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. 2021 Apr 29;6(18):12351–12360. doi: 10.1021/acsomega.1c01477

Ion-Exchange Resins for Efficient Removal of Colorants in Bis(hydroxyethyl) Terephthalate

Rong Huang †,‡, Qi Zhang †,‡, Haoyu Yao †,‡, Xingmei Lu †,‡,§,∥,*, Qing Zhou †,‡,∥,*, Dongxia Yan †
PMCID: PMC8154176  PMID: 34056387

Abstract

graphic file with name ao1c01477_0012.jpg

Bis(hydroxyethyl) terephthalate (BHET) obtained from waste poly(ethylene terephthalate) (PET) glycolysis often have undesirable colors, leading to an increased cost in the decoloration of the product and limiting the industrialization of chemical recycling. In this work, eight types of ion-exchange resins were used for BHET decoloration, and resin D201 showed an outstanding performance not only in the decoloration efficiency but also in the retention rate of the product. Under the optimal conditions, the removal rate of the colorant and the retention efficiency of BHET were over 99% and 95%, respectively. D201 showed outstanding reusability with five successive cycles, and the decolored BHET and its r-PET showed good chromaticity. Furthermore, the investigations of adsorption isotherms, kinetics, and thermodynamics have been conducted, which indicated that the decoloration process was a natural endothermic reaction. Adsorption interactions between the colorant and resin were extensively examined by various characterizations, revealing that electrostatic force, π–π interactions, and hydrogen bonding were the dominant adsorption mechanisms.

1. Introduction

Glycolysis is the most promising method in waste poly(ethylene terephthalate) (PET) recycling.1,2 It has been practiced by well-known enterprises such as DuPont, Kodak, Dow Chemicals, and Goodyear.3,4 The main glycolysis product of PET is the monomer bis(hydroxyethyl) terephthalate (BHET), which is the monomer of PET and can be used directly as a raw material for the synthesis of PET to form a closed loop. Moreover, BHET, an important chemical and industrial material, has been widely used in scientific research and industrial production. For example, it is used for the production of various high-value products such as polyurethane, hydrophobic textile dyes, alkyd resins, and waterborne coating.5 Although the recovery method of PET via glycolysis is promising,6 there are still some problems worth attention.

In general, catalyst development, BHET purification, and its decoloration are the main difficulties in glycolysis industrialization. Nowadays, many researchers focus on the first two,7−11 while the bottleneck problem is the high chromaticity of BHET, but it has attracted little attention. The heavy-colored BHET limits its high-value usage, and the r-PET regenerated from it is usually substandard in chromaticity. To reduce the chromaticity, traditional decoloration methods are adopted, which greatly increase the process cost and hinder the industrialization process. However, the problems related to BHET decoloration has not been solved yet.12−14 Bearing in mind the aforementioned dilemma, the decoloration of BHET is pressing.

In this regard, various measures have been attempted for the decoloration of BHET, including recrystallization, distillation, oxidation, and active carbon adsorption.15−17 Among these methods, the recrystallization process is quite simple, but the solvent separation is difficult; the additives from the polymer still exist and BHET loss is large. The oxidation method can achieve significant decoloration efficiency, whereas it also causes secondary pollution and even result in the destruction of BHET’s molecular structure. The vacuum distillation method is desirable with less waste generation and low requirements for raw materials. However, BHET refining is not feasible due to the likelihood of repolymerization.11 Adsorption, a common physicochemical measure, is very popular because of its excellent efficiency, minimal cost, and super less sludge accumulation. Among many adsorbents, active carbon is the most common one, while it is far from ideal due to the difficult reproduction which leads to high cost. As a result, the majority of used active carbon is treated as solid waste, which is contrary to the concept of sustainable development.18−20 The existing conventional decoloration methods and adsorbents are generally problematic. Therefore, searching for a new adsorbent that can effectively remove the colorant from BHET and can be regenerated easily is an urgent issue.

