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Toxicity to microorganisms

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The conduct of a study on toxicity to microorganisms with the target substance titanium oxychloride itself is being waived, as the substance is highly unstable in water and produces insoluble oxide after rapid hydrolysis. Nevertheless, based on the information available it can be concluded that neither the parent compound titanium tetrachloride, nor the target compound titanium oxychloride nor the final hydrolysis transformation products (namely titanium dioxide) exhibit toxicity to microorganisms.

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Additional information

The conduct of a study on toxicity to microorganisms with the target substance itself is being waived, as the substance is highly unstable in water and produces insoluble oxide after rapid hydrolysis. Thus the aquatic hazard assessment is to be based on the transformation products. As pH effects are not true toxic effects, they are not primarily relevant for assessment. 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”.

As furthermore chlorides are present in high natural background concentrations, the primary assessment bases on parent compound titanium tetrachloride and final hydrolysis product titanium dioxide.

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.

In another experiment performed with the hydrolysis product, titanium dioxide. Egeler & Goth (2009) loaded the analogue material in a magnitude corresponding to 2374 mg/L of the parent material titanium tetrachloride or to 1685 mg/l of target compound titanium oxychloride, resp., without any effect. This suggests that titanium dioxide is not likely to contribute to the overall effect of titanium tetrachloride / titanium oxychloride. 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 2009).

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 EC50at between pH 5.0 and 5.5. By analysis of the dose response curve the EC50was assigned to pH 5.2, which corresponds to 6.3 µmol/L hydronium ions.

Titanium oxychloride is an unstable chemical compound which results from the hydrolysis of titanium tetrachloride.

1. step: TiCl4 + H2OTiOCl2 + 2 HCl

2. step: TiOCl2 + H2OTiO2 + 2 HCl

Summary reaction: TiCl4 + 2 H2OTiO2 + 4 HCl

Based on stoichiometry, one mol of parent substance titanium tetrachloride results in 4 mol hydronium ions (full protolysis of final hydrolysis product hydrochloric acid). Accordingly the EC50of parent compound titanium tetrachloride to activated sludge microorganisms would be assigned to 1.575 µmol/L. This computation is based on the molecular weight of titanium tetrachloride of

189.712 g/mol and corresponds to a mass concentration of 0.3 mg/L. The result suggests strong pH related effects resulting from the hydrolysis of parent compound titanium tetrachloride.

Accordingly, one mol of target substance titanium oxychloride results in 2 mol hydronium ions (full protolysis of final hydrolysis product hydrochloric acid). Accordingly the EC50of target compound titanium oxychloride to activated sludge microorganisms would be assigned to 3.15 µmol/L. This computation is based on the titanium oxychloride molecular weight of 134.78 g/mol and corresponds to a mass concentration of 0.42 mg/L. The result suggests strong pH related effects resulting from the hydrolysis of target compound titanium oxychloride.

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