Degradation and Defluorination of Ultra Short‑, Short‑, and Long-Chain PFASs in High Total Dissolved Solids Solutions by Hydrothermal Alkaline TreatmentClosing the Fluorine Mass Balance

Emerging destructive technologies for per- and polyfluoroalkyl substances (PFASs) are receiving increased attention. To validate these emerging technologies for commercial use, rigorous testing efforts are needed to ensure that all types of PFASs can be degraded with minimal organofluorine byproduct formation. In this effort, a mixture of ultra short-, short-, and long-chain PFASs in an aqueous matrix with high total dissolved solids (TDS) content is processed using hydrothermal alkaline treatment (HALT). Degradation and defluorination are assessed at various HALT operating conditions (T = 250–350 °C, NaOH = 2–16 wt %). Broadly, perfluoroalkyl sulfonic acids are observed to be more recalcitrant, while PFASs containing a carboxylic acid functional group are readily degraded and defluorinated, even under milder treatment conditions. In all experiments, the fluorine mass balance is near-stoichiometric at optimized conditions, and a “multiple-lines-of-evidence” analytical approach including targeted LC–MS/MS analysis, free fluoride quantification, and total organic fluorine measurements strongly suggests that there is minimal undesired organofluorine byproduct formation. Additionally, the fate of inorganic anions in the aqueous matrix is tracked, evidencing a lack of competing reactions that could afford unwanted byproducts. Overall, this study robustly demonstrates that HALT can facilitate complete degradation and defluorination of ultra short-, short-, and long-chain PFASs in a challenging solution (≈10 wt % TDS).