Kinetics of methylene blue degradation in VUV/UVC irradiation process and the role of hydroxyl radical

Abstract A series of investigations have been carried out to study the decomposition of methylene blue (MB) by the VUV/UVC based AOPs. However, MB degradation by VUV/UVC direct photolysis has not been reported previously. For the first time, the degradation kinetics and mechanisms of methylene blue by a chemical-free technology VUV/UVC (185 nm/254 nm) irradiation were systematically investigated in bench scale. Firstly, we investigated the impact of reaction conditions (i.e., VUV power, temperature, initial pH, and initial MB concentration on the VUV/UVC irradiation process. We found that MB degradation by VUV/UVC irradiation is an endothermic reaction, and the apparent activation energy was calculated to be 14.72 kJ·mol-1, which is similar UVC/TiO2 and electrical discharge plasma system, while higher VUV power and lower initial MB concentration have enhancement effects. The degradation rate constant was a convex function of the solution pH and reached the optimal value at pH 9.19. Compared with VUV/UVC irradiation and UVC/H2O2 processes, hydroxyl radical (·OH) plays the dominated role in VUV/UVC irradiation system, while both ·OH and HO2· contribute to conventional UVC/H2O2 process. The degradation pathways were proposed including demethylation and chromophores oxidation, and the possible pathway was confirmed by combining the experimental observations with the density functional theory (DFT) calculation. In summary, this study provides the results that VUV/UVC is an effective and chemical-free technology for MB removal from water.