Microplastics (MPs) in wastewater treatment plants are exposed to oxidative conditions during disinfection and advanced oxidation processes (AOPs), which can alter morphology and surface chemistry and influence interactions with coexisting contaminants. Here, accelerated chemical oxidation was simulated using heat-activated potassium persulfate (K 2S 2O 8) and sodium hypochlorite (NaOCl) to examine the oxidative aging of MPs made from polyethylene (PE), polyethylene terephthalate (PET), and polypropylene (PP). Changes in particle morphology and surface chemistry before and after oxidant treatment were characterized using scanning electron microscopy (SEM) for morphological analysis and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy for chemical characterization. Carbonyl formation, an indicator of polymer oxidation, was evaluated using the carbonyl index (CI). Both oxidants induced surface morphological defects and carbonyl functional groups in the MPs, with CI increasing with degradation time. The CI trends suggest that MP oxidation varies with polymer type and oxidant. The effect of oxidative aging on MP sorption capacity was also investigated using copper ions as a model inorganic constituent. Although oxidative aging introduced oxygen-containing functional groups, no statistically significant differences in copper sorption were observed between pristine and oxidized MPs, indicating that MPs can act as vectors for copper regardless of their degree of surface oxidation. 1. Introduction Microplastics (MPs) are increasingly recognized as one of the most persistent and ubiquitous sources of contamination in aquatic ecosystems [ 1]. They are found in rivers [ 2], lakes [ 3], oceans [ 4], and drinking water systems [ 5]. Among the various sources of MPs, wastewater treatment plants (WWTPs) are major pathways for plastic particles into natural waters [ 6]. During wastewater treatment, plastic particles undergo physical, chemical, and biological processes that can modify their physicochemical properties [ 7]. However, it remains unclear how these treatment processes transform MPs and influence their interactions with other pollutants. Polyethylene (PE), polyethylene terephthalate (PET), and polypropylene (PP) are among the most detected MPs within WWTPs [ 8, 9]. These polymeric particles differ in molecular structure, surface chemistry, and oxidation resistance [ 10, 11], resulting in varied responses under wastewater treatment conditions. In modern WWTPs, persulfate-based advanced oxidation processes (AOPs) have been explored to degrade recalcitrant organic pollutants, while chlorination, a common disinfection process, is used to kill harmful pathogens. Both persulfate-based AOP and chlorination generate strong oxidants, particularly at elevated temperatures, that can attack polymer chains and modify their chemical structures [ 12, 13]. Recent studies have also investigated MP transformation under advanced oxidation processes, highlighting the role of strong oxidants such as persulfate and chlorine in altering polymer surface chemistry and degradation pathways [ 14, 15, 16]. Heat-activated K 2S 2O 8 is widely used in AOPs because it generates sulfate radicals (SO 4•−) and hydroxyl radicals (•OH), both strong oxidants, with SO 4•− being the dominant species at neutral pH [ 17]. These radicals can break polymer chains, introduce oxygen-containing functional groups, and cause surface defects [ 18]. Similarly, thermal chlorination with aqueous sodium hypochlorite (NaOCl) generates strong oxidants, such as hypochlorous acid (HOCl), at pH 3–6, which can oxidize polymer surfaces [ 19, 20]. While these processes are effective for water treatment, they may inadvertently accelerate the aging of MPs, thereby increasing their reactivity and susceptibility to further degradation [ 21]. Understanding how oxidative aging alters the chemical structure of MPs is important because surface chemistry affects their interactions with other pollutants [ 22]. This study addresses these gaps by investigating the accelerated aging of PE, PET, and PP MPs under heat-activated persulfate and chlorination conditions and by evaluating the impact of chemical oxidation on their sorption capacity. Chemical changes associated with the formation of oxygen-containing functional groups in the oxidized MPs were monitored by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and polymer degradation was quantified using the carbonyl index (CI). Changes in particle surface morphology were examined using scanning electron microscopy (SEM). By linking oxidative degradation to sorption behavior, this work provides insight into the fate of MPs in WWTPs and supports improved risk assessment of MP pollution. 