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Tin may exist in different chemical forms in the environment. These different forms may have differing potential ecotoxicological effects in the aquatic compartment. Tin can occur in different valence states, i.e., Sn2+ and Sn4+. A literature review of published data on tin ecotoxicity was conducted, which made no distinction between different inorganic tin compounds or valence states. Results from the published studies were generally reported as nominal values much of which may be present as precipitated material. There is still considerable uncertainty with regard to the form of Sn prevailing under the test conditions and the amount of dissolved, and therefore potentially bioavailable, tin. Due to this, studies which reported effects based on nominal concentrations were considered unreliable and excluded from the assessment.

A dissolution study (ITRI 2008) undertaken with a 100 mg/L loading at a wide range of pH (1 to 13) also measured the speciation of Sn(II) and Sn(IV) using voltammetry. Within the range applicable to the OECD dissolution protocol of pH 6 to 8.5, dissolved tin was only detectable at pH 8.5 and this was present exclusively as Sn (IV). Colloids were also measured instrumentally in the ITRI 2008 study. No colloids were detected except at pH 1. This suggests that the dissolution of tin ions at pH 8.5 does not lead to a dynamic process of precipitation of tin hydroxides and oxides and release of further tin ions, at least within the timescales of the testing. This suggests that the risk of large concentrations of solid tin hydroxides being formed from tin metal dissolution under environmental conditions is low. The mean dissolved tin measured after 28 days and 100 mg/L loading was 26 µg/L which is consistent with the typical maximum concentrations of dissolved tin observed in later tests. The CIMM 2008 dissolution study also confirmed minimal dissolution of tin powder at pH 6 and 7. Dissolution was observed at pH 8.5 with measured dissolved tin of 29.7 µg/L. A third dissolution study (CIMM 2010) measured 31 µg/L dissolved tin at 100 mg/L loading. These results are remarkably consistent given the non-routine nature of tin analytical methods and the possibility that some of the "dissolved" tin may be small colloids excluded from different filters on a particle size basis.

As it had been determined that the low concentrations of dissolved tin released from tin powder would be in the Sn(IV) valance state, laboratory toxicity studies were undertaken using tin (IV) chloride pentahydrate as a surrogate source of tin ions. This is consistent with regulatory guidance to test a more soluble form of the metal in order to evaluate its potential toxicity. However, it was found that the intrinsic properties of tin (IV) chloride caused confounding results in toxicity tests. At high concentrations tin (IV) chloride solutions are acidic and therefore pH effects can be observed in the media used depending on its buffering capacity. However, the principal confounding issue with the use of tin (IV) chloride solutions was found to be precipitation. As tin (IV) chloride hydrolyses, tin hydroxides and oxides precipitate from solution over hours and days. Therefore, tests where adverse effects are observed with high nominal loadings of tin were found to contain very low concentrations of dissolved tin and it is likely that effects were due to the precipitated material. For example, the CIMM 2009 long term toxicity study on Ceriodaphnia dubia found no dose response with dissolved tin but effects on reproduction were noted at high nominal loadings. It is also interesting to note that the measured dissolved tin concentrations in this study ranged from 22.6 to 37 µg/L in good agreement with the concentrations observed in the T/D tests. This, together with dissolved tin values reported in the other studies, suggests that 20 to 40 µg/L dissolved tin is likely to be the maximum possible environmental concentration in freshwaters at circumneutral pHs with most measured values at the lower end of that range.

In order to avoid the confounding contribution of precipitated tin material in the toxicity testing, two further studies were undertaken (ABC 2010a and b) which used aged solutions of tin (IV) chloride as the test material. By ageing tin (IV) chloride solutions and removing the precipitated material, in principle it allows the potential for adverse effects from precipitation to be avoided. In practice, the kinetics of the precipitation reactions appear to be slow and a final stable condition may not have been achieved over the duration of the tests even with prior ageing of the solutions. Therefore, the use of aged solutions provide a realistic but slightly pessimistic scenario for the ultimate concentrations of tin ions released from metallic tin powder and they avoid most of the confounding effects of precipitated material . The ABC 2010 studies on aged tin solutions were an algal test with acute and chronic endpoints (Pseudokirchneriella subcapitata) and acute fish test (Pimephales promelas). Both studies did not identify any adverse effects up to 100 % aged tin solution strength.

Therefore, in summary, no aquatic toxicity was observed that could be attributable to dissolved tin species.