Authors: Papalexopouloua, K., Huang, X., Ronen, A., and Aggelopoulos, C.A.

Abstract: In this study, we aim to understand the underlying mechanisms and find the optimum perfluorooctanoic acid (PFOA) degradation approach with cold plasma by investigating several critical aspects such as pulsed-plasma waveform, reactor configuration, water matrix, plasma gas and initial PFOA concentration. Nanosecond and/or microsecond high-voltage (HV) pulses were used to energize gas-liquid dielectric barrier discharge (DBD) or plasma bubbles to degrade PFOA in distilled or tap water. In parallel, the physicochemical properties and species concentration of plasma treated water were determined under all the aforementioned conditions and correlated with PFOA degradation. Under air atmosphere, much higher concentrations of long-lived species were measured in water treated with gas-liquid DBD compared to that treated with air-plasma bubbles, resulting in complete PFOA degradation which was also enhanced by the surfactant nature of PFOA. Air and argon were superior plasma gases compared to oxygen, indicating the more significant role of reactive nitrogen species and hydrated electrons compared to oxygen species in the degradation of PFOA. Air was almost equally effective at degrading PFOA in distilled and tap water, while argon was much more effective at degrading PFOA in tap water possibly due to interactions between hydrated electrons and salts resulting in water physicochemical properties (e.g. alkaline conditions) favoring its degradation. In terms of energy requirements, HV nanopulses were superior to HV micropulses under both air- and argon-plasma, with the lowest electrical energy per order achieved under nanopulsed-argon (~23.6 kWh/m3). The results of this study could contribute important knowledge to the development of plasma-based treatment of PFOA-contaminated water.

Keywords: Cold plasma, Dielectric barrier discharge, Plasma bubbles, PFOA, Water treatment, Nanosecond pulses