4.4 Bioremediation Methods

53

because complete mineralization is achieved due to the synergy of different organ-

isms. According to reports, the reduction of azo bonds can be achieved under the

reducing conditions of an anaerobic bioreactor. As a result, a colorless aromatic

amine can be formed, which is further mineralized under aerobic conditions. There-

fore, it is usually recommended to perform anaerobic decolorization first and then

to perform aerobic posttreatment to treat dye wastewater. This combined approach

is cost-competitive and applicable to various dyes [4].

4.4.3

Decolorization and Degradation of Dyes by Fungi

Fungi can quickly adapt their metabolism to various carbon and nitrogen sources

by producing a large number of intracellular and extracellular enzymes that can

degrade a variety of complex organic pollutants. This ability of fungus to degrade

various organic compounds is caused by the relative non-specificity of their

lignin-decomposing enzymes, such as manganese peroxidase, lignin peroxidase,

and laccase [5]. Most research on the biodegradation of azo dyes has focused

on fungal cultures derived from white-rot fungi that have been used to develop

biological processes for the mineralization of azo dyes. P. chrysosporium is the

most widely studied white-rot fungus, but others have also received considerable

attention, such as Aspergillus ochraceus, Bjerkandera adusta, Trametes versicolor,

species of Phlebia, and Pleurotus, Peyronellaea prosopidis, and many other isolates.

However, the application of white-rot fungi to remove dyes from textile wastewater

has some inherent disadvantages, such as long growth cycles and the need for

nitrogen-limiting conditions.

4.4.4

Decolorization and Degradation of Dyes by Yeast

There is very little work to explore the decolorization ability of yeast, and it has been

used mainly for the study of biosorption. Some yeast species, such as Debaryomyces

polymorphus, Candida zeylanoides, and Candida tropicalis, have been used to per-

form putative enzymatic biodegradation and subsequent decolorization of different

azo dyes [6]. Recently, it has been reported that Saccharomyces cerevisiae MTCC-463

plays a role in the decolorization of malachite green and methyl red [7]. In addition,

S. cerevisiae cells also showed the bioaccumulation of reactive textile dyes (Remazol

Black B, Remazol Blue, and Remazol Red RB) during growth in molasses [8]. Recently,

the decolorization of Reactive Black 5 has been studied in detail using a salt-tolerant

yeast strain Sterigmatomyces halophilus SSA-1575, and the enzymatic mechanism

and toxicity of the degradation products have also been reported [9].

4.4.5

Decolorization and Degradation of Dyes by Algae

Photosynthetic organisms are ubiquitous, distributed in many habitats around

the world, and are receiving more and more attention in the field of wastewater

decolorization. Literature surveys indicate that algae can degrade azo dyes through

an induced form of azo reductase. Several species of Chlorella and Oscillatoria are

able to degrade azo dyes into their aromatic amines and can further metabolize