Sulphur hexafluoride

Endpoint summary

Additional information

SF6 possesses a high degree of thermal and chemical stability.The stability of SF6 is explained by the symmetrical, octahedral structure of the molecule. Only the application of SF6 at high temperatures (starting at > 200°C in catalytic based reaction with metals), or in glass or metal (stainless steel, copper) tubes when heating up to 700 to 900°C will thermical discharge SF6. Reactions of SF6 with other components (non-metals like H2O, CO2, CO, rare gases, hydrogen, oxygen, nitrogen, halogens, sulphur, carbon, boron, silicon and metals) need to be activated by temperature, electric energy, pressure or other high energetic influences (Gmelin Handbuch der anorganischen chemie, 1978). Investigations related to reactions of SF6 with oxygen (O2) have shown no reaction at ambient temperature (Siegel and Breisacher, 1969).The interaction of SF6 with MgO, NiO, SiO2, and water has been investigated in the temperature range 25-500°C and the pressure range 1-4000 atm. In.each case SO2F2 and the appropriate fluoride were formed at a minimum temperature of 475°C and 130 atm.

Degradation:

Hydrolysis of sulphur hexafluoride is unlikely to occur as it contains no hydrolysable functional groups in its chemical structure.

The atmospheric lifetime of SF6 was estimated by Ravishankara et al. (1993). The reaction of SF6 with O(1D) and the photolysis mediated by Lyman radiation were considered as the possible degradation pathways. The degradation rate coefficient for reaction of SF6 with O(1D) was estimated to be 1.5E-14 cm3/mol/s (1.80 ± 0.26E-14 cm3/mol/s). The OH-mediated photodegradation was considered as not relevant.

Screening and simulation tests for biodegradation are not available as SF6 is inorganic.

Environmental distribution:

SF6 is expected to have a low potential for adsorption as the substance has a low log Kow of 1.68. Furthermore, the substance is a gas and therefore direct and indirect exposure of the sediment and soil compartments are unlikely.

A Henry's Law Constant of 185 (dimensionless) at 25 °C is reported by Giardino et al. (1988). An HLC of 4.52 atm m3/mole (458 kPa m3/mole) can by calculated by considering the conversion factor 41 between the dimensionless value and the HLC expressed in atm.m3/mol.

According to Mackay level III model (2002), SF6, released at the rate 1000 kg/h in atmosphere, will not partition significantly into water and solid compartments. The removal of the substance from the standard environment defined in the Equilibrium Criterion Model (EQC, Mackay et al, 1996) will occur mainly via advection, due to the very low atmospheric degradation rate of SF6.

Bioaccumulation:

Based on its physical chemical properties, SF6 is not expected to bioaccumulate in fish as it has a low potential of adsorption (log Kow = 1.68).

Refs:

Gmelin Handbuch der anorganischen chemieS (Schwefel), Ergänzungsband 2 Schwefelhalogenide, 1978 8.Auflage, Springer Verlag

Siegel B., Breisacher P. (1969). Oxidation of Sulfur Hexafluoride. J. inorg. Nucl.Chemical; Vol.31; pp. 675 to 683.