Towards optimizing short-chained PFCA esterification in water

Short-chained perfluorocarboxylic acids (PFCAs) are persistent, mobile aqueous contaminants which are challenging to treat. Improved treatment technologies are necessary to reduce rising short-chain PFCA emissions from high-concentration industrial point sources. We recently demonstrated that perfluorooctanoic acid (PFOA), a long-chain PFCA, undergoes chemical transformation to an insoluble ester through a reaction with decanol within spontaneously formed emulsions, (self)driving phase separation and removal from the aqueous phase. Building on this preliminary finding, here we expand the esterification-based removal paradigm to short-chained PFCAs (n_C = 4 & 6), develop a quantitative ¹⁹F NMR method for in-situ measurement of PFAS partitioning and transformation, and systematically evaluate the effect of a variety of water constituents (e.g. NaCl) and reaction additives (e.g. alcohol chain length) towards system optimization. We demonstrate that beyond chemical transformation, the removal system is also driven by emulsion formation and stabilization, as PFCAs are removed by partitioning into emulsions which are stable enough to withstand a separation step such as centrifugation. We further show that PFCA partitioning and esterification are hindered by higher ionic strengths (NaCl and Na₂SO₄ at I = 0.05 - 0.15) but only modestly inhibited by humic acid, even at concentrations up to 3000 ppm. Total PFAS removal is greater when PFCAs are present as mixtures compared to single-solutes (11-15% improvement). While the primary bottleneck for short-chain PFCA removal is reduced partitioning from the aqueous phase into emulsions, by tuning reactant concentrations and temperature, we achieve total removal yields ≥ 70% for both PFHxA and PFBA.