# UV Aging Induces Colloidal-Like Behavior in Microplastics, Mediating Contaminant Fluxes Across Interfaces

Aicha El Kharraf, Murielle Rabiller-Baudry, Ana Pradas del Real, Christophe Sandt, Isabelle Bihannic, Mark Wiesner, Dominique Bavay, Hervé Tabuteau, Imane Khatib, Maxime Pattier, et al.

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Aicha El Kharraf, Murielle Rabiller-Baudry, Ana Pradas del Real, Christophe Sandt, Isabelle Bihannic, et al. UV Aging Induces Colloidal-Like Behavior in Microplastics, Mediating Contaminant Fluxes Across Interfaces. Journal of Hazardous Materials, 2025, 500, pp.140501. 10.1016/j.jhazmat.2025.140501. insu-05373363

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Distributed under a Creative Commons Attribution 4.0 International License
UV Aging Induces Colloidal-Like Behavior in
Microplastics, Mediating Contaminant Fluxes
Across Interfaces

Aicha EL KHARRAF, Murielle RABILLER-
BAUDRY, ANA PRADAS DEL REAL,
Christophe SANDT, Isabelle BIHANNIC, Mark
WIESNER, Dominique BAVAY, Hervé
TABUTEAU, Imane KHATIB, Maxime
PATTIER, Melanie DAVRANCHE

<figure><img src="image_2.png" /></figure>

PII: S0304-3894(25)03421-1

DOI: https://doi.org/10.1016/j.jhazmat.2025.140501

Reference: HAZMAT140501

To appear in: *Journal of Hazardous Materials*

Received date: 2 September 2025

Revised date: 28 October 2025

Accepted date: 14 November 2025

Please cite this article as: Aicha EL KHARRAF, Murielle RABILLER-BAUDRY, ANA PRADAS DEL REAL, Christophe SANDT, Isabelle BIHANNIC, Mark WIESNER, Dominique BAVAY, Hervé TABUTEAU, Imane KHATIB, Maxime PATTIER and Melanie DAVRANCHE, UV Aging Induces Colloidal-Like Behavior in Microplastics, Mediating Contaminant Fluxes Across Interfaces, *Journal of Hazardous Materials*, (2025) doi:https://doi.org/10.1016/j.jhazmat.2025.140501

This is a PDF of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability. This version will undergo additional copyediting, typesetting and review before it is published in its final form. As such, this version is no longer the Accepted Manuscript, but it is not yet the definitive Version of Record; we are providing this early version to give early visibility of the article. Please note that Elsevier's sharing policy for the Published Journal Article applies to this version, see: https://www.elsevier.com/about/policies-and-standards/sharing#4-published-journal-article. Please also note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to
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© 2025 Published by Elsevier.
# UV Aging Induces Colloidal-Like Behavior in Microplastics, Mediating Contaminant Fluxes Across Interfaces

Aicha EL KHARRAF<sup>1,2</sup>, Murielle RABILLER-BAUDRY<sup>1</sup>, ANA PRADAS DEL REAL<sup>3,4</sup>, Christophe SANDT<sup>4</sup>, Isabelle BIHANNIC<sup>5</sup>, Mark WIESNER<sup>6,2</sup>, Dominique BAVAY<sup>2</sup>, Hervé TABUTEAU<sup>7</sup>, Imane KHATIB<sup>2,7</sup>, Maxime PATTIER<sup>2</sup>, Melanie DAVRANCHE<sup>2</sup>

1 Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR 6226, F-35000, Rennes, France

2 Univ Rennes, Géosciences Rennes – GR UMR6118, F-35000 Rennes, France

3 Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre, F-38000 Grenoble, France

4 Synchrotron SOLEIL, l'Orme des Merisiers, Départementale 128, 91190 Saint-Aubin, France

5 Univ Rennes, CNRS, OSEREn (Observatoire des Sciences de l'Environnement de Rennes) UAR 3343, F-35000 Rennes, France

6 Duke University – Durham, NC 27708, USA

7 Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) – UMR 6251, F-35000 Rennes, France

## Abstract

Microplastics (MPs) are often regarded as pollutants vectors, yet they undergo substantial physicochemical transformations in aquatic environments. Here, we reveal how photo-oxidation (UVs) and mechanical stress transform polyethylene-based-MPs into reactive porous particles with colloidal behaviors. Using multiscale imaging, spectroscopy, and electrokinetic analyses, we show that aged MPs (100 ± 200 µm) develop dual porosity, pronounced surface oxidation (carbonyl index 2 ± 2), and increased roughness (Ra 13 ± 5 µm). Trace elements like titanium, chromium, and iron persist within the particles and accumulate at their surface. These changes modulate their behavior: in freshwater, negative surface charges (ζ ≈ -32 mV) promote dispersion and additive release, while in brackish and sea waters, charge screening triggers aggregation and sedimentation independent of biofilm growth. Our findings redefine MPs as dynamic and reactive entities, capable of transporting legacy additives and metals vertically and laterally through aquatic systems and into the atmosphere.

**Keywords:** Polyethylene microplastics, abiotic degradation, surface oxidation, inorganic additives, vectors, mobility, adsorption, environmental fate
Graphical abstract

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# 1 Introduction

Plastic pollution in the environment constitutes a diverse pool of pollutants, varying widely in size, morphology, chemical composition, color, and physical properties¹. This diversity reflects the immense scale and variety of plastic production and non-negligible fraction of this production is mismanaged and end-up in the environment as waste¹,². Accordingly, plastic debris is now observed across all size scales, from large pieces visible on beaches to micro and nanoplastic particles that can only be detected with specialized laboratory instrumentation³. Such ubiquity, combined with the extreme durability of plastics, underscores the need to understand how these materials transform for decades or longer in the environment systems. Larger plastic items breakdown via abiotic processes (e.g., UV radiation, mechanical stress), and it has been assumed that such degradation proceeds hierarchically from macro → micro → secondary nanoplastic⁴. However, recent evidences challenge this sequential fragmentation model, demonstrating that microplastics and nanoplastics can be generated directly from macroplastic surfaces at early degradation stages (month to years), thereby immediately introducing both visible debris and invisible micro and nanoplastic contaminants into ecosystems⁵. These fragments are small enough to potentially penetrate cell membranes and travel through food webs, raising concerns about their bioavailability and toxicity⁶. These behaviors have been observed for the majority of
plastic wastes in particular for polyethylene (PE), a major buoyant plastic debris in aquatic environments⁷.

