Settling velocity of microplastic particles having regular and irregular shapes

Factors influencing the vertical distribution and transport of plastics in riverine environments: Theoretical background and implications for improved field study design

Rivers have been widely recognized as important conduits and accumulation sites for plastics. Accurately describing plastic fate and transport in these systems is essential for the development of numerical models, estimating loads to oceans, and implementing effective management strategies. However, plastic transport mechanisms within fluvial environments are not well understood, and field studies often do not provide sufficient information to test analytical models of transport. Sediment transport has dynamical similarities to plastics transport in water bodies, enough to warrant further investigation into how principles from sediment transport can be used to guide the study of plastics. In this review, we summarize fundamentals from sediment transport research and their application to plastics, then use these to make suggestions of clarifying research questions and riverine field study design with the goal of generating more insightful data that can be used to understand and predict plastic fate and transport. We focus specifically on factors influencing plastic vertical distribution and movement in the water column, as variations in this direction have historically been overlooked or oversimplified in rivers.

Transport mechanisms of particulate emissions from artificial marine structures - A review

A vast number of artificial marine structures are currently installed offshore, and the rate of new installation is increasing. Especially offshore wind farms, a sub-type of artificial marine structures, are expected to grow significantly due to ambitious installation targets from international decision-makers. With increasing numbers of installed artificial marine structures, an assessment of possible adverse effects is more important than ever. To improve the environmental friendliness of artificial marine structures, an in-depth assessment of the transport and environmental fate of particle emissions is needed. The present work provides an overview of the involved processes of particle transport in the marine environment using the example of an offshore wind turbine. In this work, a first estimation on emission quantities is given for particulate emissions from marine structures, from which it is evident that emissions will increase in the next years due to an increasing number of marine structures.

Settling velocities of microplastics with different shapes in sediment-water mixtures

The widespread distribution of microplastics (MP) in aquatic systems highlights the need for a clear understanding of how they are transported and accumulate on the bottom of water bodies. Developing predictive models for MP dispersion, sedimentation, and bioaccumulation is crucial for informing regulatory decisions and mitigating the impact of MP and related pollutants. Among the key parameters, MP settling velocity is considered the most critical for predicting their behavior in aquatic environments. Recent studies suggest an intricate and not fully understood relationship between MP settling and sediment dynamics. To date, none of the current models can predict how sediment modifies MP settling velocity. Therefore, previous understanding on MP settling does not fully account for the sedimentation of MP in aquatic ecosystems, where sediment suspension is present. This study provides further evidence that the presence of sediment alters the sedimentation rate of MP based on their shape, offering a quantitative estimate of this interaction. For the first time, the effects sediment interaction has on MP sinking trajectories and inclinations are presented. A preliminary, modified version of an existing formula is proposed to estimate MP settling velocity in the presence of sediment, laying the groundwork for more accurate predictive models of MP transport in aquatic environments.

Numerical plastic transport modelling in fluvial systems: Review and formulation of boundary conditions

In recent years, it has become clear that plastic pollution poses a significant threat to aquatic environments and human health. Rivers act as entry points for land-based plastic waste, while a certain fraction of entrained plastics is carried into marine environments. As such, the accurate modelling of plastic transport processes in riverine systems plays a crucial role in developing adequate remediation strategies. In this paper, we review the two main multiphase flow numerical approaches used in plastic transport modelling, comprising Lagrangian Transport Models (LTMs) and Eulerian Transport Models (ETMs). Although LTMs and ETMs can be regarded as complementary and equivalent approaches, LTMs focus on the transport trajectories of individual particles, whereas ETMs represent the behaviour of particles in terms of their mass or volume concentrations. Similar results of the two approaches are expected, while our review shows that plastic transport models are yet to be improved, specifically with respect to the formulation and implementation of boundary conditions, comprising plastic interactions with the channel bed, river bank, and the free surface, as well as interactions with biota. We anticipate that an implementation of these boundary conditions will allow for a better representation of different plastic transport modes, including bed load, suspended load, and surface load. Finally, we provide suggestions for future research directions, including a novel threshold formulation for free surface detachment of plastics, and we hope that this review will inspire the plastic research community, thereby triggering new developments in the rapidly advancing field of numerical plastic transport modelling.

