Characteristics of PFAS decomposition using passing-through type diaphragm discharge plasma

Among organofluorine compounds, perfluoroalkyl and polyfluoroalkyl substances (PFAS) particularly perfluorooctane sulfonic acid (PFOS) are known for their excellent physicochemical properties, such as thermal and chemical resistance, and are widely used in various industrial applications. However, in recent years, concerns have arisen due to the persistent, highly accumulative, and long-range transport characteristics of PFAS, prompting regulatory and risk management efforts in Japan. One large-scale approach to degrading these compounds is the 21-parallel plasma reactor, which can generate 21 plasma discharges simultaneously in water and has been successfully used to achieve complete detoxification of organofluorine compounds. However, systems based on in-liquid bubble discharge typically require an external gas supply. As a result, attention has shifted toward plasma treatment technologies that do not require external gas, enabling operation at smaller facilities and allowing for the recovery of fluoride ions released into the water.

In this study, we employ a passing-through diaphragm discharge method capable of generating plasma inside vapor bubbles formed by the evaporation of the treatment solution. This enhances the contact between the fluid and plasma, potentially improving decomposition efficiency over conventional diaphragm discharge methods. We investigated the decomposition characteristics of PFOS in such a system and compared its behavior to other PFAS, including PFOA and the short-chain compounds PFBS and PFBA.

The objective of this research is to evaluate the decomposition performance of the circulating passing-through diaphragm discharge system and compare it with that of standard systems. It has been confirmed that lower flow rates and higher applied voltages lead to improved efficiency and decomposition rates. Additionally, the enhanced plasma-contact ratio in the passing-through type configuration contributes to a higher overall degradation rate. Future work will focus on reducing system resistance by shortening the glass capillary to further optimize performance. This study was supported by the KIOXIA incentive Research.