During the PE-based plastic breakdown, a critical knowledge gap concerns the fate of chemical additives embedded in plastics as they degrade, knowing that plastic wastes are never pure, they often contain additives (formulated) that can represent up to 70% of their weight⁸. The release of additives from plastic matrices follows two pathways, either additives leaching or embedded inside the fragments, especially microplastics (MPs)⁹. After their release in the environment, they can be exposed to abiotic process, which can lead to further fragmentation. This phenomenon leads to several chemical and physical transformations, which markedly alter MPs behavior in aquatic systems: increased surface polarity and roughness and change how particles interact with their surrounding water interfaces¹⁰. These transformations impact how far these particles, and any additives or metals they carry, can travel in water or accumulate in sediments.

To address critical gaps in understanding the role and behavior of microplastics (MPs) in aquatic systems, we developed a multiscale analytical framework to investigate how abiotic aging governs their environmental fate. Polyethylene-based MPs were subjected to 1000 hours of combined UV and mechanical degradation, then characterized using µ-X-ray tomography, hyperspectral infrared imaging (ILIM), synchrotron µ-XRF, ICP-MS, and electrokinetic profiling. This integrated approach allowed us to assess how: (i) surface embrittlement and roughness influence fragmentation potential; (ii) oxidation introduces polar functional groups that mediate pollutant interactions; (iii) inorganic additives are redistributed or retained; and (iv) changes in surface charge affect MPs aggregation and mobility.

## 2 Materials and Methods

### 2.1 Polyethylene based material and characterization

A formulated polyethylene (PE) plastic with 1.2 mm thickness and 0.96 g·cm⁻³ was selected as the test material for the artificial weathering experiment¹¹. The polymer consists of a saturated carbon-carbon backbone, (–CH₂–CH₂–)ₙ, and contains a mixture of additives introduced during formulation processing to improve material stability and performance under environmental stress ⁸,¹².

To establish a baseline reference for the pristine plastics, the initial chemical and elemental compositions of the plastics were determined using ATR-FTIR and ICP-MS analyses. The detailed protocols and instrument settings are provided in Supplementary Information (Methods S1).
## 2.2 Experimental setup for degradation

Aiming at mimicking environmentally relevant abiotic degradation processes, the PE-based plastic was subjected to combined UVC irradiation and mechanical agitation under controlled laboratory conditions. The experimental system integrated a UVC LED source (254 nm, 2.18 W·cm⁻¹) with a PTFE-coated magnetic stirring module operating at 400 rpm, inducing continuous shear stress. Reactor temperature was stabilized between 25 and 42 °C using a ventilation thermal regulation system. Real time monitoring devices were integrated into each one of the 3 reactors to track relative humidity (maintained at 50 ± 2 %) and internal pressure (1038 mbar), ensuring environmental stability throughout the exposure period.

Triplicate degradation experiments were performed in chemically inert PTFE beakers over a continuous aging period of 1000 h. Each reactor contained 43.5 ± 0.1 g of PE and 506 ± 6 mL of Milli-Q water adjusted to an ionic strength of 1 × 10⁻³ mol L⁻¹ at natural pH = 6.35 to simulate freshwater conditions. At the end of the exposure period, 21 ± 6 mL of the aqueous phase was sampled from each reactor and sequentially filtered through a 0.8 µm glass fiber membrane. This step enabled the separation of larger microplastics (5 mm to 1 µm) from nanoscale fragments (1 µm to 1 nm).

Following filtration, microplastics retained on the membrane were carefully rinsed with Milli-Q water to recover residual particles. Samples were then stored in the dark at 4 °C to preserve physico-chemical integrity prior to downstream analyses.

## 2.3 Microplastics quantification

Quantification of MPs in the aqueous phase resulting from the exposure of PE-based plastic for 1000 h, was determined using a high-precision microbalance (Cubis SARTORIUS, sensitivity: ± 1 mg), by charging a known volumes of the solution in 1 mL vials and drying then volume for 2 h at 60°C on a hotplate in the clear room and afterwards quantifying the amount of carbon in the vials. This operation was conducted in triplicate.

## 2.4 IR laser imaging microscopy (ILIM)

Mid-infrared imaging of microplastics (MPs) was performed using a Quantum Cascade Laser (QCL MIRcat, Daylight Solutions)-based imaging microscope (Hyperion II-ILIM, Bruker Optics) coupled to a focal plane array (FPA) microbolometer detector (300 × 300 pixels) with integrated spatial coherence reduction¹³. Measurements were conducted at the SMIS beamline of Synchrotron SOLEIL. To optimize laser focusing and minimize scattering, MP samples were deposited on SeZn crystals IR-transparent discs prior to analysis. Hyperspectral images were acquired using a 15× 0.4 NA objective lens, yielding a projected pixel size of 1 µm (250 × 250 pixels). Data acquisition was carried out in sweep scan mode over the mid-infrared spectral range of 950–1800 cm⁻¹, with a spectral sampling interval of 2 cm⁻¹ and 50 scans. Each spectrum was averaged over four scans
to improve signal-to-noise ratio. Before sample acquisition, a background spectrum (n = 592 spectra) was collected from a clean region of the substrate to account for baseline variations. Final image acquisition was preceded by manual focus adjustment to ensure optimal spectral sharpness and spatial resolution.

Hyperspectral mid-IR mapping of microplastics was conducted with a focus on two diagnostic absorption bands: 1465 cm⁻¹, corresponding to CH₂ bending vibrations, to confirm the presence of polyethylene, particularly in sub-micrometric particles; and 1709 cm⁻¹, attributed to carboxylic acid groups, as a marker of oxidative degradation. Increased signal intensity at these wavelengths in spatial maps was interpreted as a higher local concentration of the corresponding functional groups. Additional technical specifications and data processing details are provided in the Supplementary Information (Methods S2).

## 2.5 Scanning Electron Microscopy (SEM)

Microplastics retained on 0.8 µm glass fiber membranes were directly analyzed on the collection substrates to avoid artefacts associated with particle transfer, including additive crystallization or salt precipitation. Surface morphology was assessed using field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7100F) equipped with Oxford EDS and EBSD detectors, operated at the ScanMat platform, University of Rennes, France.

