Mass balance analysis of perfluorocompound decomposition by DC plasma generated in gas bubbles

Summary form only given. Perfluorooctane sulfonic acid (PFOS) can be decomposed efficiently by plasmas generated in gas bubbles. However, mass balance during the degradation of PFOS is not fully understood. Mass balance is quite important for commercial applications. Thus, the purpose of this study is to clarify the value of the mass balances of fluorine, carbon, and sulfur, and to understand the degradation process of PFOS. PFOS in water was decomposed by dc plasma generated in argon gas bubbles. The reactor was made of acrylic. A ceramics plate, which was 0.3 mm in thickness and had a micro hole of 0.3 mm diameter at the center, was attached on the bottom of the reactor. Argon gas was supplied through the hole to PFOS solution at a flow rate of 100 sccm. A stainless steel mesh (SUS304, 30mesh) was used as a high voltage electrode and attached on the back side of the ceramics plate. A grounded electrode was put into the reactor and immersed in the solution. High voltage was applied to the high voltage electrode through a ballast resister (250 kΩ) and plasma was generated in argon gas bubbles. A resister (1 kΩ) was used to measure the electric current, which was regulated at 10 mA. The concentration of carbon dioxide (CO2) in the gas released from the plasma reactor was measured using gas chromatography. Other components in the gas were analyzed using Fourier transform infrared spectroscopy (FT-IR). The solution was analyzed using liquid chromatography-mass spectroscopy (LC-MS) and ion chromatography. During the degradation of PFOS, the concentrations of PFOS, perfluorocarboxylic acids (PFCAs: C_nF_2n+1COOH) with shorter carbon chains (n = 1-7), fluoride ion, and sulfate ion in the solution, and CO₂ in the gas were quantified. The mass balances of fluorine, carbon and sulfur were calculated using these concentrations, and were 59.1%, 44.6%, and 83.2% respectively after 365 min treatment. Tetrafluoro methane (CF_{4), trifluoro methane (CHF3), hexafluoro ethane(C2F6), and carbon monoxide (CO) were detected in the gas using FT-IR but the concentrations of them were not quantified yet. By the quantification of the concentrations of the gas components, it will be possible for the mass balances to amount to 100%.}