Towards A universal settling model for microplastics with diverse shapes: Machine learning breaking morphological barriers

Accurately predicting the settling velocity of microplastics in aquatic environments is a prerequisite for reliably modeling their transport processes. An increasing number of settling models have been proposed for microplastics with fragmented, filmed, and fibrous morphologies, respectively. However, none of the existing models demonstrates universal applicability across all three morphologies. Scientists now have to rely on the predominate microplastic morphology extracted from filed samples to determine the appropriate settling model used for transport modeling. Given the spatiotemporal variability in morphologies and the coexistence of diverse morphologies of microplastics in natural aquatic environments, the extracted morphological information poses significant challenges in reliably determining the appropriate model. Evidently, to reliably model the transport of microplastics in aquatic environments, a universal settling model for microplastics with diverse shapes is warranted. To develop such a universal model, a unique shape factor, which can explicitly distinguish between the distinct morphologies of microplastics, was first proposed in this study by using a specifically-modified machine learning method. Using this newly-proposed shape factor, a universal model for predicting the settling velocity of microplastics with distinct morphologies was developed by using a physics-informed machine learning algorithm and then systematically evaluated against independent data sets. The newly-developed model enables reasonable predictions of the settling velocity of microplastic fragments, films, and fibers. In contrast to purely data-driven models, the newly-developed model is characterized by its transparent formulaic structure and physical interpretability, which is conducive to further expansion and improvement. This study can serve as a paradigm for future studies, inspiring the adoption of machine learning techniques in the development of physically-based models to investigate the transport of microplastics in aquatic environments.

Sensitivity analysis of a one-dimensional microplastic transport model in turbulent rivers: Intrinsic properties and hydrodynamics

Since rivers are major transport routes for microplastics, developing novel modeling approaches has become a subject of research to better understand the transport behavior of these particles in river systems. This study aims to model the vertical transport of microplastics at selected sites of the Ergene River, Türkiye, simulate the concentration dynamics of these particles in water and sediment under different hydrodynamic and morphological conditions, and determine the sensitivity of the model results to parameters related to the physical characteristics of microplastics, as well as river hydrodynamics and morphology. A mechanistic model was developed using data on microplastics, river hydrodynamics and morphology. Mass-balance and hydrodynamic equations were utilized for model construction in GoldSim to predict the transport of microplastics between the water column and sediment. The model results revealed that the residence time of microplastics in water was directly related to flow characteristics and river hydraulics, while the initial concentration of particles in water dominated other parameters in influencing the settling and resuspension fluxes of microplastics. Turbulent conditions affected both flow rate and particle resuspension, suggesting that turbulence can either increase or decrease microplastic concentrations and their residence time in the water column and sediment. The model results for both compartments were most sensitive to changes in water and plastic density, whereas Nikuradse sand roughness was the least significant parameter affecting the model outcomes for both compartments.

A settling velocity formula for irregular shaped microplastic fragments based on new shape factor: Influence of secondary motions

Microplastic (MP) fragments are prevalent in rivers and lakes and cause considerable pollution in natural water environments. Determining the settling velocity of microplastic fragments is crucial for predicting their migration and fate in aquatic systems. Predicting the settling velocity of MP fragments is challenging because of their complex and variable geometries and the uncertainties associated with secondary motions. To better understand the secondary motions of irregular MP fragments, a numerical model was developed to study the entire settling process, and an experiment was conducted to validate the numerical model. The model results showed the temporal changes in the settling velocity and orientation of MP fragments during the settling process. The MP fragments were classified according to their shape factors into fragments undergoing stable, transitional, and oscillating settling on the basis of velocity fluctuations caused by secondary motions. To describe the shape of irregular MP fragments appropriately, a new irregular shape factor (ISF) was derived by performing a theoretical analysis of forces and demonstrated to be more suitable than the Corey shape factor (CSF) for irregular MP fragments exhibiting considerable secondary motions. The settling velocity data were fitted to obtain an explicit settling velocity formula that includes the ISF for irregular MP fragments. Compared with machine learning methods and existing formulas, the proposed formula provides more accurate predictions of the settling velocity for irregular MP fragments.

