Surface-dominated photocatalytic degradation of polystyrene microplastics in TiO 2 /Fe 3 O 4 thin films under UV irradiation

Polystyrene (PS) microplastics are persistent environmental contaminants whose degradation under realistic solid-state conditions remains poorly understood. In this study, the ultraviolet (UV-A, 365 nm, 143.5 h)-induced degradation of PS microplastics is systematically investigated using Fe 3 O 4 and Fe 3 O 4 /TiO 2 composite thin films under dry (air) conditions. This approach provides a controlled model for surface-exposed plastic aging beyond conventional aqueous systems. Structural, chemical, optical, and surface modifications were comprehensively analyzed using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), UV–visible spectroscopy (UV–vis), and X-ray photoelectron spectroscopy (XPS). XRD analysis reveals enhanced structural disorder of the PS matrix in the presence of Fe 3 O 4 /TiO 2 compared with Fe 3 O 4 alone. However, the crystalline phases of the inorganic components remain unchanged. Under identical UV irradiation conditions, FTIR carbonyl index quantification and UV–vis absorbance measurements indicate that Fe 3 O 4 alone induces only limited bulk photo-oxidation and partial aromatic degradation compared with the Fe 3 O 4 /TiO 2 system. Meanwhile, the incorporation of TiO 2 suppresses extensive bulk macromolecular chain fragmentation despite prolonged UV exposure. In contrast, XPS reveals pronounced surface oxidation for the Fe 3 O 4 /TiO 2 system, evidenced by a significant increase in oxygen-containing carbon species in the C 1s spectra. FTIR carbonyl index measurements and UV–vis absorbance changes indicate that bulk chemical and optical modifications remain modest under the applied UV conditions. Together, these results reveal that Fe 3 O 4 /TiO 2 preferentially promotes the surface-localized photo-oxidation of PS microplastics rather than uniform bulk macromolecular chain fragmentation. Based on these findings, we propose a solid-state degradation mechanism in which UV-excited TiO 2 generates charge carriers that may undergo interfacial charge interactions with Fe 3 O 4. This mechanism facilitates oxygen-mediated surface oxidation in the absence of water. Accordingly, this work provides new mechanistic insight into the spatial nature of microplastic degradation under dry conditions. The work also highlights why surface-localized photo-oxidation should be distinguished from macromolecular chain fragmentation in environmental aging processes.

Over the past decades, plastics production and consumption have continuously increased worldwide, and the improper disposal and environmental weathering of plastic waste have led to progressive fragmentation into microplastics (MPs), which are now recognized as pervasive contaminants across aquatic and terrestrial compartments (Thompson et al., 2024; Kelly et al., 2025). MPs can travel through wastewater streams and atmospheric pathways, accumulate in ecosystems, and raise concerns regarding ecological disruption and potential human exposure (Das et al., 2025). Recent studies and reviews emphasize that MPs are widely distributed in natural environments and may pose complex risks to biota and humans through ingestion, inhalation, and trophic transfer, although substantial uncertainties remain regarding exposure quantification, fate, and risk assessment frameworks (Sun and Wang, 2023; Hoang et al., 2025; Ho et al., 2025).

A common polymer, polystyrene (PS) is frequently detected in environmental samples and is widely used as a model material in mechanistic degradation studies (Gouin et al., 2024; Yuan et al., 2026; Siddiqui et al., 2023). The aromatic backbone and well-characterized photochemistry of PS facilitate the tracking of oxidation, aromatic disruption, and chain scission processes using spectroscopic techniques (Dong et al., 2024; Gautam et al., 2024). In both laboratory and environmental contexts, PS microplastics have been associated with adverse biological effects, prompting the development of feasible strategies to mitigate MP pollution or promote transformation into less harmful forms (Hoang et al., 2025; Siddiqui et al., 2023; Xiang et al., 2023). Source control and capture technologies are essential, but there is increasing interest in active degradation approaches that can accelerate polymer aging and breakdown under environmentally relevant conditions (Kelly et al., 2025; Gao et al., 2025; Pfohl et al., 2025).

