Fluorine Substitution and CO2 Production in the Treatment of Perfluorooctanoic Acid in a Radial Plasma Discharge Reactor

The paper reports and discusses the results of a detailed investigation of transient products and mineralization extent achieved in treatments of perfluorooctanoic acid (PFOA) in a radial plasma discharge reactor. The efforts were warranted by the excellent performance of this reactor in terms of process efficiency and by the need to verify that the quality of the treated water was of matching value. Minor amounts of transient products were detected and quantified, as a function of plasma treatment time, by means of LC/MS and LC/MS/MS analyses. These products arise from sequential chain-shortening, an established route for plasma induced PFOA degradation, and defluorination via fluorine substitution by -H and -OH groups. We focussed on the latter less known type of products (“substitution products”), which are formed in small amounts, cumulatively accounting, at any treatment time, for less than 2% of the total carbon content initially present as PFOA. In our system, hydroxy-containing substitution products with 8–6 carbon atoms are remarkably less reactive than their perfluoro- and hydro-substituted homologues, an effect attributed to improved solubility into the aqueous phase and removal from the plasma/liquid reactive interface. Mineralization extent and carbon mass balance were also determined by performing experiments with PFOA at high initial concentration (1∙10− 4 M) to afford quantification of the CO2 released into the gas phase by means of GC-TCD analysis. Despite the low rate of PFOA decomposition entailed by these abnormally high concentrations, remarkable carbon mass balance of 75% and mineralization extent of 67% were achieved in 90 min.