Electrochemical reduction degradation and defluorination of perfluorooctane sulfonate mediated by active hydrogen via proton-coupled electron transfer.

The simultaneous efficient degradation and defluorination of perfluoroalkyl substances in aqueous environments remains challenging due to their exceptional chemical stability. This work addresses this challenge via an underexplored electrochemical reduction strategy, employing a system comprising cobalt-biochar (CoBC) as the cathode and riboflavin (RF) as the electrolyte. Under optimized operating conditions, the CoBC/RF system achieved complete perfluorooctane sulfonate (PFOS) conversion within 20 min (kobs = 0.194 min-1), along with 75.8% total organic carbon removal and 59.8% defluorination efficiency. Mechanistic investigations identified the active hydrogen (H*) as the key reaction species. RF mediates a proton-coupled electron transfer (PCET) process that lowers the activation energy for H* generation and enables its accumulation on the electrode surface, forming a reactive 'H* cloud' that markedly enhances both degradation and defluorination performance. The PFOS decomposition pathway is proposed based on detected intermediates under sustained H* action, in which degradation initiates with desulfonation, proceeds via iterative loss of CF2 units, and ultimately yields inorganic end products. The system further demonstrates robust stability across multiple operation cycles and diverse water matrices. More broadly, this PCET-mediated electrochemical reduction system is extendable to other refractory organic pollutants beyond PFOS, outperforming previously reported results and establishing a generalizable framework that advances the remediation of persistent pollutants.