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Enhancement of terephthalic acid recovered from PET waste using a combination of citric acid and dimethyl sulfoxide extraction

Sustainable Environment Research volume 34, Article number: 16 (2024) Cite this article

Abstract

This study aimed to develop an eco-friendly, cost-efficient, and practically viable method for extracting terephthalic acid (H2BDC) from polyethylene terephthalic (PET) waste. Dimethyl sulfoxide (DMSO) was combined with either citric acid (C6H8O7) or H2SO4 to enhance the particle size of H2BDC, and the optimum conditions during the acidification step were determined. Additionally, response surface methodology was employed to examine the influence and interaction of extractant (NaOH) concentration, hydrolysis temperature, and time on the optimal H2BDC yield and recovery ratio. Experimental results demonstrated that NaOH concentration significantly impacted both H2BDC yield and recovery ratio, surpassing the effects of hydrolysis temperature and time. Under optimal conditions involving a temperature of 200 °C and a 12 h reaction time with 5% NaOH, the model predicted a 100% yield and recovery ratio, which closely matched the experimental results of 99% and 100% for yield and recovery ratio, respectively. To enhance particle size, a combination of DMSO and C6H8O7 was more effective than H2SO4. The maximum particle size achieved was 57.4 µm under the following optimum conditions: premixing 5 M C6H8O7 with DMSO at a 35:75 mL ratio and maintaining a reaction temperature of 75 °C for 40 min. The study demonstrated the stability and consistency of the method. The H2BDC yield remained between 96 and 98% with high purity over eight consecutive cycles of using the DMSO and C6H8O7 mixture. The findings highlight the importance of integrating C6H8O7 and DMSO to enhance H2BDC quality, meeting commercial product criteria with evidence of high purity and large particle size. This method presents a promising solution for extracting H2BDC from PET waste, with potential implications for the recycling industry and a positive environmental impact.

1 Introduction

Plastics have become one of the most widely used materials worldwide, significantly increasing plastic waste generation [1,2,3]. Regrettably, substantial waste finds its way into landfills and marine environments, causing environmental concerns [2, 4]. In 2015 alone, a staggering 6.3 Bt of plastic waste was generated, with only a mere 9% recycling ratio. The rest either underwent incineration (12%) or ended up in landfills and oceans (up to 79%), exacerbating the environmental issues associated with plastic pollution [4,5,6,7,8]. If this trend persists, projections indicate that by 2050, as much as 12 Mt of plastic waste could enter landfills and natural habitats. The effectiveness of plastic waste recycling is significantly impacted by the collection, sorting, and waste processing procedures adopted by each country. Developed economies, represented by Western European countries and Japan, actively promote recycling initiatives by implementing elevated taxation on landfills and incineration to encourage recycling, increasing the plastic recycling ratio by approximately 30% [9, 10]. In contrast, developed economies lacking regulatory incentives tend to rely on conventional waste management practices, such as landfills and incineration, as seen in countries like the USA and Australia, where the recycling ratio of plastic waste is less than 10%. In industrialized developing economies, collection systems are often not well-organized, resulting in a significant portion of household and industrial waste being disposed of at various unofficial and unregulated sites, with generally low recycling ratios for plastics, approximately 20%, for example, China and India [9, 10]. Polyethylene terephthalate (PET) is one of the most commonly used plastics, prevalent in various industries like food packaging, polyester fiber production, plastic films, and others [1, 11, 12]. In 2020, approximately 23.4 Mt of PET were consumed, which is expected to grow at 6.4% [2]. However, the recycling ratio for PET remains alarmingly low [4, 13]. For example, in 2020, Europe's PET plastic bottle recycling ratio was a mere 49%, resulting in a considerable number of PET bottles being discarded as waste daily, causing severe environmental issues due to their high persistence in the environment [2, 13,14,15]. It is worth noting that PET contains 85 wt% of terephthalic acid (H2BDC), a valuable material used for synthesizing carbon materials and as an essential linker for various Metal–Organic Frameworks (MOFs). As such, implementing an effective recycling strategy for PET waste would not only extend its life cycle but also help mitigate plastic contamination while producing valuable materials, ultimately reducing the reliance on non-renewable resources.

Over the past decade, research into plastic waste recycling has predominantly focused on mechanical and chemical methodologies [2, 16, 17]. Mechanical processes have been widely used to recycle PET waste into products like fibers, beverage bottles, and strapping [11, 18]. However, this method often necessitates extensive sorting and reprocessing, decreasing the quality and value of the end products [12]. Consequently, there has been a growing interest in chemical recycling techniques, which are becoming increasingly prominent within the field [16, 19,20,21]. For instance, thermochemical processes like catalytic pyrolysis have shown promise in effectively converting plastics such as high density polyethylene, polyethylene, and polypropylene into high-quality oil products [16, 19]. Conversely, depolymerization methods offer a gentler alternative characterized by high efficiency, low temperature, and low-pressure conditions, reducing costs and energy consumption for PET degradation [16, 22]. This makes depolymerization a more economically and environmentally viable approach for tackling the challenges associated with plastic recycling [19, 23].

