Removal of Zn2+ from glycolytic monomers of the polyethylene terephthalate based on electrodeposition

Abstract

The usage of zinc-containing catalysts in glycolysis causes the residue of Zn2+ in the polyethylene terephthalate (PET) degradation monomer bis-hydroxyethyl (BHET), which limits the high-value utilization of BHET. This work first used electrodeposition combined with one-time recrystallization to purify BHET. Electrodeposition was suitable for dealing the Zn2+ concentrations of degradation solutions within 50–100 ppm, and decreased the content of Zn2+ in BHET from 141.30 to 2.41 ppm, which reached the standard of repolymerization into PET bottles. The removal rate of Zn2+ was 90 % for the degradation solutions, which decreased the concentration of Zn2+ in wastewater from 100 to 8.97 ppm. Meanwhile, the Zn2+ removed from the degradation solutions was deposited on cathode, which obtained metal Zn materials and realized the recycling of Zn resources from Zn(OAc)2. Therefore, electrodeposition could realize the purification of BHET, recycling of Zn(OAc)2, reducing the environmental pollution caused by the emission of Zn2+ and the protection of human health, simultaneously. Electrodeposition had less water consumption, lower loss rate of BHET (from 60 % to 10 %) and shorter processing time, compared with six-times recrystallizations. This study first proposed the formation of quadridentate between BHET and Zn2+, and put forward a new three-steps electrodeposition mechanism of Zn2+: the addition of electric field destroyed the dissociation equilibrium of quadridentate and made it shift to release free Zn2+; the free Zn2+ gathered to form ionic groups and they migrated to copper; Zn2+ reduced on copper, of which the first step was the speed limited step.