From Waste to Worth: Glycerol-Driven Transformation of PET into Functional PolymersAbstract
Polyethylene terephthalate (PET) is one of the most widely used polymers worldwide, particularly in packaging; however, its poor recyclability greatly contributes to plastic pollution. This study investigates an alternative valorization pathway through partial depolymerization with glycerol under both microwave and conventional heating conditions, without the use of catalysts or solvents. Both methods produced white, brittle solids with distinct physicochemical properties. Thermal analysis revealed an initial mass loss (130-220 °C) due to residual glycerol, followed by degradation steps indicative of shorter polymer chains. Additionally, the lower glass transition and melting points, along with sharper crystallization peaks, support the idea of chain scission and improved nucleation. Spectroscopic analyses identified new polyester structures derived from glycerol and PET, coexisting with unmodified PET. The modified material demonstrated promising results for Rhodamine-B adsorption, with high retention (> 97%) over multiple filtration cycles. This environmentally friendly, scalable approach effectively converts PET waste into functional materials, supporting the principles of green chemistry and the circular economy.
Keywords:
PET modification; glycerol; chemical recycling; circular economy; microwave irradiation; conventional heating
Introduction
Since their synthesis in the early 20th century, synthetic plastics have been considered essential to modern society. Their low cost, lightweight nature, and superior mechanical and chemical properties have led to their widespread use in packaging, clothing, construction, transportation, and various other industries.1-3
Plastic production has mostly relied on petroleum, a non-renewable resource. In 2023, global plastic output reached 413.8 million tons, with 90.4% coming from fossil sources.4 However, only 10% of plastic waste has been recycled, 14% has been incinerated, and the remaining waste has been sent to landfills or the other disposal methods environment.5
Polyethylene terephthalate (PET) has been widely used in water bottles, food packaging, and textiles due to its lightness, durability, and versatility.6 Only in 2024, the market size of PET was USD 83.2 billion, and it is expected to increase to USD 140.0 billion by 2034 with a 5.35% Compound Annual Growth Rate (CAGR).7 In addition, projections of global PET packaging demand indicate that it will surpass 30 million tons by 2030.8
Poor plastic waste management and low recycling rates have contributed to a global environmental crisis, especially in terrestrial and marine environments ecosystems.9 Therefore, these challenges emphasize the need for more efficient and sustainable recycling technologies. Chemical recycling, in which plastics are broken into monomers, has been seen as a promising alternative for converting plastic waste into high-value products.
Among PET chemical recycling processes, glycolysis with ethylene glycol and catalysts has been extensively studied, and bis(2-hydroxyethyl) terephthalate (BHET) has been primarily produced through this route.10-19 Few studies have focused on PET glycolysis with other alcohols, such as poly(ethylene glycol).20
Glycerol, a renewable polyol and a byproduct of biodiesel production, has been linked to environmental risks when improperly disposed of due to impurities such as salts, heavy metals, and fatty acids derivatives.21 Despite its low commercial value, glycerol has been transformed into high-value derivatives, supporting green chemistry principles by decreasing waste and utilizing renewable resources.22,23
Furthermore, emerging technologies such as microwaves and biodegradable reagents have been identified as promising recycling methods. Recent studies24,25 have investigated PET depolymerization using microwaves and glycerol to develop materials with high technological value potential.
PET recycling is recognized as a vital method for reducing plastic buildup and supporting Green Chemistry and Green Engineering principles. Green Chemistry encourages waste reduction, the use of renewable materials, and safer processes, such as microwave-assisted depolymerization with glycerol, which eliminates toxic solvents and improves energy efficiency.26 Green Engineering has promoted resource efficiency, process safety, and sustainability throughout the product lifecycle.27
Using glycerol as a reagent combined with microwave energy efficiency is a sustainable approach that supports the circular economy and the Sustainable Development Goals (SDGs), especially SDG 9 (Industry, Innovation, and Infrastructure-targets 9.4 and 9.5) and SDG 12 (Responsible Consumption and Production-targets 12.4, and 12.5).28
A limited number of studies have investigated PET depolymerization using glycerol as a solvent.24,25,29,30 These studies have reacted PET with glycerol derivatives in the presence or absence of alkalis,30-32 or employed a heterogeneous catalyst in the presence of solvents and other polymers.33 However, no reports have described a functional polymer obtained from the partial depolymerization or modification of PET using only glycerol as a reactant, in a single synthesis step, without solvents, catalysts, or additives.
