Longitudinal and Vertical Transport of Microplastic Within Sediment in Rivers and Transitional Water Environments

The prevalence of microplastics in freshwater ecosystems has become a pressing environmen-tal concern, with rivers simultaneously recognized as both reservoirs and vectors for their transport to coastal areas. Although it is well-established that the presence of sediments in freshwater can significantly affect microplastics mobility, very few studies quantify this effect. Even fewer models exist capable of predicting the spatial and temporal scales of microplastics transport while accounting for sediment interactions. The reliable estimation of microplastic quantities released into marine waters through the river system, essential for the development of policies aimed at mitigating plastic pollution, requires addressing this knowledge gap. Build-ing on this conviction, this Thesis aims to advance scientific knowledge on the microplastics transport dynamics in fluvial and transitional environments through a series of experimental investigations conducted at the ‘Fluvial, Lagoon Hydraulics and Biofluidodynamics’ laboratory of the University of Florence and the ‘Environmental Physics’ laboratory of the University of Gi-rona and focused on the interaction between microplastic particles and the sedimentary ma-trix. The experiments were designed to analyze both the longitudinal microplastics transport processes (advection, turbidity currents-driven flow, and retention by vegetation - Chapters 2, 6 and 7, respectively) and the vertical transport (i.e. sedimentation, Infiltration, resuspension - Chapters 3, 4 and 5, respectively) therefore, both suspended sediments and those of the bed layer were considered. The latter led to the use of a wide range of sediment types, including clay and sand, employed both in suspension within the water column (Chapters 2, 3, 6, and 7) and as bed material (Chapter 5), as well as the use of gravel and pebbles for simulating the hyporheic zone (Chapter 4) and the coarse bed roughness (Chapter 6). Concurrently, a hetero-geneous range of microplastics, varying in size, density, and particularly shape, has been em-ployed, with a specific focus on synthetic microfibers (Chapters 2, 3, 4, 5, 6 and 7). Following the same way, the microplastic-sediment interaction mechanisms have been investigated un-der various hydrodynamic conditions, ranging from calm waters (Chapter 3) to advective flows with moderate Reynolds numbers (Chapter 2 and 7), up to high turbulence flow fields (Chapters 4, 5 and 6). Moreover, in Chapter 7, the effects of the emergent vegetation (i.e. Juncus mariti-mus) and the presence of lagoons as obstacles for the transport of microplastics by the advec-tive flow were studied. The simultaneous presence of suspended sediments and vegetation led to significant findings regarding the sedimentation capacity of microplastics in wetlands, with important implications for the design of nature-based solutions for water purification. Achieving the objectives outlined above required different setups including channel experi-ments with standard configuration, lock-exchange configuration for simulating turbidity cur-rents, the use of oscillating grids to generate homogeneous turbulence, continuous flow cir-cuits with chambers of variable hydraulic load for simulating infiltration processes, and still water tanks for detailed studies on particle settling velocity. The outcomes presented in this Thesis will enhance the ability to predict and monitor micro-plastics transport within the river system, contributing to the development of more accurate models for managing plastic pollution flows and enabling more targeted and effective interven-tion policies with a reduced use of resources. The ecological implications of the Thesis and the potential developments arising from the obtained results are discussed in Chapter 8, while Chapter 9 outlines the conclusions for each chapter as well as the general conclusions of the Thesis. In agreement with the regulations of both the University of Florence and the University of Gi-rona, Chapters 2, 3, 4, 5, 6 and 7 are a transcription of published articles in the following jour-nals: Science of the Total Environment (two of them), Environmental Pollution (two of them), Marine Pollution Bulletin and Journal of Water Process Engineering. A copy of the published articles can be also found at the end of this Thesis.