Niobium pentachloride

Endpoint summary

Additional information

Niobium pentachloride is an inorganic substance and as such not susceptible to biodegradation. According to Annex VII section 9.2.1.1 of the Regulation (EC) No 1907/2006, tests on ready biodegradability do not have to be conducted on inorganic substances. The test substance niobium 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 niobium 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 niobium at the solubility limit upon mixing NbCl₅with water in a non-buffered system, determined in triplicates at nominal loading rates of 9.1 – 14.0 g/L varied between 1.3 - 2.3 g Nb/L (hypothetically corresponding to 3.74 - 6.79 g NbCl₅/L), demonstrating an average recovery of 43.6 ± 4.3 % (relative to loading). The mean recovery values (triplicates each, given together with their standard deviation) of dissolved niobium in buffered solutions was found to be 90.6 ± 0.3 % in citrate buffer (at pH 4), 1.5 ± 1.2 % in phosphate buffer (at pH 7), and 0.02 ± 0.02 % in borate buffer (at pH 9). The Nb concentrations in water after pH adjustment to equivalent pH values differed considerably from those in buffer. The mean recovery values of dissolved niobium in water after pH adjustment to the pH values 4 and 9 (triplicates each) was found to be < 0.1 % relative to the applied loading. The calculated average recovery of dissolved niobium (triplicates, given together with its standard deviation) in water after pH adjustment to pH value 7 was found to be 1.2 ± 1.2 % 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 Nb⁵⁺. It is not possible to adjust stable and predictable Nb⁵⁺solution concentrations which is the prerequisite for any batch/equilibrium experiment investigating the adsorption/desorption behaviour of Nb⁵⁺. These constraints demonstrate that any study on the adsorption/desorption behaviour of NbCl₅would be both scientifically unjustified and technically not feasible. Available information on the sorption behaviour of Nb are published in Söderlund et al. (2015): Results demonstrate high sorption of Nb onto mineral soil. Kd values decreased with increasing soil depth and decreasing specific surface area (SSA) from 184,000 mL/g to 54,000 mL/g at 0.7 m and 3.4 m, respectively. In comparison to mineral soil the humus layer was not found to be an important component for Nb adsorption (-> maximum Kd value ca. 800 mL/g). The Kd values that were determined in soil samples from 0.7 m soil depth equilibrated with pure water were high, i.e. > 55,000 mL/g in the pH range 4.7–6.5. Above pH 6.5 Kd values decreased significantly, corresponding to the change in the major Nb species from the neutral Nb(OH)₅to the low-sorbing anionic Nb(OH)₆^–and Nb(OH)₇²⁻. In contrast, Kd values obtained from equilibration with model soil solution at slightly alkaline pH values were an order of magnitude higher than in pure water. This is probably attributed to the formation of calcium niobate surface precipitate or electrostatic interaction between surface-sorbed calcium and solute Nb.