Transport behavior of micro polyethylene particles in saturated quartz sand: Impacts of input concentration and physicochemical factors

Permeable pavement blocks as a sustainable solution for managing microplastic pollution in urban stormwater

Permeable pavement systems (PPS) designed to store stormwater and facilitate its drainage into pipeline networks also provide the added advantage of retaining particulate pollutants in the stormwater runoff. Among these pollutants, microplastics (MPs), are increasingly being detected in the atmosphere and can be deposited in the environment via rainfall. Consequently, mitigating the transport of airborne MPs through rainfall is crucial for preventing water and soil contamination, thereby reducing the potential risks to human health and ecosystems. To achieve effective pollution control, an experimental study was conducted to assess MPs removal efficiencies and permeability performance of various permeable pavement blocks. The pore structure, which is a critical factor influencing permeability, was analyzed using porosity measurements and X-ray computed tomography imaging. Additionally, computational fluid dynamics (CFD) simulations were utilized to investigate the MPs removal mechanisms within the PPS, modeling the flow of MPs through blocks with distinct pore structures and varying permeability levels. Notably, Block A, with the highest permeability (1.7 mm/s), achieved removal efficiencies exceeding 90 % for polyethylene (PE) and polyethylene terephthalate (PET). CFD analysis revealed that low-density PE particles were retained more effectively, while high-density PET particles displayed greater mobility through the pavement blocks. Furthermore, as rainfall intensity increased, the removal efficiencies of PE and PET gradually decreased. This study highlights the critical role of material design and CFD-optimised pore structures in enhancing the efficacy of permeable pavement systems for urban stormwater management. By elucidating the MPs removal mechanisms driven by distinct transport behaviors of PE and PET particles based on density differences, these systems offer a promising solution for mitigating urban stormwater contamination and advancing sustainable water resource management.

Effect of carbon chain length and concentration of perfluorinated compounds on polytetrafluoroethylene microplastics transport behavior

Perfluorooctanoic acid (PFOA) and perfluoropentanoic acid (PFPeA), as important components of perfluorinated compounds (PFAS), are not only ecologically hazardous, but also have surfactant properties that can alter the transport behavior of polytetrafluoroethylene (PTFE) in porous media. In this experiment, the effect of PFAS on the transport of PTFE in porous media under different pH, ionic strength (IS) and ion valence states was studied. The results showed that the recovery rate of PTFE decreased gradually with the decrease of pH and the increase of IS and ion valence states. When the above conditions change, the double electron layer on the microplastic surface is compressed, the absolute value of zeta potential decreases, the repulsion between each other decreases, and aggregation and deposition are more likely. In addition, it was found that the recovery rate of PTFE co-transported with long chain PFOA was higher than that of short chain PFPeA. This phenomenon may be caused by the adhesion ability of PFOA with long carbon chain on the surface of PTFE is greater than that of PFPeA with short carbon chain. On the other hand, PFAS with different carbon chain lengths produce different spatial site resistance effects after binding with particles, and the spatial site resistance produced by the long-chain PFOA is larger than that of the short-chain PFPeA, leading to a decrease in particle-to-particle aggregation and a better transport effect. This study will help to understand the effects of PFAS with different carbon chain lengths on the transport of microplastics in porous media, as well as the transport rule of PTFE under different conditions, and provide reference value for the calculation of its flux in soil.

Microplastics in soils: A comprehensive review

Microplastics (MPs) have become pervasive pollutants in terrestrial ecosystems, raising significant ecological risks and human health concerns. Despite growing attention, a comprehensive understanding of their quantification, sources, emissions, transport, degradation, and accumulation in soils remains incomplete. This review synthesizes the current knowledge on the anthropogenic activities contributing to soil MP contamination, both intentional and unintentional behaviors, spanning sectors including agriculture, domestic activities, transportation, construction, and industry. Furthermore, it examines the spatial distribution, accumulation, and abundance of MPs across various land use types, alongside a critical assessment of existing quantification methodologies. While the predominant metric for MP quantification is particle number concentration, integrating mass and area concentration enhances the ability to compare pollution levels, assess fluxes, and conduct risk analyses. Additionally, the review explores the transport behavior of MPs in soil, distinguishing between external mechanisms (abiotic factors: wind, leaching, and runoff, biotic factors: soil bioturbation and food chain interactions), and internal mechanisms that are impacted by the characteristics of MPs themselves (e.g., shape, color, size, density, surface properties), soil properties (e.g., porosity, pH, ionic strength, organic matter and mineral content), coexisting substances, and soil structural dynamics. The study of MP transport in soil remains in its early stages, with substantial gaps in knowledge. Future research should focus on integrating number, mass concentration, and area concentration for the more holistic quantification of MP abundance, and prioritize the development of more accurate and efficient methodologies. In addition, the investigation of MP transport and degradation processes under varying environmental conditions and soil management practices is critical for addressing this emerging environmental challenge.

Rainfall-induced microplastic fate and transport in unsaturated Dutch soils

Microplastic pollution has become a growing concern in terrestrial ecosystems, with significant implications for environmental and human health. Understanding the fate and transport of microplastics in soil environment is crucial for effective mitigation strategies. This study investigates the dynamics of microplastic (Low-density polyethylene (LDPE), polybutylene adipate terephthalate (PBAT), and starch-based biodegradable plastic) transport in unsaturated soils under varying rainfall intensities and soil types, aiming to elucidate the factors influencing their behavior. Effluent samples were analyzed to measure microplastic transport, with microplastic balance analysis ensuring experimental accuracy. The setup replicated real-world flow conditions, providing insights into microplastic transport in unsaturated porous media. Microplastic balance analysis revealed high recovery factors (between 64 % and 104 %), indicating the reliability of the experimental approach. Microplastic transport varied significantly between sandy loam and loamy sand soils, with loamy sand soils exhibiting higher wash-off rates due to their unique properties. LDPE microplastics showed a higher tendency to detach from soil columns compared to PBAT and starch-based particles. Higher rainfall intensity in loamy sand soil columns resulted in an increased washout of LDPE, PBAT, and starch-based particles by 92 %, 144 %, and 85 %, respectively, compared to low rainfall intensity. In sandy loam soil, increased rainfall intensity resulted in a significantly higher washout of LDPE, PBAT, and starch-based particles with percentages of 93 %, 69 %, and 45 %, respectively. This underscores the important role of water flow in mobilizing microplastics within the soil matrix.

Factors Influencing the Vertical Migration of Microplastics up and down the Soil Profile

Soil ecosystems are under serious threat from microplastics (MPs), and this is causing worldwide concern. The relationship between soil and MPs has become a popular research topic, and the vertical migration of soil MPs is of increasing interest. This Review summarizes the current status of research into the factors affecting the vertical migration of soil MPs. Published research shows that the characteristics of MPs and the physicochemical properties of the soil affect the infiltration process. Soil organisms play a key role in the vertical migration by acting as vectors or as a result of adsorption. Dissolved organic matter and metal oxides transfer MPs by adsorption-desorption. In addition, rainfall and dry-wet cycles alter the mobility of soil MPs, leading to changes in migration processes. Agricultural activities such as tillage and irrigation may distribute MPs throughout the topsoil. Vertical migration of soil MPs is a process influenced by a combination of factors, and the role of these factors in MP deposition needs to be explored further.

Advanced characterisation of groundwater contamination at a dumpsite: Methodology and assessment - Case study of a municipal solid waste dumpsite in India

Groundwater (GW) contamination due to municipal solid waste (MSW) disposal in open dumpsites is a pressing global issue. Traditional GW assessment studies are limited to single-depth sampling from nearby wells/handpumps, providing limited insights into subsurface soil characteristics and are prone to cross-contamination. The present study introduces an innovative methodology integrating advanced techniques: Cone Penetration Testing (CPT), Hydraulic Profiling Tool (HPT), Continuous Soil Sampling, and Discrete GW Sampling. Conducted at an operational dumpsite in New Delhi, India, from January to February 2023, the site investigation program covered seven distinct locations to incorporate the entire dumpsite area. The investigation proceeded in stages, starting with subsurface soil characterisation using CPT and HPT, followed by extracting soil and GW samples using CPT and HPT data. Due to restrictions in the northeast direction, GW samples were directly extracted from borewells. The results revealed maximum and minimum concentrations of 171 items/l and 26 items/l of MPs, while ionic concentration reached 13,200 ppm for Cl- and 4437 ppm for SO4-2. A maximum of 0.721 ppm, 0.663 ppm and 0.948 ppm concentration was observed for Ni, Cu and Mo in GW samples. Spearman correlation and principal component analysis underscore the influence of Ec, TDS, Na+1 and Cl-1 on GW quality. This integrated approach effectively identifies high-permeability layers, which are crucial for understanding contaminant dispersion, and ensures precise sampling at various depths with minimal cross-contamination. This research demonstrates the proposed methodology's effectiveness in providing more profound and precise insights into GW contamination dynamics and suggests its utility in forming the basis for more effective remediation and regulatory strategies.

