Effect of Chemical Agents on the Morphology and Chemical Structures of Microplastics

Increased demand for plastics leads to a large amount of plastic manufacturing, which is accompanied by inappropriate disposal of plastics. The by-products of these waste plastics are microplastics (MPs; less than 5 nm in size), which are produced because of various environmental and physicochemical factors, posing hazardous effects to the ecosystem, such as the death of marine organisms due to the swallowing of plastic specks of no nutritional value. Therefore, the collection, preparation, identification, and recycling of these microsized plastics have become imperative. The pretreatment of MPs requires numerous chemical agents comprising strong acids, bases, and oxidizing agents. However, there is limited research on the chemical resistance of various MPs to these substances to date. In this study, the chemical resistance of five species of MPs (high-density polyethylene, low-density polyethylene, polystyrene, polyethylene terephthalate, and polypropylene) to sulfuric acid, hydrochloric acid, hydrogen peroxide, potassium hydroxide, and sodium hydroxide was studied. The MPs were reacted with these chemical reagents at preset temperatures and durations, and variations in morphology and chemical structures were detected when the MPs were reacted with mineral acids, such as sulfuric acid. The data pertaining to these changes in MP properties could be a significant reference for future studies on MP pretreatment with strong acids, bases, and oxidizing agents.


Introduction
Industrialization over the last century has led to increased demand for and production of plastic materials [1,2]. However, poor disposal and the resultant fragmentation of plastics has produced small plastic particles, commonly referred to as microplastics (MPs) with sizes <5 mm [3]. Plastic fragmentation can be caused by a combination of environmental and physicochemical factors, such as mechanical abrasion, ultraviolet (UV) radiation, and others, resulting in numerous secondary MPs [4]. These resultant microscopic plastics are ubiquitously found in marine and fresh water and terrestrial ecosystems across the globe, resulting in biotic interactions [5][6][7][8]. These MPs do not immediately affect living organisms, but long-term exposure causes numerous hazardous effects by various mechanisms, such as intake of toxic additives, inflammation in organisms through sharp edges of MPs, and the resultant translocations in blood circulation [7]. Therefore, sample collections from water, sediment, and biological samples are conducted for identification and analysis of these MPs for further research [9,10]. These sample collections are then followed by sequential sample preparation utilizing a variety of techniques, including density separation, sieving, and digestion [11,12].

Experiment Procedure of Chemical Reactions of Microplastics with Acids, Bases, and Oxidizing Agents
Five sets of 0.5 g of all the MP samples were weighed and loaded in 20 mL vials. Then, 10 mL of the acids, bases, and oxidizing agents were added to each sample set for the reaction with MP samples. The reactions were conducted at 25 and 70 • C to investigate the effects of temperature on the property modification of the polymers. After one and seven days of reactions between the MPs and acids, bases and oxidizing agents, the mixtures were washed several times by DIW using a vacuum-assisted filtration kit to remove the remaining acids, bases, and oxidizing agents. The washed MP powders were dried overnight and stored in a desiccator for further characterization. The acids and oxidizing agent were used without any dilution. For chemical stability tests, 1 M of potassium hydroxide and sodium hydroxide solution were synthesized.

Characteristics of MP Samples before and after Exposure to the Acids, Bases, and Oxidizing Agents
SEM was performed using a JSM-7800 Prime microscope (JEOL Ltd., Tokyo, Japan). The functionalities of the various MP samples before and after exposure to the acids, bases, and oxidizing agents were identified by ATR (attenuated total reflection) detecting mode of Fourier-transform infrared spectroscopy (FT-IR, TENSOR27, Bruker, Borken, Germany). The detection range was 4000-400 cm −1 , the wavenumber accuracy was 0.01 cm −1 , and the resolution was 0.4 cm −1 . 13 C NMR measurements were conducted on an Advanced III HD Solid-state NMR spectrometer (Bruker, Germany). Solid-state 13 C CPTOSS NMR spectra were obtained using 4 mm C Cross-Polarisation Magic-Angle Spinning (CPMAS) probes and a 500 MHz Advanced III HD NMR spectrometer. A spinning speed of 5 kHz and pulse repetition delays of 5 s were utilized.

