Formation of perfluorooctane sulfonate on suspended water droplets via hydrolyzation of a volatile precursor.

Abstract

Perfluroroctanesulfonate (PFOS) is a ubiquitous global pollutant with no known degradation pathways. A 2001 study by Kannan et al. helped to establish PFOS as a global contaminant by measuring it in wildlife and waterways in both remote and urban areas. Atmospheric transport is believed to be the main pathway by which PFOS has been spread globally. The low vapor pressure and high water solubility combine to provide PFOS with a low Henry’s Law Constant making direct atmospheric transport unlikely. One possible explanation for the atmospheric transport of PFOS involves transport and reaction of volatile precursors. Volatile precursors such as perfluorooctane sulfonyl fluoride (POSF) have been demonstrated to react with suspended aerosol water droplets resulting in the formation of PFOS. Underwater exposure of POSF resulted in significantly decreased PFOS production compared to aerosol droplets. This indicates the importance of an air water interface, which allows POSF to position itself so that the reactive sulfonyl head is on the surface of the water and the hydrophobic tail is oriented away. Experiments involving aprotic solvents indicate the diffusion of POSF into water is faster than simultaneous surface hydrolysis. In order to gain a better understanding of the role the air-water interface plays in PFOS production two parameters were varied independently. The volume of the aerosol droplets exposed and the duration of time they were exposed for was varied in separate experiments. Observations indicate that time plays a very important role in PFOS production, as longer exposed droplets contained less POSF than those exposed for shorter time periods.