All analyses were performed under high-vacuum conditions at an accelerating voltage of 10 kV. Prior to imaging, samples were sputter-coated with a ~5 nm Pd/Au layer to enhance surface conductivity and improve imaging resolution. Microplastics were visualized at multiple magnifications (×200, ×500, ×3000, ×10,000, ×50,000, and ×65,000) to capture both overall particle morphology and fine surface features such as cracks, porosity, and erosion patterns.

Multiple regions across each membrane were systematically scanned to ensure comprehensive particle coverage and statistically robust estimation of microplastic dimensions and surface characteristics (226 particles). Data processing details are provided in the Supplementary Information (Methods S3).

## 2.6 Three-dimensional imaging and analysis of microplastics by micro-tomography

3D reconstruction of microplastics was performed using micro-X-ray computed tomography (µ-CT) on an Easytom XL 3D microtomograph (RX Solutions) equipped with a cone-beam geometry, operated at the imaging facility IMAGEO (OSERen, University of Rennes, France). The imaging system (Figure S2) comprised a micro-focus X-ray source (L10711, Open Type Microfocus X-ray source) operated at 60 kV and 200 µA, and a flat panel detector (L9181-02, Hamamatsu Photonics, Japan). A source-to-object distance of 216 mm and a source-to-detector distance of 60.15 mm yielded a final voxel size of 2.11 µm.
A total of 2,016 radiographic projections were acquired over a full 360° rotation, with three frames collected at each angular position. Median filtering across frames was applied to enhance signal quality and minimize noise artifacts. The total acquisition time per scan was approximately 2 hours. Projection images were corrected using flatfield and darkfield calibrations prior to reconstruction. Additional technical specifications and data processing details are provided in the Supplementary Information (Methods S4).

## 2.7 Elemental distribution in plastics using µ-Xray Fluorescence Analysis

The elemental composition and spatial distribution of additives within PE-based-microplastics were analyzed using synchrotron-based X-ray fluorescence microscopy (XRF). Measurements were carried out on MPs released from formulated PE plastics aged for 1000 h, using the LUCIA beamline at the SOLEIL Synchrotron (Saint-Aubin, France)¹⁴.

Same particles previously analyzed using IR laser imaging microscopy were deposited onto Ultralene XRF discs and mounted onto copper holders within a high-vacuum chamber. Analyses were performed at an excitation energy of 7800 eV and under cryogenic conditions (liquid nitrogen cooling) to minimize radiation damage. A focused X-ray beam (3.5 × 3.5 µm²) was achieved using Kirkpatrick-Baez (KB) mirrors, and fluorescence signals were collected using a 60 mm² silicon drift detector (Bruker) operating in continuous FlyScan mode.

Two scanning strategies were employed: (i) low-resolution maps (25 × 25 µm² step size, 100 ms per pixel) to screen entire samples and locate regions of interest, and (ii) high-resolution maps (3.5 × 3.5 µm² step size, 100 ms per pixel) to resolve fine-scale additive distributions within selected domains.

Fluorescence signals were processed using PyMCA software¹⁵. Raw fluorescence intensities were normalized to the incident beam flux and corrected for detector dead time, ensuring reliable inter-pixel comparisons. Spectral deconvolution was performed at each pixel using non-linear least-squares fitting to resolve overlapping peaks and accurately extract elemental signatures.

To visualize spatial relationships and possible co-localization of additives, composite RGB maps were created by overlaying three elemental distributions in a single image. This multichannel visualization approach enhanced contrast and revealed patterns of additive clustering and heterogeneity within individual microplastic particles¹⁶. All data processing and visualization steps were completed using the PyMCA software.
## 2.8 Quantification of inorganic additives in microplastics by Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

The quantification of inorganic elements within microplastics (MPs) was essential to validate, at the bulk scale, the presence of additives initially detected by synchrotron-based µ-XRF analysis. Inorganic elemental concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700X) at the GeOHeLiS analytical platform, University of Rennes, France.

Aliquots (1 mL) from the microplastic suspensions (stored in Milli-Q water at 4 °C) were processed using two distinct protocols: (i) in the first protocol protocol (applied to UV-aged MPs) MPs were directly digested in 3% HNO₃ under identical microwave-assisted conditions. (ii) In the second, MPs were placed into 5 kg.mol⁻¹ (kDa) ultrafiltration cells and leached with 1% HNO₃ for 20 min to remove surface-adsorbed additives. Samples were then ultracentrifuged at 400 rpm for 45 min. Residual acid and desorbed additives were extensively rinsed by adding Milli-Q water and repeating ultrafiltration. The leached MPs were subsequently resuspended in Milli-Q water and digested using a high-pressure microwave system (MULTIWAVE-7000, Anton Paar) at 80 °C and 40–140 bar in 3% HNO₃.

In parallel, solid samples of the pristine and 1000-h aged PE-based plastics were also digested under the same conditions, allowing direct comparison of additive concentrations between MPs and their parent materials.

Following digestion, all fractions (leached MPs_PE, MPs_PE, pristine PE, 1000-hour aged PE, and the leachates form leached MPs) were analyzed by ICP-MS. Details on instrumentation calibration are provided in supplementary information section (Methods S6).

## 2.9 Estimation of specific surface area, surface density and functional charge of MPs

The surface properties of microplastic (MP) particles were evaluated through an integrated morphological and chemical approach. For the purpose of surface area calculations, MPs were assumed to behave as non-porous particles, based on the observation that internal micro-voids identified by X-ray tomography were not fully interconnected or externally accessible.

Particle dimensions were determined from optical and scanning electron microscopy images using Feret diameter measurements. To simplify surface area estimation, particles were approximated as spheres. Under this assumption, the average surface area (S̄t) of individual particles was calculated using the following expression (1):

$$S_t = 4 \pi r^2 \qquad (1)$$

where r̄ is the average particle radius derived from Feret diameter measurements.
The specific surface area (SSA) of microplastic (MP) particles was estimated by normalizing their total surface area to mass. Assuming spherical geometry, the average particle volume was ($\bar{V}$) was calculated from the mean particle radius using (2), (3) and (4):

$$ \bar{V} = \frac{4}{3} \Pi \bar{r}^3 \qquad (2) $$

The corresponding mass of each MPs particles was computed by multiplying the volume by the bulk density of polyethylene ($\rho = 0.94 \text{ g cm}^{-3}$) knowing that the particles density is independent of their shape¹⁷:

$$ \bar{m} = \bar{V} \rho \qquad (3) $$

$$ \overline{SSA} = \frac{S_t}{m} \qquad (4) $$

To account for deviations from ideal spherical geometry, SSA values were corrected for surface roughness using 3D profilometry (5), following protocols adapted from Lowell et al. (2004) and Hanaor et al. (2014)¹⁷,¹⁸.

$$ \overline{SSA_{corrected}} = \overline{SSA} \times (1 + \frac{\text{roughness}}{r}) \qquad (5) $$

## 2.10 PCA analysis

To explore how these physicochemical surface parameters co-varied across the microplastic population, a principal component analysis (PCA) was conducted with OriginPro software using SSA, particle size, Feret aspect ratio (FAR), and chemical indices as input variables. This multivariate approach allowed identification of coupled trends between physical attributes (e.g., particle size, SSA) and chemical reactivity (e.g., functional groups), revealing key relationships that govern microplastic interactions with their environment.

