Visualization of Buoyant MP motion in response to different flow velocities and bed types

The widespread use of plastic has made it a widely dispersed product with a high impact on the environment. Through fragmentation of larger pieces or direct discharge, microplastic particles (MP) are present in almost every aquatic ecosystem. MPs based on polymers of lower density than water (ρ < 1.0 g cm⁻³), such as polyethylene (PE), are among the MPs most commonly found in the sediments of freshwater systems which is counterintuitive. Different mechanisms and theories may explain the dynamics of buoyant positive MP motion and their deposition in water systems. Thus, examining the behavior of MP particles carried in suspension is particularly relevant to assess this contaminant fate. The experimental approach is an important way to help to fill the knowledge gap exist on the transport of these particles in natural flows. In this study, Particle Image Velocimetry (PIV) has been deployed to investigate how turbulent water regime contribute affect the dynamics of buoyant particles and if it drives them towards the bed . Different sets of experiments with different flow velocities and non-cohesive bed types were conducted in a 200 cm long, 30 cm wide, 22 cm deep, rectangular, re-circulating, tilting flume. The PIV measurements were done in the centerline of the flume. A camera framed and recorded images in the laser sheet at 15 Hz to follow fast turbulence fluctuations. Pristine PE particles of around 47µm were used. A particle tracking technique was used to record, to follow the trajectory and to calculate de velocity of the particles. The analyzed images clearly show that turbulence homogenizes the particles in the water column. Also, a substantial quantities of PE particles were subject to downward vertical transport which in its turn increase the chance of particles coming in contact with the bed. Turbulent energy is an important driver in the dynamics of buoyant positive particles, and under more realistic environmental conditions, as biofilm presence on the sediment, would enlighten the trapping of these particles by the sediment.