Degradation of methyl orange by UV, O3 and UV/O3 systems: analysis of the degradation effects and mineralization mechanism

The effect of ultraviolet irradiation (UV), ozonation, and the combined UV/O3 systems on the decolorization and degradation of methyl orange, performed in a laboratory-scale reactor, was studied. Decoloration efficiency, UV–vis spectrum, chemical oxygen demand (COD), mass spectrum (MS), and total organic carbon (TOC) analyses were employed. Three oxidative processes including UV, ozonation (O3), and the combined UV/O3 system were assessed to select the most appropriate oxidative process in terms of methyl orange aqueous solution treatment and special emphasis was laid on the effect of reaction pH in the UV/O3 system. The results indicated that the pH value of methyl orange solution decreased with the treatment time, and it reached an acid value when oxidized for 150 min. The COD removal efficiencies of methyl orange were only 46.23 (UV), 44.54 (O3), and 71.17 % (UV/O3) in three processes, while the corresponding decolorization efficiencies were 94.8, 94.2, and 95.1 % after 150 min of treatment. The mineralization mechanism was suggested based on the analysis of the molecular structure of methyl orange, intermediate products, and final products by using MS. For the combined UV/O3 system, the basicity condition was good at the TOC removal.