Study on Photocatalytic mineralization of various azo dyes under solar light irradiation
Wan Fadhilah, Wan Mohd Khalik
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Azo dyes represented by azo chromophores (-N=N-) are the largest class of dyes used in many industries especially textile industries. Other chemicals that mutagenic or toxic to the environment and human life will present in the water bodies if the wastewater effluent from industries was released without treating them properly. There are many conventional treatment methods in order to remove azo dyes in wastewater, for example adsorption, membrane filtration, aerobic or anaerobic process and others. However, these conventional treatment methods required high cost and generated other pollutants which difficult to be destroyed. In order to minimize the cost for wastewater treatment but effective in removal of azo dye, solar photocatalytic had been discovered by previous researchers. Therefore, the main purpose of this research was to evaluate the mineralization of various azo dyes by solar photocatalytic process. The main photocatalyst and azo dye used in this study was zinc oxide (ZnO) and New Coccine (NC), respectively. In the first section, the study was to compare the solar photocatalytic between decolorization of Orange G and New Coccine. The comparison between the azo dyes was determined through several operating parameters such as with and without solar light irradiation, initial dye concentration, catalyst dosage, pH and with and without aeration. The results showed that concentration NC decreased rapidly in each parameter and almost achieved 100 % removal efficiency compared to OG. The second section in this study was to evaluate the effect of molecular structure of five azo dyes (Acid Orange 7, Orange G, New Coccine, Reactive Black 5, Reactive Green 19) in photocatalytic mineralization under solar light irradiation. Among these azo dyes, Reactive Green 19 showed great performance in photocatalytic process and achieved 100 % of removal efficiency under solar light irradiation. Moreover, this study also focused on the effectiveness of other photocatalysts (CuO, Fe2O3, TiO2) on the degradation of dye and showed that photocatalyst with higher band gap (ZnO) rapidly decolorized and mineralized the azo dye. The surface area and patterns of all photocatalysts were also examined. The reduction in dye concentration was determined by UV-Vis spectrophotometer. The mineralization of dye was examined by ion chromatography (IC) and chemical oxygen demand (COD) measurement. The final concentration of New Coccine achieved 100 % removal after 5 h irradiation time. Among all the photocatalysts, ZnO showed highest removal which suggesting that it absorbs large fraction of the solar spectrum and absorption of more light quanta. Batik wastewater also showed great color removal within 10 h sampling and its COD concentration decreased from 1332 mg/L to 286 mg/L after 12 h irradiated time. Both of azo dyes and photocatalysts followed pseudo-first-order rate constant and well fitted the Langmuir-Hinshelwood kinetic model. In conclusion, either synthetic dye or real textile wastewater, both of them can be degraded and mineralized by photocatalyst with presence of solar light.