Photocatalytic degradation of azo reactive dyes with ultraviolet and sunlight irradiated zinc oxide

Dye effluents are among the most persistent sources of pollution of water bodies and aquatic life. Notable dyes with known carcinogenic effects at low concentrations include azo reactive dyes. The present investigation is focused on the photocatalytic degradation of representative commercial azo dyes using zinc oxide (ZnO). The findings of this research show that ZnO irradiated with ultraviolet (UV) radiation is more effective at degrading C .I. Reactive Yellow 145, C .I. Reactive Blue 194 and C.I. Reactive Red 194 as compared to the sunlight irradiated ZnO. The crystallinity, surface morphology and band gap energy of the ZnO were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) image and UV-Visible absorption spectroscopy respectively. The ZnO has a hexagonal wurzite structure with a band gap of 3.17 eV. The degradation profile was found to decrease with increasing initial dye concentration. The degradation efficiency of the respective dyes under UV and sunlight irradiation was found to be 90% and 85% for CIRY145; 88% and 82% for CIRB194; 96% and 90% for CIRR194. Optimum degradation pH values of 6.9, 7.2 and 6.2 were also recorded for CIRY145, CIRB and CIRR194, respectively, and the general degradation profile was found to follow first order kinetics. Gas chromatographic studies of the degradation reaction intermediates also showed that the degradation profile is time-dependent and all potentially carcinogenic intermediates were degraded into smaller environmentally friendly products such as oxalic acid and acetic acid.