An excellent photochemical reduction property of Cr(vi) upon visible light illumination in a hydroxyl-functionalized zirconium-based metal–organic framework

Abstract

To design a MOF material with an appropriate band gap value for visible light absorption and a rapid photoreduction rate, a Zr-based MOF modified by hydroxyl groups was successfully fabricated, and its Cr(VI) photoreduction performance was investigated. Subsequently, the experimental data revealed an extraordinary Cr(VI) removal efficiency for HCMUE-2, achieving up to 100% reduction after 90 min under irradiation by visible light at pH = 1. In particular, the complete characterization of the material was confirmed through modern analytical procedures, including powder X-ray diffraction (PXRD), Fourier transform infrared (FT-IR) and Raman spectroscopy, thermogravimetric analysis combined with differential scanning calorimetry (TGA-DSC), scanning electron microscopy incorporating energy-dispersive X-ray (SEM-EDX), UV-vis diffuse reflectance spectroscopy (UV-vis/DRS), and X-ray photoelectron spectroscopy (XPS). Notably, HCMUE-2 retains its Cr(VI) photoreduction percentage for five consecutive cycles without any significant decrease. Additional combined experiments proved that the structural robustness and morphology of the material were maintained after the photocatalytic reduction process. A plausible photoreduction mechanism was elucidated mainly through scavenger capture investigations, showing that regulated electrons are the primary reactive species responsible for converting Cr(VI) into Cr(III) under the identified conditions. These findings illustrate that a Zr-based MOF modified by OH moieties is a promising candidate for the photoreduction of highly toxic Cr(VI) contaminants from wastewater in real-life situations.