Bacterial Degradation of Azo Dye Containing Wastes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 66

4.5 Incubation Temperature

Temperature is also an important environmental factor for all processes associated
with microbial life, including the decolorization of dye and dye ef uents by bacteria.

The bacterial cells adapt to temperature changes by biochemical or enzymatic
mechanisms. Incubation temperature affects both bacterial growth and enzyme
activity, and so, the rate of azo dyes decolorization. Narrow temperature ranges were
determined as being necessary for the decolorization of azo dyes by extremely
complex consortia of microorganisms inhabiting active sludge (Yu et al. 2001 ). It
was observed that bacteria show maximum decolorization of dye at optimum tem-
perature needed for their growth, reproduction and enzyme activities (Hazrat 2010 ).
Beyond the optimum temperature, reduction in decolorization activity is found due
to loss of cell viability, decreased rate of reproduction and denaturation of enzymes
responsible for degradation (Saratale et al. 2011 ; Solis et al. 2012 ). But, for actual
treatment of dye containing waste on commercial level requires stable bacterial
culture to a wide change in environmental temperature. Thus, application of thermo
tolerant culture for the treatment of dye ef uents is advisable. Thermophilic bacteria,

Geobacillus stearothermophilus for Orange II dye decolorization and azo reductase
enzyme of B. Badius were reported stable even at higher temperature. Most of the
reports available on azo dye degradation by bacteria indicated 25

37 ° C temperature

-

range and 35 ± 2 ° C as an optimum temperature (Hazrat 2010 ; Solis et al. 2012 ).

4.6 Medium pH

pH is also an important factor with respect to biological decolorization of azo dyes.
The medium pH has a major effect on the ef

ciency of dye decolorization because
of enzymatic activity dependence on the pH. Color of the solution and solubility of
dyes is also affected by pH. Generally, bacteria show better decolorization at neutral
or basic pH and the optimal pH for color removal is often between 6.0 and 10.0
(Solis et al. 2012 ). The rate of color removal is the highest at the optimum pH and
tends to decrease on either side of optimum pH. It is possible that pH change affects
the transport of dye molecules across the cell membrane, which is considered as the
rate limiting step for the decolorization (Saratale et al. 2011 ). An increase in pH of
the medium was observed during anaerobic dye decolorization due to reduction of
the azo bond to form aromatic amine metabolites, which are more basic than the
parent azo dye (Willmott 1997 ). Generally, uctuation in the pH slightly from

neutral to alkaline side has very little effect on the dye decolorization as compared
to acidic side (Jadhav et al. 2008 ). However, Citrobacter sp. CK3 is able to
decolorize even in strongly acidic (at pH 4) and strongly alkaline (at pH 12)
conditions (Wang et al. 2009 ). Thus, pH tolerance of decolorizing bacteria is quite
important, as it makes them suitable for the commercial treatment of dye containing
ef uents.