Estimation of the Acid Dissociation Constant of Perfluoroalkyl Carboxylic Acids through an Experimental Investigation of their Water-to-Air Transport

Abstract

The acid dissociation constants (p<i>K</i>_as) of perfluoroalkyl carboxylic acids (PFCAs) have been the subject of discussion in the literature; for example, values from −0.2 to 3.8 have been suggested for perfluorooctanoic acid (PFOA). The dissociated anionic conjugate bases of PFCAs have negligible air–water partition coefficients (<i>K</i>_AWs) and do not volatilize from water. The neutral acids, however, have relatively high <i>K</i>_AWs and volatilization from water has been demonstrated. The extent of volatilization of PFCAs in the environment will depend on the water pH and their p<i>K</i>_a. Knowledge of the p<i>K</i>_as of PFCAs is therefore vital for understanding their environmental transport and fate. We investigated the water-to-air transfer of PFCAs in a novel experimental setup. We used ∼1 μg L^–1 of PFCAs in water (above environmental background concentrations but below the concentration at which self-association occurs) at different water pH (pH 0.3 to pH 6.9) and sampled the PFCAs volatilized from water during a 2-day experiment. Our results suggest that the p<i>K</i>_as of C_4–11 PFCAs are <1.6. For PFOA, we derived a p<i>K</i>_a of 0.5 from fitting the experimental measurements with a volatilization model. Perfluoroalkane sulfonic acids were not volatilized, suggesting that their p<i>K</i>_as are below the investigated pH range (p<i>K</i>_a <0.3)