Improved degradation of tetracycline by Cu-doped MIL-101(Fe) in a coupled photocatalytic and persulfate oxidation system: Efficiency, mechanism, and degradation pathway

A bimetallic organic framework (MIL-101(Fe/Cu)) was fabricated by Cu-doped modification and applied as a catalyst for persulfate (PS) oxidation coupled with visible-light photocatalysis (Vis) to degrade tetracycline (TC). 90.5 % of TC (50 mg/L) was removed by MIL-101(Fe/Cu)/PS/Vis system at Fe to Cu molar ratio of 3:1, pH 5, PS concentration of 2 mM, and catalyst dosage of 0.05 g/L. The degradation of TC was significantly higher in the MIL-101(Fe/Cu)/PS/Vis system than that in the MIL-101(Fe)/PS/Vis system (similar to 52.0 %). The reasons for the improvement of degrading TC in a coupled photocatalysis and persulfate oxidation system by doping Cu in MOF were (1) the higher absorption energy (Eabs) to PS: based on the results of the DFT calculation, Cu-doped modified MIL-101(Fe) could remarkably improve the TC degradation by enhancing the Eabs ( 5.92 eV) to PS; (2) the decrease of the recombination rate of electrons-holes pair: doped Cu could capture the photogenerated electrons to inhibit recombination of electrons-holes pair; (3) the improved electrical conductivity: doped Cu improved electron conductivity and promoted catalytic activity by improving the density of states (DOS) of the catalyst (1.23 electron/eV). Thus, we concluded that Cu-doped MOF is an effective method for improving oxidation ability in a coupled photocatalysis and persulfate oxidation system.