The aim of this study is to demonstrate how the flow and diffusion of nanoplastics through a salinity gradient, as observed in mangrove swamps, influence their aggregation pathways. These two parameters have never yet been used to evaluate the fate and behavior of colloids in the environment, since they cannot be incorporated into classical experimental setups. Landsea continuums, such as estuaries and mangrove swamp systems, are known to be environmentally reactive interfaces that influence the colloidal distribution of pollutants. Using a microfluidic approach to reproduce the salinity gradient, and its dynamics, the results show that nanoplastics arriving in a mangrove swamp are fractionated. First, a substantial fraction rapidly aggregates to reach the micro-scale, principally governed by an orthokinetic aggregation process and diffusiophoresis drift. These large nanoplastic aggregates eventually float near the water's surface or settle into the sediment at the bottom of the mangrove swamp, depending on their density. The second, smaller fraction remains stable and is transported towards the saline environment. This distribution results from the combined action of the spatial salt concentration gradient and orthokinetic aggregation, which is largely underestimated in the literature. Due to nanoplastics' reactive behavior, the present work demonstrates that mangrove and estuarine systems need to be better examined regarding plastic pollution.
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