Most commodity plastics exhibit high chemical stability and resistance to oxidation, so numerous remediation strategies, including biodegradable or photodegradable polymer design, thermal and catalytic processing, and enzymatic degradation-have been explored (Chen et al., 2026; Ibrahim et al., 2025; Gupta et al., 2025; Tasleem et al., 2025). However, many approaches face practical limitations related to cost, scalability, selectivity, or long treatment times (Kelly et al., 2025; Thacharodi et al., 2024; Kumar et al., 2024). Photocatalysis has emerged as a green and efficient approach because it enables the generation of reactive species using light, allowing oxidative degradation under mild conditions without adding any harsh chemicals (Ramírez-Escárcega et al., 2025).

In environmental applications, titanium dioxide (TiO 2) remains one of the most established photocatalysts due to its chemical stability, photostability, and extensive history in pollutant degradation (Ramírez-Escárcega et al., 2025; Maldonado et al., 2025; Ghamarpoor et al., 2024). TiO 2 under ultraviolet (UV) irradiation generates electron–hole pairs that initiate surface redox reactions, producing reactive oxygen species (ROS) capable of oxidizing organic compounds and polymer chains (Guo et al., 2025). For microplastics, TiO 2 -based photocatalysis has shown promise in accelerating degradation, yet most existing studies focus on aqueous suspensions and slurry systems (Guo et al., 2025; Bertagna Silva and Marques, 2025). These conditions differ significantly from realistic exposure scenarios, and mechanistic interpretation is often complicated by water-mediated radical pathways, mass transfer effects, and particle dispersion (Ramírez-Escárcega et al., 2025).

For polymer photodegradation, thin-film architectures are particularly relevant because they mimic realistic environmental exposure scenarios in which plastics exist as solid materials exposed to light and atmospheric oxygen rather than being dispersed in water (Pfohl et al., 2025; Bertagna Silva and Marques, 2025). Early studies demonstrated that combining PS with TiO 2 in thin-film form enables practical photodegradation driven by surface ROS and chain scission processes (Stancu et al., 2025; Nabi et al., 2020; He et al., 2023). More recent reports have confirmed that solid-phase (air/ambient) photocatalytic degradation of PS-based composites can occur under UV irradiation (Ramírez-Escárcega et al., 2025; Mbuci et al., 2023). Nevertheless, compared with aqueous systems, dry thin-film systems remain underexplored for microplastics composites, particularly regarding the coupled roles of oxygen adsorption, interfacial charge transfer, and surface oxidation chemistry (Md Noor et al., 2024).

Critically, existing studies rarely distinguish between surface-localized photo-oxidation and true bulk polymer degradation. Studies instead often assume that increased oxidation implies extensive chain scission. However, this assumption remains largely unverified in solid-state microplastic systems, where degradation may be dominated by surface-initiated processes without substantial breakdown of the polymer bulk. Resolving this distinction is essential for accurately interpreting spectroscopic data and assessing the environmental implications of photocatalytic aging, including changes in surface chemistry, reactivity, and toxicity without complete material fragmentation (Gewert et al., 2015).

In this regard, incorporating iron oxide components offers an attractive strategy for regulating interfacial charge transfer and surface redox reactions. Magnetite (Fe 3 O 4) is abundant and environmentally relevant, and it exhibits mixed valence states (Fe 2+ /Fe 3+) that can facilitate electron transfer when coupled with TiO 2. In pollutant degradation, TiO 2 –Fe 3 O 4 composites have been widely explored due to their ability to reduce charge-carrier recombination and enhance redox activity (Ghaderpoori et al., 2025; Wei and Zheng, 2025). Fe 3 O 4 can serve as a model redox mediator in the context of polymer and microplastic degradation, enabling the investigation of how interfacial charge interaction influences surface oxidation pathways without invoking strong proton-coupled or aqueous radical mechanisms (Wei and Zheng, 2025). Therefore, comparing Fe 3 O 4 with Fe 3 O 4 /TiO 2 composite systems enables evaluation of how TiO 2 incorporation influences the surface photo-oxidation behavior of PS microplastics under identical solid-state UV irradiation conditions.

In dry thin-film systems, mechanistic interpretation requires explicit support from complementary characterization techniques because degradation may proceed through surface-driven photo-oxidation rather than bulk hydrolysis or solution-phase radical attacks. Fourier-transform infrared spectroscopy (FTIR) can track the evolution of oxygen-containing functional groups, X-ray photoelectron spectroscopy (XPS) can quantify surface enrichment of oxidized carbon species and assess oxidation states at the catalyst interface, UV–visible spectroscopy (UV–vis) can reflect photo-aging–related optical changes and aromatic disruption, and X-ray diffraction (XRD) can reveal changes in polymer structural order while verifying the phase stability of inorganic components. Together, these techniques enable a spatially resolved understanding of degradation pathways in solid-state systems (Gewert et al., 2015; Surana et al., 2024).

