Synthetic Dyes for Textile Colouration: Process, Factors and Environmental Impact

Dyes are substances that enhance the appearance of finished goods by providing uniform colour. Dyeing is the application process of dyes that are fixed in textile fibers. The dyeing process uses various steps, including pretreatment, desizing, scouring, bleaching, carbonizing, degumming, and mercerization. Water is considered the primary medium for applying dyes and other chemicals for this treatment in the dyeing process. The dyeing process contains many toxic chemicals, metals and non-soluble substances, such as wastewater, which is thrown into the environment. It is noted that, during dyeing, around 50-300 liters of water is required for 1 kg of fiber. On the other hand, 1-2 million liters of water are needed every day for 50,000 meters of daily production. This massive amount of toxic wastewater is highly harmful to the environment, human health, aquatic system, soil, air, water, plants, and animals. This review paper states various dyeing processes with different dyes, including acid dye, direct dye, sulfur dye, reactive dye, vat dye, metal complex dye, azo dye, disperse dye, and basic dye. In contrast, it also describes the influencing factors of dyeing processes and methods. Moreover, this review demonstrates the impacts of textile dyes on the environment (water, soil, air emissions, human health) and illustrates possible remedial actions regarding wastewater.