Preparation Of Mo-Zn-Al-O Catalyst And Wet Air Oxidation Of Dye Wasterwater

Catalytic wet air oxidation process requires high temperature and high pressure which lead to high installation costs. In order to solve this problem, the catalyst was prepared and modified, which has ability to treat cationic red GTL dye wastewater under room temperature and atmospheric pressure.Firstly, the carrier of catalyst was selected. Al2O3,pseudoboehmite,LDHs or molecular sieve was as the carrier and Fe, Mn or Mo was as active compound to prepare the catalysts using impregnation method. The results show that Mo-Zn-Al-O catalyst prepared with Zn-Al as the carrier and Mo as active compound had a better catalytic activity. The XRD and XPS characterization results show that a part of Mo was present as MoO2and a part as ZnMoO4. Al was not in the crystal and was present as AlOx.The effect of Zn and Al molar ratio on the catalytic activity was studied to investigate the effect of M2+/M3+ratio in LDHs on the structure of the catalyst. A series of Zn-Al LDHs with different Zn/Al molar ratios were prepared using copreicipitation and Mo-Zn-Al-O catalyst was obtained. The structure of the catalysts was characterized using ICP-OES, Zeta potential, XRD, BET, FT-IR, H2-TPR, and O2-TPD. The catalytic activity of these catalysts on the degradation of cationic red GTL wastewater under room condition was investigated. The experimental results show that the highest color removal efficiency and TOC removal efficiency of cationic red GTL can reach90.9%and65.8%by the Mo-Zn-AI-O catalyst with1:1of Zn/Al molar ratio respectively. Associated with the characterization results of these catalysts, Mo-Zn-Al-O catalyst with1:1of Zn/Al molar ratio with highest catalytic activity could be contributed to its lowest zeta potential (-17.5eV), largest specific surface area (146.98m2/g), special crystalline phases (ZnMoO4, ZnO and MoO2), special Mo valences (reducibility from Mo6+to Mo5+and from Mo5+to Mo4+), largest number of active adsorption sites (31.5a.u./g).The factors in CWAO process on degradation of cationic red GTL were investigated. Application of Plackett-Burman Design and its following statistical analysis indicated that pH value of system, initial cationic red GTL concentration, the amount of catalysts were the three key factors significantly influencing catalytic activity. Considering decolorization efficiency as the response objective, a quadratic model was respectively obtained by Central Composite Design, Box-Behnken Design and D-optimal Design. Base on the analysis of variance, the coefficient of determination of three models were0.97,0.99and0.94repectively. This means that BBD model is closer to reality and97%of decolorization of cationic red GTL in CWAO over Mo-Zn-Al-O mixed oxide achieved. The optimal catalytic conditions were:pH value was4.5, the initial concentration of cationic red was284mg/L and the amount of catalysts was0.97g/L.The kinetic of the CWAO process on the degradation of284mg/L cationic red GTL dye wastewater was investigated. The results showed that the activation energy is-312.36J/mol, Reaction rate constant is9×10-5.The degradation mechanism of cationic red GTL was studied by density fuctional theory and electron spin resonance (ESR), FT-IR, GC-MS, acid measuring through GC, inhibit of radicals experiments and ESR technique. The results show that dye pollutants are adsorbed on the surface of catalysts by a hydrogen bonding interaction between hydroxyl groups and O atoms. The Mo-Zn-Al-O catalyst can efficiently react with adsorbed oxygen/H2O to produce·OH and'O2. Azo bond of cationic red GTL adsorbed on the surface of catalyst is relatively easier to be attacked by free radicals in CWAO process. Once the-N=N-bond is broken, the color of dye is removed. After azo bond were attacked, the free radicals continued to react with the intermediates. With the continuous oxygen and the reaction time, these intermediates can be oxidized to acid, aldehyde, alcohol and hydrocarbons.