Controlling factors of microplastic fibre settling through a water column

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.

Evaluation of the migration behaviour of microplastics as emerging pollutants in freshwater environments

Microplastics, as an emerging pollutant, are widely distributed in freshwater environments such as rivers and lakes, posing immeasurable potential risks to aquatic ecosystems and human health. The migration behaviour of microplastics can exacerbate the degree or scope of risk. A complete understanding of the migration behaviour of microplastics in freshwater environments, such as rivers and lakes, can help assess the state of occurrence and environmental risk of microplastics and provide a theoretical basis for microplastic pollution control. Firstly, this review presents the hazards of microplastics in freshwater environments and the current research focus. Then, this review systematically describes the migration behaviours of microplastics, such as aggregation, horizontal transport, sedimentation, infiltration, stranding, resuspension, bed load, and the affecting factors. These migration behaviours are influenced by the nature of the microplastics themselves (shape, size, density, surface modifications, ageing), environmental conditions (ionic strength, cation type, pH, co-existing pollutants, rainfall, flow regime), biology (vegetation, microbes, fish), etc. They can occur cyclically or can end spontaneously. Finally, an outlook for future research is given.

Global microplastic fiber pollution from domestic laundry

The rapid expansion of fast fashion has significantly increased microplastic fiber (MPF) release during laundry practices, accounting for approximately one-third of primary microplastics entering the ocean. Currently, a significant gap exists in global-scale research on the release of MPFs from washing textiles. This study introduces an innovative empirical model to assess the spatial distribution of MPF emissions. The model estimates an annual global emission of 5.69 million tons of MPFs from laundry. Of this total, machine washing accounts for the majority (93.7 %), with hand washing contributing the remaining 6.3 %. As the primary source of MPF pollution, Asia's emissions reach 3.71 million tons, far exceeding those of North America (1.18 million tons) and Europe (0.45 million tons). The primary issue is that wastewater management efficiency varies significantly worldwide. In Asia, there is persistently high discharge of MPFs into natural waters, and the removal efficiency of wastewater treatment plants is still comparatively low. In contrast, the United States and many European countries exhibit better MPF retention. The global nature of this challenge mandates international collaboration for comprehensive environmental conservation. Our study provides the first high-resolution global distribution map of MPF emissions and discharge into natural waters, establishing a data foundation for global and regional management of microplastics originating from household laundry sources.

3D analysis of microplastic settling in algal suspensions

The influence of algae presence in surface water on the settling velocities of microplastics is unknown, and determining it is challenging due to the turbidity of algal suspensions. Measuring the settling velocity of microplastics has traditionally relied on either manual measurement techniques or 2D Particle Tracking Velocimetry (PTV). This study introduces a 3D-PTV method tailored to determine the effects of algae (Synechoccocussp.) on microplastic settling speeds in semi-large columns. We demonstrated that 3D PTV produces much more accurate results than 2D particle tracking. Testing the method in six experiments with varying algae concentrations revealed consistent results across the experiments and alignment with some theoretical approximations. The results were concurrent with calculated 2D speeds. No influence of algal density on settling velocities was found, which is highly relevant for microplastic fate modeling in eutrophic systems. We highlight the applicability and accuracy of 3D particle tracking velocimetry in further understanding microplastic settling behavior.

Deciphering morphology patterns of environmental microfibers: Insights into source apportionment

Microfibers, a prevalent form of microplastics, undergo diverse environmental interactions resulting in varied morphological changes. These changes can offer insights into their environmental trajectories. Despite its importance, comprehensive studies on microfiber morphology are scarce. This study collected 233 microfibers from the East China Sea and South China Sea. Based on morphological features observed in microscopic images of microfibers, such as curvature, cross-sectional shapes, diameter variations, and crack shapes, we identified a general morphological pattern, classifying the environmental microfibers into three distinct morphological types. Our findings highlight noticeable differences in morphological metrics (e.g., length, diameter, and surface roughness) across three types, especially the diameter. Microfibers of Type I had an average diameter of 19.45 ± 4.93 μm, significantly smaller than Type II (263.00 ± 75.15 μm) and Type III (299.68 ± 85.62 μm). Within the three-dimensional (3D) space fully defined by these quantitative parameters, the clustering results of microfibers are also consistent with the proposed morphology pattern, with each category showing a potential correlation with specific chemical compositions. Type I microfibers correspond to synthetic cellulose, while 94.79 % of Types II and III are composed of polymers. Notably, we also validated the great applicability of the morphology categories to microfibers in diverse environmental compartments, including water and sediments in nearshore and offshore areas. This classification aids in the efficient determination of microfiber sources and the assessment of their ecological risks, marking a significant advancement in microfiber environmental studies.

