Mechanical Fragmentation of Environmentally Aged Macroplastics from the Rhine River under Vessel-Induced Propeller-Jet Forces

Abstract

The fragmentation of plastic particles is strongly influenced by both the intrinsic properties of the plastic item, such as size and shape, and by the external environmental characteristics. Once introduced into river systems, larger plastics are subject to weathering and fragmentation processes, ultimately resulting in secondary microplastics (<5 mm). Plastics suspended in the water column may be exposed to strong hydrodynamic forces produced by propeller jets of riverine vessels. This study experimentally determines the hydrodynamic forces required to fragment macroplastic items collected from the Rhine River in the Netherlands, to assess whether the drag forces exerted by the propeller jets of riverine vessels can cause plastic fragmentation. The results demonstrate clear differences in fragmentation susceptibility among plastic categories. “Plastic film 2.5–50 cm (soft)” exhibited the lowest median breaking force (3.8 N) and the highest breakage probabilities when exposed to high jet-induced velocities generated by vessels, exceeding 50% at velocities ≥5 m s–1 and reaching up to ∼80% at 10 m s–1.