Chemical Stability Tests of Microplastic Samples
To evaluate the chemical stability, the MP samples obtained from all the polymer pellets and resins are immersed in the solutions of acid, alkali, and oxidizing agent in 20 mL vials ( Figure 1). The reaction of the polymer microparticles with the strong acids, bases, and oxidizing agent were performed for one and seven days at 25 and 70 • C. After 1 and 7 d, the reacting MP samples were repeatedly filtrated and washed by ultrapure deionized water until the remaining chemicals were completely removed. The washed MP samples were dried over 24 h and stored in a desiccator wrapped with aluminum foil to protect the MPs from ultraviolet light for further chemical characterization and observation of sample morphologies.
CPTOSS NMR spectra were obtained using 4 mm C Cross-Polarisation Magic-Angle Spinning (CPMAS) probes and a 500 MHz Advanced III HD NMR spectrometer. A spinning speed of 5 kHz and pulse repetition delays of 5 s were utilized.

Chemical Stability Tests of Microplastic Samples
To evaluate the chemical stability, the MP samples obtained from all the polymer pellets and resins are immersed in the solutions of acid, alkali, and oxidizing agent in 20 mL vials (Figure 1). The reaction of the polymer microparticles with the strong acids, bases, and oxidizing agent were performed for one and seven days at 25 and 70 °C. After 1 and 7 d, the reacting MP samples were repeatedly filtrated and washed by ultrapure deionized water until the remaining chemicals were completely removed. The washed MP samples were dried over 24 h and stored in a desiccator wrapped with aluminum foil to protect the MPs from ultraviolet light for further chemical characterization and observation of sample morphologies.  Figure 2 shows the changes in colors, functionalities, carbon structures, and morphologies of HDPE MPs after the reaction with acids, bases, and oxidizing agents. After completing the chemical stability testing procedures, including washing and drying, the HDPE MP samples were collected and characterized. As observed with the naked eye, the HDPE MPs reacted with H2SO4 at 70 °C for 1 and 7 d, resulting in color alterations from white to black, indicating chemical changes (Figures 2a and S6b). However, there was no color change observed for the HDPE MP samples reacted with HCl, H2O2, KOH, and NaOH (Figures 2a and S6). Solid-state 13 C NMR and FT-IR characterizations were conducted to monitor the carbon structures and functionalities of the HDPE MPs before and after the chemical stability tests. Because of the monoclinic crystalline component, a minor resonance signal at 33.4 ppm can be detected in Figures 2b and S7a-c for all the HDPE MP samples [28,29]. A strong resonance signal at 32.0 ppm is ascribed to the orthorhombic crystalline component of the HDPE MPs. The weak signal at 30.3 ppm is assigned to the  Figure 2 shows the changes in colors, functionalities, carbon structures, and morphologies of HDPE MPs after the reaction with acids, bases, and oxidizing agents. After completing the chemical stability testing procedures, including washing and drying, the HDPE MP samples were collected and characterized.  [28,29]. A strong resonance signal at 32.0 ppm is ascribed to the orthorhombic crystalline component of the HDPE MPs. The weak signal at 30.3 ppm is assigned to the non-crystalline component of the HDPE MPs [28,29]. After the reaction with the chemical agents, there were no changes or new signals in the spectra of any of the HDPE MP samples, indicating that the carbon structures of the HDPE MP samples were not altered. Figures 2c and S7d-f depict a series of attenuated total reflection (ATR) spectra of the HDPE MP samples prior to and following the chemical reactions with all the chemical agents at all predetermined times and temperatures. For the pristine HDPE MPs, the bands at 2913.9, 2845.9, 1471.9, and 717.4 cm −1 denote CH 2 asymmetric stretching, CH 2 symmetric stretching, CH 3 bending deformation, and CH 2 rocking deformation, respectively [30]. Further, consistency in the FT-IR ATR spectra suggests the HDPE MPs were stable against HCl, H 2 O 2 , KOH, and NaOH. In contrast to the other chemical agents, the HDPE MPs that were exposed to H 2 SO 4 exhibited several new bands, indicating surface sulfonation. At 25 • C