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4 Bioremediation of Toxic Dyes for Zero Waste

dye-contaminated waste, because microorganisms change the chemical structure

in an environmentally friendly and cost-effective way. The microbial treatment

produces less sludge and requires less water, and the final product is less toxic when

compared to various physical and chemical treatment processes. The microbial

remediation method has several advantages because it can be performed on-site, is

cost-effective, has few problems, and can be integrated with physical and chemical

methods. However, all these methods differ in terms of efficiency, cost, and envi-

ronmental impact. Therefore, there is a crucial need for all researchers to look for

efficient, inexpensive, and environmentally friendly systems to reduce the dye con-

centration in wastewater to acceptable levels [2]. This chapter summarizes the latest

research on the use of biodegradation methods to remove dyes, including microbial

treatment, recombinant DNA (rDNA) technology, enzyme-mediated dye removal,

immobilization technology, and phytoremediation. The secondary focus is to

discuss the combination of appropriate technologies to form an eco-friendly system.

4.2

Background to Dye(s)

Colorants are chemicals that impart color to the materials in which they are used.

Colorants can be divided into pigments and dyes, and the main difference between

them is their solubility. The pigment retains its granular nature during application

and always combined in a medium applied to the surface. On the other hand, the dye

is soluble and diffuses into the material, becoming an indispensable part. Among

all types of synthetic dyes used for commercial purposes, azo dyes are the most

widespread used and most toxic among all. This dye is an aromatic compound with

one or more —N=N— groups in the chemical structure. They are extensively used in

many industries, such as textile printing and dyeing, food, cosmetics, paper printing,

etc., among which the textile industry is the largest consumer. A dye has at least one

chromophore group which possesses the color. Besides chromophores, most of the

dyes also contain groups called auxochromes, examples of which are sulfonic acid,

carboxylic acid, hydroxyl, and amino groups. Even though these are not responsible

for the color of dye, their presence can change its color and is most commonly used

to affect its solubility.

4.3

The Toxicity of Dye(s)

The toxicity of dyes has been studied by many researchers and their acute toxicity

is usually low. US regulatory agencies believe that only a few dyes and pigments are

carcinogenic. Except for some azo dyes with free amino groups, azo dyes are rarely

mutagenic or carcinogenic. Under reducing conditions, the azo group can be broken

down to form two aromatic amines, and these intermediate products cause serious

harmful effects on humans and aquatic life. For humans, these intermediates can

damage important organs such as the liver, brain, kidneys, reproductive system, and

central nervous system. They are also known to cause cancer of the human bladder,