Comparative study of decolorization processes for dyed cotton waste: enabling circularity through white feedstock

The growing volume of textile waste requires effective decolorization strategies to enable recycling routes. This study compares two-bath and one-bath alkaline-reductive decolorization processes for mixed multi-colored reactive-dyed post-consumer cotton waste using sodium carbonate (Na₂CO₃) and sodium dithionite (Na₂S₂O₄). The decolorizing agents (Na₂CO₃ and Na₂S₂O₄) were selected based on the safe and sustainable by design (SSbD) framework. Process conditions were optimized based on colorimetric measurements, and the properties of the treated fabrics were evaluated using tensile strength retention, weight loss, intrinsic viscosity, and degree of polymerization, X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, and CHNS elemental analysis. The two-bath decolorization processes achieved upto 99% decolorization and and one-bath achieved ≥97% decolorization and lightness values above 80, while maintaining high tensile strength retention and preserving the cellulose crystalline structure. Elemental analysis confirmed efficient removal of sulfur-containing dyes, and thermogravimetric analysis showed that the thermal degradation behavior of decolorized cotton closely approached that of undyed cotton. Compared with the two-bath route, the integrated one-bath process delivered higher lightness, more uniform decolorization, reduced energy and water consumption, and consequently lower wastewater treatment demand. These results demonstrate that the continuous one-bath alkaline-reductive process can provide recyclable cellulosic white feedstock from reactive-dyed cotton waste with improved process efficiency and lower environmental impact, supporting scalable textile-to-textile circularity.