2. Materials and Methods Chemicals and reagents. Potassium persulfate (K 2S 2O 8) and nitric acid (trace metal grade) were purchased from Fisher Scientific (Waltham, MA, USA). Sodium hypochlorite (NaOCl) solution (reagent grade, available chlorine 4.00–4.99%) was supplied by Sigma-Aldrich (St. Louis, MO, USA). An internal standard mix for inductively coupled plasma-mass spectrometry (ICP-MS) systems (P/N: 5188-6525) was purchased from Agilent Technologies (Santa Clara, CA, USA). A certified copper reference standard was purchased from Millipore Sigma (Darmstadt, Germany), and ultrapure water was produced using a Milli-Q water purification system (Millipore, Bedford, MA, USA). Model microplastic samples. Polypropylene (PP) polyballs and granular polyethylene terephthalate (PET) were supplied by Polysciences (Warrington, PA, USA) and Sigma-Aldrich (St. Louis, MO, USA), respectively. Polyethylene (PE) pellets were purchased from a local store in Waco (TX, USA). All MP samples were thoroughly rinsed with ultrapure water before use. Surface morphology characterization. The surface features of the MPs (PE, PET, and PP) before and after treatments with the oxidants (NaOCl and K 2S 2O 8) were examined using a FEI Versa 3D scanning electron microscope (SEM, FEI, Hillsboro, OR, USA). Prior to imaging, MP samples were mounted on aluminum stubs using carbon tape and sputter-coated with iridium (EM ACE 600, Leica Microsystems, Wetzlar, Germany). For each polymer type and treatment condition, multiple MP particles ( n = 3) were analyzed. For each particle, several regions of the surface were imaged to account for surface heterogeneity. The images presented are representative of the observed surface features across the analyzed particles. Spectroscopic analysis of microplastic samples. Polymer identities were verified by comparison with reference spectra from the Thermo Scientific TM Hummel Polymer and Additives FTIR Spectral Library (Thermo Fisher Scientific, Waltham, MA, USA). Chemical changes in the polymers were evaluated by comparing spectra of pristine and treated samples using a Nicolet iS50R FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with an ATR diamond crystal. Spectra were collected over 4000 − 400 cm −1 at a resolution of 4 cm −1 with 32 scans. Additionally, chemical oxidation was assessed by monitoring the appearance of oxygen-containing functional groups (carbonyl (C=O), carboxyl (C–O), and hydroxyl (O–H)). The carbonyl index (CI) was calculated from the FTIR spectra to quantify oxidative aging of the MPs. Quality control (QC). Quality control samples were included in the oxidative aging experiments. For each polymer type, water served as the control, and samples were prepared and incubated under the same conditions as the oxidant treatments. These controls allowed attribution of chemical changes to temperature, mixing, and contact with water in the absence of an oxidant. FTIR spectra of the oxidant solutions ( Figure S1A,B) and the solid form of K 2S 2O 8 ( Figure S1C) were collected to verify that the appearance of C=O, C–O, and O–H in the aged MPs originated from polymer oxidation rather than spectral contributions from the oxidants. All glassware and tools used for aging and sorption experiments were acid-washed and rinsed with ultrapure water before use. To confirm that the MPs did not contain measurable copper before the sorption experiments, procedural blanks and a solution containing only MPs were prepared and analyzed. No copper was detected above the limit of detection in the control samples, indicating that the MPs used in the study were copper-free before the sorption experiment. Oxidative aging of microplastics. Oxidative aging of PE, PET, and PP MPs was conducted using K 2S 2O 8 and NaOCl as oxidants. Experiments were performed in an incubator shaker set to 60 °C and 180 rpm (MaxQ 5000, Thermo Fisher Scientific, Waltham, MA, USA). Separate volumetric flasks made of glass were used for each polymer type and oxidant. K 2S 2O 8 oxidation experiments. Oxidation using K 2S 2O 8 was conducted in 50 mL glass volumetric flasks. Seven intact MP pieces of each polymer type were all introduced into a 30 mL solution of 100 mM K 2S 2O 8, with the oxidant concentration selected based on conditions reported in a previously published oxidative aging study [ 27]. Samples were agitated in an incubator shaker at 60 °C for 7 days. Oxidant solutions were replaced every 12 h, and one piece was removed at each sampling time point (24, 48, 72, 96, 120, 144, and 168 h). After the experiment, all samples were rinsed multiple times with ultrapure water using an analog vortex mixer (VWR, Radnor, PA, USA) and air-dried before ATR-FTIR characterization. All experiments were conducted in triplicate using separate glass volumetric flasks for each MP type. NaOCl oxidation experiments. Oxidative aging with NaOCl was conducted in 50 mL glass volumetric flasks in an incubator shaker at 60 °C and 