Introduction

Plastics are indispensable in modern society due to their versatility and durability; however, their increasing production and improper disposal have caused severe contamination of terrestrial, aquatic, and atmospheric environments1. Among synthetic polymers, PVC is of particular concern because of its high production volume, long service life, and resistance to natural degradation. Environmental stressors such as ultraviolet radiation, temperature fluctuations, and chemical oxidants can induce PVC aging through polymer chain scission, surface oxidation, and fragmentation, ultimately generating secondary microplastics2,3. Despite extensive documentation of PVC aging pathways, the environmental implications of chemically aged PVC microplastics, particularly their altered interactions with organic pollutants, remain insufficiently resolved.

MPs, defined as plastic particles smaller than 5 mm, are now ubiquitously detected in freshwater and marine systems4. Their persistence, hydrophobic nature, and large surface-to-volume ratio enable them to adsorb, concentrate, and transport organic contaminants over long distances while participating in photochemical and oxidative reactions in aquatic environments5,6,7. PVC-derived MPs, in particular, have demonstrated strong adsorption affinity toward a wide range of pollutants, including dyes, polycyclic aromatic hydrocarbons, pharmaceuticals, pesticides, and heavy metals7,8,9. However, most adsorption studies implicitly assume pristine or environmentally weathered plastics, without systematically isolating the role of specific chemical aging processes on pollutant uptake capacity.

Synthetic dyes represent a major class of aquatic pollutants due to their complex aromatic structures, high chemical stability, and resistance to biodegradation10. The adsorption of dye molecules is strongly governed by the surface functionality, charge distribution, and structural disorder of the adsorbent. Studies on various adsorbent systems, including bacterially produced amorphous calcium carbonate, have highlighted the critical role of surface functional groups in enhancing dye–surface interactions11. Among industrial dyes, CV is widely used in textile dyeing, printing, and biological staining, and is recognized for its high toxicity, genotoxic potential, and environmental persistence12,13. The cationic nature and conjugated aromatic structure of CV promote strong adsorption through electrostatic attraction, π–π stacking, and hydrogen bonding with oxidized or negatively charged surfaces13,14. Nevertheless, the extent to which chemically induced oxidation of PVC microplastics specifically alters CV adsorption mechanisms has not been quantitatively established.

Recent studies demonstrate that aging processes significantly modify microplastic surface properties, increasing surface roughness and introducing oxygen-containing functional groups that enhance adsorption of organic pollutants, particularly cationic dyes15,16,17. In addition to physicochemical aging, biological processes such as microbial colonization and biofilm formation further accelerate polymer oxidation and structural alteration, thereby influencing pollutant adsorption behavior18. Advances in polymer transformation research also indicate that deliberate modification of surface chemistry and functional groups can critically influence polymer–pollutant interactions19. Despite these advances, a clear mechanistic understanding linking a defined oxidative aging process to changes in adsorption performance for PVC microplastics is still lacking.

Ozone is a powerful oxidant extensively applied in water and wastewater treatment and is capable of inducing polymer chain scission, surface oxidation, and microcrack formation in plastic materials20,21. While ozonation has been widely studied for polymer degradation and disinfection purposes, its role as a controlled aging pathway that modulates the sorption behavior of PVC microplastics toward organic dyes has received little attention. Existing studies predominantly emphasize physical degradation or mass loss, rather than the adsorption consequences arising from ozone-induced surface functionalization.

Accordingly, this study explicitly addresses this knowledge gap by systematically investigating how controlled ozone aging alters the surface chemistry of PVC microplastics and how these changes govern the adsorption behavior of crystal violet. By linking ozonation-induced physicochemical transformations to adsorption performance, this work provides mechanistic insight into the role of ozone-aged PVC microplastics as dynamic pollutant vectors in oxidizing aquatic environments, thereby improving the understanding of microplastic–pollutant interactions under realistic water treatment and environmental conditions.

Materials and methods

Chemicals and reagents

PVC grade 65 (< 425 μm) was sourced from Abadan Petrochemical Company (Iran), and CV dye (≥ 90% dye content) from Sigma-Aldrich (Germany).

