Titanium 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

In general, the hazard assessment is based in the concentration of the Ti ion in environmental media. As the T/D test could not demonstrate release of Ti from TiC (method detection limit 0.4 µg/L), hazards for the environment were not identified. Thus, derivation of PNECs is not necessary.

Conclusion on classification

Aquatic toxicity classification of metals and inorganic metal compounds is conducted by comparing transformation/dissolution (T/D) data for the substance, generated using the standard protocol (UN GHS, Annex 10) with toxicity data for the most soluble metal substance as described in the CLP technical guidance (section IV. 5, application of classification criteria to metals and metal compounds; EU, 2008). The T/D data is ideally tested at the pH at which the highest dissolution is expected, within the range defined by the test protocol. In the T/D pre-test on TiC no metal ion release above the method detection limit was detected at any of the pH values assayed (pH 6, 7, and 8). Since the solubility of most metals is higher at lower pH values, pH 6 was chosen to perform the full T/D test (24-hour, 7-, and 28-day T/D testing) (CIMM, 2012).

 

According to the ECHA CLP Guidance (v.3, 2012) “Reading across metal compounds can [...] be conducted by comparing the soluble metal ion concentration (µg Me/L) causing the ecotoxicity effect and translating this towards the compound under investigation. [...] The comparison is therefore directly done by comparing the soluble fraction measured after transformation/dissolution with the ecotoxicity reference values of the soluble metal ion (based on the UN GHS, 2009).”

In the current case of TiC, the ecotoxicity reference values (ERVs) are based on available ecotoxicity data of TiO₂. However, the ERVs are in this case not derived from and/or normalized to the soluble Ti ions released from TiO₂ (3.4 µg/L at 21.9 °C and pH 6), but from the nominal TiO₂ concentrations applied in the different tests. This is due to the fact that the toxic mode of action of TiO₂ is not solely based on the Ti ions released, but also on indirect and physical effects (photocatalytic activity or biological surface coating) of non-dissolved, dispersed TiO₂ in solution.

 

TiO₂, especially nano-sized material, is a well-known photocatalyst. “Namely, the TiO₂ crystalline forms are semiconductors, meaning that they can be photo-excited to generate electron-hole pairs on their surfaces, which results in their strong oxidizability. This characteristic enhances the formation of reactive oxygen species (ROS), which is among the main toxic mechanisms proposed for the observed toxic effects of photo-irradiated, nano-sized TiO₂.” (Hirakawa et al., 2004, cited in Jemec et al., 2008). But this mechanism for (nano-sized) TiO₂ has also been described without photo-activation. “The ability of nano-sized TiO₂to induce ROS formation without photo-activation has been related to its crystallinity and electronic configurations and to an indirect effect on the antioxidant system of the cell” (Xia et al., 2006, and Hussain et al., 2001, cited in Jemec et al., 2008).

 

This photocatalytic effect is not anticipated for TiC, and significant release of Ti ions from TiC could not be demonstrated in the T/D test (Ti ionic release < 0.4 µg/L), showing that TiC is practically insoluble. Therefore, using nominal TiO₂ concentrations as ERVs is considered adequately protective.

 

The T/D values obtained for titanium carbide were compared to the corresponding acute (based on the E_rC₅₀) and chronic (based on the E_rC₁₀) aquatic toxicity reference values derived from testing titanium dioxide in algae, as the most sensitive standard aquatic species for titanium dioxide. The results of this comparison demonstrate that titanium carbide does not require classification for aquatic toxicity (see table below).

 

Classification of titanium carbide using T/D data comparison to toxicity data according to the CLP technical guidance (European Chemicals Bureau, 2009):

 

Test type/duration	T/D loading amount as TiC [mg/L]	T/D results [µg Ti/L] pH 6	Toxicity reference value [µg Ti/L]	Comparison of T/D and toxicity values [µg Ti/L]	Aquatic toxicity classification result
Screening test / 24 h	100	BDL	Acute = 36600 (ErC50 = 61 mg TiO2/L)	BDL < 36600	Refer to full T/D test for classification
Acute full test / 7 d	10	0.2 (control: 0.2)	Acute = 36600 (ErC50 = 61 mg TiO2/L)	0 < 36600	No Chronic 2 classification
Acute full test / 7 d	100	0.2 (control: 0.2)	Acute = 36600 (ErC50 = 61 mg TiO2/L)	0 < 36600	No Chronic 3 classification
Chronic full test / 28 d	1	0.5 (control: 0.5)	Chronic = 3940 (ErC10 = 9.9 mg TiO2/L)	0  < 5940	No Chronic 2 classification

BDL = below detection limit

 

The CLP classification scheme for evaluating aquatic toxicity of metals and metal compounds is the same as that used to classify metals and metal compounds under the Dangerous Substances Directive (DSD), with the exception of the name of the classifications (e.g. DSD cites R phrases, CLP uses acute and chronic categories). Although the DSD does not specifically cite the classification scheme for metals and metal compounds, the scheme was outlined in the ECB documents used in the classification of nickel metal (massive and powder). In addition, this classification scheme was used to evaluate aquatic toxicity of nickel metal and some copper compounds (ECB, 2001; ECB, 2005).