## 2.11 Electrophoretic mobility of microplastics

To assess the electrostatic properties of microplastics released from PE-based plastic during abiotic degradation, their electrophoretic mobility ($\mu$) was measured using a micro-electrophoresis instrument (Zetaphoremeter IV, CAD Instrumentations, Les Essarts le Roi, France). Experiments were conducted at the Laboratoire Interdisciplinaire des Environnements Continentaux (LIEC, CNRS, Université de Lorraine, Nancy, France). Measurements were performed at neutral pH in a quartz cell at 24 °C as a function of electrolyte salinity. Prior to analysis, the conductivity (0.308 mS·cm⁻¹) and pH (6.78) of the MPs solution were recorded, and pH monitoring was conducted throughout the experiments.

The technique consists of tracking the velocity of charged particles suspended in electrolyte solutions under an applied electric field of 800 V·m⁻¹. Particle displacement was converted to electrophoretic mobility values using CAD image analysis software. Simultaneously, the number
of observable particle trajectories was monitored to detect potential aggregation phenomena associated with increasing ionic strength ([NaCl] = 2.5–250 mmol L⁻¹). Each experimental condition was tested in triplicate to ensure reproducibility.

Changes in ionic strength were anticipated to impact the electrostatic behavior of MPs by compressing the electrical double layer, as described by Debye–Hückel theory and Elimelech et al. (1995)¹⁹. Therefore, the Debye length (κ⁻¹) was calculated for each electrolyte concentration to quantify the thickness of the electrical double layer and its variation with salinity.

The Debye length was determined based on classical Gouy–Chapman theory according to the following expression (6):

$$k = \left( \frac{2 \cdot 10^3 e^2 N_a I}{\varepsilon_0 \varepsilon_r k_B T} \right)^{1/2} \quad (6)$$

where ε₀ is the vacuum permittivity, εᵣ is the relative permittivity of water, kₛ is the Boltzmann constant, T is the absolute temperature, Nₐ is Avogadro's number, e is the elementary charge, and I is the ionic strength of the solution.

The Debye length (κ⁻¹) was obtained as the reciprocal of κ and expressed in nanometers. The calculations assumed standard aqueous conditions and monovalent electrolyte behavior.

Aggregation behavior was evaluated using the extended Derjaguin–Landau–Verwey–Overbeek (X-DLVO) theory, together with zeta potential calculations derived from electrophoretic mobility measurements. Detailed calculation procedures and equations are provided in the Supplementary Information (Methods S7). (Methods S7).

## 2.12 Surface charge density estimation

The surface charge density (σ, C·m⁻²) of PE-based-MPs was estimated using the linearized Gouy–Chapman model under room temperature conditions (25 °C), following established electrokinetic methodologies for environmental colloids and plastic particles. The surface potential (Ψ₀) was approximated from experimentally measured zeta potential (ζ), assuming the thin double layer condition (κa ≫ 1), where particle radius exceeds the Debye length. Ionic strength (I, mol·L⁻¹) was controlled using NaCl, and σ was calculated using the following expression (7)²⁰:

$$\sigma = 0.1174 \sqrt{I} \sin h (19.47 \zeta) \quad (7)$$

where ζ is in volts and σ is in C m⁻². This formulation assumes a planar interface, symmetric 1:1 electrolyte, and uniform distribution of ionizable surface groups. The estimated σ values were further used to back-calculate the surface site density (mol m⁻²) via rearrangement of the relation (8)²⁰:

$$\sigma = -F \sum_i [SO^-]_i \quad (8)$$
where F is the Faraday constant (96 485 C mol⁻¹), and the sum accounts for all deprotonated acid groups contributing to the surface charge.

## 3 Results and Discussion

Environmentally-relevant MPs, were generated from polyethylene (PE) based plastic through a protocol of 1000 h of UVC exposure combined with mechanical agitation. The PE based plastic underwent extensive fragmentation, producing surface embrittlement and a morphologically diverse. SEM analysis of three distinct areas (Figure 1a-c) revealed a heterogeneous mix of angular fragments, film-like flakes, and partially rounded particles. These features are consistent with known abiotic degradation pathway in polyolefins, where photo-oxidative weakening of the amorphous phase initiate cracking and surface embrittlement under stress⁷,²¹. The diversity observed mirrors that of MPs isolated from freshwater and estuarine systems²², where light exposure and physical abrasion shape particle geometry²³.

Quantitative image analysis of 226 particles revealed a mean Feret diameter of 100 ± 200 µm (Figure 1d), with a broad size distribution dominated by smaller fragments. The Feret aspect ratio (FAR) of 0.5 ± 0.1 (Figure 1e) indicating a predominance of irregular, asymmetric particles (FAR < 0.6), is consistent with field observations where shape complexity modulates aggregation, settling, and bio-availability²⁴.

The mass of MPs released (larger than 0.8 µm threshold) reached 213.50 ± 0.06 µg g⁻¹, underscoring the high fragmentation potential of even stabilized PE under realistic abiotic conditions. However, this value likely underestimates the total degradation output, as it excludes NPs and dissolved by-products²⁵.