Characterization techniques

To identify functional groups on the PVC surface and assess bond changes from aging, Fourier Transform Infrared (FTIR) analysis was performed. For high-resolution imaging of morphological features, surface characteristics, and the presence of pores, cracks, and roughness in aged MPs, Field Emission Scanning Electron Microscopy (FESEM) was used. Energy Dispersive X-ray Spectroscopy (EDX) determined the elemental composition of sample surfaces. Dynamic Light Scattering (DLS) measured particle size and its variations after aging. ZP measurements assessed the surface properties of aged PVC particles22.

Aging process

In this study, a PVC microplastic suspension (1 g/L) was prepared by dispersing the particles in 100 mL of distilled water. The mixture was then subjected to an ozonation treatment using the Anyzone Gold generator (Model BIO-2002 A). Based on preliminary optimization tests, the maximum stable ozone output of the system (1.4 mg L⁻¹. min⁻¹) was selected as the operational dose, and the suspension was exposed to ozone for 1 h at room temperature.

Following the aging process, the ozonated PVC particles were collected using Whatman filter paper (0.45 μm pore size) and dried at 60 °C for 24 h to remove residual moisture before further analysis23.

Batch adsorption experiment

To evaluate the adsorption of CV onto PVC before and after ozonation, the following procedure was conducted. A 1 g/L dose of both aged and raw PVC was separately added to a 5 mg/L CV solution. The mixtures were stirred at 1500 rpm for 45 min, then centrifuged. The absorbance of the supernatant was measured at 590 nm using a UV-Vis spectrophotometer. Adsorption efficiency was calculated using the following equation:

$$\:\text{C}\text{V}\:\text{a}\text{d}\text{s}\text{o}\text{r}\text{p}\text{t}\text{i}\text{o}\text{n}\:=\frac{{C}_{0}-{C}_{e}}{{C}_{0}}\:\times\:100$$
(1)

Where C₀ is the initial CV concentration, and Cₑ is the final CV concentration.

Result and discussion

Characterization

Figure 1 presents the FTIR spectra of PVC before and after aging. In Fig. 1a, the band at 610.60.

cm− 1 corresponds to C-Cl stretching, a characteristic structural feature of PVC24. The bands at 692.44 cm⁻¹ and 833.80 cm− 1 are associated with C-H bending vibrations25, while the peak at 1097.48 cm− 1 corresponds to C-O stretching. Additionally, the peak at 3444.68 cm− 1 represents OH stretching, indicating the presence of hydroxyl groups or moisture on the sample surface11. As shown in Fig. 1b, significant spectral changes in PVC occur after ozonation. The C-Cl stretching band shifts to 607.05 cm− 1, indicating partial dichlorination1. The C-H peaks decrease and shift to 689.27 cm− 1 and 833.59 cm− 1, suggesting oxidative degradation26. Additionally, the increased peak intensities at 2816.27 cm− 1 and 2846 cm− 1 indicate the formation of aldehyde and carboxylic acid groups27.

Fig. 1
figure 1

FT-IR spectra of (a) virgin PVC and (b) aged PVC.

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FESEM was employed to evaluate the surface morphology and structural alterations of PVC before and after ozonation. As illustrated in Fig. 2a, the pristine PVC exhibited a relatively smooth and compact surface with minimal irregularities and no evident cracks or pores. In contrast, the ozonated PVC sample (Fig. 2b) displayed pronounced surface deterioration, including irregular cracks, microscopic cavities, and porous regions28. These changes clearly indicate that ozone induces oxidative degradation at the polymer surface, resulting in polymer chain scission and partial material erosion20.

Compared to UV-aged microplastics, which typically show gradual surface pitting and discoloration, the ozone-treated PVC in this study exhibited a more heterogeneous and aggressive degradation pattern. This observation suggests that ozonation triggers a stronger and more rapid oxidative mechanism, leading to intensified surface disruption20. The presence of interconnected pores and fissures likely stems from the cleavage of carbon–carbon and carbon–chlorine bonds, which weakens the structural integrity of the polymer matrix and increases surface disorder.

Such structural deterioration not only increases brittleness but also significantly enlarges the effective surface area and the number of available binding sites, thereby enhancing the interaction potential between PVC microplastics and dissolved contaminants29. Similar phenomena have been reported by Hu et al., who observed that treatment of polyethylene microplastics with ozone and hydrogen peroxide induced the formation of oxygen-containing functional groups, increased surface negativity, promoted fragmentation, and altered adsorption behavior1. Likewise, Kye et al. (2024) demonstrated that increasing ozone doses progressively intensified surface abrasion and crack development in various microplastic types20.