Recently, resins have been widely used in the decoloration and separation of sugar liquors,21−23 medicine,24,25 foods26 and so on,27−29 owing to its easy regeneration and high selectivity. Qiang et al. used resins to removal the hydrophobic caramel pigments from cane molasses, and the anion exchange resin can achieve 99% decoloration rate.30 The colorants used in PET products, especially in textile, are mainly disperse dyes and reactive dyes, among which, reactive dyes are more difficult to treat.31,32 The reactive dyes usually appear in an ionic state in solution, which makes the decoloration through ion-exchange resins highly advantageous. However, the theoretical research is lacking, and we hope to reveal a gentle and effective method for removing colorants from BHET and usher in the new plastic economy.

Therefore, eight ion-exchange resins were used to evaluate the decoloring performance according to their color removal rate and BHET retention rate for the first time. The decoloration factors and the interactions between the adsorbent and colorant were investigated. The adsorption isotherm, kinetic, and thermodynamic experiments together with different adsorption models were analyzed. Numerous characterization methods were employed to discover the interactions between the adsorbent and colorant. A comprehensive and detailed recognition of the decoloration mechanism was obtained via this theoretical study.

2. Results and Discussion

2.1. Adsorption Parameters and Reusability

2.1.1. Adsorbent Selection

Ion-exchange resins with different functional groups were carried out to select the most suitable adsorbent for the decoloration and purification of the PET glycolysis product. As shown in Figure 1, the decoloration rate and loss ratio of BHET on different resins were distinct.33,34 Among these resins, D201, JK206, and D202 showed decent efficiencies in the decoloration process, which can achieve 99.16%, 99.87%, and 83.65%, respectively. Moreover, the decoloration rates of strong base ion-exchange resins were higher than those of weak base resins: strong [JK206 (99.87%) > D201 (99.16%) > D202 (83.65%) > D296 (74.95%) > IRA-410 (69.33%)] > weak [D301 (51.98%) > D900 (43.44%) > D392R (31.09%)]. Thus, the functional group played a vital role in the decoloration stage, and the more basic the functional group of the resin, the better its decoloration performance. Besides, the superiority in the decoloring performance of D201 and JK206 may be caused by its pore diameter distribution. In addition, in separate and purification technology, product selectivity is also an essential evaluation index. Li et al. used modified activated carbon to remove the color of BHET, and the decoloration rate was 97%. However, the retention efficiency of BHET was not mentioned.17 In this study, the loss ratio of BHET was investigated for the first time. All these resins caused BHET loss in a more or less amount. The loss ratio of BHET was JK206 > D202 > D201. Therefore, to reduce the loss of BHET, further investigations on the decoloration of BHET were carried out using D201.

Figure 1.

Figure 1

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Properties of different ion-exchange resins for BHET decoloration.

2.1.2. Influence of Temperature

Temperature’s impact on BHET decoloration by D201 was investigated using a fixed original concentration in 1 h (Figure 2a). The increase in temperature was in favor of the sorption process of the colorant, which hinted that the adsorption of the colorant onto D201 was a natural endothermic reaction. On the one hand, the increased temperature endowed colorant molecules with a much higher kinetic energy; hence, the adsorbates can reach active sites with a higher energy. On the other hand, the boundary layer thickness around the resin decreased with an increase in temperature; hence, the mass transfer resistance of the colorant decreased consequently.35 Therefore, the increase in temperature promoted adsorption. However, the loss degree of BHET was also sensitive to temperature change, and heating was not conducive to the retention of BHET. Hence, room temperature was the relatively optimal condition, which can not only save energy but also achieve the ideal purification goal in both the decoloration and selectivity of BHET.

Figure 2.

Figure 2

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Different effects on the decoloration of BHET: (a) contact temperature, (b) resin dosage, (c) contact time, and (d) the influence of regeneration cycles via D201.

2.1.3. Influence of Resin Dosage

An optimal concentration of the adsorbent not only can achieve the ideal effect but can make the process more economic. As evident from Figure 2b, the adsorption rate of the colorant increased dramatically with an increase in the adsorbent dosage, and from 0.1 g/50 mL to 0.15 g/50 mL, the prominent changeless decoloration rate over 99% kept constant with an increase in the dosage. In contrast, a slight increase in BHET loss was caused with an increase in the mass of resin. These trends probably caused by the available active adsorption sites and the competition intensity between the colorant and BHET. First, with a larger amount of resin, redundant effective adsorption sites were provided, and the colorant was preferentially adsorbed on the resins rapidly. Then, with an excess amount of adsorbent and an extremely low concentration of the colorant after adsorption, the adsorption competitiveness of the colorants was largely “lost” even the interaction between BHET and the adsorbent was quite weak.36 Hence, the loss rate of BHET was increased to some extent.

