Adopting activated carbons on the PET depolymerization for purifying r-TPA

1 Introduction

Polyethylene terephthalate (PET), a common polyester used in various applications such as packaging and textiles, is synthesized through the polymerization of ethylene glycol (EG) and terephthalic acid (TPA) or its dimethyl ester [1]. This process results in long chains of repeating ester units, contributing to desirable properties of PET, including transparency, durability, and light weight. However, the extensive use of PET has unfortunately led to a significant accumulation of plastic waste, posing substantial challenges [2,3,4]. This accumulation leads to overflowing landfills, pollution of waterways and oceans, and harm to wildlife through ingestion and entanglement. Economically, this waste represents a loss of valuable resources and necessitates costly waste management solutions. To mitigate this issue, chemical recycling has emerged as a promising strategy to convert PET waste back into its original monomers or oligomers [5,6]. Here, hydrolysis is a widely used technique that breaks down PET using water in the presence of a catalyst. These catalysts, including tetraoctylammonium bromide (TOMAB), hydrotalcite, tetrabutylammonium bromide (TBAB), and tetrabutylammonium iodide (TBAI), have demonstrated promising results in promoting the breakdown of PET [7,8,9,10]. These phase transfer catalysts enhance the solubility of reactants by shuttling ions between the aqueous and organic phases, significantly boosting the reaction rate. Despite its effectiveness, the depolymerization of PET often generates impurities that can negatively impact the purity and quality of the recycled terephthalic acid (r-TPA). These impurities can hinder the performance and limit the applications of recycled PET. To overcome this challenge, various purification techniques have been explored. In many previous studies, adsorption using activated carbon has gained considerable attention due to its efficiency, versatility, and cost-effectiveness [11,12,13]. Yet, its application in purifying r-TPA during the depolymerization process remains relatively unexplored. Previous research has primarily focused on using activated carbon for treating wastewater from PET production or removing specific contaminants from PET flakes before depolymerization.

This study aims to investigate the effects of incorporating activated carbon treatment on the purity of r-TPA during PET depolymerization. The research involves a two-step process, illustrated in Figure 1: first, depolymerizing PET via hydrolysis to break it down into its constituent monomers, and second, purifying the resulting TPA using activated carbon to remove any contaminants generated during hydrolysis. This two-step process may offer a potentially efficient and sustainable approach to PET recycling, contributing to waste reduction and valuable monomer recovery.

Figure 1

Two-step process to recycle PET.

2 Materials and methods

All reagents were used as received without further purification. Three quilting cotton samples were randomly collected from a municipal waste disposal site (Figure 2). The samples, which contained various impurities such as adhesives and surfactants, underwent depolymerization to recover TPA (Figure 3). Acid hydrolysis, employing a 2 M acetic acid solution at 280°C for 4 h, was used for PET depolymerization. This process facilitated an exchange reaction between the carboxylic acid and ester, resulting in the breakdown of PET into TPA and EG. TPA was then separated from EG via filtration. To enhance the purity of the recovered TPA and remove residual impurities, the solid TPA was dissolved in a NaOH solution and subjected to a multi-step purification process. This process involved centrifugation and filtration through a glass fiber filter and activated carbon. The purified sample was then isolated by titrating with HCl to a pH of 2, followed by washing.

Figure 2

Physical outlook of samples collected from the garbage dump.

Figure 3

Route for recycling PET using activated carbons to remove impurities.

Effective impurity removal is crucial in material processing. Adhesive residues in the quilting cotton samples posed a significant challenge to TPA purification using activated carbon. These residues can block pores and hinder adsorption. To address this, a centrifugation step was introduced to remove the adhesive component in the initial stage of the purification process. A Cryste VARISPIN L6R centrifuge was utilized for this purpose, operating at 300 rpm for 3 h. This centrifugation-based pre-treatment effectively removed the adhesive residues, preventing pore blockage during the subsequent activated carbon purification. The final TPA product obtained after activated carbon treatment exhibited high purity, confirming the effectiveness of this approach.