Prediction of vertical transport of microplastics: Shape- and aging-dependent drag models

The prediction of vertical transport of microplastics (MPs) is essential for understanding their multidimensional transport, fate, and environmental risks, but drag models applicable to aging MPs are currently understudied. In this study, pristine and UV-aged polyethylene terephthalate (PET) and polystyrene (PS) MPs were used for settling experiments. Combining physicochemical properties and transport data, a shape-dependent drag model based on the Corey shape factor was optimized with average errors of 9.73 % and 10.42 % and coefficients of determination of 0.6878 and 0.8359 for predicting the settling terminal velocities (ut) for PET and PS MPs, respectively. Meanwhile, aging-dependent drag models were constructed by incorporating the carbonyl index as functional forms of the newly defined aging index, which can be used to differentiate the effects of shape and aging characteristics on the vertical transport of MPs. These aging-dependent models showed better predictive abilities with average errors of 3.97 % and 4.56 % in predicting ut for PET MPs, and of 5.89 % and 6.91 % for PS MPs. Additionally, the drag models in this study improved applicability to predict vertical transport of environmentally-collected weathered MPs. With the continuous improvement of the transport database of diverse MPs, this study is expected to provide scientific support for predicting the environmental behaviors of MPs and formulating targeted pollution control strategies.

Shear induced remobilization of buried synthetic microfibers

Microplastics are known to accumulate in sediment beds of aquatic environments where they can be buried. Once buried they can remobilize due to high energetic events, entering the water column again. Here, turbulence induced by an oscillating grid device was used to investigate the remobilization of microfibers (MF) buried into the sediment bed. Four different types of plastic fibers commonly used for several industrial applications (PET, PP, PA and LDPE) and two types of soils (cohesive and non-cohesive) were investigated. Particles were in depth characterized via 3D reconstruction to estimate important parameters like the Corey shape factor and the settling velocity. Experimental runs explored a wide range of shear stresses. Measurements were taken at different time steps (between 15 min and 240 min from the start of each run). The results have shown that the remobilization of MFs is directly proportional to the value of the shear rate and the duration of the disturbance. Also, buoyant MFs were found more prone to remobilize respect to the denser ones. Drawing from experimental observations of the key parameters affecting MF remobilization, a non-dimensional predictive model was developed. A comparison with previous studies was performed to validate the model in order to predict MF remobilization in aquatic environments.

Insight into the removal of nanoplastics and microplastics by physical, chemical, and biological techniques

Plastic pollutants create health crises like physical damage to tissues, upset reproductive processes, altered behaviour, oxidative stress, neurological disorders, DNA damage, gene expression, and disrupt physiological functions, as the biosphere accumulates them inadvertently through the food web. Water resources have become the generic host of plastic wastes irrespective of their particle size, resulting in widespread distribution in aquatic environments. The pre-treatment step of the traditional water treatment process can easily remove coarse-sized plastic wastes. However, the fine plastic particles, with sizes ranging from nanometres to millimetres, are indifferent to the traditional water treatment. To address the escalating problems, the upgradation of different traditional physical, chemical, and biological remediation techniques offers a promising avenue for tackling tiny plastic particles from the water environment. Further, new techniques and hybrid incorporations to the existing water treatment techniques have been explored, specifically removing tiny plastic debris. A detailed understanding of the sources, fate, and impact of plastic wastes in the environment, as well as an evaluation of the above treatment techniques and their limitations and challenges, can only show the way for their upgradation, hybridization, and development of new techniques. This review paper provides a comprehensive overview of the current knowledge and techniques for the remediation of nanoplastics and microplastics.

