Transport of plastic particles in natural porous media under freeze-thaw treatment: Effects of porous media property

Current research status on the distribution and transport of micro(nano)plastics in hyporheic zones and groundwater

Micro(nano)plastics, as an emerging environmental pollutant, are gradually discovered in hyporheic zones and groundwater worldwide. Recent studies have focused on the origin and spatial/temporal distribution of micro(nano)plastics in regional groundwater, together with the influence of their properties and effects of environmental factors on their transport. However, the transport of micro(nano)plastics in the whole hyporheic zone-groundwater system and the behavior of co-existing substances still lack a complete theoretical interpretation. To provide systematic theoretical support for that, this review summarizes the current pollution status of micro(nano)plastics in the hyporheic zone-groundwater system, provides a comprehensive introduction of their sources and fate, and classifies the transport mechanisms into mechanical transport, physicochemical transport and biological processes assisted transport from the perspectives of mechanical stress, physicochemical reactions, and bioturbation, respectively. Ultimately, this review proposes to advance the understanding of the multi-dimensional hydrosphere transport of micro(nano)plastics centered on groundwater, the microorganisms-mediated synergistic transformation and co-transport involving the intertidal circulation. Overall, this review systematically dissects the presence and transport cycles of micro(nano)plastics within the hyporheic zone-groundwater system and proposes prospects for future studies based on the limitations of current studies.

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.

Colloid mobilization and transport in response to freeze-thaw cycles: Insights into the heavy metal(loid)s migration at a smelting site

Smelting sites often exhibit significant heavy metal(loid)s (HMs) contamination in the soil and groundwater, which are inevitably subjected to environmental disturbances. However, there is limited information available regarding the migration behaviors of HMs in a disturbed scenario. Thus, this work explored the migration of HMs-bearing colloids in response to freeze-thaw treatments by laboratory simulation and pore-scale study. Ultrafiltration results of soil effluents revealed that 61.5 %, 47.6 %, 68.0 %, and 59.2 % of Zn, Cd, Pb, and As were present in colloidal phase, and co-transported during treatments. Nanoparticle tracking analysis (NTA) further confirmed that freeze-thaw cycles were conducive to the generation of colloidal particles and showed the heteroagglomeration among different particles. Pore-network model (PNM) was used to quantify the soil macropore characteristics (macropore diameter, macropore number, coordination number, and Euler value) after treatments. It is evident that freeze-thaw cycles induced the formation of larger macropores while simultaneously enhancing macropore connectivity, thereby establishing an optimal pathway for colloid migration. These findings underscored the importance of environmental disturbances as a trigger for the release and migration of HMs in the smelting site, offering valuable insights for controlling HMs pollution. ENVIRONMENTAL IMPLICATION: The contaminated site has been subjected to prolonged environmental disturbances, causing the exacerbation of pollutants leaching and frequent occurrences of unstable pollution situations. This work explored the migration of HMs-bearing colloids in response to freeze-thaw treatments by laboratory simulation and pore-scale study. The distinct effects of freeze-thaw treatment on colloidal particle number concentration and macropore characteristics may explain the generation and migration of colloid-associated HMs driven by environmental disturbances. This work revealed the underlying mechanisms driving the redistribution of HMs under freeze-thaw cycles, offering valuable insights for risk assessment of soil and groundwater associated with HMs migration.

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.

Microplastics and nanoplastics in soil: Sources, impacts, and solutions for soil health and environmental sustainability