In this study, we therefore investigate the UV-induced degradation of PS microplastics using Fe 3 O 4 and Fe 3 O 4 /TiO 2 composite thin films under dry (air) conditions, representing an environmentally relevant solid-state exposure scenario. We integrate XRD, FTIR, UV–vis, and XPS analyses to systematically evaluate structural disorder, chemical bond modification, optical degradation, and surface oxidation of PS microplastics. Particular attention is given to distinguishing surface-localized photo-oxidation from bulk polymer degradation by correlating XPS-derived oxidation indices with FTIR carbonyl indices and UV–vis absorbance loss. Based on this multi-technique approach, we propose a degradation mechanism that highlights oxygen-mediated surface reactions and interfacial charge-transfer effects in the absence of water. The results of this approach provide mechanistic insight into photocatalytic PS degradation under solid-state conditions and extend current microplastics research beyond conventional aqueous environments.

Materials

Monodisperse polystyrene (PS) microspheres were obtained from Cospheric LLC (United States). Supplied as dry powder (200 mg), the particles had a nominal diameter of 4.8–5.8 µm and a density of 1.07 g/cc. Titanium dioxide (TiO 2) was also purchased from Cospheric LLC. Black iron oxide (magnetite, Fe 3 O 4) powder was sourced from Alpha Chemicals (United States). The material was derived from natural magnetite and supplied as a bulk powder with an average particle size corresponding to approximately 500 mesh (∼30 µm). For the preparation of all suspensions, deionized (DI) water was used for the preparation of all suspensions.

Preparation of stock suspension

A stock suspension of PS microplastics was prepared by dispersing 200 mg of PS microspheres in 2.0 mL of DI water, corresponding to a concentration of approximately 9 wt%. The suspension was transferred to sealed glass vials wrapped with polytetrafluoroethylene (PTFE) tape and sonicated in an ultrasonic bath for 60 min to ensure homogeneous dispersion and minimize particle aggregation. Sonication was performed in a standard laboratory ultrasonic bath under ambient conditions without active temperature control. No deliberate heating was applied, and the dispersion step was completed prior to UV irradiation.

For preparation of the PS–Fe 3 O 4 composite suspension, 7 mg of Fe 3 O 4 powder was added to 300 µL of the PS stock suspension, yielding final concentrations of approximately 8.9 wt% PS and 2.1 wt% Fe 3 O 4. The mixture was sealed and sonicated for an additional 60 min to promote uniform distribution of Fe 3 O 4 particles within the PS suspension.

To prepare the PS–Fe 3 O 4 /TiO 2 composite suspension, 7 mg of Fe 3 O 4 and 10 mg of TiO 2 were first dispersed in 100 µL of DI water and sonicated for 60 min to prevent agglomeration and ensure intimate mixing of the oxide components. This Fe 3 O 4 –TiO 2 suspension was then added to 400 µL of the PS stock suspension, followed by further sonication for 60 min (Figure 1). The resulting composite suspension contained approximately 7.2 wt% PS, 1.3 wt% Fe 3 O 4, and 1.8 wt% TiO 2.

The selected compositions were optimized to facilitate film formation and solid-state interfacial interactions, rather than bulk aqueous photocatalytic degradation. Under these conditions, UV-induced transformations are expected to be dominated by surface-localized oxidation processes at the PS–oxide interfaces.

Thin film preparation

Thin films were prepared by depositing 100 µL of each sonicated suspension onto clean glass substrates. For PS, PS–Fe 3 O 4, and PS–Fe 3 O 4 –TiO 2 samples, separate films were fabricated. The deposited suspensions were uniformly spread using a spin-coating technique under identical processing conditions.

Following deposition, the coated films were oven-dried at 90 °C for 30 min to remove residual moisture and promote film stabilization. All films were allowed to cool to room temperature after drying and were then stored in the dark prior to irradiation and characterization to prevent any unintended photo-induced reactions.

