Effects of clay minerals on the transport of nanoplastics through water-saturated porous media

Heteroaggregation and deposition behaviors of carboxylated nanoplastics with different types of clay minerals in aquatic environments: Important role of calcium(II) ion-assisted bridging

The widespread utilization of plastic products ineluctably leads to the ubiquity of nanoplastics (NPs), causing potential risks for aquatic environments. Interactions of NPs with mineral surfaces may affect NPs transport, fate and ecotoxicity. This study aims to investigate systematically the deposition and aggregation behaviors of carboxylated polystyrene nanoplastics (COOH-PSNPs) by four types of clay minerals (illite, kaolinite, Na-montmorillonite, and Ca-montmorillonite) under various solution chemistry conditions (pH, temperature, ionic strength and type). Results demonstrate that the deposition process was dominated by electrostatic interactions. Divalent cations (i.e., Ca2+, Mg2+, Cd2+, or Pb2+) were more efficient for screening surface negative charges and compressing the electrical double layer (EDL). Hence, there were significant increases in deposition rates of COOH-PSNPs with clay minerals in suspension containing divalent cations, whereas only slight increases in deposition rates of COOH-PSNPs were observed in monovalent cations (Na+, K+). Negligible deposition occurred in the presence of anions (F-, Cl-, NO3-, CO32-, SO42-, or PO43-). Divalent Ca2+ could incrementally facilitate the deposition of COOH-PSNPs through Ca2+-assisted bridging with increasing CaCl2 concentrations (0-100 mM). The weakened deposition of COOH-PSNPs with increasing pH (2.0-10.0) was primarily attributed to the reduce in positive charge density at the edges of clay minerals. In suspensions containing 2 mM CaCl2, increased Na+ ionic strength (0-100 mM) and temperature (15-55 ◦C) also favored the deposition of COOH-PSNPs. The ability of COOH-PSNPs deposited by four types of clay minerals followed the sequence of kaolinite > Na-montmorillonite > Ca-montmorillonite > illite, which was related to their structural and surface charge properties. This study revealed the deposition behaviors and mechanisms between NPs and clay minerals under environmentally representative conditions, which provided novel insights into the transport and fate of NPs in natural aquatic environments.

Transport of polyethylene and polypropylene microplastics under the action of agricultural chemicals: Role of pesticide adjuvants and neonicotinoid active ingredients

Understanding the impact of various agricultural chemical components on the fate and transport of microplastics (MPs) in the subsurface is essential. In this study, column experiments on saturated porous media were conducted to explore the influence of the coexistence environment of pesticide adjuvants (surfactants) and active ingredients (neonicotinoids) on the transport of polyethylene (PE) and polypropylene (PP) MPs. An anionic surfactant (sodium dodecyl sulfate (SDS)), a nonionic surfactant (nonylphenol ethoxylate (NP-40)), and three neonicotinoid insecticides (acetamiprid, dinotefuran, and nitenpyram) could independently increase MP migration by 9.31%-61.01% by improving the hydrophilicity. Acetamiprid or dinotefuran reduced the adhesion work of the binary system by competing with SDS for adsorption sites, thereby inhibiting PE mobility. However, nitenpyram in the mixture was not easily adsorbed on the surface of PE MPs together with SDS because of nitenpyram's high hydrophilicity. Neonicotinoid molecules could not reduce the hydrophilic modification of SDS on PP MPs by competing for adsorption sites. Owing to their weak charge and adhesion work of nonionic surfactants (-4.80 mV and 28.45 kT for PE and -8.21 mV and 17.64 kT for PP), neonicotinoids tended to occupy the adsorption sites originally belonging to NP-40. The long molecular chain of NP-40 made it difficult for high-concentration neonicotinoids to affect the adhesion on MPs. In addition, NP-40 was harder to peel off from the MP surface than SDS, leading to a larger MP transport ability in the sand column.

