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

Abstract

The Henry's law constant ( K H) of perfluorooctanoic acid (PFOA, C 7F 15C(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 7F 15C(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 7F 15C(O)OH aggregates, even at low concentrations of C 7F 15C(O)OH in aqueous sulfuric acid solutions, and the adsorption of gaseous C 7F 15C(O)OH on the walls of the experimental apparatus. We derived overall gas-to-water partition coefficients ( K H′) 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 K H′ 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 7F 15C(O)OH at 298 K were determined at 9.9 ± 1.5 mol dm −3 atm −1 for p K a = 2.8 and 5.0 ± 0.2 mol dm −3 atm −1 for p K a = 1.3. The p K a value of 1.3 seems to be most probable among the reported three values for C 7F 15C(O)OH, taking into account dependence of K H′ on sulfuric acid concentrations for aqueous sodium chloride and sulfuric acid mixtures. Despite the low p K a value, the relatively small K H of C 7F 15C(O)OH obtained at 298 K suggests a substantial partitioning of C 7F 15C(O)OH in air in the environment.