Aqueous PFCA Destruction by Hydroxyl Radical at Low pH: A Putative H-atom Abstraction Mechanism

Hydroxyl radical is a potent oxidant invoked in numerous advanced oxidation processes as a species responsible for destroying recalcitrant organic contaminants. However, there has been little direct evidence for the reaction between hydroxyl radical and perfluoroalkyl carboxylic acids in the aqueous phase. Nevertheless, we discovered that at low pH, perfluoroalkyl carboxylic acids are degraded by hydroxyl radical. When perfluorobutanoic acid was treated with H2O2 under steady-state 254 nm irradiation in 1 M HClO4, it was transformed to shorter-chain perfluoroalkyl carboxylic acid homologues and F–/HF. Empirical and computational mechanistic studies suggested an H-atom abstraction mechanism. Namely, more perfluorobutanoic acid was removed as the pH was decreased, which corresponds to a larger fraction of protonated perfluorobutanoic acid vs. conjugate base. Meanwhile, density functional theory calculations indicated that single electron transfer oxidation of protonated perfluoroalkyl carboxylic acids has a much larger free energy of activation relative to single electron transfer oxidation of the conjugate base anion, suggesting that single electron transfer is unfavorable at low pH. This is the first clear description of aqueous perfluoroalkyl carboxylic acid destruction by hydroxyl radical under simple reaction conditions.