Study Of Wet Oxidation For Treatment Of Pesticide Wastewater From Imidacloprid Production

Imidacloprid becomes a typical representative of the new generation pesticide with higher efficiency, lower toxicity and residue. Liquid effluents from imidacloprid production containing high acrylonitrile, toluene and DMF concentrations are difficult of degradation organic wastewater. It will pollute the environment seriously if discharged directly. However, the conventional processes, such as biological systems are inefficient. By taking the actual-producing wastewater of imidacloprid pesticide as the subject, study on base and application of organic contamination degradation were carried out by wet oxidation technology roundly in this paper in order to solve the problem for enterprise and to provide a effective way for the treatment of high concentrated wastewater.Oxidation experiments were conducted in a 2L autoclave which can withstand high temperature and high pressure. The various factors, such as reaction temperature, oxygen partial pressure, concentration and pH influent, intensity of stir on the wastewater treatment efficiency were investigated. The reaction temperature was a key factor of influence on the efficiency of the WAO. Oxidation reaction was limited notably when oxygen was insufficient, but as oxygen partial pressure exceeded to a certain value, treatment efficiencies were less significant. The influence of the pH in influent on the WAO was relatively small. As long as the amount of oxygen is enough, WAO still had a good processing effect within the range of wider concentration in influent.Wet peroxide oxidation (WPO) which totally used hydrogen peroxide as oxidant was developed. The results showed that the efficiency of WPO was influenced by temperature, pH and dosage of hydrogen peroxide. WPO was found to attain the same COD removal efficiency 60% as compared to WAO but with the reaction temperature lowered from 210℃to 130℃, and the total pressure from 8MPa to 0.6MPa.The homogeneous catalyst can improve the COD removal efficiency greatly. Among all the homogeneous catalysts tested, copper salt was found to be more effective in treatment of imidacloprid production wastewater. Cupric nitrates were more active than the copper sulfates. While using cupric nitrate as the catalyst, the COD removal efficiency was reached to 97.5% after reacting 30 min.The complex oxide catalysts of Cu/Mn, Cu/Ni, Mn/Ce were prepared by the co-precipitation method and applied to the treatment of pesticide wastewater of imidacloprid production by catalytic wet air oxidation. The preparation processes including the types of precipitator, aging temperature of precipitator, the drying temperature, the calcination temperature and time, active component proportion conditions which might effect on the activity of the bi-metal catalysts, were optimized. Among all preparation conditions, calcination temperature and active component composition were more important influences on catalytic activity than others, and results were as follows. NaOH was the precipitator, aging temperature was 80℃, drying temperature was 110℃for 12h, calcination time was 8h. For Cu/Mn catalyst, calcination temperature was 800℃and the Cu/Mn molar ratio was 1:2; For Cu/Ni catalyst, calcination temperature was 700℃and Cu/Ni was 2:1; For Mn/Ce catalyst, calcination temperature was 300℃and Mn/Ce was 7:3.XRD data revealed that the 'solid solutions' were formed in all the three series catalysts that improved greatly activity and stability of the catalysts. Mn/Ce composition-activity relationship was investigated mainly. XPS analysis indicated that Ce existed of Ce⁴⁺ on the surface of catalyst, and Mn had several oxide phases, which varies with the composition of Mn/Ce. Catalytic active sites have been ascribed to manganese oxide species exhibiting higher oxidation state. The analysis of catalytic mechanism showed that Mn/Ce complex oxide catalyst had higher catalytic activity. The effect of operation parameters of CWAO including reaction temperatures, oxygen partial pressure and pH value of wastewater on the activity and stability of Cu/Mn catalyst were studied, and the optimum condition of CWAO was confirmed, which was temperature of 190℃, oxygen partial pressure of 1.6MPa, total pressure 4.8MPa, pH of 6.21. Using Mn/Ce catalyst, 93.1% COD removal was obtained in CWAO of wastewater from imidacloprid production after 120min under the optimized reaction conditions. BOD₅/COD of the wastewater increased from 0.19 to more man 0.65, the concentration of leached Mn and Ce were 0.0299mg/L and 0.0316mg/L, respectively.A type of CWPO catalysts, Cu-Ni-Ce/SiO₂ prepared by the impregnation method was found to have high catalytic activity. The preparation process were optimized by CWPO, considering the activity and stability synthetically, the optimum conditions were total loading amount of 4%, 0.16% of Ce content and calcination temperature of 700℃. CWPO process was carried out with 5g Cu-Ni-Ce/SiO₂ catalyst to deal with the imidacloprid wastewater under 110℃, pH =7.0, total pressure of 0.6Mpa which were gentler greatly than that of CWAO process, 89.1 %COD removal was obtained after 60min.All catalysts of Cu-Ni-Ce/SiO₂ prepared under different calcination temperature and Ce content were characterized by XPS, XRD, SEM and nitrogen adsorption techniques. It was discussed that the mechanism of that adding certain amount of Ce improved the catalyst activity. SEM image revealed that addition of Ce promoted the dispersion of Ni and Ce particles on the surface of catalyst It was proved by BET that the surface area of Cu-Ni-Ce/SiO₂ was the largest when the Ce content was 0.16%. The catalyst structure was analyzed by XRD. CuO and complex oxide were observed on the catalyst surface. XPS was used to analyze the composition and the chemical state of the catalyst surface elements of Cu, Ni, Ce and O. Moreover the percentage of the three kinds oxygen, the lattice oxygen, hydroxy oxygen and chemisorbed oxygen, to total oxygen was obtained. The amounts of chemisorbed oxygen were increased on the surface of Cu-Ni-Ce/SiO₂ catalysts with decreasing calcination temperature and adding feasible Ce content, and the oxygen absorbing on the catalyst surface was promoted. Therefore, the oxygen reactive specie O₂^- was produced. This was favorable to improve the catalyst activity.The reaction dynamics for wet oxidation of imidacloprid wastewater in four react systems which were WAO, CWAO (added with Mn/Ce catalyst), WPO, CWPO (added with Cu-Ni-Ce/SiO₂ catalyst) were studied in the batch slurry reactor. The rate of chemical oxygen demand (COD) reduction was described by two steps first-order kinetics. The specific react rate coefficient and activation energies were determined for WAO, WPO, CWAO and CWPO, respectively. The kinetic model was found to agree well with experimental results. It is useful for the application of wet oxidation in industrial wastewater treatment...