Synchronous Photocatalytic Redox Conversion of Chromium(VI) and Arsenic(III) by Bimetallic Fe/Ti Metal–Organic Frameworks

In this work, bimetallic organic frameworks NH2-MOFs(Fe, Ti) with different Fe3+/Ti4+ molar ratios were prepared by a hydrothermal method for the synchronous redox transformation of Cr(VI) and As(III). These results showed that NH2-MIL-125(Ti) was less effective in the photocatalytic removal of Cr(VI), whereas NH2-MIL-88B(Fe) was less effective in the photocatalytic oxidative removal of As(III). Due to the introduction of Fe3+, the photocatalytic reduction removal of Cr(VI) (23.04 → 42.56%) and the photocatalytic oxidation removal of As(III) (5.58 → 26.09%) by NH2-MOFs(Fe, Ti) were significantly enhanced. Among them, NH2-MIL-88B(Fe0.6Ti0.4) exhibited the best performance in the photoreduction of Cr(VI) and photo-oxidation of As(III), which balanced the insufficiency of monometallic MOFs(Fe/Ti). In this case, the total removal of Cr(VI) and As(III) by NH2-MIL-88B(Fe0.6Ti0.4) was found to be 94.19 and 83.54%, respectively. The excellent photocatalytic property could account for the ligand-to-metal charge transfer (LMCT), where photogenerated e- generated by -NH2 group excitation was transferred to the active site Fe-O clusters, as well as e- transported along Ti → O → Fe (metal-to-metal charge transfer, MMCT). Therefore, the synergetic effect of LMCT and MMCT could efficiently inhibit the recombination of e--h+ pairs, improving the photocatalytic performance of NH2-MOFs(Fe, Ti).