Photocatalytically Generated Nitrate Radicals Drive Decomposition of Perfluorooctanoic Acid

Nitrate radicals (NO₃•) generated in irradiated aqueous TiO₂ suspensions at acidic pH drive decomposition of perfluorooctanoic acid (PFOA). The key role played by NO₃• is demonstrated by the dependence of the reaction rate on the initial concentration of nitrate ions and by scavenging tests. The amount of nitrate ions remains constant during the process, indicating its catalytic behavior. The existence of nitrate radicals under the given conditions is here for the first time directly demonstrated by time-resolved transient absorption spectroscopy. The process achieves ∼90% degradation of PFOA in 9 h with ca. 57% fluoride liberation. Theoretical results suggest that nitrate radicals trigger water-mediated hydrogen atom transfer from the protonated PFOA, followed by decarboxylation and hydroxylation to the corresponding alcohol (experimentally detected), which eventually evolves to shorter-chain intermediates. The proposed process represents the first proof of concept of the reactivity of NO₃• with PFAS, but more importantly may be of practical relevance when coupled with the acidic regeneration of adsorbents currently used for remediation of water effluents containing PFAS.