Settling Velocity of plastic particles in Salinity-Stratified Water Columns: An Experimental Investigation

The prediction of plastic particles’ spatial distribution and their fate in water environments depends on a range of particle properties and environmental factors. Salinity-stratified water columns, commonly found in estuaries, have a major influence on the transport behaviour and the settling velocity of these particles. Water density gradients with depth modify the buoyancy force acting on plastic particles, leading to non-uniform vertical velocity and heterogeneous distribution throughout the water column. On the other hand, intrinsic particle properties, such as polymer density, size and shape, also play an important role, affecting their behaviour within the same environment. In this study, the effects of salinity gradients in settling velocity of plastic particles of different shapes, sizes and polymers are investigated through several experiments carried out in the Laboratory of Hydraulics at the University of Coimbra. To perform the experiments, granular particles of four different types of polymers (PMMA, PS, PVC and PET) were used, with particle densities ranging from 1.047 g/cm³ to 1.372 g/cm³. The experiments were conducted in a transparent acrylic tank with dimensions of 32 cm (width) x 32 cm (length) x 100 cm (height). The tank was filled with water at different salt concentrations, which were previously measured using a conductivity sensor. By means of an auxiliary valve located at mid-height of the tank, it was possible to introduce water with different salinities, producing different stratification profiles. The particles were then carefully released 5 cm below the water surface. Following the particles’ release, images were recorded using high resolution cameras, placed in front of the tank. The acquired images were treated and post-processed using a Particle Tracking Velocimetry (PTV) method to determine the settling velocity of the plastic particles. Based on the experiments carried out, this study highlights the importance of accounting for salinity effects when determining the settling velocity of plastic particles, as higher salinity concentrations lead to reduced settling velocities. As expected, the study demonstrates that higher salinity gradients promote a decrease in the settling velocity of the particles along the water column. This effect is particularly clear in the experiments where the halocline is more evident. In addition, the particles’ density and shape also prove to be important factors, directly influencing the settling velocity.