An accurately measured equilibrium acid dissociation constant (pKa) is essential for understanding and predicting the fate of perfluorocarboxylic acids (PFCAs) in the environment. The aqueous pKa of perfluorooctanoic acid (PFOA) has been determined potentiometrically using a standard water-methanol mixed solvent approach and was found to be 3.8 +/- 0.1. The acidity of PFOA is thus considerably weaker than its shorter-chain PFCA homologues. This was attributed to differences in molecular and electronic structure, coupled with solvation effects. The pKa of PFOA was suppressed to approximately 2.3 at higher concentrations because of the aggregation of perfluorooctanoate (PFO). Often, PFCA partion coefficients are determined at concentrations above those found in the environment. Thus, it was suggested that a pKa correction factor, which accounts for this concentration-dependent shift in acid/base equilibrium, should be applied to PFCA partition efficients before they are implemented in environmental fate models. A pKa of 3.8 +/- 0.1 suggests that a considerable concentration of the PFCA exists as the neutral species in the aqueous environment for example, in typical Ontario rainwater, it is approximately 17%. Transport, fate, and partitioning models have often ignored the presence this species completely. The environmental dissemination of PFCAs could, in part, be explained by considering the role of the neutral species.

Aerosol-mediated transport of perfluorooctanoate (PFO) from a water body to the atmosphere and the subsequent emission of gas-phase perfluorooctanoic acid (PFOA) was investigated. The potential for this process to facilitate long-range transport of PFOA/PFO was assessed. In a laboratory experiment, aerosols were generated and collected from deionized, fresh, and ocean waters spiked with PFO and analyzed by LC-MS/MS. Gas-phase samples were also collected from the system and analyzed for PFOA. Aerosols were found to have significantly higher concentrations of PFO than the parent water body (< or = 80 times for ocean waters). The PFOA, at equilibrium with the PFO in the aqueous aerosol, partitioned rapidly (t 1/2 = 7.2 s) out of the aerosol droplet. This suggests that rainout rates are likely to be longer than previously hypothesized. These results imply that water bodies are not a permanent sink for atmospheric PFOA as previous studies have suggested. The occurrence of contamination in remote regions may not depend solely on the previously hypothesized indirect sources but also on the long-range transport, via the gas phase, of direct releases of PFOA to both the aquatic and atmospheric environments.

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