4.4 Bioremediation Methods

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carboxylated azo dyes. Also, the chemical composition of the dye determines the

induction of the enzyme in bacteria for dye decolorization.

Oxygen and Agitation Different groups of bacteria under anaerobic and aerobic con-

ditions are strictly involved in the decolorization of azo dyes. In order to increase the

biomass and also transfer oxygen between bacterial cells and the nutrient medium,

aeration and agitation are typically necessary. Moreover, it also increases enzyme

activity during the aerobic growth, but reductive enzymes are mostly susceptible to

oxygen presence. However, oxidative enzymes play an important role in aerobic dye

degradation, requiring the presence of oxygen [17].

Carbon and Nitrogen Supplements For the fast degradation of contaminants, microor-

ganisms require nutrient supplements. In order to achieve high and rapid dye degra-

dation rates for both pure and mixed cultures, organic sources such as peptone, yeast

extract or a combination of carbohydrates and complex organic sources have been

reported. The efficiency of dye degradation can be improved by adding glucose as the

major carbon source and phosphorus has been identified as a significant growth fac-

tor [18]. Lignocellulosic agricultural waste has also been used by some researchers

as a substitute for successful decolorization, thereby making the process commer-

cially profitable and theoretically useful. Through adequate production of lignolytic

enzymes in the presence of lignocellulosic substrates can enhance color removal effi-

ciency.

Dye Concentration Enzymes that are secreted by dye degrading bacteria may not

detect low dye concentration. High dye concentration, on the other hand, is harmful

to bacteria and also affects dye degradation by blocking active sites of the enzyme.

However, it was noted that this increasing dye concentration effect was reduced

when bacterial coculture was used instead of pure culture, possibly due to the com-

bined effect of both microorganisms.

Electron Donor and Redox Mediator Electron donors and redox mediators play a major

role in achieving a successful anaerobic decolorization process, as azo dye and var-

ious other organic textile wastewater material are not adequate substrates for the

growth of anaerobic bacteria. The application of electron donors, such as sodium

succinate, sodium formate, sodium acetate, sodium citrate, and sodium pyruvate,

has shown to increase the decolorization efficiency. Flavin-based compounds such

as flavin adenine dinucleotide (FAD) and flavin adenine mononucleotide (FMN)

and quinone-based compounds such as anthraquinone-2,6-disulfonate, riboflavin,

and cyanocobalamin are the majority of the recorded redox mediators.

Soluble Salts Dye industry wastewater has high electrical conductivity due to the

use of high salt concentration. In the dye treatment plant, salts like NaCl, Na2SO4,

and NaNO3 are typically added to increase ionic strength and fix the dye on fab-

ric. Therefore, salts are also released into industrial wastewater when dye pollutants

are released. Effluents containing high salt concentrations may reduce the rate of

biodegradation by inhibiting the biological movement [19].