It should be noted, however, that FESEM provides information primarily on surface-level morphological changes and does not capture internal or sub-surface chemical restructuring. Therefore, while the observed surface damage strongly supports enhanced adsorption capacity, it represents only one aspect of the comprehensive aging process affecting PVC microplastics. These morphological modifications collectively suggest that ozonated PVC microplastics possess an increased potential to adsorb and transport environmental contaminants, thereby elevating their ecological significance and potential risk in aquatic ecosystems29.

Fig. 2
figure 2

FESEM images of the (a) virgin PVC and (b) aged PVC.

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As shown in Fig. 3a, the weight percentages of carbon, chlorine, and oxygen in raw PVC were 57.12%, 36.97%, and 5.07%, respectively. Minor amounts of phosphorus, sulfur, and potassium were also detected at 0.52%, 0.18%, and 0.14%, respectively. In Fig. 3b, chlorine content increased to 43.16%, indicating a structural rearrangement of chlorine groups after aging process. Carbon content decreased to 51.92%, likely due to polymer chain degradation and conversion of carbon compounds into volatile substances30. Oxygen content declined to 3.48%, attributed to the degradation of oxygen-containing functional groups during ozonation and subsequent surface reactions. These trends do not contradict expected ozonation effects because ozone primarily induces surface-limited oxidation, forming carbonyl and other oxygenated groups at the nanoscale, while EDX averages signals over a slightly deeper region, potentially underestimating oxygen enrichment at the outermost surface. The increase in chlorine reflects surface reorganization rather than a net gain of chlorine atoms. Additionally, phosphorus, sulfur, and potassium levels showed a slight increase, likely due to surface chemical reactions and adsorption of environmental impurities post-ozonation. Meng et al. (2021) reported a similar trend, where EDX analysis revealed decreased oxygen and increased chlorine in aged PVC treated with H₂O₂31. Likewise, Zahmatkesh et al. (2024) observed a reduction in carbon content following the aging of cigarette butt microfibers subjected to ultrasonic treatment32.

Fig. 3
figure 3

EDX results for (a) virgin PVC and (b) aged PVC.

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DLS analysis revealed noticeable changes in the particle size of PVC following ozonation. As shown in Table 1, the average diameter decreased from 405.9 nm in raw PVC to 353.4 nm after aging, indicating partial fragmentation and surface degradation. Similar trends have been reported for other polymer types exposed to strong oxidants, though the magnitude of change varies depending on polymer crystallinity, additive content, and exposure conditions21,33. Ziembowicz et al. (2024) reported a reduction in PVC particle size after short-term ozone exposure under different pH conditions, which was attributed to surface chain scission and structural weakening20. However, some studies have also observed negligible changes, or even apparent size increases, due to aggregation or surface swelling during aging, suggesting that particle size evolution is highly system-dependent and cannot be generalized across all polymer–oxidant combinations34. Therefore, while the decrease observed here is consistent with ozone-induced chain breakdown, it should be interpreted as a combined effect of oxidative scission, surface etching, and possible loss of low-molecular-weight fragments rather than simple physical shrinkage.

Table 1 DLS and ZP values for Virgin PVC, and aged PVC.
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In addition, ZP shifted from − 25.6 mV to − 31.6 mV after aging, reflecting a significant modification of surface chemistry. This shift is generally associated with the introduction of oxygen-containing functional groups, such as carbonyl and carboxyl moieties, during oxidative aging35,36. While several studies support this trend for aged plastics, the extent of the change depends strongly on the intensity of oxidation and the initial polymer composition37. A more negative surface charge typically enhances electrostatic repulsion between particles, improving colloidal stability and reducing aggregation in aqueous systems1. However, increased stability does not necessarily imply reduced environmental risk; rather, more stable and dispersed particles may remain suspended for longer periods and exhibit enhanced mobility. Furthermore, the deprotonation of newly formed functional groups in aqueous environments increases the density of negative charges on the surface, which may intensify electrostatic attraction toward positively charged pollutants and dyes, thereby potentially enhancing adsorption performance35.