Modeling the settling and resuspension of microplastics in rivers: Effect of particle properties and flow conditions

Microplastics have numerous different shapes, affecting the fate and transport of these particles in the environment. However, theoretical models generally assume microplastics to be spherical. This study aims to develop a modeling approach that incorporates the shapes of microplastics to investigate the vertical transport of microplastics in rivers and simulate the effect of particle and flow characteristics on settling and resuspension. To achieve these aims, a mechanistic model was developed utilizing the mass-balance and hydrodynamic equations. Scenario analysis was implemented assigning different values to model parameters, such as bed shear stress, shape factor and particle size to simulate the effect of flow patterns and particle properties. The model outcomes revealed that the residence time of microplastics in the water column was longest in medium bed shear stress, whilst it was shortest in low bed shear stress. This suggests that the influence of turbulence is not unidirectional; it can both increase and decrease microplastic concentrations and residence time in the water column. According to the scenario analysis, the settling flux of microplastics was the highest for near-spherical particles and increased with the size of the particles, as well as with increasing bed shear stress. However, the resuspension of particles was primarily influenced by increasing bed shear stress, but the ranking of resuspension flux values for different shaped and sized microplastics exhibited alterations with changing flow patterns. Turbulent conditions predominantly influenced the resuspension of near-spheres and large microplastics. On the contrary, the settling of fibers and small microplastics were significantly influenced by changing flow patterns, whereas near-spheres and largest particles were least affected. The model results were sensitive to changes in shape factor developed for this model, therefore this parameter should be improved in future studies.

Settling velocity of microplastics in turbulent open-channel flow

The settling behavior of microplastics (MPs) plays a pivotal role in their transport and fate in aquatic environments, but the dominant mechanisms and physics governing the settling of MPs in rivers remain poorly understood. To gain mechanistic insights into the velocity lag of MPs in an open-channel flume under different turbulent flow conditions, an experimental study was conducted using three types of MPs: polystyrene, cellulose acetate, and acrylic, of sphere-shaped particles with diameters ranging from 1 mm to 5 mm. A particle tracking technique was employed to record and analyze the MPs velocity within turbulent flows. The results showed a variation in the vertical settling velocity of MPs ωMP ranging from -26 % to +16 %, when compared to their counterparts in still water (ωs). A new formula for the drag coefficient (Cd) of MP particles was developed by introducing the suspension number (u∗/ωs). The developed Cd formula was used to calculate the resultant velocity lag VMP, with a mean relative error of 16 % compared with the measured values. Further, the study highlighted that the MPs with large Stokes numbers are mainly driven by their own inertia and turbulence has less influence on their settling behavior. This study is crucial for understanding the settling behavior of MPs in turbulent flows and developing their transport and fate models for MPs in riverine systems.

An equilibrium criterion for plastic debris fate in wave-driven transport

The nearshore zone turns out to be the area with the higher concentration of plastic debris and, for this reason, it is important to know the processes that affect the transport and the fate of this type of litter. This study focuses on investigating the dynamics of various plastic types under several hydrodynamic conditions primarily induced by waves. 2D tests were carried out at the Hydraulic Laboratory of the University of Messina reproducing the main phenomena that occurred during the wave propagation on a planar beach. More than 200 different conditions were tested changing the wave characteristics, the water depth, the plastic debris characteristics (density and shape), and the roughness of the fixed bottom. In general, it can be observed that the reduction in particle displacement occurs due to: i) a decrease in wave steepness; ii) an increase in depth; iii) an increase in particle size; iv) an increase in plastic density. However, the experimental investigation shows that some plastic characteristics and bed roughness, even when hydraulically smooth, can alter these results. The experimental data analysis identified a criterion for predicting the short-term fate of plastic debris under wave action. This criterion to determine equilibrium conditions, based on an empirical relationship, takes into account the wave characteristics, the bed roughness and slope, and the weight of the debris.

Quantifying the influence of size, shape, and density of microplastics on their transport modes: A modeling approach

Microplastics (MPs) pose significant risks to marine ecosystems and human health, necessitating accurate predictions of their distributions in aquatic environments for effective risk mitigation. However, understanding MP transport dynamics is challenging because of the inadequate representation of MP characteristics such as size, shape, and density in numerical models. Further, the accuracy of the MP vertical profiles in existing models has not been thoroughly validated. Thus, we developed an MP transport model within the Finite Volume Community Ocean Model framework (FVCOM-MP) by integrating MP characteristics. We validated FVCOM-MP against experimental and analytical data, focusing on various MP transport modes and transitions. FVCOM-MP successfully replicates MP profiles in different transport modes, including the bedload, surface load, suspended load, and mixed load modes. Additionally, we introduce phase diagrams for classifying MP transport modes based on particle characteristics, enhancing our understanding of MP dynamics in aquatic systems. The transport modes for a number of real-world MP particles, including fishing line, plastic bag/bottle fragments, synthetic fibers, tire wear particles, polyvinyl chloride and expanded polystyrene pellets, were analyzed with these phase diagrams.

