The impact of riverine particles on the vertical velocities of large microplastics

Microplastics interact with other suspended particles in aquatic systems, which may impact their environmental fate. Little is known about aggregation between suspended sediment and larger microplastics (1-5 mm), and how this impacts the vertical velocities of microplastics, although it was hypothesised these are size limited. Consumer items made of five common polymers: polypropylene (PP), high density polyethylene (HDPE), polyethylene terephthalate (PET), polyvinyl chloride (PVC) and polystyrene (PS), were fragmented by cryomilling and their vertical velocities (rising/settling) measured experimentally before and after 24-hours of aggregation with riverine particles. Microplastic size (microscopy), zeta potential and density (density gradient column) were measured, with aggregation quantified using microscopy. PP had an experimental density of 1052 kg·m^-3, and sank in river water, although it is often stated as being buoyant based on literature density values. Aggregation occurred with all five polymers: 39 %-72 % of microplastics were observed to have sediment and/or organic particles adhered, depending on the polymer type. PVC had the least negative zeta potential, -8.0 ± 3.0, and showed a much higher number of adhered sediment particles than all other polymers: on average 4.55 particles, compared with <1.72 particles for other polymers. For four polymers, aggregation did not significantly change vertical velocities. However, PP particles showed a significantly slower settling velocity after aggregation: a decrease of 6.3 % based on mean averages, from 9.7 × 10^-3 to 9.1 × 10^-3 m·s^-1. Theoretical calculations showed the amount of adsorbed sediment or biofilm required to induce a microplastic density change of ∼50 kg·m^-3 was much higher than observed experimentally. Overall, this study indicates that the vertical velocities of larger microplastics are less influenced by interactions with natural particles than smaller microplastics.