Experimental investigation of the roles of non-oxidative species in plasma-based water treatment*

Poly -and perfluoroalkyl substances (PFAS) are a group of toxic non-oxidizable compounds that are used in the manufacturing of many consumer goods. While plasma treatment has demonstrated efficacy in degrading PFAS into shorter chain compounds, the conditions required for complete mineralization and the plasma species capable of degrading these non-oxidizable compounds remain unknown.

This work deconvolutes the individual contributions of photons and H radicals in perfluorooctanoic acid (PFOA) degradation and defluorination by employing the COST jet operating in humid helium at atmospheric pressure. Plasma was generated using a 13.56 MHz radio-frequency (RF) power supply, and the jet was employed to treat 2.5 mL aqueous samples.

Preliminary results revealed that the jet is capable of degrading concentrations of PFOA up to 50 ppm, albeit at very low conversion rates. Following a 90-minute treatment, the maximum of about 20 % of PFOA was degraded in experiments where applied voltage, gas flowrate and parent compound concentration were varied. The low PFOA degradation rates were explained by low hydrogen radical and photon fluxes generated by the COST jet compared to other plasma sources. The main byproduct of degradation was perfluoroheptanoic acid (PFHpA), indicating stepwise degradation of PFOA. The jet performance was compared to that of an RF-driven dielectric barrier discharge atmospheric pressure plasma source.