2.1.4. Influence of Contact Time

Figure 2c shows the effect of time on adsorption. The decolorization rate increased in the first 3 h and for this period, colorant adsorption onto the resins occurred quickly. The motor force of this stage may be caused by abundant blank active sorption sites and the huge difference of the colorant concentration between the liquid–solid interphase.37 After this, the colorant concentration difference gradually decreased, the active sites on the adsorbent surface tended to saturation, and the decolorization rate of the resin increased more slowly and flattened out at 4 h; at this time, the decoloration rate was nearly 99%, and the BHET retention efficiency was over 95%. Then, the colorant adsorption achieved equilibrium at 4 h. After 4 h, there was an increased loss in the BHET product while the decoloration rate remained constant.

2.1.5. Reusability

In general, the recyclability of the selected resin should be fully studied to gain economic viability when applied in practical application.38 As shown in Figure 2d, D201 showed outstanding reusability with a slight decrease in the desorption rate and the decoloration rate after five cycles can still exceed 99%. Besides, the appearance of the reused D201 after five cycles was almost unchanged compared to fresh D201 at low magnification, and only a slight change was observed at a higher magnification with tiny cracks (Figure S1). Thus, D201 resin showed outstanding performance and reusability for colorant removal, which makes it a potential candidate for the decoloration of BHET.

2.2. Adsorption Isotherms

Adsorption isotherms show the distribution of adsorbates in two phases in terms of concentration and pH under uniform temperature.39 To further elucidate the mechanism of colorant adsorption using D201, adsorption isotherm experiments were carried out. As shown in Figure 3a, it was noteworthy that all the isotherms showed the same trends and shapes, and the higher the temperature, the stronger the adsorption capacity of the colorant, suggesting that the decoloration process was endothermic. To have a more comprehensive understanding of the decoloration mechanism, Temkin, Freundlich, and Langmuir isotherm models were proposed, which were expressed in the following equation, respectively.40,41

2.2. 1
2.2. 2
2.2. 3

Figure 3.

Figure 3

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Adsorption isotherm of the BHET decolorization system via D201: (a) adsorption isothermal curve, (b) Temkin, (c) Freundlich, and (d) Langmuir.

In these equations, Inline graphic and Inline graphic are the equilibrium concentration and the adsorption capacity of the colorant, respectively; A is the binding constant of equilibrium relevant to maximum binding energy; and B equals to Inline graphic (T is the temperature in kelvin, R is the molar gas constant (8.314 J·mol–1·K–1), and b is the adsorption heat). Both n and Inline graphic are Freundlich constants, reflecting the sorption amount and intensity, respectively. The Inline graphic is the Langmuir constant concerning the affinity of binding sites. Inline graphic is the maximum sorption capacity (mg·g–1). The fitting parameters of the adsorption isotherm are summarized in Table 1.

Table 1. Isotherm Model Parameters of Colorant Adsorption onto D201.

Temkin isotherm model
temperature (K) A (L·g–1) B (J·mol–1) R2
288.15 38.07 11.15401 0.9784
298.15 1722.80 9.02364 0.8705
308.15 3.29 × 105 7.51728 0.9616
318.15 2.21 × 1015 4.17278 0.9833
328.15 7.42 × 106 11.06643 0.8890
Freundlich isotherm model
temperature (K) N kF ((mg·g–1)(L·mg–1)1/n) R2
288.15 6.23 43.26 0.9938
298.15 10.63 69.95 0.9301
308.15 15.43 95.80 0.9736
318.15 38.52 147.73 0.9872
328.15 18.11 175.28 0.8962
Langmuir isotherm model
temperature (K) kL (L·mg–1) qmax (mg·g–1) R2
288.15 0.00109 107.41 0.9910
298.15 0.00089 131.58 0.9911
308.15 0.00034 142.05 0.9962
318.15 0.00009 173.31 0.9998
328.15 0.00003 231.48 0.9987
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According to the fitting correlation coefficients, the Temkin and Freundlich isotherms did not describe the experimental data well, while the correlation coefficient R2 of the Langmuir isotherm was above 0.991. This suggested that the Langmuir model can depict the adsorption data well and indicated that the decoloration process was uniform monolayer adsorption.42,43 Besides, the equilibrium capacity Inline graphic from the experiments and the calculated maximum capacity Inline graphic from the model were very close. From 288.15 to 328.15 K, the values of Inline graphic were 117.79, 129.37, 143.04, 172.05, and 232.01 mg·g–1, respectively. The values of Inline graphic were 107.41, 131.58, 142.05, 173.31, and 231.48 mg·g–1, respectively. The values of the separation factor RL that are related to the adsorption process were in the range of 0–1 (Table S1), indicating that the adsorption was favorable. The pH value of solution is a significant factor that affects the equilibrium, and the adsorption isotherm under different pH values is shown in Figure 4.

