Textiles dyes as a source of wastewater contamination: screening of the toxicity and treatment methods
Dyes are widely used for industrial, printing, food, cosmetic and clinical purposes as well as textile dyeing because of their chemical stability, ease of synthesis, and versatility. Their stability, however, causes pollution once the dyes are released into the environment in effluents. More than 800,000 tons of dyes are annually produced worldwide, of which 60 to 70% are azo dyes. Considering the heterogeneous composition of these latter dyes, their degradation usually requires a chain of physical, chemical and biological treatments assuring the elimination of different pollutants in successive steps. In addition, some azo dyes are toxic and mutagenic and thus the biological treatment of these dyes is now of major scientific interest. Physical-chemical treatments (adsorption, coagulation/flocculation precipitation, etc.) are usually used for industrial effluents. In spite of their rapidity, these methods have turned out to be ineffective in attaining the standards required for these discharges. As a viable alternative, biological processes are receiving increasing interest owing to their cost effectiveness and their ability to produce less sludge. It has been found that some microorganisms can transform azo dyes into colourless products. Bacterial degradation of azo dyes is often initiated by an enzymatic biotransformation step that involves cleavage of azo linkages with the aid of an azoreductase and an electron donor. As the azoreductase in some microorganisms can catalyze the reductive cleavage of azo groups, they have potential advantages in developing bio-treatment methods of wastewater containing azo compounds.