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Photoaging dynamics of tire wear particles in the terrestrial-aquatic interface: The crucial role of soil solution chemistry and dissolved organic matter evolution
Summary
Tiny plastic particles from tire wear, one of the biggest sources of microplastic pollution, don't stay the same once they wash off roads into soil and water; this study shows that the type of soil they pass through changes how quickly and thoroughly these particles break down and react chemically, based on the soil's acidity, metal content, and natural organic matter. This matters because how these particles age affects what harmful substances they might release or absorb before reaching rivers, lakes, and eventually our water supply, meaning soil isn't just a passive filter but an active chemical processor that shapes the risks microplastics pose to ecosystems and human health.
Tire wear particles (TWPs) represent one of the primary sources of microplastic pollution entering aquatic systems via road runoff and soil leaching. Once deposited at the terrestrial-aquatic interface, the inevitable photoaging of TWPs in soil solutions dictates their subsequent environmental fate and toxicological profile. This study investigates the time-dependent photoaging behavior of TWPs within soil solutions derived from three representative soil types: brown soil (BS), paddy soil (PS), and fluvo-aquic soil (FS). The results demonstrate that the molecular evolution of dissolved organic matter (DOM) at the soil-water interface is closely associated with the aging trajectory of TWPs. During the initial 30 days of photoaging, the higher content of humic substances in DOM from both the FS soil solution and the TWPs conferred greater photochemical reactivity, leading to a higher degree of TWPs oxidation in FS compared to BS and PS. Conversely, as photoaging progressed, the high metal concentrations and low pH in BS facilitated a greater accumulation of quinone-like components, which potentially enhanced electron-accepting capacity and increased the production of hydroxyl radicals and triplet-state DOM. These shifts ultimately correlated with more extensive fragmentation and surface oxidation of TWPs in BS. By elucidating how soil solution chemistry co-regulates the dynamic evolution of DOM and particle aging trajectories, this study highlights the role of soil as a reactive gateway that alters the characteristics of microplastics before their transport into the aquatic environment. These findings provide essential insights into incorporating plastic-derived organic carbon reactivity into future environmental fate assessments of microplastics.