Decolorization of Real Post-Consumer Polyester Textiles Using a Recyclable Deep Eutectic Solvent

The removal of dyes from textile waste remains a major bottleneck in closed-loop textile recycling. Post-consumer textiles contain complex and often unidentified dye mixtures at unknown concentrations, creating a significant challenge for the development of efficient decolorization processes. To address this issue, a deep eutectic solvent (DES) based on thymol and propionic acid was developed at different molar ratios and evaluated for the decolorization of post-consumer black polyester textiles without prior knowledge of the dye composition. Among the investigated systems, a thymol:propionic acid with a molar ratio of 1:2 achieved dye removal efficiencies of up to 99.35% at a fabric loading of 0.01 g mL−1 after 4 h, while the 1:1 DES achieved >96% removal within 2 h, demonstrating that both formulations are highly effective, with the 1:2 system achieving marginally higher overall extraction. Control experiments with propionic acid alone achieved high dye extraction but left visible dye residues on the fabric, demonstrating that the presence of thymol contributes to more uniform decolorization, faster dye extraction time, and effective dye sequestration. A thymol-rich crude solid was recovered through water-induced crystallization and reused without loss of performance, achieving an average decolorization efficiency of 99.44 ± 0.27%. In addition, the DES was directly reused for at least five consecutive cycles without regeneration while maintaining dye removal efficiencies between 96.9 and 99.3%. FT-IR analysis did not reveal detectable chemical changes in the polyester structure after treatment; however, further mechanical and thermal characterization will be required to assess fiber preservation fully. These findings demonstrate that the thymol:propionic acid DES is a highly effective and reusable solvent system for polyester textiles decolorization, offering a promising and sustainable pretreatment approach for future textile recycling processes.