Hafnium tetrachloride

Endpoint summary

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 to give HfO2 and HCl.

To confirm this hypothesis, the hydrolysis of HfCl4 has been studied in a test complying with OECD guidance 111 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 did not 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.

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).

Complexes with sulphates, fluorides and chlorides may be poorly soluble in aqueous solution, but complexation with natural organic matter may increase the concentrations of Hf in natural freshwater.

Biodegradation:

Concerning degradation, HfCl4 cannot be degraded in a biotic way because it is an inorganic element.

Bioaccumulation:

Hafnium compounds in water are very insoluble, and with a strong affinity with particulate organic matter showed by high Kd value for Hf, Hf compounds and thus HfCl4 and its degradation proucts are therefore very unlikely to be bioavailable to aquatic organisms, fish and then fish-eating predators. No potential for bioaccumulation is then expected.

Adsorption:

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.

 

Atmospheric distribution:

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