for 1 and 7 d, new bands assigned to S-O-C stretching (888.6 cm −1 ), O=S=O stretching (1024.1 and 1159.4 cm −1 ), C=O stretching (1697.1 cm −1 ), and O-H stretching (3271.6 cm −1 ) [31] were observed. At 70 • C for 1 and 7 d, the peak at 1577.6 cm −1 can be attributed to C=C stretching in addition to the newly generated peaks [31]. Figure 2d depicts  non-crystalline component of the HDPE MPs [28,29]. After the reaction with the chemical agents, there were no changes or new signals in the spectra of any of the HDPE MP samples, indicating that the carbon structures of the HDPE MP samples were not altered. Figures 2c and S7d-f depict a series of attenuated total reflection (ATR) spectra of the HDPE MP samples prior to and following the chemical reactions with all the chemical agents at all predetermined times and temperatures. For the pristine HDPE MPs, the bands at 2913.9, 2845.9, 1471.9, and 717.4 cm −1 denote CH2 asymmetric stretching, CH2 symmetric stretching, CH3 bending deformation, and CH2 rocking deformation, respectively [30]. Further, consistency in the FT-IR ATR spectra suggests the HDPE MPs were stable against HCl, H2O2, KOH, and NaOH. In contrast to the other chemical agents, the HDPE MPs that were exposed to H2SO4 exhibited several new bands, indicating surface sulfonation. At 25 °C for 1 and 7 d, new bands assigned to S-O-C stretching (888.6 cm −1 ), O=S=O stretching (1024.1 and 1159.4 cm −1 ), C=O stretching (1697.1 cm −1 ), and O-H stretching (3271.6 cm −1 ) [31] were observed. At 70 °C for 1 and 7 d, the peak at 1577.6 cm −1 can be attributed to C=C stretching in addition to the newly generated peaks [31].  3.3. Chemical Stability of LDPE against Acids, Alkalis, and Oxidizing Agents Figure 3 displays the changes in colors, functionalities, carbon structures, and morphologies of LDPE MPs after the reaction with acids, bases, and oxidizing agents. The LDPE MP samples were collected and analyzed in the same manner as the HDPE MP samples following all the chemical stability tests. After the reaction with H 2 SO 4 at 70 • C after 1 and 7 d, the LDPE MP samples exhibit a color change from white to brown and black, respectively (Figures 3a and S9b). Figures 3a and S9 reveal that, similarly to the color change observed in the HDPE MP samples, the LDPE MP samples exposed to the various chemical agents did not exhibit a color change. Similarly, solid-state 13 C NMR and FT-IR spectra were obtained to observe carbon structures and functionalities of the LDPE MPs before and after the chemical stability tests. In Figures 3b and S10a-c, the monoclinic crystalline component, which appears as a minor resonance signal at 33.4 ppm, was not detected in the LDPE MP samples. This is unlike the observation in the HDPE MP samples. However, a strong resonance signal at 32.0 ppm owing to the orthorhombic crystalline component of the LDPE MPs is comparable to the result of the HDPE MPs [28,29]. The noncrystalline component from the LDPE MPs at 30.2 ppm appears as a weak signal [28,29]. Moreover, there was no change in the carbon structures of any of the LDPE MPs, suggesting that they are chemically resistant to all the chemical agents tested regardless of preset temperature. This chemical stability is attributed to the main chain of the LDPE samples. In Figures 3c and S10d-f, the functionality changes in the LDPE MP samples subjected to acids, bases, and oxidizing agents are depicted to examine their surface chemical alterations. The pristine LDPE MPs exhibited 2916.1, 2848.5, 1471.5, and 717.4 cm −1 of bands, designated as CH 2 asymmetric stretching, CH 2 symmetric stretching, CH 3 bending deformation, and CH 2 rocking deformation, respectively [30]. Similarly to the results of the HDPE MPs, the LDPE MP exhibited chemical resistance to HCl, H 2 O 2 , KOH, and NaOH with consistent FT-IR spectra at all testing conditions (Figures 3c and S10d-f). With H 2 SO 4 at 25 • C for one day, no new bands or band shifts in the FT-IR spectrum of the LDPE MP sample are observed, suggesting good chemical stability ( Figure S10d). Alternatively, with H 2 SO 4 at 25 • C for 7 d and 70 • C for 1 and 7 d, a chemical functionality change on the surface of the LDPE MPs is observed (Figures 3c and S10e-f). An S-O-C stretching band at 885.2 cm −1 , O=S=O stretching band at 1189.9 cm −1 , a C=O