Sustainable electrochemical dyeing of indigo with Fe(Ⅱ)-based complexes

In order to avoid the intensive burden of sewage treatment in the traditional dyeing process of indigo caused by the heavy usage of sodium dithionite (Na₂S₂O₄), an indirect electrochemical reduction dyeing method for denim fabrics was carried out. The indirect electrochemical reduction dyeing with different iron-based complexes was explored by cyclic voltammetry. In addition, the stability of the complex solution, the reduction potential of the dye solution, the rate of dye reduction (Re) and ferrous ion conversion (Xe) in the electrochemical process were investigated, which showed that the Fe(Ⅱ)-DGS-Abal B complex containing ferrous sulfate, sodium gluconate and Abal B (a complexing agent with high alkali resistance and electrical conductivity based on triethanolamine) had stronger reducing capacity. Furthermore, orthogonal experiments and gray clustering analysis were utilized to optimize the indirect electrochemical dyeing process for denim fabric. Under the optimized electrochemical reduction dyeing process, the rate of dye reduction could be up to 91.23% and the K/S value of dyed denim fabric could reach 14.75, which is 6.58% higher than that of the traditional dyeing process. Besides, the dyeing depth is nearly invariable within 7 times of cyclic dyeing and the dyeing liquid is more susceptible to biological and chemical degradation. Compared with the traditional reduction dyeing with Na₂S₂O₄ at higher temperature, the electrochemical reduction dyeing strategy uses electrical energy to reduce indigo at room temperature, and the reduction medium could be sustainably utilized by the circulation pump after the cyclic electrochemical reduction dyeing is completed, which not only decreases the COD value of dyeing wastewater, but also saves 63.5% of the economic cost. The eco-friendly electrochemical dyeing strategy presented here with obvious economic benefits could significantly contribute to enhance the sustainability of dyeing process for denim production.