Adsorption and decomposition of perfluorooctane sulfonic acid on plasma-water interface

Summary form only given. Perfluorooctane sulfonic acid (PFOS) is environmentally harmful and persistent substance. The decomposition of PFOS in water using plasma attached to the water surface has been successfully conducted and the energy efficiency is relatively high compared to other methods such as photochemical process. This reason will be discussed by comparing the mole fraction of PFOS on plasma-water interface to that in bulk water. During the decomposing process using plasma, the surface concentration of PFOS on the plasma-water interface increases due to adsorption, whereas decreases due to decomposition. To estimate the surface concentration and mole fraction, measurements of surface tensions and decomposition experiment using oxygen plasma on the PFOS solution were carried out. Surface concentration of PFOS without plasma was calculated from the static and dynamic surface tensions. Time variation of surface concentration at the PFOS bulk concentration of 50 mg/L gave the adsorption rate of PFOS as a function of the surface concentration. PFOS solution of 50 mg/L, 50 mL was treated by a DC plasma in a closed vessel with atmospheric oxygen gas. A needle electrode was placed 2 mm above the solution and a grounded electrode was inserted to the solution. Applying positive voltage initiated oxygen plasma between the needle and the solution surface. After 10-minute treatment with the discharge current of 7 mA, 0.27 mg PFOS was decomposed. It coincides with the decomposition rate of 7.8x10^-8 mol/(cm²s). Assuming that the reaction rate of plasma with PFOS is proportional to the PFOS surface concentration, and the surface concentration comes to equilibrium during the decomposing process, we obtained the surface concentration of 9.4x10^-13 mol/cm² and the mole fraction of 6.0x10^-4. This value is much larger than the mole fraction in bulk water of 1.8x10^-6. Therefore, the plasma could effectively reac- with PFOS on the water surface and showed higher energy efficiency than other methods using bulk reactions.