Niobium carbide

Endpoint summary

Additional information

Several lines of evidence indicate that aquatic toxicity of niobium carbide is unlikely to occur:

Niobium carbide (NbC) is highly insoluble in water. The maximum water solubility of the substance analysed under the conditions of the T/D test (OECD Series on Testing and Assessment No. 29) is 6.47 µg/L after 7 days at 21.5 °C at a loading of 10 mg/L. In addition, the water solubility of niobium carbide at environmentally relevant conditions is assumed to be even lower. Therefore, only very low bioavailability is also expected for aquatic organisms. Data for the acute toxicity of niobium carbide to aquatic organisms are only available for fish. In a study conducted according to OECD guideline 203 Zebrafish (Danio rerio) were exposed to niobium carbide at nominal concentrations of 0 and 100 mg/L under semi-static conditions (limit test). The 96-h LC50 of NbC was > 100 mg/L (nominal concentration, highest tested concentration), and the 96-h LC0 was ≥ 100 mg/L.

This result is further supported by acute fish toxicity data on niobium pentachloride (NbCl₅). In a study conducted according to OECD guideline 203, zebrafish (Danio rerio) were exposed to niobium pentachloride at nominal concentrations of 0 and 100 mg/L under semi-static conditions (limit test). No mortality could be observed. Due to methodological constraints, effect levels are provided as nominal loading rates. Hence, the 96-h LL50 (nominal) is > 100 mg/L.

Further acute aquatic toxicity studies investigating the effects of niobium carbide to aquatic invertebrates, algae and microorganisms are not available. In order to fulfil the REACH Annex VIII registration requirements, niobium pentachloride (NbCl₅) was used as read-across partner for these endpoints. Due to lower water solubility for niobium carbide compared to niobium pentachloride the resulting toxicity is also expected to be lower. Hence, the read-across to the substance niobium pentachloride is sufficiently protective. For further details please refer to the read-across report.

In a study in accordance with OECD guideline 202 the 48-h acute toxicity of niobium pentachloride to Daphnia magna was studied under semi-static conditions at nominal concentrations of up to 4500 mg/L. The 48-hour EL50 was 1498 mg/L. The 48-h NOELR based on immobilization was 889 mg/L and the LOELR was 1333 mg/L.

The toxicity of niobium pentachloride to cultures of Pseudokirchneriella subcapitata after 72-h (growth inhibition test) was investigated in accordance with OECD Guideline 201 at nominal concentrations of up to 2000 mg/L. Results based on growth rate after 72 h were as follows:

-         EL50: 533 mg/L (95 % C.I. 393 – 725 mg/L);

-         LOELR: ≤ 125 mg/L;

-         NOELR: < 125 mg/L.

An activated sludge respiration inhibition (limit) test with niobium pentachloride according to OECD 209 resulted in an EL50 > 1000 mg/L (nominal concentration).

Please note that in aquatic toxicity tests conducted with niobium pentachloride it was not possible to determine effect levels based on measured effective test substance concentrations. Addition of NbCl₅to water leads to strong temperature rise and a strong shift in pH value (<1). Thus, pH values had to be adjusted in all tests by the addition of NaOH. After pH adjustment, the measured Nb⁵⁺concentrations were low and showed high variability. Therefore, no concentration-response relationship could be established. Concentrations of the test item varied due to precipitation, agglomeration and adsorption reactions. In addition, slight changes in pH had profound impact on the test item concentrations. Observed effects in daphnia and algae at very high test substance concentrations may therefore rather predominantly refer to elevated sodium chloride (NaCl) concentrations, originated from neutralization, than released Nb5+ ions. Reported EC50 values in daphnia and inhibitory concentrations in algae for NaCl vary but may be as low as EC50 = 874 mg/L (ECHA registry for sodium chloride) and IC50 = 0.87-2.5 g/L (IC = inhibition of fluorescence; Santos et al., 2007, in Heisterkamp, 2015; Geis et al., 2000), respectively. Due to concentrations of approximately 4.68 g/L NaCl and 2.92 g/L NaCl present at the highest loading rate in OECD test 202 (daphnia; 4500 mg/L test substance) and OECD test 201 (algae; 2000 mg/L test substance), respectively, and the sensitivity of D. magna and the freshwater alga P. subcapitata to salinity, any hypothetical Nb⁵⁺related effects cannot be distinguished from NaCl induced inhibition in the test. However, effects can be rather attributed to NaCl than to the release of Nb⁵⁺ions. This assumption is supported by the results of the available tests investigating the toxicity of NbC towards fish where no adverse effects could be observed at limit test concentrations of 100 mg/L.