Box–Behnken Design to Optimize Herbicide Decomposition Using an Eco-Friendly Photocatalyst Based on Carbon Dots from Coffee Waste Combined with ZnBi2O4 and Its Antibacterial Application

A Box–Behnken design (BBD) for a response surface methodology with five factors and three levels was applied to design 2,4-D degradation experiments under visible light. To optimize the experimental conditions, the five factors included the amount of Cdots in a Cdots (x%)-ZnBi₂O₄ catalyst (x = 0–2%), the decomposition time (90–120 min), the initial 2,4-D concentration (30–40 mg/L), the catalyst dosage (0.5–1.5 mg/L), and the pH (2–7), and these were selected as independent variables. The BBD method proposed a second-order polynomial equation that fitted the experimental data perfectly. The results of the analysis of variance (ANOVA) confirmed the appropriateness of the proposed model, resulting in the relationship between the predicted and adjusted values having an R² value of 0.9980. The optimal conditions for the photodecomposition of 2,4-D were found to be an initial 2,4-D concentration of 30 mg/L, a degradation time of 120 min, a Cdots(2%)-ZnBi₂O₄ dosage of 1.0 mg/L, and a pH of 4.0. Under these conditions, the highest 2,4-D photodecomposition of 91.1% was obtained, which was in reasonable agreement with the predicted value of 91.67%. After 6 consecutive reaction cycles, the photodecomposition efficiency still exceeded 81%. The results confirmed that the Cdots(2%)-ZnBi₂O₄ photocatalyst has excellent reusability. Moreover, the lowest concentration of Cdots(2%)-ZnBi₂O₄ that inhibited the growth of E. coli (ATCC 8793) and S. aureus (ATCC 6538) was 150 µg/mL, with an inhibition zone of 18–19 nm for E coli and about 15 mm for S. aureus.