Impact of freeze-thaw cycles on the remobilization behaviors of microplastics in natural soils

Freeze-thaw (FT) cycle would greatly influence the fate of plastic particles (one of emerging contaminants with great concerns) in soils, yet its impacts and mechanisms remain unclear. The vertical migration/release behaviors of plastic particles (with diameters of 0.2 μm and 1 μm) in two natural soils and one model soil (i.e. quartz sand) without/with FT treatments (1 and 3 cycles) were examined. Owing to the presence of Fe/Al oxide minerals, finer pore structure, and uneven surfaces of natural soils, the breakthrough ratio of plastics in two natural soils was over 25% lower than in quartz sand. However, regardless of porous media type, FT processes (increasing cycles) significantly promoted the remobilization of plastics initially retained in three media during the subsequent water flushing processes. Via theoretical calculation, tracer experiments, and visible chamber experiments, the mechanisms driving plastics release from natural soils subjected to FT treatments during the water elution processes were determined to be different from those from pure quartz sand. The change of sand local configuration (the rearrangement of local sand pore spaces) during FT process mainly drove to plastics released from quartz sand columns. While the alteration in local soil configuration, the formation of preferential pathways, and increased release of soil particles contributed to plastics remobilization from soil columns subjected to FT. Clearly, FT processes significantly increased the vertical migration of plastics in soils potentially to groundwater, enhancing environmental risks of plastics.

Perfluorobutanoic acid weakens the heterogeneous aggregation of microplastics and microalgae: Perspective from physicochemical properties, extracellular polymeric substances secretion and DLVO theory

Microplastics (MPs) and per- and poly-fluoroalkyl substances extensively coexist in aquatic environments and potentially endanger organisms. Microalgae may decrease the effective concentration of pollutants via hetero-aggregation with MPs and adsorption of emerging contaminants. However, the potential influence of coexistent pollutants on hetero-aggregation of MPs and microalgae remains unknown. This study investigated the hetero-aggregation process involving different sizes of polystyrene (PS, 3.0 and 50.0 μm) with Chlorella sorokiniana (C. sorokiniana) in the presence or absence of perfluorobutanoic acid (PFBA) along settling experiments, scanning electron microscope, and Derjaguin-Landau-Verwey-Overbeek (DLVO) model. We found that the hetero-aggregation between C. sorokiniana and 3 μm PS was more pronounced than with 50 μm PS, while PFBA inhibited this process. ΔOD1 values (reflected hetero-aggregation level) for 3PS-cells and 50PS-cells were 0.189 and 0.087, respectively, and PFBA decreased these values to 0.134 and 0.033. Furthermore, extracellular polymeric substances, known as inducer of hetero-aggregation, increased by 14.33% when exposed to 3 μm PS alone, whereas the co-exposure group showed a decrease of 4.52% compared to 3PS-cells group. PFBA also significantly decreased the protein/polysaccharide ratios in both MPs sizes, reducing hetero-aggregation. DLVO theory revealed that microalgae lowered the energy barrier significantly, while PFBA elevated it, indicating that hetero-aggregation was inhibited by PFBA. This study provides new perspectives for pollutant removal and toxicity variation in aquatic environments.

Global distribution, drivers, and potential hazards of microplastics in groundwater: A review

Since microplastics (MPs) were first detected in groundwater, an increasing number of studies have focused on groundwater pollution by MPs. However, knowledge of the global properties of groundwater MPs: distribution, concentration, composition, and morphology remains limited, while potential factors regulating their transport and distribution in groundwater, especially the hydrogeological background and climate warming conditions, have been omitted from most analyses. Furthermore, previous field investigations did not assess the risks posed by groundwater MPs to the environment and to human health, a necessary preliminary to remediation. In this work, to promote future MP pollution studies and remediation policies, we assimilated and synthesized the current knowledge on this topic. We reviewed current data on global groundwater pollution by MPs, analyzed the driving factors of their transport and distribution, and summarized the ecological and health hazards posed by MPs, before discussing current knowledge limits and suggesting perspectives for future work.

Pore-Scale Visualized Transport and Retention of Fibrous and Fragmental Microplastics in Porous Media under Various Surfactant Conditions

For advancing current knowledge on the transport of microplastics (MPs) in the environment, this study used a real-time pore-scale visualization and quantitative system to examine the motions and mobility of fibrous and fragmental MPs under various surfactant (AEO, CTAC, and AES) and electrolyte conditions. The videos showed that fibrous MPs formed tangles through entanglement, which moved in an axial direction aligned with the flow streamline. Both fibrous and fragmental MPs showed suspended movement as well as surface movement (e.g., sliding, rolling, and saltating) in the porous media. Some deposited fibrous MPs showed flexible deformation due to shear flow. Compared to fragmental MPs, fibrous MPs showed lower mobility due to the tendency to deposit and clog the porous media. The mobility of fragmental MPs was enhanced in the presence of AEO but remained relatively unchanged with AES. In the presence of CTAC, the mobility of fragmental MPs was slightly inhibited under low ionic strength (IS) conditions but remarkably enhanced under high IS conditions. However, the mobility of fibrous MPs was largely unaffected by the surfactants. Both the numerical model and FDLVO calculations effectively described the transport and deposition of MPs in porous media.

Effect of background ions and physicochemical factors on the cotransport of microplastics with Cu2+ in saturated porous media

Microplastics (MPs) in subsurface environments are migratory and can carry heavy metals, increasing the extent of MP and heavy metal pollution. This study used quartz sand-filled column experiments to investigate the adsorption and cotransport behaviours of PS-MPs, O3, UV-aged PS-MPs, and Cu2+ at different MP concentrations, ionic strengths, and ionic valences in a saturated porous medium. The results showed that when MPs migrate alone in the absence of an ionic background, higher concentrations have increased mobility. In contrast, an increase in the background ion concentration or ion valence inhibits the individual transport capacity of PS-MPs. An increase in the concentration of background ions or elevation in the valence state promotes Cu2+ transport because of the action of the double electric layer on the surface of the colloid and the electrostatic repulsive forces combined with the background ions. The adsorption capacity of aged PS-MPs was stronger than that of PS-MPs because of the binding of the aged PS-MPs to Cu2+ through complexation and electrostatic attraction. In the binary system of PS-MPs/Cu2+, PS-MPs promoted Cu2+ transport and the mobility of Cu2+ loaded by PS-MPs decreased with increasing background ion concentration. The cotransport results showed that MPs promote Cu2+ transport in the following order: O3-aged Ps > UV-aged Ps > Ps, as the increasing cation concentration in the MPs and Cu2+ occupies the PS surface adsorption sites. Overall, PS is an effective carrier for Cu2+. These findings offer fresh exploration concepts for the joint migration of MPs and heavy metals in underground settings.

Effect of shape on the transport and retention of nanoplastics in saturated quartz sand

Nanoplastics (NPs) pose great challenges to soil-groundwater systems. This study investigated the transport and retention of self-synthesized 0.5-μm polystyrene NPs with different shapes using column experiments. The regular NPs were with spherical shapes, while the irregular NPs were with toroid-like shapes. The toroid-like shapes were the irregular shapes (with low aspect ratio) which have not been studied yet. The explorations were carried out in both 5-25 mM NaNO3 and 1-10 mM Ca(NO3)2 solutions. Both breakthrough curves (BTCs) and retained profiles (RPs) were monitored. Our findings uncovered a clear disparity in the transport of irregular and regular NPs, with irregular particles exhibiting lower transport ability compared to the regular ones. For example, the average breakthrough plateaus of the regular and irregular NPs were ∼0.9 and ∼0.5, respectively, in 10 mM NaNO3. In-depth theoretical analysis indicated that the lower XDLVO interaction energy barrier between the irregular NPs and quartz sand was one factor, and the greater margination of irregular NPs on quartz sand, as verified by the numerical simulation, was another factor leading to the decreased transport and increased retention of the irregular NPs. The obtained results highlighted the significance of considering particle shape in future modelling and predicting the fate of NPs in real environmental circumstances.