The present review discusses the growing concern of microplastics (MPs) and nanoplastics (NPs) in soil, together with their sources, concentration, distribution, and impact on soil microorganisms, human health, and ecosystems. MPs and NPs can enter the soil through various pathways, such as agricultural activities, sewage sludge application, and atmospheric deposition. Once in the soil, they can accumulate in the upper layers and affect soil structure, water retention, and nutrient availability. The presence of MPs and NPs in soil can also have ecological consequences, acting as carriers for pollutants and contaminants, such as heavy metals and persistent organic pollutants. Additionally, the leaching of chemicals and additives from MPs and NPs can pose public health risks through the food web and groundwater contamination. The detection and analyses of MPs and NPs in soil can be challenging, and methods involve spectroscopic and microscopy techniques, such as Fourier-transform infrared spectroscopy and scanning electron microscopy. To mitigate the presence and effects of MPs and NPs in soil, it is essential to reduce plastic waste production, improve waste management practices, and adopt sustainable agricultural practices. Effective mitigation measures include implementing stricter regulations on plastic use, promoting biodegradable alternatives, and enhancing recycling infrastructure. Additionally, soil amendments, such as biochar and compost, can help immobilize MPs and NPs, reducing their mobility and bioavailability. This review article aims to provide a comprehensive understanding of these emerging environmental issues and identify potential solutions to alleviate their impact on soil health, ecosystem functioning, and community health.

Topic modeling discovers trending topics in global research on the ecosystem impacts of microplastics

The ecological threats of microplastics (MPs) have sparked research worldwide. However, changes in the topics of MP research over time and space have not been evaluated quantitatively, making it difficult to identify the next frontiers. Here, we apply topic modeling to assess global spatiotemporal dynamics of MP research. We identified nine leading topics in current MP research. Over time, MP research topics have switched from aquatic to terrestrial ecosystems, from distribution to fate, from ingestion to toxicology, and from physiological toxicity to cytotoxicity and genotoxicity. In most of the nine leading topics, a disproportionate amount of independent and collaborative research activity was conducted in and between a few developed countries which is detrimental to understanding the environmental fates of MPs in a global context. This review recognizes the urgent need for more attention to emerging topics in MP research, particularly in regions that are heavily impacted but currently overlooked.

Occurrence and emission characteristics of microplastics in agricultural surface runoff under different natural rainfall and short-term fertilizer application

Land-based microplastics (MPs) are considered the primary source of MPs in aquatic environments, with runoff being a major pathway for their transfer from soil to surface water. However, the transportation characteristics of MPs via agricultural surface runoff remain unclear. In this study, we investigated the occurrence and emission characteristics of MPs in agricultural surface runoff under various short-term fertilizer applications and natural rainfall events using laser direct infrared imaging analysis (LDIR). MPs from fertilizers and soils co-migrated with the agricultural runoff. The abundance and concentration of MPs in runoff were 145.90 ± 22.48-2043.38 ± 89.51 items·L-1 and 39.17 ± 21.94-523.04 ± 47.85 µg·L-1, respectively. Small and low-density MPs, such as polyethylene (PE), chlorinated polyethylene (CPE), and polyurethane (PU) in film/fragment form with 20-50 µm exhibited a higher mobility. No statistical differences were observed in the distribution of runoff MPs with the application of different fertilizers. There was a significant positive relationship between runoff MP abundance and rainfall intensity. The annual emission load in this study area was 116.73 g·hm-2, indicating that the transportation of MPs via agricultural surface runoff cannot be ignored. This study is conducive to understanding the migration behavior of MPs in soil-water environments in a better manner.

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.

Revealing the Freezing-Induced Alteration in Microplastic Behavior and Its Implication for the Microplastics Released from Seasonal Ice

Ice can serve as a significant temporary repository and conveyance mechanism for microplastics (MPs). MPs present in the water column can become entrapped within developing ice formations, subsequently being sequestered and transported by ice floes. With changing temperatures, MPs stored in ice can be released back into the environment, while freezing conditions can alter the properties of MPs, ultimately affecting the fate of MPs in the environment. Freezing of MPs in freshwater ice results in the aggregation of MP particles due to physical compression, leading to an increase in particle size once the MPs are released from the ice. The freezing-induced aggregation enhances buoyancy effects, accelerating the settling/rising velocity of MPs in water. Additionally, freezing can lead to enhanced surface wetting alterations, thus improving the dispersion of hydrophobic MPs. The presence of salt in the water can mitigate the effect of freezing on MPs due to the formation of a brine network within the ice structure, which reduces the pressure on MPs entrapped by ice. In cold regions, numerous MPs undergo freezing and thawing, re-entering the water column.

