Flocculation and its impact on microplastic transport mechanisms in rivers

Rivers serve as primary pathways for transporting microplastics from land to oceans, but they also can retain these small particles within their sedimentary deposits. A critical factor in this process is the role of fine cohesive sediments, which can adhere to microplastic particles and form aggregates. These aggregates can be transported as part of the bedload, resulting in enhanced accumulation of both microplastics and sediments. This study explores the mechanisms of microplastic-sediment aggregation and their influence on the transport and deposition of microplastics in rivers through laboratory experiments. A rotating wheel (34 cm diameter, 7 cm depth) was used to keep the mud in suspension and promote aggregation, while a settling column equipped with a high-resolution camera was utilised to analyse floc properties and their settling velocities. Five experiments were conducted, analysing approximately 4,000 flocs per experiment. Four experiments involved mixtures of water, sediment (1 g/L), and 500 µm-long microplastic fibres (MPs-sediment ratio, in weight, of 1:25), using four different plastic polymers: Polypropylene (PP, ρ=0.9 g/cm³), Polyamide (PA, ρ=1.14 g/cm³), Polyester (PES, ρ=1.38 g/cm³), and Aramid (AR, ρ=1.44 g/cm³). A fifth experiment, serving as a control, was conducted to observe floc formation without microplastics. Mud with a median grain size of 40 µm sampled from a real river (Arno River, Tuscany, central Italy) was used as the sediment. The mixtures were subjected to a shear rate of 10 s⁻¹ in the rotating wheel for two hours. After complete settling, flocs were collected and moved into the settling column. The recorded videos were analysed using an image analysis tool to determine floc size, shape, and settling velocity. Results showed that including microplastic fibres within flocs formed larger flocs that settled faster than flocs containing only sediment. Furthermore, while flocs formed from plastic-free mud generally exhibit a rounded shape, flocs containing microfibres display greater variability in shape, with some maintaining a rounded morphology but others exhibiting more elongated forms.