Some companies have successfully implemented depolymerization to transform PET into valuable products. Eastman, for example, has developed polyester renewal technology, utilizing alcoholysis and methanolysis to break down PET into ethylene glycol (EG), dimethyl terephthalate, and other components, reducing greenhouse gas emissions by 20–30% [16, 24]. Hydrolysis is a particularly efficient method for PET conversion, breaking down PET waste into terephthalate salt and EG [11, 19, 22] (as shown in Scheme 1). The recovered terephthalate salt can then be further processed into its constituent monomer, Hydrogenated H2BDC, through an acidification reaction (Scheme 2). However, conventional acidification processes using strong acids like sulfuric, nitric, or hydrochloric acid result in significant amounts of strongly acidic and corrosive wastewater, posing environmental challenges and increasing operational costs [11, 25]. To address these concerns, researchers are investigating greener acidification methods to minimize wastewater production and reduce environmental impact. Alternative approaches include employing milder acids or exploring different catalysts and processes to achieve the desired reaction with fewer environmental consequences. The previous literature results indicated that the depolymerization of PET into H2BDC and EG using the hydrolysis with a catalyst, subsequently utilizing H2BDC to synthesize Cr-MOF, achieving high performance for hydrogen storage [5].

Scheme 1
scheme 1

PET hydrolysis process using alkaline

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Scheme 2
scheme 2

Acidification process using sulfuric acid

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Prior research has predominantly centered on extracting H2BDC from PET waste via hydrolysis and acidification techniques, showcasing promising yield and recovery ratio results. However, these studies were constrained by the reliance on hydrolysis catalysts and strong acid solutions for acidification. Moreover, previous investigations need to pay more attention to main aspects such as acid solution reusability and the determination of the optimum conditions for efficient H2BDC extraction from PET waste. In 2020, Jung et al. identified optimal conditions for hydrolyzing powdered PET utilizing an ultrasonicator method, yielding a substantial H2BDC output [26]. Nevertheless, their approach involved using tetrabutylammonium iodide (TBAI) as a catalyst and sulfuric acid for H2BDC precipitation without addressing acid reusability and particle size modulation. Meanwhile, it is known that particle size is the critical factor in evaluating whether the quality of PET waste-derived H2BDC product meets the criteria of a commercial product or not. The American Standard Sieve (ASTM E11-22 standard) [27] specifies nominal aperture sizes ranging from 20 µm to 125 mm. Particles finer than 20 µm tend to agglomerate, leading to the clogging of sieve meshes [28, 29]. It is exemplified by the fine particle size of H2BDC, which has necessitated higher drying temperatures for the product, spanning from 100 °C for 24 h [5]. In contrast, Lee et al.(2021) employed a blend of 4.5 M H2SO4 and dimethyl sulfoxide (DMSO) to augment H2BDC particle size up to 25 µm, with the drying temperature a mere 40 °C for 3 h [28]. However, this study did not delve into the acid-DMSO mixture's reusability or the optimum hydrolysis conditions. These endeavors underscore the challenges linked to H2BDC extraction from PET waste, particularly the absence of insights into the optimal hydrolysis conditions and the most suitable solvent for washing. Previous studies have employed various solvents, such as methanol and ethanol, for these processes, resulting in a notable escalation of production costs [5, 30]. The oversight of interdependencies between independent variables has further contributed to suboptimal H2BDC productivity in earlier studies [28, 31, 32]. A shift towards eco-friendly acids and solvents is advised to address these challenges. Citric acid emerges as a potential substitute for inorganic acids, offering impressive stability constants, chelate effects, and a robust capacity for metallic ion complexation, rendering it an ideal choice for H2BDC extraction. Additionally, DMSO is environmentally conscious and capable of dissolving substantial quantities of H2BDC (105 mg L−1). The property leads to an augmentation in particle size, as demonstrated by previous literature [33,34,35]. The amalgamation of acid and DMSO promotes gradual supersaturation, ultimately bolstering H2BDC particle size [28].

In summary, the cost-efficient method demonstrated in this research, employing Response Surface Methodology (RSM) and reusing a mixture of C6H8O7 and DMSO, offers significant economic and environmental benefits. RSM optimized hydrolysis conditions, reducing chemical and solvent costs, conserving energy, and minimizing wastewater treatment expenses. Reusing C6H8O7 and DMSO cuts chemical costs and alleviates wastewater treatment burdens. This eco-friendly approach, utilizing benign solvents like DI water and DMSO, showcases a commitment to sustainability. Additionally, the efficient use of citric acid with DMSO presents a promising alternative to harsh acids like H2SO4 and HNO3, resulting in wastewater with lower pollutant concentrations. Therefore, the main objectives of this study are to (1) investigate the interactions of independent variables, such as NaOH concentration, temperature, and time, through RSM, aiming to optimize the hydrolysis procedure and maximize H2BDC yield and recovery ratio; (2) evaluate the effectiveness of combining citric acid and DMSO for the acidification phase, with a focus on enhancing particle size and H2BDC yield, while determining the optimal conditions for this combined approach; and (3) assess the purity of PET-derived H2BDC under optimal conditions, making a comparison with commercially produced H2BDC, and gauge the reusability of the citric acid-DMSO mixture based on yield, particle size, and purity considerations.