Differential carbon accumulation of microbial necromass and plant lignin by pollution of polyethylene and polylactic acid microplastics in soil

The wide microplastics (MPs) occurrence affects soil physicochemical and biological properties, thereby influencing its carbon cycling and storage. However, the regulation effect of MPs on soil organic carbon (SOC) formation and stabilization remains unclear, hindering the accurate prediction of carbon sequestration in future global changes under continuous MP pollution. Phospholipid fatty acids, amino sugars and lignin phenols were used in this study as biomarkers for microbial community composition, microbial necromass and plant lignin components, respectively, and their responses to conventional (polyethylene; PE) and biodegradable (polylactic acid; PLA) MPs were explored. Results showed PLA MPs had positive effects on soil microbial biomass, while the positive and negative effects of PE MPs on microbial biomass varied with MP concentration. PE and PLA MPs increased microbial necromass contents and their contribution to SOC, mainly due to the increase in fungal necromass. On the contrary, PE and PLA MPs reduced lignin phenols and their contribution to SOC, mainly owing to the reduction in vanillyl-type phenols. The response of microbial necromass to PLA MPs was higher than that to PE MPs, whereas the response of lignin phenols was the opposite. MPs increased SOC level, with 83%-200% and 50%-75% of additional SOC in PE and PLA treatments, respectively, originating from microbial necromass carbon. This finding indicates that the increase in SOC pool in the presence of MPs can be attributed to soil microbial necromass carbon, and MPs increased capacity and efficacy of microbial carbon pump by increasing microbial turnover and reducing microbial N limitation. Moreover, the increase in amino sugars to lignin phenols ratio in PE treatment was higher than that in PLA treatment, and the increase in SOC content in PLA treatment was higher than that in PE treatment, indicating a high possibility of SOC storage owing to PLA MPs.

Traversing the prevalence of microplastics in soil-agro ecosystems: Origin, occurrence, and pollutants synergies

The ubiquity of plastics in modern life has made them a significant environmental concern and a marker of the Anthropocene era. The degradation of plastics results in the formation of microplastics (MPs), which measure 5 mm or less. The coexistence of MPs with other pollutants found in sludge, water treatment plant effluents, surface water, and groundwater, shapes the environmental landscape together. Despite extensive investigation, the long-term implications of MPs in soils remain uncertain, underscoring the importance of delving into their transportation and interactions with soil biota and other contaminants. The present article provides a comprehensive overview of MPs contamination in soil, encompassing its sources, prevalence, features, and interactions with soil flora and fauna, heavy metals, and organic compounds. The sources of MPs in soil agroecosystems are mulching, composting, littering, sewage sludge, irrigation water, and fertilizer application. The concentration of MPs reported in plastic mulch, littering, and sewage sludge is 503 ± 2760 items per kg-1, 4483 ± 2315 MPs/kg, and 11,100 ± 570 per/kg. The transport of MPs in soil agroecosystems is due to their horizontal and vertical migration including biotic and abiotic mobility. The article also highlighted the analytical process, which includes sampling planning, collection, purification, extraction, and identification techniques of MPs in soil agroecosystems. The mechanism in the interaction of MPs and organic pollutants includes surface adsorption or adhesion cation bridging, hydrogen bonding, charge transfer, ligand exchange, van der Waals interactions, and ion exchange.

Fate and transport of fragmented and spherical microplastics in saturated gravel and quartz sand

Microplastics in urban runoff undergo rapid fragmentation and accumulate in the soil, potentially endangering shallow groundwater. To improve the understanding of microplastic transport in groundwater, column experiments were performed to compare the transport behavior of fragmented microplastics (FMPs ∼1-µm diameter) and spherical microplastics (SMPs ∼1-, 10-, and 20-µm diameter) in natural gravel (medium and fine) and quartz sand (coarse and medium). Polystyrene microspheres were physically abraded with glass beads to mimic the rapid fragmentation process. The experiments were conducted at a constant flow rate of 1.50 m day-1 by injecting two pore volumes of SMPs and FMPs. Key findings indicate that SMPs showed higher breakthrough, compared to FMPs in natural gravel, possibly due to size exclusion of the larger SMPs. Interestingly, FMPs exhibited higher breakthrough in quartz sand, likely due to tumbling and their tendency to align with flow paths, while both sizes (larger and smaller relative to FMPs) of SMPs exhibited higher removal in quartz sand. Therefore, an effect due to shape and size was observed.

Migration characteristics of microplastics in riparian soils and groundwater

Investigations have revealed the presence of microplastics in both soil and groundwater, but the migration characteristics from soil to groundwater remain incompletely understood. In this study, two sampling sections consisting of soil-groundwater-river water were established near Lianxi Bridge and Xilin Bridge along the Jiuxi River in Xiamen. A total of 22 soil samples, 36 groundwater samples, and 18 river water samples were collected. Microplastics were detected in all samples with an abundance range of 392-836 n/kg in soil (mean, 655 ± 177 n/kg), 0.58-2.48 n/L groundwater (mean, 1.23 ± 0.42 n/L), and 0.38-1.80 n/L in river water (mean, 0.86 ± 0.41 n/L). Flakes predominantly constituted the shape of microplastics found in soil, while fibers dominated those present in water. Black, yellow, and red were the dominant color types. Polyamide (PA) and polyethylene (PE) were the main components of microplastics within soils, whereas polyethylene terephthalate (PET), polypropylene (PP), and PA prevailed within water. Microplastic particle sizes ranged from 39 to 2498 μm in soils, mainly from 29 to 3394 μm in water. The upstream section displayed higher abundances of microplastic compared to the downstream, revealing the soil particles having an intercepting effect on microplastics. The distribution and migration of microplastics in soil and groundwater are affected by many factors, including natural and anthropogenic factors, such as soil depth, soil properties, pore structure, hydrodynamics, hydraulic connections between groundwater and surface water, the extensive utilization and disposal of plastics, irrational exploitation of groundwater, and morphology and types of microplastics. These research findings contribute to a better understanding of the pathways, migration capacity, and influencing factors associated with microplastic entry into groundwater, thereby providing valuable technical support for the development of strategies aimed at controlling microplastic pollution.

Impact of Type and Shape of Microplastics on the Transport in Column Experiments

The pervasive nature of plastic and the longevity of plastics leaves a legacy of microplastics (MPs) that contaminate our environment, including drinking water sources. Although MPs have been documented in every environmental setting, a paucity of research has focused on the transport and fate of MPs in groundwater. Previous field and laboratory studies have shown that MPs can migrate through aquifer material and are influenced by environmental factors. This study used controlled column experiments to investigate the influence of polymer type (polyamide, polyethylene, polypropylene, and polyester) and particle shape (fragment, fiber, and sphere) on MP retardation and retention. The results showed that all individual MP types investigated were retarded compared to the NaCl tracer, with a retardation factor ranging from 1.53 to 1.75. While hypothesized that presence of multiple types and shapes could change mobility, the results indicate that this hypothesis is not correct for the conditions tested. This study provides new insights into MP transport in groundwater systems based on the characteristics of MP particles. In addition, this study demonstrates the need for further research on types of MPs and under more conditions, especially in the presence of a mixture of types and shapes of MPs to gauge what is occurring in natural systems where many MPs are present together.

Comparative study of polystyrene microplastic transport behavior in three different filter media: Quartz sand, zeolite, and anthracite

Microplastics (MPs) are emerging contaminants that are attracting increasing interest from researchers, and the safety of drinking water is greatly affected by their transportation during filtration. Polystyrene (PS) was selected as a representative MPs, and three filter media (quartz sand, zeolite, and anthracite) commonly found in water plants were used. The retention patterns of PS-MPs by various filter media under various background water quality conditions were methodically investigated with the aid of DLVO theory and colloidal filtration theory. The results show that the different structures and elemental compositions of the three filter media cause them to exhibit different surface roughnesses and surface potentials. A greater surface roughness of the filter media can provide more deposition sites for PS-MPs, and the greater surface roughness of zeolite and anthracite significantly enhances their ability to inhibit the migration of PS-MPs compared with that of quartz sand. However, surface roughness is not the only factor affecting the migration of MPs. The lower absolute value of the surface potential of anthracite causes the DLVO energy between it and PS-MPs to be significantly lower than that between zeolite and PS-MPs, which results in stronger retention of PS-MPs by anthracite, which has a lower surface roughness, than zeolite, which has a higher surface roughness. The transport of PS-MPs in the medium is affected by the combination of the surface roughness of the filter media and the DLVO energy. Under the same operating conditions, the retention efficiencies of the three filter materials for PS-MPs followed the order of quartz sand < zeolite < anthracite. Additionally, the conditions of the solution markedly influenced the transport ability of PS-MPs within the simulated filter column. The transport PS-MPs in the simulated filter column decreased with increasing solution ionic strength and cation valence. Naturally, dissolved organic matter promoted the transfer of PS-MPs in the filter layer, and humic acid had a much stronger facilitating impact than fulvic acid. The study findings might offer helpful insight for improving the ability of filter units ability to retain MPs.