UV irradiation

The PS–Fe 3 O 4 and PS–Fe 3 O 4 –TiO 2 thin-film samples were irradiated under UV light (365 nm, UV-A) using a Uvitec UV irradiation chamber (model CV-415.LL, United Kingdom) equipped with two low-pressure UV lamps (15 W each, total power 30 W). The irradiation was conducted in the absence of ambient illumination to simulate the controlled photo-aging of airborne polystyrene microplastics (PS-MPs). The samples were exposed to light for 143.5 h. In parallel, pristine PS film (control) was maintained under identical temperature and humidity conditions but stored in complete darkness, serving as control sample to ensure any observed physicochemical changes were attributable solely to UV irradiation.

The experimental workflow for thin-film preparation and UV irradiation is illustrated in Figure 1.

Characterization techniques

Both control (dark-stored) and UV-irradiated thin-film samples were characterized using complementary techniques to investigate surface-dominated optical, structural, chemical, and morphological changes in PS-MPs and their composites. To monitor changes in optical absorption associated with photo-oxidation, UV–vis spectroscopy was performed using a Thermo Scientific Evolution 201 UV–vis spectrophotometer. UV–vis absorption spectra were recorded directly from the thin-film samples deposited on glass substrates, representing solid-state measurements rather than solution-phase analysis. FTIR analysis was applied to identify modifications in surface functional groups induced by UV exposure, using a Shimadzu IRSpirit-T FTIR spectrophotometer equipped with a single-reflection ATR accessory (QATR-S). Spectra were collected in the 4,000–400 cm -1 range at a resolution of 4 cm -1 with 32 accumulated scans. The carbonyl index (CI) was evaluated from FTIR spectra by calculating the relative change in the intensity of the carbonyl absorption band with respect to an invariant reference band of the PS matrix, enabling assessment of photo-oxidation progression. To analyze surface elemental composition and oxidation states, XPS measurements were performed using a Thermo Scientific X-ray Photoelectron Spectroscopy system (ESCALAB Xi+), providing detailed insight into surface and interfacial chemical changes induced by UV irradiation. Scanning electron microscopy (SEM) was utilized to examine surface morphology and particle distribution within the thin films, using a field-emission scanning electron microscope (FE-SEM, JEOL JSM-7600F) operated at an accelerating voltage of 5.0 kV under high-vacuum conditions. XRD was performed to evaluate potential changes in structural ordering of the polymer matrix and oxide phases following irradiation, using a Rigaku Ultima IV X-ray diffractometer operated with Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 40 mA in θ–2θ geometry over a 2θ range of 5°–90°. All measurements were carried out using calibrated instruments under standard laboratory operating conditions.

XPS, UV–vis, and FTIR analyses were conducted at the Research Laboratories Building, College of Science, Umm Al-Qura University, Makkah, KSA, while SEM and XRD analyses were performed at the Center of Nanotechnology, King Abdulaziz University, Jeddah, KSA.

Effect of Fe 3 O 4 and Fe 3 O 4 /TiO 2 composites on PS-MPs degradation under UV irradiation

The interaction between PS-MPs and iron oxide–based materials was investigated by comparing Fe 3 O 4 alone with Fe 3 O 4 /TiO 2 composites under UV irradiation. To assess microplastic degradation, the structural and chemical modifications were investigated using XRD, FTIR, UV–vis, SEM, and XPS. These techniques enabled a comprehensive evaluation of structural disorder, chemical bond modifications, optical degradation, and surface oxidation.

XRD structural changes

XRD patterns (Figure 2a) show that pristine PS exhibits a broad amorphous halo centered around 2θ ≈ 22°, characteristic of the semi-amorphous polymeric structure of pristine PS. With the incorporation of Fe 3 O 4, only minor changes in the diffraction profile were observed, indicating limited structural alteration of the PS matrix. In contrast, the PS–Fe 3 O 4 /TiO 2 system displayed a noticeable reduction in diffraction intensity across the entire 2θ range. This reduction suggests enhanced disruption of the polymer structure occurred under UV exposure. Regarding the PS matrix, the attenuation reflects an increased degree of structural disorder, without direct evidence of extensive bulk chain scission.