At higher magnification, SEM reveals pronounced surface roughness, micro-voids, and incipient cracks (Figure 1a-c, zoomed panels), suggesting that many of the resulting PE-based-MPs are not stable end-products, but rather in transient state still undergoing physical transformation. The presence of such structural instabilities reflects incomplete degradation, where oxidation-induced embrittlement continues propagating through the particle matrix²⁶. Comparable microfractures have been reported in field-aged PE and polypropylene (PP) based debris, where crack development and surface fatigue are key drivers of progressive downsizing into smaller fragments via abiotic processes²⁷.
<figure><img src="image_4.png" /><figcaption>**Figure 1:** Morphological characterization of microplastics (PE-based-MPs) generated by abiotic degradation of formulated PE plastic.</figcaption></figure>

(a–c) SEM images of PE-based microplastics retained on a 0.8 µm glass fiber filter captured from three distinct regions. Insets highlight individual particles with diverse morphologies, including angular fragments, flakes, and partially rounded debris. (d) Size distribution (Feret diameter) of 226 particles, showing a predominance of small fragments. (e) Feret aspect ratio histogram, with most particles displaying irregular shapes (FAR < 0.6), consistent with asymmetric and fracture-derived geometries.
## 3.1 Abiotic weathering drives structural transformation and dual porosity in PE based microplastics

Three-dimensional X-ray microtomography of PE MPs revealed extensive morphological degradation consistent with long term weathering. Particles exhibited irregular, fragmented, and filamentous geometries, often bearing secondary fragments still attached to the primary surface (Figure 2a, red arrows). These structural features provide direct evidence of ongoing *in situ* fragmentation rather than terminal disintegration. This morphology is consistent with previous UV-aging studies, where prolonged exposure leads to surface embrittlement, crack propagation, and the detachment of both smaller micro- and nanoscale debris<sup>7,28</sup>.

Volumetric segmentation further revealed a dual porosity structure within the MPs (Figure 2b-c). Surface connected-pits and fissures (hallmarks of photo-oxidative degradation), were primarily located in the amorphous regions of the polymer<sup>29</sup>. These less-ordered domains are particularly susceptible to oxidative attack, triggering chain scission and the formation of cracks and surface voids. As degradation proceeds, the progressive loss of amorphous material leads to a relative enrichment of the crystalline phase. In contrast, confined cavities were observed deeper within the particle matrix. These internal voids, distinguishable by grayscale contrast, likely originated from residual heterogeneities, such as clusters of inorganic additives<sup>30,31</sup>. With time, environmental aging amplifies these structural defects, promoting deeper cracking and increased mechanical instabilities. This process is further intensified by the high additive content of many PE plastic formulations (often exceeding 50% by mass) composed of non-covalently bonded constituents that can leach or migrate during aging, generating internal porosity patterns analogous to surface erosion<sup>32</sup>.

Furthermore, topographic profiling of PE-based-MPs revealed high-amplitude surface roughness with an average roughness (Ra) of 14 ± 5 µm (more than twice that of pristine PE :6 ± 1 µm). The root mean square roughness (Rq) also increased to 18 ± 5 µm compared to Rq = 8 ± 1 µm for the pristine plastic. Surface pits reached depth of 70 ± 20 µm for MPs (supplementary Table S1) and rough zones accounted for ~36% of the total surface area (supplementary Figure S4b). These values are consistent with environmental weathering patterns observed for PE-based-MPs exposed over long periods to sunlight, temperature fluctuations, and mechanical abrasion<sup>33,34</sup>. Oxidative chain scission introduces carbonyl groups that compromise the integrity of the polymer matrix, while mechanical abrasion strips away oxidized layers, continuously exposing fresh surfaces to degradation.

Importantly, surface roughness is not merely a by-product of degradation, it actively shapes how MPs interact with their surrounding environment. The increase in specific surface area (SSA), combined with micro-crevice formation enhances the sorption of hydrophilic contaminants and metal ions through mechanisms such as complexation or van der Waals interactions<sup>34</sup>.
<figure><img src="image_5.png" /><figcaption>**Figure 2:** µX-ray tomography reveals in situ fragmentation and dual-porosity architecture in aged PE-based-MPs.</figcaption></figure>

(a) External 3D volume rendering of a representative microplastic showing filamentous morphology, irregular contours, and surface-attached secondary debris (red arrows), indicative of active fragmentation under combined oxidative and mechanical stress. Multiple orientations are displayed to illustrate the full extent of surface complexity and morphological heterogeneity.

(b) Tomographic reconstruction with porosity segmentation highlighting surface-connected pits (red) and deeper confined cavities.

(c) Grayscale-contrast rendering of internal voids not connected to the surface, likely corresponding to additive-rich microdomains inherited from the parent polymer matrix.

## 3.2 Microplastics are not inert

High-resolution µ-XRF mapping and spectroscopy of PE-base-MPs revealed a heterogeneous distribution of inorganic additives within individual particles (Figure 3a-c). Elemental mapping shows clear co-localization of titanium (Ti), chromium (Cr), and iron (Fe) as concentrated hotspots across the MPs matrices, indicating that these elements are not merely surface-bound but are embedded throughout the MPs interior. The corresponding µ-XRF spectra (Figure 3a-c, right panels) confirmed the presence of these elements, consistent with known additive profiles in formulated plastics<sup>8,30</sup>. These findings demonstrate that PE-based-MPs are vectors for pollutants but also act as intrinsic sources of inorganic contaminants by retaining legacy additives inherited from the parent material.
To confirm the origin of these elements, we compared the µXRF spectra of MPs with those of their corresponding macroplastic precursors: pristine PE plastic and artificially aged PE-1000h (Figure 3d). The parent materials exhibited markedly higher concentrations of Ti, Cr, and Fe, however shared identical elemental signatures with the derived MPs. This confirms that the metals detected in MPs were inherited during degradation.

Quantitative ICP-MS analysis (Figure 3e) further corroborated the µ-XRF findings showing that MPs contain lower, but still substantial, concentrations of Ti, Cr, and Fe compared to both pristine and aged PE-based plastic. This supports the hypothesis that MPs act as reservoirs for inorganic additives, although at diminished levels due to weathering and fragmentation³⁵⁻³⁸. The acid leaching experiments conducted on PE-based-MPs (Figure 3f) revealed that a significant fraction of these elements can be released from the MPs surface, as highlighted by the red arrow in the Figure 3f, denoting a distinct decrease in metal concentration corresponding to leached-out species. This result confirms that a fraction of these metals was loosely adsorbed onto oxidized MPs surfaces, whereas the remaining fraction was strongly retained within their matrix, representing embedded additives that are not readily mobilized by leaching. This dual behavior underscores the complex environmental role of MPs which act both as source, releasing inherited additive, but also as vectors capable of adsorbing and transporting additional contaminants³⁹.