Simulation of the dynamic processes of microplastic suspension and deposition in a lake sediment-water system

The occurrence of microplastics in aquatic environments has attracted increasing interest from both the public and scientists, especially their migration behaviors. Although several environmental behaviors of microplastics have been studied, the issue of microplastic suspension and deposition in lake sediment-water systems remains to be elucidated. In this study, we built an indoor sediment-water system with input and output rivers that simulated the actual situations in lakes, and aimed to explore the suspension and deposition behavior of microplastics using eight group experiments. The abundance of microplastics in overlying water and sediments in different periods was analyzed, and the characteristics of hydrodynamic disturbance on microplastic suspension and deposition were identified. Importantly, the exchange of microplastics in sediments and water under dynamic flow conditions was assessed. The results showed that the middle-scale experiment designed in this study effectively simulated the dynamic transport process of microplastics in lakes, and the hydrodynamic force had a significant impact on the suspension and deposition behaviors of microplastics. The average abundance of polystyrene, polyethylene terephthalate and polyamide microplastics was 1.07, 0.60 and 0.83 particles/L in overlying water during the suspension experiments, respectively. This showed a pattern of first rising and then falling with the extension of suspension time. Even in the environment with the maximum input water volume (8000 ml/min) in this study, only microplastics at a depth of 0 to 2 cm from the sediment were suspended. The average abundance of microplastics was 313.02 particles/kg during the deposition experiments, which gradually increased with the extension of deposition time in sediments. Finally, microplastic sizes in water of the suspension experiments and in sediments of the deposition experiments were concentrated in the range of 500 to 1500 μm and 300 to 1000 μm, respectively.

Settling velocity of atmospheric particles in seawater: Based on hydrostatic sedimentation method using video imaging techniques

When atmospheric particles deposit to the ocean, their settling velocities and residence times associated are critical for their effects on oceanic ecosystems. We developed a hydrostatic sedimentation method using video imaging techniques to track particles of 5-20 μm in diameter falling into seawater and determine the particle settling velocities in relation to their diameter, shape, organic matter contained, and seawater salinity. The measured settling velocities varied from 0.025 to 0.41 mm/s. Irregular particle shape and organic matter contained in particles also, however, reduced the values. The settling velocities were decelerated by the dissolution process of particle in seawater. Combined with the experimental results, a formula for calculating the settling velocity formulae for atmospheric particles was estimated. Using this equation, the residence time of particles is estimated to be less than one month in continental shelf sea and more than 100 days in the oceans.

Towards better predicting the settling velocity of film-shaped microplastics based on experiment and simulation data

The properties of microplastics determine their settling velocities and affect the fates and migration pathways of microplastics. This paper has simulated the settling velocities of film-shaped microplastics, which are present in natural aquatic environments. The numerical results provided more data to fit the terminal settling velocities of film-shaped microplastics. Comparison between the particle definition and the equivalent spherical diameter confirmed that the particle definition is more suitable for film-shaped microplastics. In the transitional flow regime, CD decreases linearly with Re. As Re further increases, CD gradually converges at approximately 1.20. By integrating the experimental and simulated data, a new explicit formula for predicting the settling velocity of film-shaped microplastics has been presented with the optimal shape parameter f. The presented formula achieves better performance (MAPE = 6.6 %, RMSE = 16.8 %, and R2 = 0.99) than the existing formulas for settling velocity for film-shaped microplastics, closely rivaling that of the ensemble learning algorithm.

