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

• The acid dissociation constants (pK(a)s) 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 (K(AW)s) and do not volatilize from water. The neutral acids, however, have relatively high K(AW)s and volatilization from water has been demonstrated. The extent of volatilization of PFCAs in the environment will depend on the water pH and their pK(a). Knowledge of the pK(a)s 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 similar to 1 mu g 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 pK(a)s of C4-11 PFCAs are <1.6. For PFOA, we derived a pK(a) of 0.5 from fitting the experimental measurements with a volatilization model. Perfluoroalkane sulfonic acids were not volatilized, suggesting that their pK(a)s are below the investigated pH range (pK(a) <0.3).

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NATURVETENSKAP  -- Geovetenskap och miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences (hsv//eng)

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