Plastic microbeads from cosmetic products: an experimental study of their hydrodynamic behaviour, vertical transport and resuspension in phytoplankton and sediment aggregates

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.

A Critical Examination of the Role of Marine Snow and Zooplankton Fecal Pellets in Removing Ocean Surface Microplastic

“…We do this to calculate a separate detrital export pool (fecal pellets), with a faster sinking rate than the rest of the detritus. These new rates produce a decent fit to observed ocean nutrient profiles (Figure 1) and reflect the settling approximations of diatoms from Moehlenkamp et al (2018) and copepod fecal pellets (Cole et al, 2016), where diatoms settle at approximately half the rate as pellets. What proportion of all zooplankton particulate losses is appropriate to represent egestion is un-constrained, therefore 50% is assumed.…”

“…Detritus in the UVic ESCM is formed by phytoplankton mortality, and zooplankton mortality, sloppy feeding and egestion. We assume MP aggregates in fecal pellets via zooplankton ingestion (Cole et al, 2016) and physically aggregates in marine snow (Moehlenkamp et al, 2018), though we do not explicitly trace MP, nor model marine particle aggregation. We introduce a new detritus tracer to divert 50% of zooplankton particulate losses into a detrital pool representing fecal pellets with an initial sinking rate of 18 m d −1 (double the initial sinking rate of the original detrital pool).…”

“…Even if it is assumed that one MP particle is transported by 1 organic marine snow aggregate or pellet, biological export can easily explain all of the missing MP. In ideal conditions, aggregates can hold 20 or more MP particles (see the photos in Cole et al, 2016;Moehlenkamp et al, 2018). In less ideal FIGURE 4 | Number of days from year 2010 it took for marine aggregate export at the seafloor to deposit an amount of MP equivalent to the surface concentration, if it is assumed 1 MP particle is transported by 1 organic particle.…”

“…MP particle modeling suggests MP might oscillate within the upper 100 m as a product of biofouling/de-fouling (Kooi et al, 2016). Microplastic aggregation in marine snows (e.g., Long et al, 2015;Moehlenkamp et al, 2018;Porter et al, 2018) and ingestion (and subsequent egestion) by zooplankton (e.g., Cole et al, 2013Cole et al, , 2016 offer an intriguing potential pathway for MP export out of the surface layer and in to the deep ocean and sediments, but to date no global estimate of the potential of marine aggregates (marine snow and fecal pellets) to transport MP has been made.…”

Spatial distribution of microplastics in sediments and surface waters of the southern North Sea

“…This makes it less likely for MP to accumulate in North Sea surface waters. (Möhlenkamp et al, 2018;Porter et al, 2018). This exceeds general sinking velocities of phytoplankton aggregates (53±22 m d -1 ) (Möhlenkamp et al, 2018) and fecal pellets, which range, depending on the composition of the phytoplankton bloom, between 70-100 m d -1 (Frangoulis et al, 2001).…”