2 Materials and methods

2.1 Materials

This study collected PET waste from household recycling bins in Taoyuan City, Taiwan, and was prepared for experimentation. The PET waste underwent a series of processes: initial drying in an oven at 90 °C for 24 h, followed by thorough washing with tap water. The chemicals employed in the experiment, TBAI, 1,4 benzene dicarboxylate, ethanol, and DMSO of 99.9% purity, were procured from Sigma-Aldrich. Citric acid monohydrate (99.9% purity), H2SO4, and NaOH were provided by Avantor Performance Materials Inc (Radnor, PA, US). Aqueous citric acid monohydrate, sulfuric acid, and sodium hydroxide solutions were prepared with Milli-Q water. All chemicals were used without the need for further purification. The PET waste's characteristics were thoroughly examined through proximate and ultimate analyses by Taiwan Environmental Protection Administration standards (NIEA R213 and R205). It encompassed the determination of moisture, ash content, volatile matter, and fixed carbon. This analytical procedure was carried out in triplicate to ensure accuracy and reliability. The ultimate analysis of PET samples was conducted using an elemental analyzer (Elementary Analyzer, Vario MICRO). As detailed in Table S1, the PET waste exhibited a notable carbon content, with a dominant presence of approximately 57.8 ± 1.9%. The energy content of the PET samples was quantified using a bomb calorimeter (Parr 1341 calorimeter) following NIEA E214.01C protocol.

2.2 The extraction process of H2BDC derived from PET waste

2.2.1 Alkaline hydrolysis process

The extraction procedure was undertaken in a dual-stage process encompassing hydrolysis and subsequent acidification, as illustrated in Fig. 1. For the hydrolysis phase, one gram of PET flakes was immersed in a mixture comprising varying quantities of NaOH (5, 10, and 15%) and distilled water (in mL). The mass ratio of NaOH to PET was set at 3:1, and for DI water to PET, it was maintained at 10:1. The hydrolysis procedure was conducted under two conditions: one without the incorporation of a catalyst, and the other involving the introduction of a phase-transfer catalyst named TBAI. The hydrolysis reaction was performed within a 50 mL Teflon-coated steel autoclave reactor. The reactor was subjected to different temperature conditions, specifically 160, 180, and 200 °C, and the reaction duration was set at 8, 10, and 12 h. The reactor was cooled to room temperature upon reaching the designated reaction time. 20% NaOH concentration was added to neutralize the mixture until the solution reached a pH of 14, ensuring complete dissolution of any remaining H2BDC [36]. Subsequently, the unreacted solid phase of PET was separated from the mixture using vacuum filtration.

Fig. 1
figure 1

A schematic diagram of PET–derived H2BDC extraction procedure

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2.2.2 Acidification process and computation of H2BDC production yield

The precipitation capability was determined by the acidification reactions separately using two different acid solutions, namely 4.5 M H2SO4 and C6H8O7. H2SO4 was gradually added to the filtrate until the pH reached 1.0, following the approach outlined by Manju et al. in 2013 [37]. Previous research has not explored the precipitation of H2BDC using C6H8O7, and this led to our examination of various C6H8O7 concentrations (1, 3, and 5 M) to determine the ideal concentration and final pH value for precipitation. The goal was to maximize the yield of H2BDC and control particle size while minimizing acid consumption. The experiments were conducted with various pH values (2.5, 3.5, and 4.0) to determine the H2BDC yield. Additionally, the efficacy of DMSO in enhancing H2BDC particle size was evaluated. In this research, 5 M C6H8O7 was combined with DMSO at different volume ratios (1:1, 1:2, and 1:3) while maintaining a pH of 4.0 in the experiments.

Further experiments involved mixing different concentrations of citric acid (1, 3, and 5 M) with DMSO and subjecting them to acidification at various reaction temperatures (25, 50, and 75 °C) and durations (5, 40, and 60 min). Throughout these experiments, the precipitation reactions were maintained at a constant stirring speed of 300 rpm. The resulting H2BDC product was collected through filtration, washed with deionized (DI) water or ethanol (EtOH) and dried at 60 °C for 12 h in a vacuum oven [12]. The conversion percentage of PET and the yield percentage of H2BDC were subsequently calculated. It is important to note that, following the precipitation reaction and the cooling process, the aqueous phase containing citric acid and DMSO solvent was separated through filtration. This aqueous phase was recycled for subsequent precipitation reactions, and its reusability was assessed regarding yield and particle size.

PET conversion and H2BDC yield (%) were determined as follows: Eqs. (1) and (2):

$$\text{PET conversion }\left({\%}\right)=\frac{({\text{W}}_{\text{PET},\text{ i}} -{\text{W}}_{\text{PET},\text{f}} ) }{{\textrm{W}}_{\textrm{PET},\textrm{ i}}}\times 100\%$$
(1)

WPET, i is the initial weight of PET and WPET,f is the final weight of PET that remains on filter paper after filtration.

$$\text{Yield }({\%}) =\frac{{\text{W}}_{\text{precipitate}} }{{\textrm{W}}_{\textrm{theoretical}}}\times 100\%$$
(2)

Wtheoretical is the weight of H2BDC recovered theoretically ( 100% recovery of 3 g of PET is 2.596 g of H2BDC [26, 38], and Wprecipitate is the weight of H2BDC extracted from designed experiments.