Effects of arsenic on the transport and attachment of microplastics in porous media

Understanding the impact of arsenic (As(III), inorganic pollutant widely present in natural environments) on microplastics (MPs, one type of emerging contaminants) mobility is essential to predict MPs fate and distribution in soil-groundwater systems, yet relevant research is lacking. This study explored the effects of As(III) copresent in suspensions (0.05, 0.5, and 5 mg/L) on MPs transport/attachment behaviors in porous media containing varied water contents (θ = 100 %, 90 %, and 60 %) under different ionic strengths (5, 10, and 50 mM NaCl) and flow rates (2, 4, and 8 m/day). Despite solution ionic strengths, flow rates, porous media water contents, sizes, and surface charges of MPs, with coexisting humic acid, and in actual water samples, As(III) of three concentrations increased MPs transport in quartz sand and natural sandy soil. The increased electrostatic repulsion between MPs and sand caused by the altered MPs surface charge via the adsorption of As(III) together with steric repulsion from As(III) in solution contributed to the promoted MPs mobility in porous media. The occupying attachment sites by As(III) partially contributed to the increased mobility of MPs with negative surface charge in porous media. Clearly, As(III) coexisting in suspensions would enhance MPs transport in porous media, increasing MPs environment risks.

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.

Freeze-thaw aged polyethylene and polypropylene microplastics alter enzyme activity and microbial community composition in soil

In cold regions, microplastics (MPs) in the soil undergo freeze-thaw (FT) aging process. Little is known about how FT aged MPs influence soil physico-chemical properties and microbial communities. Here, two environmentally relevant concentrations (50 and 500 mg/kg) of 50 and 500 µm polyethylene (PE) and polypropylene (PP) MPs treated soils were subjected to 45-day FT cycles (FTCs). Results showed that MPs experienced surface morphology, hydrophobicity and crystallinity alterations after FTCs. After 45-day FTCs, the soil urease (SUE) activity in control (MPs-free group that underwent FTCs) was 33.49 U/g. SUE activity in 50 µm PE group was reduced by 19.66 %, while increased by 21.16 % and 37.73 % in 500 µm PE and PP groups compared to control. The highest Shannon index was found in 50 µm PP-MPs group at 50 mg/kg, 2.26 % higher than control (7.09). Compared to control (average weighted degree=8.024), all aged MPs increased the complexity of network (0.19-1.43 %). Bacterial biomarkers of aged PP-MPs were associated with pollutant degradation. Aged PP-MPs affected genetic information, cellular processes, and disrupted the biosynthesis of metabolites. This study provides new insights into the potential hazards of MPs after FTCs on soil ecosystem in cold regions.

Statuses, shortcomings, and outlooks in studying the fate of nanoplastics and engineered nanoparticles in porous media respectively and borrowable sections from engineered nanoparticles for nanoplastics

This review discussed the research statuses, shortcomings, and outlooks for the fate of nanoplastics (NPs) and engineered nanoparticles (ENPs) in porous media and borrowable sections from ENPs for NPs. Firstly, the most important section was that we reviewed the research statuses on the fate of NPs in porous media and the main influencing factors, and explained the influencing mechanisms. Secondly, in order to give NPs a reference of research ideas and influence mechanisms, we also reviewed the research statuses on the fate of ENPs in porous media and the factors and mechanisms influencing the fate. The main mechanisms affecting the transport of ENPs were summarized (Retention or transport modes: advection, diffusion, dispersion, deposition, adsorption, blocking, ripening, and straining; Main forces and actions: Brownian motion, gravity, electrostatic forces, van der Waals forces, hydration, filtration, bridging; Affecting elements of the forces and actions: the ENP and media grain surface functional groups, size, shape, zeta potential, density, hydrophobicity, and roughness). Instead of using the findings of ENPs, thorough study on NPs was required because NPs and ENPs differed greatly. Based on the limited existing studies on the NP transport in porous media, we found that although the conclusions of ENPs could not be applied to NPs, most of the influencing mechanisms summarized from ENPs were applicable to NPs. Combining the research thoughts of ENPs, the research statuses of NPs, and some of our experiences and reflections, we reviewed the shortcomings of the current studies on the NP fate in porous media as well as the outlooks of future research. This review is very meaningful for clarifying the research statuses and influence mechanisms for the NP fate in porous media, as well as providing a great deal of inspiration for future research directions about the NP fate in porous media.

