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Long-term toxicity to aquatic invertebrates

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No chronic effects (mortality or reproduction) for Ceriodaphnia dubia were seen from the concentrations of soluble tin that could arise from the dissolution of tin metal in the environment. Both the key study (CIMM 2009) and the supporting study (Parametrix 2009) reported reproductive effects at higher nominal tin loadings which are probably due to physical effects from the precipitated material. The high nominal tin concentrations and subsequent effects from precipitation would not occur from the dissolution of tin metal and are only an outcome from the use of tin (IV) chloride as a surrogate material for testing.

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Additional information

Two studies (CIMM 2009, Parametrix 2009) were sequentially commisioned to determine the long term toxicity of soluble tin to

C. dubia. A third study using aged solutions of tin (IV) chloride was undertaken in 2010 but did not complete successfully as the validity criteria were not met.

From the first study (CIMM 2009), no chronic toxicity of tin on C. dubia survival and reproduction was observed for nominal tin concentrations under 100 µg/L. At higher nominal concentrations, however, an effect of tin on the reproductive rate was observed, starting at 200 µg/L of tin. The effect was more marked at higher nominal tin concentrations, up to 1600 µg/L, where the mean number of offspring was about one third the mean control number. At 3200 µg/L no further decrease was observed. Over the entire concentration interval tested, from 6.25 to 3200 µg/L of nominal tin concentration, no parental lethality was observed. However, the concentration of mean measured dissolved tin reaches a plateau of around 25 µg/l at higher nominal tin loadings. The purpose of the testing is to evaluate the potential toxicity of free tin ions and therefore care needs to be taken in separating effects seen from high nominal concentrations of tin, which will mostly be in the particulate phase, from any effects from truly soluble tin. No dose response was seen against soluble tin and any effects observed cannot be distinguished from those due to precipitated tin in the form of hydroxides.

The objective of the chronic Ceriodaphnia dubia Parametrix 2009 tests were to further investigate the toxicity of Sn (IV) ions under different conditions. As Sn (IV) ions hydrolyse in solution leading to precipitation reactions, this creates difficulty in maintaining stable concentrations of soluble Sn ions. The hydrolysis of tin salts reduces the pH of the test solution and creates precipitated tin hydroxides which may confound test data. This study tested effects on C. dubia with and without filtration at two different pHs. C. dubia were exposed to different combinations of tin (IV) and pH (6.5 and 8.5) for 7 days starting as neonates (<24hrs old). Tin (IV) chloride stock solutions were made in MOPS buffered laboratory water and the pH adjusted to either 6.5 or 8.5. These stock solutions were mixed for 2 hours on a stir plate after which they were divided in half and one half was filtered through a 0.2 µm filter. At pH 6.5, no tin was detected in the prefiltered samples suggesting that after 2 hours all the measurable tin was in the particulate phase. At pH 8.5, 828 µg/L of tin were detected in the prefiltered samples at the highest initial loading concentration. However, of the 828 µg/L tin in the prefiltered sample, only 21.9 µg/L were found to be dissolved after later filtration of the sample and measurement. The data suggests that tin continued to precipitate after the initial 2 hour equilibration and stirring time such that the majority of the tin species in the test conditions were not soluble. In all four combinations of tests (pH 6.5 and 8.5 with and without initial filtration) there was no significant effect on organism survival. There was also no effect on organism reproduction in the pH 8.5 filtered treatment. The pH 6.5 unfiltered and filtered tests showed significant effects on reproduction at the highest concentration only. Reproduction effects in the pH 6.5 filtered treatment are difficult to explain as the total tin reported in all tests were below the detection limit of 5 µg/L. The reproduction effects observed in the two unfiltered tests are likely to be due to the mass of precipitated tin hydroxides and not soluble tin and so these test data are not considered relevant for assessing the toxicity of soluble tin.

In summary, no chronic effects (mortality or reproduction) for Ceriodaphnia dubia were seen from the concentrations of soluble tin that could arise from the dissolution of tin metal in the environment. Both studies reported reproduction effects at higher nominal tin loadings which are probably due to physical effects from the precipitated material. The high nominal tin concentrations and subsequent effects from precipitation would not occur from the dissolution of tin metal and are only an outcome from the use of tin (IV) chloride as a surrogate material for testing.