Therefore, an alternative PET depolymerization and modification process using glycerol as a reagent in a catalyst- and solvent-free, environmentally friendly approach has been explored in this study. In addition to complementing the synthesis data in the literature involving PET and glycerol, the present study produced a material with unique properties from PET waste, which shows potential as an adsorbent for dye removal in water solutions. Two recovery routes have been investigated: (i) microwave-assisted (PG-MW) and (ii) conventional heating (PG-CH). Meanwhile, microwave processes have been recognized as more efficient.12 Conventional heating has proven to be a viable alternative due to its ease of operation and scalability, making it an appealing choice for industrial applications.
Experimental
Sample preparation and reagents
A colorless PET bottle, typically used for water or soda, was thoroughly sanitized, carefully cut open with scissors, and dried using absorbent paper. A colorless bottle was selected to prevent interference from dyes in the processes and thermoanalytical analyses. After cleaning, the PET was cut into small pieces, approximately 5.0 mm2, using scissors.
Deuterated chloroform (CDCl3, 99.96% D), deuterated trifluoroacetic acid (TFA-d, 99.5% D), glycerol (> 98%), and Rhodamine-B (RhB, ≥ 95%) were obtained from Sigma-Aldrich. Except for PET, all reactants were used without further purification.
Methodology
PET modification by microwave irradiation (PG-MW)
Approximately 15 g of glycerol and 3 g of PET were introduced into a 150 mL Teflon® reactor. All synthesis routes were carried out in a modified microwave oven (presented in Figure S1, Supplementary Information (SI) section), connected to a thermocouple and a manometer. The Incon® V1.5 software controlled the entire system. The efficiency of system was already demonstrated in a previous work.34 The synthesis was performed at ambient pressure in an air atmosphere. The system was heated to 180 °C at a rate of 10 °C min-1 and maintained at this temperature for 30 min. After the reaction, the system was allowed to cool to room temperature. The resulting product, a solid white plate was then filtered and washed with distilled water to remove excess glycerol. Finally, it was dried in an oven at 110 °C for 1 h until a constant mass was achieved. The sample was named PG-MW.
PET modification by conventional heating (PG-CH)
Approximately 3 g of PET was weighed and transferred to an aluminum container. The material was heated on a plate at 290 °C until it melted (using 180 °C, as in the microwave procedure, was not enough to produce the final material, as no reaction occurred between the reactants). Then, 15 g of glycerol, previously heated to 150 °C, was added and stirred for about 15 min until a homogeneous mixture was formed. Afterward, the sample was cooled to room temperature. Finally, it was filtered to remove excess glycerol and dried in an oven at 110 °C for 2 h. The sample was named PG-CH.
Instrumentation
Thermal analysis
Simultaneous thermogravimetry - differential thermal analysis (TG-DTA) curves were obtained using a STA 449 F3 equipment (Netzsch). Approximately 10 mg of sample were used in an open α-alumina (150 μL), at a heating rate of 10 °C min–1, within a temperature range of 30-800 °C, and under a flow rate of dry air (70 mL min–1). The thermal stability and temperature intervals of mass loss were determined using TG curves and their first derivative curve (DTG); therefore, DTG curves are present alongside the TG-DTA curves. When necessary, TG-DTA curves were obtained under a nitrogen atmosphere using the same conditions.
Differential scanning calorimetry (DSC) was performed using a DSC1 Star equipment (Mettler-Toledo). The DSC curves were recorded using 5 mg of sample in closed aluminum crucibles (40 μL) with perforated lids, under a flow rate of dry air at 50 mL min–1. The samples were subjected to cyclic analysis between 25-260 °C, with the first heating performed at 10 °C min–1, cooling at 20 °C min–1, and the final heating at 20 °C min–1. Before the DSC curves were taken, the modified PET samples (PG-MW and PG-CH) were heated to 230 °C and then analyzed and were subsequently named PG-MW-230 and PG-CH-230.