FTIR chemical bond modification

To investigate UV-induced chemical bond modifications in PS microplastics in the presence of Fe 3 O 4 and Fe 3 O 4 /TiO 2, FTIR was employed (Figure 2b). Pristine PS exhibited characteristic aromatic absorption bands, including C–H stretching vibrations (3,025–2,850 cm -1), aromatic C=C stretching modes (∼1,600 and 1,490 cm -1), and out-of-plane C–H bending vibrations (∼700–760 cm -1), confirming the preservation of the polymer backbone prior to irradiation (Chen et al., 2001; Kumar et al., 2015). Following UV exposure in the presence of Fe 3 O 4, a moderate increase in absorption within the carbonyl region (∼1700–1750 cm -1) was observed. The increase was accompanied by slight attenuation of aromatic bands, indicating partial photo-oxidation of the polymer (Zan et al., 2006). The carbonyl index (CI) was calculated as a descriptive indicator of carbonyl band intensity, where pristine PS exhibited a CI of 0.806. This finding reflects minimal carbonyl content. Meanwhile, UV irradiation with Fe 3 O 4 resulted in a slight increase to 0.810 (+0.49%), suggesting limited formation of oxidation products. Because the FTIR spectra were obtained from a single measurement (n = 1), these small CI differences (≤1%) should be interpreted cautiously and may fall within experimental uncertainty. In contrast, the PS–Fe 3 O 4 /TiO 2 system showed discernible enhancements in carbonyl and C–O stretching bands (1,000–1,300 cm -1), which indicates oxidative modification at the polymer surface (Zan et al., 2006). However, the calculated CI for this system (0.800) remained comparable to that of pristine PS and thus implies that carbonyl formation was modest despite the presence of TiO 2. This limited CI variation suggests that bulk carbonyl formation remains modest under the applied UV conditions. Overall, these results demonstrate that Fe 3 O 4 alone induces limited bulk photo-oxidation, while the incorporation of TiO 2 enhances UV-driven surface oxidation processes.

UV–vis optical degradation

UV–vis absorption spectra (Figure 2c) further clarify bulk optical changes. Pristine PS exhibits a characteristic absorption band near 280–300 nm associated with π–π* transitions of aromatic rings (Zhang et al., 2021). After UV irradiation in the presence of Fe 3 O 4, a measurable decrease in absorbance intensity was observed, indicating partial loss of aromatic conjugation. In contrast, the PS–Fe 3 O 4 /TiO 2 system showed no net absorbance loss in this region. The system also exhibited only minor spectral variations. The UV–vis absorbance changes do not strictly follow the same trend observed in surface-sensitive XPS measurements. This difference likely arises because UV–vis spectroscopy probes bulk electronic transitions within the polymer matrix, whereas XPS primarily reflects chemical modifications occurring at the outermost surface. Consequently, surface-localized oxidation detected by XPS may not necessarily produce proportional changes in bulk optical absorbance.

XPS surface oxidation

XPS C1s deconvolution (Figure 3) further clarified surface changes. Pristine PS was dominated by C–C/C–H peaks (∼284.8 eV) with minimal oxygen groups (Figure 3b). UV irradiation with Fe 3 O 4 (Figure 3c) increased C=O and O–C=O components, indicating partial oxidation (Flak et al., 2018). The PS–Fe 3 O 4 /TiO 2 (Figure 3d) exhibited a more pronounced presence of these oxygen-containing carbon species, indicating enhanced surface-localized chemical modification. Figures 3c,d correspond to two different composite systems prepared under identical UV irradiation conditions. Figure 3c represents the PS–Fe 3 O 4 thin film, whereas Figure 3d represents the PS–Fe 3 O 4 /TiO 2 composite thin film. The slight differences observed in the C1s spectra reflect the influence of TiO 2 incorporation on the surface chemical environment of the PS matrix.

Overall, Fe 3 O 4 induces only modest surface modification of PS microplastics under UV irradiation, while the Fe 3 O 4 /TiO 2 system promotes comparatively stronger surface-localized oxidation, whereas bulk-sensitive measurements indicate limited bulk chemical transformation.