2.3 Experimental design for hydrolysis reaction using RSM

RSM is a crucial statistical tool for optimizing experimental parameters and minimizing the number of investigations. It effectively optimizes, maximizes, or minimizes output responses influenced by multiple input factors [39, 40]. The interactive effects of essential input variables on the output response are simulated through RSM. Within RSM, the Box-Behnken design (BBD) holds significance for evaluating quadratic response surfaces and developing second-order polynomial models. The BBD is a spherical, 3-level fractional factorial design, including a central point and the middle points on the edges of a circumscribed circle [41]. In this research, NaOH concentration, temperature, and time are critical factors influencing the extraction efficiency of H2BDC from PET waste. The BBD method is employed to determine the optimum conditions for PET waste hydrolysis using the hydrothermal method in an autoclave. The experimental design incorporates three levels of BBD (-1, 0, + 1) with 17 tests and five duplications at the central point. NaOH concentration, temperature, and time are independent variables, while H2BDC yield is the response variable [26]. These values are determined based on prior research outcomes and preliminary screening experiments, ensuring comprehensive parameter space coverage [26, 42]. For instance, NaOH concentrations ranging from 5 to 15%, temperatures spanning from 160 to 200 °C, and hydrolysis times varying from 8 to 12 h are considered (as detailed in Table S2). Utilizing Minitab software version 20 further facilitates the analytical process, aiding in data interpretation and model development. This systematic approach not only streamlines the optimization process but also enhances the overall robustness of the experimental design. The RSM experimental results are fitted using the second-order polynomial equation, represented as Eq. (3) [26, 43].

$$\text{Y}={\upbeta }_{0}+ \sum_{\rm{i}=1}^{3}{\upbeta }_{\text{i}}{\rm{X}}_{\text{i}}+\sum_{\rm{i}-1}^{3}{\upbeta }_{\text{ii}}{\rm{X}}_{\text{i}}^{2}+\sum_{\rm{i}<1}^{\text{n}}{\upbeta }_{\rm{ij}}{\text{X}}_{\rm{i}}{\text{X}}_{\rm{j}}+\upvarepsilon$$
(3)

where Y is the predicted response, Xi and Xj are independent variables; β_0 is an intercept coefficient, β_i is a linear coefficient, β_ii is a quadratic coefficient; β_ij is an interactive coefficient; n is the number of independent variables, ε is the experimental error. Variance (ANOVA) analysis is conducted to assess the statistical significance of the polynomial model at a 95% confidence level and to reveal the interaction between independent variables and the response variable.

2.4 Characterization

The elemental composition of PET samples and H2BDC production are analyzed using an elemental analyzer (Vario MICRO Elementary Analyzer). The energy content of PET samples is determined by a bomb calorimeter (Parr 1341 calorimeter, NIEA E214.01C). H2BDC diffraction profiles are obtained through X-ray diffraction (XRD, Brucker, D8, Advance) with CuKα radiation (λ = 1.5406 Å) at a scan rate of 0.03° s−1 and the 2θ range of 5–80°. Fourier transform infrared (FTIR) spectra (Frontier MIR/FIR) are recorded in the wavelength range of 650–4,000 cm−1 with a high resolution of 0.4 cm−1. Nuclear magnetic resonance (NMR) spectroscopy (Varian 500 NMR spectrophotometer) is employed to gather structural information regarding hydrogen and assess the purity of the H2BDC production yield. The morphological characteristics of H2BDC are examined using field emission-scanning transmission electron microscopy (FE-SEM, Leica, Stereoscan 420).

3 Results and discussion

3.1 Optimization of the hydrolysis process using the RSM

3.1.1 Development of regression model

The preliminary results of the H2BDC extraction from PET waste are presented in Table S2, revealing that the conversion of PET and the yield of H2BDC were significantly influenced by NaOH concentration, reaction temperature, and time. The highest H2BDC production yield of 97 ± 1% was obtained under 10% NaOH, temperature 200 °C, and 12 h, while the lowest yield of 38 ± 3% occurred at 5% NaOH, temperature 160 °C, and 10 h. A hydrolysis reaction without alkanile was conducted to understand the role of NaOH in PET hydrolysis further. The result indicated that the recovery ratio was only about 18.7%, highlighting the crucial role of NaOH in hydrolysis (as shown in Table S2). Notably, the TBAI catalyst did not significantly enhance H2BDC extraction efficiency from PET flake, and citric acid demonstrated superior potential in precipitating H2BDC compared to sulfuric acid, resulting in a high H2BDC production yield. The study also revealed that DI water could replace ethanol in washing H2BDC production (as indicated in Table S2 and Fig. S1), with the 1H-NMR spectrum in Fig. S1 confirming identical purity using ethanol and DI water.

The results presented in Table S2 provided a valuable basis for selecting and designing the experimental conditions of independent variables for the RSM. The experimental results obtained under various designed conditions of independent variables were used to establish the regression model (as shown in Table 1). The statistical parameters in Table S3 demonstrated that the quadratic model best predicted the H2BDC yield, supported by a high predicted R-squared (R-pred = 0.961), a very low P-value (< 0.0001), and a small predicted residual error sum of squares (PRESS*) of 178. The quadratic model also indicated the highest determination coefficient (R-squared = 0.995) and the smallest value of PRESS* compared to other models (as depicted in Table S3), explaining 99.5% of the response variability.

