for bioaccumulation of tributyltin. Limited evidence of weak biomagnification of tributyltin in marine food chains has been reported (e.g., Takahashi et al., 1997; Rouleau et al., 1998; Mamelona and Pelletier, 2003). Reported biomagnification factors are typically less than 10.

Huang et al. (1993) reported a BCF of 11 400 for triphenyltin in the alga Scenedesmus obliquus over a 7-day exposure, compared with a BCF of >33 200 for tributyltin. Borghi and Porte (2002) reported elevated levels of triphenyltin in livers of certain deep-sea fish (up to 4.2 µg/g wet weight) in the Mediterranean Sea, with lower concentrations of mono-, di- and tributyltins. This indicates that triphenyltin is bioaccumulative and sufficiently persistent to reach deep-sea areas. Although biomagnification of triphenyltins has not been widely studied, results reported by Borghi and Porte (2002) suggest that triphenyltins have the potential to biomagnify. These authors estimated a biomagnification factor of close to 1 for triphenyltins in one fish species from the Mediterranean Sea.

It should be noted that many of the BCFs cited above were based on field studies. Therefore, the term “bioaccumulation factor” (BAF) would be more appropriate than BCF, because the organisms would be exposed to organotins in water, food and particulate matter, not just in water. Furthermore, several of the studies reported tissue concentrations on a dry weight basis. BAFs (or BCFs) calculated on a dry weight basis would be several times higher than if reported on a wet weight basis.

Based on this information, tributyltin and triphenyltin compounds meet the criteria for problematic bioaccumulation (BCF values >= 5000 or log K_ow >= 5) as specified in the Persistence and Bioaccumulation Regulations of CEPA 1999 (Government of Canada, 2000).

Predicted Environmental Concentrations (PECs) for mono- and dialkyltins and for tributyltins and tetrabutyltin released from shipping containers, storage tanks and transfer lines were calculated to range from 4.1 × 10⁻⁴ to 2.0 µg/L for average and low flow rate Canadian river systems, assuming instantaneous dilution (Environment Canada, 2006).

With appropriate industry-wide stewardship practices in place, PECs would be reduced to 8 × 10⁻⁸ to 8.1 × 10⁻³ µg/L for average and low flow rate Canadian river systems (Environment Canada, 2006).

Environment Canada (2006) estimated a sediment PEC of 7.8 mg/kg dry weight for tributyltins, using an equilibrium partitioning approach based on the highest estimated concentration of tributyltin in overlying waters.

Chau et al. (1997) presented the results of a survey of organotin substances in water and sediment (freshwater and marine) from across Canada carried out in 1994. Samples were collected at various locations, including marinas, harbours and shipping lanes, as well as some areas less likely to be receiving substantial input from pesticidal (antifouling) uses of organotins. A summary of the results of this survey is presented in Table 2 (concentrations in Canadian fresh waters) and Table 3 (concentrations in Canadian freshwater sediments). Organotins were detected in surface waters and sediments throughout Canada, with the highest concentrations of the various organotin substances almost always being in harbours and marinas. It is thought that butyltin substances found in these areas resulted mainly from the degradation of tributyltin substances used in antifouling paints. As stated above, organotins may also enter the environment from other sources. The highest organotin concentration in water, 0.0593 µg dimethyltin/L, was found in the Welland River, Ontario, downstream from a PVC manufacturing plant (Chau et al., 1997).

^a Detection limits and concentrations were originally reported as µg Sn/L.

^a Detection limits and concentrations were originally reported as µg Sn/g dry weight.

Restrictions were placed on tributyltins in antifouling paints under Canada's Pest Control Products Act in 1989. Surveys carried out in 1982-1985, before these restrictions were put in place, show that organotins were generally detected more frequently and at higher concentrations in water during that period than in 1994, as shown in Table 4 and Table 5.

^a Detection limits and concentrations were originally reported as µg Sn/L.

^a Detection limits and concentrations were originally reported as µg Sn/g dry weight.

¹ Estimated concentrations are expressed on a whole-molecule basis, including the anionic moiety.
² Measured concentrations are expressed on a hydrolyzed basis (e.g., monobutyltin), not including the anionic moiety.