Treatment of methylene blue waste water using silver- doped titaniumdioxide illuminated by flourescent light

Abstract

Textile industries produce large amounts of wastewater due to high consumption of water in the dyeing and finishing operations. At the present time, great efforts are being expended to minimize the quantity and toxicity of industrial effluents. Methylene blue is the most common among all other dyes in its category. Apart from being a textile dye, it is also used in the lab as an indicator, for staining in bacteriology and as an antiseptic. The color that remain in the unexhausted dye effluent cause disturbance to the ecological system. It can also cause health problems to humans when ingested including diarrhea, gastritis, mental confusion, increased heart rate among others. Thus removal of methylene blue from waste water has become a major environmental concern. The objective of this study was to investigate the use of silver-doped titanium dioxide catalyst in the photo-degradation of methylene blue waste water using fluorescent light. The effect of initial pH and catalyst dosage were investigated at the optimum conditions. It was found that alkaline medium favored the adsorption of the dye onto the catalyst surface. Increase in catalyst dosage led to an increase in degradation rate but dropped when the optimum catalyst dosage was exceeded. Minitab statistical tool was used to design the experiments based on the central composite design. Sol-gel acid catalyzed method was used to prepare the catalyst. The absorbance of the samples was measured using a UV spectrophotometer and percentage degradation determined. A fully quadratic regression equation that related the variables and the response was developed. The two way variable interactions of the regression model were discussed. At these conditions, the percentage degradation was found to be 90% with the chemical oxygen demand (COD) reduction of 95.5% indicating complete mineralization of the dye. The colour index was found to be 37 platinum cobalt units after degradation which is within the required Environmental water discharge standards. When pure (undoped) catalyst was used under the same light intensity, the percentage degradation was found to be 16% indicating that doping the catalyst with silver enhanced its excitation by fluorescent light. Kinetic studies indicate that the reaction process is a first order and follow Langmuir-Hinshelwood adsorption kinetics which means that the rate of reaction is controlled by the reaction of adsorbed molecules on the catalyst surface. The catalyst was reused three times without appreciable loss of its catalytic activity. More research need to be done on doped titanium dioxide catalyst immobilized on a surface used in the treatment of textile dyes because it can be more economical in terms of catalyst recovery. Pilot plant design and process development should be done before actual full scale implementation.