Decolorization and mineralization of reactive dyes by a photocatalytic process using ZnO and UV radiation

Reactive dyes are one of the major pollutants in textile wastewater and a concern because they are not easily degraded by conventional wastewater treatments. Heterogeneous photocatalysis has been considered an effective option for treating wastewater containing those dyes. This research work assesses the photocatalytic degradation of reactive dyes using UV irradiation and pure or impregnated ZnO. In addition to photocatalysis, separate photolysis and adsorption experiments were conducted but showed low efficiency. The dye degradation was monitored by UV-Vis spectroscopy and mineralization was determined by total organic carbon (TOC) analyses. Total color removal was achieved after 30 min of irradiation using pure ZnO. The Black 5 dye photocatalytic decolorization reaction followed first-order kinetics, while Yellow 145, Red 4 and Blue 21 dyes followed zero-order kinetics. TOC removals in the range of 70-80% were achieved after 240 min of individual photocatalytic treatment with ZnO. The performance of each photocatalyst was also compared when the four dyes were mixed together and the order of efficiency in the mineralization process was as follows: Fe/ZnO > ZnO > Co/ZnO. This result was explained by the crystal field theory.