The DSC images were obtained using the same equipment previously described, coupled to an SC 30 digital camera, which incorporates a 3.3-megapixel CMOS sensor and a Navitar 1-6232D mechanical optical subassembly with a 6.5× zoom. The measurements were carried out under the same conditions for the DSC curves, except for the crucible, which was an open α-Al2O3 crucible (40 mL).
Spectroscopy analysis: MIR, 1H NMR, solid UV-Vis spectra
Mid-infrared spectroscopy (MIR) analysis was performed using a Vertex 70 FTIR spectrometer (Bruker) equipped with an attenuated total reflectance (ATR) diamond crystal accessory. The spectra were recorded within the range of 4000-400 cm–1, with 32 scans and a resolution of 4 cm–1. Proton nuclear magnetic resonance (1H NMR) spectra were acquired using an Ascend III 600 MHz spectrometer (Bruker). The samples were solubilized in a deuterated solvent mixture (CDCl3:TFA-d) at a 3:1 v/v ratio.35 Ultraviolet-visible (UV-Vis) spectra of solid samples were obtained in the range of 200-600 nm using a PerkinElmer Lambda 1050 double-beam spectrophotometer.
Morphology - scanning electron microscopy (SEM)
The polymer morphology was examined using a Zeiss EVO LS15 scanning electron microscope. Samples were placed on a standard carbon adhesive and subsequently coated with a thin gold layer for 10 min. Imaging was performed under vacuum conditions (ca. 10–3 Pa) with an accelerating voltage of 15 kV.
UV-Vis spectroscopy - adsorption tests
Initially, preliminary qualitative tests involved exposing 100 mg of the polymer to a Rhodamine-B solution (10 mg L–1), then allowing the systems to rest for 24 h. At this stage, the samples PET, PG-MW, and PG-MW-230 were tested.
Following the preliminary test, PG-MW was selected for quantitative analysis based on observed visual changes. These tests were performed in triplicate by mixing 2.0 mL of an aqueous RhB solution (10.4 mg L–1) with 1.0 mg of PG-MW powder in 10.0 mL Erlenmeyer flasks. The mixtures were stirred at 300 rpm in a shaking incubator (Marconi, model MA 420, Brazil) at a constant temperature of 25 °C. During the experiment, aliquots were collected at different time intervals (3, 5, 10, 20, 30, 60, 120, 180, 240, and 480 min) until the final time point (720 min). After the adsorption process, the samples were filtered with filter paper. The maximum wavelength absorption (λ = 554 nm for RhB, as shown in Figure S2, SI section) was measured using UV-Vis spectroscopy with a Cary 8854 spectrophotometer (Agilent Technologies) at room temperature, using a 1 cm quartz cuvette. The final concentration of each sample at its respective time was calculated from the slope (0.122) obtained through linear regression of an analytical curve of RhB at different concentrations (Figure S3, SI section). The dye removal percentage and adsorption capacity at time t were calculated according to equations 1 and 2, respectively.36
where C0 (mg L–1) represents the initial concentration, Ct (mg L–1) is the concentration at time t, m is the adsorbent mass used, V is the volume of the dye solution, R (in percentage) is the dye removal percentage, and qt (mg g–1) is the adsorption capacity at time “t”.
After assessing the RhB adsorption capacity of PG-MW, the polymer was used as an adsorptive agent in a filtration system for this dye. To do this, a syringe was used to mimic a pipeline, where approximately 500 mg of the polymer was inserted and secured between two layers of cotton, as shown in Figure 1. The syringe was then filled with 5 mL of the same RhB aqueous solution (10.4 mg L–1), which was passed through the filtration system by manual pressing. The filtered liquid was collected and analyzed using UV-Vis spectroscopy, with the dye concentration determined according to the previously described procedure. This process constituted one cycle, and a total of 15 cycles were performed without recovering the PG-MW material used as the filter.
