Controls on microplastic breakdown due to abrasion in gravel bed rivers

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, particularly in gravel bed rivers where there are a range of grain sizes present with the bed sediment. Here we report a series of experiments designed to explicitly quantify the influence of sediment grain size on microplastic degradation and understand how this varies by microplastic type.

Four sediment beds ((i) 0.8mm uniform sand; (ii)10mm uniform gravel; (iii) 20mm uniform gravel and (iv) bimodal sand gravel mix D₅₀ 14mm)) were seeded with either Nylon pellets (d= 1.2 g/cm³), Polycarbonate fragments (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 100 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 which has implications for the ability of the plastics to potentially host microbial taxa.