Experimental observations on synthetic microfibers re-suspension under silty and sandy soil conditions

Microplastic (MP) pollution in aquatic environments has emerged as a critical environmental issue with far-reaching implications. In freshwater MPs are known to accumulate in sediment beds where they can bury. Once buried they can resuspend due to high energetic events, re-entering the water column again. Therefore, riverbed sediments should be considered as a source rather than a sink for MPs. Synthetic microfibers (MFs) are becoming the focus of attention from the scientific community because of their abundance in natural environment. Moreover, due to their highly elongated shape, their behaviour significantly differs from other organic or inorganic particles that can be found in the aquatic environment, consequently theoretical and empirical models already developed for the sediment re-suspension or for the pollutant transport and successfully applied to 3d-shaped MPs do not fit for MFs. With the aim to fill this knowledge gap, in this work the turbulence induced by an oscillating grid device was used to investigate the resuspension of microfiber (MF) buried into the sediment bed. Four different types of plastic fibres commonly used for several industrial applications (PET, PP, PA and LDPE) and two types of soils (cohesive and noncohesive) were investigated. Particles were in depth characterized via 3D reconstruction to estimate important parameters like Corey shape factor and settling velocity. Experimental runs explored a wide range of shear stresses, and they varied over time. The concentration of resuspended MFs were correlated with the main geometrical and physical particles properties. Particularly, we examined the effects of the shear rate, the length and diameter of fibres, the polymer type, and the sediment characteristics showing that resuspended sinking MF concentration resulted inversely proportional to their settling velocity. In contrast, the concentration of floating MFs increased as their buoyancy velocity increased. The shear rate increased the resuspension of both floating and sinking MFs. Besides, the characteristics of the soil also played a key role, with more MFs resuspended under silty than in sandy soil conditions. Starting from statistically based observations we defined a model able to predict the concentration of both sinking and floating MFs resuspended along the water column. The physical meaning of the obtained non-dimensional model was discussed, and outcomes were compared with experimental findings from other works. Results of our work could serve as a guide for the advancement in the development of new MPs transport models that aim to include the contribution of those MFs resuspended from the bottom sediment layer.