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Influence of sediment size on microplastic fragmentation
Summary
Researchers examined how sediment grain size influences the physical fragmentation of microplastics in river environments, where the mechanical controls on microplastic storage, remobilization, and transfer pathways remain poorly understood. The study found that sediment size plays a meaningful role in breaking down plastic particles, contributing to the generation of smaller microplastic fragments in fluvial systems.
Microplastic contamination of river sediments has been found to be pervasive at the global scale however, the physical controls governing the storage, remobilization and pathways of transfer in fluvial sediments remain largely unknown. The properties that make plastics useful - strength, flexibility, durability and resistance to degradation - also make their transport through the environment difficult to predict. Specifically, the risk profile associated with microplastic transfer is dynamic because their physical and chemical properties change over time as they persist in, or move through, the environment. For example, mechanical breakdown, due to abrasion, likely decreases the size of microplastic particles, increases their surface roughness and surface area to volume ratio, and influences the diversity and abundance of the microbial taxa that colonise them. However, the processes controlling the mechanical breakdown of plastic particles rivers by abrasion is poorly understood. This paper reports a series of experiments designed to explicitly quantify the influence of river sediment grain size on microplastic degradation and understand how this varies by microplastic type. Four sediment samples composed of ((i) uniform sand (D50 = 0.3mm); (ii) small uniform gravel (D 50 = 7.9 mm); large uniform gravel (D50 = 11.9mm) and (iv), sand (20%) gravel (80%) mix (D 50 = 7.1 mm) were seeded with either Polycarbonate fragments (d=1.2 g/cm3), PVC beads (d= 1.2 g/cm3) or Nylon fibres (d = 1.15g/cm3) at 0.005% concentration by mass. The sediment and plastic were placed into a cement mixer with 20L of water and tumbled for 83 hours. During each experiment, the cement mixer was periodically stopped and a sample removed to assess microplastic abrasion. Results indicate that fibres are abraded to the greatest degree in comparison to beads and fragments. Results also indicate a clear relationship with sediment size where microplastic fragmentation rates increase with river sediment grain size. In all plastic types surface complexity increases with time.