Results and Discussion
Figure 2 displays the images of the PET used in this study. As expected, the material appeared as a homogeneous and translucent solid under room light. Both modified PET samples were also solid; however, they exhibited a white, opaque appearance and were easily macerated, as shown in Figure 2 (for visual similarity, only the sample obtained through microwave irradiation is displayed). Under UV light (370 nm), all samples were luminescent.
Images of polyethylene terephthalate (PET) used in the present work and the resulting material after modification with glycerol under microwave irradiation (PG-MW).
The MIR spectra are shown in Figure 3. The main bands observed in the PET spectrum include those centered at 1712 cm–1 (highlighted in red) and 1240 cm–1, which correspond to the stretching of the carbonyl group (C=O) and the C–O–C bond, respectively, both characteristic of the ester chain.37-39 Additionally, the band at 1407 cm–1 (indicated by a blue arrow) is associated with the aromatic ring.37,38
After the reaction with glycerol, the spectra of PG-MW and PG-CH samples remained similar to that of PET, except for the appearance of a broad band between 3670 and 3020 cm–1 (highlighted in pink in Figure 3). This band may be due to residual glycerol in the material, as it is also present in the glycerol spectrum, or to glycerol-derived oligomers, or to lower molecular weight PET, which results in free hydroxyl groups along their chains. This aspect will be further discussed.
The TG/DTG-DTA curves of PET and the modified samples are shown in Figures 4a to 4c. At the same time, Table 1 lists the temperature ranges (θ) for each stage of mass loss, along with detailed information on mass changes (∆m), maximum degradation rate (MDR), temperature of maximum degradation rate (TMDR), and peak temperatures from their respective TG/DTG-DTA curves.
TG/DTG-DTA curves of (a) PET, (b) PG-MW, (c) PG-CH, (d) PG-MW-230, (e) PG-CH-230, (f) PG-MW-230 under N2 atmosphere, and (g) magnification of its DTA curve.
Temperature ranges (θ), mass losses (Δm), peak temperatures (TP), maximum degradation rates (MDR), and temperature of maximum degradation rate (TMDR) observed for each step in TG/DTG-DTA of samples
The PET sample (Figure 4a) remained thermally stable until 329 °C, after which it decomposed in two consecutive mass loss steps, as shown by its TG/DTG curves. Before decomposition, an endothermic peak appeared at 244.0 °C in the DTA curve, indicating the melting point.40,41 The first mass loss step can be attributed to the breakdown of polymer chains, while the second corresponds to the oxidation of the remaining organic matter. Both steps were accompanied by exothermic events in the DTA curve (Table 1). The thermal decomposition of PET was similar to those described in the literature in an air atmosphere.42,43 TG/DTG-DTA curves of glycerol are shown in Figure S4 (SI section). It remains thermally stable up to 170.0 °C (Table 1) and exhibits two consecutive steps of mass loss, with the first related to sample evaporation. The second corresponds to glycerol degradation. Both mass-loss stages produce, respectively, one endothermic and one exothermic event on the DTA curve (Table 1).
After modifying PET with glycerol, both PG-MW and PG-CH materials exhibited thermal stability up to approximately 120 °C (Table 1). At this temperature, they experienced an initial mass loss of around 10%, along with a significant endothermic event in their respective DTA curves (Table 1). This mass loss results from the release of residual glycerol, which may have been trapped inside the polymer matrix due to a cage effect, similar to other materials containing excess glycerol in their composition.44 Subsequently, as evidenced by the variation of their respective TG/DTG curves, both modified samples exhibited mass loss starting at approximately 245 °C (Table 1), significantly lower temperature compared to PET (329 °C). The beginning of each mass loss for PET, PG-MW, and PG-CH is better visualized in the SI section (Figures S5 to S7).
This indicates a partial depolymerization process, leading to shorter chain lengths (lower molecular weight PET). The following mass loss steps were similar to those observed for PET (Table 1), suggesting that a mixture of partially depolymerized PET and PET may be present in the system.
