Direct measurement of fluorocarbon radicals in the thermal destruction of perfluorohexanoic acid using photoionization mass spectrometry

Thermal destruction is a critical cornerstone of addressing the rampant contamination of natural resources with per- and polyfluoroalkyl substances (PFAS). However, grave concerns associated with stack emissions from incineration exist because mechanistic studies have thus far relied on ex-situ analyses of end-products and theoretical calculations. In this manuscript, we used synchrotron-based vacuum ultraviolet photoionization mass spectrometry to study the pyrolysis of a representative PFAS – perfluorohexanoic acid, and for the first time provide direct evidence of fluorocarbon radicals and intermediates. A key reaction pathway from perfluorocarboxylic acids to ketenes via acyl fluorides is proposed. We furthermore propose CF2/CF3-radical-centered pyrolysis mechanisms and explain their roles in the formation of other products which may form in full-scale incinerators. These results have not only unveiled the role of radicals and intermediates in thermal PFAS decomposition and recombination mechanisms, but also provide unique insight into improving the safety and viability of industrial PFAS incineration.

Methods
The data for this experiment were collected using the synchrotron vacuum ultraviolet photoionization mass spectrometer at the National Synchrotron Radiation Laboratory, Hefei, China. The entire analytical detection process consists of four main steps: vaporization of the reactant, thermal decomposition, photoionization, and reflectron time-of-flight mass spectrometry (Re-TOF-MS) analysis. During operation, 0.3% PFHxA vapor was seeded in argon carrier gas, passing through an alumina laminar flow tube with a temperature range from 400 to 975 °C. The residence time is calculated to be around 10 milliseconds as molecules traverse through the flow tube, where the high temperature caused thermal decomposition of PFHxA molecules. Successively, the molecules expanded into vacuum to generate a supersonic molecular beam. The beam containing all thermal decomposition intermediates and products then entered the interaction region and was intersected by VUV photons from the synchrotron.