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

• Published in: Environmental science & technology Vol. 47; no. 19; pp. 11032 - 11039
• Main Authors: Vierke, Lena, Berger, Urs, Cousins, Ian T
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.10.2013
• Subjects: Acidity; Acids; Air; Applied sciences; Atmospheric pollution; Biological and physicochemical phenomena; carboxylic acids; Carboxylic Acids - chemistry; Chemical compounds; dissociation; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Estimating techniques; Exact sciences and technology; Experiments; Fluorocarbons - chemistry; Natural water pollution; partition coefficients; perfluorooctanoic acid; Pollutants physicochemistry study: properties, effects, reactions, transport and distribution; Pollution; Pollution, environment geology; sulfonic acids; Volatilization; Water - chemistry; Water Pollutants, Chemical - chemistry; Water treatment and pollution; Emerging contaminant; Organic perhalocompound; Pollutant behavior; Dissociation constant; Environmental factor; Acidity; Air water interface; Persistent organic pollutant; Transport process; Carboxylic acid; Perfluoroalkyl acid; Volatilization; Organic fluorine compounds; pH; Air pollution; Water pollution; Organic compounds
• ISSN: 0013-936X; 1520-5851; 1520-5851
• DOI: 10.1021/es402691z

The acid dissociation constants (pK 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 (K AWs) and do not volatilize from water. The neutral acids, however, have relatively high K 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 pK a. Knowledge of the pK 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 pK as 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 as are below the investigated pH range (pK a <0.3).