Experimental determination of Henry's law constant of perfluorooctanoic acid (PFOA) at 298 K by means of an inert-gas stripping method with a helical plate

The Henry's law constant (K(H)) of perfluorooctanoic acid (PFOA, C(7)F(15)C(O)OH) was determined at 298 K in aqueous sulfuric acid solutions and in aqueous sodium chloride and sulfuric acid mixtures by an inert-gas stripping method in which a helical plate was used to increase the residence time of the gas bubbles in the solutions. The partial pressures of C(7)F(15)C(O)OH in the purge gas (P(PFOA)) were determined by means of Fourier-transform infrared spectroscopy. Time-courses of P(PFOA) and concentrations of PFOA in the test solutions (C(PFOA)) differed from those typically obtained by an inert-gas stripping, indicating both the presence of C(7)F(15)C(O)OH aggregates, even at low concentrations of C(7)F(15)C(O)OH in aqueous sulfuric acid solutions, and the adsorption of gaseous C(7)F(15)C(O)OH on the walls of the experimental apparatus. We derived overall gas-to-water partition coefficients (KH) by simulating the time-courses of P(PFOA) and C(PFOA) simultaneously to optimize parameters of the model relating to the partitioning, the aggregation, and the adsorption. The KH value for 0.31 mol dm(-3) sulfuric acid solutions at 298 K was determined at 3.8 +/- 0.1 mol dm(-3) atm(-1). From the relationship between K(H') and the ionic strength of aqueous sulfuric acid solutions, the K(H) values of C(7)F(15)C(O)OH at 298 K were determined at 9.9 +/- 1.5 mol dm(-3) atm(-1) for pK(a) = 2.8 and 5.0 +/- 0.2 mol dm(-3) atm(-1) for pK(a) = 1.3. The pK(a) value of 1.3 seems to be most probable among the reported three values for C(7)F(15)C(O)OH, taking into account dependence of K(H) on sulfuric acid concentrations for aqueous sodium chloride and sulfuric acid mixtures. Despite the low pK(a) value, the relatively small K(H) of C(7)F(15)C(O)OH obtained at 298 K suggests a substantial partitioning of C(7)F(15)C(O)OH in air in the environment. (C) 2008 Elsevier Ltd. All rights reserved.