Volcano-shaped PFAS defluorination efficiency as a function of process intensification during electrocatalytic reductive treatment

Pervasive and recalcitrant per- and polyfluoroalkyl substances (PFAS) contamination has motivated development of a plethora of treatment approaches aimed at their degradation, with increased interest in process intensification to enhance defluorination. However, limited attention has been given to the unintended consequences of the empirical intensification of tunable electrochemical treatment systems. Here, we consider the reductive defluorination of 4-(trifluoromethyl)hexafluoropent-2-enoic acid (PFMeUPA), a lesser studied PFAS with growing health concerns, using elemental palladium (Pd(0))-coated carbon fiber paper cathodes. Pd(0) catalyzed the formation of surface-adsorbed atomic hydrogen (H•), which was confirmed as the reactive species responsible for defluorination via scavenger experiments with 2,4 dichlorophenol. Fluoride release (serving as direct evidence of defluorination) followed a volcano-shaped relationship with applied current, revealing an optimal operating point at 2.5 mA where defluorination was maximized. At higher currents, substantial H 2 bubble formation indicated wasteful H• recombination and dominance of the competing hydrogen evolution reaction (HER). These results represent a caveat that increasing energy input for process intensification may eventually hinder PFAS degradation. Thus, process design and operation should not overlook HER to optimize H• utilization for energy-efficient PFAS defluorination. • Electrocatalytically generated H• on Pd(0) reductively defluorinated PFMeUPA • Applied current must be optimized to maximize H• utilization for PFAS reduction • Too much current favors the competing HER, leading to wasted treatment capacity • Process intensification thus has a volcano-shaped effect on PFAS degradation