Wave-induced transport of non-buoyant microplastic particles: Phase-resolved experiments and excess-Shields scaling

Non-buoyant pollutant sources and particles in displacement ventilation

A majority of microplastic particles (MPs) in marine waters are transported with rivers from land-based sources. Traffic is estimated to be one of the largest sources of MPs, hence stormwater and subsequently urban waterways are expected to be important transportation routes of MPs to marine waters. However, there is currently little knowledge of MP fate from land sources to marine waters. The aim of this study is to investigate the transport of traffic-related microplastic particles in a receiving freshwater body using hydrodynamic modelling. A 16 km stretch of the Gota River, Sweden’s largest river, was set up using MIKE 3 FM software. The model builds on data on water flows in the river and its tributaries, water levels and salinity stratification in the Kattegat strait, and meteorological conditions. Concentrations of MPs in stormwater and MP characteristics data, including prevalent particle sizes and density of commonly occurring polymers, were found in the literature. The simulations show that peak concentrations of MPs have a short duration; however, elevated concentrations of MPs may be present for hours after rainfall. If the MPs do not settle, as is the case for low density MPs including tyre rubber, a high load of MPs from the city of Gothenburg will reach the marine environment. Biofouling and MPs adhering to mineral particles, as has been shown in marine waters, may considerably change the characteristics of MPs and should be considered in future studies.

Using a numerical model, we analyse the effects of shape on both the orientation and transport of anisotropic particles in wavy flows. The particles are idealized as prolate and oblate spheroids, and we consider the regime of small Stokes and particle Reynolds numbers. We find that the particles preferentially align into the shear plane with a mean orientation that is solely a function of their aspect ratio. This alignment, however, differs from the Jeffery orbits that occur in the residual shear flow (that is, the Stokes drift velocity field) in the absence of waves. Since the drag on an anisotropic particle depends on its alignment with the flow, this preferred orientation determines the effective drag on the particles, which in turn impacts their net downstream transport. We also find that the rate of alignment of the particles is not constant and depends strongly on their initial orientation; thus, variations in initial particle orientation result in dispersion of anisotropic-particle plumes. We show that this dispersion is a function of the particle’s eccentricity and the ratio of the settling and wave time scales. Due to this preferential alignment, we find that a plume of anisotropic particles in waves is on average transported farther but dispersed less than it would be if the particles were randomly oriented. Our results demonstrate that accurate prediction of the transport of anisotropic particles in wavy environments, such as microplastic particles in the ocean, requires the consideration of these preferential alignment effects.

The present study provides indications and underlying drivers of wave-induced transport and retention potential of microplastic particles (MP) in marine vegetation canopies having different densities. The anthropogenic occurrence of MP in coastal waters is well documented in the recent literature. It is acknowledged that coastal vegetation can serve as a sink for MP due to its energy dissipating features, which can mimic a novel ecosystem service. While the transport behavior of MP in vegetation has previously been investigated to some extent for stationary flow conditions, fundamental investigations for unsteady surf zone flow conditions under irregular waves are still lacking. Herein, we demonstrate by means of hydraulic model tests that a vegetation's retention potential of MP in waves increases with the vegetation shoot density, the MP settling velocity and decreasing wave energy. It is found that particles migrating by traction (predominantly in contact with the bed) are trapped in the wake regions around a canopy, whereas suspended particles are able to pass vegetated areas more easily. Very dense canopies can also promote the passage of MP with diameters larger than the plant spacing, as the canopies then show characteristics of a solid sill and avoid particle penetration. The particle migration ability through a marine vegetation canopy is quantified, and the key drivers are described by an empirical expression based on the particle settling velocity, the canopy length and density. The findings of this study may contribute to improved prediction and assessment of MP accumulation hotspots in vegetated coastal areas and, thus, may help in tracing MP sinks. Such knowledge can be considered a prerequisite to develope methods or new technologies to recover plastic pollutants and rehabilitate valuable coastal environments.

Microplastic (MP) particles are assumed to be potentially harmful to organisms in the hydrosphere. To better assess the exposure and the associated risk it is essential to quantify the transport and sedimentation behavior of MP particles in aquatic environments.Using the Delft3D Flexible Mesh Suite we set up a three-dimensional hydrodynamic and MP transport model for lakes and reservoirs. Our focus is on modeling polymers with different densities and particle sizes to identify patterns of particle residence time and sedimentation. The reservoir Großer Brombachsee in Germany serves as the research site with realistic forcings and boundary conditions.We present first results for horizontal and vertical distribution patterns for different polymer types. We found that the distribution of MP in the computational domain is strongly affected by both particle density and particle size. Smaller, lighter particles are spread over the entire horizontal extent of the reservoir, but particles of higher density or of larger size settle within a limited area around the inflow location, indicating a much higher settling velocity.