A radar plot (Figure 3j) illustrates the broad range of inorganic additives detected across all samples, including B, Al, Ti, Cr, Fe, Ni, Cu, Zn, Ba, Ce, and Pb. Both MPs and subsequent leached-MPs exhibit consistently lower but overlapping, additive profiles relative to pristine PE, highlighting the partial, but incomplete, loss of inorganic constituents during environmental aging and leaching. This compositional persistence indicates that MPs contribute to aquatic contamination not only as vector pathways but also as intrinsic sources of legacy additives, thereby amplifying their environmental risk ⁸,⁴⁰.

From an environmental perspective, our results challenge the conventional view of MPs as inert pollutant vectors by revealing their dual role: aged MPs not only retain legacy additives embedded within their polymer matrix but also exhibit surface-enrichment in elemental species. Although no external tracer was introduced, surface quantification (Figure 3f) revealed localized accumulations of Ti, Cr, and Fe at the particle surfaces. These patterns are in agreement with the potential of MPs to develop chemically active surfaces able to interact with their surrounding environment³⁸.
<figure><img src="image_6.png" /><figcaption>Figure 3: Elemental composition and additive retention in PE-based-MPs.</figcaption></figure>

(a-c) Synchrotron µ-XRF maps of three representative MPs (MP_1, MP_2, MP_3), showing the spatial distribution and relative intensity of Ti, Cr, and Fe within the particle matrices. (d) Normalized µ-XRF signal intensities for Ti, Cr, and Fe across pristine PE-based plastic, aged PE-based plastic (1000 h), and derived MPs, highlighting inheritance of elemental signatures during degradation. (e-f) ICP-MS quantification of key elements in pristine PE-based plastic, MPs, and leached MPs, showing partial but persistent additive retention after aging and leaching obtained from acid treatment. (j) Radar plot summarizing the concentration profiles of multiple inorganic additives (B, Al, Ti, Cr, Fe, Ni, Cu, Zn, Ba, Ce, Pb) across all sample types.

### 3.3 Reactive by design: how degradation sculpts the surface chemistry of microplastics

To investigate the surface chemistry and oxidation state of PE-based-MPs containing embedded additives, infrared laser imaging microscopy (ILIM) were performed in transmission mode on the same MPs as those analyzed by µ-XRF (Figure 4a-c). Spatial mapping of the CH₂ bending vibration at 1464 cm⁻¹ confirmed the PE identity of the particle, with each map cross-referenced against the corresponding IR spectrum to validate polymer composition (Figure 4a-c, IR average spectra). In contrast, absorbance at 1709 cm⁻¹ (characteristic of C=O of carboxylic acid groups) exhibited a highly heterogeneous spatial distribution (Figure 4a-c, distribution maps of acid groups), indicating localized and non-uniform oxidative modifications. Although transmission mode captures IR signals across the full particle thickness, the presence of discrete intensity hotspots and patchy C=O signals indicates that oxidation is predominantly confined near the particle surface, where UV light and reactive oxygen species exhibit the highest penetration and reactivity. This pattern aligns with established oxidation mechanisms in PE, where surface-initiated radical reactions and oxygen diffusion drive the progressive formation of polar functional groups during
environmental aging⁷. The corresponding averaged spectra (~30,000 spectra per MP; Figure 4a-c, third panels), further confirm the development of oxidative bands⁴¹, particularly in the C=O (1670–1800 cm⁻¹) and C-O (1000–1300 cm⁻¹) regions, highlighting ongoing chemical transformation of the polymer matrix.

To quantify the extent and nature of surface oxidation, the carbonyl region of the spectra was deconvoluted for each particle to calculate acid, ketone, and ester indices. The results (Figure 4a-c, right panels) revealed a clear predominance of acid functional groups on MPs surfaces, with a mean acid index value of 1 ± 1, higher than those of ketones (0.3 ± 0.2) and esters (1.0 ± 0.6) Figure 5b). This surface chemical profile is particularly relevant to discuss the environmental behavior of MPs: the acid groups formed upon PE oxidation are predominantly deprotonated *at neutral pH* (99% at pH 6.98) ⁴¹ (supplementary Table S3), a form that enhance the MPs capacity to adsorb and complex either polar or cationic contaminants, respectively, from surrounding waters⁴²,⁴³. This reactive surface chemistry not only reflects the oxidative history of the MPs but also reinforces our earlier finding that oxidation increases their pollutant-binding potential (Figure 3f).

To assess the statistical robustness and environmental relevance of these findings, the ILIM analysis was extended to a population of 31 PE-based-MPs (Figure 5a). Image analysis enabled quantification of particle size (100 ± 200 µm; Figure 5b), carbonyl index (2 ± 2) versus pristine plastic = 0.11 ± 0.02; Figure 5b), and vinyl index (0.6 ± 0.5; Figure 5b), two features associated with instability and a higher propensity for fragmentation³⁵. Spatial mapping of the 1709 cm⁻¹ band further confirmed the widespread yet heterogeneous distribution of surface acid groups across the particle population. Crucially, we calculated the specific surface area of each particle and, after correction for measured roughness, evidenced an increase in SSA (from 0.2 ± 0.4 m² g⁻¹ for smooth particle to 0.4 ± 1 m² g⁻¹ for rough particles). This result underscores the role of microtopography (estimated from µ-CT analysis; supplementary Figure S4) in enhancing surface reactivity and, by extension the potential role of vector of MPs for pollutant adsorption⁴³.

Principal component analysis (PCA) integrating morphological and chemical descriptors of PE-based-MPs revealed strong co-variation between surface oxidation and structural properties (Figure 5a). The first two principal components accounted for nearly 80% of the total variance, with the carbonyl index, vinyl index, SSA, and FAR emerging as the dominant loading variable. This co-variation indicates that oxidative degradation and physical fragmentation are mechanistically linked: as MPs weather, polymer chain scission, evidenced by increased carbonyl and vinyl indices, is accompanied by surface erosion and elongation, which increase SSA and distort particle shape. These trend align with known photo-oxidation pathways in Polyolefin, where bond cleavage leads to the formation of oxidized groups such as carbonyls, acids, and vinyl moieties ⁴⁴, while mechanical destabilization drives further restructuring and embrittlement²⁸.
The PCA clustering underscores that the most environmentally reactive MPs are those that combine surface oxidation, high roughness, and irregular geometry.