SEM morphological changes

To complement the spectroscopic and structural analyses, SEM was employed to visualize surface morphological alterations in the PS thin films following UV irradiation in the presence of Fe 3 O 4 and Fe 3 O 4 /TiO 2 composites. Pristine PS microspheres exhibited a uniform, spherical morphology with smooth surfaces and minimal defects (Figure 4a). After 143.5 h of UV exposure in the PS–Fe 3 O 4 system, the microspheres largely retained their spherical shape; however, subtle surface modifications were apparent (Figure 4b). A small protrusions or adhered particles - likely Fe 3 O 4 aggregates - were visible on the PS surfaces, accompanied by minor roughening or pitting. These changes suggest initial surface interactions and limited oxidative erosion, consistent with the spectroscopic observations indicating that bulk chemical and optical modifications remain modest under the applied UV conditions, which reflects early-stage photo-oxidation without substantial macromolecular chain fragmentation (Surana et al., 2024; Zhang et al., 2021). In contrast, the PS–Fe 3 O 4 /TiO 2 system displayed more pronounced morphological degradation under identical UV conditions (Figure 4c). SEM images revealed increased surface roughness, with evident cracks, flakes, and fragmented debris on the microsphere exteriors. Some particles exhibited partial disintegration or exfoliation-like features, indicative of enhanced surface erosion. This intensified surface alteration corroborates the XPS findings of increased oxygen-containing carbon species, supporting a surface-dominated photo-oxidation process that may be associated with TiO 2 -mediated surface reactions and possible interfacial charge interactions within the composite system (Ramírez-Escárcega et al., 2025; Md Noor et al., 2024). Notably, the core spherical structure remained intact, reinforcing the decoupling between surface modification and bulk polymer breakdown, as minimal bulk changes were detected via FTIR and UV–vis. Overall, the SEM observations demonstrate that while Fe 3 O 4 induces minor surface perturbations, the incorporation of TiO 2 accelerates localized surface degradation, consistent with environmental weathering scenarios where MPs are exposed to UV and atmospheric oxygen (Pfohl et al., 2025). These morphological insights, when integrated with spectroscopic data, highlight the potential of Fe 3 O 4 /TiO 2 composites for promoting surface-specific transformations in solid-state PS-MP systems, which may influence their environmental persistence and toxicity without necessitating complete fragmentation.

Taken together, the combined spectroscopic results indicate that UV-induced chemical modification of PS microplastics under these conditions is primarily localized at the polymer surface, while bulk-sensitive measurements show only modest changes.

Mechanism of PS degradation in PS–TiO 2 /Fe 3 O 4 thin films

Upon UV irradiation, TiO 2 is photo-excited, generating electron–hole pairs (TiO 2 + hν → e − + h +). In TiO 2 -based composite systems, interfacial electron transfer between TiO 2 and Fe 3 O 4 has been widely reported in the literature and may contribute to improved charge separation (Saszet et al., 2025; Orero et al., 2025; Zare and Mehrabani-Zeinabad, 2022). In this process, photogenerated electrons (e −) may transfer to Fe 3 O 4, where Fe 3+ species can act as electron acceptors and be reduced to Fe 2+. Meanwhile, photogenerated holes (h +) participate in oxidation reactions occurring at the PS surface (Figure 5). The presence of mixed Fe 2+ /Fe 3+ states observed in XPS is consistent with the redox-active nature of Fe 3 O 4 (Nabi et al., 2020). In the absence of water, photogenerated electrons may react with oxygen adsorbed on the film surface to form superoxide radicals (e − + O 2 → •O 2 −), as widely reported for TiO 2 -based photocatalytic systems, while photogenerated holes may also participate in oxidation reactions involving surface-exposed PS chains. These surface-confined reactive processes can promote oxygen incorporation into the polymer backbone, leading to the formation of oxygen-containing functional groups detected by XPS. Due to the limited mobility of ROS in the solid matrix, oxidation remains localized at the surface rather than propagating uniformly into the polymer bulk.

The XRD patterns indicate that the amorphous nature of polystyrene becomes more disordered after UV irradiation. However, the crystalline phases of TiO 2 and Fe 3 O 4 remain unchanged. This finding confirms that the structural changes originate from modification/disordering of the PS matrix rather than the phase transformation of the metal oxides.

FTIR analysis provides evidence of oxidation-related functional group evolution. The attenuation of characteristic C–H vibrations and the emergence of oxygen-containing functional groups (e.g., C–O and C=O) suggest oxidative modification consistent with surface-initiated photo-oxidation processes.

XPS results further confirm this mechanism by showing an increased contribution of oxidized carbon species relative to the C–C/C–H component after UV exposure. This increase is consistent with oxidation occurring primarily at the film surface. Additionally, the presence of mixed Fe 2+ /Fe 3+ states may indicate the redox activity of Fe 3 O 4 as an electron acceptor, which may facilitate interfacial charge transfer from TiO 2 to Fe 3 O 4 and potentially suppress charge carrier recombination.

