Chain-length dependent ultrasonic degradation of perfluoroalkyl substances

Per- and polyfluoroalkyl substances (PFAS) have been found all over the world and are particularly persistent, potentially carcinogenic, and bioaccumulative in the environment. Degradation of short-chain perfluorinated carboxylic acids of varying carbon chain lengths (from 4 to 8 carbons), higher-chain perfluoro carboxylic acids of varying carbon chain lengths (from 9 to 14 carbons), and perfluorosulfonic acids of varying carbon chain lengths (6 and 8 carbons) were tested in a flow through ultrasonic cavitation reactor to determine the efficacy of the high frequency ultrasound process. Temperature, frequency, power density, pH, sodium chloride, and sodium bicarbonate concentrations are examined as process parameters. The frequency and length of the PFAS chain were found to be vital components in the sonolytic degradation process. Degradation of all PFAS was shown to be particularly rapid at room temperature, basic pH, and a power density of 252 W/L. At a power density of 144 W/L, all of the PFAS were degraded by more than 97% in 8 h, with the exception of perfluorobutonic acid (83%) and perfluorohexanoic acid (94%). The bond dissociation energy of C-F bonds was found to be much higher than experimental sonolytic activation energies, supporting cavitation bubble as a catalyst for thermolytic destruction of PFAS compounds. Optimizing the reactor geometry has the potential to make this approach even more appealing for treating small volumes of concentrated wastes.