Trajectory, fate, and magnitude of continental microplastic loads to the inner shelf: A case study of the world's largest coastal shallow lagoon

The Patos Lagoon estuary is a highly significant ecosystem where freshwater from a vast and densely populated area continuously flows into the Atlantic Ocean by coastal plumes, exporting not only freshwater but also sediment, nutrients, plastics, and other contaminants. In this work, numerical modeling tools together with field data were used to assess for the first time the capacity of the coastal plume to export microplastics (MPs) to the inner shelf under different hydrodynamic conditions. Two field surveys were conducted during plume events to quantify MP concentrations and validate the model approach. A bottom-up approach was employed to estimate the potential MP export from the estuary's domain to the Atlantic Ocean. MP concentration in surface plume waters ranged from 0.20 items m-3 to 1.37 items m-3, confirmed by FTIR as synthetic polymers in a 90 %, being Polypropylene (PP) and Polyethylene (PE) the most abundant in a 73 %. The accumulation pattern was observed on the plume's frontal system, consistent with simulation results. The estimated average MP potential export rate attained 9.0 million items day-1 during moderate plume events and 47.5 million items day-1 during high discharge plume events. Strong discharge events, coupled with intense northeast winds, facilitated rapid southwestward export of MPs. Conversely, moderate to weak discharge events retained MPs closer to the estuary's mouth, enabling either longer trajectories or earlier deposition. Significant MP accumulation hotspots were identified in the gyre between the jetties and Cassino beach, as well as in the saline front within the plume boundaries. These accumulation zones may function as reservoirs for MP particles, potentially posing threats to local ecosystems. Understanding these dynamics is crucial for ongoing monitoring efforts to assess potential harmful interactions over time.

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.

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.

Settling of microplastics in mucus-rich water column: The role of biologically modified rheology of seawater

Non-buoyant microplastics (MPs) sink through the marine water column, adversely affecting the ecosystem. The manner in which MPs influence the water environment depends to a large extent on their settling dynamics, driven by their properties and the physio-chemical characteristics of water column. However, some properties of seawater remain elusive, limiting our ability to fully explain the sinking processes of MPs. One of the gaps in our understanding relates to the elevated content of exopolymers (EPSs) secreted by algae and bacteria, which locally transform seawater into a non-Newtonian liquid, altering the hydrodynamics of particle transport. In this study, we present a series of lab-scale experiments on the dynamics of isometric (spheres and irregular particles) and anisometric (disks, rods, and blades) MPs settling in artificial seawater with the addition of polysaccharides. We find that upon the appearance of EPSs in seawater, the sinking velocity of MPs diminishes and may fluctuate, the orientation pattern changes in a non-intuitive way, and MPs may tumble. As measured in rheological tests, these consequences result from seawater gaining viscoelastic and shear-thinning properties. Our findings raise concerns that mucus-rich seawater may favor the aggregation of MPs with organic matter, interaction with biota, and biofouling, which can affect the biogeochemistry of the marine ecosystem. Based on these findings, we recommend that seawater rheology, modified by excessive amounts of EPSs during algal blooms, should be considered in biogeochemical and microplastic transport models.

Microplastic transport and deposition in a beach-dune system (Saunton Sands-Braunton Burrows, southwest England)

Although microplastics (MPs) are ubiquitous contaminants that have been extensively studied in the marine setting, there remain gaps in our understanding of their transport and fate in the coastal zone. In this study, MPs isolated from surface sediments sampled from a large beach-dune complex in southwest England have been quantified and characterised. Concentrations above a detectable size limit of 30 to 50 μm ranged from about 40 to 560 MP kg-1 dry weight but, despite local sources of plastics such as an estuary and seasonal tourism, there were no significant differences in median concentrations between different orthogonal foreshore transects and the dunes or according to zonal location on the beach. The majority of MPs were black and blue fibres of <1 mm in length that were constructed of polymers of density > 1 g cm-3 (e.g., rayon, polyester, acrylic). A significant correlation was found between MP concentration and the proportion of very fine sand (100 to 250 μm) but relationships with other granulometric or compositional markers of sediment (e.g., volume-weighted mean diameter, circularity, calcium content) were not evident. An association of MP concentration with very fine sand was attributed to similar particle depositional characteristics and the entrapment of fibres within small interstitial spaces. Overall, the observations reflect the wavelaid and windlaid deposition of MPs from a diffuse, offshore source, and, despite their role as accumulators of particles from the foreshore, dunes do not appear to act as a landward barrier of MPs.

