Development of a novel process for the removal of selected organic compounds from waste streams

The aim of this research work was to develop a solid regenerable catalytic adsorbent for the removal of organics from industrial wastewater. This was to be achieved by a two-step process. The first step involved the removal of the aqueous contaminant of concern by adsorption onto a selective adsorbent/catalyst. The second step involved the oxidation of this adsorbed pollutant into carbon dioxide, water and nitrogen whilst minimising the formation of nitrogen oxides. 2-nitrophenol was selected as the model pollutant for this study because it is a simple nitrogen containing organic compound. The adsorbent chosen in this study was Beta zeolite with varying silica to alumina ratios of 25:1; 75:1; 150:1 and 300:1. Beta zeolite was chosen due to its high surface area, it catalytic properties and its ion exchange capacity. Experimental results reported 2-nitrophenol adsorption levels of 40-50 mg g-1,80-90 mg g-1, 80-90 mg g-1 and 80-90 mg g -1 on the unmodified Beta zeolite with varying silica-to-alumina ratios, 25:1, 75:1, 150:1 and 300:1 respectively. The adsorption process was found to follow the Langmuir model for β(25), β(75) and β(150), while the Freundlich model was found to fit the adsorption data of β(300). The adsorption process was found to follow the pseudo-second-order model which suggested that the adsorption on 2-nitrophenol was dependent on the availability of outer surface sites on the Beta zeolite. The thermodynamic parameters show that the adsorption process is mildly exothermic in nature. The magnitude of the enthalpy change indicate that the adsorption is physical in nature involving weak forces of attraction on the outer surface of the Beta zeolite. The adsorption process for 2-nitrophenol on β(25), β(75), β(150) and β(300) is largely independent on the pH of the adsorbate solution below pH 7. The Beta zeolite was modified by a catalytic component in order to enhance its potential adsorption and catalytic capabilities. The Beta zeolite was modified with copper in a one-step ion exchange procedure. The introduction of extra-framework copper cations into all the Beta zeolite samples was found to enhanced the adsorption capacity of 2-nitrophenol. The adsorption of 2-nitrophenol was also found to increase with increasing copper loadings on all the Beta zeolite samples. The introduction of copper cations was found to alter the adsorption model from Langmuir to Freundlich model which suggests a change in the nature of adsorption process from monolayer to multilayer. The thermodynamic study showed that the adsorption of 2-nitrophenol onto the copper modified Beta zeolite samples was influenced by the presence of extra-framework copper cations as it increased the spontaneous nature of the overall adsorption process. During the catalytic oxidation step the presence of copper on the Beta zeolite was found to enhance the oxidation process by increasing the total oxidation of the adsorbed 2-nitrophenol into CO2. While carbon dioxide was the main species detected in the exit steam during the temperature programmed oxidation of the prepared samples, other minor products were also detected such as phenol, benzene, nitrogen oxide, nitrogen dioxide and carbon monoxide/nitrogen. However, the presence of copper greatly reduced the formation of these minor products. In summary, this study demonstrated the potential of copper modified Beta zeolites to treat industrial wastewater containing 2-nitrophenol by adsorption and catalytic oxidation of the adsorbed 2-nitrophenol into carbon dioxide, water and nitrogen.