Hydroxide-Promoted Transformation of Fluorotelomer Carboxylic Acids at Ambient Temperature

Abstract

Fluorotelomer carboxylic acids (FTCAs) are key intermediates in the environmental transformation of fluorotelomer-based precursors, yet their abiotic degradation pathways remain poorly understood. This study investigates the hydroxide-promoted transformation of n:2 FTCAs at ambient temperature and under controlled laboratory conditions to elucidate reaction mechanisms and kinetics. Equilibrium experiments were performed across sodium hydroxide (NaOH) concentrations ranging from 1 × 10^-5 to 1 M, revealing transformation onset between 1 ×10^-4 and 2 × 10^-4 M. Both 6:2 and 8:2 FTCA were fully converted to the corresponding unsaturated products (FTUCAs) at ≥5 × 10^-4 M NaOH, followed by secondary loss of FTUCA at higher base concentrations (>2 × 10^-3 M) to undetected nontarget products. Minor yields (<7%) of perfluorocarboxylic acids (PFCAs) were detected at ≥0.1 M NaOH. Kinetic experiments at 1 × 10^-2 M NaOH showed first-order transformation of 6:2 and 8:2 FTCA, with observed rate constants (k_obs) of 0.09 and 0.48 h^-1, respectively. When repeated with 0.3% ammonia (NH₃;[OH-] ~ 1.74 × 10^-3 M) and 6:2 FTCA, the k_obs decreased sixfold (0.015 h^-1) relative to NaOH, consistent the approximately sixfold lower [OH^-]. Because 0.3% NH₃ is commonly used in PFAS extraction methods, these results suggest that prolonged extractions may lead to underestimation of FTCAs and potentially other precursors. Experiments with a base-containing consumer cleaning product confirmed that this hydroxide-promoted transformation of 6:2 FTCA to 6:2 FTUCA also occurs in more complex matrices. Mechanistic analysis supports a reversible E1cb pathway in which deprotonation of the α-hydrogen generates a stabilized carbanion intermediate, followed by unimolecular C-F bond cleavage. This study provides the first evidence of abiotic FTCA transformation via an E1cb mechanism and highlights the potential for mild alkaline environments, including those in analytical and household contexts, to promote PFAS precursor transformation.