Hafnium

Endpoint summary

Additional information

In aqueous solution, hafnium is always quadrivalent and precipitates as the Hf(OH)₄. Hf(OH)₂²⁺ is the hydrated form of HfO₂, and is the only species present in solution in the absence of other stabilising ligands. This complex 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 materials may increase the concentrations of Hf in natural freshwater.

Studies suggest that when hafnium is detected in surface water samples it is associated with particles in the water column (Tanisaki et al. (1992); Taylor et al. (1990)).

Due to this behaviour of Hf in aquatic compartment, all tests required have been carried out on the hydrated form HfO2 (asHf(OH)₂²⁺). (Acute toxicity to aquatic invertebrates, algae and fish).

 

Aquatic compartment:

Three studies with aquatic invertebrates, algae and fish were carried out with HfO2 at saturated concentrations.

The first one assessed the acute toxicity to Zebrafish (Brachydanio rerio), over an exposure period of 96 hours in flow through conditions.

The second one was performed on Daphnia magna, over an exposure period of 48 hours in a semi-static system.

And the third assessed the effect of HfO2 on algal growth using the unicellular green alga Pseudokirchneriella subcapitata (Selenastrum capricornutum), over an exposure period of 72 hours.

One concentration at the solubility limit of the test item in the test medium (100 % v/v saturated solution) plus an untreated control were tested in limit tests.

In the conditions of these three tests, Hafnium dioxide had no toxic effect at water solubility limit (<0,008 mg Hf/L) on fish, invertebrate and algae; the LC50 results and the NOEC are higher than the solubility limit of the test item in the test medium.

 

These three tests showed no hazard to these three trophic levels due to short term exposure of HfO2. Consequently, there is no need to derive any aquatic PNEC.

 

Microorganisms/ sewage treatment plant:

The extremely low water solubility of Hf and its affinity to form complexes with organic molecules in water (see explanation for Kd values just above), makes it not bioavailable to aquatic organisms. Moreover, the nature of the substance (inorganic), suggests that no biological treatment is expected for this substance. But in case a WWTP with several treatment steps exists on site, Hf will then be removed in the primary settling tank, due to adsorption on particulate matter and exposure of micro-organisms is unlikely. For these reasons, no PNEC was derived for STP.

Sediment organisms:

There is no data for sediment species, but considering the very low water solubility of Hf, it is not expected that it is present in the pore water of sediment, where sediment-dwelling organisms can be found. It is furthermore assumed that due to complexation reactions, Hf will not be bioavailable to benthic organisms. In addition, no effects have been observed in aquatic organisms (daphnia and fish) in acute tests with HfO2 used for read across. In conclusion, it can be reasonably assumed that Hf concentrations will not reach sufficiently high levels to be able to exert toxic effects to benthic organisms.

Moreover, due to tonnage band, the potential hazard assessment to sediment is not mandatory.