Rapid Destruction and Defluorination of Perfluorooctanesulfonate by Alkaline Hydrothermal Reaction

Here, we report a promising new strategy for achieving rapid and complete destruction of perfluorooctanesulfonate (PFOS) through the application of hydrothermal conditions (condensed water, 200–350 °C, 2–16.5 MPa) to solutions amended with NaOH. Initial screening experiments with a wide range of solution amendments (e.g., acids, alkalis, oxidants, reductants) revealed highly variable extents of PFOS defluorination, ranging from 0% to 80% after 90 min of reaction at 350 °C. The most reactive amendments, regardless of type, shifted solution pH to highly alkaline conditions (pH ≥ 9), suggesting a base-promoted mechanism. For NaOH-amended solutions, rates of PFOS degradation increased with temperature and followed a second-order rate law, −d[PFOS]/dt = k2[OH–][PFOS], with k2 = 0.052 ± 0.004 M–1 min–1 at 350 °C, and 19F-NMR measurements show complete conversion of C–F bonds to F– (2.5 g/L PFOS) within 40 min for the reaction with 1 M NaOH. Small quantities of short-chain perfluorocarboxylic acids (≤1.5% [PFOS]init) were detected as transient intermediates, indicating that an initial OH–-catalyzed cleavage of the sulfonate headgroup is followed by rapid sequential decarboxylation reactions, eventually leading to complete mineralization. These findings suggest a promising technology for destruction of PFOS-containing wet concentrates (e.g., aqueous film-forming foam (AFFF) stockpiles, industrial waste, sorbent regenerate, and membrane reject waste streams).