Bacterial Degradation of Textile Dyes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 248

contains many organic and inorganic pollutants. As textile wastewaters result from
different classes of dyes, they vary in their composition. Every microorganism has
some physico-chemical properties to work for biodegradation process, even if the
microbes are in consortia these physico-chemical properties in

uence the biodeg-

radation process (Mohana et al. 2008 ).
One of the factors, like pH has a major effect on the ef ciency of textile dye

degradation and the optimal pH for color removal in bacteria is often between pH
6.0 and 10.0 (Yoo et al. 2001 ; Kolekar et al. 2008 ; Kolekar and Kodam 2012 ). Most
of fungi and yeast showed better decolorization at acidic or neutral pH (Mielgo et al.
2001 ). Many reports showed that the rate of dye degradation is higher at the optimal
pH and gradually decreases at either side of optimum pH (Yoo et al. 2001 ; Saratale
et al. 2009 ; Chaudhari et al. 2013 ). The effects of pH may be related to the transport
of dye molecule across the cell membrane, which is considered as the rate limiting
step for the decolorization (Kodam et al. 2005 ). The decolorization of Brilliant blue
G by consortium of Galactomyces geotrichum and Bacillus sp. was found to have
broad pH range from 5.0 to 9.0 (Jadhav et al. 2008 ).
During the biodegradation process, temperature also affects the biodegradation

ef ciency. The maximum rate of dye degradation by most of microbes depends on

the optimum growth and temperature. It was observed that degradation potential
was proportional to an increase of temperature, but within the optimum temperature
range (Nachiyar and Rajkumar 2003 ; Kolekar et al. 2008 ). However, the rate of
decolorization was reduced beyond optimum temperature. The reduction in
decolorization can be attributed to the loss of cell viability or the denaturation of
azoreductase enzyme which is a key enzyme for dye degradation (Saratale et al.
2009 ). It was also found that the dye degradation depended largely on the microbial
optimum growth temperature (Stolz 2001 ; Khan et al. 2013 ). Recently, the deg-
radation of reactive Orange M2R dye and chromate by Lysinibacillus sp. KMK-A
was reported in the temperature range of 20 C, but the degradation gradually

decreased with increasing temperature (Chaudhari et al. 2013 ).
Microbial decolorization often gets affected in the presence of high amounts of
salts (up to 10 g l − 1 NaCl or Na 2 SO 4 ) (Uddin et al. 2007 ). Many microbial species
are able to decolorize azo dyes within a certain limit of salts. But in most of cases,
they were unable to decolorize azo dyes at high salinity conditions. High salt
concentration may cause plasmolysis or loss of activity of cells. It was observed that
85

50 °

-

95 % decolorization of acid orange 10 and disperse blue 79 by Bacillus fusi-
formis was found within 48 h for 0.5

-

3 % salt concentration, However, further

-

increase in the salt (4 %) signi cantly decreased dye decolorization (Kolekar et al.

2008 ). There are only some bacteria which have the potential to grow, multiply in
high salt concentrations and can degrade dye ef ciently. Gracilibacillus sp. GTY

strain, grown in the media containing 15 % (w/v) of NaCl, showed the best per-
formance in dye decolorization (Uddin et al. 2007 ).
The dye decolorization affects with contaminants present in the wastewater.
Many heavy metals are known to be toxic and can reduce the microbial growth
which affects the biodegradation process. There are strict regulations to limit the
amount of heavy metal present in the dye ef

uent in different countries, as some of