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Toxicity to microorganisms

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Description of key information

The hydrolysis transformation products of titanium tetrachloride do not exhibit true toxic effects.

Key value for chemical safety assessment

Additional information

A 9-hour IC50 (inhibitory concentration) of 20 mg/L (0.42 mmol/L) has been determined by Sauvant et al (1995) for effects of titanium tetrachloride (CAS 7550-45-0) on population growth of a ciliate protozoan (Tetrahymena pyriformis). The direct application of titanium tetrachloride into aqueous media, as done in this experiment, leads to hydrolysis and the eventual formation of titanium dioxide (CAS 13463-67-7) and hydrogen chloride (CAS 7647-01-0). Sauvant et al (1995) did not adjust the pH so that effects of the hydrochloric acid were not excluded.

The recent version of the OECD TGD 209 (2010) recommends “Solutions / suspensions of test substance in water should be neutralised prior to inoculum addition, if necessary. However, since neutralisation may change the chemical properties of the substance, further testing depending on the purposes of the study, could be performed to assess the effect of the test substance on the sludge without pH adjustment”.

In order to assess the contribution of acidity to the overall effect, read across from the experiment of Daniels (2008) was made, where the pH effects to activated sludge respiration were studied using hydrochloric acid. The latter has been shown to have an inhibitory effect on respiration rates of activated sewage sludge at pH 6.0 to 4.0, with an EC50 at between pH 5.0 and 5.5. By analysis of the dose response curve the EC50 was assigned to pH 5.2. This was normalized to 0.23 mg/L, which corresponds to 6.3 µmol/L. The EC50 hydrochloric acid molarity would result from the forth concentration of the target chemical. Accordingly the EC50 of titanium tetrachloride to activated sludge microorganisms would be assigned to 1.575 µmol/L. This corresponds based on the titanium tetrachloride molecular weight of 189.712 g/mol to 0.3 mg/L and suggests strong pH related effects.

A third experiment performed with the other hydrolysis product, titanium dioxide. Egeler & Goth (2009) loaded the analogue material in a magnitude corresponding to 2374 mg/L of the target material, titanium tetrachloride, without any effect. This suggests that titanium dioxide is not likely to contribute to the overall effect of titanium tetrachloride. Accordingly and as the isolated pH effects should be disregarded in hazard assessment the relevant measure is the result of the latter study (Egeler & Goth 2000).

In conclusion the toxicity of the hydrolysis products of titanium tetrachloride are considered not toxic to aquatic micro-organisms and no threshold level is derived.