stretching band at 1675.9 cm −1 , and an O-H stretching band at 3394.3 cm −1 are observed, demonstrating that H 2 SO 4 induces sulfonation on the surface of the LDPE MP samples [31]. The morphological changes in the LDPE MP particles detected by SEM were similar to those in HDPE MP particles; the LDPE MP particles are spherical shaped and 20 µm in size (Figure 3d). This is indicative of surface deformation and aggregation when reacted with H 2 SO 4 at 70 • C for 7 d (Figure 3e). Regardless of temperature and reaction duration conditions, H 2 SO 4 induced surface deformation in the LDPE microparticles ( Figure S11a,f,k). However, the other chemical agents (HCl, H 2 O 2 , KOH, and NaOH) did not affect the morphology of the LDPE MPs (Figures 3f-i and S11).   Figure 4 presents the changes in colors, functionalities, carbon structures, and morphologies of PS MPs after the reaction with acids, bases, and oxidizing agents. After treating the PS MP samples with the acids, bases, and oxidizing agents, the samples were characterized. In the bulk state, the PS MP samples exhibit a color change from white to orange and swelling with H2SO4 at 70 °C for 1, and with H2SO4 at 70 °C and 7 d (Figures 4a and S12b). The PS MP samples exhibit a shift in hue from white to yellow during the HCl reaction at 25 °C for 1 and 7 d ( Figure S12a,c). In contrast, the other chemical agents did not affect PS MP samples in the bulk state regardless of temperature and reaction time (Figures 4a and S12). The 13 C NMR spectra of the PS MP samples show resonance signals at 144.8, 127.4, 45.1, and 39.4 ppm, which were attributed to non-protonated, protonated aromatic carbons, methylene, and methyl carbons of the pristine PS carbon structures, respectively (Figures 4b and S13a-c) [32,33]. The PS MP samples exhibit chemical resistance to HCl, H2O2, KOH, and NaOH at all preset conditions, indicated by the unchanging 13 C NMR spectra (no new bands and no chemical shifts). In addition, H2SO4 at 25 °C for 1 and 7 d cannot derive any chemical reactions to PS MP samples. However, with H2SO4 at 70 °C for 1 and 7 d, the resonance signals at 144.8, 45.1, and 39.4 ppm were suppressed. Furthermore, the protonated aromatic carbon peak at 127.4 ppm, which is designated as the mono-substituted toluene sulfonic acid signal, was firmly generated [34]. To  Figure 4 presents the changes in colors, functionalities, carbon structures, and morphologies of PS MPs after the reaction with acids, bases, and oxidizing agents. After treating the PS MP samples with the acids, bases, and oxidizing agents, the samples were characterized. In the bulk state, the PS MP samples exhibit a color change from white to orange and swelling with H 2 SO 4 at 70 • C for 1, and with H 2 SO 4 at 70 • C and 7 d (Figures 4a and S12b). The PS MP samples exhibit a shift in hue from white to yellow during the HCl reaction at 25 • C for 1 and 7 d (Figure S12a,c). In contrast, the other chemical agents did not affect PS MP samples in the bulk state regardless of temperature and reaction time (Figures 4a and S12). The 13  Furthermore, the protonated aromatic carbon peak at 127.4 ppm, which is designated as the mono-substituted toluene sulfonic acid signal, was firmly generated [34]. To further investigate the functionality changes in the PS MP samples, FT-IR characterization was conducted before and after the chemical stability tests (Figures 4c and S13d-f). Several bands at 3028.7, 2907.2, 1492.6, and 1451.6 cm −1 are observed, denoting aromatic stretching (=C-H), asymmetric stretching (-CH 2 -), and aromatic stretching (C=C), respectively, for the pure PS MPs [35].    3.5. Chemical Stability of PET against Acids, Alkalis, and Oxidizing Agents Figure 5 illustrates the changes in colors, functionalities, carbon structures, and morphologies of PET MPs after the reaction with acids, bases, and oxidizing agents. The PET MP samples were characterized after being treated with all the chemical agents used in this study. The PET MP samples reacted with H 2 SO 4 at 70 • C for 1 and 7 d, change color from white to black in the bulk state (Figures 5a and S15b). In contrast, the PET MP samples exhibit chemical resistance to HCl, H 2 O 2 , KOH, and NaOH under all predetermined conditions in the bulk state (Figures 5a and S15). As depicted in 13 C NMR spectra of