Figure 4.

Figure 4

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Adsorption isotherm at various pH values.

As shown in Figure 4, with an increase in the pH value, the equilibrium adsorption capacity decreased rapidly and then tended to be flat. This may be caused by a change in resin’s surface charge properties. As shown in Figure S2, the point of zero charge (pHpzc), in which the surface of the resin was electrically neutral, was determined, and the pHpzc of D201 was found to be 6.70. When pH < pHpzc, the D201 surface was occupied by excess H+ ions and in a positive charge state. Besides, the colorant was anionic and contained deprotonated sulfonic clusters in solution.44 Hence, a strong electrostatic force occurred between the colorant and D201.20 When pH > pHpzc, the resin was occupied by excess OH– ions and in a negative charge state; hence, the electrostatic repulsive force was observed between the resin and colorant, and the adsorption attraction was very week.45 An increase in the equilibrium capacity at a lower pH implied that decoloration was probably due to electrostatic force along with ion exchange.

2.3. Adsorption Kinetics

Adsorption kinetics is an important factor reflecting chemical reaction and mass-transfer processes. The colorant adsorptions via D201 resin with time were investigated. Pseudo-first-order and pseudo-second-order kinetic models were used to fit the experimental data.46,47

2.3. 4
2.3. 5
2.3. 6
2.3. 7

where Inline graphic and Inline graphic are the sorption capacity in equilibrium and at time t, respectively; k1 and k2 refer to first-order and second-order rate constant, respectively; H is the indicator of the initial adsorption rate; and Inline graphic and C represent the diffusion coefficient and the intercept of the curve, respectively. Figure 5 and Table 2 show the fitting results. Both the pseudo-first-order and pseudo-second-order kinetic models can elucidate the adsorption behavior of the colorant onto the anion exchange resin properly. In detail, the decoloration process of BHET was more in line with the second model (R2 > 0.990), which suggested that the adsorption process was dominated by chemiadsorption.48 The value of H for decoloration increased rapidly with a higher temperature and then decreased slightly, which also reflected that the adsorption rate at higher temperature was significantly faster than that at lower one.

Figure 5.

Figure 5

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Adsorption kinetics study of the decolorization system.

Table 2. Kinetic Parameters of Colorant Adsorption onto D201 Ion-Exchange Resin.