Transport and retention of positively charged zinc oxide nanoparticles in saturated porous media: Effects of metal oxides and clays

The effects of metal oxides and clays on the transport of zinc oxide nanoparticles (ZnO-NPs) in saturated porous media were investigated under different ionic strength (IS) conditions. We studied the transport and retention behavior of ZnO-NPs for different types of porous media (untreated, acid treated, and acid-salt treated sand). The selected untreated sand was used as a representative sand, coated with both metal oxide and clay. The acid treated and acid-salt-treated sands were used and compared to investigate the effects of clays on the surface of the sand. In addition, the effects of clay particles in bulk solutions on the mobility and retention of ZnO-NPs were observed using bentonite as a representative clay particle. We found that the increased mobility of positively charged ZnO-NPs can be attributed to increasing charge heterogeneity of silica sand with metal oxides (mainly, iron oxide) and clays in untreated sand. No breakthrough of ZnO-NP was observed for acid-treated (presence of clays and absence of metal oxides) and acid-salt-treated sand (absence of both metal oxide and clays). Most of the injected ZnO-NPs were deposited on the surface of the sand near the column inlet. The transport of bentonite-facilitated ZnO-NPs was improved at the lowest IS (0.1 mM) (∼20%), whereas there was no difference in the mobility of ZnO-NPs at high IS solutions (1 mM and 10 mM). In particular, the breakthrough amount improved with increasing bentonite concentration. Classical Derjaguin-Landau-Verwey-Overbeek interactions help explain observed interactions between ZnO-NPs and sand as well as bentonite and sand.

Different inhibitory mechanisms of flexible and rigid clay minerals on the transport of microplastics in marine porous media

Colloidal interactions between clay minerals and microplastics (MPs) in high salinity seawater are crucial for determining MP fate in marine environments. Montmorillonite (MMT) forms thin and pliable films that tightly cover MPs, while the thick and rigid lamellae of kaolinite (KLT) have limited contact with MPs, resulting in unstable bonding. However, a small quantity of small-sized KLT can create relatively stable heteroaggregates by embedding into the interstitial spaces of MPs. Both MMT and KLT colloids can decrease the mobility of MPs in seawater-saturated sea sand, but their breakthrough curves (BTCs) show distinct phenomena of "blocking" and "ripening", respectively. The "blocking" phenomenon occurs when flexible MMT adheres to the sand surface, depleting attachment sites quickly and inhibiting the retention of subsequent heteroaggregates of MMT-wrapped MPs. The transport of single MMT also experiences colloid competition for attachment sites, but pre-equilibration experiments reveal no competition between MMT and bare MPs for attachment sites. Instead, the attached MMT provides additional attachment sites for MPs. These results suggest that the wrapping of MPs by MMT plays a dominant role in the "blocking" of cotransport. In contrast, rigid KLT forms a three-dimensional stack on the sand surface, offering more attachment sites for subsequent MPs and heteroaggregates.

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.

The long-term release and particle fracture behaviors of nanoplastics retained in porous media: Effects of surfactants, natural organic matters, antibiotics, and bacteria

The transport of nanoplastics (NPs) in porous media has received a lot of attention, but the studies on the long-term release of NPs retained in porous media and the particle fracture during this process are seriously lacking. For filling this deficiency, we examined the individual or synergistic effects of surfactants, natural organic matters (NOMs), antibiotics, and bacteria on the desorption, long-term release, and particle fracture behaviors of polystyrene NPs (PS-NPs) retained in porous media. It was found that the change in hydrophilicity of PS-NPs dominated the long-term release of PS-NPs retained in porous media when surfactants were present. In the single system of surfactants and the dual system of surfactants and NOMs, the release of PS-NPs were improved owing to the increasing hydrophilicity of PS-NPs, although cationic surfactants also reduced the electrostatic repulsion between PS-NPs and porous media. Increasing antibiotic concentration reduced the electrostatic repulsion between PS-NPs and porous media to inhibit the release of PS-NPs. When bacteria were present whether containing antibiotics or not, the effects on roughness of PS-NPs dominated the release of PS-NPs. The effects of surfactants and NOMs on the PS-NP desorption were similar with the long-term release, with changes in hydrophilicity dominating the process. Whereas the effects of antibiotics and bacteria on the PS-NP desorption were different with the long-term release. Surfactants and NOMs in the presence of surfactants inhibited the fracture of PS-NPs by increasing the hydrophilicity of PS-NPs brought about the coating of water molecules on PS-NPs for protection. Antibiotics had no significant effects on the fracture of PS-NPs due to unaltered vertical forces on PS-NPs and no protective effect. Bacteria in the presence or absence of antibiotics inhibited the fracture of PS-NPs by coating PS-NPs retained in porous media to protect PS-NPs from fracture.