Journal Pre-proof
<figure><img src="image_7.png" /><figcaption>Figure 4: Surface chemical mapping of PE-based- microplastics reveals advanced oxidation and predominance of acid groups.</figcaption></figure>

(a-c) ILIM maps showing the spatial distribution of CH₂ groups (1464 cm⁻¹) and carboxylic acid groups (1709 cm⁻¹) on three representative MPs. Corresponding average IR spectra (n ≈ 30,000 pixels per particle, right panels) display distinct oxidative features, including carbonyl bands (1680-1800 cm⁻¹) and C-O stretching bands (1000-1300 cm⁻¹). Scatter plots present calculated acid, ketone, and ester indices, highlighting the dominance of acid functional groups on the three MPs.
<figure><img src="image_8.png" /></figure>

<table><caption>Figure 5: Statistical analysis links oxidation, surface structure, and reactivity across a population of 31 polyethylene-based microplastics.</caption><thead><tr><th>Sample</th><th>Size (μm)</th><th>FAR</th><th>SSA (m² g⁻¹)</th><th>SSA<sub>rough</sub> (m² g⁻¹)</th><th>Carbonyl index</th><th>Acid index</th><th>Ketone index</th><th>Ester index</th><th>Vinyl index</th></tr></thead><tbody><tr><td>MPs</td><td>100 ± 200</td><td>0.5 ± 0.1</td><td>0.1 ± 0.2</td><td>1 ± 1</td><td>2 ± 2</td><td>1 ± 1</td><td>0.3 ± 0.2</td><td>1.0 ± 0.6</td><td>0.6 ± 0.5</td></tr></tbody></table>

(a) PCA of 31 MPs (upper left) integrates size, specific surface area (SSA), carbonyl index, and vinyl index, identifying key drivers of surface reactivity. Representative ILIM maps (right and lower panels) show the heterogeneous distribution of acid groups (1709 cm⁻¹), reflecting variability in oxidative state and surface morphology. (b) Summary table of morphological and chemical surface properties for the 31 analyzed MPs, including mean and standard deviation values for particle size, carbonyl index, vinyl index, SSA, and roughness-corrected SSA. Results illustrate the impact of environmental weathering on surface reactivity and potential for contaminant interaction.

## 3.4 When rivers meet the sea: how salinity reshapes microplastic behavior and fate

The electrokinetic properties of PE-based-MPs are key determinants of their mobility, aggregation behavior, and pollutant reactivity in aquatic systems. To investigate these properties, we
measured their electrophoretic mobility (allowing the determination of their zeta potential, surface charge density and total surface site concentration) across a salinity gradient ([2.5–250] × 10⁻³ mol L⁻¹ NaCl) to simulate transitions from freshwater to estuarine (brackish water) conditions. Under low ionic strength conditions as in freshwater (<10⁻² mol L⁻¹), PE-based-MPs exhibited highly negative electrophoretic mobilities (−2.49 to −2.27 × 10⁻⁸ m² V⁻¹ s⁻¹) and ζ-potentials near −32 mV (Table S4; Figure 6a), consistent with a thick electric double layer and strong colloidal stability. In this regime, MPs remain highly mobile and suspended, increasing their likelihood of interacting with trace contaminants, nutrients, and microbial communities in the water column⁴⁵,⁴⁶. The persistence of long-range electrostatic repulsion, further promotes dispersion and delays aggregation or sedimentation.

As salinity increased beyond 70 ×10⁻³ mol L⁻¹, corresponding to the transition to representative estuarine conditions, the zeta potential decreased sharply to −6 mV, and the Debye length contracted below 1 nm (Figure 6b). This reflects the compression of the diffuse electric double layer and the increase of the effective screening of surface charges, which reduces electrostatic repulsion and promotes particles’ aggregation⁴⁷. Concomitant with this collapse, dynamic tracking revealed pronounced drop in the number of observable MPs (Figure S7), suggesting the onset of rapid aggregation. To mechanistically interpret this transition, we computed extended Derjaguin-Landau-Verwey-Overbeek (X-DLVO) interaction energies that incorporate electrostatic, van der Waals, and Lewis acid-base contributions⁴⁸. The resulting energy profiles (Figure 6c) showed a progressive decline in the total interaction energy barrier with increasing ionic strength. Beyond 70. 10⁻³ mol·L⁻¹, interaction energies became negative, indicating that attractive forces dominate and aggregation becomes thermodynamically favored⁴⁹, a trend consistent with critical coagulation concentrations (CCC) observed in colloidal systems⁵⁰.

Electrokinetic modeling revealed that both surface charge density and total reactive site concentrations increased with salinity, peaking at 50 ×10⁻³ mol L⁻¹ NaCl (1.26 × 10⁻⁷ mol m⁻²; Table S4). This rise likely reflects enhanced counter-ion association and surface ionization, which may facilitate the pollutant desorption from previously oxidized domains, consistent with the IR-ILIM topographic patterns described above (Figures 2–5). However, beyond the ionic threshold, aggregation led to a marked decline in both parameters, likely due to interfacial occlusion and reduced accessibility of surface site during coagulation. This shift is critical since it suggests that PE-based-MPs might reach a peak in pollutant exchange capacity just before aggregation, after which their interactions with their environment sharply decreases due to surface shielding and reduce diffusional access²⁰.

These coupled electrostatic and colloidal transitions directly govern the environmental fate of PE-derived microplastics across freshwater-marine interfaces.
In riverine systems, MPs remain electrostatically stabilized, buoyant, and chemically reactive. Their oxidized and roughened surfaces provide favorable sites for contaminant adsorption and microbial colonization, reinforcing their role as dynamic interfacial agents in aquatic biogeochemical processes⁵¹.

As MPs are transported toward estuarine waters, the progressive increase in ionic strength compresses their electrical double layer, diminishing electrostatic repulsion and promoting aggregation. This physicochemical shift gives rise to two divergent fate pathways:

(i) Sedimentation pathway: In estuarine and near-coastal zones, MPs can heteroaggregate with mineral particles or biofilms, increasing their effective density and favoring deposition within sediments⁵²⁻⁵⁴. Our observations reveal an alternative, abiotic mechanism for MP settling that occurs independently of biofilm or eco-corona formation, traditionally considered prerequisites for sinking⁵⁵⁻⁵⁷. Photooxidative aging alone, through the combined effects of surface oxidation, porosity generation, additive loss, and charge collapse under saline conditions, is sufficient to drive rapid aggregation and sedimentation. This mechanism challenges prevailing models that attribute sinking primarily to biological colonization and suggests a complementary, purely physicochemical route. However, such sequestration may be temporary, as sediment resuspension or redox fluctuations could remobilize trapped MPs and their associated contaminants back into the water column.

