Niobium carbide

Toxicity to aquatic algae and cyanobacteria

Description of key information

No data are available on the acute aquatic toxicity of NbC to algae. Thus, information on NbCl5 is used in a read-across approach. In the conducted static algae growth inhibition test with Pseudokirchneriella subcapitata, applying NbCl5 the nominal EL50 was 533 mg/L (95 % C.I. = 393–725 mg/L). The values indicate effects at nominal loading rates. Due to methodological constraints, it was not possible to determine effect levels based on measured test item concentrations. Observed growth inhibition effects were predominantly related to elevated NaCl concentrations, originating from neutralisation of the test solutions, instead of dissolved Nb5+ ions.

Additional information

Available data on niobium pentachloride are appropriate for read-across to the target substance niobium carbide. Due to lower water solubility of niobium carbide compared to niobium pentachloride the resulting toxicity is also expected to be lower. Hence read-across from niobium pentachloride is sufficiently protective.

In a 72-h growth inhibition test, cultures of Pseudokirchneriella subcapitata (strain No. 61.81 SAG) were exposed to niobium pentachloride at nominal concentrations of 0 (control), 125, 250, 500, 1000 and 2000 mg/L under static conditions in accordance with OECD guideline 201 (March 2006) under GLP. The % growth inhibition in the treated algal culture as compared to the control ranged from 57.9 % at a nominal concentration of 125 mg/L to 100.7 % at a nominal concentration of 2000 mg/L. Measured values ranged from 16 to 382 µg/L in the nominal loading rates of 125 to 2000 mg/L at the start of the test.

Specific results based on growth rate after 72 h were as follows: EL50 = 533 mg/L (95 % C.I. = 393–725 mg/L); EL20 = 164 mg/L (95 % C.I. 79.5–242 mg/L); EL10 = 88.3 mg/L (95 % C.I. = 31.6–149 mg/L); LOELR: ≤ 125 mg/L; NOELR: < 125 mg/L. The values indicate effects at nominal loading rates. After pH adjustment the measured Nb5+ concentrations were low and showed high variability. Therefore, no concentration-response relationship can be established. Concentrations of the test sample varied due to precipitation, agglomeration and adsorption reactions. In addition, slight changes in pH had significant impact on the test concentrations.

Observed growth inhibition effects may therefore be predominantly related to elevated NaCl concentrations, originating from neutralisation, instead of dissolved Nb5+ ions. 50 % inhibitory concentrations of sodium chloride inP. subcapitatareported in the literature are slightly variable, ranging between 2.5 and 0.87 g/L (Geis et al., 2000 and Santos et al., 2007, cited in Heisterkamp, 2015). Due to a concentration of approximately 2.92 g/L NaCl, present at the highest loading rate of 2000 mg/L test substance, and the sensitivity of P. subcapitata to salinity, any hypothetical Nb5+ related effects cannot be distinguished from NaCl induced inhibition observed in this test.