Microplastic particles are easily captured by microorganisms and enter the food chain, which poses a threat to ecological health. These particles are abundant in coastal areas because of the influence of anthropic activities and the interaction between the sea and land. Although much research on microplastics has been done, predicting the transportation of microplastic particles in coastal zones is still a challenge. In this paper, the trajectories of microplastic particles released from four river mouths around Laizhou Bay are investigated using the lattice Boltzmann method coupled with the Lagrangian particle-tracking method, involving inter-particle and particle-wall collisions. The trajectories of particles released from four river mouths are recorded within 30 days.

Plastic debris is one of the most significant organic pollutants in the aquatic environment. Because of properties such as buoyancy and extreme durability, synthetic polymers are present in rivers, lakes, and oceans and accumulate in sediments all over the world. However, freshwater sediments have attracted less attention than the investigation of sediments in marine ecosystems. For this reason, river shore sediments of the rivers Rhine and Main in the Rhine-Main area in Germany were analyzed. The sample locations comprised shore sediment of a large European river (Rhine) and a river characterized by industrial influence (Main) in areas with varying population sizes as well as sites in proximity to nature reserves. All sediments analyzed contained microplastic particles (<5 mm) with mass fractions of up to 1 g kg⁻¹ or 4000 particles kg⁻¹. Analysis of the plastics by infrared spectroscopy showed a large abundance of polyethylene, polypropylene, and polystyrene, which covered more than 75% of all polymer types identified in the sediment. Short distance transport of plastic particles from the tributary to the main stream could be confirmed by the identification of pellets, which were separated from shore sediment samples of both rivers. This systematic study shows the emerging pollution of inland river sediments with microplastics and, as a consequence thereof, underlines the importance of rivers as vectors of transport of microplastics into the ocean.

&amp;lt;p&amp;gt;Microplastic (MP) delivery from the terrestrial to aquatic environments is a global concern to many ecosystems and potentially also to humans. Currently, a limited number of models can accurately predict how MPs move through streams and rivers toward the oceans. The limited predictive power of classical colloid filtration theory and the lack of models that take into account the interactive effect between streambed characteristics, flow conditions and particle characteristics limit our ability to model the deposition of MP in streambeds. This study combines improved mechanistic prediction of colloid attachment with a model that predicts flow and transport of particles in a moving streambed to quantify MP deposition in streams. A set of numerical simulations were conducted using sand with D&amp;lt;sub&amp;gt;50&amp;lt;/sub&amp;gt; of 0.3 mm and hydraulic conductivity of 0.12 cm/s. Such sand is predicted to form ripples with a length of approximately 17 cm and a height of 1.9 cm. Coefficient of attachment (K&amp;lt;sub&amp;gt;att&amp;lt;/sub&amp;gt;) was predicted for simulated MP particles of four different densities (900, 1050, 1140, and 1350 (Kg/m&amp;lt;sup&amp;gt;3&amp;lt;/sup&amp;gt;), which are typical densities of Polypropylene [PP], Polystyrene [PS], Polyamide [PA], Polyethylene terephthalate [PET], respectively. In addition, model scenarios included three colloidal sizes (0.5, 1, 10 &amp;amp;#956;m) and various overlying stream velocities of 0.1-0.5 m/sec. Such stream velocities were predicted to yield bed celerities between 0-130 cm/hr. Hyporheic exchange flux between the stream and the bed increased non-linearly with celerity and was found to be ten times greater for the fast celerity (130 cm/hr at stream velocity of 0.5 m/sec) as compared to slow-moving bedform with the same geometry (10 cm/hr at stream velocity of 0.2 m/sec). Difference hyporheic exchange fluxes are also expected to influence the rate of MP delivery to the bed and their deposition. Initial simulations show that increased bedform celerity and K&amp;lt;sub&amp;gt;att&amp;lt;/sub&amp;gt; lead to a shallower depth of MP deposition and a more compact distribution in the bed. Increased celerity reduces deposition depth by flattening hyporheic exchange flow paths. Therefore, despite an increased flux of MP into the bed under high stream water velocity, deposition occurs at shallower depths, and the chance for resuspension due to erosion of the bed sediment increases. Quantifying the deposition rates and residence time in the bed is essential for understanding the transfer of MP through streams and rivers toward the oceans, developing sampling strategies, and finding long-term solutions for reducing their concentrations and the associated risks.&amp;lt;/p&amp;gt;

Distribution characteristics and mechanism of microplastics mediated by soil physicochemical properties

Microplastics are an anthropogenic contaminant widely recognized for their effect on marine and freshwater systems, but their terrestrial effects remain less well studied. The inclusion of microplastics in soils has the potential to affect a range of different soil properties, including bulk density, hydraulic conductivity and aggregation. Soil properties affect the susceptibility of soils to wind erosion, and it is therefore likely that where the quantity of microplastics present in soils is sufficient to change soil properties, it may also change the response of soils to wind erosion. This paper quantifies whether the presence of microplastics in sediments affects the development of small-scale soil surface roughness (SSR) properties during wind erosion, and whether there are any relationships between indices of SSR and microplastic flux due to wind erosion. Two contrasting substrates (well-sorted sand and poorly sorted soil) and two types of microplastic (polyethylene beads and polyester fibres) are used. SSR is quantified using geostatistically derived indicators calculated from high-resolution laser scans of the soil surface with and without microplastics, and before and after wind erosion simulated using a wind tunnel. Our results reveal the relative size of the microplastic to the mineral sediment is key to controlling microplastic flux.This article is part of the Theo Murphy meeting issue 'Sedimentology of plastics: state of the art and future directions'.

