Calcium-iron-D-gluconate complexes for the indirect cathodic reduction of indigo in denim dyeing: A greener alternative to non-regenerable chemicals

Abstract

With an annual production of approximately 7–10 billion m2 the manufacturing of indigo dyed denim takes a substantial share in global textile production. At present the vast majority of dyeing processes with indigo still bases on the use sodium dithionite as reducing agent. For a successful replacement of the non-regenerable reducing agent by electrochemical processes, a combination of a regenerable redox system with an appropriate cell design is required. In the presented study, the use of embroidered electrodes in an 85 mA flow-through electrolyser for electrochemical indigo reduction has been investigated for the first time. Indirect cathodic reduction of indigo was achieved with use of a 0.1 M solution of a binuclear complex of calcium and iron with D-gluconate as a ligand. The electrochemically reduced indigo was then used in lab scale dyeing (1–3.5 g L−1 indigo), and the results were compared to reference dyeing experiments with sodium dithionite as reducing agent, by measuring the colour strength and CIElab colour coordinates. Comparison of the electrochemical technique for indigo reduction with the dithionite-based standard process indicates substantial potential for an improvement of the ecological profile of the dyeing process used at present. The electrical power demand for a full scale electrolyser is estimated to be in the range of 2.8–6.9 kW, leading to an energy consumption of 58–145 kWh d−1 as compared to a sodium dithionite consumption of 50–126 kg d−1. Introduction of biobased and regenerable chemical systems instead of non regenerable chemicals in indigo dyeing offers significant potential for reduction of the chemical load released into wastewater. A reduction of the sulphate concentration in the wastewater from the current values of 640–1600 mg L−1 to values well below the legal limit of 200 mg L−1 sulphate will be possible. By reuse of the washing water and the reversible reducing agent, a reduction of both water consumption (by 80%) and chemical consumption (by 50%) are expected.