Hafnium tetrachloride

Endpoint summary

Additional information

For ecotoxicological studies and according to OECD guidance document (OECD 2000), if a substance is likely to be unstable, a decision to test the parent substance and/or its degradation products, if identified, should be based on a consideration of its half-life under test and realistic environmental conditions. The following decision criteria are suggested in the OECD guidance document only as a guide for static and semi-static tests with medium renewal times of 24 hours:

- Half-life >3 days: test parent substance;

- Half-life <3 days and >1 hour: consider on a case-by-case basis, and include possible testing of degradation products;

- Half life <1 hour: test degradation products.

As showed in the hydrolysis test (see section 5.1.2.), HfCl4 decomposes quasi instantaneously into HfOCl2 and finally HfO2. Hf(OH)₂²⁺which is the hydrated form of HfO_2, is very stable and resistant to protonation (Hagfeldt et al. 2004). Complexes with sulphates, fluorides and chlorides may be poorly soluble in aqueous solution, but complexation with natural organic matter may increase the concentrations in natural freshwater.

Due to this behaviour of HfCl4 in aquatic compartment, all tests required have been carried out on the hydrated form HfO2.

These tests (acute toxicity to aquatic invertebrates, algae and fish) are then used in read across and all give the same results: at aquatic saturation (< 0.007 mg Hf/L), the LC50 and LOEC results are higher than the solubility limit of the test item. These results indicate that there is no concern for ecotoxicological potential effect with hafnium for aquatic compartment, and then no further testing is considered for chronic toxicity to aquatic organisms.

Moreover, the extremely low water solubility of HfO2 (< 8 µg/L, see section 4.8.) and its affinity to form complexes with organic matter (Kd values are reported for Hf metal with a range of 1500- 8500 L/kg, that show a strong affinity for particulate matter), make HfO2 not bioavailable to aquatic organisms. In addition, with the inorganic nature of the substance, no biological treatment is expected for industrial site. In case a STP exists, Hf compounds will be removed in the primary settling tank and exposure of micro organisms is unlikely. For all these reasons, there is no need to investigate the effects on aquatic micro organisms.