Experimental pK a Determination for Perfluorooctanoic Acid (PFOA) and the Potential Impact of pK a Concentration Dependence on Laboratory-Measured Partitioning Phenomena and Environmental Modeling

• Published in: Environmental science & technology Vol. 42; no. 24; pp. 9283 - 9288
• Main Authors: Burns, Darcy C, Ellis, David A, Li, Hongxia, McMurdo, Colin J, Webster, Eva
• Format: Journal Article
• Language: English
• Published: American Chemical Society 15.12.2008
• Subjects: Environmental Measurements Methods
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es802047v

An accurately measured equilibrium acid dissociation constant (pK a) is essential for understanding and predicting the fate of perfluorocarboxylic acids (PFCAs) in the environment. The aqueous pK a 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 pK a of PFOA was suppressed to ∼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 pK a 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 pK a 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 ∼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.