Table 1 Comparison results between experimental and predicted of H2BDC yield and recovery rate by Box-Behnken matrix
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The reliability of the quadratic model was evaluated using statistical parameters and an ANOVA table, as presented in Table S4. The F-value of 195 indicated a significant difference in the means of other groups, making this model highly significant. The P-value for H2BDC production yield was less than 0.0001, and the "Prob > F" values for almost all terms in Table S4 were less than 0.05, indicating the statistical significance of the quadratic model and its terms at the 95% confidence level. The Lack of Fit F-value was 2.06, which was more significant than 0.05, indicating its insignificance, with only a 25.1% chance that the Lack of Fit F-value could be attributed to noise [26, 44]. The R-sq(adj) of 0.989 and R-sq(Pred) of 0.961 were not significantly different, suggesting that the quadratic model can effectively navigate the design range. The variance inflation factor of 1.00, obtained from the RSM result using Design of Experiments, demonstrated the independence of variables in the quadratic model. In summary, the statistical results of the quadratic model are highly significant and adequate for interpreting experimental data. Therefore, the quadratic regression equation for studying H2BDC yield is as follows Eq. (4):

$$\begin{array}{c}\text{Yield}\left({\%}\right)=64.2+30.27{\text{X}}_1-2.135{\text{X}}_2-6.80{\text{X}}_3+0.0730\text{X}_1^2+0.01141\text{X}_2^2+0.669\text{X}_3^2-0.13120{\text{X}}_1{\text{X}}_2-0.4834{\text{X}}_1{\text{X}}_3\\\end{array}$$
(4)

X1, X2, and X3 are symbols of NaOH concentration (%), temperature (°C), and time (h), respectively. Yield (%) is H2BDC production.

3.1.2 Proposed regression model analysis

The adequacy of the proposed quadratic model was further evaluated through Fig. 2 and S2. The normality assumption of the model was validated by the bell-shaped histogram of residuals illustrated in Fig. S2. Figure 2a presented the H2BDC yield obtained from experimental observations was compared with the quadratic regression model, revealing a commendable agreement between the experimentally observed and predicted values. This demonstrates that the proposed quadratic model can accurately predict the experimental results [45, 46]. Moreover, Fig. 2b showed that the difference between internally studentized residuals and predicted responses remained within the 2.0 range without any abnormal observations, confirming that the experimental data were suitable for the predicted ones calculated from Eq. (2). The normal probability percent and residuals are presented in Fig. 2c, where residuals were employed to examine the assumption of intimate variance. The uniform scattering of data about both sides of the line suggests the appropriateness of the proposed model for this study. The diagnostic plot points closely resembled a straight-line model, with no necessity for data smoothing. Consequently, the proposed quadratic model is deemed adequate for predicting experimental results.

Fig. 2
figure 2

Predicted plots of H2BDC yield: a actual and predicted values of the quadratic model; b Internally studentized residuals vs. predicted values; c Normal probability and internally studentized residual for H2BDC yield; d Pareto charts of standardized effects for H2BDC yield; and e Pareto charts of standardized effects for H2BDC recovery ratio

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3.1.3 Optimum condition and interactive effects between variables using Pareto chart and three-dimensional response surface

The BBD was implemented in this research to optimize the independent variables, including NaOH concentration, temperature, and time, to achieve maximum H2BDC yield and recovery ratio. Desirability served as a criterion factor for selecting each independent parameter, with desirability value ranges between 0 (indicating an utterly undesirable response) and 1 (representing a highly desirable response). The BBD optimization results, illustrated in Fig. S3, suggest that the optimal conditions for 5% NaOH concentration, temperature (200 °C), and time (12 h) yielded a maximum H2BDC yield and recovery ratio of 100%, with a desirable value of 1.00. The model-based H2BDC yield closely aligned with experimental observations at 99% (Residual of 1.3 as indicated in Table 1), and there was no significant difference between model-based H2BDC recovery ratio and experimental values at the optimum conditions. Thus, it can be concluded that RSM is an efficient approach for optimizing PET waste hydrolysis. Figures 2d and e present the standardized effects of independent variables on H2BDC production yield and recovery ratio. NaOH concentration exhibited a more significant effect than temperature and time, with temperature having a more significant effect than time (as shown in Figs. 2d, e, and S4). The interaction between NaOH concentration and temperature was more prominent than between NaOH and time. However, the interactive effect of temperature and time on H2BDC yield and recovery ratio was insignificant (as shown in Fig. S5). 3D response surface plots in Fig. 3 reveal the interaction among independent variables and their effects on H2BDC yield and recovery ratio. Based on the proposed quadratic response equation, the polts demonstrate a significant increase in H2BDC yield and recovery ratio with higher NaOH concentration, temperature, and reaction time. However, a slight decrease in production yield and recovery ratio was observed at a constant 15% NaOH concentration when the time was increased from 8 to 10 h at higher temperatures (160 and 180 °C) (as shown in Fig. S6). This could be attributed to the higher evaporation of DI water at higher temperatures, which reduces water molecules and increases NaOH concentration. As water is essential in promoting PET hydrolysis [11, 12], a decrease in water molecules can negatively affect the hydrolysis process and consequently decrease the H2BDC yield and recovery ratio. Therefore, it is crucial to determine the optimum NaOH dosage corresponding to temperature and time to avoid setting up beyond the optimum conditions. The study determined the optimum PET: water mass ratio as 1:10, aligning with previous studies [12, 26]. Analyzing results through a Pareto chart emphasizes the significant influence of NaOH on the H2BDC production yield and recovery ratio, followed by temperature and time (as shown in Figs. S4, S5, and S7). Thus, under optimal conditions (200 °C for 12 h), the depolymerization of 2.0 g of PET waste achieved 1.72 g of H2BDC. This process consumed 6 mL of 5% NaOH solution and 20 mL of DI water to hydrolyze PET waste and 2 mL of 20% NaOH solution to neutralize the hydrolysis mixture. In conclusion, RSM proved effective in determining optimum conditions and interactive effects of variables for the PET waste hydrolysis process.

