Terminal settling and rising velocity prediction of macroplastics: Medical face masks as newly emerged objects of concern

The widespread use of medical face masks during the SARS-CoV-2 pandemic has significantly increased plastic waste, with a considerable proportion of these masks ending up in the environment. As these masks are transported through wind and surface runoff, they accumulate in water bodies, leading to pollution and potential environmental risks. Understanding the transport behavior of these macroplastic items is crucial for addressing the pollution problem effectively. This study focuses on predicting the terminal settling and rising velocities of medical face masks, considering their unique shape and composition, aiding upcoming research with base data for, e.g., numerical transport simulations. Three different mask types, including surgical face masks, FFP2-standard face masks, and non-medical reusable face masks, were investigated in various shapes, and modified transport formulas that take into account the shape factor and sphere-equivalent radius of the masks to accurately predict their terminal settling and rising velocities were tested for applicability. The results reveal that the unique shapes of masks influence the terminal settling and rising velocity to a greater extent than their density difference to water. The absolute mean terminal velocities ranged from 0.05 to 0.3 m/s. Understanding the transport behavior of the studied face masks provides valuable insights for managing and mitigating the pollution caused by discarded face masks in water bodies and helps to develop effective strategies for environmental protection. Furthermore, the findings highlight the need for comprehensive laboratory studies to investigate the rising and settling velocities of common macroplastic items, as they are expected to vary in their hydrodynamic behavior significantly.