Activation of oxalic acid via dual-pathway over single-atom Fe catalysts: Mechanism and membrane application

Oxalic acid (OA), together with its ligand, appears in diverse biotic and abiotic processes in nature. However, the applications of the photochemical property of OA and its complexes in environmental purification are scarce. Herein, we reported a Fe single-atom catalyst with Fe-N4 configuration supported on g-C3N4 (Fe-CN) to activate OA for ultrahigh-efficiency removal of Cr(VI) under visible light irradiation through dual pathways: the photo-dissociation of [FeⅢ(C2O4)]+ and the reaction between OA and photogenerated holes, achieving the generation of •CO2-. The introduction of Fe-N4 favors the formation of configuration [FeⅢ(C2O4)]+ and fabricates the electron transfer channel from catalyst to OA, further decreases the energy barriers of •CO2- generation, and thereafter efficiently facilitates the C-C cleavage for the generate of •CO2-. Meanwhile, the strong coordination interaction over Fe single atoms and N atoms could enhance the separation efficiency of photo-carriers and further promote the reduction activity. Under optimal conditions, Fe-CN showed a 5.1-fold increasement in photocatalytic kinetics contrast to pristine CN. The Fe-CN catalyst attached on fibrous membrane for treatment of flowing Cr(VI) wastewater exhibits 90% reduction efficiency over 10 h, which is expected to facilitate the practical application of the catalyst. The mechanism of OA activation with dual-pathway was unveiled by in-situ Fourier transform infrared spectra and theoretical calculations. This work provides an important guidance for the application of OA and its complexes to achieve the attenuation of toxic and harmful heavy metal in water restoration.