Arbuscular Mycorrhizal Fungus Alleviates Charged Nanoplastic Stress in Host Plants via Enhanced Defense-Related Gene Expressions and Hyphal Capture

Contamination of small-sized plastics is recognized as a factor of global change. Nanoplastics (NPs) can readily enter organisms and pose significant ecological risks. Arbuscular mycorrhizal (AM) fungi are the most ubiquitous and impactful plant symbiotic fungi, regulating essential ecological functions. Here, we first found that an AM fungus, Rhizophagus irregularis, increased lettuce shoot biomass by 25-100% when exposed to positively and negatively charged NPs vs control, although it did not increase that grown without NPs. The stress alleviation was attributed to the upregulation of gene expressions involving phytohormone signaling, cell wall metabolism, and oxidant scavenging. Using a root organ-fungus axenic growth system treated with fluorescence-labeled NPs, we subsequently revealed that the hyphae captured NPs and further delivered them to roots. NPs were observed at the hyphal cell walls, membranes, and spore walls. NPs mediated by the hyphae were localized at the root epidermis, cortex, and stele. Hyphal exudates aggregated positively charged NPs, thereby reducing their uptake due to NP aggregate formation (up to 5000 nm). This work demonstrates the critical roles of AM fungus in regulating NP behaviors and provides a potential strategy for NP risk mitigation in terrestrial ecosystems. Consequent NP-induced ecological impacts due to the affected AM fungi require further attention.

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.

Mechanisms of increased small nanoplastic particle retention in water-saturated sand media with montmorillonite and diatomite: Particle sizes, water components, and modelling

The processes by which small nanoplastics (NPs) accumulate in soil are unclear. To clarify the different deposition processes that affect small NPs (< 30 nm) compared to larger NPs in the soil environment, due to their interaction with clays as major soil components, the transport behavior of two-sized NPs (20 and 80 nm) with two clays (diatomite (Diat) and montmorillonite (Mont)) in NaCl and CaCl2 solutions were investigated in water-saturated quartz sand columns. The experimental results showed that more 20 nm NPs could enter the lattice structure of Diat than Mont in NaCl solution. This contributed to the stronger deposition of 20 nm NPs by Diat on sand, which was associated with a lower k1d/k1 value (obtained from two-site kinetic attachment model). In contrast, 80 nm NPs had a stronger reversible retention than 20 nm NPs with Mont, even though both sizes of NPs-Mont displayed a similar transportability. In CaCl2 solution, the larger NPs-Mont hetero-aggregates formed with a stronger suppressed depth of φmax based on Derjaguin-Landau-Verwey-Overbeek theory. Thus, Mont had a stronger transport inhibition than Diat for both NPs sizes, with a lower k1d/k1. These findings could benefit in predicting the size-based deposition of NPs in a heterogenous soil environment.

Cotransport of fullerene nanoparticles and montmorillonite colloids in porous media: Critical role of divalent cations of montmorillonite

While the cotransport of carbon nanoparticles (CNPs) and clay colloids in porous media has been widely studied, the influence of the cation exchange capacity (CEC) of clay colloids on the transport process remains unclear. In this study, batch adsorption and column transport experiments were conducted to investigate the fate and transport of CNPs and clay colloids in quartz sand, with respect to the effect of monovalent-cation exchange capacity (mono-CEC), divalent-cation exchange capacity (di-CEC) and total CEC of clays. Fullerene nanoparticles (nC60) and six types of montmorillonite (ML) with different CEC were selected as modeled CNPs and clay colloids, respectively. Transport behavior of nC60 and ML was characterized using breakthrough curves (BTCs) and fitted with two-kinetic-sites colloid transport model. Results of the adsorption experiments showed a good linear correlation between the deposition of nC60 on the sand surface and the di-CEC of ML. Transport of ML and nC60 was inhibited by each other. The calculated mass recovery of nC60, as well as the fitted maximum deposition capacity and attachment rate coefficients of nC60 exhibited a strong linear relationship with the di-CEC of ML. These results indicate that divalent cations in ML interlayers play a significant role in aggregation between nC60 and ML and their cotransport. Through measurements of the particle size and zeta potentials of sole nC60 and mixtures of ML and nC60, FTIR and XPS analysis of nC60 under different conditions, and a release experiment of nC60 in a sand column, it demonstrated cation bridging (Ca2+-π) between nC60 and ML mediated by the divalent cations in ML interlayers. The study highlighted the potential of using di-CEC of clays as an indicator to predict the mobility of nC60 in clay-containing porous media and added insights to the transport behavior of CNPs in porous media.