Fate and cotransport of Pb(II) and Cd(II) heavy ions with bentonite colloidal flow in saturated porous media: The role of filter cake, counter ions, colloid concentration, and fluid velocity

This study examines the transport and retention of colloidal particles and heavy ions in porous sand, focusing on the environmental risks associated with waste from oil and gas drilling. Experimental and numerical models assess the influence of flow rate, external filter cake layer, and ionic strength on bentonite clay particles and heavy ions, such as cadmium (Cd) and lead (Pb), in near-wellbore (high-flux) and far-field (low-flux) scenarios. Colloidal filtration theory and the one-dimensional convection-dispersion equation with two-site kinetic model for attachment and detachment were utilized to calibrate and predict the transport of colloidal suspension in porous media. The research investigates the role of internal and external filter cakes on sand column pressure distribution and heavy ion absorption. Results indicate that the mobility of colloids and heavy ions is influenced by the ionic strength and pH of the carrying fluid. Colloidal clay suspensions show a higher affinity for Pb (II) absorption, while Cd (II) exhibits increased mobility in both clean sand and colloidal environments. Notably, the formation of an external filter cake significantly delays the breakthrough of heavy ions, up to four times longer than in clean sand, and reduces Cd (II) and Pb (II) outlet concentrations by 86% and 93%, respectively. This cake also limits clay concentration and particle size passage. High clay concentrations or injections under high ionic conditions induce clay bridging in pore throats, enhancing internal filtration and heavy ion retention. Conversely, low clay fluxes allow freer particle passage, increasing heavy ion loads and outlet concentrations.

Transport of polystyrene nanoplastics with different functional groups in goethite-coated saturated porous media: Effects of low molecular weight organic acids and physicochemical properties

The influence of low molecular weight organic acids (LMWOAs) and goethite on the migration of nanoplastics in the soil environment remains poorly understood. To elucidate the mechanism of influence, the study investigated the impact of LMWOAs on the migration ability of functionalized polystyrene nanoplastics (PSNPs-NH2/COOH) in quartz sand (QS) and goethite (α-FeOOH)-coated quartz sand (FOS). We investigated the effect of changes in iron valence induced by LMWOAs on the migration of PSNPs. The results revealed that the migration ability of polystyrene nanoplastics (PSNPs) declined as the ionic strength (IS) increased and the pH decreased, primarily due to the compression of the double layer and protonation reactions. The migration of PSNPs is facilitated by LMWOAs through distinct mechanisms in the two media. Specifically, LMWOAs were adsorbed on the FOS and QS surfaces through complexation and hydrogen bonding, respectively. At pH 4.0, LMWOAs exhibit redox activity, resulting in the generation of additional Fe(III). This redox process enhances the electrostatic attraction between the media and PSNPs, thereby reducing the competition at specific points and spatial resistance associated with LMWOAs. In contrast to FOS, LMWOAs at pH 4.0 reduced the migration ability of PSNPs in QS, following the trend of MA > TA > CA. This difference was attributed to the pKa of LMWOAs and the weak hydrogen bonding on the QS surface. The relevant mathematical models effectively validate the migration results. The above conclusions suggest that LMWOAs can alter the valence state of iron on the surface of goethite, thereby influencing the migration of plastic particles in environmental media.

