How the Dyes Are Degraded/Mineralized in a Photocatalytic System? The Possible Role of Auxochromes

The capability of W6+-impregnated ZnO photocatalysts for sunlight mineralization of a variety of structurally different dyes has been investigated. Compared to bare ZnO, the W6+-loaded photocatalysts showed significantly higher activity for the decolorization as well as mineralization of dyes, and complete mineralization was noticed in a short span of 150 min. The results obtained by various analytical tools were correlated to estimate the mechanistic aspects of the decolorization/mineralization process and to identify the nature of the oxidizing species involved in the process. A strong dependence of the decolorization/mineralization process was observed on the nature and number of auxochromes attached to color-generating conjugated system. The rapid decolorization/mineralization of the dyes and release of corresponding anions with the decolorization of dyes suggested the involvement of charged rather than radical reactive oxygen species (ROS) in the oxidation process. Langmuir/Hinshelwood kinetic model was found to be best suited for evaluating the kinetics of mineralization process. The effectiveness of the catalysts for the decolorization/mineralization of a mixture of dyes was also examined. The suitability of the catalysts for successive use in sunlight exposure was also evaluated.