Removal of Zn2+ from polyethylene terephthalate (PET) glycolytic monomers by sulfonic acid cation exchange resin

Abstract

Glycolysis of polyethylene terephthalate (PET) has always been a research hotspot in PET waste recovery due to its high efficiency and sustainability. Zinc acetate (ZnAc2•2H2O) and urea/(ZnAc2•2H2O) are usually regarded as the most promising catalysts for the glycolysis of PET, while the residue of Zn2+ in glycolytic monomers (bis(hydroxyalkyl) terephthalate (BHET)) was identified as a big hassle, which caused the detrimental effect on the chromaticity of down-stream product. However, the removal of Zn2+ from BHET has almost not been reported previously. In the present work, ion exchange was adopted as effective measure to eliminate trace Zn2+ from BHET and more than 99% of Zn2+ can be removed by 001*7 sulfonic acid resin with resin dosage of 1.6 g/L at 298.15 K, 160 rpm for 120 min. Additionally, based on prevailing adsorption kinetics, isotherms, thermodynamic models and various precise instruments, comprehensive adsorption mechanism were exploited. Pseudo-second-order model (R2 > 0.999) and Langmuir model (R2 > 0.999) were found to match well with the experimental data, indicating the sorption of Zn2+ on 001*7 was homogeneous monomolecular layer adsorption and dominated by chemisorption. The thermodynamic model revealed the sorption was thermodynamically spontaneous. Fourier-Transform Infrared Spectroscopy (FTIR) illuminated the sulfonic acid groups on 001*7 resin were the main binding sites for Zn2+. X-ray Photoelectron Spectroscopy (XPS) elucidated ion exchange and electrostatic interaction between the sulfonic acid groups and Zn2+ were responsible for the sorption of Zn2+. All the results obtained can prove 001*7 sulfonic acid resin is an efficient material for removal of Zn2+ from BHET.