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.

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.

A review of methods for modeling microplastic transport in the marine environments

Microplastic (MP) pollution is ubiquitous in the oceans and poses serious threats to the marine ecosystems. Nowadays numerical modeling has become one of the widely used tools for monitoring and predicting the transport and fate of MP in marine environments. Despite the growing body of research on numerical modeling of marine MP, the advantages and disadvantages of various modeling methods have not received systematic evaluation in published works. Important aspects such as parameterization schemes for MP behaviors, factors influencing MP transport, and proper configuration in beaching are essential for guiding researchers to choose proper methods in their work. For this purpose, we comprehensively reviewed the current knowledge on factors influencing MP transport, classified modeling approaches according to the governing equations, and summarized up-to-date parameterization schemes for MP behaviors. Critical factors such as vertical velocity, biofouling, degradation, fragmentation, beaching, and washing-off were reviewed in the frame of MP transport processes.

Settling velocity of microplastic particles having regular and irregular shapes

The settling velocities of 66 microplastic particle groups, having both regular (58) and irregular (eight) shapes, are measured experimentally. Regular shapes considered include: spheres, cylinders, disks, square plates, cubes, other cuboids (square and rectangular prisms), tetrahedrons, and fibers. The experiments generally consider Reynolds numbers greater than 102, extending the predominant range covered by previous studies. The present data is combined with an extensive data set from the literature, and the settling velocities are systematically analyzed on a shape-by-shape basis. Novel parameterizations and predictive drag coefficient formulations are developed for both regular and irregular particle shapes, properly accounting for preferential settling orientation. These are shown to be more accurate than the best existing predictive formulation from the literature. The developed method for predicting the settling velocity of irregularly-shaped microplastic particles is demonstrated to be equally well suited for natural sediments in the Appendix.

Effect of monsoon on microplastic bioavailability and ingestion by zooplankton in tropical coastal waters of Sabah

Plankton seasonality in tropical coastal waters is becoming more apparent as a result of monsoon-driven changes in environmental conditions, but research on the monsoonal variation of microplastics (MP) is still limited. We examined the monsoonal variation of MP in the water column and their ingestion by zooplankton in Sepanggar Bay, Sabah, Malaysia. MP concentrations were significantly higher during the Southwest monsoon whereas MP ingestions showed no monsoonal difference across major zooplankton taxa. Canonical Correspondence Analysis (CCA) and Generalized Additive Models (GAM) indicate that MP concentrations were driven by changes in rainfall and salinity while MP bioavailability to zooplankton was consistent regardless of monsoon. MP ingestion increased progressively up the planktonic food chain, and bioavailability of fibers and small-sized MP of high-density polymers to zooplankton was proportionately higher. Distinct changes in the MP concentration relative to the monsoons provide new insights into the seasonal variation of MP in tropical coastal ecosystems.

Human occupational exposure to microplastics: A cross-sectional study in a plastic products manufacturing plant

Microplastics are ubiquitous in the natural environment, and their potential impact on health is a key issue of concern. Investigating exposure routes in humans and other living organisms is among the major challenges of microplastics. This study aims to examine the exposure level of plastic factory staff to microplastic particles before and after work shifts through body receptors (hand and facial skin, saliva and hair) in Sirjan, southeast of Iran. Moreover, the effect of face masks, gloves, cosmetics (e.g: face powder cream, lipstick and eye makeup products) and appearance on the exposure level is investigated. In total, 19 individuals are selected during six working days. Then, the collected samples are transferred to the laboratory for filtration, extraction, identification and counting of microplastic particles. Moreover, 4802 microplastic particles (100-5000 μm in size) in strand, polyhedral and spherical shapes and color spectra of white/transparent, black, blue/green, red and purple are observed. The nature of most of the observed samples is fiber with a size ≥1000 μm. Analyzing the selected samples using micro-Raman spectroscopy indicate polyester and nylon are the main identified fibers. Hair and saliva samples have the highest and lowest number of microplastics, respectively. Using gloves and sunscreen among all the participants, wearing a scarf and hair size among women and having a beard and mustache among men could have an effective role in the exposure level to microplastics. Results of this study could reveal the exposure route to microplastic particles in the human body and highlight the importance of providing higher protection to reduce exposure.

