Microplastics (<5 mm), a pervasive pollutant, pose significant environmental risks. While the mass of marine plastics is projected to triple by 2025, our understanding of their degradation in marine environments remains limited. Existing knowledge relies heavily on ageing simulations and short-term field observations, lacking the necessary long-term perspective. These limitations hinder the development of accurate risk assessments and effective policies. Environmental factors like UV radiation, mechanical forces, and microbial activity influence plastic degradation, although the process is significantly slower in marine environments compared to terrestrial settings due to lower temperatures and UV intensity. This study addresses this knowledge gap by providing the first comprehensive, long-term time-series investigation into the natural weathering of plastics in the marine environment. The research aims to (i) understand the impacts of natural marine weathering on the physicochemical properties of plastics; (ii) analyze time-dependent changes in the properties of marine plastics in situ; and (iii) determine weathering's role in modulating chemical sorption and desorption. This research is expected to enhance our understanding of the behavior, fate, and hazards of marine plastics, contributing towards informed environmental risk assessments.

Previous research on weathered plastics is primarily based on artificial ageing simulations and observations of field-collected plastics, both of which have significant limitations. Artificial ageing studies offer valuable insights into the impacts of weathering processes such as flaking, cracking, and changes in surface roughness, along with the release of microscopic particles. However, these studies often fail to capture the complexities of natural weathering. Few studies have explored natural weathering behavior, mostly focusing on limited polymer types and short time frames. The absence of advanced analytical tools has further hindered comprehensive characterization of weathering behavior. Changes in surface properties (hydrophobicity, surface charge, roughness) affect contaminant adsorption, interaction with natural colloids, and microbial biofilm formation. Bulk properties such as crystallinity and molecular structure influence mechanical stability and chemical adsorption-desorption processes. A significant limitation in current research is the disproportionate focus on virgin plastic particles in effect studies, despite the clear impact of weathering on their properties. This mismatch hinders accurate assessment of the actual environmental risks associated with weathered plastics, underscoring the need for comprehensive studies to understand the weathering mechanisms of microplastics and their toxic effects.

The study utilized a variety of commonly found plastic polymers: linear-low-density polyethylene (LLDPE), polypropylene (PP), expanded polystyrene (ePS), polyethylene terephthalate (PET), polyvinyl chloride (PVC), aramid (PA), and polycaprolactone (PCL). These plastics were placed in ageing units deployed in the top 100 cm of the water column at ten sites in the Hunter Region of NSW, Australia, for twelve months. Samples were collected at six time points (0, 1-2, 3-4, 5-6, 8-10, and 12 months) for analysis. A rigorous cleaning procedure removed loosely attached material without damaging the plastic surfaces. Several analytical techniques were employed to assess the changes in plastic properties. Scanning electron microscopy (SEM) visualized nano-scale surface changes and biofouling. Brunauer-Emmett-Teller (BET) analysis measured specific surface area changes. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) characterized changes in surface chemistry, calculating hydroxyl, carbonyl, and carbon-oxygen indices. X-ray diffraction (XRD) determined changes in crystallinity. Simultaneous thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) assessed thermal properties and stability. Crystallinity was assessed using FTIR, XRD, and DSC data, comparing the results from these different techniques to understand their respective suitability for assessing long-term changes in weathered plastics. Finally, SEM images were analyzed to quantify changes in particle size, and statistical analysis (two-sample t-tests) compared virgin and 12-month-aged plastic pellet diameters.

SEM revealed surface biofouling and morphological changes such as flaking, cracking, pitting, and the formation of micronanoplastics (<1 μm). The BET analysis showed a significant increase in the surface area of weathered plastics (up to 1265% for LLDPE), indicating surface modification. ATR-FTIR detected new functional groups (O-H, C=O, C-O), indicating surface oxidation and hydrolysis. The hydroxyl, carbonyl, and carbon-oxygen indices showed significant polymer-specific changes over time. XRD revealed a decrease in crystallinity for most polymers, except ePS, which showed an increase. This was attributed to the preferential degradation of amorphous regions and the potential for adsorption of crystalline inorganic matter. TGA and DSC analysis indicated slight decreases in the thermal decomposition temperature for some polymers (ePS, PVC, PA), suggesting decreased thermal stability. The particle size analysis indicated a decrease in the mean diameter of PCL and PVC after twelve months, indicative of degradation. However, LLDPE showed an increase, possibly due to water absorption and swelling. PP and PET showed no significant changes in particle size. Based on the change in mean particle size, the estimated degradation time for PCL was 112 years and for PVC was 401 years. The study estimated a degradation rate of up to 469.73 μm per year, significantly greater than previous estimates. Overall, the extent of weathering varied by time and polymer type, with PCL and PVC showing the most significant degradation. Plastic-associated inorganic and organic matter (PIOM) was found to accumulate on the weathered plastic surfaces, suggesting the potential for enhanced contaminant adsorption.

The findings highlight the significant and varied degradation of microplastics in the marine environment over time, challenging previous estimations. The observed formation of micronanoplastics suggests a potential pathway for increased bioavailability of toxic compounds. The observed rates of degradation significantly exceed previous estimates, emphasizing the need for revised risk assessments. The study demonstrates the value of employing a multi-faceted analytical approach that combines various techniques (SEM, BET, FTIR, XRD, TGA, DSC) for comprehensively characterizing the complex weathering processes in plastics. The varying responses of different polymer types to environmental weathering also point to the need for polymer-specific risk assessments. The accumulation of PIOM on the weathered plastics points to a potentially important role in contaminant transport in the marine environment. The limitations of the study, particularly the assumption of constant surface area in degradation rate calculations, need to be considered when interpreting the results.

This study presents a comprehensive analytical approach for assessing the long-term degradation of microplastics in marine environments. The findings highlight significant polymer-specific differences in degradation rates and mechanisms, with rates surpassing previous estimations. The formation of secondary micronanoplastics and the role of PIOM in contaminant adsorption are noteworthy. Further research is needed to assess the leaching of plastic degradation products to determine their ecological and human health risks. The development of alternative, biodegradable materials should be prioritized to mitigate the impacts of plastic pollution.

The study's field-based nature introduced some limitations. Precise quantification of micronanoplastic release rates during in-situ degradation was not possible due to the uncontrolled nature of field studies. The assumption of constant surface area in estimating degradation rates, although a common approach, is a simplification of the complex changes occurring during long-term weathering. The potential influence of varying environmental conditions (temperature, UV radiation, salinity) across the sampling sites might also affect the generalizability of the results. Further investigations are needed to refine the degradation rate estimations and investigate the influence of environmental variability on weathering processes. The study focuses on a limited range of polymers; expansion to a wider range of plastics and environmental conditions is needed for comprehensive risk assessment.