Catalytic Degradation of Hexafluoropropylene Oxide Trimeric Acid during the Hydrothermal Regeneration of Spent Activated Carbon

Regeneration of granular activated carbon (GAC) after adsorption of perfluoroalkyl and polyfluoroalkyl substances is an emerging challenge. This study reports a successful regeneration of spent GAC with hexafluoropropylene oxide trimer acid (HFPO–TA) at 120 °C. For the first time, we observed that GAC significantly catalyzed the degradation and defluorination of adsorbed HFPO–TA. The new catalytic degradation mechanism of HFPO–TA on GAC was proposed. Reaction intermediates' analysis confirmed that the ether group of HFPO–TA near the carboxyl group was first split into hexafluoropropylene oxide dimer acid (HFPO–DA) and trifluoroacetic acid (TFA). Subsequently, HFPO–DA was further broken down to produce PFPrA and TFA. These intermediates were detected only when HFPO–TA was adsorbed on GAC and were not found when HFPO–TA was dissolved in water without GAC. Bond dissociation energy calculations revealed that the ether group close to the carboxylic acid had a smaller dissociation energy and was more susceptible to ether group breakage compared to the decarboxylation reaction of HFPO–TA. The gas-phase products contained significant amounts of 1H-substituted HFPO–TA and 1H-substituted HFPO–DA, indicating the decarboxylation of HFPO–TA. Furthermore, density functional theory calculations indicated that the ether group in the HFPO–TA breakage site carried a more positive charge on GAC, facilitating the breakage through nucleophilic reaction. Finally, the regeneration efficiency of GAC remained at 81% at the fifth adsorption–degradation cycle. This study offers a novel and cost-effective strategy for the degradative regeneration of HFPO–TA-adsorbed GAC.