Hafnium tetrachloride

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

The hydrolysis of HfCl4 has been described by Kirk-Othmer in 1998, Benamira in 1997 and Beden in 1969: Hafnium tetrachloride reacts with water at room temperature, forming first hafnium oxide chloride (HfOCl2) and hydrochloric acid (HCl):

HfCl4 + 9H2O = > HfOCl2 •8H2O + 2HCl

Then, HfOCl2 reacts with H2O and gives HfO2 and HCl.

To confirm this hypothesis, the hydrolysis of HfCl4 has been studied in a test complying with OECD guidance and rate with reliability 1. The experiment showed that the reaction occurred immediately and the stabilization of the reaction is showed by the pH and also the chloride content, which didn't vary at all after the reaction. The instantaneous hydrolysis was also seen visually with a precipitate formation that could demonstrate the formation of HfO2, a very insoluble compound.

The Hf(OH)₂²⁺ is the hydrated form of HfO2 and is therefore taken into consideration for the behaviour of HfCl4 in the aquatic compartment, since HfCl4 is very hydrolytically unstable. On the contrary, this Hf(OH)₂²⁺ complex is very stable and resistant to protonation (Hagfeldt et al. 2004).

Concerning the second degradation product; HCl, the regulatory classification and the dossier submitted in 2010 in the frame of REACh do not show a concern for the environment, except for pH effects. That is why only HfO2 was considered for aquatic hazard and the pH variations were carefully followed in the performed studies.

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 aquatic 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 following 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 HfO2 and its affinity to form complexes with organic matter 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 compounds 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.

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 all soils in general.

These values and IAEA assessor were considered reliable enough to be used for HfCl4 in a read across approach, as the common metal form Hf is retrieved in soils.

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 HfCl4.

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 the relevant degradation product (HfO2) used for read across, 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, HfO2 will not be bioavailable to benthic organisms. In addition, no effects have been observed in pelagic organisms (daphnia and fish) in acute tests. In conclusion, it can be reasonably assumed that HfO2 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.

Air compartment:

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

HfCl4, or its decomposition product HfO2, is not concerned by global warning potential nor ozone depletion/ formation, since there are non-volatile substances, containing no carbon nor fluoride molecule. The HCl formation from HfCl4 hydrolysis is assessed in its specific dossier. Moreover, there is no available method 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 low water solubility of HfO2, with a strong affinity with particulate organic matter showed by high Kd value, make HfO2 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 (used for read across; see chapter 6.3 of the dossier) and HfCl4 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 HfCl4 will not bioaccumulate in the food chain, and no PNEC have been derived for secondary poisoning.

Conclusion on classification

As showed in the hydrolysis test (see section 5.1.2.) and in several publications, HfCl4 decomposes quasi instantaneously in water in HfO2 (as Hf(OH)₂²⁺) and HCl. These two by products have then to be taken into account for classification for the environment.

HCl regulatory classification does not concern the environment (only pH effects are demonstrated), that is why only HfO2 is considered for aquatic hazard.

Three studies with aquatic invertebrates, algae and fish were carried out with HfO2 (used as read across for HfCl4, as substance form retrieved in water) at saturated aquatic concentrations. These tests showed no hazard to these three trophic levels following short term exposure of HfO2.

Moreover, HfO2 is very poorly soluble in water and has a strong affinity with particulate matter. For these reasons, it is very unlikely that HfO2 bioaccumulate since it is no bioavailable to aquatic organisms.

Consequently, there is no reason to consider that HfO2 may have any effect to aquatic organisms and it is not classified for the environment.

For the same reasons, a classification for the environment for HfCl4 is not justified.