Design and optimization of a photocatalytic system for Cr(VI) ion reduction using ZnO-NiO nanocomposite by response surface method and their antibacterial studies

Zinc oxide and nickel oxide nanocomposite was successfully synthesized using the co-precipitation technique and evaluated for their photocatalytic efficiency in the degradation of hexavalent chromium. The synthesized nanocomposite was characterized using various techniques such as X-ray diffraction, Fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy to analyze its structure, morphology, chemical bonding, photoabsorption properties, and elemental composition. X-ray diffraction exhibits the presence of zinc oxide, nickel oxide, and zinc oxide and nickel oxide nanocomposite, with average crystallite sizes of 37.92 nm, 56.23 nm, and 41.77 nm, respectively. Fourier transform infrared spectroscopy was used to analyze the chemical composition of zinc oxide and nickel oxide nanocomposite. The ultraviolet-visible spectroscopy exhibited an absorption peak at 447 nm, indicating a bandgap of 2.77 eV. A novel industrial-oriented photoreactor was designed for the reduction of hexavalent chromium under ultraviolet-visible illumination with varying operational parameters. The effectiveness of the experiment was assessed using response surface methodology based on central composite design, which demonstrated a remarkable 95% reduction of hexavalent chromium under optimized conditions. This study validates the accuracy of central composite design in predicting levels of hexavalent chromium reduction. The rate constant for the degradation kinetics of hexavalent chromium was studied to be 0.00450 min-1, indicating adherence to pseudo-second-order kinetics. Additionally, the prepared nanocomposite exhibited significant antibacterial activity against Bacillus subtilis, Escherichia coli, and Pseudomonas putida at different concentrations.