Bacterial Degradation of Azo Dye Containing Wastes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 64

azo dye cannot pass through bacterial cell membrane easily. The speci city of azo

reductase was found to be strongly dependent upon the electron-withdrawing ability
of functional groups in the proximity of the azo linkage (Saratale et al. 2011 ).

4.2 Concentration of Azo Dye

A survey of the literature suggests that increasing the dye concentration gradually
decreases the decolorization rate, probably due to the toxic effect of dyes on the
bacteria. It is also due to inadequate biomass concentration and blockage of active
sites of azo reductase by dye molecules with different structures. It was also observed
that azo dyes with reactive groups as sulfonic acid (SO 3 H) on their aromatic rings
greatly inhibited the growth of microorganisms at higher dye concentrations (Chen
et al. 2003 ). However, Saratale et al. ( 2009 ) found that the increasing concentration
effect was reduced when bacterial co-culture was used instead of pure culture, due to
the synergistic effect of both microorganisms. Moreover, Dubin and Wright ( 1975 )
did not observe any effect of dye concentration on its reduction rate. This obser-
vation is compatible with a non-enzymatic reduction mechanism that is controlled by
processes that are independent of the dye concentration.

4.3 Aeration and Agitation

Decolorization of azo dyes occurs under strictly anaerobic, facultative anaerobic
and aerobic conditions by different groups of bacteria. Biodegradation process of
azo dye is highly in uenced by presence or absence of oxygen. Aeration and

agitation may favour or inhibit the azo dye decolorization by bacteria. Generally,
aeration and agitation increase biomass and oxygen transfer between bacterial cells
and nutrient medium. It also increases enzyme activity, if mechanism is aerobic, but
most of the reductase enzymes are sensitive to oxygen. The effect of oxygen on azo
reduction is irreversible, so in presence of oxygen, azo reductases show lower
activity. Hence, for ef cient reduction of color, aeration and agitation, which

increase the concentration of oxygen in the solution, should be avoided. However,
under anaerobic conditions, reductive enzyme activities are higher, but a small
amount of oxygen is also required for the oxidative enzymes, which are involved in
the degradation of azo dyes. Some studies have reported that during bacterial
degradation of azo dyes, both oxidative and reductive enzymes play a signi cant

role. The intermediates, formed during azo dye reduction reaction, like the simple
aromatic compounds, are degraded via hydroxylation and ring-opening in the
presence of oxygen. The aerobic condition is required for the mineralization of the
azo dye molecules. Thus, for the most effective ef uent treatment, an anaerobic

process with subsequent aerobic treatment can be used to decolorize wastewaters
containing dyes (Saratale et al. 2011 ; Solis et al. 2012 ).