Fig. 3
figure 3

3D response surface plots for H2BDC yield (a, b, and c) and recovery ratio (d, e, and f) illustrating the interactive effects at optimization solution: a and d NaOH and temperature; b and e: NaOH and time; c and f temperature and time

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3.2 Optimum conditions of the acidification stage

3.2.1 The efficiency of sulfuric acid and citric acid for H2BDC precipitation

The precipitation reactions were conducted under the same ambient temperature conditions, reaction time (with a slow feeding rate over 40 min), and agitation speed of 300 rpm. To gain 1.72 g of H2BDC from acidification reactions of the hydrolyzed mixture, which consumed approximately 8.5 mL 4.5 M H2SO4 or 20 mL 4.5 M C6H8O7 to adjust the pH to 1.0 and 4.0, respectively. Then, the product was washed thrice with 30 mL of DI water. Figure 4 illustrates the PXRD spectra and SEM images of PET-derived H2BDC precipitated by 4.5 M citric acid and sulfuric acid. The results indicated an insignificant difference in H2BDC yield and recovery ratio between citric and sulfuric acid. However, H2BDC precipitated with citric acid exhibited superior quality and characteristics compared to sulfuric acid, as shown in Fig. 4a. Remarkably, the particle size of H2BDC produced using citric acid was approximately 10.4 µm, which is larger than that produced by sulfuric acid in Fig. 4b. Additionally, the diffraction peaks obtained using citric acid for H2BDC precipitation were sharper and stronger than those obtained using sulfuric acid. These results suggest that the H2BDC crystals precipitated with citric acid were more well-defined and ordered, indicating higher crystallinity. It is important to note that strong acids such as H2SO4 or HCl pose safety and corrosion concerns, leading to elevated environmental pressure and increased production costs. In contrast, citric acid is a more environmentally friendly alternative. Moreover, the pH of wastewater after the precipitation period was lower when using sulfuric acid (pH = 2.0) compared to using citric acid (pH = 5.0). To further compare the precipitation performance by H2SO4 and citric acid, the nitrate levels were 0.5 mg L−1 for H2SO4 and 0.04 mg L−1 for citric acid, respectively. However, it is important to note that using citric acid for neutralizing and precipitating H2BDC not only is more eco-friendly but also results in a significantly higher quality and larger particle size of H2BDC as compared to sulfuric acid.

Fig. 4
figure 4

PXRD spectra and SEM image of PET-derived H2BDC precipitated by (a) C6H8O7 (4.5 M) in 40 min; (b) by H2SO4 (4.5 M) in 40 min; (c) (C6H8O7 5 M/DMSO (1:1), 25 °C, 40 min); (d) ((C6H8O7 5 M/DMSO = 35/75 mL), 75 °C, 40 min)

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3.2.2 Optimum conditions for H2BDC particle size and yield at the acidification stage

This research determined a final pH value of 4.0 for the precipitation reaction, as the H2BDC yield showed no significant difference between pH 2.5 and 4.0. To obtain 1.72 g of H2BDC from the acidification reactions of the hydrolyzed mixture using a combination of DMSO and citric acid, which consumed approximately 35 mL 5 M C6H8O7 and 75 mL DMSO were utilized to adjust the pH to 4.0. Subsequently, the product was washed with 30 mL DI water. Particle size, a critical factor for H2BDC production quality [17], was enhanced using citric acid mixed with DMSO. Figure 4c displays H2BDC particle size under precipitation conditions, where 15 mL 5 M C6H8O7 was remixed with 15 mL DMSO at 25 °C for 40 min. Experimental results showed a significant improvement in H2BDC particle size from 10.4 to 33.2 µm (as shown in Fig. 4a and c), while yield remained insignificantly about 97%. This occurrence arises due to the water-insolubility of H2BDC, resulting in its rapid precipitation and subsequent formation of fine particles. Citric acid was mixed with DMSO to achieve a slow degree of supersaturation and increase the equilibrium concentration of H2BDC, resulting in larger crystals.

