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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 oxide sulphate 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 target compound titanium oxide sulphate nor the final hydrolysis transformation products titanium dioxide and sulphuric acid (when being neutralised) ) 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 sulphates are present in high natural background concentrations, the primary assessment shall be based on final hydrolysis products titanium dioxide and sulphuric acid.

In an experiment performed with the hydrolysis product titanium dioxide, Egeler & Goth (2009) loaded the analogue material in a magnitude corresponding to 2002 mg/l of target compound titanium oxide sulphate, without any effect. This suggests that titanium dioxide is not likely to contribute to the overall effect of titanium oxide sulphate.

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.

Hydrochloric acid in this case shall be used as source chemical for read across to target chemical sulphuric acid, which in fact is the relevant transformation product for pH effect assessment of target chemical titanium oxide sulphate. Read across is deemed to be applicable as both hydrochloric acid as well as sulphuric acid are strong mineral acids with more or less comparable acidity. There is however one major difference, as hydrochloric acid is a singly protonated acid, whereas sulphuric acid has two protolysis steps. Still it is considered conceivable that the acidity of two-protonated acids can be approximated by considering the first step of protolysis only.

In the experiment of Daniels (2008) hydrochloric acid 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 mg/L 0.23 mg/L, which corresponds to 6.3 µmol/L.

As one mol of target substance titanium oxide sulphate results in one mol sulphuric acid, and as only the first protolysis step shall be considered, from above stated hydrolysis reaction results approximately one mol hydronium ions. The EC50of target compound titanium oxide sulphate to activated sludge microorganisms therefore would be assigned to 6.3 µmol/L. Based on the titanium oxide sulphate molecular weight of 159.93 g/mol, this corresponds to a mass concentration of 1.0 mg/L. The result suggests strong pH related effects resulting from the hydrolysis of target compound titanium oxide sulphate.

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