pristine PET, signals at 167.4, 136.7, 133.2, and 64.7 ppm were assigned to COO, aromatic C adjacent to the ester, aromatic C, and COOCH 2 , respectively (Figures 5b and S16a-c) [37]. Chemical deformation or degradation was not detected with the treatment of HCl, H 2 O 2 , KOH, and NaOH at any predetermined temperature and treatment duration, demonstrating the chemical resistance of PET to these chemical agents (Figures 5b and S16a-c). However, PET breakdown into terephthalic acid monomer was observed when treated with H 2 SO 4 (Figures 5b and S16a-c). The signal for carboxylic acid at 172 ppm appears, and the signal for the C adjacent to the ester shifts from 136.7 to 135.5 ppm because the carboxylic esters hydrolyze to carboxylic acid. The ethylene carbon adjacent to the hydroxyl resonance signal (64.7 ppm) disappears because ethylene glycol is eliminated from the material during decomposition [37,38]. This decomposition is not completed with the treatment of H 2 SO 4 at 25 • C after 1 d ( Figure S16a), but harsher predetermined temperatures and longer reaction durations yield a complete conversion of the PET MPs into the terephthalic acid monomer (Figures 5b and S16a-c). The FT-IR ATR spectra of PET before and after the chemical stability test was observed to confirm the chemical resistance of the PET MP samples against the acids, bases, and oxidizing agents (Figures 5c and S16d-f). For the pristine PET MPs, a typical signal at 1714.5 cm −1 corresponding to the C=O stretching of ester bonds is confirmed in addition to the C-C phenyl ring band at 1407.8 cm −1 , and C-O stretching band at 1242.1 and 1093.5 cm −1 . Two peaks at 1016.3 and 873.7 cm −1 were designated as the distinctive peaks of pristine PET (Figures 5c and S16d-f) [39,40]. The PET MP samples are chemically resistant to HCl, H 2 O 2 , KOH, and NaOH, as evidenced by unchanging ATR spectra of the PET MPs (Figures 5c and S16d-f). In contrast, after reaction with H 2 SO 4 at all temperatures and reaction durations, the ATR spectra of the PET MPs exhibit typical bands were designated as aromatic dicarboxylic acids at 3062. 6 Figure 6 shows the changes in colors, functionalities, carbon structures, and morphologies of PP MPs after the reaction with acids, bases, and oxidizing agents. After the chemical stability test, the PP MP samples were collected and their bulk states were recorded. Similarly to the LDPE MP particles, the PP MPs undergo a color change from white to brown and black after reacting with H2SO4 at 70 °C for 1 and 7 d, respectively, (Figures 6a and  S18b). In addition, the PP MP samples reacted with HCl, H2O2, KOH, and NaOH do not undergo a color change (Figures 6a and S18). To examine the carbon structure of the PP MP samples, 13 C NMR spectroscopy was conducted on all the PP MP samples before and after chemical stability testing (Figures 6b and S19a-c). For the pristine PP MPs, resonance signals appear at 42.8, 24.9, and 20.3 ppm, denoting carbon components of-CH2-,-CH-, and-CH3 in the polymer main chains [42]. No new resonance signals are observed in the 13 C NMR spectra of the PP MP samples for reactions with any of the chemical agents or under any of the predetermined conditions, demonstrating the chemical resistance of the carbon structures. This is also confirmed by the ATR-FT-IR spectra (Figures 6c and S19df). As shown in Figures 6c and S19d-f, the pristine PP MPs exhibit four prominent peaks at 2951.5 and 2868.6 cm −1 for CH3 asymmetric and symmetric stretching vibrations, and at 2917.3 and 2838.2 cm −1 for CH2 asymmetric and symmetric stretching vibration bands. In addition, the two peaks at 1456.9 and 1375.7 cm −1 are attributed to the CH3 asymmetric 3.6. Chemical Stability of PP against Acids, Bases, and Oxidizing Agents Figure 6 shows the changes in colors, functionalities, carbon structures, and morphologies of PP MPs after the reaction with acids, bases, and oxidizing agents. After the chemical stability test, the PP MP samples were collected and their bulk states were recorded. Similarly to the LDPE MP particles, the PP MPs undergo a color change from white to brown and black after reacting with H 2 SO 4 at 70 • C for 1 and 7 d, respectively, (Figures 6a and S18b). In addition, the PP MP samples reacted with HCl, H 2 O 2 , KOH, and NaOH do not undergo a color change (Figures 