Microplastics (MPs) from predominantly terrestrial sources are transported by rivers to the oceans. Yet, the underlying riverine transport processes remained insufficiently understood. This comprehensive review synthesizes some laboratory studies published between 2021 and 2025, including also earlier work where relevant, on MPs transport and transformation in river-like flows, focusing on characterization, biofouling and abiotic weathering, and unconventional experimental designs quantifying settling, dispersion, vertical distributions, near-bed transport, incipient motion, and infiltration. Findings indicated that particle properties and environmental factors jointly determine MPs fate. Shape-dependent drag, along with changes in particle density and surrounding hydrodynamic profile from biofouling or UV aging, control settling behavior and can accelerate either sinking or rising. Flow conditions (e.g., turbulence and presence of vegetation), govern longitudinal dispersion and vertical mixing, while bed roughness (ks) modulates near-bed transport regimes, shifting particles between rolling, saltation, and suspension. Existing MPs and sediment transport models capture many observed behaviors but perform poorly for sub-millimeter and irregular MPs. Standardized metrics and cross-scale experiments are needed to improve predictive models, with key gaps remaining in small-particle dynamics, non-spherical forms, and biofilm–turbulence interactions.
Lau et al. (Thu,) studied this question.