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CAS# 756-13-8 is a completely fluorinated ketone.  The substance is a liquid at room temperature with a high vapor pressure (40 kPa at 25°C).  The water solubility of CAS# 756-13-8 is very low, in the range of 1-25 ppm.  A precise value cannot be measured owing to its extremely short half life at all pH values measured.  Most releases of CAS# 756-13-8 are expected to be atmospheric emissions based upon its intended uses.  Fugitive emissions may occur at transfer points.  During routine use, there is no anticipated release to water or wastewater in the EU.  Therefore, this compound will remain in the atmosphere when released from industrial applications.

 

The half-life of CAS# 756-13-8 by direct photolysis in the atmosphere is approximately one week.(1,2) Photolysis products were not definitively assessed, however the expected photolytic pathways result in formation of trifluoroacetic acid (TFA, CAS# 76-05-1), CO2 and hydrofluoric acid (HF,CAS# 7664-39-3).  Perfluoropropionic acid (PFPA, CAS# 422-64-0) is also expected to form under conditions of limited NOx concentration. Indirect photolysis of CAS# 756-13-8 was determined to be insignificant. CAS# 756-13-8 solubility in water is low.  Upon accidental release to aquatic systems, CAS# 756-13-8 will degrade via hydrolysis to form perfluoropropionic acid and heptafluoropropane (HFP).  Once dissolved into water, the hydrolytic half-life of CAS# 756-13-8 was found to be ca. 2.5 minutes. However, the rate of CAS# 756-13-8 hydrolysis in other studies was limited by dissolution of the chemical into liquid water.   The ultimate fate of CAS# 756-13-8 in the atmosphere therefore depends on the relative contributions of photolysis and hydrolysis.  In a supporting study addressing this question through a theoretical approach, the two processes were compared.  The contribution of hydrolysis in liquid water droplets to overall atmospheric fate was estimated to be approximately 120,000 times less than the contribution of photolysis.  The two factors that led to this decrease were the low tendency of CAS# 756-13-8 to partition from vapor phase to liquid water (estimated Kaw of 5300) and the extremely low fraction of liquid water as droplets in the atmosphere (ca. 1x10-8 on a volume basis).  This study was deemed reliable with restrictions, in that not all degradation pathways were considered, no attempt was made to assess the variability of the input parameters used, and no sensitivity was done to determine robustness of the analysis to variability in input parameters. A reanalysis of the available data using the supporting study’s approach suggests that the relative rate of photolysis may be 6000 to 120,000 times greater than that of hydrolysis, with photolysis still the overwhelming process for degradation in the atmosphere.

 

The primary degradation products, TFA and HF, and the negligible amount of PFPA formed in the atmosphere are expected to rapidly undergo wet deposition with no further significant transformation.  Partitioning of these acids in the environment is driven by the fact that they are completely ionized at environmental pH values, are miscible in water, and are not likely to bind with organic matter based on low Kocs and low log Kows.  Thus, HF, TFA and PFPA will be associated with the aqueous phase of any environment where they are released.  HF, TFA and PFPA that have deposited in aquatic compartments are expected to remain in the aquatic compartment. Please note that a published environmental risk assessment on TFA is available in the literature(3).

 

The formation of HFP in the atmosphere is negligible. It is a highly volatile gas (vapor pressure = 3410 mm Hg (454.6 kPa) at 25°C) with an estimated atmospheric lifetime of 34.2 years, forming TFA, HF and CO2(4).

 

1)    N. Taniguchi, T. J. Wallington, M. D. Hurley, A. G. Guschin, L. T. Molina, and M. J. Molina.  2003.  Atmospheric Chemistry of C2F5C(O)CF(CF3)2: Photolysis and Reaction with Cl Atoms, OH Radicals, and Ozone.  J. Phys. Chem. A Vol. 107, No. 15, pp. 2674-2679

 

2)    B. D'Anna, S. R. Sellevåg, K. Wirtz, and C. J. Nielsen.  2005.  Photolysis Study of Perfluoro-2-methyl-3-pentanone under Natural Sunlight Conditions.  Environ. Sci. Technol.  Vol. 39, No. 22, pp. 8708–8711

 

3)    Boutonnet (Ed.), 1999.  Environmental Risk Assessment of Trifluoroacetic Acid.  Human and Ecological Risk Assessment:  Vol. 5, No. 1, pp. 59-124.

 

4)    Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland, 2007: Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.