(Invited) Electrical Discharge Plasma Technology for the Treatment of Fab Wastewater

Abstract

This work investigates plasma-assisted degradation of triflate and perfluorobutane sulfonate (PFBS), two short-chained per- and polyfluoroalkyl substances (PFAS) regularly found in the effluents of semiconductor fabrication processes. Recent studies have shown that triflate and PFBS are persistent in the environment and associated with negative health impacts. Here, the degradation of short-chain PFAS was investigated using a point-ring plasma reactor electrode configuration in which plasma is formed in argon gas and is contacting the surface of contaminated water. Experiments were first carried out in lab-prepared solutions and subsequently in a fab effluent membrane concentrate containing dozens of long- and short-chain PFAS. Time-dependent concentration profiles for PFAS were followed for a range of operational parameters and degradation byproducts identified and when possible quantified.Results indicate that the removal rates of short-chain PFAS are significantly lower than those of their long-chain counterparts due to their inability to accumulate and concentrate at gas-liquid interfaces. The addition of cetyltrimethylammonium bromide (CTAB), a cationic surfactant, significantly improved degradation rates of short-chained PFAS in lab-prepared solutions by transporting them to the plasma-liquid interface, the region of chemical reactivity. Experiments conducted with different surfactant types revealed that only cationic surfactants such as CTAB affect PFBS degradation, mainly through electrostatic attraction. In fab wastewater, the CTAB-PFAS binding may be hindered by the presence of different PFAS and CTAB may be competing with long-chain PFAS for the sites at the plasma-liquid interface. Nevertheless, through careful process design one can achieve near-complete destruction of the majority of PFAS allowing the electrical discharge plasma process to maintain its position as the leading PFAS destruction technology.