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 (KH) of perfluorooctanoic acid (PFOA, C7F15C(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 C7F15C(O)OH in the purge gas (PPFOA) were determined by means of Fourier-transform infrared spectroscopy. Time-courses of PPFOA and concentrations of PFOA in the test solutions (CPFOA) differed from those typically obtained by an inert-gas stripping, indicating both the presence of C7F15C(O)OH aggregates, even at low concentrations of C7F15C(O)OH in aqueous sulfuric acid solutions, and the adsorption of gaseous C7F15C(O)OH on the walls of the experimental apparatus. We derived overall gas-to-water partition coefficients (KH′) by simulating the time-courses of PPFOA and CPFOA 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 KH′ and the ionic strength of aqueous sulfuric acid solutions, the KH values of C7F15C(O)OH at 298 K were determined at 9.9 ± 1.5 mol dm−3 atm−1 for pKa = 2.8 and 5.0 ± 0.2 mol dm−3 atm−1 for pKa = 1.3. The pKa value of 1.3 seems to be most probable among the reported three values for C7F15C(O)OH, taking into account dependence of KH′ on sulfuric acid concentrations for aqueous sodium chloride and sulfuric acid mixtures. Despite the low pKa value, the relatively small KH of C7F15C(O)OH obtained at 298 K suggests a substantial partitioning of C7F15C(O)OH in air in the environment.