Microplastics and potentially toxic elements: A review of interactions, fate and bioavailability in the environment

In recent years, microplastics (MPs) have obtained growing public concern due to widespread distribution and harmful impacts. Their distinctive features including porous structure, small size, as well as large specific surface area render MPs to be carriers for transporting other pollutants in the environment, especially potentially toxic elements (PTEs). Considering the hot topic of MPs, it is of great significance to comb the reported literature on environmental behaviors of co-occurrence of MPs and PTEs, and systematically discuss their co-mobility, transportation and biotoxicity to different living organisms in diverse environmental media. Therefore, the aim of this work is to systematically review and summarize recent advances on interactions and co-toxicity of MPs and PTEs, in order to provide in-depth understanding on the transport behaviors as well as environmental impacts. Electrostatic attraction and surface complexation mainly govern the interactions between MPs and PTEs, which are subordinated by other physical sorption processes. Besides, the adsorption behaviors are mainly determined by physicochemical properties regarding to different MPs types and various condition factors (e.g., ageing and PTEs concentrations, presence of substances). Generally speaking, recently published papers make a great progress in elucidating the mechanisms, impact factors, as well as thermodynamic and kinetic studies. Bioavailability and bioaccumulation by plant, microbes, and other organisms in both aquatic and terrestrial environment have also been under investigation. This review will shed novel perspectives on future research to meet the sustainable development goals, and obtain critical insights on revealing comprehensive mechanisms. It is crucial to promote efficient approaches on environmental quality improvement as well as management strategies towards the challenge of MPs-PTEs.

Nano- and micro-plastic transport in soil and groundwater environments: Sources, behaviors, theories, and models

With the increasing use of plastics, nano- and micro-plastic (NMP) pollution has become a hot topic in the scientific community. Ubiquitous NMPs, as emerging contaminants, are becoming a global issue owing to their persistence and potential toxicity. Compared with studies of marine and freshwater environments, investigations into the sources, transport properties, and fate of NMPs in soil and groundwater environments remain at a primary stage. Hence, the promotion of such research is critically important. Here, we integrate existing information and recent advancements to compile a comprehensive evaluation of the sources and transport properties of NMPs in soil and groundwater environments. We first provide a systematic description of the various sources and transport behaviors of NMPs. We then discuss the theories (e.g., clean-bed filtration and Derjaguin-Landau-Verwey-Overbeek theories) and models (e.g., single-site and dual-site kinetic retention and transport models) of NMP transport through saturated porous media. Finally, we outline the potential limitations of current research and suggest directions for future research. Overall, this review intends to assimilate and outline current knowledge and provide a useful reference frame to determine the sources and transport properties of NMPs in soil and groundwater environments.

Oxalic-activated minerals enhance the stabilization of polypropylene and polyamide microplastics in soil: Crucial roles of mineral dissolution coupled surface oxygen-functional groups

Low-molecular-weight organic acids (LMWOAs) prevalent in soil environments may influence the transport, fate, and orientation of microplastics (MPs) by mediating mineral interfaces. Nevertheless, few studies have reported their impact on the environmental behavior of MPs in soil. Here, the functional regulation of oxalic at mineral interfaces and its stabilizing mechanism for MPs were investigated. The results showed that oxalic drove MPs stability onto minerals and new adsorption pathways, which are dependent on the bifunctionality of minerals induced by oxalic acid. Besides, our findings reveal that in the absence of oxalic acid, the stability of hydrophilic and hydrophobic MPs on kaolinite (KL) mainly displays hydrophobic dispersion, whereas electrostatic interaction is dominant on ferric sesquioxide (FS). Moreover, the amide functional groups ([NHCO]) of PA-MPs may have positive feedback on the stability of MPs. In the presence of oxalic acid (2-100 mM), the MPs stability efficiency and property onto minerals were integrally increased in batch studies. Our results demonstrate the oxalic acid-activated interfacial interaction of minerals via dissolution coupled O-functional groups. Oxalic-induced functionality at mineral interfaces further activates electrostatic interaction, cation bridge effect, hydrogen forces, ligand exchange and hydrophobicity. These findings provide new insights into the regulating mechanisms of oxalic-activated mineral interfacial properties for environmental behavior of emerging pollutants.