Investigation of the simultaneous interactions of experimental variables and mechanism pathway in the photodegradation of methylene blue by binary ZnO/Cu2O photocatalyst

Abstract

The pollution of water suppliers by various organic compounds, especially synthetic dyes, has been known as a critical global risk. An efficient removal technology is a heterogeneous photocatalysis based on the illumination of semiconducting materials, especially as coupled systems and on the nanoscale. This work synthesized and coupled ZnO and Cu2O nanoparticles (NPs) to enhance the photocatalytic activity due to the decreased electron/hole (e/h) pairs recombination. The pHpzc values of 8.5, 8.0, and 8.4 were estimated for ZnO, Cu2O, and Cu2O-ZnO samples. In the response surface methodology (RSM) study of methylene blue (MB) photodegradation, a significant model was confirmed based on model F-value of 55.8 > F0.05, 14, 15=3.18 and a lack-of-fit (LOF) of 2.52 < F0.05, 10, 5 =4.77. All correlation coefficients (R2 = 0.9812, the adjusted R2 = 0.9636, and the predicted R2 = 0.9109) are close to each other and unity. The optimal conditions run included pH 3, an illumination time of 45 min, and a binary catalyst dose of 0.55 g/L. In the presence of Na2CO3, ascorbic acid, Na2SO4, and NaCl, the initial degradation efficiency of 50.9% (in the absence of any scavenger) was decreased to 31.9%, respectively 36.3%, 39.7%, and 44.4%, respectively, within an irradiation time of 30 min. This order confirms the hydroxyl radicals, superoxide radicals, free electrons, and finally, the holes have relative importance in MB photodegradation process. Based on the results, a direct Z-scheme mechanism was suggested for the MB photodegradation. The direct Z-scheme is a favor to produce more hydroxyl and superoxide radicals as the main centers of MB photodegradation by the proposed system. The recovery percentages of 48, 45, 43, and 39% were obtained for the first to fourth reusing runs, respectively. The catalyst holds above 81% of its initial activity after four reusing runs, which is satisfactory.