Reductive Decolorization of Azo Dye by Bacteria - Microbial Degradation of Synthetic Dyes in Wastewaters - page 124

6 Toxicity of Azo Dyes and Their Decolorization Products

It has been found that the puri ed forms of many azo dyes are directly mutagenic

and carcinogenic (Chen 2002 ). In addition, the transformed intermediates of azo
dyes are also highly toxic and mutagenic in nature (An et al. 2007 ; Pandey et al.
2007 ). Thus, merely showing decolorization of azo dyes is not enough for the
publication of a study, but veri cation after decolorization is also

essentially required in recent years. The toxicity study of azo dyes and their met-
abolic intermediates was usually carried out by assessing the phytotoxicity or
microbial toxicity of the sample before and after bacterial decolorization. In phy-
totoxicity, the seeds of model plant were treated with certain concentration of dye
and its decolorization metabolites and incubated for a given period. Then germi-
nation percentage, lengths of plumule and radicle of the treated seeds were three
most investigated parameters for the phytotoxicity assessment (Khan et al. 2013 ).
For microbial toxicity test, the zones of inhibition in the presence of azo dye or its
decolorization metabolites were usually measured and compared (Khan et al. 2013 ).

cation of detoxi

7 Future Prospectives

Based on the development of bacterial decolorization during the past two decades,
now we have to focus on: (i) Mineralization and detoxi cation besides decolor-

ization. As the list of dye decolorizing bacteria continues to extend, more attention
should be paid to the isolation of strains capable of removing aromatic amine
metabolites. The combination of certain physicochemical method with bacterial
decolorization could also be a good effort. (ii) Adaption better than genetic mod-
i cation. Considering the present situation of ineffective intracellular azoreductase

and strict regulation on gene-modi ed organisms, we believed that the acclimati-

zation of natural microbes to practical operation conditions should be the rst

choice of dye decolorizer. (iii) Design of more biodegradable azo dye. The
knowledge of effects of dye structure on its biodegradability should be provided to
dye chemists to direct the synthesis of more environment-friendly dye molecules.
(iv) New process and technology for dye production and application industries. The
development of novel process and technology, which can bring down water use and
recycle the treated wastewater, would reduce azo dye pollution from the source.
This is more important than treatment at the nal stage.

8 Conclusion

Pollution caused by azo dye wastewater not only creates aesthetic problems, but
may also affect the equilibrium of natural ecosystem and human health. Regulations
on azo dye use and discharge are becoming more and more stringent. Hence,