To further examine the samples and remove any remaining glycerol, PG-MW and PG-CH were heated to 230 °C, producing PG-MW-230 and PG-CH-230, respectively. Their MIR spectra remained similar to those of the unheated samples (Figure 3), except in the region between 3670 and 3020 cm–1. Instead of a broad and intense band, a smaller one appeared between 3640 and 2380 cm–1, indicating hydroxyl groups and supporting the formation of polymer chains different from those of PET, such as lower molecular weight PET. Analysis of the TG/DTG-DTA curves of PG-MW-230 (Figure 4d) and PG-CH-230 (Figure 4e) confirmed the absence of residual glycerol, as both samples showed three consecutive mass loss steps associated with exothermic events in their DTA curves.
Melting processes in PG-MW-230 and PG-CH-230 were identified by endothermic peaks (Tp) in their DTA curves at 245 and 239 °C, respectively. The similarity between these processes and those of PET will be further detailed in the DSC analysis. As expected, these samples exhibited lower thermal stability (ca. 260 °C) than PET. The first mass loss (ca. 6%, Table 1) in these samples can be attributed to the initial decomposition of polymer chains modified with glycerol (lower molecular weight PET), which may have overlapped with the decomposition of unmodified PET chains, leading to the subsequent mass loss.
In addition to exhibiting similar decomposition behavior to PET partially modified with other alcohols,41 the partial modification of chains was supported by the comparable values of PG-MW-230 (16.7% min-1 at 434.0 °C) and PG-CH-230 (17.1% min-1 at 434.0 °C), which were lower than PET’s (30.8% min-1 at 426.0 °C). Furthermore, the events observed in the DTA curve within this temperature range differed. Simultaneously, the PET sample displayed a well-defined exothermic peak at Tp = 444 °C; the modified samples showed broad, overlapping exothermic events between 380 and 460 °C (Figures 4d and 4e, Table 1). These events became more prominent under an inert atmosphere (N2), as illustrated in Figure 4f, with a magnified DTA curve emphasizing this temperature range in Figure 4g.
The cyclic DSC curves for each sample are shown in Figure S8 (SI section). After synthesizing PG-MW-230 and PG-CH-230, the cooling process was not controlled. To eliminate the thermal memory of the polymers, only the cooling and second heating stages were analyzed in this discussion, removing the influence of thermal history. During cooling (Figure 5a), PET showed an exothermic peak related to crystallization, with an enthalpy value (Table 2) similar to those reported in literature (36 J g–1).41 PG-MW-230 and PG-CH-230 also crystallized during cooling, with peak temperatures and enthalpy values comparable to PET (Table 2). However, the exothermic peaks of PG-MW-230 (201-155 °C) and PG-CH-230 (201-155 °C) were sharper than PET’s (218-123 °C), indicating that the modified polymer chains crystallized more quickly than those of PET. This may be due to the formation of smaller chains modified with glycerol.
DSC curves of PET, PG-MW-230, and PG-CH-230 in (a) the cooling stage and (b) the second heating.
Values of peak temperature values (Tp) and enthalpy (ΔH) observed for each crystallization (Crist) and melting (Melt) process observed in DSC curves
During the second heating stage (Figure 5b), all samples exhibited a glass transition (Tg). The Tg values of PG-MW-230 and PG-CH-230 were similar but lower than those of PET. Subsequently, PET displayed a sharp endothermic peak (Table 2), corresponding to its melting.40,41 Although PG-MW-230 and PG-CH-230 also displayed endothermic events, at least two consecutive overlapping peaks were observed, as shown in Figure 5b. In addition, the modified samples (PG-MW-230 and PG-CH-230) exhibited a melting peak (Tp) at different temperatures compared to PET (Table 2), with a minimum difference of at least 10 °C, which can be attributed to chains of varying lengths. This profile was also observed in the DSC curves of PET containing small chains resulting from glycolysis with other alcohols.20,41
Figure S9 (SI section) displays representative images taken during the heating and cooling of PET and PG-MW. The PET sample was a translucent solid, and as it was heated, it began to melt, completing the process at 254 °C. During cooling, solidification started at 210 °C, as shown by small solid portions at the sample edges (indicated by blue arrows in Figure S9a). As the temperature continued to drop, solidification progressed and finished at 180 °C (see Figure S9a). The second heating cycle closely resembled the first: the sample showed a glass transition, followed by melting process.
