Mechanisms for PFAS Degradation at the Plasma-Liquid Interface

Long-chain per- and polyfluoroalkyl substances (PFAS) are chemicals with surfactant-like properties that persistently reside at the water surface. Atmospheric pressure plasmas (APPs) have been shown to chemically activate the liquid and defluorinate the long-chain surfactants [1]. APPs propagate as surface ionization waves (SIWs) on the liquid. This close proximity delivers energetic electrons, ions, metastable species and VUV photons which directly impact the surface, and generates solvated electrons which initiate carbon-carbon and carbon-fluorine bond breaking. PFAS molecules, which have a high acid-dissociation constant K_a, and their conjugate bases are degraded through the decarboxylation-hydroxylation-elimination-hydrolysis (DHEH) mechanism as well as the direct impact of these photons, electrons, and ions. To optimize these interfacial processes, the contributions of the reactants should be controlled. In this work, the degradation of surface resident PFAS molecules by APPs was investigated using 2-D (nonPDPSIM) and 0-D (GlobalKin) models. In the 2-D model, dissociation of the surrogate C₃F₈ was studied while varying the contact area of SIWs on the water surface. The degradation of PFOA was investigated with the 0-D model over hundreds of pulses and the defluorination rates were evaluated based on voltage, repetition rate, and flow rate.

[1] M. Vasilev et al, Chem. Engr. J. 473, 144833 (2023).