Insights into Electrochemical Technology for Dyeing of Textiles with Future Prospects

Vat and sulfur dyes are water-insoluble dyes, and in their reduced form, they have water solubility and exhibit affinity for the application to cellulosic materials. After dyeing, these dyes are oxidized in-situ to return to their original water-insoluble pigment structure. Sodium dithionite (hydrosulfite) and sodium sulfide are the most commonly used reducing agents in conjunction with alkali in aqueous solutions. These sulfur-based reducing agents employed for reduction are toxic, increase the effluent load, and are corrosive. Additionally, these most often used reducing agents cannot be recycled, and the disposal of coloring baths and rinse water is expensive. In an attempt to add sustainability and increase the eco-efficiency of the dyeing of vat and sulfur dyes, several alternative techniques have been investigated for the reduction of dyes. Electrochemistry is a promising alternative where the reduction of dyes is achieved by transferring electrons from the electrode using an electric current. The advancement of electrochemical reduction, including direct electrochemical reduction and indirect electrochemical reduction, has been explored. Electrocatalytic hydrogenation is a new electrochemical reduction process in which in-situ generated hydrogen from the electrolysis of water reacts with dye at the electrode surface for reduction. These techniques are more promising for textile dyeing and offer a lot of environmental and economic benefits.