Removal of disperse dye from alcoholysis products of waste PET fabrics by nitric acid-modified activated carbon as an adsorbent: Kinetic and thermodynamic studies

Decolorization technology is a critical problem of high-quality chemical recycling and recovery of waste poly(ethylene terephthalate) (PET) textiles. In order to deal with this problem, nitric acid-modified activated carbon (AC-HNO ₃ ) was utilized as an adsorbent for removal of C.I. Disperse Red 60 (DR60) from the glycolysis products of waste PET fabrics. The glycolysis product was bis(2-hydroxyethyl) terephthalate (BHET). The pore structure and surface properties of AC and AC-HNO ₃ samples were characterized by N ₂ adsorption, differential thermogravimetric analysis (DTG) and elemental analyses (EA). Modification with nitric acid increased the amount of oxygen groups of the activated carbon from 12.3% to 18.0% and enhanced the electrostatic attraction between dye molecules and activated carbon. The average color removal ratio of DR60 on AC-HNO ₃ increased into 97.6 ± 0.5%, which is better than that on raw AC (85.0 ± 1.6%). Relative whiteness of BHET increased from 40.3 ± 0.9% to 98.3 ± 1.1%. The adsorption kinetics for DR60 on the AC-HNO ₃ were studied using pseudo-first order and pseudo-second order models and fitted to the latter well ( R ² = 0.9999). To investigate the adsorption equilibrium behavior, Freundlich and Langmuir models were examined. The results showed that the Langmuir model provided better correlation ( q _max = 163.9 mg/g). The adsorption isotherms at different temperatures were used for the determination of thermodynamic parameters. Based on the data of Δ G ⁰ , Δ H ⁰ and Ea, the adsorption process was physisorption.