(ii) Buoyancy and surface persistence pathway: When aggregation does not substantially increase particle density, MPs remain buoyant due to the inherently low density of polyethylene⁵⁸. Their internal porosity and the progressive loss of dense inorganic additives further enhance this buoyancy. Consequently, MPs can persist at the air-water interface, where prolonged exposure to sunlight and mechanical stress accelerates additional oxidation, fragmentation into nanoplastics, and additive release³⁵,⁵⁹. Surface-floating MPs may also be aerosolized under strong winds, introducing both micro- and nanoplastics into the atmosphere⁶⁰.

Beyond influencing particle transport, salinity also modulates the behavior of metal species associated with MPs. The attenuation of electrostatic interactions under high ionic strength can displace weakly bound metals from oxidized surface groups, altering their loading dynamics. Aggregation and aerosolization processes can thus redistribute surface-enriched elements into new environmental compartments, positioning airborne MPs as potential vectors for trace-metal dispersion⁶¹. These findings highlight estuarine environments as active biogeochemical regulators rather than passive transition zones. The physicochemical transformations occurring at these interfaces govern whether MPs are ultimately buried and sequestered, incorporated into food webs, or dispersed into the atmosphere.

While this study emphasizes the river-to-sea transition, plastic oxidation can also initiate directly within marine environments, where halide-mediated radical quenching slow photooxidation and
promote early aggregation. Under such saline conditions, MPs tend to form denser, less reactive
particles compared to those in freshwater. These contrasts underscore the need for dedicated
investigations on the degradation in sea water to fully capture the environmental pathways of
plastic transformation across aquatic systems<sup>7,62,63</sup>.

<figure><img src="image_9.png" /></figure>
<figure><img src="image_10.png" /><figcaption>Figure 6: Electrostatic evolution and aggregation thresholds of aged PE-based- microplastics across freshwater-estuarine salinity gradients.</figcaption></figure>

(a) Electrophoretic mobility (orange, left axis) and Debye length (blue, right axis) of PE microplastics (PE-based-MPs) as a function of NaCl concentration. A clear transition from electrostatic stability (green zone, freshwater) to destabilization (red zone, estuarine) is observed beyond 0.07 mol L⁻¹. (b) Correlation between electrophoretic mobility and Debye length evidences that reductions in electric double layer thickness are directly linked to diminished colloidal mobility, highlighting the physicochemical basis for salinity-induced destabilization. (c) Extended X-DLVO interaction energy profiles between PE-based-MPs under different salinity conditions. The progressive reduction and inversion of the energy barrier beyond 70.10⁻³ mol L⁻¹ NaCl indicates that spontaneous aggregation becomes thermodynamically favorable.
## 4 Conclusion

This study demonstrates that polyethylene-based microplastics (MPs) formed through abiotic degradation are reactive, structurally dynamic particles whose environmental fate depends on evolving physico-chemical properties. Using high-resolution tomography, infrared imaging, and electrokinetic modeling, this study reveals how degradation processes (including surface oxidation, internal porosity, additive redistribution, and charge changes) govern dispersion, aggregation, and pollutant interactions. Aging enhances both surface reactivity and structural complexity, influencing whether MPs float, sink, or remain suspended across salinity gradients. Our findings move beyond static MPs classifications, offering a predictive framework linking degradation history to environmental behavior, and position MPs as transitional particles whose evolving properties must be integrated into fate and risk models in aquatic systems.

## 5 Acknowledgments

The authors acknowledge financial support from CNRS 80PRIME (UltimatePlast project) and the French National Research Agency (ANR) [ref. ANR23-CE01-0002-01]. Through the support of the GeOHeLiS analytical platform at Rennes University, this publication is also supported by the European Union through the European Regional Development Fund (FEDER), the French Ministry of Higher Education and Research, the French Region of Brittany, and Rennes-Metropole.

The authors gratefully acknowledge the Laboratoire Interdisciplinaire des Environnements Continent ((LIEC), UMR 7360 CNRS – Université de Lorraine, MetzNancy, France) for providing access to their facilities for electrophoretic mobility measurements. We also sincerely thank Dr. Céline Caillet (Pôle de Physico-Chimie Environnemtale du LIEC) for her valuable assistance and technical support during the experimental work (Electrophoretic mobility measurements).

We are grateful to the ImagéO IMAGEO (OSERen platforms, Rennes, France) (tomography), and SCANMAT platforms for their technical resources.

Finally, we acknowledge SOLEIL for providing synchrotron radiation facilities, and we thank the staff of the "SMIS" and "LUCIA" beamlines for their valuable assistance under proposal 20240749.
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**Declaration of interests**

[x] 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.

[ ] The author is an Editorial Board Member/Editor-in-Chief/Associate Editor/Guest Editor for [*Journal name*] and was not involved in the editorial review or the decision to publish this article.

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## Environmental Implication

Our study reveals that polyethylene-derived microplastics undergo profound physicochemical transformations during environmental aging, redefining their role from passive vectors to dynamic colloidal entities. Photo-oxidation and mechanical stress generate porous, oxidized, and roughened surfaces, while embedded additives such as Ti, Cr, and Fe persist and accumulate at particle interfaces. These changes enhance pollutant adsorption, modulate dispersion in freshwater, and trigger aggregation and sedimentation under estuarine salinities, independent of biofilm growth. Furthermore, porosity and buoyancy may favor atmospheric transfer. By integrating chemical aging, additive release, and mobility across aquatic gradients, our findings highlight overlooked pathways of contaminant transport and ecosystem exposure.

## Highlights

* Polyethylene microplastics aged under UV and mechanical stress acquire dual porosity, roughness, and oxidized surfaces.

* Inorganic additives (Ti, Cr, Fe) persist within and accumulate at microplastic surfaces, acting as legacy pollutant sources.

* Surface oxidation introduces acid functional groups that enhance pollutant binding and reactivity.

* Electrokinetic analyses reveal charge-dependent dispersion in freshwater and aggregation in estuarine conditions.

* Findings highlight an abiotic pathway for microplastic sinking and potential aerosolization, independent of biofilm growth.