*3.6. E*ff*ect of Initial Dye Concentration*

The photocatalytic degradation of dye depended on its initial concentration, and it was necessary to investigate the effect of dye concentrations in view of their practical application. The solar photocatalytic degradation with different initial MB dye concentrations in the presence of CuO/ZnO was studied, as shown in Figure 6. It was observed that as initial MB dye concentrations increased from 5 mg/L to 20 mg/L, the dye degradation efficiency gradually decreased [36,37]. In this study, methylene blue of 10 mg/L solution was selected to evaluate the photodegradation of dye under sunlight, because of the high concentration of real wastewater.

**Figure 6.** Effect of initial dye concentration on the photodegradation of MB with CuO/ZnO.