180 rpm for 7 days, following the sampling design described above, in which seven intact MP pieces were introduced into each flask and one piece removed at each sampling time point. Prior to the experiment, 30 mL of chlorine water containing 5000 ppm free chlorine at pH 6.5, adjusted with 0.1 M HNO 3, was added to each flask before the MP particles were introduced. The free chlorine concentration and pH conditions were selected based on a previously published study investigating the accelerated aging of polymeric materials via chlorination [ 28]. Oxidant solutions were replenished every 12 h, and MP particles were removed at different time intervals (24, 48, 72, 96, 120, 144, and 168 h). After the experiment, all MP samples were rinsed multiple times with ultrapure water and air dried before ATR-FTIR characterization. Sorption experiment. The sorption capacities of pristine and oxidized MPs were compared for each polymer type. Oxidized MPs used in the sorption experiments were obtained from the K 2S 2O 8 oxidation experiments. In 50 mL glass volumetric flasks, 100 mg of MPs and 20 mL of 20 µg/L copper ion solution, prepared by diluting a 1 mg/L copper standard solution with ultrapure water, were mixed in an incubator shaker at room temperature and 180 rpm. The initial pH of the copper solution was 2.56, and no pH adjustment or monitoring was performed during the sorption experiment; significant pH changes were not expected. The acidic pH conditions ensure that copper remains predominantly in its soluble ionic form, allowing for consistent comparison of sorption behavior across polymer types. After 5 days, all MP particles were removed prior to analysis of the final copper concentration. Preliminary experiments indicated that sorption equilibrium was reached before 5 days. Sorption experiments were conducted in triplicate for each polymer type. Copper quantification by mass spectrometry. The initial and final concentrations of copper ions before and after the sorption experiments were quantified using the 63Cu isotope. All MP particles were removed before analysis, and the solutions were acidified and analyzed using an Agilent 7900 ICP-MS (Agilent Technologies, Santa Clara, CA, USA) operated in helium collision mode. Matrix-matched calibration standards prepared in 1% ( v/ v) HNO 3 were used to quantify copper ions in the solution. Scandium was used as the internal standard to correct for matrix effects and instrumental drift. Copper ions sorbed to the MPs were calculated using Equation (1), where ΔCu represents the copper ions sorbed to the MPs; Cu i is the initial concentration of copper ions before interaction with the MPs; and Cu f is the final concentration of copper ions after interaction with the MPs. ΔCu = Cu i − Cu f (1) Carbonyl index (CI) analysis. Carbonyl group formation in MPs during oxidative aging was evaluated using CI values calculated from FTIR spectra. The CI is calculated as the ratio of the integrated area of the C=O band to that of a specific reference band of the polymer [ 29]. The reference bands used for normalization were selected from values reported in the literature because they remain relatively unchanged during oxidation [ 30, 31, 32]. The integration ranges used for the carbonyl and reference bands for each polymer are summarized in . The carbonyl bands were identified based on characteristic carbonyl wavenumbers in the FTIR spectra of the treated MPs. The CI was calculated using these polymer-specific integration ranges. The CI was plotted against degradation time to compare the extent of oxidation between the two oxidants. For each polymer type and time point, three individual MP particles were analyzed to account for potential surface heterogeneity. For each particle, three ATR-FTIR measurements were collected at different locations on the particle surface, and the average value was used for CI calculation. The reported CI values represent the mean of the measurements obtained from the three particles. Statistical analysis. Statistical analysis was performed using OriginPro 2025 (OriginLab, Northampton, MA, USA) and Microsoft Excel 2025 (Microsoft, Redmond, WA, USA). Differences in sorption capacity between pristine and oxidized MPs across the three polymer types were evaluated using the Kruskal–Wallis test, with statistical significance set at p 0.05; Figure 6), though measurable sorption occurred under all conditions. This observation is consistent with previous work reporting adsorption to both pristine and aged MPs [ 51]. Oxidative aging introduced oxygen-containing functional groups; however, these changes did not lead to increased Cu 2+ sorption. This may be due to the relatively low density or limited accessibility of the newly formed functional groups. Although pristine PE and PP are nonpolar and hydrophobic, the observed sorption of