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4 Bioremediation of Toxic Dyes for Zero Waste

aromatic amines into simpler organic compounds. According to reports, Chlorella

vulgaris, Chlorella pyrenoidosa, Oscillatoria tenuis, Anabaena flosquae UTCC64,

Phormidium autumnale UTEX1580, and Synechococcus sp. PCC7942 can decom-

pose and decolor more than 30 kinds of azo compounds into simpler aromatic

amines [10, 11]. Therefore, the above results may mean that algae play an important

role in the removal of azo dyes and, moreover, this biosorption process can be used

as a cost-effective method for wastewater decolorization.

4.4.6

Bacterial Decolorization and Degradation of Dyes

Generally, the decolorization of azo dyes occurs by different kinds of bacteria under

conventional anaerobic, facultative anaerobic, and aerobic conditions. The work to

isolate pure bacterial cultures capable of degrading azo dyes began in the 1970s, and

Bacillus subtilis, Aeromonas hydrophila, and Bacillus cereus were reported. Recently,

a large number of studies have been conducted on decolorization using pure strains

of bacterial cultures such as species of Enterobacter, Enterococcus, Acinetobacter,

Proteus, Pseudomonas, etc. In addition, there are several studies describing the decol-

orization mechanisms of reactive azo dyes mediated by pure bacterial cultures. The

use of a pure culture system ensures reproducible data, so the interpretation of exper-

imental observations becomes easier. The results of bacterial species reported as dye

degraders are summarized in Table 4.2 [12–16].

Due to their ability to function by consortia or synergistic alliances that function as

biological inducers, bacterial activity in the degradation of azo dyes is enhanced. The

combination of the catabolic function of each microorganism makes them a more

useful substitute, which can increase the decolorization rate. The most important

competitive advantages that place bacteria as the most successful microorganisms

in the degradation of azo dyes are summarized in Table 4.3 [12].

There are also some drawbacks to the use of bacteria to remove dyes: (i) the decol-

orization process does not depend solely on these microorganisms, but also on exter-

nal variables such as aeration, agitation, pH, temperature, concentration of the dye,

structure of the dye, sources of carbon and nitrogen, electron donor, and redox medi-

ator; (ii) under anaerobic conditions, the dye penetrates through the cell membrane

with difficulty, affecting the degradation rate; (iii) they produce noxious and recalci-

trant aromatic amines as a result of the anaerobic degradation process; and (iv) pure

strains of bacteria do not degrade the azo dyes completely, so bacterial consortia are

required to make the process more efficient. Bacteria, however, are the most resilient

microorganisms, which become possible degraders of recalcitrant pollutants such as

azo dyes because of their structure and genome. Among other things, the compet-

itive advantages of bacteria are their ability to adapt and their metabolic activity,

short life cycle, and capable of degrading and detoxifying the secondary metabolites

produced during the process of decolorization.

4.4.6.1

Factors Affecting Dye Decolorization and Degradation

Changes in different physicochemical parameters, namely aeration, agitation, pH,

temperature, concentration of the dye, structure of the dye, sources of carbon and