Destruction and Defluorination of Ultrashort-Chain and Short-Chain Perfluoroalkyl Substances by Alkaline Hydrolysis

Abstract

Per- and polyfluoroalkyl substances persist in the environment due to the strength of the carbon-fluorine bond; in particular, ultrashort- and short-chain species are hardest to treat because of their high mobility, as they cannot be captured by most conventional methods. This dissertation examines the end-of-life treatment of three such compounds, C-1 trifluoromethanesulfonic acid (TFMS), C-3 perfluoropropanoic acid (PFPrA), and C-4 perfluorobutanoic acid (PFBA), under a high-temperature, alkaline environment in a continuous-flow reactor. The dissertation describes mechanistic insights and an evaluation of differences in their destruction and defluorination.For TFMS, two expanded operating regimes were developed that utilize phase transition of the subcritical water rather than increasing the hydroxide concentration, to enhance destruction and defluorination. (i) Superheated ionically modulated alkaline hydrolysis (SIM-AH) raises the critical point of pure water (374 °C and 22.1 MPa) by electrolyte amendment, e.g., to 406.6 °C and 29.7 MPa with 3 wt% NaCl. (ii) Distillation alkaline hydrolysis (DAH) uses high-temperature, low-pressure operation to force phase separation, concentrating alkali in the liquid/brine phase. SIM-AH reached above 99.9% destruction of TFMS at 2 M NaOH and DAH at 0.5 M NaOH whereas conventional alkaline hydrolysis requires 5 M NaOH to achieve similar destruction efficiencies. Mechanistic insights were provided by experimental findings and are supported by density functional theory, which identifies heterolytic carbon-sulfur bond cleavage as the rate-determining step, with a competing thermal pathway that becomes significant only when hydroxide is limited, evidenced by fluoroform detected in the absence of base. For the two perfluorocarboxylic acids (PFCAs), ultrashort-chain PFPrA and short-chain PFBA, degradation and defluorination are shown to occur sequentially. Parent compound destruction is independent of alkali concentration, exceeding 99.9% by 225 °C for PFBA and PFPrA at every alkali concentration including zero, so the initial scission step is thermally driven. That scission yields volatile fluorinated fragments whose mineralization cannot proceed without hydroxide. The mineralization rate of the volatile fragments relative to the residence time governs defluorination efficiency. Carbon and fluorine balances, gas-phase product identification, and computed elimination barriers inform the pathways and the fate of the decarboxylation product. The three-carbon fragment is re-activated by hydroxide and returns to the degradation cascade, while mineralization of the two-carbon fragment (pentafluoroethane) requires higher activation energy; the computed base-assisted barrier difference between the two fragments is 7.5 kcal/mol. The experimental and computational analysis suggests that defluorination of these PFCAs is limited not by the degradation of the parent compound but by the mineralization of the reaction byproducts. In both classes, the hydroxide-dependent step sets the limit. For TFMS that step is cleavage of the carbon-sulfur bond; for the PFCAs it is reactivation of the fluorinated fragment, set by the fragment’s elimination barrier.