Vertical distribution of microplastics in an agricultural soil after long-term treatment with sewage sludge and mineral fertiliser

Sewage sludge applications release contaminants to agricultural soils, such as potentially toxic metals and microplastics (MPs). However, factors determining the subsequent mobility of MPs in long-term field conditions are poorly understood. This study aimed to understand the vertical distribution of MPs in soils amended with sewage sludge in comparison to conventional mineral fertiliser for 24 years. The depth-dependent MP mass and number concentrations, plastic types, sizes and shapes were compared with the distribution of organic carbon and metals to provide insights into potentially transport-limiting factors. Polyethylene, polypropylene and polystyrene mass concentrations were screened down to 90 cm depth via pyrolysis-gas chromatography/mass spectrometry. MP number concentrations, additional plastic types, sizes, and shapes were analysed down to 40 cm depth using micro-Fourier transform-infrared imaging. Across all depths, MP numbers were twice and mass concentrations 8 times higher when sewage sludge was applied, with a higher share of textile-related plastics, more fibres and on average larger particles than in soil receiving mineral fertiliser. Transport of MPs beyond the plough layer (0-20 cm) is often assumed negligible, but substantial MP numbers (42 %) and mass (52 %) were detected down to 70 cm in sewage sludge-amended soils. The initial mobilization of MPs was shape- and size-dependent, because the fractions of fragmental-shaped and relatively small MPs increased directly below the plough layer, but not at greater depths. The sharp decline of total MP concentrations between 20 and 40 cm depth resembled that of metals and organic matter suggesting similar transport limitations. We hypothesize that the effect of soil management, such as ploughing, on soil compactness and subsequent transport by bioturbation and via macropores drives vertical MP distribution over long time scales. Risk assessment in soils should therefore account for considerable MP displacement to avoid underestimating soil exposure.

Bark and biochar in horizontal flow filters effectively remove microplastics from stormwater

Organic materials such as bark and biochar can be effective filter materials to treat stormwater. However, the efficiency of such filters in retaining microplastics (MPs) - an emerging stormwater pollutant - has not been sufficiently studied. This study investigated the removal and transport of a mixture of MPs commonly associated with stormwater. Different MP types (polyamide, polyethylene, polypropylene, and polystyrene) were mixed into the initial 2 cm material of horizontal bark and biochar filters of 25, 50, and 100 cm lengths. The MP types consisted of spherical and fragmented shapes in size ranges of 25-900 μm. The filters were subjected to a water flow of 5 mL/min for one week, and the total effluents were analyzed for MPs by μFTIR imaging. To gain a deeper insight, one 100 cm bark filter replica was split into 10 cm segments, and MPs in each segment were extracted and counted. The results showed that MPs were retained effectively, >97%, in all biochar and bark filters. However, MPs were detected in all effluents regardless of filter length. Effluent concentrations of 5-750 MP/L and 35-355 MP/L were measured in bark and biochar effluents, respectively, with >91% of the MP counts consisting of small-sized (25 μm) polyamide spherical particles. Combining all data, a decrease in average MP concentration was noticed with longer filters, likely attributed to channeling in a 25 and 50-cm filter. The analyses of MPs in the bark media revealed that most MPs were retained in the 0-10 cm segment but that some MPs were transported further, with 19% of polyamide retained in the 80-90 cm segment. Overall, this study shows promising results for bark and biochar filters to retain MPs, while highlighting the importance of systematic packing of filters to reduce MP emissions to the environment from polluted stormwater.

Groundwater systems under siege: The silent invasion of microplastics and cock-tails worldwide

Microplastics (MPs) contamination is one of the significant escalating environmental concerns worldwide, and this stems from the increasing production and unlawful disposal of plastic materials. Regretfully, the synthesis of plastic materials is expected to triple in the upcoming years. Nevertheless, MPs pollution in marine, aquatic, and terrestrial settings has received much attention, unlike in groundwater systems. This study exhaustively reviewed varying degrees of recent publications in various search engines and provided a detailed state of current knowledge and research progress vis-à-vis MPs and cock-tail pollution in groundwater systems. Evidently, groundwater sources are severely contaminated as a result of growing anthropogenic activities and vertical movement of MPs and cock-tails from the atmospheric, terrestrial, and aquatic environments, however, fewer researchers have fixated their attention on estimating the occurrence of MPs in groundwater resources, while sufficient information regarding their sources, sampling methods, abundance, transport pathways, fate, modeling techniques, appropriate and adequate data, sorption properties, separation from other environmental media, toxicity, and remedial measures are extensively lacking. In addition, MPs may combine with other toxic emerging contaminants to improve migration and toxicity; however, no research has been conducted to fully understand cock-tail migration mechanisms and impacts in groundwater systems. Over time, groundwater may be regarded as the primary sink for MPs, if effective actions are neglected. Overall, this study detected a lack of concern and innumerable voids in this field; hence, vital and nascent research gaps were identified for immediate, advanced, and interdisciplinary research investigations.

Microplastics/nanoplastics in porous media: Key factors controlling their transport and retention behaviors

Till now, microplastics/nano-plastics(M/NPs) have received a lot of attention as emerging contaminant. As a typical but complex porous medium, soil is not only a large reservoir of M/NPs but also a gateway for M/NPs to enter groundwater. Therefore, the review of the factors controlling the transport behavior of M/NPs in porous media can provide important guidance for the risk assessment of M/NPs in soil and groundwater. In this study, the key factors controlling the transport behavior of M/NPs in porous media are systematically divided into three groups: (1) nature of M/NPs affecting M/NPs transport in porous media, (2) nature of flow affecting M/NPs transport in porous media, (3) nature of porous media affecting M/NPs transport. In each group, the specific control factors for M/NPs transport in porous media are discussed in detail. In addition to the above factors, some substances (colloids or pollutants) present in natural porous media (such as soil or sediments) will co-transport with M/NPs and affect its mobility. According to the different properties of co-transported substances, the mechanism of promoting or inhibiting the migration behavior of M/NPs in porous media was discussed. Finally, the limitations and future research directions of M/NPs transport in porous media are pointed out. This review can provide a useful reference for predicting the transport of M/NPs in natural porous media.

Decreased transport of nano- and micro-plastics in the presence of low-molecular-weight organic acids in saturated quartz sand

Low-molecular-weight organic acids (LMWOAs) and nano- and micro-plastics (NPs and MPs) are both widely distributed in terrestrial systems. To better understand the influence of LMWOAs on the transport of NPs and MPs, the effects of 0.5 mM citric- (CA), malic- (MA), and tartaric- (TA) acid on the transport of nano- (0.51 μm, PS NPs) and micro- (1.1 μm, PS MPs) polystyrene particles (2 mg L-1) in saturated quartz sand were investigated. All three LMWOAs decreased the transport of PS NPs and MPs, regardless of ionic composition or strength (0.1-10 mM NaCl and 0.1-1 mM CaCl2). Further investigation revealed that the interfacial interactions between PS-quartz sand surfaces and PS-PS were altered by LMWOAs. LMWOAs adsorbed to quartz sand surfaces could serve as new deposition sites, as evidenced by the decreased transport of PS NPs and MPs in quartz sand that was subjected to pre-equilibration with selected MA, the low inhibition of PS transport with low concentrations of LMWOAs (0.1 mM), and also the adsorption of LMWOAs onto quartz sand surfaces by batch experiments. Meanwhile, the adsorption of LMWOAs on PS, hydrodynamic measurement and visual TEM observation together clarified the slight aggregation of PS NPs and MPs in suspensions, inducing the subsequent decrease in transport. Among them, the adsorption of LMWOAs onto quartz sand surfaces was found to be the main factor dominating the decreased transport of both PS NPs and MPs in saturated quartz sand.

Experimental and simulated microplastics transport in saturated natural sediments: Impact of grain size and particle size

Microplastics (MPs) present in terrestrial environments show potential leaching risk to deeper soil layers and aquifer systems, which threaten soil health and drinking water supply. However, little is known about the environmental fate of MPs in natural sediments. To examine the MPs transport mechanisms in natural sediments, column experiments were conducted using different natural sediments and MPs (10-150 µm) with conservative tracer. Particle breakthrough curves (BTCs) and retention profiles (RPs) were numerically interpreted in HYDRUS-1D using three different models to identify the most plausible deposition mechanism of MPs. Results show that the retention efficiency for a given particle size increased with decreasing grain size, and RPs exacerbated their hyper-exponential shape in finer sediments. Furthermore, the amounts of MPs present in the effluent increased to over 85 % as MPs size decreased to 10-20 µm in both gravel and coarse sand columns, while all larger MPs (125-150 µm) were retained in the coarse sand column. The modeling results suggested that the blocking mechanism becomes more important with increasing particle sizes. In particular, the attachment-detachment without blocking was the most suited parameterization to interpret the movement of small MPs, while a depth-dependent blocking approach was necessary to adequately describe the fate of larger particles.