On the vertical structure of non-buoyant plastics in turbulent transport

Plastic pollution is overflowing in rivers. A limited understanding of the physics of plastic transport in rivers hinders monitoring, the prediction of plastic fate and restricts the implementation of effective mitigation strategies. This study investigates two unexplored aspects of plastic transport dynamics across the near-surface, suspended and bed load layers: (i) the complex settling behaviour of plastics and (ii) their influence on plastic transport in river-like flows. Through hundreds of settling tests and thousands of 3D reconstructed plastic transport experiments, our findings show that plastics exhibit unique settling patterns and orientations, due to their geometric anisotropy, revealing a multimodal distribution of settling velocities. In the transport experiments, particle-bed interactions enhanced mixing beyond what established turbulent transport theories (Rouse profile) could predict in low-turbulence conditions, which extends the bed load layer beyond the classic definition of the bed load layer thickness for natural sediments. We propose a new vertical structure of turbulent transport equation that considers the stochastic nature of heterogeneous negatively buoyant plastics and their singularities.

A modified drag coefficient model for calculating the terminal settling velocity and horizontal diffusion distance of irregular plume particles in deep-sea mining

Deep-sea mining inevitably produces plumes, which will pose a serious threat to the marine environment with the continuous movement and diffusion of plumes along with ocean currents. The terminal settling velocity (wt) of irregular particles is one of the crucial factors for determining the plumes' diffusion range. It is generally calculated by drag coefficient (CD), while most existing CD models only consider single shape characteristic parameter or have a smaller range of Reynolds number (Re). In this study, a new shape factor (γ) of irregular particles is proposed by considering the thickness (one-dimension), the projected area (two-dimension), and the surface area (three-dimension) of irregular particles as well as their coupling effect to establish a modified CD model for calculating the wt. A modified Gaussian plume model is proposed to predict the horizontal diffusion distance of the plume particles by considering the settling velocity and diffusion effect of irregular particles. Research results show that the wt increases nearly linearly, with a gradually decreased slope and slightly then greatly with the increasing of γ, dp (diameter) and ρp (density), respectively. The modified CD model is verified to be more valid with a wider application range (Re < 3×105) than five existing CD models by the test results. The larger the ρp or dp, the larger the wt and thus the smaller the Sh. This study could provide a theoretical basis for calculating the plume diffusion range to further study the impact of deep-sea mining on the ocean environment.

Settling Velocities of Small Microplastic Fragments and Fibers

There is only sparse empirical data on the settling velocity of small, nonbuoyant microplastics thus far, although it is an important parameter governing their vertical transport within aquatic environments. This study reports the settling velocities of 4031 exemplary microplastic particles. Focusing on the environmentally most prevalent particle shapes, irregular microplastic fragments of four different polymer types (9-289 μm) and five discrete length fractions (50-600 μm) of common nylon and polyester fibers are investigated, respectively. All settling experiments are carried out in quiescent water by using a specialized optical imaging setup. The method has been previously validated in order to minimize disruptive factors, e.g., thermal convection or particle interactions, and thus enable the precise measurements of the velocities of individual microplastic particles (0.003-9.094 mm/s). Based on the obtained data, ten existing models for predicting a particle's terminal settling velocity are assessed. It is concluded that models, which were specifically deduced from empirical data on larger microplastics, fail to provide accurate predictions for small microplastics. Instead, a different approach is highlighted as a viable option for computing settling velocities across the microplastics continuum in terms of size, density, and shape.

Assessing the Settling Velocity of Biofilm-Encrusted Microplastics: Accounting for Biofilms as an Equivalent to Surface Roughness