Kinetic models

To elucidate the adsorption kinetics of CV onto aged PVC MPs, the pseudo-first-order (PFO), pseudo-second-order (PSO), and intra-particle diffusion (IPD) models were applied, and their fitting parameters are summarized in Table 2. Among these models, the PFO equation provided the best fit across all concentrations, with high correlation coefficients (R² = 0.98–0.99) and consistent equilibrium adsorption capacities (qₑ = 0.96–0.99 mg g⁻¹). This strong agreement suggests that the adsorption process is predominantly governed by physical interactions and diffusion-controlled transport rather than by chemical bond formation. The observed increase in the PFO rate constant (k₁) with increasing CV concentration further indicates that elevated dye levels enhance the initial sorption rate, likely by increasing the driving force for mass transfer38.

In contrast, the PSO model exhibited considerably poorer performance, particularly at higher dye concentrations, where the R² value decreased sharply (down to 0.24 at 15 mg L⁻¹). Even at the lowest concentration tested, the PSO fit remained inferior to that of the PFO model (R² = 0.80 at 5 mg L⁻¹). Moreover, the large fluctuations in the k₂ values, ranging from 13.8 min⁻¹ at 10 mg L⁻¹ to nearly zero at ≥ 15 mg L⁻¹, suggest that the PSO assumptions of chemisorption-controlled kinetics do not adequately describe this system. While several studies have reported PSO-dominated kinetics for dye adsorption onto specific plastic polymers, such as malachite green on polypropylene39 or organic pollutants on polyamide MPs40, these observations cannot be generalized. Adsorption kinetics are highly dependent on factors such as polymer composition, surface functional groups, crystallinity, and the physicochemical characteristics of the target pollutant. Therefore, discrepancies across studies underscore the need to evaluate multiple kinetic models rather than assume a universal adsorption mechanism for all microplastic, pollutant systems.

The IPD model was also employed to explore the contribution of internal diffusion to the overall rate-limiting step41. The moderate to high correlation values (R² = 0.80–0.98) indicate that intra-particle diffusion plays a role during the adsorption process but is not the sole controlling mechanism. The diffusion rate constant (kₚ) increased from 0.15 to 0.47 mg g⁻¹ min⁻⁰·⁵ with increasing CV concentration, suggesting that higher concentration gradients promote greater mass transfer into the PVC matrix. However, the intercept values (not passing through the origin) imply the presence of boundary layer effects, confirming that film diffusion and surface interactions also contribute significantly.

Overall, the kinetic analysis demonstrates that CV adsorption onto aged PVC MPs is best described by the PFO model, indicating a predominantly physical adsorption mechanism influenced by both external mass transfer and intra-particle diffusion processes.

Table 2 Kinetic and isotherm model factors for the sorption of CV onto aging PVC.
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Isotherm model

To further elucidate the adsorption behavior of CV on ozonated PVC microplastics, three widely used equilibrium models, the Langmuir, Freundlich, and Temkin isotherms, were applied, and their respective parameters are summarized in Table 2. These models represent different assumptions regarding surface heterogeneity, interaction energies, and adsorption mechanisms, which allows for a more nuanced interpretation of the adsorption process.

The equilibrium data were well described by the Langmuir model (R² = 0.97), suggesting that adsorption predominantly occurred through monolayer coverage on a surface with a finite number of energetically similar sites12. The maximum adsorption capacity (qmax) obtained from this model was 5.55 mg/g, while the Langmuir constant (kL = 0.13 L/mg) reflects a moderate affinity between the CV molecules and oxidized PVC surfaces. This behavior is consistent with adsorption onto polymeric materials that have undergone surface functionalization but still retain relatively limited site diversity.

In contrast, the Freundlich model yielded a slightly higher correlation coefficient (R² = 0.99), indicating that it also provides an excellent mathematical fit. The Freundlich constant (kF = 1.51 mg/g·(L/mg)¹⁄ⁿ) and the heterogeneity factor (n = 1.77) imply favorable adsorption on a surface with non-uniform energy distribution and the possibility of multilayer formation. Such behavior has frequently been reported for aged or weathered plastics, where surface oxidation, cracking, and oxygen-containing functional groups introduce heterogeneity into an otherwise smooth polymer matrix42. However, Freundlich fitting alone does not necessarily confirm multilayer adsorption but rather reflects variability in surface energies—a factor that must be interpreted with caution.

The Temkin model also showed a strong correlation (R² = 0.97), providing further insight into adsorption energetics. The Temkin constant (B1 = 1.2 L/mg) indicates a decrease in the heat of adsorption as surface coverage increases, likely due to repulsive interactions between adjacent adsorbed CV molecules or preferential occupation of high-energy sites during the initial stages43. This trend is commonly observed in systems where surface functional groups are gradually saturated, leading to weaker binding at higher coverage44.