Prolonged drying impedes the detachment of microplastics in unsaturated substrate: Role of flow regimes

The detachment of microplastics (MPs) from porous media under different moisture conditions and flow regimes has garnered limited attention within the research community. The present study investigates the detachment of MPs from porous media under wet and dry conditions combined with steady and transient flow. For both the wet and dry conditions, the increase in flow rates is found to decrease the detachment of hydrophobic polyethylene of two sizes and of hydrophilic polymethylmethacrylate. Intermittent flow is found to result in effluent peaks and a higher rate of MP detachment compared to steady flow. The ionic strength of inflow drops in a stepwise manner, leading to abrupt peaks followed by a tail corresponding to the arrival of each ionic strength front. Each step increase in flow rate leads to a steep peak followed by slow release over several pore volumes. Although transient flow facilitates the detachment of MPs, drying significantly impedes the detachment of MPs irrespective of flow regime. Ultraviolet weathering of MPs for 60 days weakens the inhibition effect of drying on hydrophilic polymethylmethacrylate, facilitating their detachment. Furthermore, the release of MPs decreases markedly with an increase in air-drying duration from 0 h to 72 h. Hydrus-1D two-site kinetic models are used to successfully simulate time-dependent processes, implying that drying heightens the energy barrier for MPs to detach. Our analysis confirms the significance of moisture in determining the remobilization of MPs, providing valuable insights concerning the fate of MPs in unsaturated substrate under prolonged drought conditions.

Retention and Transport of Nanoplastics with Different Surface Functionalities in a Sand Filtration System

The efficiency of sand filtration was investigated in terms of the behavior of the nanoplastics (NPLs) with different surface functionalities. The initial condition concentrations of NPLs were varied, and their effects on retention and transport were investigated under a constant flow rate in saturated porous media. The behavior of NPLs in this porous system was discussed by considering Z- average size and zeta (ζ) potential measurements of each effluent. The retention efficiencies of NPLs were ranked as functionalized with amidine [A-PS]+ > with sulfate [S-PS]- > with surfactant-coated amidine [SDS-A-PS]-. The reversibility of the adsorption process was revealed by introducing surfactant into the sand filter system containing adsorbed [A-PS]+ at three different initial state concentration conditions. The deposition behavior on sand grain showed that positively charged NPLs were attached to the quartz surface, and negatively charged NPLs were attached to the edge of the clay minerals, which can be caused by electrical heterogeneities. The homoaggregates made of positively charged NPLs were more compact than those made of negatively charged NPLs and surfactant-coated NPLs. An anti-correlation was revealed, suggesting a connection between the fractal dimension (Df) of NPL aggregates and retention efficiencies. Increased Df values are associated with decreased retention efficiencies.The findings underscore the crucial influence of NPL surface properties in terms of retention efficiency and reversible adsorption in the presence of surfactants in sand filtration systems.

Limited effects of different real groundwaters from three coastal cities in China on the transport of low-concentration nanoplastics in quartz sand

Nanoplastics (NPs) have been widely detected in soil-groundwater systems. However, to date, the effect of real groundwater on the fate and transport of NPs has been poorly understood. In this study, the transport and retention behaviors of both polystyrene and poly(lactic-co-glycolic acid) NPs (PS NPs and PLGA NPs) in different real groundwaters from three coastal cities in China were explored using column experiments. PS (0.51 and 1.1 μm) and PLGA (1 μm) NPs with a low concentration of 2 mg L-1 were employed. Close observation showed that the transport of PS NPs was much higher than PLGA NPs in different groundwaters, with an average breakthrough curve plateau (C/Co) of ∼0.81 for PS NPs and ∼0.19 for PLGA NPs, respectively. As observed for PLGA, the plastic shape- and size-induced straining may be the reason for the minimal transport. Interestingly, we found that although the physicochemical characteristics of different real groundwaters varied significantly, the transport of certain NPs in real groundwater was similar with negligible differences. Closer inspection indicated that similar pHs of different groundwaters may be the reason contributing to these findings. Further investigation revealed that the transport behaviors of PS and PLGA NPs in real groundwater did not follow the classical DLVO theory. These findings suggest that the fate and transport of NPs in real soil-groundwater systems are much more comprehensive than the prediction based on DLVO theory and need intensive investigation.

Transport of polystyrene nanoplastics in porous media: Combined effects of two co-existing substances

Both surfactants and natural organic matters (NOMs) are substances commonly found in aqueous environments, and their effects on the transport of nanoplastics that is gradually gaining widespread attention in porous media are currently in their infancy, while their combined effects are absent. We investigated innovatively the combined effect of surfactants and NOMs on the transport of polystyrene nanoplastics (PS-NPs) in saturated porous media. Adsorption tests of surfactants and NOMs onto PS-NPs, adsorption tests of PS-NPs onto quartz sand, and transport tests of PS-NPs in saturated quartz sand were conducted. Hydrophobicity and Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy were measured and calculated. A mathematical model was employed to fit the transport of PS-NPs in porous media. It was found that the effects and action mechanisms of cationic cetyl trimethylammonium bromide (CTAB) and anionic sodium dodecylbenzene sulfonate (SDBS) on the transport of PS-NPs in porous media were distinct. In the presence of CTAB, 1 mg/L humic acid (HA) and 10 mg/L sodium alginate (SA) could promote aggregation of PS-NPs by decreasing the absolute zeta potential of PS-NPs, and reducing the energy barrier between PS-NPs and porous media and increasing the blocking and straining, thus inhibiting the transport of PS-NPs. In the presence of SDBS, SA and HA could improve the adsorption of SDBS onto PS-NPs by bridging and increasing adsorption sites, thus increasing the hydrophilicity of PS-NPs and improving the transport of PS-NPs. Whether or not NOMs were present, the transport of PS-NPs in porous media was mainly governed by the DLVO interaction energy in the presence of cationic surfactants and by hydrophobicity in the presence of anionic surfactants. This innovative observation has led to an understanding on the environmental behaviour of nanoplastics in porous media under complex environments.

How effective is the retention of microplastics in horizontal flow sand filters treating stormwater?

Microplastics accumulate in stormwater and can ultimately enter freshwater recipients, and pose a serious risk to aquatic life. This study investigated the effectiveness of lab-scale horizontal flow sand filters of differing lengths (25, 50 and 100 cm) in retaining four types of thermoplastic microplastics commonly occurring in stormwater runoff (polyamide, polyethylene, polypropylene, and polyethylene terephthalate). Despite the differences in particle shape, size and density, the study revealed that more than 98% of the spiked microplastics were retained in all filters, with a slightly increased removal with increased filter length. At a flow rate of 1 mL/min and after one week of operation, 62-84% of the added microplastics agglomerated in the first 2 cm of the filters. The agglomerated microplastics included 96% of high-density fibers. Larger-sized particles were retained in the sand media, while microplastics smaller than 50 μm were more often detected in the effluent. Microplastics were quantified and identified using imaging based micro Fourier Transform Infrared Spectroscopy. The efficient retention of microplastics in low-flow horizontal sand filters, demonstrated by the results, highlights their potential importance for stormwater management. This retention is facilitated by various factors, including microplastic agglomeration, particle sedimentation of heavy fibers and favorable particle-to-media size ratios.

Exposure pathways, environmental processes and risks of micro (nano) plastics to crops and feasible control strategies in agricultural regions

Micro/nanoplastics (MPs/NPs) pollution may adversely impact agricultural ecosystems, threatening the sustainability and security of agricultural production. This drives an urgent need to comprehensively understand the environmental behavior and effects of MPs/NPs in soil and atmosphere in agricultural regions, and to seek relevant pollution prevention strategies. The rhizosphere and phyllosphere are the interfaces where crops are exposed to MPs/NPs. The environmental behavior of MPs/NPs in soil and atmosphere, especially in the rhizosphere and phyllosphere, determines their plant accessibility, bioavailability and ecotoxicity. This article comprehensively reviews the transformation and migration of MPs/NPs in soil, transportation and deposition in the atmosphere, environmental behavior and effects in the rhizosphere and phyllosphere, and plant uptake and transportation pathways. The article also summarizes the key factors controlling MPs/NPs environmental processes, including their properties, biotic and abiotic factors. Based on the sources, environmental processes and intake risks of MPs/NPs in agroecosystems, the article offers several feasible pollution prevention and risk management options. Finally, the review highlights the need for further research on MPs/NPs in agro-systems, including developing quantitative detection methods, exploring transformation and migration patterns in-situ soil, monitoring long-term field experiments, and establishing pollution prevention and control systems. This review can assist in improving our understanding of the biogeochemistry behavior of MPs/NPs in the soil-plant-atmosphere system and provide a roadmap for future research.

