Removal of Synthetic Textile Dyes From Wastewaters: A Critical Review on Present Treatment Technologies

Azo dyes represent the largest class of industrial colorants. These are no longer used only for the coloration of textiles, plastics, paints, inks, and lacquers, but rather serve as key components in high-tech applications such as optical data storage, reprographics, display devices, dye-sensitized solar cells, energy transfer cascades, light-emitting diodes, laser welding processes, or heat management systems. Azo dyes are also of growing importance in the medical and biomedical fields. In most of these applications, the color is largely irrelevant and it is the ability of the colorants to absorb visible electromagnetic radiation with high efficiency, or other functional property, that is exploited. With the growing awareness and environmental concerns, it is imperative that the hierarchy of reduce, reuse, and degrade be adopted and measures be taken to remove color from the industrial discharge. The present review (a) embodies a comparison of the present decolorization/degradation techniques for water-soluble and insoluble dyes, (b) describes their advantages and limitations, (c) discusses various mechanisms, and (d) focuses on the present literature on microbial decolorization of textile dyes specifically by using bacteria, fungus, yeast, and algae. Also, for the first time an attempt has been made to comprehensively compile chemical and biological decolorization/degradation of disperse dyes. The research on the decolorization of textile dyes has mainly been focused on water-soluble dyes, while decolorization of disperse dyes that are water insoluble have received only scant attention and constitutes a topical area of concern as these dyes persist for a longer duration in textile effluents. Given the limitations (vide infra) of the chemical treatment methods, decolorization using biological means is an interesting option.