13.8 Metal-Complexed Dyes

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difficult to use in high-scale process because of its high cost [34]. Many researchers

preferred very inexpensive and other natural sources such as bagasse, bark, chitosan,

wood, peat, and fly ash. In addition to that, they found microbial sources such as

bacteria, fungi, and yeast which have the potential to degrade the dyes. The reactive

functional groups of azo-based dyes will impart with the active sites after taking part

with acidic polysaccharides, chitin, amino acids, fatty acids, and other constituents

of the microbial consortium. In addition, the ionic strength shows a greater impact

on the increase of the interaction between the cell surface and dyes. The interaction

between the ligands present on the cell surface and dyes is carried out by electrostatic

interaction and hydrogen bonding. The efficiency of the color removal by the yeast

decreases at higher concentration of dyes due to the nonavailability of active sites

to support the biosorption. Moreover, the binding capacity of dyes on the surface of

the microbial species depends on several factors such as structure, ligands present

on the surface, surface area, and differences observed in morphology and division of

the yeast. In another study [35], it was explained about the removal or detoxification

of the reactive dyes (Drimarene dyes) in the presence of Aspergillus foetidus.

13.8

Metal-Complexed Dyes

The negatively charged highly solubilized metal-chelated dyes are used for improv-

ing the light fastedness and dyeing of the polyamide fibers and proteins in the

textile and tanning industries. The important metals used for the formation of

metal-complexed dyes are cobalt, copper, and chromium. Different types of colored

dyes from bright black to greenish-yellow can be developed depending on the

usage of metal ions, functional groups of the dyes, and the complexation of metals

and dyes. At the same time, the aggregation of dye molecules and decrease in the

solubility of the dyes can be achieved by the addition of salts like NaCl in the dyeing

and finishing industries. The salts change the salinity of the water bodies and it

affects the aquatic species [5]. The biosorption of textile dyes on the microbial

surface mainly depends on various factors such as the chemistry of the dye, micro-

bial type, surface properties of the microbes, and physiochemical characteristics

such a pH, temperature, ionic strength, and the presence of organic and inorganic

ligands in the solution. The fungal biomass A. niger, Rhizopus arrhizus, Neurospora

crassa, and Phanerochaete chrysoporium are the economically cheap biomass

which was often used in the remediation of dyes. Yellow RL or cobalt complex

formazan dye derivatives are used in tannery and fabric industries for coloring the

polyamide, natural silk, leather, and wool materials. The molecular structure of

cobalt-complexed dye (Formazans) is shown in Figure 13.5.

The formazan dyes are produced from the reduction of aqueous soluble tetra-

zolium salts which have the capability of formation of complex with metal ions

to produce symmetric and unsymmetrical metal (iron)-complexed dyes. Cobalt,

copper, and chromium trivalent MCDs are used in several industries, but these

synthetic dyes cause environmental toxicity. The iron-complexed formazan dyes are