Continents of plastics: An estimate of the stock of microplastics in agricultural soils

While there are estimates of the stock of microplastics in the marine environment, there are no estimates for soils. The main objective of this work is to estimate the total mass of microplastics in global agricultural soils. Microplastic abundance data from 442 sampling sites were collected from 43 articles. From these, the median of the abundance values, as well as the abundance profile of microplastics in soils were calculated. Thus, 1.5 to 6.6 Mt of microplastics would be present in soils on a global scale, i.e. one to two orders of magnitude higher than the estimated ocean surface microplastic stock. However, many limitations exist to accurately calculate these stocks. This work should therefore be considered as a first step in addressing this question. In the long term, in order to better assess this stock, it seems important to obtain more diversified data, e.g. better representing certain countries, or certain land uses.

Exploring the vertical transport of microplastics in subsurface environments: Lab-scale experiments and field evidence

Microplastics (MPs) defined as smaller 5 mm plastic particles have received increasing attention due to their global occurrence and potential toxicity. This study investigated the effects of environmental factors (rainfall intensity, 13 and 29 mm/h) and MP characteristics (morphology (fiber, flake, and film), polymer type (polypropylene (PP), polyethylene terephthalate (PET), and polystyrene (PS)) and size (100-300, 300-500, and 500-1000 μm)) on the vertical transport of MP in unsaturated soil conditions using lab-scale column experiments. Additionally, the occurrence and characteristics of MP detected in soil/sediment (total 13 samples) and groundwater samples (total 6 samples) were explored in the field study. Laboratory-scale column experiments revealed that heavy rainfall intensity (29 mm/h) increased the degree of MP vertical transport in unsaturated soil conditions and MP fibers showed the greatest vertical mobility among the various morphologies of MPs assessed. For the polymer type and size, the lighter PP polymer or the larger size of MP (500-1000 μm) showed higher mobility. In the field study, a statistical difference in MP abundance was observed depending on the population density and degree of urban development in both soil and groundwater samples. Comparing to the two different types of environmental media samples obtained from the same site, there was a significant difference in the composition of polymer types present while statistically no difference in MP abundance was observed between the two media samples (i.e., soil or sediment and groundwater). In addition, MP fibers and polyethylene (PE) were predominantly detected in our two study areas. These results suggest that various types of MP can pass through the unsaturated zone by water infiltration, even if it takes a long time to reach groundwater. Overall, we found that the degree of vertical transport of the MPs was highly sensitive to environmental conditions and MP characteristics.

Influence of microplastics on the transport of antibiotics in sand filtration investigated by AFM force spectroscopy

Microplastics and antibiotics were frequently detected in the effluent of sand filtration, while the presence of microplastics may change the interactions between the antibiotics and the quartz sands. However, the influence of microplastics on the transport of antibiotics in sand filtration has not been revealed. In this study, ciprofloxacin (CIP) and sulfamethoxazole (SMX) were respectively grafted on AFM probes to determine the adhesion forces to the representative microplastics (PS and PE) and the quartz sand. CIP and SMX exhibited low and high mobilities in the quartz sands, respectively. Compositional analysis of the adhesion forces indicated that the lower mobility of CIP in sand filtration columns could be attributed to the electrostatic attraction between the quartz sand and CIP compared with repulsion for SMX. Moreover, the significant hydrophobic interaction between the microplastics and the antibiotics could be responsible for the competitive adsorption of the antibiotics to the microplastics from the quartz sands; meanwhile, the π-π interaction further enhanced the adsorption of PS to the antibiotics. As a result of the high mobility of microplastics in the quartz sands, the carrying effect of microplastics enhanced the transport of antibiotics in the sand filtration columns regardless of their original mobilities. This study provided insights into the mechanism of the microplastics on enhancing the transport of antibiotics in sand filtration systems from the perspective of the molecular interaction.

Effect of different aging treatments on the transport of nano-biochar in saturated porous media

Widely used for soil amendment, carbon sequestration, and remediation of contaminated soils, biochars (BCs) inevitably produce a large number of nanoparticles with relatively high mobility. Geochemical aging alters chemical structure of these nanoparticles and thus affect their colloidal aggregation and transport behavior. In this study, the transport of ramie derived nano-BCs (after ball-milling) was investigated by different aging treatments (i.e., photo (PBC) and chemical aging (NBC)) as well as the managing BC under different physicochemical factors (i.e., flow rates, ionic strengths (IS), pH, and coexisting cations). Consequences of the column experiments indicated aging promoted the mobility of the nano-BCs. Compared to the nonaging BC, consequences of spectroscopic analysis demonstrated the aging BCs exhibited a number of tiny corrosion pores. Both of these aging treatments contribute to a more negative zeta potential and a higher dispersion stability of the nano-BCs, which is caused by the abundance of O-functional groups. Also the specific surface area and mesoporous volume of both aging BCs increased significantly, with the increase being more pronounced for NBC. The breakthrough curves (BTCs) obtained for the three nano-BCs were modelled by the advection-dispersion equation (ADE), which included first-order deposition and release terms. The ADE revealed high mobility of aging BCs, which meant their retention in saturated porous media was reduced. This work contributes to a comprehensive understanding of the transport of aging nano-BCs in the environment.

Preliminary investigation on effects of size, polymer type, and surface behaviour on the vertical mobility of microplastics in a porous media

Due to the ubiquitous nature of microplastic (MP), knowledge of its fate and migration in subsurface environments like soil becomes extremely important to understand underlying ecological risk. The fate and migration of MP in subterranean settings like sand are governed by the retention/transport properties influenced by the interaction of sand and MPs. In this study, sand column experiments under simulated rainfall conditions were conducted for 180 days to assess the vertical migration of mixed MPs consisting of polypropylene (PP), polyethylene (PE), and polyethylene terephthalate (PET). Sand column experiments were subjected to 60 wet-dry cycles over 180 days. The effects of polymer type, microplastic size, sand particle size, and surface roughness on the migration of MPs were evaluated. Results showed that the smallest-sized fragmented PE particles had the highest migration potential compared to PET and PE. The ratios of the diameters of MP particles and sand particles (d_MP/d_sand) played a significant role in determining the penetration depth of the various sizes of MPs. The MP particles with d_MP/d_sand 0.11 showed greater penetration depth in sand media and were detected in the column effluent water after 60 days of a column run. In addition, surface roughness, low ionic strength water, irregularly shaped particles, and wet and dry cycles contributed to the migration of MPs in the sand column. Three new absorbance peaks corresponding to the hydroxyl, CO stretch, and carbonyl groups evolved in the extracted PE MPs sample from different depths, as shown by FTIR analysis, suggesting that PE MPs had been oxidised. XPS analysis revealed changes in the surface properties of the MPs, indicating that oxidation occurred at the top layer, causing structural deterioration of the PE MPs. However, oxidation of the surface bonds was restricted in the layers underneath due to a lack of oxygen. The finding of the study suggests that in a natural environment, such accumulation and migration of MPs in sandy soil can increase the possibilities to the underlying groundwater contamination.

Understanding the underestimated: Occurrence, distribution, and interactions of microplastics in the sediment and soil of China, India, and Japan

Microplastics (MPs) are non-biodegradable substances that can sustain our environment for up to a century. What is more worrying is the incapability of modern technologies to annihilate MPs from om environment. One ramification of MPs is their impact on every kind of life form on this planet, which has been discussed ahead; that is why these substances are surfacing in everyday discussions of scholars and researchers. This paper discusses the overview of the global occurrence, abundance, analysis, and remediation techniques of MPs in the environment. This paper primarily reviews the event and abundance of MPs in coastal sediments and agricultural soil of three major Asian countries, India, China, and Japan. A significant concentration of MPs has been recorded from these countries, which affirms its strong presence and subsequent environmental impacts. Concentrations such as 73,100 MPs/kg in Indian coastal sediments and 42,960 particles/kg in the agricultural soil of China is a solid testimony to prove their massive outbreak in our environment and require urgent attention towards this issue. Conclusions show that human activities, rivers, and plastic mulching on agricultural fields have majorly acted as carriers of MPs towards coastal and terrestrial soil and sediments. Later, based on recorded concentrations and gaps, future research studies are recommended in the concerned domain; a dearth of studies on MPs influencing Indian agricultural soil make a whole sector and its consumer vulnerable to the adverse effects of this emerging contaminant.

