Niobium carbide

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

no hazard identified

Marine water

Hazard assessment conclusion:

no hazard identified

STP

Hazard assessment conclusion:

no hazard identified

Sediment (freshwater)

Hazard assessment conclusion:

no hazard identified

Sediment (marine water)

Hazard assessment conclusion:

no hazard identified

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

no hazard identified

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

A study investigating the acute aquatic toxicity of niobium carbide (NbC) to fish (OECD Guideline 203) demonstrates that the test substance does not exhibit acute toxicity to zebrafish at nominal concentrations of 100 mg/L (96-h LC0 ≥ 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.

For the 48-h acute toxicity of niobium pentachloride to Daphnia magna an EL50 = 1498 mg/L (nominal concentration) was determined. The 72-h growth inhibition study in cultures of Pseudokirchneriella subcapitata revealed an EL50 of 533 mg/L (nominal concentration). An activated sludge respiration inhibition test with niobium pentachloride 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 and soil invertebrates where no adverse effects could be observed at limit test concentrations of 100 mg/L and 1000 mg/kg soil d.w., respectively.

Based on the very low water solubility (6.47 µg/L, T/D test) and thus very low bioavailability, toxic effects in aquatic organisms are not expected. Available results on NbC (fish, soil invertebrates) do not give rise to any concern.

Conclusion on classification

Niobium carbide is not toxic to fish and soil invertebrates at limit test concentrations as demonstrated in available OECD 203 and OECD 207 guideline studies. Data on the toxicity of NbC to aquatic invertebrates, algae and microorganisms are not available and niobium pentachloride (NbCl₅) is used as read-across partner.

Niobium pentachloride is not toxic to fish in an OECD 203 test at a nominal concentration of 100 mg/L. The substance did not induce inhibition of the respiration rate of activated sludge at a concentration of 1000 mg/L (nominal) in a test according to OECD 209. Effects were observed in studies with aquatic algae and invertebrates at very high test substance concentrations far exceeding the threshold of 100 mg/L relevant for classification and labelling. These effects are considered to be rather related to elevated NaCl concentrations generated by neutralization of test solutions by means of NaOH than to the release of Nb⁵⁺ions:

Reported EC50 values in daphnia and inhibitory concentrations in algae for NaCl vary but may be as low as 874 mg/L (ECHA registry for sodium chloride) and 0.87-2.5 g/L (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 P. subcapitata to salinity, any hypothetical Nb⁵⁺-related effects cannot be distinguished from NaCl induced inhibition in the test. As sodium chloride is a consequence of the artificial test conditions and is neither legally classified nor self-classified for environmental effects by any of the notifying parties (1772; as of April 06, 2016), a classification for environmental hazards for NbCl₅ related to these indirect effects is also not warranted.

In studies investigating the ecotoxic effects of NbCl₅to daphnia and algae, test solutions used in the ecotoxicity studies were adjusted to near neutral pH values prior to commencement of testing, as addition of NbCl₅to aqueous solutions leads to a strong shift in pH (< 1). Any hypothetical ecotoxicological effects after release of NbCl₅to environmental compartments which would be related to changes in the pH value are considered to be of physical nature. Beyond that, such effects are not relevant under environmental conditions due to the inherent buffer capacity of natural surface waters. In conclusion, niobium pentachloride does not need to be classified with regard to potential environmental hazards, the substance is considered as non-hazardous.

Based on available results for NbC and with the read-across substance NbCl₅, NbC can be considered as non-hazardous and a classification for environmental hazards is not justified.