Azo dyes: past, present and the future

Dyes have a long history and constitute an important component in our daily lives. The dye industry began by using natural plant and insect sources, and then rapidly turned to synthetic manufacturing processes. Unfortunately, several of the synthetic dyes, especially azo dyes, have been found to be toxic and mutagenic, and are banned throughout the world. However, because of their low cost and other desirable properties, the use and manufacture of azo dyes continues even today. Removal and treatment of azo dyes from wastewater presents a very special challenge. Since azo dyes are recalcitrant to the conventional aerobic biological treatment, and physical/chemical treatment processes are neither ecofriendly nor economical, innovative remediation approaches need to be explored. In the current scenario, a combination of biological and advanced oxidation processes seem to be the most desired solution. Moreover, mechanistic studies of the pathways and enzymes involved in dye degradation and detoxification need to be undertaken for process optimization. This paper critically and comprehensively reviews recent research advances and the knowledge-base of the previously mentioned areas, including history and chemistry of dyes, mechanistic studies on their persistence, ecotoxicity, and carcinogenicity, and potential remediation methods. Particular emphasis has been given towards critical evaluation of the existing dye wastewater treatment methods and their full-scale applications; based on which recommendations have been made for future developments of the treatment technologies and their assessment methods. In the Authors' opinion, a more holistic and large-scale approach is required to tackle the chronic problem of dye pollution.