In the absence of water, photogenerated electrons may interact with oxygen adsorbed on the film surface to generate reactive oxygen species that initiate surface-confined oxidation reactions within the PS matrix. The synergistic interaction between TiO 2 and Fe 3 O 4 may facilitate charge separation and sustains oxygen-mediated surface oxidation, leading primarily to surface chemical modification and increased structural disorder, rather than uniform bulk chain scission. This mechanism explains the pronounced surface oxidation detected by XPS and the limited bulk degradation indicated by the FTIR carbonyl index and UV–vis spectroscopy.

From a broader perspective, the solid-state thin-film setup used in this study closely reflects the conditions experienced by airborne and soil-bound microplastics, which are often exposed to sunlight under dry or semi-dry environments rather than submerged in water. Although this system is not proposed as a remediation approach, it serves as a practical model for examining how mineral components commonly found in dust, soils, and urban aerosols - such as iron oxides and TiO 2 - can influence the surface aging of microplastics under natural sunlight. These results suggest that, in real environmental settings, microplastic transformation is more likely to occur through gradual, cumulative surface oxidation over long timescales, rather than through rapid breakdown, highlighting the need to account for solid-state photochemical processes in environmental fate and transport assessments.

It should be noted that the present work does not directly measure reactive oxygen species generation or charge-carrier dynamics. Therefore, the mechanism proposed here represents a plausible interpretation based on the observed spectroscopic changes and previously reported photocatalytic behavior of TiO 2 -based composite systems rather than direct experimental confirmation.

A limitation of this study is the relatively narrow time scale and experimental scope. The work is intended to elucidate the initial mechanisms of UV-induced surface degradation in microplastics, rather than to demonstrate complete polymer mineralization or long-term environmental fate. Consequently, the findings describe how degradation begins at the surface, without addressing how far or how completely this process may proceed under real environmental conditions. Future work could include systematic evaluation of parameters such as catalyst loading, environmental conditions, and irradiation intensity to better understand how these factors influence the photocatalytic aging behavior of microplastics.

This study systematically elucidates the UV-induced degradation behavior of PS-MPs in Fe 3 O 4 and Fe 3 O 4 /TiO 2 thin-film systems under dry conditions, offering a realistic framework for understanding solid-state plastic aging. Via the use of XRD, FTIR, UV–vis, and XPS, our comprehensive analysis demonstrates that Fe 3 O 4 alone induces limited surface oxidation accompanied by measurable but modest bulk chemical and optical changes, whereas TiO 2 incorporation significantly enhances surface-localized oxidation without causing extensive bulk chain scission.

XRD confirms increased disorder within the PS matrix while preserving the crystalline integrity of TiO 2 and Fe 3 O 4. FTIR and UV–vis analyses indicate only modest bulk chemical and optical modifications, even after prolonged UV irradiation. In contrast, XPS reveals substantial enrichment of oxygen-containing carbon species highlighting the dominance of surface oxidation processes. The correlation of surface- and bulk-sensitive techniques underscores a clear decoupling between surface photo-oxidation and bulk polymer degradation.

Based on the study results, we propose a mechanistic model in which UV-excited TiO 2 generates charge carriers that may interact with Fe 3 O 4 via interfacial charge transfer, enabling oxygen-mediated surface oxidation in the absence of water.

Overall, this work demonstrates that photocatalytic degradation of PS-MPs in thin-film, dry systems is governed primarily by surface-dominated oxidation mechanisms, which fundamentally differs from aqueous degradation pathways. These findings ultimately advance the mechanistic understanding of microplastic transformation under environmentally relevant conditions and establish TiO 2 /Fe 3 O 4 thin films as an effective platform for probing and accelerating solid-state plastic aging while clearly distinguishing surface modification from true bulk polymer breakdown.

Although the polystyrene particles did not undergo extensive fragmentation, the UV-driven oxidation of their surfaces remains environmentally important. The introduction of oxygen-containing functional groups increases surface polarity, which can make microplastics more chemically reactive, enhance their ability to bind pollutants and metals, and potentially facilitate the release of residual additives. Together, these changes suggest that in natural environments, microplastic aging is likely to occur mainly through gradual surface chemical modification rather than complete polymer breakdown, with meaningful consequences for how environmental risk is evaluated.