Bacterial Degradation of Textile Dyes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 239

Dyes are dif cult to be decolorized due to their complex structure, synthetic origin

and recalcitrant nature. Therefore, it is obligatory to remove them from industrial
ef

uents before being disposed into water bodies. The color of water polluted with
organic colorants gets reduced with the cleavage of the bonds as

well as heterocyclic and aromatic rings. The absorption of light by the associated
molecules shifts from the visible to the ultraviolet or infrared region of the elec-
tromagnetic spectrum (Strickland and Perkins 1995 ). There are about 12 classes of
chromogenic groups, among which, the most common are the azo dyes, which
make up to 60

C=C , N=N

- - - -

70 % of all textile and tannery dyestuffs produced (Carliell et al.
1996 ), followed by the anthraquinone type. Discharge of colorless ef

-

uents loaded
with toxic and hazardous pollutants to water bodies is not usually objected. On the
other hand, the discharge of colored ef uents, though less toxic, is often objected by

the public on the assumption that color is an indicator of pollution. It is, therefore,
not surprising to note that color in wastewater has now been considered as a
pollutant that needs to be treated before discharge. Wastewater treatment using
biological process for decolorization of industrial ef

uents is still ambiguous and

divergent.

2 Microbial Treatment of Waste Water Containing Dyes

Rapid industrialization releases an array of environmental pollutants which include
several xenobiotics (Khan et al. 2013 ). These compounds are highly toxic and most
of them are carcinogenic in nature. Therefore, they have been listed as highly toxic
pollutants by the United States Environment Protection Agency ( http://www.epa.
gov ) . The synthetic dyes constitute the largest class and are widely used in the
textile, cosmetic, printing, drug, and food processing industries. These dyes are
toxic, mutagenic and carcinogenic in nature and have negative impact on the living
organisms (Dos Santos et al. 2007 ). In the dyeing process, most of dyes remain
unbound and get released into the environment with wastewater. This is an
alarming situation. A dye house ef 0.8 g dye l − 1 (Gahr

et al. 1994 ). Microbial degradation of textile dyes has become a key research area in
environmental sciences. The microbes adapt to the toxic wastes and develop new
resistant mechanism, they can biodegrade various toxic chemicals into less harmful
forms. Many bacterial, fungal and algal species have developed the ability to absorb
and/or degrade azo dyes (Pandey et al. 2007 ).

uent typically contains 0.6

-

2.1 Bacterial Biodegradation

Microbial decolorization and degradation is an eco-friendly and cost-competitive
alternative to the chemical decomposition process (McMullan et al. 2001 ). Most of
studies on dye biodegradation have focused on bacteria and fungi, in which bacteria