A comparison with previous studies reveals important distinctions. For instance, Chandrasekaran et al. investigated CV adsorption onto SDS-coated magnetic nanoparticles and reported R² values above 0.9 for all three models, with the Freundlich model (R² = 0.957) providing the best fit45. However, their system involved engineered nanomaterials with extremely high surface area and designed chemical heterogeneity. In contrast, PVC microplastics represent a comparatively simple polymer matrix, and their heterogeneity is primarily induced by oxidative aging rather than intentional surface modification. Therefore, directly transferring mechanistic interpretations from engineered adsorbents to environmentally aged plastics may lead to overgeneralized conclusions.

Although the Freundlich model statistically showed a slightly better fit, its fundamental assumption of a highly heterogeneous and multilayered adsorption system may not fully reflect the physicochemical nature of ozonated PVC. Instead, the Langmuir model offers a more mechanistically realistic representation, as it assumes adsorption on a limited number of equivalent sites, consistent with polymer surfaces that possess constrained chemical diversity even after oxidation. The simultaneous applicability of both Langmuir and Freundlich models suggests that CV adsorption on aged PVC occurs on partially heterogeneous sites generated by ozonation, rather than on a highly porous or architecturally complex surface typical of engineered adsorbents such as activated carbon or nanocomposites.

Effect of aging MPs on the adsorption rate

The adsorption rates of CV onto both virgin and aged PVC microplastics, presented in Fig. 4, clearly demonstrate the significant influence of aging on adsorption performance. Aged PVC exhibited an adsorption rate of 76.55%, which is substantially higher than the 52.62% observed for virgin PVC. This marked increase confirms that aging processes, particularly ozonation, transform PVC from a relatively inert polymer into a more chemically active and adsorptive surface.

The enhanced performance of aged PVC can be attributed primarily to chemical oxidation and structural modification of the polymer surface. Ozonation introduces oxygen-containing functional groups such as carbonyl (–C = O) and hydroxyl (–OH) moieties, which increase surface polarity and promote electrostatic and hydrogen-bond interactions with the cationic and polar CV molecules20. In addition, the increase in surface oxygen content reduces hydrophobicity and improves wettability, facilitating greater interaction between the dye molecules and the polymer surface. Similar trends were reported by Liu et al. (2019), who demonstrated that aged microplastics exhibited higher adsorption capacities due to decreased hydrophobicity and the development of polar functional groups46. However, such chemical modifications should not be interpreted as beneficial in an environmental sense, as they increase the ability of microplastics to act as vectors for toxic pollutants rather than contributing to remediation.

Beyond chemical oxidation, physical transformations induced by aging, such as surface roughening, crack formation, and pore development, also play a crucial role in enhancing CV uptake47. Increased surface roughness expands the effective contact area between the microplastics and surrounding solution, while newly formed pores can act as localized accumulation sites for CV molecules48. These structural irregularities generate additional adsorption sites that are absent in smooth, virgin PVC. A similar observation was made in studies where aged PVC microplastics showed increased pore volume, surface fissures, and a higher number of active adsorption sites, collectively contributing to enhanced dye uptake49. Nonetheless, unlike highly porous engineered adsorbents, these features remain irregular and non-uniform, resulting in a partially heterogeneous adsorption environment rather than an efficient, well-defined adsorption matrix.

Furthermore, changes in the internal structure of the polymer also contribute to the increased adsorption rate. Aging has been shown to reduce the crystallinity of PVC, increasing the proportion of amorphous regions that are more accessible to organic contaminants50. Since crystalline domains restrict molecular diffusion, a decrease in crystallinity allows easier penetration and binding of CV molecules within the polymer matrix. In addition, oxidation-driven chain scission and polymer fragmentation can enhance internal diffusion pathways, thereby increasing the rate of pollutant transport toward interior adsorption sites51.

Taken together, these findings indicate that the increased adsorption rate observed for aged PVC MPs results from a synergistic combination of chemical functionalization, surface morphological changes, and altered internal polymer structure. Importantly, this enhancement does not signify an improvement in environmental quality; rather, it underscores the role of aged microplastics as more effective carriers and concentrators of hazardous compounds in aquatic environments. Therefore, aging intensifies the ecological risk associated with PVC microplastics by reinforcing their capacity to sequester and transport toxic organic pollutants such as crystal violet.

