Fundamental principles and application of heterogeneous photocatalytic degradation of dyes in solution

Synthetic dyes are a major part of our life as they are found in the various products ranging from clothes to leather accessories to furniture. An unfortunate side effect of their widespread use is the fact that up to 12% of these dyes are wasted during the dyeing process, and that approximately 20% of this wastage enters the environment (mostly in water supply). Not surprisingly, various approaches have been developed to remove and degrade these carcinogenic dyes from the natural environment. Advanced oxidation processes (AOPs) are the most widely used approach that is employed for dye degradation studies. Over the past few years, there has been an enormous amount of work that has been done with AOPs and as a result various kinds of AOPs have been developed. The aim of this review is to address the fundamentals of one kind of AOP, namely, photocatalytic, and how it is used for dye degradations in aqueous suspensions using TiO₂ as a catalyst. Since AOPs rely on the generation and subsequent reaction of highly reactive oxygen radicals with dyes, there are many factors that can affect the efficiency of this process. Hence, this review will attempt to summarize and highlight the effect of a variety of conditions on TiO₂-photocatalysed decoloration of dyes, such as amount of catalyst, reaction pH, light intensity, concentration of organic dye, and the presence of additives such as ions. This review also summarizes the degradation pathways that azo dyes undergo, with some of the intermediates that are generated during their degradation. Finally, a survey is presented of the various classes of dyes and their relative ease of degradation by AOPs.