Microbial Degradation of Basic Dyes in Wastewaters - Microbial Degradation of Synthetic Dyes in Wastewaters - page 96

biochemically characterized and the gene encoding the enzyme was cloned (Jang
et al. 2005 ). Further, the TMR crystal structure was worked out at a resolution of
2.0

Å (Kim et al. 2008 ).

5 Mechanism and Pathways of Degradation

Many studies have revealed that a two-step mechanism, namely physical adsorption
and enzymatic degradation, are involved in dye decolorization by white rot fungi
and other fungal species. Both live and dead forms of diverse fungal genera were
reported to be employed in the decolorization process of a wide variety of dyes (Fu
and Viraraghavan 2001 ). Adsorption is considered as an effective process for color
removal from dye wastewater, which is dependent on the dye properties, such as
molecular structure and type, number and position of substituents in the dye
molecule (Reife and Freeman 1996 ). Limited information is available on the
interactions between microbial biomass (living and dead cells) and dyes to enable
decolorization which occurs through several complex mechanisms such as surface
adsorption, ion-exchange, complexation (coordination), chelation and micro-pre-
cipitation (Crini 2006 ). The bacterial cell wall comprises of polysaccharides, pro-
teins and lipids offering varied functional groups. The dyes can interact with these
active groups on the cell surface through adsorption process. The presence of
hydroxyl, nitro and azo groups in the dye molecule enables increased adsorption,
while decreased adsorption is attributed to the sulfonic acid groups (Reife and
Freeman 1996 ). It is now widely accepted that ion-exchange mechanisms are

involved in the ef ciency and the selectivity of adsorption by microbial biomass.

The ef ciency of dye treatment is dependent on various environmental conditions

and variables used for the adsorption process, such as pH, ionic strength, temper-
ature, contact time and adsorbent concentration as well as the properties of the
adsorbent and adsorbate (Crini 2006 ).
Limited information is available on the interactions between dead fungal bio-
mass and different types of dyes with complex molecular structures. Different
functional groups, such as carboxyl, amino, phosphate and lipid fractions present in
the fungal biomass from A. niger played an important role in the biosorption of four
different dyes (Fu and Viraraghavan 2002b ). Carboxyl and amino groups were the
main binding sites involved in the biosorption of Basic blue 9 by A. niger, while in
the biosorption of Congo red, the amino, carboxylic acid, phosphate groups and
lipid fractions were found to be important binding sites. In addition to electrostatic
attraction, other mechanisms were also responsible to be involved in biosorption.
Some studies have demonstrated effective dye removal properties with dead Rhi-
zopus arrhizus biomass (Aksu and Tezer 2000 ; Aksu and Karabayir 2008 ).
In P. chrysosporium, different isozymes of LiP have been shown to decolorize
azo, triphenylmethane and heterocyclic dyes in the presence of veratryl alcohol and
H 2 O 2 (Ollika et al. 1993 ). Complete decolorization of two triphenylmethane dyes
(Bromophenol blue and Malachite green) was achieved by submerged cultures