Efficient Decolorization of Recovered Bis(Hydroxyethylterephthalate) from Waste Polyester Textiles by Hydrophobic Deep Eutectic Solvents

Glycolysis is one of the most promising closed-loop recycling technologies for PET textiles; however, its efficiency is compromised by the presence of dyes, which inhibit the repolymerization of the depolymerized monomer, bis(hydroxyethylterephthalate) (BHET), into high-quality recycled PET (rPET). In this study, hydrophobic deep eutectic solvents (DES) were employed as extractants to remove colored impurities from the glycolysis solution (GS) of PET glycolysis. An optimized DES system composed of oleic acid and menthol (Men:OleA) achieved a decolorization ratio of up to 89.4 % in a simulated GS with Disperse Blue 56 doped BHET (BHET-DB56). Kinetic analysis revealed that the dye extraction by Men:OleA DES follows a pseudo-second-order kinetic model. Furthermore, DES successfully decolorized nine different colored PET textile GSs, achieving a maximum decolorization rate of 98.2 % after a secondary extraction process. Even after multiple recycling cycles, Men:OleA DES maintained consistent decolorization performance. Through characterization and quantum chemical calculations, the decolorization process was found to be driven primarily by physical interactions, specifically hydrogen bonding and van der Waals forces between dye molecules and DES. This study seeks to provide an effective method for the decolorization of BHET and proposes a strategy for the closed-loop recycling of colored PET textiles.