model temperature (K) k1 (h–1)     R2
pseudo-first-order 288.15 0.94     0.9710
298.15 1.35     0.9875
308.15 1.58     0.9823
318.15 2.38     0.9847
328.15 1.62     0.9809
  temperature (K) k2 (g·mg–1·h–1) H (mg·g–1·h–1)   R2
pseudo-second-order 288.15 0.03 28.59   0.9909
298.15 0.04 100.32   0.9995
308.15 0.06 210.07   0.9961
318.15 0.07 202.81   0.9973
328.15 0.09 192.00   0.9989
temperature (K) kdif1 (mg·g–1·h–1/2) kdif2 (mg·g–1·h–1/2) kdif3 (mg·g–1·h–1/2) R12
intraparticle diffusion 288.15 19.36 16.66 8.85 1.0000
298.15 58.11 20.79 2.54 0.9984
308.15 63.02 13.34 2.34 0.9685
318.15 62.73 12.83 0.98 0.9962
328.15 58.78 10.55 0.88 0.9997
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The diffusion of the colorant into the pores of fresh D201 is of great significance. Thus, the intraparticle diffusion model was taken into consideration. According to the fitting result of the intraparticle diffusion model, all plots in three linear parts showed similar characteristics (Figure 6). The deviations of the straight lines from the origin in Figure 6 indicated that the intraparticle diffusion was not the only rate-limiting step. Moreover, the higher the intercept C value, the more important role the external diffusion played in the decoloration process. Moreover, the values of Inline graphic in every segment followed the order: Inline graphic; they represent the exterior surface adsorption step, the interior surface adsorption step, which is controlled by intra-particle diffusion, and the equilibrium adsorption step for which the colorant moves from larger pores to micropores in a tardy rate, respectively.40 In the initial segment, as a result of vastly available adsorption sites on the surface of D201, the removal rate of the colorant was instantaneous. Besides, the maximum value of Inline graphic hinted that the external diffusion played a dominant role in the decoloration dynamics.

Figure 6.

Figure 6

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Intraparticle diffusion model.

2.4. Thermodynamic Study

The increased value of qe with a higher temperature elucidated that the decoloration process was endothermic. The thermodynamic parameters Inline graphic, Inline graphic, and Inline graphic related to the decoloration were calculated from the van’t Hoff plot equation. The activation energy Inline graphic was calculated by the Arrhenius equation (eq 11), where K is equal to k2 in the pseudo-second-order model.47

2.4. 8
2.4. 9
2.4. 10
2.4. 11

As shown in Table 3, the value of Inline graphic was positive and exceeds 40 kJ·mol–1, indicating a chemical adsorption process, and a higher temperature was conducive to decoloration.49 The enhancement in the randomness of the decoloration system can be confirmed by the positive value of entropy Inline graphic. Above room temperature, the negative value of Inline graphic hinted the spontaneous decoloration process and Inline graphic gradually decreased with a higher temperature, which indicated that within the scope of experimental investigation, the temperature was one of the controlling factors of the decolorization rate, and the increase in temperature can promote the spontaneous reaction. The Inline graphic value for the colorant adsorption was found to be 21.52 kJ·mol–1, indicating that the BHET decoloration procedure was favorable.20

Table 3. Thermodynamics Parameters and Activation Energy.

R2 △H (kJ·mol–1) △S (J·K–1·mol–1) △G (kJ·mol–1)
 Ea (kJ·mol–1)
      288.15 K 298.15 K 308.15 K 318.15 K 328.15 K  
0.9988 40.02 134.80 1.18 –0.17 –1.52 –2.87 –4.22 21.52
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2.5. Characterization

Figure 7 shows the FTIR results of BHET, colorants, and D201 before and after adsorption. A small stretching vibration peak of carbonyl appeared at 1715 cm–1, which represented that the resin still had a small adsorption effect on BHET.50 A new strong absorption peak appeared at 1130 cm–1, which was the characteristic absorption peak of the sulfonate S=O, and the peak in 1486 cm–1 was attributed to the methyl C–H of the quaternary ammonium group in D201 resin; the increase in intensity hinted that the quaternary ammonium played an indispensable role in the decoloration process. The new vibration absorption peak of C–Cl at 615 cm–1 further proved the adsorption of the colorant. The peak at 1630 cm–1 red-shifted to 1607 cm–1, and the width of the peak also expanded significantly, suggesting that the benzene ring in D201 and colorant showed π–π interactions during the decolorization process. Thus, the adsorption process was mainly influenced by electrostatic forces, π–π interactions, etc. In addition, according to the molecular structure of BHET, colorant, and D201, hydrogen bond could work. However, this interaction is too weak to be proved by FTIR. Thus, the possible decoloration mechanism is shown in Figure 8.

Figure 7.

Figure 7

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FTIR spectra of the decoloration system.

Figure 8.

Figure 8

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Schematic diagram of the decolorization mechanism.