The acidity of r-TPA samples purified with activated carbons was analyzed by the following method. To confirm the amount of KOH required to neutralize free fatty acids, the degree of free fatty acids present in samples was analyzed by the reaction of R-COOH + KOH → R-COOK + H₂O. Functional groups of each sample were identified using FT-IR 6100 (Jasco) through Fourier transform infrared spectroscopy (FT-IR) analysis. Thermal properties were observed by a thermogravimetric analysis (TGA) using TGA/DSC STARR_e2 (Metler Toledo). To analyze the type of contaminants, ultraviolet-visible (UV) analysis was performed using UV-1900i (Shimadzu). UV analysis was conducted within the range of 200–400 nm using a data interval of 0.1 nm and a medium scan speed. For a better insight in the regeneration efficiency using carbonaceous materials, high-performance liquid chromatography (HPLC) analysis was added to UV using 1200 Infinity Series (Agilent) referring to the standard (ASTM D 7884-20).

3 Results and discussion

The depolymerization of PET can generate various contaminants, including silicone oil, chloroform, benzoic acid, p-toluic acid, and 4-carboxybenzaldehyde. Figure 4 illustrates the physicochemical adsorption mechanism of these contaminants by activated carbon.

Figure 4

Physiochemical adsorption mechanism of contaminants via functional groups on the surface of activated carbon.

The process involves physisorption, where contaminants are absorbed into the micro-, meso-, and/or macro-pores of the activated carbon via van der Waals forces. This mechanism is consistent with previous studies that have demonstrated the effectiveness of activated carbon in removing a wide range of organic contaminants from aqueous solutions through physisorption [14,15]. The remaining contaminants, which may not be effectively removed through physisorption, are subsequently adsorbed onto the surface functional groups of the activated carbon. This secondary adsorption mechanism can involve various interactions, such as electrostatic interactions, hydrogen bonding, and π–π interactions, depending on the nature of the contaminants and the surface chemistry of the activated carbon [16].

To analyze the TPA yield in the PET depolymerization process and TPA production, Equations (1) and (2) were applied [17]. The principle is to assess the efficiency of the TPA production process by determining how much of the potential TPA is actually obtained. A higher yield indicates a more efficient process with minimal losses due to side reactions, incomplete conversion, or purification issues. The calculated result showed a yield of 91.5% each.

The acidity of the r-TPA samples was analyzed according to the American Society for Testing and Materials D 8032-20 standard and compared to a reference TPA sample. As shown in Table 1, the reference sample exhibited an acidity of 675 mg KOH/g. Samples 1 and 2 showed minor deviations within an error range of ± 1 mg KOH/g compared to the reference sample, while Sample 3 displayed an acidity value identical to the reference sample. These results indicate that the activated carbon treatment did not significantly alter the acidity of the r-TPA. This finding is crucial because the acidity of TPA can influence its reactivity and performance in subsequent polymerization reactions to produce recycled PET.

HPLC-UV analysis was conducted to determine the levels of contaminants in the r-TPA samples. The reference sample showed trace amounts of 4-carboxybenzaldehyde, benzoic acid, and p-toluic acid (Figure 5). In contrast, Samples 2 and 3 were free of these contaminants, indicating the effectiveness of the activated carbon treatment in removing these impurities. Interestingly, Sample 1 exhibited a significant increase in 4-carboxybenzaldehyde content compared to the reference sample, along with a minor amount of p-toluic acid. This observation suggests that the depolymerization process, particularly for Sample 1, may have favored the formation of 4-carboxybenzaldehyde, possibly through the oxidation of p-toluic acid or other intermediates.

Figure 5

HPLC-UV analyses of the reference sample and Sample 1.

The results from Figures 4 and 5 suggest that the physical state of the PET waste can influence the effectiveness of activated carbon treatment. Samples that were physically less damaged, such as Sample 1, may have exhibited stronger cross-linking between the contaminants and the PET matrix, hindering the access of activated carbon to these contaminants. This highlights the importance of proper pre-treatment of PET waste, such as shredding or grinding, to enhance the efficiency of activated carbon treatment.

