Tin

Toxicity to aquatic algae and cyanobacteria

Description of key information

100% aged tin solution represents the maximum concentration of soluble tin ions possible at the test conditions. No significant adverse effects of growth rate or yield were seen. ABC (2010)

Additional information

Growth inhibition tests on the unicellular green algae Pseudokirchneriella subcapitata reported by the Chilean Mining and Metallurgy Research Center (CIMM 2009) were performed on tin (IV) chloride in accordance with OECD 201 guidelines. A preliminary growth inhibition test for a broad concentration range of tin(IV) allowed the 72h-E_rC₅₀to be calculated by extrapolation. A value of 19.7 mg/L was obtained. As high nominal concentrations of tin(IV) caused significant pH drift of the test media, another set of tests was performed in the presence of 5mM HEPES buffer, pH 8.The results from the second inhibition test with P. subcapitata showed a 72h-E_rC₅₀concentration of 9.8 mg/L of tin(IV). All the total tin (IV) concentrations are nominal. However, the measured dissolved tin at the start of the test varied between 14.0 µg/L and 16.0 µg/L across the range of nominal tin concentrations. Therefore, the dose response observed was not attributable to free tin ions and this test data cannot be used to assess the toxicity of soluble tin arising from the dissolution of tin metal powder.

In order to remove the confounding effects caused by precipitation from high nominal concentrations of tin salts, the growth inhibition study on Pseudokirchneriella subcapitata was repeated using aged solutions of tin ions (ABC 2010). The protocol for ageing tin (IV) solutions used in the ABC 2010 green algal study was developed following difficulties encountered during earlier toxicity tests with a number of species with precipitation of tin hydroxides. The methodology used for preparing the aged solutions is identical as used in the ABC 2010 short term toxicity test on Pimephales promelas.

The objective of this and the other key aquatic toxicity tests is to assess any adverse effects from soluble tin released from tin metal. Attempts to investigate the potential toxicity from tin ions using salts such as tin (IV) chloride have been confounded by lack of stability of tin ions in solution. This instability produces dissolved tin concentrations of only 10 to 20 µg/L against a nominal tin concentration of 1 mg/L. The high concentrations of precipitated tin hydroxides and the associated pH effects at higher doses do not reflect the true behaviour of tin ions released from the transformation and dissolution of tin metal. Therefore, to overcome these difficulties, aged solutions containing stable tin concentrations have been prepared just prior to the testing in a consistent manner such that the solutions will all be of the same age. Even after a total of seven days of ageing the tin (IV) ions are not yet at equilibrium conditions as slow precipitation and mineralisation reactions continue to take place. However, the aged solutions are considered to represent the maximum dissolved tin that can be achieved in the test media, and therefore a worst case exposure to dissolved tin.