The majority of microplastics (MPs) in marine environments originate from terrestrial sources and are transported by rivers and streams. In fluvial systems, interactions between particles, biota and sediments influence particle mobility and retention. It has been shown that the transport behaviour of MPs differs from the one of natural sediments. However, specific transport mechanisms for MPs are not yet fully understood, mainly because of the limited availability of reliable experimental data. As part of this study, transport mechanisms for MPs in fluvial systems (surface flow and hyporheic sediments) were investigated using an experimental flume environment. Realistic flow conditions were represented by varying sediment characteristics (e.g. glass beads or sand) and bedform structures (e.g. riffle-pool sequences, ripples and dunes). To track MPs in i) surface flow, ii) at the streambed interface and iii) within hyporheic sediments, we developed a quantitative method for fluorescent MP particles (1-10 μm) based on state-of-the-art fluorometric techniques. Particle velocities in surface flow were measured by Particle-Image Velocimetry and Laser-Doppler-Velocimetry. With this setup, for the first time it was possible to quantitatively track advective MP transfer from surface flow into the streambed sediments. Empirical results obtained from this study can be used in a next step to establish and validate transport models for MPs.

Transport of ellipsoidal microplastic particles in a 3D lid-driven cavity under size and aspect ratio variation

Wave-induced particle transport during ion cyclotron heating in Tokamak plasmas is studied. Diffusion takes place both in real and velocity space. The real space diffusion occurs due to absorption of the waves' toroidal angular momentum. This process can lead to a pump-out of resonating ions in regions with large power densities giving rise to hollow density profiles, lower energy of the heated ions, flattening of the profile of power transfer to the background ion species and toroidal acceleration of the plasma. The transport in real and velocity space is studied by reducing the problem to a time-dependent 2-D diffusion problem, 1-D in velocity space and 1-D in real space. This is done by using various models for the distribution in pitch angle. The resulting 2-D diffusion problem is then solved numerically.

&lt;p&gt;An estimated 8.3 billion tons of non-biodegradable plastic has been produced over the last 65 years. Much of this is not recycled and is disposed into the natural environment, has a long environmental residence time and accumulates in sedimentary systems worldwide, posing a threat to important ecosystems and potentially human health. We synthesize existing knowledge of seafloor microplastic distribution, and integrate this with process-based sedimentological models of particle transport, to provide new insights, and critically, to identify future research challenges. Compilation of published data shows that microplastics pervade the global seafloor, from abyssal plains to submarine canyons and deep-sea trenches (where they are most concentrated). However, few studies relate microplastic accumulation to sediment transport and deposition. Microplastics may enter directly into the sea as marine litter from shipping and fishing, or indirectly via fluvial and aeolian systems from terrestrial environments. The nature of the entry-point is critical to how terrestrially sourced microplastics are transferred to offshore sedimentary systems. We present models for physiographic shelf connection types related to the tectono-sedimentary regime of the margin. Beyond the shelf, the principal agents for microplastic transport are: (i) gravity-driven transport in sediment-laden flows; (ii) settling, or conveyance through biological processes, of material that was formerly floating on the surface or suspended in the water column; (iii) transport by thermohaline currents, either during settling or by reworking of deposited microplastics. We compare microplastic settling velocities to natural sediments to understand how appropriate existing sediment transport models are for explaining microplastic dispersal. Based on this analysis, and the relatively well-known behavior of deep-marine flow types, we explore the expected distribution of microplastic particles, both in individual sedimentary event deposits and within deep-marine depositional systems. Residence time within certain deposit types and depositional environments is anticipated to be variable, which has implications for the likelihood of ingestion and incorporation into the food chain, further transport, or deeper burial. We conclude that the integration of process-based sedimentological and stratigraphic knowledge with insights from modern sedimentary systems, and biological activity within them, will provide essential constraints on the transfer of microplastics to deep-marine environments, their distribution and ultimate fate, and the implications that these have for benthic ecosystems. The dispersal of anthropogenic across the sedimentary systems that cover Earth’s surface has important societal and economic implications. Sedimentologists have a key, but as-yet underplayed, role in addressing, and mitigating this globally significant issue.&lt;/p&gt;

Microplastic particle trapping through microfluidic devices with different shaped pillars