TREATMENT OF NITROGENOUS AROMATIC COMPOUNDS CONTAMINATED WATER BY CATALYTIC WET PEROXIDE OXIDATION

Abstract

Nitrogenous aromatic compounds such as Congo red, quinoline, aniline, etc. bearing wastewaters have attracted the attention of the scientific communities because of their potential ecotoxicological risks. Due to low nitrogenous pollutants removal efficiency of conventional wastewater treatment plants, worldwide concerns are raised for efficient and eco-friendly technologies. Advanced oxidation processes (AOPs), such as catalytic ozonation, photocatalysis, Fenton process and catalytic wet peroxide oxidation (CWPO)/heterogeneous Fenton-like processes have been investigated as promising technologies for the wastewater treatment. Among these AOPs, CWPO could achieve complete degradation of the pollutants into CO2, N2 and inorganic ions with the help of heterogeneous catalysts using hydrogen peroxide as oxidant under mild operating condition. The optimized catalyst design is still required to bring this technique to the forefront of the most efficient AOP technologies. In last years, copper containing heterogeneous catalysts have been reported suitable for industrial applications due to their wide working pH range as well as good redox properties. Particularly promising are copper incorporated zeolite Y and copper hydroxyl phosphate because of their high degradation efficiency. Although conventionally used, aqueous ion exchange (AIE) method for the preparation of Cu/zeolite Y, and hydrothermal method for the preparation of copper hydroxyl phosphate and copper phosphate are associated with various limitations. A further discussion is required in context of more advanced and green routes for their synthesis as well as evaluation of their catalytic performances in different type of reactors to increase their suitability for the large scale processes. Keeping in mind all the above discussed points, in the present research, wet-impregnation (IMP) and precipitation-impregnation (PI) for cu/zeolite Y and ultrafast sonochemical method for copper hydroxyl phosphate (CHP) and copper phosphate (CP) have been developed. Moreover, the catalytic activities of these catalysts have been tested in batch and continuous fixed-bed reactors. The IMP and PI methods not only overcome the limitations of AIE but also generate hierarchical mesoporosity in zeolite Y and increase surface area in considerable extent. In present work, firstly, copper was incorporated on zeolite Y framework using IMP method with different targeted loading (1wt%, 2.5wt%, 5wt%, 7.5wt% and 10wt%) and tested for the degradation of Congo red in batch reactor. The maximum surface area of 667 m2 g−1 and generation of mesoporosity was observed for 7.5wt% CuY (actual loading ~ 5wt%). The copper was present in well-dispersed monovalent and divalent states in this sample. The maximum degradation, decolorization and mineralization of 93.58%, 95.34% and 79.52% were exhibited by iii Cu 7.5 wt% after optimum times of 2.5, 2 and 4 h, respectively. The optimized condition was observed at pH = 7, H2O2 concentration = 52.24 mM, catalyst concentration = 1 g L−1 and T = 60 °C. The kinetic studies revealed that the degradation and decolorization profiles for Congo red were well fitted to first-order kinetic model. Secondly, copper was incorporated on zeolite Y by three different methods (AIE, IMP and PI) and as-synthesized samples were tested for the CWPO of quinoline in continuous fixed-bed reactor. CuYAIE promoted isolated species similar to CuYIMP; however, large CuO crystallites of different sizes (4.7-6.1 nm) were present on the external surface of precipitation-impregnation (CuYPI) catalysts depending upon the Cu loadings. The Langmuir surface area and pore volume (Vp) increase surprisingly from 567 to 909 m2 g−1 and 0.26 to 0.51 cm3 g−1, respectively, for 5CuYPI. The mesoporosity generation in CuYPI was result of higher desilication from zeolite framework due to synergetic effect of copper and NaOH. Almost comparable mineralization (61-65%) and H2O2 stoichiometric efficiencies (44.2-45.7%) were observed for CuYAIE and CuYIMP samples in continuous fixed-bed reactor. Higher catalytic activities of both catalysts in comparison to CuYPI suggest that isolated sites are the most redoxactive sites for H2O2 activation and play important role than high surface area, i.e., for CuYPI. The optimised removal was observed at LHSV = 4 h−1, particle size = 1.2-1.7 mm, H2O2/quinoline = 48 and T = 80 °C. The system followed Eley-Rideal mechanism and kinetic parameters were calculated using model based on this mechanism. Finally, copper hydroxyphosphate (CHP) and copper phosphate (CP) were synthesized using ultrafast sonochemical route and tested for the CWPO of aniline in aqueous solution. CHP was obtained at pH 5 in 2 h, and the formation of CP was noticed at pH = 3 and pH = 7. In CHP samples, small nanorods of size ~50 nm were selfassembled to larger nanospindles-like hierarchical architecture of size 500-700 nm. At low sonication energy, one dimensional nanorods was obtained. In CP samples, thin nanoflakes of size 100-200 nm were obtained. The *OH radicals generated through ultrasonic irradiation of water are thought to be responsible for formation of CHP phase. The optimized aniline degradation, TOC removal and H2O2 conversion of 99.86%, 74.73% and 68.84% were achieved for nanospindle-like architecture at pH = 7, H2O2 amount = 2.4 mL, catalyst concentration = 1 g L−1 and T = 80 °C. The recyclability up to five continuous cycles has shown a remarkable operational stability of CHP catalyst.