Cu 2+ to pristine MP surfaces may be attributed to non-uniform surface properties and possible trace surface functionalities introduced during polymer processing [ 52]. Chemical oxidation introduces carbonyl-containing groups that can increase surface polarity and potentially enhance sorption via surface complexation and electrostatic interactions. Overall, these findings indicate that MPs can act as vectors for copper ions regardless of their degree of surface oxidation. It should be noted that the experiments were conducted in simplified aqueous systems that do not fully represent the complexity of real wastewater. In WWTP environments, the presence of organic matter and competing ions (e.g., Ca 2+, Mg 2+) may influence both oxidative aging and metal interactions with MPs. Therefore, the role of MPs as vectors for copper ions under environmental conditions may be more limited or variable than observed under these accelerated conditions and should be interpreted with caution. 4. Conclusions This study investigated the effects of chemical oxidation on the physicochemical properties of three MP types (PE, PP, and PET) and evaluated how surface alterations resulting from chemical oxidation influence MP interaction with copper ions. Thermal-assisted oxidative aging of MPs using NaOCl and K 2S 2O 8 altered the surface morphology, as observed by SEM, and carbonyl formation was detected by ATR-FTIR. These results confirm that chemically driven oxidative processes relevant to engineered aquatic environments can alter the physicochemical properties of MP. Sorption experiments demonstrated that both pristine and oxidized MPs could sorb copper ions across all three polymer types. Although oxidative aging altered surface chemistry, there were no statistically significant differences in copper sorption between pristine and oxidized MPs across all the investigated polymers. These findings indicate that MPs can interact with copper ions regardless of the MPs’ oxidation state. From an environmental perspective, these results highlight that MPs may act as carriers of inorganic contaminants either in relatively unaged or minimally altered states. The observation that pristine MPs sorb copper underscores their potential to participate in metal transport soon after entering aquatic systems. Together, these findings emphasize the importance of considering both pristine and environmentally aged MPs when assessing the role of plastic particles as vectors for trace metals in aquatic environments. Supplementary Materials The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/microplastics5020115/s1, Figure S1: Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra of the oxidants used in the chemical oxidation experiments, including (A) sodium hypochlorite (NaOCl) solution, (B) potassium persulfate (K 2S 2O 8) solution, and (C) solid-phase potassium persulfate (K 2S 2O 8) (dry salt). Spectra are included to document the characteristic infrared features of the oxidants and to confirm that the oxygen-containing functional groups observed in aged microplastics originate from polymer oxidation rather than spectral contributions from the oxidants.. Author Contributions Conceptualization, T.A. and C.M.S.; methodology, T.A. and C.M.S.; validation, T.A., A.K.C.-S., W.C.H. and C.M.S.; formal analysis, T.A., W.C.H. and C.M.S.; investigation, T.A. and A.K.C.-S.; resources, C.M.S.; data curation, T.A. and C.M.S.; writing—original draft preparation, T.A. and C.M.S.; writing—review and editing, T.A., A.K.C.-S., W.C.H. and C.M.S.; visualization, T.A., A.K.C.-S. and C.M.S.; supervision, C.M.S.; project administration, C.M.S.; funding acquisition, C.M.S. All authors have read and agreed to the published version of the manuscript. Funding This work was supported by the Food and Chemical Safety Committee of the Institute for the Advancement of Food and Nutrition Sciences (IAFNS). IAFNS is a nonprofit science organization that pools funding from industry collaborators and advances science through the in-kind and financial contributions from public and private sector participants. We also thank the C. Gus Glasscock, Jr. Endowed Fund for Excellence in Environmental Science for partial financial support of this research. Institutional Review Board Statement Not applicable. Informed Consent Statement Not applicable. Data Availability Statement Dataset available on request from the authors. Acknowledgments The authors thank the Department of Environmental Science at Baylor University for their support. The authors also thank the Center for Microscopy and Imaging (CMI), the Mass Spectrometry Center (MSC), and the Department of Chemistry at Baylor University for access to equipment and sample preparation. Conflicts of Interest The authors declare no conflicts of interest. 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