While it is well established that a biofilm contributes to the sinking of plastics, the underlying mechanisms of how it influences the vertical transport of plastics have not been well explained. In this context, our study dives into the intricate effects of biofouling on the settling velocity (Ws) of microplastics (MPs) within the fluid. We adopt the perspective that the biofilm is a form of surface roughness impacting the drag coefficient (Cd) and vertical settling of MPs. By advancing the biofouling process model, we simulate the temporal variations of density and biofilm thickness of biofouled floating MPs, accounting for realistic parameters and assuming a layer-by-layer growth of biofilm on plastisphere surfaces. MPs of polyethylene (PE) exhibit a quicker initiation of descent compared to their polypropylene (PP) counterparts. Furthermore, leveraging computational fluid dynamics (CFD) simulation, the method to predict the Cd of spherical MPs with surface roughness is established. By treating the thickness of the biofilm as roughness height, an explicit method to predict the Ws of biofouled MPs is derived. The settling experiments for biofouled MPs conducted not only support the combination of the biofouling model and the explicit method to predict the Ws of biofouled MPs but also enhance the prediction accuracy by introducing a ratio parameter Co to better relate the equivalent surface roughness height (k) to the biofilm thickness (σ), i.e., k = Co·σ, where the recommended value of Co for spherical PP and PE MPs is between 0.5 to 0.8. This study, thus, provides new insights into the dynamics of biofouled MPs in hydraulic ecosystems.

Settling velocity of submillimeter microplastic fibers in still water

Microplastic fibers (MPFs) are one of the most important MP contaminants of aquatic environments. However, little research has been conducted on the movement of submillimeter MPFs in water. Herein, the settling of 519 submillimeter MPFs in still water was measured and the settling velocity was analyzed. Observations of the settling velocity of MPFs with lengths of 300, 500, and 600 μm showed that most MPFs settled individually or in pairs. The sedimentation of a single fiber could be divided into three patterns, that is, horizontal, inclined, and vertical. The average settling velocity increased with an increase in the MPFs length and orientation angle. As the MPFs length increased, the probability of inclined settlement decreased but that of horizontal settlement increased. The horizontal velocity of single fibers also was investigated, and the horizontal and vertical settling of MPFs exhibited minimal horizontal velocity. Because of the considerable difference between the calculated drag coefficients from existing drag coefficient models and experimental values, a drag coefficient model was developed with a deviation of <3 %. Four settling patterns were identified for two fibers, that is, X shaped, inverted-T shaped, cross shaped, and overlapping. The average velocity of the overlapping settlement of two fibers was considerably higher than that of the other three settling patterns. The average settling velocity of 600-μm two fibers was 1.47 times that of single fibers, indicating that their corresponding drag coefficient was ~46 % that of a single fiber.

Investigations on microplastic infiltration within natural riverbed sediments

Several studies focused on the role of rivers as vectors of microplastics (MPs) towards the sea. It is well known that during their path through the fluvial environment, MPs interact with riverbed sediments; however, the main factors impacting the mobility of MPs within the upper part of the hyporheic zone are not clear yet. The present work investigates the role of different sediment size layers in affecting the mobility of the most common MP (Polyethylene terephthalate - PET - spheres, PET 3D-ellipsoids, polystyrene - PS - fragments and polyamide - PA - fibers) within sediment porous media under different hydraulic loads (HL) and time scales (t) conditions. Results indicated the relationship between the characteristic MP diameter and that of the grains as the main parameter for the MP infiltration into the sediment layer. The maximum infiltration depth was found to not depend on HL and t. However, HL was able to influence the percentage of MPs penetrating the superficial layer and their distribution within the first 10-15 cm of the sediment layer. None of the MPs were found at depths >20-25 cm, where only PET spheres were detected. Starting from the suffusion theory, a model able to predict the MP maximum infiltration depth in the range of parameter values was provided. The outcome indicates the importance of considering geometrical and hydrodynamic aspects of the riverbed sediment layer to better characterize the spatial and temporal scales of MP transport in freshwater environments.

Depth profiles of microplastics in sediments from inland water to coast and their influential factors