New Perspective on the Mobilization of Microplastics through Capillary Fringe Fluctuation in a Tidal Aquifer Environment

The presence of plastic fragments in the environment is a growing global concern. In this study, we explored the effects of dynamic fluctuations of capillary fringe on the transport of microplastics (MPs) in the substrate combining various environmental and MP properties. Both experimental and Hydrus-2D modeling results confirmed that increasing cycles of water table fluctuation led to the rise of capillary fringe. An increase in the cycles of water table fluctuations did not significantly change the overall MP retention percentages in 0.5 mm substrate but altered the MP distribution along the column. In 1 and 2 mm substrate, the increase in cycle numbers enhanced the MP transport from substrate to the water below. In terms of the size of the MPs, more 20-25 μm polyethylene (PE2) were retained in the substrate compared to 4-6 μm polyethylene (PE1) under the same number of fluctuation cycles. High-density polytetrafluoroethylene (PTFE, 5-6 μm) exhibited higher retention percentages compared to PE1 particles. Ultraviolet aging for 60 days enhanced PE1 transport along the column, while 60 days of seawater aging did not affect PE1 transport greatly. For PTFE, ultraviolet and seawater aging enhanced its retention in the substrate. The retention percentages of both PE1 and PTFE in the column increased with the elevated ionic strength and the decrease of fluctuation velocity. This work highlights that capillary fringe fluctuation can serve as a pathway to relocate MPs to the tidal aquifer.

Microplastic Textile Fibers Accumulate in Sand and Are Potential Sources of Micro(nano)plastic Pollution

Agricultural soils have been identified as sinks for microplastic fibers; however, little information is available on their long-term fate in these soils. In this study, polyester and nylon fibers were precisely cut to relevant environmental lengths, using novel methodology, and their behavior in sand columns was studied at environmental concentration. The longer fibers (>50 μm) accumulated in the upper layers of the sand, smaller fibers were slightly more mobile, and nylon showed marginally higher mobility than polyester. Previous studies have overlooked changes in microplastic morphology due to transport in soil. Our study is the first to show that fibers exhibited breakage, peeling, and thinning under flow conditions in soil, releasing smaller, more mobile fragments. Furthermore, the peelings exhibited different adsorption properties compared to the core fiber. This suggests that microplastic fibers can become a source of smaller micro(nano)plastics and potential vectors for certain molecules, risking continuous contamination of nearby soils, surfaces, and groundwater.

Transport of degradable/nondegradable and aged microplastics in porous media: Effects of physicochemical factors

The degradable properties of degradable plastics allow them to form microplastics (MPs) faster. Therefore, degradable MPs may easily be transported in the underground environment. Research on degradable MPs transport in porous media is necessary and urgent. In this study, polylactic acid (PLA) and polyvinyl chloride (PVC) were selected to compare the transport differences between degradable and nondegradable MPs under different factors (flow rates, ionic strengths (ISs), pH, and coexisting cations) through column experiments, and UV irradiation was used to further simulate the effect of aging on different types of MPs. Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) were used to characterize functional groups and to determine the surface elements of MPs, respectively. The results showed that MPs were more mobile at higher flow rate, lower IS, higher pH, and monovalent cations. The order of transport capacity of MPs was PVC < aged PVC < PLA < aged PLA. This result was mainly attributed to the more negative Zeta potential and higher dispersion stability of aged PLA and PLA, which were caused by abundant O-functional groups. Compared with PVC, the O/C ratio of PLA increased significantly after aging, indicating that PLA was more prone to aging. The advection-dispersion-equation (ADE) fitted the transport data of MPs well. The interaction energy of MPs and quartz sand was accurately predicted by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. This work contributes to a comprehensive understanding of the transport of degradable MPs in the environment.

Impact of nanoplastic debris on the stability and transport of metal oxide nanoparticles: role of varying soil solution chemistry

The release of metal-based nanoparticles (MNPs) and nanoplastic debris (NPDs) has become ubiquitous in the natural ecosystem. Interaction between MNPs and NPDs may alter their fate and transport in the sub-surface environment and have not been addressed so far. Therefore, the present study has explored the role of NPDs on the stability and mobility of extensively used MNPs, i.e., CuO nanoparticles (NPs) under varying soil solutions (SS) chemistry. In the absence of NPDs, a very high aggregation of CuO NPs observed in SS extracted from black, lateritic, and red soils, which can be correlated with ionic strength (IS) and type of ionic species. The sedimentation rate (k_sed(1/h)) for CuO NPs was >0.5 h^-1 in the case of these SS. Interestingly, the stability and sedimentation behavior of CuO NPs varied significantly in the presence of NPDs. The k_sed for CuO NPs decreased to half and found <0.25 h^-1 in the presence of NPDs in all SS. C/C₀ values in breakthrough curves increased drastically (black < alluvial < laterite < red) in presence of NPDs. Results suggest that the release of NPDs in the terrestrial ecosystem is a potential threat leading to increased mobility of MNPs in the environment.

Attachment and detachment of large microplastics in saturated porous media and its influencing factors

Groundwater contamination by microplastics (MPs) has been gradually regarded as a potential human health risk, which calls for detailed investigation of MPs transport behavior in saturated zone. In this study, a series of sand column experiments were carried out to investigate the transport characteristics of large MPs with its diameter of 10-20 μm in porous media, in which the effects of different hydrological conditions and MPs characteristics were examined. Experimental results showed that the increase of water flow rate from 2.2 to 7.5 mL/min significantly increased the maximal outlet MPs concentration by two orders of magnitude, while a larger ratio of MPs diameter to soil particle diameter decreased its mobility. The increase of water salinity from 0 to 25 mmol/L (NaCl) decreased the maximal outlet MPs concentration by 50.5-68.4% for different sized MPs. Since chemical aging would lead to the formation of oxygen-containing functional groups and make MPs more negatively charged, it greatly increased the maximal outlet MPs concentration by 0.53-5.67 times. Compared with the traditional attachment model (AM), the attachment-detachment model (ADM) could better simulate the gradual desorption of large MPs from soil in the process of clean water flushing, indicating the nonnegligible detachment of large MPs from soil. In ADM, the desorption coefficient gradually decreased in the process of clean water flushing, which was only 31.6% of the initial value after flushing kept for 10 PV. Moreover, the equations to calculate the adsorption and desorption coefficients of MPs in the saturated zone were developed, which considered both MPs and aquifer characteristics. Results from this study described the desorption of large MPs in porous media under various conditions, which expands our knowledge about the fate and risk of MPs in underground environment.

State of knowledge and future research needs on microplastics in groundwater

Microplastics (MPs) are widespread in aquatic and soil environments. This study targets the issue of MPs' transfer from soil to groundwater. Scientific papers were collected and analyzed using a text-mining approach that classifies text segments. This allowed the identification of four research topics and the organization of the results into a summarizing table. Those four topics are sources of groundwater MPs, main types of MPs (physico-chemical properties, polymer units, shapes, and size), human exposure (mainly drinking water), and potential environmental and human effects. Compared to the research of MP on aquatic or soil compartments, scientific data on MP in groundwater are less substantial. Current results show a divergence due to differences in context (alluvial aquifer, fractured rock aquifer, karst aquifer, etc), collecting, sampling, and analytical methods. This divergence requires further research with standardized analytic protocols and reference materials. The associated research gaps were identified by using the same approach. The following five topics emerged: (1) the transfer of MPs from soil to underground, (2) the contribution of groundwater to drinking water microplastic pollution, (3) the interaction with other contaminants, (4) the human and environmental effects, and (5) the protective and remediation solutions.

Effect of fragmentation on the transport of polyvinyl chloride and low-density polyethylene in saturated quartz sand

Microplastics are an obstinate pollutant in terrestrial environments, posing a risk to the subsurface soil matrix and potentially to groundwater. In this study, the transport and retention behaviour of two major plastic polymers, 125-300 μm Polyvinyl chloride (PVC) plastic fragments and 300 μm Low-density polyethylene (LDPE) spherical particles, were explored in saturated quartz sand (1.6-2.0 mm) columns. The PVC used in this study represented secondary microplastics, while the LDPE represented primary microplastics. Retention profiles at different ultrapure water flow rates (2.0-3.5 ml/min) were compared and analysed. At the beginning and end of each column test, the microplastic particles were scrutinized, identified, and quantified by light microscopy. The results showed that the transport distance of microplastic particles increased with their decreasing diameter. Small-sized PVC microplastic particles, whose morphology was more 1-dimensional, were more susceptible to fragmentation within the column, promoting migration. Spherical LDPE remained at their initial position without fragmenting. Microplastic degradation into fragments appeared to play an important role in improving the movement of particles. This study offers initial indications of infiltration depths and shape-dependent fragmentation of secondary microplastics in coarse sand based on the lab experiments.

