58

4 Bioremediation of Toxic Dyes for Zero Waste

4.4.7

Microbial Decolorization and Degradation Mechanisms

The microbial treatment of dye-rich wastewater may follow two main mechanisms:

biosorption and enzymatic degradation. It can also happen through a combination

of the above two methods.

4.4.7.1

Biosorption

The biosorption potential of the selected bacterial species depends on the charac-

teristics of the lipid component and heteropolysaccharides on the cell wall. They

have several functional groups such as −NH2, −COOH, −OH, −PO4, etc., and other

charged groups. Several pretreatment processes may cause modifications in the cell

surface and change the capacity of the binding site. Compared with the living form,

the lack of nutrition, long-term barrier-free storage and use, and the convenient

regeneration of organic solvents and detergents make dead cells more suitable for

biosorption.

4.4.7.2

Enzymatic Degradation

Owing to the presence of azo bonds (—N=N—), azo dyes are essentially elec-

tron defective and are related to other electron withdrawing moieties (such as

sulfonic acid groups) in many cases. Under optimal conditions, these dyes may be

degraded by various enzymes (such as reductase, laccase, oxidase, etc.). Among

these laccase-producing species have higher bioremediation potential, due to their

nonspecific oxidizing ability, non-requirement for cofactors, and use of oxygen as

an electron acceptor [20].

4.4.8

Decolorization and Degradation of Dyes by Plants

(Phytoremediation)

Phytoremediation refers to the use of plants to remove environmental pollutants.

The use of living plants as bioremediation agents is promising for the degradation

of various dyes and other organic and inorganic pollutants. Due to cost-effective

technology, effectiveness, and environmentally friendly technology, it is an in

situ biological treatment method that pays more attention to the treatment of

dye-contaminated sites. Plants are highly sensitive to pollutants and have the

potential to degrade textile dyes [21]. The use of plant systems for bioremediation

is still limited because of the poor understanding of the basic mechanisms and

processes involved. Many green plants, including herbs, shrubs, and trees (ter-

restrial and aquatic plants), have a good potential to regenerate and rebuild the

contaminated ecosystem, according to research. These plant species can remove

the pollutants by acting as excluders, accumulators, and hyper accumulators. The

excluders accumulate contaminants from the substrate into the roots but restrict

their transportation into the aerial parts such as shoots. Compared with other plant

species, accumulators can concentrate pollutants and convert them into inert forms

in air tissues, while hyper accumulators can accumulate unusually large amounts

of pollutants.