Hafnium

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 exposure of sediment expected

Sediment (marine water)

Hazard assessment conclusion:

no exposure of sediment expected

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

no exposure of soil expected

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

In aqueous solution, hafnium is always quadrivalent and precipitates as Hf(OH)_4. Hf(OH)₂^{2 +} 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 (Hf(OH)₂^{2 +)}

(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 (< 0.007 mg Hf/L or <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 therefore exposure of micro-organisms is unlikely. For these reasons, no PNEC was derived for STP.

Terrestrial compartment:

In the terrestrial compartment, the availability of metal compounds for uptake by biota can differ from site to site and may change over time due to many processes, including weathering and (de)sorption processes. It should also be noted that Kd values are accurate only during an equilibrium state, which is difficult to reach for metals in the environment. As a consequence, part of the metal present in the solid phase may be encapsulated in the mineral fraction and is therefore not available.

The IAEA Technical Reports Series No. 364 (Handbook of Parameter Values for the Prediction of Radionuclide Transfer in Temperate Environments, 1994) publication reported Kd values for Hf within the range of 1500 to 5400 L/kg, with a Kd of 2500 L/kg for soils in general.

The typical value was considered for assessment: Kd of 2500 L/kg. This coefficient confirmed a high adsorptivity of the substance on particulate matter and low mobility in soils, leading to a poor availability to biota via interstitial water.

Moreover local exposure for the soil compartment could be due to deposition of particles emitted in the atmospheric compartment, and no volatilization is assumed for metal compounds, there is then no way to expect soil exposure to Hf.

That is why no PNEC was derived for soil compartment.

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 expected that due to complexation reactions, Hf will not be bioavailable to benthic organisms. In addition, no effects have been observed in aquatic organisms (algae, 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 to sediment is not mandatory.

 

Air compartment:

Volatilization can be ignored for metals compounds, except for mercury compounds and several organometallic compounds.

Hf is not concerned with global warning potential nor ozone depletion/ formation, since it is a non-volatile substance, containing no carbon or fluoride molecule. Moreover, there is no available methods for the determination of effects of chemicals on species arising from atmospheric contamination. For all these reasons, no PNEC was derived for the air compartment.

 

Secondary poisoning:

 

The very low water solubility of Hf and the strong affinity with particulate organic matter showed by high Kd value, make Hf very unlikely to be bioavailable to aquatic organisms, fish and then fish-eating predators. Moreover, there is no way to expect soil exposure to Hf (see chapter 6.3 of the dossier) and Hf is not classified for toxic acute or repeated effects, carcinogenicity, mutagenicity or toxicity to the reproduction. For these reasons, it could be reasonably assumed that Hf will not bioaccumulate in the food chain, and no PNEC have been derived for secondary poisoning.

 

Conclusion on classification

Three studies with aquatic invertebrates, algae and fish were carried out with HfO2 (used as read across, as substance form retrieved in water) at saturated concentrations. These tests showed no hazard to these three trophic levels following a short term exposure of HfO2.

Moreover, Hf is not concerned by biodegradation, is very poorly soluble in water, and has a strong affinity with particulate matter. For these reasons, it is very unlikely that Hf bioaccumulate since it is not bioavailable to aquatic organisms.

Consequently, there is no reason to consider that Hf may have any effect to aquatic organisms and it is therefore not classified for the environment.