To better understand the physicochemical characteristics of D201 resin before and after the decoloration process, SEM and BET were also performed. As observed in Figure 9, the appearance and structure of the D201 particle did not change at a low magnification, which indicated that the resin had strong mechanical properties and did not break easily. At a higher magnification, the fresh D201 had a smooth surface and a large number of pores, while the one after adsorption appeared much rougher and its pores became smaller. Table 4 presents the BET characteristics of anion exchange resins, and it shows that the D201 resin had a large average pore size, which could be conducive to the colorant diffusion.27 Moreover, according to the BET analysis, the pore diameter and pore volume of fresh D201 were larger than those of the used one, which was consistent with the phenomenon observed in Figure 9.

Figure 9.

Figure 9

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SEM images of ion-exchange resin (a) before adsorption and (b) after adsorption.

Table 4. Physicochemical Properties of D201 before and after Adsorption.

resin surface area (m2·g–1) pore volume (cm3·g–1) pore diameter (nm)
D201 6.933 0.0484 31.96
D201 after adsorption 6.857 0.0435 29.21
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2.6. Chroma Analysis

The BHET samples before and after decolorization with D201 and the regenerated r-PET are shown in Figure 10, and their chroma analysis results are shown in Table 5. Evidently, the color of BHET changed significantly from red to white after decoloration, and the color of r-PET regenerated from undecolorized BHET was yellow, while the one from the decolored BHET was obviously white, and its whiteness value L ≥ 80 and b value ≤ 2, which met the requirements of the bottle-grade PET. Moreover, when compared with other decolorization methods, decolorization method using the D201 resin not only can show an excellent decoloring performance but also can achieve a good quality in r-PET (Table S2).11,17,51 Thus, the decoloration method of BHET by the D201 resin was feasible.

Figure 10.

Figure 10

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BHET and r-PET: (a and c) before decoloration and (b and d) after decoloration.

Table 5. Chroma Analysis Results of BHET and r-PET.

sample chromaticity
  L a B
BHET before decoloration 72.48 25.73 –6.34
BHET after decoloration 94.12 –0.10 0.79
r-PET from colored BHET 78.99 2.55 13.87
r-PET from decolored BHET 94.05 –0.39 1.44
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3. Conclusions

An efficient decoloration method for waste PET glycolysis product BHET was developed using ion-exchange resins with two main evaluation indexes (the decoloration rate of the colorant and loss ratio of BHET) for the first time. Under the optimal condition (25 °C, 0.1 g·50 mL–1, and 4 h), the decoloration rate of the colorant and the retention rate of BHET were over 99% and 95%, respectively. The regeneration performance of D201 was outstanding with five successive cycles, and the decolored BHET and its r-PET showed good chromaticity. The interaction between the resin and colorant was mainly attributed to electrostatic forces, π–π interactions, and hydrogen-bonding interactions. Finally, the investigation of the adsorption isotherms at various concentrations and pH values, kinetics, and thermodynamics indicated that the sorption was a monolayer endothermic chemical sorption conformed to the pseudo-second-order model. In conclusion, the aforementioned excellent performances of D201 resin provide a good prospect for BHET decoloration, and we believe that this green and effective decoloration method can provide waste PET glycolysis product a wider area of application.

4. Experimental

4.1. Materials

PET particles were obtained from Jingdong Commercial Co., Ltd. Zinc acetate, urea, ethylene glycol, methanol, hydrochloric acid (HCl), sodium chloride (NaCl), sodium hydroxide (NaOH), active carbon, and ethylene glycol antimony (C6H12O6Sb2) were obtained from Kepujia Reagent Co., Ltd. The colorant (Reactive Red X-3B, the chemical structure is shown in Figure S2) was purchased from Karma Reagent Co. Ltd. The reagents were directly used without purification. Ion-exchange resins (IRA-410, D201, D301, D900, D392R, JK206, D202, and D296) were obtained from Langfang Sennart Chemical Company, and their basic properties are listed in Table S3. The resins were used after pretreatment.25

