Efficient decolorization of reactive dyed cotton fabric with a two-step NaOH/Na₂S₂O₄ process

Waste cotton textiles can be decolorized with the NaOH/Na2S2O4 reducing system with mild and effective results. Nevertheless, this system consumes substantial amounts of Na2S2O4, resulting in the decomposition of Na2S2O4 (insurance powder) into H2S and SO2, which is ineffective decomposition. The present study proposed a two-step NaOH/Na2S2O4 method to decolorize the reactive-dyed cotton fabric was proposed to improve the decolorization efficiency of insurance powder. The decolorization mechanism of NaOH and Na2S2O4 was investigated based on the decolorization kinetics. The results showed that Na2S2O4 decolorized the dyed cotton fabric more quickly than NaOH. In the reduction system, the decolorization mechanism of NaOH and Na2S2O4 individually followed a three-order kinetic law. During the decolorization process, a hydrolysis reaction between NaOH and cotton fiber destroyed the covalent bond between the dye and the fiber, which was an ether bond, and a reduction reaction between Na2S2O4 and the dye destroyed the chromophore. The two-step NaOH/Na2S2O4 method improved the decolorization of dyed cotton fabric by reducing the ineffective decomposition of Na2S2O4. Compared to traditional methods, the two-step method endowed the decolorized fabric with a 16% higher CIE L value. Notably, the decolorized cotton fabric retained about 90% of its breaking strength.