Abstract
Hydrodynamic behaviour and the transport pathways of microplastics within the ocean environment are not well known, rendering accurate predictive models for dispersal management of such pollutants difficult to establish. In the natural environment, aggregation between plastic microbeads and phytodetritus or suspended sediments in rivers and oceans further complicate the patterns of dispersal. In this laboratory study, the physical characteristics and hydrodynamic behaviour of a selection of common plastic microbeads, as used in exfoliation skincare cosmetic products, were investigated. Additionally, the potential for aggregation of these microbeads with phytodetritus and suspended sediments, as well as the subsequent sinking and resuspension behaviour of produced aggregates, were investigated with roller tanks, settling columns and erosion chamber. Physical characteristics of the plastic microbeads showed great heterogeneity, with various densities, sizes and shapes of plastic material being utilised in products designed for the same purpose. The majority of the plastics investigated were positively buoyant in both freshwater and seawater. Aggregation between plastic microbeads and phytoplankton was observed to be swift, with even extremely high concentrations of plastics being rapidly scavenged by suspended algal material. Following aggregation to sizes of 300 to 4400 μm diameter, some formerly buoyant plastics were observed to settle through the water column and enter the benthic boundary layer with settling velocities ranging between 32 and 831 m day–1. These aggregates could be resuspended in the laboratory under critical shear velocities of 0.67–1.33 cm s–1 (free stream velocities of > 10 cm s–1). This rapid aggregation and subsequent settling indicates a potentially important transport pathway for these waste products, a pathway that should be considered when modelling discharge and transport of plastic microbeads and determining the ecosystems that may be at risk from exposure.
Highlights
The global distribution of plastic pollution in the aquatic environment is a current topic in pollution research and policy making
Our results indicate that plastic microbeads can aggregate with algal biomass rapidly, with even high concentrations of microplastics being removed from suspension swiftly by phytoplankton concentrations typically found in coastal waters
Discharge of plastic microbead wastes from cosmetic products following use has been identified as a potentially important primary source of microplastics into the marine environment
Summary
The global distribution of plastic pollution in the aquatic environment is a current topic in pollution research and policy making. Though the presence of large-sized plastic wastes in the world oceans has been studied for decades, microplastics and their potential impacts on ecosystem functioning has recently come to prominence as a topic of serious concern (Ivar and Costa, 2014). A common source of primary microplastics are the microbeads found in consumer skincare products, such as facial cleansers, body and shower scrubs and tooth pastes. These can be characterized as synthetic, non-degradable, water insoluble, solid materials comprising a range of polymers and additives (Leslie, 2014) which vary greatly in size (100 μm to more than 1000 μm), shape (from amorphic to spherical) and quantity used in different commercial products (F endall and Sewell, 2009). Various studies have shown that waste water treatment plants are often incapable of capturing microplastics efficiently during the purification process
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