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Microplastics (MPs) are one of the most abundant and widespread anthropogenic pollutants worldwide. In addition to the global spread and threats of plastic to native species by carrying toxic substances, its slow degradation rate and resulting long retention time in the environment constitute a problem that is still poorly understood. In this study, five of the most manufactured plastic types were weathered under simulated beach conditions for 18 months in freshwater, brackish water, and seawater. Those included polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), and polyvinyl chloride (PVC). PP was the first polymer type that fragmented after 9 months of weathering and influenced the pH of the surrounding water. Molecular surface changes were detected for all polymers, just after the first week. Hydroxyl bonds were one of the first groups incorporated into the polymers, weakening 2–3 weeks later. Carbonyl groups were also measured early, but with significantly different developments with time between the polymer types. Differences in degradation rates were proven between the water media, with the fastest degradation in seawater compared to brackish water and freshwater for PE and PP. These results are consistent with previous findings on MPs aged under environmental conditions and provide initial long-term observations of MP degradation pathways under simulated environmental conditions. These findings are valuable for assessing the fate and hazards of MPs in aquatic systems.

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