Addition of biochar as thin preamble layer into sand filtration columns could improve the microplastics removal from water

The release of microplastics (MPs) especially those with sizes less than 10 μm from effluent of wastewater treatment plants (WWTPs) is one of the major sources of plastics into aquatic environment. To reduce the discharge of MPs into environment, it is essential to further enhance their removal efficiencies in WWTPs. In present study, to boost the removal performance of MPs in sand filtration systems (units that commonly employed in WWTPs to remove colloidal pollutants), six types of biochar fabricated from three raw biomass materials (i.e. lignin, cellulose, and woodchips) at two pyrolysis temperatures (400 °C and 700 °C) was respectively amended into sand columns as thin permeable layer. We found that adding all six types of biochar into sand columns as thin permeable layer could greatly improve the retention of MPs with the diameter of 1 μm under either slow (4 m/d) or fast flow rates (160 m/d) due to the high adsorption capability of biochar. Woodchip-derived biochar exhibited the highest MPs retention performance, which was followed by cellulose-derived biochar and then lignin-derived biochar. Moreover, for biochar derived from three raw biomasses, increasing pyrolysis temperature could improve MPs retention performance. The direct observation of real-time plastics retention processes on different types of biochar via a visible flow chamber showed that woodchip-derived biochar especially that fabricated at 700 °C exhibited more MPs trapping processes relative to lignin and cellulose-derived biochar due to their more complex surface morphology. Thus, the highest MPs retention performance was achieved in sand columns with amendment by 1 wt% woodchip-derived biochar fabricated at 700 °C. More importantly, we found that for these modified sand filtration column systems, complete MPs removal could be achieved in real river water and actual sewage water, in multiple filtration cycles, longtime filtration process (100 pore volumes injection) as well as with interval flow conditions. Moreover, biochar could be regenerated and reused as thin permeable layer to effectively remove MPs. The results of this study clearly showed that biochar especially woodchip-derived biochar fabricated at 700 °C had the potential to immobilize MPs especially those with small sizes in WWTPs.

Effect of land use on microplastic pollution in a major boundary waterway: The Arvand River

The occurrence of microplastics (MPs) was investigated in the Arvand River (Iran). The Arvand River (200 Km) is a major water body that flows through land with diverse use and it meets the Persian Gulf. This study constitutes the first assessment of MP pollution (prevalence and physico-chemical characteristics) in the Arvand river, both in the sediment and in the water. MP monitoring has been carried out in 24 stations located along the river. The MP pollution found ranged between 1 and 291 items·L^-1 and 70 to 15,620 items·kg^-1 (dw), in water and sediment, respectively. The majority of MPs were fibres, black/grey and yellow/orange in colour, and mainly 250-500 μm and >1000 μm in size. Polyethylene terephthalate (PET), polypropylene (PP), nylon (NYL), high-density polyethylene (HDPE), and polystyrene (PS) were found in sediment samples. All these polymers, except HDPE, were also identified in the water samples. PET and PP were dominant in the water samples; whereas PET and PS were the most abundant in the sediments. The vicinity of urban wastewater effluents could be behind MP pollution in both water and sediments. Significant differences (p < 0.05) of MP concentrations were affected by different land uses when comparing MP levels in undisturbed natural area with urban areas. A strong correlation between MP fibres and fragments found with PCA biplots revealed their similar distribution in water. In the sediment samples, fibre and fragment MP particles were significantly correlated with colloidal particles (e.g., clay and organic matter) suggesting a relevant role of colloidal particles in the aquatic ecosystem of the Arvand River in transporting MPs. This study contributes to the better understanding of the presence of MP in major rivers, which are systems that have been scarcely investigated for this type of pollution, and it can inform interventions to reduce MP inputs to the river and sea.

Transport and retention of microplastics in saturated porous media with peanut shell biochar (PSB) and MgO-PSB amendment: Co-effects of cations and humic acid

Biochar particles are extensively used in soil remediation and interact with microplastics (MPs), especially metal oxide-modified biochar may have stronger interactions with MPs. The mechanism of interactions between humic acid (HA) and different valence cations is different and the co-effect on the transport of MPs is not clear. In this study, the co-effects of HA and cations (Na⁺, Ca²⁺) on the transport and retention of MPs in saturated porous media with peanut shell biochar (PSB) and MgO-modified PSB (MgO-PSB) were systematically investigated. Breakthrough curves (BTCs) of MPs were fitted by the two-site kinetic retention model for analysis. In the absence of HA, the addition of PSB and MgO-PSB significantly hindered the transport of MPs in saturated porous media, and the retention of MPs increased from 34.2% to 59.1% and 75.5%, respectively. In Na⁺ solutions, the HA concentration played a dominant role in controlling MPs transport, compared to the minor role of Na⁺. The transport capacity of MPs always increased gradually with the increase of HA concentration. Whereas, in Ca²⁺ solutions, Ca²⁺ concentrations had a stronger effect than HA. The transport ability of MPs was instead greater than that in Na⁺ solutions as the HA concentration increased at low ionic strength (1 mM). However, the transport capacity of MPs was significantly reduced with increasing HA concentrations at higher ionic strength (10, 100 mM). The two-site kinetic retention model indicated that chemical attachment and physical straining are the main mechanisms of MPs retention in the saturated porous media.

Vertical transport and retention behavior of polystyrene nanoplastics in simulated hyporheic zone

The ecological risk of microplastics (MPs) usually depends on their environmental behavior, however, few studies focused on the impact of hydrodynamic perturbations on the fate of MPs in hyporheic zone. This study chose quartz sand (250-425 μm) as simulated porous medium to investigate the transport of 100 nm polystyrene nanoplastics (PSNPs) under hydrodynamic factors, including flow rates (0.5, 1.0, and 2.0 mL/min), flow orientations (up-flow, down-flow, and horizontal-flow), and water saturations (50%, 80%, and 100%), as well as different salinities and temperatures. The breakthrough curves (BTCs) and retained profiles (RPs) of PSNPs were compared and analyzed by Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Due to the small size and moderate density of PSNPs, as well as high flow rates, the flow orientation exhibited little effect on the PSNP transport. However, high flow rate, low salinity, high water saturation, and low temperature would facilitate the mobility of PSNPs. The increase in salinity from zero to 35 PSU (practical salinity units) caused the compression of electrical double layer and weakened the electrostatic repulsion between PSNPs and sands, which dramatically decreased the penetration rate from 100% to zero. Especially, the lower energy barrier of PSNPs-PSNPs at 3.5 and 35 PSU (16.45 k_BT and zero, respectively) facilitated the adsorption of PSNPs on sand via ripening mechanism. Due to the strong adsorption of PSNPs by sand at high salinity, the effect of flow rate on PSNP transport was more pronounced at low salinity. The mobility of PSNPs at 0.035 PSU was enhanced by 41.4%-75.3% as the flow rate increased from 0.5 to 2.0 mL/min, which was contributed from the reversible deposition in lower secondary energy minimum depth at low salinity and the stronger hydrodynamic drag force generated by the high flow rate. However, the sufficient molecular diffusion at low flow rate promoted the occupation of PSNPs on adsorption sites. In addition, the penetration rate of PSNPs decreased by 25.0% as the water saturation decreased from 100% to 50%, indicating that the film straining at the air-water interface would hinder the transport of PSNPs. Finally, temperature increase impeded the penetration of PSNPs by 6.26%-23.1% via blocking mechanism. Our results suggest that low-salinity, high-flow river systems may be at greater risk of MPs contamination due to enhanced vertical transport capability.

Transport of Microplastics in Shore Substrates over Tidal Cycles: Roles of Polymer Characteristics and Environmental Factors

Tidal zones providing habitats are particularly vulnerable to microplastic (MP) pollution. In this study, the effects of tidal cycles on the transport of MPs (4-6 μm polyethylene, PE1; 125 μm polyethylene, PE2; and 5-6 μm polytetrafluoroethylene, PFTE) in porous media combined with various environmental and MPs properties were systemically investigated. The results indicated that smaller substrate sizes exhibited higher retention percentages compared to those of larger substrate sizes under different tidal cycles. In terms of the size of MPs, a larger size (same density) was found to result in enhanced retention of MPs in the column. As the number of tidal cycles increased, although the transport of MPs from the substrate to the water phase was enhanced, PE1 was washed out more with the change in water level, compared to PTFE. Additionally, more MPs were retained in the column with the increase of salinity and the decrease of flow velocity under the same tidal cycles. Ultraviolet and seawater aged PE1 showed enhanced transport, while aged PTFE showed enhanced retention under the same tidal cycles. These results can help understand the MP behaviors in the shoreline environment and provide support for future cleanup and sampling in tidal zones.