Study On The Removal Of Trace Metal Impurities From PET Glycolytic Product

The glycolysis of polyethylene terephthalate(PET)is a depolymerization method that has industrial prospect and conforms to the principle of green chemistry development.Studies have found that the metals in the glycolysis catalysts and polycondensation agents would remain in the glycolysis product,resulting in difficulties in product purification and pose a serious threat to the chromaticity and safety of PET.We used inductively coupled plasma atomic emission spectroscopy(ICP)determined that the residual metals in glycolysis product including Zn,Cr,Ca,Ni,Mn,Co and Cd.Considering the harms caused by trace metals,we used active carbon and a series of resins such as 001*7,201*7,D401,D418,D311,XDA-7 and so on to remove trace metals in glycolysis product,and discussed the removal of metal ion in single-metal impurity system and multi-metal impurities system.The main research contents and results were as follows:(1)Removal of Zn(II)in single-metal impurity system.The common adsorbent such as activated carbon and resins were screened,and the best adsorbent for the purification system was 001*7 sulfonic acid cation exchange resin.Subsequently,the conditions such as the dosage of resin,stirring rate,adsorption time and temperature were optimized.The results indicated that more than 99%of Zn2+ can be removed by 001*7 resin with resin dosage of 1.6 g/L at 298.15 K,160 rpm for 120 min and the loss of BHET was only about 2%.In addition,series of characterizations were used to characterize the interaction between metal ion and resin.Various characterization methods indicated that the resin and BHET have undergone π-π interaction and hydrogen bonding interaction,and the adsorption mechanism of 001*7 resin for Zn(Ⅱ)was ion exchange and coordination of lone electron pairs.(2)Removal mechanism of Zn(II)in single-metal impurity system.Common kinetic models,isothermal adsorption models and thermodynamic models were used to fit the adsorption data.The pseudo-second-order kinetic model indicated that the adsorption of Zn(II)on 001*7 resin was dominated by chemical adsorption.Furthermore,the Langmuir model indicated that the adsorption process was a uniform monolayer adsorption.The thermodynamic study of resin adsorption indicated that the adsorption was a spontaneous endothermic reaction,and the disorder at the solid-liquid interface increased during the adsorption process.Furthermore,the purified BHET was re-polymerized into r-PET.After polymerization,the chromaticity of PET was significantly improved.(3)Removal of trace multiple metals and the analysis of removal mechanism.001*7 resin was used as effective adsorbent to remove trace multiple metals in glycolysis product.The results showed that trace metal ions in BHET can be removed effectively with resin dosage of 4.4 g/L at 298.15 K,120 rpm for 120 min,and the chromaticity of r-PET was significantly improved.FTIR,SEM-EDS and XPS were also used to characterize the resin structure after adsorption.Each characterization method showed the similar changes with the single-metal impurity system,manifesting that the main adsorption mechanism of resin for each trace metal were ion exchange and coordination of lone electron pairs.The kinetic analysis of multiple metals impurities system revealed that the adsorption of each trace metal on the 001*7 resin was dominated by chemical adsorption.