The images obtained for PG-MW are shown in Figure S9b. At 25 °C, it appeared white and opaque, in contrast to PET. Upon heating, slight camera fogging was observed, becoming evident at 200 °C. The sample then began to melt, as indicated by the red arrows in Figure S9b, and the melting process was completed at 240 °C. During cooling, solidification initiated at the sample edges (highlighted by the red ellipse). No fog release was observed during the second heating, consistent with the suggestion that the initial mass loss in the TG curve (Figure 4) was due to the release of residual glycerol. In the second heating, two consecutive and overlapping events were observed, in contrast to the single event seen for PET. At 217 °C (Figure S9b), partial melting occurred at several points within the sample. By 230 °C, the sample consisted of both solid and liquid phases (as indicated by the purple circle), with complete melting occurring at ٢٣٧ °C. These findings support the existence of chains of different lengths in the PG-MW sample, indicating that transesterification with glycerol had taken place, even though some PET chains remained.
Therefore, it is suggested that lower molecular weight PET resulting from the partial glycolysis of PET with glycerol is present, along with non-modified PET chains. Furthermore, based on MIR, TG/DTG-DTA, and DSC analyses, it can be affirmed that both microwave irradiation and conventional heating were effective in yielding similar samples. Therefore, from this point forward, only systems derived from microwave irradiation will be evaluated to avoid duplicates results.
1H NMR spectra of PET, PG-MW, and PG-MW-230, and the most important signals are highlighted in Figure 6a. In the PET spectrum, the singlet at 8.14 ppm is related to aryl hydrogens (terephthalic ring - highlighted in red), and the singlet at 4.80 ppm is attributed to methylene protons of the ethylene glycol backbone (highlighted in blue).19,35,41,45,46
(a) 1H NMR (600 MHz, (CDCl3:TFA-d) 3:1 v/v) spectra for PET, PG-MW, and PG-MW-230; (b) amplified 1H NMR spectra; and (c) monomers and proposed chemical structures for polymer and polymeric fragments.
After the microwave reaction with glycerol, new signals appeared in the spectrum. For instance, the double doublet between 3.91 and 3.97 ppm is related to the hydrogen of primary alcohol present in glycerol, and a doublet at 4.02 ppm is associated with the hydrogen of secondary alcohol (highlighted in green); all these are related to the remaining free glycerol in the system. As expected, the signals for free glycerol disappeared after heating the PG-MW to 230.0 °C. However, some signals persisted, which differ from those observed in the PET spectrum. Signals at 4.05, 4.20, 4.43, 4.62, 4.69, and 4.83 ppm correspond to hydrogens attached to the glycerol backbone of the new esters and to alcohol groups (Figure 6b). The presence of multiple new signals indicates that the polymeric product includes various polyester chains, such as esters of secondary and primary alcohols derived from glycerol, along with polyesters of ethylene glycol (remaining PET structures). Therefore, producing a pure polymeric matrix of a single, uniform polyester is not feasible. Some chemical structures of the partially modified PET with glycerol are shown in Figure 6c. Consequently, this polymeric mixture exhibits diverse properties and physicochemical characteristics that influence the final product.
The resulting spectra of solid UV-Vis are present in Figure 7a. The PET spectrum exhibited a large absorption band between 245 and 322 nm, with two maxima observed: the first at 270 nm and the second at 299 nm. After modification, the sample PG-MW exhibited the same bands and a new, small band at 234 nm. This additional band may result from the residual glycerol in the polymer (cage effect), which disappears after heating the sample to 230 °C (due to the glycerol being released), as observed in the PG-MW-230 spectrum. Thus, based on the UV-Vis spectra, no significant changes in absorption properties were detected after modification and the release of residual glycerol, corroborating the obtention of lower molecular weight PET.