Fig. 4
figure 4

CV adsorption rate for both virgin and aged PVC MPs.

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Based on Table 3, the adsorption capacity obtained for aged PVC microplastics (0.96 mg/g) is substantially lower than values commonly reported for engineered sorbents, which often range from several tens to hundreds of milligrams per gram. This difference is expected, as materials such as activated carbon, graphene-based composites, and biochar are intentionally designed to maximize sorption through highly developed pore structures, large specific surface areas, and abundant heteroatom-containing functional groups that promote strong interactions with organic contaminants. In contrast, PVC microplastics originate from consumer products and were never engineered to function as adsorptive media. Their intrinsic properties, including a hydrophobic polymer backbone, limited native porosity, low density of surface functional groups, and relatively large particle size, impose natural constraints on their capacity to bind crystal violet and similar organic molecules.

Although ozonation induced measurable modifications in surface chemistry, including the incorporation of oxygenated functional groups and a shift toward more negative surface charge, these alterations remained largely confined to the outermost layers of the particles. The extent of oxidation was therefore insufficient to generate the hierarchical porosity or chemical complexity typical of advanced sorbents. As a result, the adsorption capacity of aged PVC remains modest when evaluated in isolation.

However, the environmental significance of these findings extends beyond absolute adsorption capacity. Microplastics represent a unique class of environmental risk not because they rival engineered sorbents in efficiency, but because of their ubiquity, persistence, and mobility in aquatic systems. Even relatively low sorption capacities become environmentally relevant when multiplied across the vast quantities of microplastic particles distributed in rivers, lakes, and oceans. Our results demonstrate that aged PVC microplastics are capable of accumulating quantifiable amounts of crystal violet under conditions representative of natural waters, confirming their function as passive yet persistent contaminant carriers.

Moreover, oxidative aging appears to enhance the environmental reactivity of PVC microplastics by increasing their affinity for dissolved pollutants. This aging-mediated transformation strengthens their role as vectors that can facilitate the transport of adsorbed contaminants across hydrological compartments. Once associated with microplastics, dyes and other organic pollutants may be transported over long distances, become bioavailable through ingestion, and migrate through aquatic food webs. Consequently, even low-level sorption assumes ecological significance when considered in the context of chronic exposure, long-term persistence, and the cumulative transport potential of microplastic pollution.

Table 3 Comparison of crystal Violet absorption by aged PVC and other adsorbents.
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Effect of intervening parameters on CV adsorption onto aged PVC

The results presented in Fig. 5 indicate that ambient water chemistry, specifically pH, EC, and temperature, exerts a strong influence on the adsorption efficiency of CV by aged PVC microplastics. As shown in Fig. 5a, the highest dye removal occurred under acidic conditions (pH 4), followed by a slight decrease at neutral pH7 and a pronounced reduction in alkaline media (pH 10). This trend suggests that oxidized PVC surfaces interact more favorably with cationic dye molecules under acidic conditions, where surface functional groups are more likely to be protonated and electrostatic attraction is enhanced. In contrast, under alkaline conditions, the abundance of hydroxide ions, coupled with increased deprotonation of surface groups, likely intensifies competition for adsorption sites and promotes electrostatic repulsion, thereby reducing dye uptake. Similar pH-dependent behavior has been reported in other microplastic–dye systems. For example, Wang et al. (2023) demonstrated that hydrogen bonding and electrostatic interactions govern the adsorption of malachite green and methyl orange onto polyamide microplastics60, while Du et al. (2022) observed that increasing pH and subsequent surface deprotonation significantly reduced CV adsorption onto polypropylene microplastics8.

Electrical conductivity also had a marked inhibitory effect on CV adsorption, as illustrated in Fig. 5b. When EC increased from 0 to 1500 and 5000 µmho/cm, a corresponding decrease in adsorption efficiency was observed. This phenomenon can be attributed to competition between dissolved ions and CV molecules for available adsorption sites on the aged PVC surface. Furthermore, higher ionic strength compresses the electrical double layer surrounding the negatively charged microplastic particles, thereby weakening electrostatic interactions with cationic dye species61,62. Together, these mechanisms reduce the overall driving force for adsorption under saline or ion-rich conditions, which is particularly relevant in estuarine and industrial wastewater environments.

