Electrochemical reduction enables clean production of high-concentration sulfur dyes

Sulfur black is the most widely used sulfur dye for coloring cellulose fibers. However, its conventional reduction with sodium sulfide generates sulfur-laden effluents and toxic hydrogen sulfide, which creates severe environmental and operational challenges. Electrochemical reduction offers a sustainable alternative, However, its adoption has been constrained by sluggish electron-transfer kinetics in traditional dye vats. A detachable three-dimensional electrolytic cell was developed that enables efficient electrochemical reduction of sulfur black, using electrons in place of sodium sulfide. Process parameters were optimized by monitoring redox potential, and high-concentration reduction (300 g/L) was achieved under simulated factory conditions. Electron microscopy and UV-visible spectroscopy revealed that, compared with sulfide reduction, electrochemically reduced dye exhibited finer dispersion, higher absorbance, and better preservation of conjugated structures. Dyeing trials demonstrated an 11.5% increase in the K/S value of cotton fabrics, yielding a deeper black with higher chromatic purity, reduced fiber damage, and improved mechanical strength. This approach eliminates hydrogen sulfide emissions and reduces alkali demand, coupling environmental benefits with enhanced dyeing performance. These findings establish a sustainable strategy for sulfur dyeing and advance clean production in the textile industry.