Microplastics, plastic particles with a size smaller than 5 mm, are widely observed in the global environments and pose a growing threat as they accumulate and affect the environments in numerous ways. These particles can be transported from inland water to coast and disperse from surface water to deep sediments, especially the latter, while knowledge of the hidden microplastics in sediment layers is still lacking. Understanding the characteristics and behavior of microplastics in deep sediments from inland water to coast is crucial for estimating the present and future global plastic budget from land to seas. Herein, present knowledge of microplastic sedimentation from inland water to coast is reviewed, with a focus on the physical characteristics of microplastics and environmental factors that affect sedimentation. The abundance, shape, composition, and timeline of microplastics in sediment layers in rivers, floodplains, lakes, estuaries and coastal wetlands are presented. The abundance of microplastics in sediment layers varies across sites and may exhibit opposite trends along depth, and generally the proportion of relatively small microplastics increases with depth, while less is known about the vertical trends in the shape and composition of microplastics. Timeline of microplastics is generally linked to the sedimentation rate, which varies from millimeters to centimeters per year in the reviewed studies. The spatiotemporal characteristics of microplastic sedimentation depend on the settling and erosion of microplastics, which are determined by two aspects, microplastic characteristics and environmental factors. The former aspect includes size, shape and density influenced by aggregation and biofouling, and the latter includes dynamic forces, topographic features, bioturbation and human activities. The comprehensive review of these factors highlights the needs to further quantify the characteristics of microplastic sedimentation and explore the role of these factors in microplastic sedimentation on various spatiotemporal scales.

Settling processes of cylindrical microplastics in quiescent water: A fully resolved numerical simulation study

The settling process of marine microplastics (MPs) is crucial research concerning the transport and movement of MPs. The settling processes of MP fibers that possess a cylindrical geometry are affected by environmental factors and properties. In this study, a three-dimensional numerical model for the still water settling of MPs with complex shapes was constructed using the lattice Boltzmann method (LBM) and the immersed boundary method (IBM). The fully resolved settling simulation of cylindrical MPs was achieved, and the model results demonstrated good agreement with the semi-empirical settling velocity formulas. Based on the simulation results, the critical aspect ratio of the cylindrical MP was found to be between 0.93 and 0.94. Near this critical aspect ratio, there is a decline in the drag force. Additionally, it was found that the angular displacement and aspect ratio influence horizontal movement but not the vertical settling velocity, while the density only affects vertical movement.

Settling velocities of coarse organic solids

The settling velocity of a particle is an integral parameter in stormwater modeling and design. The settling velocity can be used to predict the fate and transport of stormwater particles and if the particles contribute to nutrient loading in a watershed. Prediction of settling velocity for inorganic particles is generally well-researched and well-understood. Organic particles tend to vary widely in their physical properties and there are currently no set standards or empirical equations for estimating the settling velocity of organic particles. This paper presents data from tree leaves and seeds settling velocity experiments to better understand how organic particles settle in the context of settling velocity equations such as the one developed by Ferguson and Church. Analysis of the collected data showed that the second of the two drag coefficients (C₂) used in the Ferguson and Church Equation was sensitive to particle type and shape. By averaging C₂ by particle type and species, there was a correlation between the observed settling velocity and the settling velocity predicted by the Ferguson and Church Equation (R² = 0.83). With these results, stormwater modelers and designers are equipped with a better understanding of how to represent common organic particles in terms of settling velocity. Additional research on a wider variety of organic particle types and species would expand on the dataset presented here.

Simulation and Characterization of Nanoplastic Dissolution under Different Food Consumption Scenarios

Understanding of the potential leaching of plastic particles, particularly nanoplastics (NPs), from food packaging is crucial in assessing the safety of the packaging materials. Therefore, the objective of this study was to investigate potential exposure risks by simulating the release of NPs from various plastic packaging materials, including polypropylene (PP), general casting polypropylene (GCPP) or metalized casting polypropylene (MCPP), polyethylene (PE), polyethylene terephthalate (PET), and polyphenylene sulfone (PPSU), under corresponding food consumption scenarios. Surface-enhanced Raman scattering (SERS) and scanning electron microscopy (SEM) were utilized to identify and characterize the NPs leached from plastic packaging. The presence of separated NPs was observed in PP groups subjected to 100 °C hot water, GCPP plastic sterilized at a high temperature (121 °C), and PE plastic soaked in 100 °C hot water, exhibited a distorted morphology and susceptibility to aggregation. The findings suggest that the frequent consumption of takeaway food, hot beverages served in disposable paper cups, and foods packaged with GCPP materials may elevate the risk of ingestion of NPs. This reminds us that food packaging can serve as an important avenue for human exposure to NPs, and the results can offer valuable insights for food safety management and the development of food packaging materials.