The study found that acid concentration, reaction time, and temperature significantly influenced H2BDC particle size but did not affect yield. In the case of ambient temperature and 40 min minutes of reaction time, the particle size increased from 13.5 to 33.2 µm with an increase in the C6H8O7 concentration from 1 to 5 M (as indicated in Table S5, Figs. S8a and S8b). This phenomenon can be attributed to the fact that the C6H8O7 concentration increases, leading to a reduction in the ratio of the aqueous phase to DMSO and an increase in dissolved H2BDC in DMSO. However, in the case of the relatively high C6H8O7 concentration (6 M), the particle size significantly decreased from 11.2 to 3.6 µm (Figs. S8c and S8d), resulting in a sudden jump in supersaturation from the instant conversion of Na2-H2BDC into H2BDC and rapid nucleation. Therefore, 5 M C6H8O7 was deemed suitable for obtaining the desired particle size of H2BDC.

By increasing the reaction time from 5 to 40 min, a significant increase in particle size from very fine to 11.2 µm was observed under the slower feeding speed and the same conditions (25 °C and 5 M citric acid concentration)(as shown in Figs. S8d and S8e), 5 min of reaction time resulted in fast precipitation and a sudden rise in supersaturation while 40 min allowed for the assembly of H2BDC solubles into larger crystals. However, H2BDC particle size did not significantly differ under 40 and 60 min of reaction time. The study highlighted that reaction temperature during the acidification step significantly affects particle size (as shown in Fig. 4d). The mixture of DMSO and 5 M C6H8O7 promoted the largest particle size and crystallinity at 75 °C, with an average particle size of 57.4 µm. This phenomenon can be ascribed to the significant solubility of H2BDC in DMSO at 75 °C. The H2BDC particle size produced is larger than reported by previously published literature [28] (as indicated in Table 2). Therefore, this study creates a large H2BDC particle size, preventing product loss and saving energy during filtration and drying. Larger crystals have lower moisture content, allowing for a drying temperature and time of 60 °C in 12 h for this study, compared to fine particles that require 90 °C for 24 h [47] or 100 °C for 24 h [5]. In summary, the optimum conditions for particle size and yield of H2BDC at the hydrolysis and precipitation stages were recorded as follows: a PET: H2O mass ratio of 1:3, NaOH (5%), 200 °C, and the reaction time of 12 h for the hydrolysis stage, and the 5 M C6H8O7: DMSO volume ratio of 1:2 (35 mL C6H8O7 mixed with 75 mL DMSO), 75 °C with a reaction time of 40 min and 300 rpm during the precipitation stage.

Table 2 Comparison results of PET-derived H2BDC extraction efficiency
Full size table

3.3 Reusability test of the mixture of citric acid and DMSO

Reusing the citric acid and DMSO mixture from the H2BDC precipitation process is crucial for the subsequent acidification reaction. By reusing this mixture, extraction costs are minimized, and wastewater generation is avoided, reducing the demand for further treatment. After cooling the reaction solution to room temperature, filtration is employed to recover H2BDC and separate the aqueous solution. The aqueous phase, which contains citric acid and DMSO, serves as the reaction solution for the subsequent acidification process. Assessing the reusability of the citric acid and DMSO mixture involves evaluating the yield, particle size, and purity of the produced H2BDC. The chemical structure of produced H2BDC was determined using 1H and 13C NMR spectroscopy. The results depicted in Fig. 5a–c illustrate that the spectra of H2BDC remain consistent after eight cycles, aligning with the commercial H2BDC and initial cycle. Consequently, the chemical structure and purity of H2BDC remain unchanged. In addition, the particle size exhibits a slight change after eight cycles, ranging from 47.1 to 60.1 µm, as shown in Fig. 5d. The yield of produced H2BDC remains stable, maintaining between 96 to 99% over eight cycles of continuous reuse. It suggests that recycling the citric acid and DMSO mixture has no detrimental effect on the purity and yield of H2BDC. Thus, DMSO retains its high potential for enhancing the particle size of H2BDC over eight cycles of the aqueous phase.

Fig. 5
figure 5

1H and 13C NMR spectra of (a) PET-derived H2BDC of cycle-0; (b) commercial H2BDC; (c) PET-derived H2BDC of cycle-8; (d) The effect of recycling the aqueous phase on the yield and particle size of H2BDC

Full size image

3.4 Evaluation of PET-derived H2BDC quality at optimum conditions

FTIR analysis was employed to determine the chemical structure of PET waste, PET-derived H2BDC, and commercial H2BDC, as illustrated in Fig. 6a. The peaks observed at 1,578, 1,511, and 1,422 cm−1 are attributed to the benzene ring vibration of H2BDC and PET [25]. Peaks around 1,282 and 2,548–3,062 cm−1 correspond to the stretching vibrations of carboxylic groups [16]. The peak at 1,678 cm−1 signified the carbonyl groups' stretching vibration, while the 728 cm−1 peak indicated the out-of-plane bending vibrations of aromatic rings [26, 48]. Peaks at 1,730 and 1,252 cm−1 in the original PET spectra are attributed to the stretching vibrations of C = O and C-O of the ester bonds, respectively. A remarkable feature in Fig. 6a is the presence of identical characteristic peaks between PET-derived H2BDC and commercially produced H2BDC, confirming the successful recovery of H2BDC from PET bottles. The PXRD diffraction spectra in Fig. 6b exhibit multiple sharp Bragg and strong diffraction peaks at 17.2, 25.3, 27.9, 29.8, and 39.8°, indicating high crystallinity of PET-derived H2BDC. Characteristic peaks for PET-derived H2BDC and commercial H2BDC coincide well and are consistent with previously reported studies [5, 26, 31].

