Surface roughness and biofouling affect the settling behavior of plastics in quiescent water

Plastic particles are transported in aquatic environments under flow action, which can lead to changes in the particle's surface texture, due to physical abrasion and biofilm colonization. These dynamic alterations complicate our understanding of plastic transport dynamics. To address this issue, we investigated how artificially roughened surfaces and water-worked roughened surfaces influence the settling behavior of macroplastics, and how their motion is impacted by biofouling. A dual-camera tracking system combined with TRex software was used to capture the three-dimensional motion of rectangular plate plastic samples (2 × 1 × 0.1 cm³) during settlement in a 0.3 × 0.3 × 1 m³ tank. The results showed that surface roughening can significantly increase settling velocity, whereby motion was accompanied by increased variability in vertical velocity and mode transitions. While biofilm colonization caused the macroplastics to exhibit more chaotic trajectories, increased horizontal dispersion and higher oscillatory frequencies resulted in decreased settling velocity. These findings highlight surface texture and biofouling are critical factors governing macroplastic transport in aquatic environments, with important implications for environmental modeling and plastic pollution assessment.