Nb-MOG as a High-Performance Photocatalyst for Cr(VI) Remediation: Optimization and Reuse Cycles

This study describes the removal of Cr(VI) using Nb-MOG (Niobium Metal–Organic Gel) as a photocatalyst. The characterization was performed using various techniques: Scanning Electron Microscopy–Energy Dispersive X-ray Spectroscopy (SEM–EDS), Point charge zero charge (PZC) determination, Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD) and thermogravimetric analysis (TG). The characterization results indicated an amorphous structure with predominance of Nb on the catalytic surface. Photoreduction tests were performed under different experimental conditions, following a two-factor central composite design with 11 experiments—including triplicates of the central point—to evaluate the influence of catalyst concentration (0.146 to 0.854 g L−1) and pH (1.46 to 8.54) on the Cr(VI) removal efficiency after 60 min of photocatalytic treatment. Experimentally, complete Cr(VI) removal was achieved at pH 5.00 using 0.854 g·L−1 of Nb-MOG, and the response surface analysis indicated optimal performance at higher catalyst concentrations and pH values around 5.00. In contrast, lower efficiencies were observed at extreme pH values, particularly at higher pH and lower catalyst concentrations. These results suggest that the photocatalytic performance of Nb-MOG for Cr(VI) removal is very susceptible to operating conditions, underscoring the importance of optimizing pH and catalyst concentration for effective treatment.