Efficient decolorization of reactive dyed cotton fabric with a two-step NaOH/Na 2 S 2 O 4 process

Waste cotton textiles can be decolorized with the NaOH/Na 2 S 2 O 4 reducing system with mild and effective results. Nevertheless, this system consumes substantial amounts of Na 2 S 2 O 4 , resulting in the decomposition of Na 2 S 2 O 4 (insurance powder) into H 2 S and SO 2 , which is ineffective decomposition. The present study proposed a two-step NaOH/Na 2 S 2 O 4 method to decolorize the reactive-dyed cotton fabric was proposed to improve the decolorization efficiency of insurance powder. The decolorization mechanism of NaOH and Na 2 S 2 O 4 was investigated based on the decolorization kinetics. The results showed that Na 2 S 2 O 4 decolorized the dyed cotton fabric more quickly than NaOH. In the reduction system, the decolorization mechanism of NaOH and Na 2 S 2 O 4 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 Na 2 S 2 O 4 and the dye destroyed the chromophore. The two-step NaOH/Na 2 S 2 O 4 method improved the decolorization of dyed cotton fabric by reducing the ineffective decomposition of Na 2 S 2 O 4 . 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. Graphical abstract