6a and S18). To examine the carbon structure of the PP MP samples, 13 C NMR spectroscopy was conducted on all the PP MP samples before and after chemical stability testing (Figures 6b and S19a-c). For the pristine PP MPs, resonance signals appear at 42.8, 24.9, and 20.3 ppm, denoting carbon components of-CH 2 -,-CH-, and-CH 3 in the polymer main chains [42]. No new resonance signals are observed in the 13 C NMR spectra of the PP MP samples for reactions with any of the chemical agents or under any of the predetermined conditions, demonstrating the chemical resistance of the carbon structures. This is also confirmed by the ATR-FT-IR spectra (Figures 6c and S19d-f). As shown in Figures 6c and S19d-f, the pristine PP MPs exhibit four prominent peaks at 2951.5 and 2868.6 cm −1 for CH 3 asymmetric and symmetric stretching vibrations, and at 2917.3 and 2838.2 cm −1 for CH 2 asymmetric and symmetric stretching vibration bands. In addition, the two peaks at 1456.9 and 1375.7 cm −1 are attributed to the CH 3 asymmetric and symmetric deformation vibrations. The peak at 1167.7 cm −1 is designated to the C-C asymmetric stretching vibrations. The peaks at 997.9, 972.9, and 899.2 denote the CH 3 asymmetric rocking vibrations. Lastly, the peaks at 841.4 and 809.4 are attributed to the CH 2 rocking vibrations (Figures 6c and S19d-f) [43]. With HCl, H 2 O 2 , KOH, and NaOH, no peak shifts or new peaks are observed owing to the chemical resistance of the PP MPs. Moreover, reactions with H 2 SO 4 at 25 • C for 1 d did not induce chemical modifications on the PP MP surface. However, the longer reaction duration (7 d) or higher temperature (70 • C for 1 and 7 d) using H 2 SO 4 can induce surface chemical modifications. The O-H stretching band at 3414.4 cm −1 , the bands in the 1250-840 cm −1 region, and the peak at 582 cm −1 are attributed to the SO 3 H groups on the surface of the PP MPs. The peak at 1657.5 cm −1 denotes the stretching vibrations of C=O [44]. The surfaces of the approximately 20 µm-sized PP microparticles (Figure 6d) (Figures 6c and S19d-f) [43]. With HCl, H2O2, KOH, and NaOH, no peak shifts or new peaks are observed owing to the chemical resistance of the PP MPs. Moreover, reactions with H2SO4 at 25 °C for 1 d did not induce chemical modifications on the PP MP surface. However, the longer reaction duration (7 d Table  1).

Conclusions
In this study, five types of polymer-based MP samples were manufactured and reacted with strong acids, bases, and an oxidizing agent under four preset reaction conditions (reaction duration of 1 and 7 d and temperature of 25 and 70 • C) to investigate their chemical resistance. The HDPE, LDPE, and PP MPs exhibited chemical resistance to HCl, H 2 O 2 , KOH, and NaOH at all reaction conditions, as indicated by the unchanged morphology and chemical structures before and after the chemical stability tests. When treated with H 2 SO 4 , the chemical backbones of these three MPs were retained; however, under the temperature of 70 • C, they demonstrated morphological deformation and surface sulfonation. PS exhibited surface abrasion when reacted with bases at 70 • C, while the bases had no influence on its chemical structure. Only the reaction with H 2 SO 4 at an elevated temperature (70 • C) induced sulfonation of the PS MPs with morphological swelling and aggregation. In the case of PET MPs, H 2 SO 4 promotes hydrolysis of the PET MPs into nanoflower shaped terephthalic acid under all reaction conditions, while the other chemical agents could not induce any morphological or chemical changes. In conclusion, based on the results of the chemical stability tests, chemical digestion utilizing mineral acids, such as sulfuric acid, can induce morphological deformation and transformation in the chemical structures. As such, using sulfuric acid during the pretreatment of MPs should be avoided when researchers want to prepare MPs without chemical damaging based on findings on the results of chemical stability tests. Furthermore, chemical structure changes of microsized plastic particles showed almost the same results in their bulk states, such as film, which are reported in the related literature. Thus, these results can contribute to future research on chemical digestion of polymer-based MPs.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.