Transport of aged microplastics in river confluence zones: Settling behavior and influencing factors

Clarifying microplastic transport in river confluences is critical for predicting their environmental fate. However, the transport mechanisms of environmentally aged microplastics in complex turbulent flow remain a fundamental knowledge gap. To address this, we conducted open-channel confluence flume experiments to investigate the effects of particle properties, Fenton oxidation aging, and hydrodynamic conditions on the transport of polyethylene terephthalate (PET), polyamide (PA), and polyvinyl chloride (PVC) microplastics. The results indicate that: (1) In the confluence zone, the migration-deposition behavior of PET particles reflects the spatially heterogeneous flow field: in the high-velocity mainstream region, particles undergo enhanced horizontal transport; in the low-velocity separation zone near the tributary side, deposition is promoted. Furthermore, the time-averaged streamwise velocity of PET particles decreases significantly with increasing confluence ratio. (2) The influence of Fenton aging on the migration of PET particles depends strongly on flow field properties: in confluence zones with high turbulence, the time-averaged streamwise velocity of aged particles is reduced to approximately 70.0% of that of pristine particles, whereas in stable flow, aging effects are minimal, with an average velocity difference of only 1.1%. (3) Density and size drive divergent vertical behaviors: PA particles exhibit high vertical mobility due to low density, while PVC particles preferentially settle with greater inertial resistance. Based on a multi-variable experimental approach, this study elucidates the key factors and coupling mechanisms controlling microplastic transport in confluences, providing a mechanistic basis for interpreting microplastic fate and assessing ecological risk in river networks.