Rapid Destruction and Defluorination of Perfluorooctanesulfonate by Alkaline Hydrothermal Reaction

Here, we report a promising new strategy for achieving rapid and complete destruction of perfluorooctane-sulfonate (PFOS) through the application of hydrothermal conditions (condensed water, 200-350 degrees 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 degrees 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 = k(2)[OH-][PFOS], with k(2) = 0.052 +/- 0.004 M-1 min(-1) at 350 degrees C, and F-19-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).