Giovanni Di Lollo

Every day, plastic wastes of different sizes and compositions are poured into rivers from industrial and civil waste plants. Some of the debris seeps into the soil, contaminating groundwater, while most of it is transported into lakes, seas and oceans., where they interact with the flora and fauna present, with drastic impacts on their health and growth. Finally, they return to the human being through the food chain. In recent years, the scientific community has developed the idea of using green barriers to block the transport of debris. Numerous experimental works, in situ and in the laboratory, have shown how vegetation acts as a collector of plastic debris in the marine environment. On the other hand, the aquatic vegetation along the rivers is still little analysed.

The aim of the research project is to experimentally investigate the ability of aquatic vegetation to trap plastic waste and to understand whether different biotic factors, seasonality conditions or types of waste influence this capacity. Furthermore, the aim of the research is to experimentally investigate the effects induced by the presence of vegetation on the dynamics of currents in transition zones, such as estuaries.

Real plant samples inside a recirculating channel. A known amount of plastic debris is inserted into the flow. The ability in trap plastic is quantified by the ratio of the trapped items to the total thrown. The experiments are repeated by changing the species of plants, the area occupied by vegetation and the height of the water column. The debris is produced with different sizes, to analyse the retention of macro, meso and microplastics. Transition zones are simulated through gravity currents, flows generated by the density gradient between two fluids. The currents are reproduced using the lock-release technique. Vegetation is simulated using different obstacle configurations. The dynamics analysed are the velocity and the density field.

The amount of debris trapped is affected by the species of plants and the density of the area occupied. A more articulated and complex structure of vegetation leads to a greater retention of debris, especially microplastics. Higher plant density means more debris retention. The presence of obstacles has a significant influence on the dynamics of gravity currents. The redistribution of turbulent kinetic energy from the production zones to the rest of the current is interrupted by the presence of an adverse pressure gradient induced by the obstacle, resulting in a reduction in turbulent terms. The presence of obstacles also increases the dilution between the two fluids.