CATALYTIC WET OXIDATION OF WASTEWATER USING DEPARTMENT OF INDIAN INSTITUTE OF TECHNOLOGY ROORKEE NANOCATALYST

Abstract

Catalytic wet air oxidation (CWAO) using heterogeneous catalyst is a promising advanced oxidation process (AOP) for the treatment of complex industrial wastewaters. The drastic operating conditions during CWAO are disadvantageous as maintaining the process at extreme conditions is not favourable. Catalysis research to develop the economical, active and stable catalyst is a key factor in the development of CWAO at mild operating conditions. CeO2 containing materials attracted a lot of interest due to their wide range of applications. Oxygen storage capacity (OSC) of ceria is the most important property which makes it an excellent catalytic material. Despite of its widespread applications, pure CeO2 has poor thermal stability and it sinters at high temperature, leading to its deactivation. In recent years, a lot of efforts are devoted in designing the CeO2-based mixed oxide systems, with enhanced thermal stability. The mixed oxides of ceria with transition metals have attracted great attention in various heterogeneous catalytic applications. The redox properties of ceria based mixed oxides are dependent on particle size, lattice defects and chemical nonstoichiometry. The high specific surface area and porosity can be attained by tuning the particle size in nanometer scale. The aim of this work was to study the activity of Ce-Fe, Ce-Cu, Ce-Co, Ce-Zn and Ce-Ni mixed oxide nanoparticles in CWAO of industrial wastewater. A facile co-precipitation method was adopted for the preparation of mixed oxides. The physicochemical properties of prepared samples were studied by various characterization techniques, i.e, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (RS), N2-adsorption/desorption, Field emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM) and Energy-dispersive X-ray spectroscopy (EDX). XRD indicated the successful preparation of nanosized mixed oxides with high lattice defects. FT-IR confirmed the interaction between metal oxides in mixed phases. XPS and Raman studies revealed the oxygen storage capacity of mixed oxides due to high oxygen vacancies and Ce3+ content. FE-SEM and TEM micrographs indicated the decrease in particle size with increasing transition metal oxide content. High surface area and porosity of catalysts was assured by N2-adsorption/desorption analysis. Thus characterization results indicated the suitability of these nanocatalysts for the oxidation application. ii The prepared mixed oxides were used in CWAO of paper industry wastewater at atmospheric pressure. The efficiency of catalysts was quantified in terms of Chemical oxygen demand (COD), Total organic carbon (TOC), Biological oxygen demand (BOD), Color, Adsorbable organic halides (AOX) and Chlorophenolics (CHPs) removal. The optimized process variables i.e., initial pH of wastewater, catalyst dose, treatment time and treatment temperatures were found to be pH 4, 1 gL-1, 2 h and 90°C, respectively. These process variables were optimized for Ce-Fe mixed oxides, and same conditions were utilized for the removal study over other mixed oxides. The catalyst was recovered from the treated wastewater and supernatant was analyzed for the leaching of metal ions. Fe-Ce mixed oxides were found to be most efficient with 74% COD, 82% color, 72% TOC, 68% AOX and 71% CHPs reduction. Co-Ce and Cu-Ce mixed oxides presented comparable removal efficiency in terms of COD, Color, TOC and AOX removal. In term of CHPs, higher removal was observed over Cu-Ce mixed oxide. Zn-Ce and Ni-Ce mixed oxides, exhibited comparable removal of COD, color and AOX. In terms of TOC and CHPs removal, higher efficiency was attained for Zn-Ce mixed oxides. The removal efficiency of mixed oxides was found to follow the order of Fe-Ce > Co-Ce ≈ Cu-Ce > Zn-Ce > Ni-Ce. The leaching study indicated that the mixed oxides are stable catalysts. The efficiency of mixed oxides was in good agreement with their structural and textural properties. High removal efficiency of Fe-Ce mixed oxide was related to its high specific surface area, uniform pores and high oxygen storage capacity (Ce3+ content). The efficiency of Co-Ce and Cu-Ce mixed oxides was also in accordance with their characterization results. The low efficiency of Zn-Ce mixed oxides may be attributed to the presence of single oxidation state of Zn (i.e., 2+). While others exhibit the multiple oxidation states, like Fe (3+, 2+), Co (3+, 2+), Cu (2+, 1+) which can increase the oxidation property of catalyst.