Azo dyes: Sources, occurrence, toxicity, sampling, analysis, and their removal methods

Population growth, rapid industrialization, and urbanization over the years have contributed to the presence of chemical compounds in the aquatic environment. There are lots of reports on the presence of pesticides, steroids, and antibiotics as aquatic pollutants. However, little is known about synthetic organic dyes as contaminants of water sources which are a serious threat to public health. Azo dyes are the most important and the largest class of commercial dyes, and they represent almost 65%–75% of all products of textile dyes. They are classified as aromatic compounds with NN groups. The presence of the nitro group, which is mutagenic, makes azo dyes highly toxic, and after breakdown, they generate poisonous products such as 1,4-phenylenediamine, o-tolidine. Due to their large-scale production and widespread application, azo dyes are readily spread into environmental waters and hence, they are considered micropollutants in aquatic ecosystems. Owing to the mutagenic native of azo dyes, it is of high importance that their occurrence in aquatic systems should be monitored continuously. In this chapter, we will first classify dyes, and then will try to present scientific data considering application areas, toxicity, sources, environmental occurrence, and the fate of azo dyes and their ecological implications. Their presence in different parts of the world's waters (occurrence of azo dyes in different countries, especially in developing countries) will be discussed and at the end, analytical methods for azo dye determination and methods of their removal from (waste) water will be described.