Modeling riverine non-buoyant microplastic transport under ultraviolet aging: A framework based on stochastic theory

Understanding of microplastic (MP) transport in riverine environments is crucial for developing efficient pollution control strategies and reducing the riverine MP load. This study primarily focused on the influence of ultraviolet (UV) aging on the transport behavior of non-buoyant MPs (polyethylene terephthalate and polystyrene). The stream flume experiments revealed notable differences in the transport behavior of MPs under aging effects compared to the theoretical transport patterns based on the Rouse number. Specifically, unaged particles exhibited pronounced aggregation tendencies, leading to predominantly floating or suspended transport modes. In contrast, aged MPs showed less aggregation, resulting in transport behaviors more aligned with theoretical predictions dominated by settling suspended load, bed load, or immobile modes. This study proposed a new stochastic theory-based model, the "MP Transport with Random-walk Advection Mechanism (MP-TRAM)" model, to simulate the transport dynamics of MPs under aging effects. The model effectively captured the initial variation characteristics but underperformed in simulating the tailing features of particle transport. Therefore, this study introduced a new parameter into the probability density function to couple the effects of particle properties, aging characteristics, and flow conditions. The adapted model demonstrated predictive performance improvements of 1.52% to 56.02%, enabling more accurate capture of MP transport characteristics, while this parameter proved applicable for the quantitative characterization of aging effects. This study underscores the influence of aging effects on MP transport behavior and provides a novel modeling framework for predicting the fate and transport of riverine MP pollution.