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

摘要

The Henry's law constant (K_H) of perfluorooctanoic acid (PFOA, C_7F_(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_7F_(15)C(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 C_7F_(15)C(O)OH aggregates, even at low concentrations of C_7F_(15)C(O)OH in aqueous sulfuric acid solutions, and the adsorption of gaseous C_7F_(15)C(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 PPFOA and CPFOA 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_(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_7F_(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_7F_(15)C(O)OH obtained at 298 K suggests a substantial partitioning of C_7F_(15)C(O)OH in air in the environment.