Temperature likewise influenced the adsorption behavior of CV onto aged PVC MPs (Fig. 5c). The highest adsorption occurred at 4 °C, followed by a decline at 25 °C. Interestingly, a partial increase in adsorption was observed again at 40 °C, indicating that the process is temperature-dependent and governed by competing thermodynamic and kinetic factors. At moderate temperatures, decreased adsorption may reflect reduced dye–surface affinity, whereas at higher temperatures, enhanced molecular mobility of both dye molecules and microplastic particles can improve collision frequency and facilitate adsorption63. Comparable trends have been reported in previous studies, including that of Muthuraja et al. (2024), who observed a decrease in adsorption of positively charged species onto polypropylene and polystyrene microplastics between 25 °C and 35 °C64. Hao et al. (2024) further reported that increasing temperature enhances CV ionization, thereby strengthening electrostatic interactions between CV⁺ and negatively charged regions on microplastic surfaces65.

Importantly, the enhanced adsorption capacity observed for aged PVC microplastics should not be interpreted as a beneficial or remedial outcome. On the contrary, it represents an escalation of environmental risk. By acting as mobile reservoirs of toxic compounds, aged microplastics can facilitate the transport, concentration, and eventual release of pollutants under changing pH, redox, or biological conditions. This dynamic behavior increases the likelihood of long-range contaminant dispersal and bioavailability, reinforcing the role of aged microplastics not as passive debris, but as chemically reactive vectors within aquatic ecosystems.

Fig. 5
figure 5

Effect of various factors on adsorption rate onto aged MPs: (a) pH, (b) EC, and (c) Temperature.

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Environmental implications

The results indicate that ozone-induced aging significantly modifies the physicochemical properties of PVC microplastics by increasing surface oxidation (e.g., carbonyl formation), decreasing particle size, and shifting the zeta potential toward more negative values. These alterations markedly enhance the adsorption affinity of aged PVC particles for cationic organic pollutants such as crystal violet, primarily through strengthened electrostatic attraction, π–π interactions, and hydrogen bonding with newly generated oxygen-containing functional groups. In aquatic environments, such aged PVC microplastics may act as efficient sorbents, transforming from relatively inert particles into active carriers capable of transporting adsorbed pollutants across environmental compartments65.

The higher adsorption capacity observed for aged PVC particles suggests an increased residence time of pollutants on microplastics within the water column, sediments, and biofilm interfaces. This elevated affinity may promote the long-range dispersal of dyes and other cationic contaminants, including during passage through water-treatment systems and natural hydrological flows. The stronger binding behavior also raises concerns regarding bioavailability and trophic transfer: organisms that ingest aged microplastics may encounter elevated local concentrations of associated contaminants, which could desorb under gastrointestinal conditions (e.g., reduced pH and elevated ionic strength), resulting in internal exposure66. Such processes may contribute to bioaccumulation in lower trophic-level organisms and facilitate the upward transfer of pollutants through the food web.

From an applied perspective, these findings highlight the need for careful consideration of strong oxidants such as ozone in water-treatment systems. Although effective for microbial disinfection, ozone may unintentionally increase the environmental hazard of PVC-derived microplastics by enhancing their capacity to bind and transport chemical contaminants. Mitigation strategies may include coupling ozonation with advanced filtration (e.g., membrane separation) to remove oxidized microplastics or implementing downstream adsorption units to capture pollututant-laden particles prior to discharge or reuse.

Conclusion

This study examined the effects of ozonation on the physical and chemical properties of PVC MPs and their adsorption of CV dye. Virgin and ozone-aged PVC samples were analyzed using FTIR, FESEM, EDX, DLS, and ZP measurements, and CV adsorption was evaluated for both. The results showed that ozonation reduced chlorine-containing groups, introduced carbonyl and carboxyl groups, and altered the surface structure and elemental composition of the particles. The aged particles exhibited smaller sizes and greater surface stability.

Notably, the adsorption rate of CV for aged PVC increased to 76.55%, compared to 52.62% for raw PVC. These findings suggest that ozonation enhances the adsorption capacity of PVC MPs, which may influence their environmental impact. CV adsorption followed the Freundlich isotherm, indicating a heterogeneous surface and multilayer adsorption. The maximum adsorption capacity reached 5.55 mg/g, as suggested by the Langmuir model. Kinetic analysis showed that PFO model provided a better fit than PSO model, indicating that CV adsorption onto PVC is primarily governed by chemical interactions.