(a) UV-Vis spectra obtained for samples in solid state, and SEM micrograph for (b) PG-MW, and (c) PG-MW-230.
Figures 7b and 7c present the scanning electron microscopy (SEM) micrographs for PG-MW and PG-MW-230, respectively. Both have a magnification of 3k and a scale bar of 10 mm. The PG-MW exhibits a bulk structure composed of an agglomeration of particles of different sizes (red circle); consequently, small particles create surface roughness (blue arrows). PG-MW-230 shows a rough surface (blue arrows) and some holes (green arrows) that may form after glycerol evaporation.
A qualitative test involving the adsorption behavior of the obtained material is shown in Figure 8a. The PG-MW sample demonstrated the ability to adsorb RhB dye, as evidenced by a noticeable color change in the solution after 24 h, with the solution becoming lighter while PG-MW acquired a pink color. This behavior was not observed for PET, which was submitted to the same conditions. Therefore, despite the incomplete depolymerization of PET by glycerol, the resulting material exhibited properties distinct from those of PET waste.
(a) Qualitative tests performed using a RhB (10.4 mg L-1); adsorption performance of PG-MW according to (b) removal percentage of RhB and (c) adsorption capacity; (d) cycles of RhB retention using PG-MW as an adsorbent filter.
The removal percentage and adsorption capacity over time are shown in Figure 8. As seen in Figure 8b, the dye removal percentage during the first 30 min stayed below 15.0%. After 60 min, equilibrium was reached, with R% values stabilizing at 16.1% and staying near this level until 720 min. Additionally, as shown in Figure 8c, the amount of dye adsorbed reached 2.85 mg g–1 after 30 min. It remained almost unchanged until the end of the experiment, as expected once equilibrium was achieved. Although the adsorption efficiency was not very high, PG-MW showed potential as an adsorbent, which will be further studied in future work by adjusting factors like adsorbent amount, dye concentration, temperature, and pH.
Furthermore, the adsorption potential was examined by using the polymer as an adsorptive agent in a filtration system for RhB removal. As shown in Figure 8d, retention rates above 99% were maintained during the initial three cycles. During the next eight cycles, values stayed above 97%. Finally, after the twelfth cycle, the dye removal percentage started to decrease, reaching 93.5%, and ultimately dropped to 82.7% after the fifteenth cycle. Therefore, the effective retention of RhB from an aqueous solution can be attributed to the large amount of PG-MW used and its associated adsorption capability.
Conclusions
This work presents a green and efficient method for PET modification through partial glycolysis with glycerol, using either microwave or traditional heating, without requiring solvents or catalysts. The resulting materials show decreased thermal stability, distinct glass transition temperatures, altered chemical structures, and potential as adsorbents, confirming successful polymer transformation. This approach allows for the valorization of PET waste into functional materials, promoting sustainable recycling technologies aligned with circular economy principles.
Supplementary Information
Supplementary data are available free of charge at http://jbcs.sbq.org.br as PDF file.
Data Availability Statement
Data is available in “Repositório Institucional UNESP” https://hdl.handle.net/11449/312908.
Acknowledgments
The authors gratefully acknowledge the financial support provided by: The São Paulo Research Foundation - FAPESP (grants 2024/14279-1; 2024/00779-2; 2024/03936-1; 2021/14879-0), CAPES (grants 024/2012 - Pro-equipment, and 011/2009), and National Council for Scientific and Technological Development - CNPq (grant 303968/2024-9). In addition, we would like to express our gratitude to Prof Dr José Humberto Dias da Silva and Prof Dr Luiz Carlos da Silva Filho, as well as their research groups, for their kindness in allowing us to use their laboratories and equipment to obtain the UV-Vis spectra of the solid and liquid samples, respectively.
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Edited by
-
Editor handled this article:
Paulo Augusto Netz (Associate)
Publication Dates
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Publication in this collection
20 Oct 2025 -
Date of issue
2025
History
-
Received
30 June 2025 -
Accepted
01 Sept 2025
