Microbial Degradation of Basic Dyes in Wastewaters - Microbial Degradation of Synthetic Dyes in Wastewaters - page 91

of the functional groups like carboxyl, hydroxyl and amino groups of the fungal or
bacterial cell wall (Fu and Viraraghavan 2002b ; Won et al. 2009 ). At low pH, the
biosorbent is rich in positive dye binding sites which attract the anionic dye mol-
ecule, while at higher pH, the biosorbent carries a net negative charge, resulting in
electrostatic repulsion with the dye molecule. Lower biosorption at higher pH
values may be due to the presence of excess hydroxyl ions competing with the
negatively charged dye for the biosorption sites (Won et al. 2009 ). The decolor-
ization process can be conducted with a whole fermentation broth (mycelium and
enzymes) or with isolated enzymes. There needs to be a clear distinction among the
optimum pH required for growth and enzyme production, the action of isolated
enzymes and dye degradation. Therefore, optimum pH depends on the medium,
microbial strain and its enzyme system, as well as on the decolorization or deg-
radation process.

3.6 Temperature

Temperature is another critical parameter which needs to be optimized from
organism to organism. Temperature has its in uence on the growth, enzyme pro-

duction, decolorization rate and the temperature of the waste stream. Different fungi
and most white-rot fungi are mesophiles and exhibit optimal growth and dye
decolorization at temperatures ranging between 25 and 35 C (Fu and Viraraghavan

2001 ; Parshetti et al. 2007 ). This is mainly due to the increased surface activity and
kinetic energy of each dye molecule (Kaushik and Malik 2009 ). The optimal
temperatures for enzymatic reactions are usually higher, however, at temperatures
of >65

°

C may cause enzyme instability and degradation. Under dyeing operation
conditions, various textile and dye ef

°

uents are generated at a temperature range of

50 - 60 ° C. Hence, optimal temperature for decolorization process varies from case

to case.

3.7 Agitation and Aeration

Ligninolytic fungi are obligate aerobes and therefore, require oxygen for growth
and maintenance of their viability. In addition, dye degradation also requires
oxygen, either for the mycelial generation of H 2 O 2 for peroxidases or for the direct
action of oxidases. The oxygen demand mainly depends on the fungus and its
ligninolytic system. The major limitation of oxygen is its low water solubility,
which is only 8 mg l − 1 at 20

°

C. Aeration and agitation are necessary to meet the
microbial oxygen demand during the cultivation so as to increase better oxygen
transfer and nutrient distribution under shaking conditions which enhance dye
decolorization rates as compared to the stationary cultures (Swamy and Ramsay
1999 ). However, this may affect the morphology of lamentous fungi leading to the