Incipient Motion of Exposed Microplastics in an Open-Channel Flow

Impact of Fenton aging on the incipient motion of microplastic particles in open-channel flow

Upon entering the environment, Microplastics (MPs) experience aging processes that modify their properties and integrity. Previous methods for predicting the incipient motion of MPs have been validated using pristine plastics, which do not account for the effects of aging. This can lead to uncertainties in both quantification and characterization. This study investigates the effect of aging on the incipient motion of MPs with different bed roughness (smooth and rough beds) and MP properties (e.g., grain sizes and densities) in an open-channel flow. Five types of MPs were subjected to four different degrees of aging using the Fenton reagent, and their incipient velocities were tested on beds with two distinct roughness. The results suggest that the incipient velocity of MPs increases linearly with aging. However, this increase is not uniform across different particles and bed roughness. Upon comparing various commonly employed sediment incipient velocity equations, experimental results are in agreement with Ruijin Zhang's equation as the most precise. The parameters in Ruijin Zhang's equation are modified to enhance its applicability for predicting the incipient velocity of aged MPs. This study provides novel insights into the incipient motion of aged MPs in an open-channel flow, highlighting the intricate interaction between aged MP characteristics and bed roughness.

Short-term buoyant microplastic transport patterns driven by wave evolution, breaking, and orbital motion in coast

Recently, there has been a notable rise in social and scientific interest regarding microplastic pollution in coasts where waves significantly influence flow patterns and material transport. This study explores typical short-term movement of buoyant microplastics driven by surf zone processes including wave transformation, breaking, and orbital motion. To track microplastics, Lagrangian Particle Tracking Model (PTM) coupled with Eulerian wave-current interaction model appropriate for coastal hydrodynamics was used. From the simulations, several important findings were observed. (i) In alongshore uniform beaches, lighter and larger buoyant microplastics tended to reach beach more readily. (ii) Accurate predictions of microplastic transport in the surf zone required the consideration of wave breaking. (iii) In alongshore non-uniform coastal bathymetry, rip-currents can send buoyant microplastics offshore, beyond the surf zone.

Machine learning-based prediction for settling velocity of microplastics with various shapes

Microplastics can easily enter the aquatic environment and be transported between water bodies. The terminal settling velocity of microplastics, which affects their transport and distribution in the aquatic environment, is mainly influenced by their size, density, and shape. Due to the difficulty in accurately predicting the terminal settling velocity of microplastics with various shapes, this study focuses on establishing high-performance prediction models and understanding the importance and effect of each feature parameter using machine learning. Based on the number of principal dimensions, the shapes of microplastics are classified into fiber, film, and fragment, and their thresholds are identified. The microplastics of different shape categories have different optimal shape parameters for predicting the terminal settling velocity: Corey shape factor, flatness, elongation, and sphericity for the fragment, film, fiber, and mixed-shape MPs, respectively. By including the dimensionless diameter, relative density and optimal shape parameter in the input parameter combination, the machine learning models can well predict the terminal settling velocity for the microplastics of different shape categories and mixed-shape with R2 > 0.867, achieving significantly higher performance than the existing theoretical and regression models. The interpretable analysis of machine learning reveals the highest importance of the microplastic size and its marginal effect when the dimensionless diameter D* = dn(g/v2)1/3 > 80, where dn is the equivalent diameter, g is the gravitational acceleration, and ν is the fluid kinematic viscosity. The effect of shape is weak for small microplastics and becomes significant when D* exceeds 65.

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.

Interventions of river network structures on urban aquatic microplastic footprint from a connectivity perspective

Microplastic footprint in urban river networks can be disturbed by multiple urbanization features, and regional river structures are generally overlooked. In this research, we analyzed the distribution of microplastics and potential impact pattern of river structures on it in a typical urban river network in Nanjing, China. Surface waters of the river network were jointly detected by multiple methods, and the Renkonen similarity index was used to study spatial variabilities of microplastics characteristics. Microplastics were ubiquitous and abundant, showing five (>50 μm) and six (20∼50 μm) hotspots, and heterogeneities in the shape and type of microplastics larger than 100 μm were prominent, presumably influenced by river network scale and connectivity. River structure parameters associated with network connectivity were obtained by combining graph theory and an entropy-based set-pair analysis model. Aiming at the action pathway of river structures, by using correlation and partial least squares regression analysis, we found that river node (confluences and sluices) ratio, river frequency, river network density, and water system circularity were significantly positively correlated with microplastic abundance, and confluences with poor connectivity had a greater indirect intervention intensity on the microplastic distribution. The land use characteristics dominated the fitting of microplastic abundance, which was about 1.2 times better than river structures, and the comprehensive land use intensity and river network connectivity were the critical factors, respectively. Potential ecological risks of microplastics were evaluated, resulting in relatively severe levels. This study proposed targeted measures to control urban microplastic pollution by combining the perspective of river network characteristics. To summarize, our exploration of microplastic footprint based on urban river network structures from the perspective of river network connectivity provides new insights into microplastic management.

Threshold migration conditions of (micro) plastics under the action of overland flow

Surface runoff is a major pathway for the transport of plastics. However, most previous studies focus on the transport of microplastics in aquatic environment, whereas the migration of plastics from terrestrial environment to aquatic ecosystems receives limited attention. In this work, we investigated the migration of plastic on different surfaces via surface runoff. Results indicate that the threshold migration condition increases with the size and density of plastics, while decreases as the surface inclination increases. Plastics show a higher degree of mobility on smooth surfaces, but plastic films exhibit lower mobility due to the frictional forces induced by the downward pressure exerted by the water flow. Conversely, rough surfaces such as concrete and macadam can trap small fragments and rigid film plastics, and plastics can be embedded within the soil matrix under the water flow. In summary, smaller size, steeper incline, and greater water flow rate facilitate the movement of plastics on surfaces. Results from this work improve the understanding of the process of plastic migration from land to water, and are of great significance for the prevention and control of plastic pollution.