4.2. Preparation of BHET and r-PET

The glycolysis product BHET was obtained from the waste PET decomposition using the depolymerizing agent EG and catalyst. The synthesis of the catalyst (n(urea)/n(ZnCl2) 4/1) was the same as reported previously, and the mass of the catalyst and EG was 0.05 and 4 times of waste PET, respectively.9 A three-necked rounded-bottom glass flask with a reflux condenser, magnetic stirrer, and thermometer was employed. The reaction was conducted at 449.15 K until the complete degradation of PET. Then, deionized water was used for the separation of BHET from the reaction mixture, and the obtained crude BHET products were purified by active carbon treatment and water recrystallization several times, and then they were dried at 343.15 K for 24 h. Then, the colored glycolysis product solution was simulated by adding the colorant (Reactive Red X-3B) to control it as the only colorant source, since it was commonly used in polyester blended fabrics and was an important source of color. The concentrations of BHET and the colorant were 10 g/L and 100 mg/L (the volume of methanol was equal to that of distilled water), respectively. The synthesis of r-PET was carried out in a sealed flask under mechanical agitation. The usage amount of the catalyst (ethylene glycol antimony) was 0.1 wt % of BHET. The reaction conditions were kept at a nitrogen atmosphere and heated and evacuated.

4.3. Adsorbent Performance Test

A fixed mass of ion-exchange resins (IRA-410, D201, D301, D900, D392R, JK206, D202, and D296) of 0.1 g were put into Erlenmeyer flasks containing 50 mL of colored BHET solution. The flasks were shaken at 298.15 K for 12 h with a constant agitated rate. The supernatant after adsorption was determined by UV and HPLC to obtain the concentration of the colorant and BHET, respectively. The equilibrium adsorption capacity of the colorant, Inline graphic (mg·g–1), decoloration rate (R%), and the loss ratio of BHET (L%) were calculated by the following equations:

4.3. 12
4.3. 13
4.3. 14

where Inline graphic (mg·L–1) and Inline graphic (g·L–1) are the initial concentrations of the colorant and BHET, respectively; Inline graphic (mg·L–1) and Inline graphic (g·L–1) are the residual concentrations of the colorant and BHET after reaching adsorption equilibrium, respectively; Inline graphic (g) is resin mass; and Inline graphic (L) represents the volume of colored BHET solution.

4.4. Batch Adsorption and Desorption

The adsorption experiments of the colorant and BHET onto resin were carried out and possible influence factors including sorption time, temperature, and resin dosage were analyzed. The temperature range was 288.15–328.15 K. The resin dosage was varied from 0.02 g/50 mL to 0.15 g/50 mL. The contact time was in the range of 0.5–6 h. The sorption–regeneration cycles were repeated five times under constant conditions. Desorption experiments were conducted with 20 wt % HCl solution (the usage amount was 1 g of resin for 1 L of eluent and the volume of methanol was equal to distilled water) for a day. Then, the resin was removed, washed to neutral, and reused in the following decoloration tests. The efficiency of regeneration (RE%) was computed via eq 15.

4.4. 15

where Inline graphic, Inline graphic, and Inline graphic are defined as in eq 12; Inline graphic is the colorant concentration in the regeneration liquid after desorption; and Inline graphic is the desorption liquid volume.

4.5. Isotherm and Kinetic Experiments

Decoloration isotherm studies using D201 as an adsorbent were investigated at different initial colorant concentrations and pH values, which were in range of 100–700 mg/L and 1–11, respectively. The adsorption temperature was in the range between 288.15 and 328.15 K. Decoloration kinetics studies were carried out by placing D201 contacted with colored BHET solution at different temperatures and time intervals. The adsorbed amounts of the colorant on resin were determined by eq 12, where the adsorption capacity Inline graphic at time t (mg/g) replaces Inline graphic. Other parameters of eq 12 represent the same meaning.

Acknowledgments

This work was financially supported by the National Key Research and Development Program of China (no. 2019YFC1908204), the National Natural Scientific Fund of China (nos. 21776289, 21978291, 21878292, and 21908232), the Innovation Academy for Green Manufacture, Chinese Academy of Sciences (nos. IAGM2020C21 and IAGM2020C12), and the K. C. Wong Education Foundation (no. GJTD-2018-04). The authors sincerely thank Prof. Suojiang Zhang (IPE, CAS) for his careful academic guidance and great support.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c01477.

  • SEM characterization of the fresh D201 and reused D201, determination of point of zero charge, information about the colorant, RL values of the adsorption isotherm, comparison of different BHET decoloration methods, and resin’s basic information (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c01477_si_001.pdf (251.7KB, pdf)

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