Fig. 6
figure 6

a Comparison FT-IR spectrum of PET-derived H2BDC vs. commercial H2BDC and original PET; b PXRD pattern of PET-derived H2BDC vs. commercial H2BDC

Full size image

Furthermore, NMR spectroscopy was conducted to analyze the chemical structure and purity of PET-derived H2BDC, as shown in Fig. 5a. In the 1H NMR spectra, peaks at 8.04 and 13.29 ppm are attributed to the aromatic protons of the benzene ring and the hydroxyl protons, respectively. The 13C NMR spectra displayed signals at 129.38, 134.41, and 166.58 ppm, corresponding to the aromatic (CA), quaternary aromatic (CB), and carbonyl (CC) carbon atoms of H2BDC. The PET-derived H2BDC spectra match well with 1H NMR and 13C NMR spectra of commercial H2BDC (as shown in Fig. 5a and b) and align with previously published results [9, 25]. No other peaks, except those from the NMR solvent, were observed, indicating the high purity of the produced H2BDC. The elemental analysis results showed that carbon content in commercial H2BDC production and PET-derived H2BDC production were 58 ± 0.37% and 57 ± 1.90%, respectively. Meanwhile, the hydrogen content of H2BDC from commercial H2BDC and PET-derived processes were quite similar, ranging between 3.7 ± 0.12% and 3.5 ± 0.08%, respectively. These findings affirm that PET-derived H2BDC has a consistent structure, quality, and purity comparable to commercial H2BDC (purity: 99%), making it suitable for commercial development.

Table 2 compares the extraction efficiency of PET-derived H2BDC under optimal conditions in this study with values reported in previous studies [11, 12, 26, 28, 31, 37, 49, 50]. Notably, particle size plays a crucial role in determining the production quality of PET-derived H2BDC, impacting its compliance with commercial product standards. However, several previous studies have overlooked critical parameters such as precipitation time and particle size. The current research has made significant advancements, achieving an improved particle size of 57.3 µm and a high yield of 98% by applying a combination of citric acid and DMSO during the acidification step within a short reaction time of 40 min. This result contrasts the report by previous research [17], which used a mixture of sulfuric acid and DMSO, resulting in a particle size of 25 µm and yield (92%) with a reaction time of 55 min. As seen in Table 2, recent researchers have shown interest in using catalysts for hydrolysis and strong acids for acidification, leading to increased operating costs and hazardous substrates. In contrast, the current study determined the optimum conditions for hydrolysis and acidification without using catalysts and strong acids, resulting in cost-efficiency and eco-friendly operation.

4 Conclusions

This study successfully identified the optimal conditions for PET hydrolysis using hydrothermal processes, specifically at 200 °C for 12 h with 5% NaOH. It represents a significant step forward by enhancing the quality of PET waste-derived H2BDC and reducing pollutant concentrations in wastewater resulting from the acidification process. A unique combination of citric acid and DMSO was used in this research to precipitate H2BDC from the hydrolyzed PET mixture. This innovative method substantially improved the particle size of H2BDC, meeting commercial production standards. Key insights from the experiments highlighted the critical role of DMSO in controlling the particle size of H2BDC. Notably, the results showed that the reuse of the DMSO and citric acid mixture did not compromise the quality or yield of H2BDC production. The method's environmentally friendly and cost-effective nature, utilizing benign precursors and promoting high reusability of the citric acid and DMSO mixture, underscores the sustainability of the process. Consequently, this study addresses the limitations of prior research that relied on strong acids and catalysts without determining the optimal hydrolysis conditions or considering the reuse of acid and the particle size of H2BDC during the acidification process. In summary, the findings highlight the promising potential for industrializing H2BDC derived from PET, offering an environmentally conscious and economically viable pathway for its production. Future research will focus on applying PET waste-derived H2BDC to synthesize various MOFs and evaluating their capacity for adsorption and separating contaminants in water, wastewater, and air research fields.

Availability of data and materials

The datasets collected and analyzed during this study are mentioned in the submitted article and supplementary materials.

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Acknowledgements

The authors would like to thank the National Science and Technology Council (NSTC) Taiwan for financially supporting this work (Grand No. NSTC-111-2221-E-008-012-MY3). The authors also appreciate the Precious Instruments Utilization Center of National Central University in providing analysis facilities.

Funding

This research was supported by the Taiwan National Science and Technology Council (NSTC).

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Thi Hong Nguyen is the first author responding to collect, analyze data, and write a draft manuscript. Prof Kung-Yuh, Chiang is the corresponding author who supervised and revised the whole research. All authors have read and approved the final manuscript.

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Nguyen, T.H., Chiang, KY. Enhancement of terephthalic acid recovered from PET waste using a combination of citric acid and dimethyl sulfoxide extraction. Sustain Environ Res 34, 16 (2024). https://doi.org/10.1186/s42834-024-00220-2

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