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Weathering of microplastic creates nanometre-sized surface-near cavities – A 129Xe NMR spectroscopic study
Summary
When plastic breaks down in the environment, it doesn't just get smaller, it develops tiny hidden pockets and pores on its surface, some smaller than 1/1000th the width of a human hair. Using a specialized imaging technique, researchers discovered these microscopic nooks are too small for standard tools to detect, but they matter because they can trap and hold onto toxic chemicals and pollutants. This means weathered microplastics may be better at soaking up and carrying harmful substances than previously realized, which is important since these particles can end up in water, food, and eventually our bodies.
Microplastic particles (MP) undergo structural transformations during weathering that directly influence their ability to adsorb and transport contaminants. Conventional surface-analytical techniques such as atomic force microscopy, scanning electron microscopy and gas adsorption provide useful information on external morphology and total surface area but are inherently limited when absolute surface areas are small and structural changes are dominated by small cavity sizes. While microscopy primarily reveales larger surface defects, smaller cavities remain unresolved and cannot explain the modest surface increase observed by physisorption measurements. To access these environments, hyperpolarized 129 Xe NMR spectroscopy is established as a sensitive, benefiting from a signal enhancement of up to five orders of magnitude non-destructive tool to probe surface-near cavities, enabling resolution of cavities smaller than 50 nm. Reference materials with well-defined pore sizes were used to calibrate xenon chemical shifts based on the Terskikh model, providing a framework for size-dependent interpretation. Application of this approach to artificially weathered polystyrene particles reveals the formation of surface near micro and meso porosities to small to detect with AFM or SEM, that contribute largely to total surface area. These results demonstrate that weathering generates confined adsorption environments inaccessible to classical techniques and highlight the relevance of microporosity as preferential sites for the adsorption and retention of potentially toxic inorganic and organic compounds, thereby contributing to the role of microplastics as vectors for their transport in environmental systems.