Azo dyes are considered as xenobiotic compounds, which are often recalcitrant to be biodegraded. Many dyes are relevant for staining in industries, especially textile and food related. Among other environmental problems, one obvious issue is the coloring effect on effluents from industrial sites and thus the release of xenobiotics into nature. Microorganisms (algae, fungi, yeast, and bacteria) have been found to decolorize a number of azo dyes. Decolorization of azo dyes by microbial cultures is summarized and this is mostly linked to initial activation or even cleavage of the azo bond (e.g., by azoreductases). However, it does not necessarily mean that the compound is degraded to non-toxic products. Various mechanisms of microbial decolorization processes were discovered, including adsorption, enzymatic degradation, or a combination of both. Oxidases and reductases were found to be involved, which contain azoreductase, lignin peroxidase, Mn peroxidase, laccase, tyrosinase, and so on. A focus is on the azoreductases including classification, activity, and applicability.
Kinetic and mechanistic analysis of azo dyes decolorization by ZVI-assisted Fenton systems: pH-dependent shift in the contributions of reductive and oxidative transformation pathways