Investigation of dynamic change in microplastics vertical distribution patterns: The seasonal effect on vertical distribution

This paper analyzes the variability of microplastics vertical distributions in the oceanic water column. Data were obtained from targeted sampling in the Bay of Marseille (France) and from a numerical simulation forced by realistic physical forcings. By fitting model and in-situ data in a simplified vertical dimension, three microplastics classes may be deduced: settling, buoyant and winter neutrally-buoyant microplastics. Buoyant microplastics are mainly concentrated at the surface but they can be mixed throughout the whole water column during episodes with strong winds and no water stratification, inducing an implicit underestimation of buoyant microplastics in surface sampling. Almost symmetrical to the distribution of buoyant microplastics, settling microplastics are mainly found at the bottom but they can sometimes reach the surface under the mixing conditions cited above. They could thus contribute to surface sampling. Winter neutrally-buoyant microplastics are more homogenously mixed during the winter but are under the stratified layers during summer.

A baseline assessment of the relationship between microplastics and plasticizers in sediment samples collected from the Barcelona continental shelf

It has been suggested that the seafloor may be a sink for the plastic debris that enters the ocean. Therefore, the collection of data in the seafloor sediments regarding the co-presence of microplastics (MPs) and contaminants associated to plastic is considered a relevant topic. However, the number of studies addressing their possible correlation in this environment is still limited, and very little is known about the mechanisms that determine the release of plastic additives from plastic items. Starting from this basis, we investigated the presence of MPs and eleven phthalic acid esters (PAEs) in the continental shelf offshore Barcelona. Following a shelf-slope continuum approach, we sampled sediments from five stations, and we performed analysis by means of infrared micro spectroscopy (µFTIR) and liquid chromatography tandem mass spectrometry (LC-MS/MS). MPs were found to range from 62.0 to 931.1 items/kg d.w. with maximum concentration in the submarine canyon Besòs and at the highest depth. Moreover, different trends in the size distribution of fibers and non-fibers were observed, indicating the occurrence of a size dependent selection mechanism during transport and accumulation. PAEs resulted comprised between 1.35 to 2.41 mg/kg with Di(2-ethylhexyl)phthalate (DEHP) the most abundant congeners (1.04 mg/kg). Statistical analysis revealed no correlation between the Σ₁₁PAEs and the total MPs concentration, but correlation between DEHP and fibers (σ = 0.667, p = 0,037), that resulted both correlated to the distance to the coast (ρ = 0.941 with p = 0,008 and ρ = 0.673 with p = 0.035, respectively).

Characteristics of microplastic pollution in golden pompano (Trachinotus ovatus) aquaculture areas and the relationship between colonized-microbiota on microplastics and intestinal microflora

Microplastic (MPs) pollution is a global marine environmental problem. The effects of MPs on the gut microbiota of aquatic organisms have received considerable attention. For example, microbes colonizing MPs in pond cultures alter the structure and function of the intestinal microbes of shrimp and fish. It was hypothesized that bacteria on MPs in natural mariculture areas also interact with the intestinal flora of golden pompano (Trachinotus ovatus) because biofilms can form on the surface of MPs during long-term floating in seawater. To our knowledge, this study is the first to investigate MPs pollution in T. ovatus aquaculture. DNA sequencing and bioinformatics analysis confirmed the effect of microbial colonization of MPs on the intestinal flora of T. ovatus. The MPs detected in the gut wet weight (w.w.) of golden pompano (546 ± 52 items/g) were mainly pellets and fragments of blue or green, whereas the sediment MPs dry weight (d.w.) (4765 ± 116 items/kg) were mainly black fibers. The MPs richness in the sediment gradually increased from the open-sea aquaculture area to the estuarine aquaculture area and was positively correlated with the MPs richness in the intestinal tract of golden pompano. MPs 20-200 μm were the most common in the gut and sediment. The intake of MPs increased the abundance of Proteobacteria and decreased that of Firmicutes in the intestinal flora. The functional compositions of MP-colonizing microbes and gut microbiota were similar, suggesting that the two communities influence each other. Network analysis further confirmed this and revealed that Vibrio plays a key role in the intestinal flora and surface microorganisms of MPs. Overall, the intake of MPs by aquatic animals not only affects the intestinal flora and intestinal microbial function, but also poses potential risks to aquaculture.