FT-IR analysis was performed to assess the impact of 4-carboxybenzaldehyde on the functional groups of r-TPA (Figure 6(a)). The characteristic functional groups identified in each sample are listed in Table 2. All samples, including the reference and r-TPA samples, exhibited similar FT-IR spectra, indicating the presence of C–H phenyl stretching, C═O stretching, OH group, C–O stretching, C═C stretching, C–OH deformation, and C–H out-of-plane bands [18,19]. This consistency in the FT-IR spectra suggests that the activated carbon treatment did not significantly alter the functional groups of r-TPA, even in the presence of residual 4-carboxybenzaldehyde.

Figure 6

FT-IR results are shown in (a), while TGA and DSC results are shown in (b) and (c), respectively.

TGA was conducted to evaluate the thermal stability of the r-TPA samples and to identify any undetected contaminants (Figure 6(b) and Table 3). The TGA curves of all samples, except Sample 3, exhibited similar thermal characteristics. This similarity indicates that the activated carbon treatment did not significantly affect the thermal stability of r-TPA. The deviation observed for Sample 3, which was physically damaged during collection, suggests that the physical state of the r-TPA can influence its thermal behavior. The relatively weak cross-linking in Sample 3 may have contributed to its altered thermal characteristics.

UV absorbance analysis was performed to further characterize the r-TPA samples (Figure 7). All samples showed a similar absorbance pattern in the 200–400 nm range. Figure 7(b)–(d) exhibited a slight increase in the peak around 250 nm, which, considering the margin of error, suggests the presence of residual TPA. However, Figure 7(e) showed a negligible peak at 250 nm and a distinct peak increase at 260 nm, indicating the presence of organic impurities other than TPA. These findings highlight the importance of optimizing the activated carbon treatment process to minimize residual impurities in r-TPA.

Figure 7

UV absorbance analysis of (a) blank test, (b) reference sample, (c) Sample 1, (d) Sample 2, and (e) Sample 3.

The r-TPA content in the samples was quantified using HPLC analysis, and the numerical values were calculated using Equation (3) (Table 4). Here, A, B, V, D, and W refer to the concentration of TPA in HPLC measured test solution (μg/mL), the concentration of TPA in HPLC measured blank solution (μg/mL), the amount of test solution (mL), the dilution of test solution (g), and the sampling amount (g), 10,000: unit conversion coefficient (μg/mL, ppm → %). The results revealed that Sample 1 had an average TPA content of 96.22%, Sample 2 had 94.56%, and Sample 3 had 94.90%. In comparison, the reference sample had an average TPA content of 97.52%. These values are higher than those reported for hydrolysis using TBAI as a catalyst (90%) but lower than those achieved with TOMAB and TBAB catalysts (99%) [20]. However, it is important to consider that the activated carbon treatment is a relatively environmentally friendly and cost-effective purification method compared to catalyst-based approaches.

4 Conclusions

This study investigated a novel approach to purifying r-TPA obtained from the depolymerization of post-consumer PET waste, specifically quilting cotton, using activated carbon. The two-step process involved acid hydrolysis of the PET followed by purification of the resulting r-TPA with activated carbon. The results demonstrated the potential of this method to produce high-purity r-TPA, achieving a purity of 96.28% and a yield of 91.5%. The characterization of the r-TPA samples also revealed important insights. Acidity measurements indicated that the activated carbon treatment did not significantly alter the acidity of the r-TPA. HPLC analysis confirmed the effectiveness of activated carbon in removing contaminants such as 4-carboxybenzaldehyde, benzoic acid, and p-toluic acid. FT-IR analysis confirmed the preservation of the expected functional groups in the r-TPA. TGA results corroborated this, showing consistent thermal stability between r-TPA and reference for most samples, except Sample 3, which was noted as being physically compromised. UV analysis indicated the presence of residual TPA and other organic impurities, reinforcing the need for optimization and further research. By addressing these future research directions, the activated carbon purification method can be further refined and optimized for large-scale implementation, contributing significantly to more sustainable PET waste management and a circular economy.