Experimental study of the settling characteristics of spherical microplastics under oscillating grid turbulence

The settling process of microplastics plays a key role in their fate in marine environments. However, the settling characteristics of microplastics in turbulent environments have not been fully elucidated. In response to this problem, the settling characteristics of microplastics (50-5000 μm) under turbulence were analyzed using an oscillating grid and a total of approximately 6000 samples. The results show that the average settling velocity of microplastics under identical turbulence is not deterministic and varies within a range because of the randomness of the occurrence of turbulent mechanisms. The fluctuation range of the settling velocities of the microplastics increases with increasing turbulent shear rate and decreases with increasing diameter. Microplastics' settling velocity follows a skewed distribution under relatively strong turbulence, leading to the proposal of median settling velocity. Turbulence reduces microplastics' median settling velocity, with the minimum occurring at relatively moderate turbulence. Turbulence mechanism occurrence depends on microplastic-turbulence coupling; relative turbulence intensity is an effective indicator. A new explicit formula for predicting median settling velocities was proposed whose R² was 0.998. This study can improve the accuracy of predicting microplastic fate, thereby providing reference data for the assessment and efficient treatment of microplastic pollution.