Photocatalytic degradation of a trichromatic direct dye under direct solar irradiation: reaction pathway and identification of intermediate products

ZnO was synthesized, characterized by scanning electron microscopy, X-ray diffraction, UV-visible spectrophotometry, and infrared spectroscopy analysis, and then it was applied as a photocatalyst for trichromatic direct dye: Direct Yellow 86 (DY86), Direct Red (DR243), and Direct Blue (DB71) under solar irradiation. The progress of photocatalytic degradation was observed by monitoring the change in substrate concentration, employing UV-visible spectrophotometer and high-performance liquid chromatography. The order of degradation was DB71 ≥ DR243 > DY86. A detailed study was also carried out using liquid chromatography/tandem mass spectrometry to determine the degradation pathway of the dye, as well as to confirm the order of degradation and to identify some of the intermediate products formed; a possible degradation mechanism was also proposed. The effects of parameters such as time, pH, and salt were studied, and the mineralization was confirmed by chemical oxygen demand measurement. The synthesized ZnO nanoparticles could be reused, and the water obtained after treatment might be reused for a second dyeing process.