Degradation process analysis of the azo dyes by catalytic wet air oxidation with catalyst CuO/γ-Al2O3

Abstract

Three azo dyes (Methyl Orange, Direct Brown and Direct Green) were treated by catalytic wet air oxidation (CWAO) with the catalysts CuO/γ-Al2O3 prepared by consecutive impregnation. The relationship of decolorization extent, chemical oxygen demand (COD) removal extent and total organic carbon (TOC) in dye solution were investigated. The results indicated that the CuO/γ-Al2O3 catalyst had excellent catalytic activity in treating azo dyes. Almost 99% of color and 70% of TOC were removed in 2h. The high removal extent of color and TOC indicated that the CWAO obtained perfect decomposition for pollutants. The degradation pathway of azo dyes was analyzed by UV–Vis, FTIR and MS. According to the examined results, the hydroxyl (OH) radicals induced strong oxidizing effects in the target solution and destroyed the chromophoric groups of azo-benzene conjugated of the molecular structure. Considering characteristics of the dye structure, the azo bond (NN) would first be attacked by the hydroxyl radical and other free radicals. With the continuous oxidization and the long reaction time at high temperature, these intermediates could be oxidized to the final oxidation products, such as water and carbon dioxide.