Decolorization of azo dyes in a heterogeneous persulfate system using FeS as the activator

Textile effluents containing synthetic refractory azo dyes are one of the most important sources of water pollution. However, these kinds of refractory organic pollutants were resistless to a persulfate (PS) oxidation process which was proper activated. In this study, PS was activated by ferrous sulfide (FeS) in a heterogeneous system to decompose azo dyes wastewater. The results showed that all five selected azo dyes were efficiently decomposed by the PS/FeS system, except DY 12, more than 95% of azo dyes were decolored within 60 minutes. The decolorization efficiency of DR 81 in PS/FeS system was comparable to PS activated with heat (60 °C) or Fe2+, and was slightly superior to Fe0 powders under the same conditions. The quenching studies indicated that both SO4−• and •OH were formed in the FeS surface and diffused into the solution to facilitate the successive transformation of DR 81, and •OH reacted with DR 81 might the crucial reaction. The coexisted chelating agents in real azo dyes effluents with high concentration had negative influence on azo dyes decolorization by PS/FeS. However, the superiority of the PS/FeS system was the regenerability and reusability of the heterogeneous catalyst.

Microbial Degradation of Azo Dyes

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.

Microbial Degradation of Azo Dyes

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.