Methylene blue (MB) serves as a widely employed dye within both industrial sectors and laboratory settings. Its utility extends to the biological staining of tissues, cells, and various samples in biology and microbiology laboratories, as well as industrial applications including textile, paper, plastic, and chemical coloring processes. Consequently, its presence is commonly detected in liquid effluents from these industries, posing ecological hazards due to its inherent toxicity and potential for bioaccumulation. Methylene blue, characterized by its multi-core aromatic composition, can induce acute toxicity and engender serious health issues such as respiratory distress, chest pain, burning sensations, nausea, vomiting, and diarrhea.

To address this issue, various methods are employed to remove methylene blue from wastewater, such as adsorption, biodegradation, ion exchange, and advanced oxidation processes. In this study, the degradation of methylene blue using a photocatalytic process with copper (II) oxide (CuO) was explored. The performance of CuO as a photocatalyst was optimized using response surface methodology (RSM) combined with a Box–Behnken Design (BBD) under visible light. Variables such as MB concentration (10-40mg/L), photocatalyst CuO dosage (0.1-1g/L), and irradiation time (10-90 min) were evaluated for their impact on the MB degradation rate (%). An optimal photodegradation efficiency of 99.85% was achieved with an MB concentration of 10 mg/L, a CuO dosage of 1 g/L, and an irradiation time of 90 min.