Efficient decomposition of perfluoroether carboxylic acids in water with a combination of persulfate oxidant and ultrasonic irradiation

The decomposition of perfluoroether carboxylic acids CF3OC2F4OCF2COOH, CF3OC2F4OC2F4OCF2COOH, CF3OC3F6COOH, C2F5OC2F4OCF2COOH, and C4F9OC2F4OC2F4OCF2COOH and perfluoroalkylether sulfonates C2F5OC2F4SO3− and C3F7OC2F4SO3− in water with a combination of persulfate oxidant and ultrasonic irradiation was investigated. In the absence of persulfate, the perfluoroether acids decomposed quite slowly. For example, the pseudo-first-order rate constants for the decomposition of perfluoroether carboxylic acids ranged from 2.03 × 10−2 to 3.34 × 10−2 h−1. After 24 h, 44.2–58.0% of the initial concentrations (ca. 50 μM) of the substrates remained, and the F− yields were 27.7–59.7%. In contrast, the addition of persulfate dramatically accelerated the reactions. When 10 mM persulfate was used, the pseudo-first-order rate constants for the decomposition of perfluoroether carboxylic acids were 2.5–3.9 times those in the absence of persulfate. However, the addition of persulfate did not enhance the decomposition of perfluoroalkylether sulfonates. Enhancement of the perfluoroether carboxylic acid decomposition can be explained by acceleration of substrate decarboxylation, induced by sulfate radical anions formed from the persulfate during ultrasonic irradiation.