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Weathering of microplastic creates nanometre-sized surface-near cavities – A 129Xe NMR spectroscopic study
Original title: Weathering of microplastic creates nanometre-sized surface-near cavities – A 129Xe NMR spectroscopic study
Summary
When plastic waste breaks down in the environment, it doesn't just crumble into smaller pieces—it also develops tiny pits and rough patches on its surface, almost like microscopic craters. Using advanced imaging and a specialized gas-based scanning technique, researchers found that these nanoscale nooks and crannies can act like sponges, potentially trapping pollutants and toxic chemicals. This matters because it means weathered microplastics may carry more hitchhiking contaminants than fresh plastic, which could affect how these particles transport harmful substances through water, soil, and eventually into our bodies.
Microplastic (MP) particles undergo simultaneous structural and morphological transformations during weathering. These processes are expected to generate surface features across multiple length scales, ranging from micrometere-sized cracks and fractures to nanometre scale roughness and surface-near cavities. In particular, the latter strongly influences the adsorption, and thus the transport, of environmental contaminants. However, the characterisation of these surface features is severely hampered by the low overall surface areas of typical MP particles. Here, we demonstrate that a multiscale characterisation approach combining scanning electron microscopy (SEM), atomic force microscopy (AFM), krypton physisorption and hyperpolarised 129 Xe NMR spectroscopy allows to identify the complex surface morphology induced by environmental weathering. Therefore, an important gap for understanding the development of MP particle surface evolution and the uptake of environmental pollutants can be closed. For different batches of artificially weathered polystyrene (PS) microplastic particles, this approach allowed to determine the micrometre scale surface damage, the resulting nanoscale topographical features and the trends for the specific adsorptive surface area during weathering The hyperpolarised 129 Xe NMR spectra enabled to identify the corresponding surface near confined structures and small cavities. The NMR spectroscopic analysis was based on reference materials with well-defined pore sizes that provide a calibration of the 129 Xe chemical shifts using the Terskikh model, thereby providing a cavity size-to-chemical-shift relation. These findings highlight the importance of microporous surface domains as potential adsorption sites for ions and small organic molecules, including environmentally relevant toxic compounds. The results illustrate the use of hyperpolarised 129 Xe NMR spectroscopy as a powerful tool for investigating weathering-induced nanostructural changes in microplastics. The implications for contaminant uptake and transport are generally relevant of MP in environmental systems.