Engineered nanostructured ZnO for water remediation: operational parameters effect, Box-Behnken design optimization and kinetic determinations

The photocatalytic oxidation of methylene blue (MB) in aqueous media is explored using nanoscale ZnO nanoparticles (ZnO NPs) for maximal dye removal within the high-surface-area nanoparticle photocatalyst. The operating parameters such as illumination time, initial MB load, ZnO NP dose, solution flow rate and pH were examined. The experimental results revealed the alkaline pH (12.0) corresponding to the higher oxidation rate within only 20min of reaction time; however, increasing the initial MB load decreased the reaction rate at the optimum circulation flow rate of 460mL/min and ZnO NP dose of 0.4g. Additionally, for providing maximum performance, the interaction between the most effective independent parameters (ZnO NP dose, flow rate, and initial pH) were explored using Box-Behnken experimental design based on the response surface methodology. The results showed a good fitness of the model (R-2=96.86%) with the experimental data. The optimum values were recorded after 20min of reaction time with the values: 0.45g/L for ZnO NP dose, 370mL/min for flow rate and pH 11, showing a 94% maximum dye removal compared to 92% using manual optimization. Finally, the kinetic models were applied and the data were described by second-order kinetic model.