Application of Box-Behnken Design to Optimize Visible-Light Degradation of 2,4-Dichlorophenoxyacetic Acid Using Hybrid CNQDs/CuBi₂O₄ Catalyst

The narrow band gap and rapid recombination rate of photogenerated carriers in CuBi2O4 limits the photocatalytic performance. Because it possesses unique properties such as up-conversion photoluminescence (PL) and in-plane quantum confinement effect, graphitic carbon nitride quantum dots (CNQDs) have outstanding application prospects in many fields. By combining solid-phase, coprecipitation and reconstruction methods, a new Z-scheme CNQDs/CuBi2O4 photocatalyst was successfully fabricated. The Box-Behnken design was optimized to study the complex interaction between reaction parameters. The CNQDs percentage in the catalyst, reaction time, 2,4-Dichlorophenoxyacetic acid (2,4-D) concentration, catalyst dose, and pH were varied from 0 to 2%, 90 to 120 min, 30 to 40 mg/L, 0.5 to 1.5 g/L, and 2.0 to 7.0, respectively, to maximize the 2,4-D removal efficiency after exposure to visible light. The following reaction parameters led to the highest photocatalytic removal efficiency: 1.0% of CNQDs, 110 min of reaction time, 33.0 mg/L of 2,4-D, 1.0 g/L of CNQDs/CuBi2O4, and a pH of 4.78. The predicted efficiency of 94.34% was very close to the experimental efficiency of 92.35%, which confirmed the accuracy of the design. The excellent activity of the CNQDs(1%)/CuBi2O4 catalyst in the photocatalytic degradation of 2,4-D is due to the enhanced charge separation efficiency and improved light absorption in the Z-scheme heterojunctions. Additionally, CNQDs(1%)/CuBi2O4 can be recycled with outstanding stability over four consecutive cycles. Superoxide radical (O-center dot(2)-) and photoinduced hole (h(+)) play major roles, while the hydroxyl radical (OH center dot) plays a minor role in the photodegradation process. This study demonstrates that the design of the heterojunction is effective in achieving superior photocatalytic performance. The newly-developed Z-scheme CNQDs(1%)/CuBi2O4 photocatalyst is a promising visible-light catalyst for the decomposition of organic pollutants.