Rivers are the main source of marine plastic pollution due to high levels of land-based sources, such as mismanaged waste 1,2. Estimates of plastic entering the ocean vary annually between 4.8 to 12.7 million metric tons, while floating marine plastic is calculated to be only 268,940 tons, accounting for just 2-6% of the estimated plastic entering marine systems every year1,3. Consequently, the transport processes that determine the fate and the ecological threat of microplastics are still largely unknown. Our study aimed to increase our knowledge of the factors that impact aquatic microplastic transport as it moves from rivers out into the oceans and whether it follows similar patterns to sediment. This will aid in predictive models to determine hotspots, ecological risk and mitigation.
To determine how microplastics move and where they accumulate, you first have to understand what factors impact their transport, which is summarised in Figure 1. First, the density of the particle is determined by the polymer type and will influence its rate of settling in water. The shape and size of the particle will also influence its trajectory. For example, a fragment or a fibre will move in a different way to a spherical microplastic.
As a microplastic moves from a freshwater to marine environment, it will undergo a range of different biological, physical and chemical surroundings such as changes in salinity. Biofouling will also occur, where microorganisms grow on the surface of a microplastic within minutes to hours of it entering an aquatic environment5,6. Figure 2 demonstrates the changes in a microplastic surface that can occur after biofilm growth which may alter the particle buoyancy and therefore the relative density to the ambient fluid. In addition, interactions of microplastics and suspended sediment may result in the development of flocs (aggregates of suspended material), which is known to affect settling velocity of particles particularly in river to estuary transition zones7. The combination of these factors will ultimately impact how a microplastic is transported, in addition to other aspects such as turbulence and weathering which may cause fragmentation. Settling velocity is a key parameter often used to predict sediment transport, yet no comprehensive study has yet experimentally quantified the combination of these effects (biofouling, salinity and sediment concentration) on microplastic movement.
For our study, we analysed how the settling velocity of various types of microplastics was impacted by biofouling, salinity and sediment concentration that is typically observed as a particle moves from freshwater to marine environments. Three types of non-buoyant microplastics, often found in aquatic environments were tested: PET, Polyvinyl chloride (PVC) fragments and nylon, polyester and acrylic (NP&A) fibres. Our experiments were conducted in a a Laboratory Spectral Flocculation Characteristics (LabSFLOC) plexiglass water column which has a LED light panel and high-resolution video camera that records particle settling. For each polymer, measurements were taken for clean and biofilmed particles under 3 salinities and 3 sediment concentrations. The images collected were analysed using a self-developed code. Each particle in each image was identified and matched between images to calculate the distance travelled over a given time frame to determine settling velocity, which can be seen in Figure 3.
Our results demonstrate how biofouling significantly increased the settling velocity of various types of microplastics, (on average 40%) through changes in specific density, and that impacts can occur within one week. Settling of microplastics also depended on both polymer type and shape, with settling regimes differing according to both salinity and sediment concentration. NP&A . Flocculation of microplastics with sediment did not occur, but settling was still impacted, with overall settling decreasing at higher sediment concentrations for PET and PVC fragments. This was unexpected, as higher sediment concentration was thought to increase flocculation rates and therefore particle size and settling velocity.
In addition, as microplastic transport is often thought to follow the same patterns as natural sediment transport, we compared our results to those predicted from a widely applied sediment transport formula of Ferguson & Church (2004) 11. The theoretical settling velocity was calculated to be much higher than our experimental results, supporting previous studies who also show overestimation of theoretical values. Any predictive models using this formula may inaccurately predict settling of microplastics potentially resulting in greater microplastic load in suspension than expected. However, we emphasise a need for a new generation of transport formulae that considers irregular microplastic shapes, biofouling and sensitivity to changes in salinity.
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