Study On Decomposition And Decolorization Of Waste Poly(Ethylene Terephthalate) Textiles

Polyethylene terephthalate（PET）has been widely used in food packaging,engineering plastics and textile industries due to its excellent physical,thermal,optical and mechanical properties.In 2020,the global annual production capacity of PET had reached 106 million tons per year,and the output had climbed to 78.2 million tons,resulting in large consumption of non-renewable petroleum resources and a serious threat to the global ecosystem.According to the published data,77.5%of PET production capacity in our country is occupied by synthetic fiber or textile products,while bottle-grade PET accounts for about 18.8%.It can be concluded that the recycling of waste PET textiles is more pressing,but most of the PET recycling is still focused on clean,high-purity,bottle-grade PET without colors.PET textiles containing dyes and auxiliaries are supposed to be economically inefficient in recycling,which are landfilled or incinerated at the end of their service life.Based on the vision of efficient recycling of waste PET textiles,this paper studied on the decomposition of PET and focused on the optimization of reaction conditions and simplify the products purification while improving the efficiency of PET conversion and monomer yield.Subsequently,PET textiles colored by anthraquinone and azo dyes were decomposed and recycled.The feasibility and reaction mechanisms of synchronous degradation and decolorization of PET textiles were investigated.We anticipate that the study will be an important theoretical basis for efficient recycling of waste PET textiles.The main contents are as follows:（1）PET glycolysis was investigated.Magnetically recyclable catalyst Zn-MNPs were synthesized by solvothermal method and applied in PET glycolysis in place of traditional soluble catalysts.The adjustment can simplify the purification of products and minimize the consumption of EG.Results reveal that the as-synthesized Zn-MNPs are endowed with uniform hollow morphology,excellent thermal stability,magnetic recyclability and adsorption capacity.When applied to PET glycolysis,the Zn-rich surface and acid sites of Zn-MNPs significantly improve their catalytic activity.When the dosage of Zn-MNPs is 0.8 wt%and the EG/PET molar ratio is 7:1,PET can be completely decomposed by glycolysis at 196°C within 2 hours with a BHET yield of 79.82%.Meanwhile,different types of PET can be decomposed in glycolysis catalyzed by Zn-MNPs.Compared with other catalysts,the prepared Zn-MNPs are low-cost and thermal stable,which can be separated from the glycolysis system by magnetic decantation and maintain high catalytic activity in a relatively mild glycolysis temperature and EG addition in their recycling.（2）The synthesized catalysts were applied in the recycling of PET textiles colored by different disperse dyes.Results reveal that the decolorization of disperse dyes took place in PET glycolysis,the color removal rate of glycolysis catalyzed by Zn-MNPs was more effective than traditional catalysts,especially for azo dyes.The cleavage of the azo bond in glycolysis increased the color removal rate to 71.91%,but only a few substituents of anthraquinone dyes were removed in glycolysis.After taking the adsorption of Zn-MNPs into account,the average color removal rate of anthraquinone dyes was 54.85%.In order to improve the recyclability of excess EG in glycolysis solution,we took advantage of electro-Fenton technology to decolorize the benzene ring and stubborn additives in residue dyes.The Zn-MNPs recovered by glycolysis were used as dispersing electrodes in electro-Fenton.In this way,the color removal rate of dyes in glycolysis liquid can be enhanced to 97.54%within 2 h,with 2.6 vol%of H₂O₂concentration,0.35 wt%of Zn-MNPs dosage,and 0.19 m A/cm²of current density.The discolored glycolysis liquid can be purified by rectification to recover pure EG.The results show that the simultaneous PET glycolysis and decolorization can be preliminarily realized through the upgraded glycolysis technology,but the consumption of solvent EG is also a waste of resources,so electro-Fenton technology should be supplied for decolorization of residue dyes in glycolysis liquid.（3）For further adjustment of PET decomposition towards efficient recycling of PET wastes and release the burden of solvent consumption,hydrolysis assisted by reactive processing was proposed and applied in degradation of PET wastes.We investigated the main component,the purification and quantification methods of products,as well as the mechanisms of PET decomposition.PET was transformed to a naturally degradable powder by auto-catalytic degradation of Ca（OH）₂and PET in an anhydrous melt mixing,and the powder was purified to trihydrate calcium terephthalate（Ca TP·3H₂O）by further cooking in water.The synthesized Ca TP·3H₂O exhibits good crystallinity and high purity,which can be converted to anhydrous Ca TP when coordinate with carboxyl groups of PET in different modes.The target product Ca TP can be transformed into soluble calcium salts after chelating with sodium hexametaphosphate and quantified.Results reveal that the conversion of PET to Ca TP is 88.2%,the selectivity and yield of Ca TP can reach 95.9%and 84.6%,separately.The up-recycling of PET was realized by an efficient,economic effective methods with no assumption of organic solvents.（4）PET textiles colored by different disperse dyes were decomposed by hydrolysis assisted by reactive processing.The combined action of mechanical shearing force,high temperature sublimation and alkaline hydrolysis accelerates the decolorization of dyes on textile surfaces while the PET is converted to high-purity Ca TP in the reactive process.The obtained Ca TP was acidified to TPA_cafor comparison with TPA_nasynthesized by traditional hydrolysis.It can be concluded that TPA_caobtained from hydrolysis assisted by reactive processing not only exhibits higher selectivity（98.35%）and yield（88.51%）but presents lower residue color from disperse dyes than traditional hydrolysis.The color removal rates of anthraquinone dyes in solid and liquid products are 93.08%and 96.59%,respectively.Meanwhile,the average decolorization rates of azo dyes are 95.37%and 98.71%,respectively.It can be concluded that synchronous degradation and decolorization of colored PET have been successfully achieved through hydrolysis assisted by reactive processing.The success is attributed to higher water and thermal stability of the intermediate product Ca TP,which can not only store the obtained TPA_caduring reactive processing but also serve as a carrier for the alkaline hydrolysis and calcination of residue dyes.