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Environmental fate & pathways

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Tantalum pentachloride is an inorganic substance and as such not susceptible to biodegradation. According to Annex VII section of the Regulation (EC) No 1907/2006, tests on ready biodegradability do not have to be conducted on inorganic substances.

The test substance tantalum pentachloride readily decomposes and reacts violently with water as demonstrated by the results of the available hydrolysis study (Klink, 2015; Hedrich, 2015). According to the experimental results on hydrolysis of tantalum pentachloride, a strong temperature rise during the addition of water was observed due to the exothermic reaction. The resulting turbid solution is strongly acidic (pH value < 1) and contains insoluble residues. The concentration of dissolved tantalum at the solubility limit upon mixing TaCl5with water in a non-buffered system, determined in triplicates at nominal loading rates of 8.9- 13.5 g/L varied between 2.0-3.5 g Ta/L (hypothetically corresponding to 3.92- 6.91 g TaCl5/L), demonstrating an average recovery of 49.3 ± 4.6 % (relative to loading). The mean recovery values (triplicates each, given together with their standard deviation) of dissolved tantalum in buffered solutions was found to be 38.1 ± 8.1 % in citrate buffer (at pH 4), 12.8 ± 2.3 % in phosphate buffer (at pH 7), and 0.02 ± 0.02 % in borate buffer (at pH 9). The Ta concentrations in water after pH adjustment to equivalent pH values differed considerably from those in buffer. The mean recovery values of dissolved tantalum in water after pH adjustment to the pH values 4 and 7 (triplicates each) was found to be < 0.1 % relative to the applied loading. The calculated average recovery of dissolved tantalum (triplicates, given together with its standard deviation) in water after pH adjustment to pH value 9 was found to be 0.7 ± 1.0 % relative to the applied loading. Results indicate that, in addition to the fast decomposition upon contact with water, slight changes in pH have profound impact on the test concentration of soluble Ta5+.

It is not possible to adjust stable and predictable Ta5+solution concentrations which is the prerequisite for any batch/equilibrium experiment investigating the adsorption/desorption behaviour of Ta5+. These constraints demonstrate that any study on the adsorption/desorption behaviour of TaCl5would be both scientifically unjustified and technically not feasible.