Photochemical degradation pathways and near-complete defluorination of chlorinated polyfluoroalkyl substances

Abstract

Chlorinated polyfluoroalkyl substances (Clx–PFAS) are an emerging class of pollutants worldwide. Here we demonstrate near-complete defluorination (that is, cleaving most C–F bonds) of diverse Clx–PFAS structures and the reaction mechanisms. First, we used ultraviolet/sulfite to degrade various carboxylic acids (n = 1, 2, 4 and 8 Cl–CnF2nCOO− and n = 1, 2 and 3 Cl–(CF2CFCl)nCF2COO−) and an ether sulfonic acid surfactant (F-53B, Cl–(CF2)6–O–(CF2)2SO3−). The initial reaction with a hydrated electron cleaved the C–Cl bond. The resulting fluorocarbon radicals (•CF2– on the terminal and –•CF– in the middle) yield C–H or C–SO3−, or form a dimer. In particular, we identified a novel reaction pathway with the unexpected HO• radical to cleave the C–C bond (for –•CF–) and yield –COO−. This pathway is critical for rapid and deep defluorination at 90–96% from carboxylic acids and 76% from F-53B. The following treatment with heat/persulfate at pH >12 achieved ~100% overall defluorination from carboxylic acids and 93% from F-53B without producing toxic ClO3− from Cl−. This study advances the understanding of PFAS transformation in engineering systems. It also suggests a synergy between molecular design and degradation technology towards sustainable management of fluorochemicals. The achievement of almost complete defluorination of chlorinated polyfluoroalkyl substances reveals insight into the degradation pathways of these persistent contaminants.