Settling Behaviors of Microplastic Disks in the Acceleration and Terminal Processes in Stagnant Water

Abstract

The amount of plastic production grows since the 1950s. Due to poor management and high durability of plastics, they enter and accumulate in the environment. Microplastics (MPs) defined as small (< 5 mm) plastic particles have been widely found in aquatic environments, which threaten biotas and the environment. Although MPs become one of the global major concerns, hydrodynamic properties of MPs are still poorly understood. In oceans, MPs predominantly accumulate on the seafloor, indicating the importance of MP settling process. So far, settling process of microplastic (MP) disks have not been reported, which is the focus of this MSc thesis. In this thesis, systematic experiments were conducted on MP disk settling processes with three shapes (square, rectangle and triangle) and four common-used plastic materials (ABS, PC, PET and PVC with densities of 1.038 - 1.343 g/cm3). The thesis mainly includes two parts: Part Ⅰ “Terminal Settling of Microplastic Disks in Stagnant Water”; and Part Ⅱ “Acceleration Fall of Microplastic Disks in Stagnant Water”. Part I and Part II examine MP settling behaviors after and before reaching their terminal settling velocity, respectively. In Part I, during the terminal settling process, lighter MP disks (with density ρs = 1.038 g/cm3 and length l ≤ 5 mm) followed rectilinear vertical trajectories, while heavier MP disks (ρs = 1.161 - 1.343 g/cm3 and l = 5 mm) followed zigzag trajectories with oscillations and rotations. The mean terminal settling velocities of MP disks were 19.6 - 48.8 mm/s. Instantaneous settling velocities of heavier MP disks fluctuated. Existing formulas could not accurately predict the settling velocity of MP disks; and thus, a new model was proposed with an error of 15.5%. The Red - I* diagram (Red is the disk Reynolds number and I* is the dimensionless moment of inertia) was extended for MP disks to predict settling trajectories. In Part II, the initial acceleration process of MP disks was investigated from the point of release and before they reach the terminal settling velocities. In fact, this process has not been studied for any MP particles (not just MP disks). MP disks were released in stagnant water in five ways (different sides of MP disks facing downward) and three angles (0° - horizontal, 45°, and 90° - vertical). It was found that the MP disks with release angles of 0° start to zigzag immediately after release, while the MP disks with release angles of 45° and 90° experience an adjustment phase first where they adjust to the horizontal position before zigzagging. The adjustment distances in the vertical (Lx) and horizontal (Ly) directions, as well as the maximum vertical settling velocity (Wx,max), are influenced by the density, size, release angle and release way of MP disks. The detailed instantaneous settling trajectory and velocity were analyzed, and the relationships for predicting Lx, Ly and Wx,max were proposed. The drag coefficient of MP disks in the acceleration process, which changes with time, was also examined and discussed. Finally, general conclusions and future work directions were provided at the end of the thesis.