Sustained hydrated electron production for enhanced reductive defluorination of PFAS in groundwater

Hydrated electrons (e_aq^‒; ‒2.9 V) are effective at defluorinating per- and polyfluoroalkyl substances (PFAS), but production of e_aq^‒ often requires excess source chemicals, anoxic environment, and harsh pH conditions. To improve the feasibility of the reductive process, we harnessed phenol as a source chemical yielding four e_aq^‒ stoichiometrically and utilized dithionite (DTN) to catalyze phenol cycle for sustained e_aq^‒ yields. The added DTN not only scavenges dissolved oxygen, the e_aq^‒ trap, but also reductively transforms phenol degradation product, p-benzoquinone, to hydroquinone which yields more e_aq^‒ upon UV irradiation. In the UV/phenol/DTN system, up to 70 % defluorination of PFOA solution was achieved while the impact of groundwater matrix was minor on the degradation performance of PFOA, PFOS and GenX. Especially in acidic conditions, ^•H, the conjugate acid of e_aq^‒, is the dominant radical for decomposing the three tested PFAS. Density functional theory calculations reveal hydrogen bonding and van der Waals interactions between PFAS and phenol, facilitating both decarboxylation and fluorine elimination in PFAS structures. The combined experimental and theoretical evidence demonstrated the capability of the new UV/phenol/DTN method to sustain e_aq^‒ production for effective defluorination of PFAS in the groundwater matrix.