New Indole Derivative Heterogeneous System for the Synergistic Reduction and Oxidation of Various Per-/Polyfluoroalkyl Substances: Insights into the Degradation/Defluorination Mechanism

The hydrated electron (e_aq^-) system is typically suitable for degrading perfluoroalkyl substances (PFASs). To enhance e_aq^- utilization, we synthesized a new indole compound (DIHA) that forms stable nanospheres (100-200 nm) in water via a supramolecular assembly. Herein, the DIHA nanoemulsion system exhibits high degradation efficiencies toward a broad category of PFASs, regardless of the headgroup, chain length, and branching structure, under UV (254 nm) irradiation. The strong adsorption of PFAS on the DIHA surface ensures its effective degradation/defluorination. Quenching experiments further demonstrated that the reaction took place on the surface of DIHA nanospheres. This specific heterogeneous surface reaction unveiled novel PFAS degradation and defluorination mechanisms that differ from previously reported e_aq^- systems. First, the photogenerated surface electrons nonselectively attacked multiple C-F bonds of the -CF₂- chain. This plays a dominant degrading/defluorinating role in the DIHA system. Second, abundant hydroxyl radicals (^•OH) were also produced, leading to synergistic reduction (by surface electron) and oxidation (by surface ^•OH) in a single system. This facilitates faster and deeper defluorination of different structured PFASs through multiple pathways. The new mechanism inspires the design of innovative organo-heterogeneous e_aq^- systems possessing synergistic reduction and oxidation functions, thereby making them potentially effective for treating PFAS-contaminated water.