Sustained hydrated electron production for enhanced reductive defluorination of PFAS in groundwater
Xingaoyuan Xiong, Zirui Luo, Shuang Luo, Lu Bai, Yanan Shang, Allyson L. Junker, Zongsu Wei
Abstract
• UV/phenol/dithionite effectively degrade both PFAS and phenol in one process. • Dithionite reverses phenol degradation cycle to sustain e aq ‒ production. • Hydrogen bond between PFAS and phenol facilitates the degradation of PFAS. • Decarboxylation as first step is favored over F-abstration in H • -driven degradation. 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.