Bibliometric analysis and systematic review of the adherence, uptake, translocation, and reduction of micro/nanoplastics in terrestrial plants

Micro/nanoplastics are emerging agricultural pollutants globally. Micro/nanoplastics can adhere to terrestrial plant surfaces, be absorbed and transported by plants, and accumulate in the edible parts of plants, leading to the possibility of enrichment and transmission through the food chain and threatening human health. However, the underlying mechanism remains unclear. With increased studies on the internalization of micro/nanoplastics in terrestrial plants, a comprehensive and systematic review summarizing the current research trends and progress is warranted to provide a reference for further relevant research. Based on bibliometric analysis, this study focused on the mechanisms, study methods, and reduction techniques of micro/nanoplastics adherence, uptake, and translocation by terrestrial plants. The results showed that micro/nanoplastics can adhere to the surfaces of plant tissues such as seeds, roots, and leaves. Root uptake (root-to-leaf translocation) and foliar uptake (leaf-to-root translocation) are the two simultaneous internalization pathways of MNPs in plants. The observation methods included scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS), and inductively coupled plasma-mass spectrometry (ICP-MS). We highlighted the necessity and urgency of reducing the uptake and translocation of MNPs by plants and found that the application of silicon may be a promising approach for reducing internalization. This study identifies current knowledge gaps and proposes possible future needs.

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.

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.

Combined effects of bacteria and antibiotics on surface properties and transport of nanoplastics in porous media

Currently, research on the individual effects of bacteria and antibiotics on the transport of nanoplastics (NPs) in porous media is in its infancy, while research on their combined effect is absent. It is well known that bacteria and antibiotics also interact with each other, so this synergistic transport of bacteria, antibiotics, and NPs in porous media must be very interesting. For exploring this aspect, we investigated the individual and combined effects of bacteria and antibiotics on the transport of polystyrene NPs (PS-NPs) in saturated porous media. Hydrophobicity, roughness, and the Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy were measured and calculated. The PS-NPs' transport in porous media was fitted using a mathematical model. Enhanced roughness and size of PS-NPs with increased bacterial concentration dominated and inhibited the PS-NPs' transport in porous media, although the hydrophilicity of PS-NPs and the energy barrier between PS-NPs and porous media were also increased. An increase in antibiotic concentration reduced the energy barrier between PS-NPs and porous media, thereby decreasing the PS-NPs' transport. Combined effects of bacteria and antibiotics on the PS-NPs' transport were complex and distinct from individual effects, but the mechanisms were clear. Roughness and hydrophilicity of PS-NPs and the DLVO interaction energy between PS-NPs and porous media together influenced this process. In the presence of bacteria, antibiotics could alter the bacterial surface roughness by altering bacterial extracellular polymeric substances, and thus alter the PS-NPs' surface roughness, thereby affecting the PS-NPs' transport in porous media. When antibiotics were present, enhanced bacterial concentration increased the PS-NPs' hydrophilicity and the energy barrier between PS-NPs and porous media, thus promoting the PS-NPs' transport. The findings of this study provided a theoretical basis for clarifying the transport of NPs in porous media under complex environments, facilitating a reduction in environmental pollution of NPs.

Influence of humic acid and bovine serum albumin on colloid-associated heavy metal transport in saturated porous media

Colloid-facilitated contaminant transport in porous media has been widely observed in laboratory and field studies. In this study, the influence of two dissolved organic matters (DOMs), humic acid (HA) and bovine serum albumin (BSA), on the colloid-associated heavy metal transport, was investigated. Soil colloids with particle sizes <2 μm were prepared from bentonite. Glass bead was used as porous media for the column tests. The influence of DOM on the adsorption of Pb2+ and Cu2+ onto colloids was tested. Colloid mobility and colloid-metal co-transport in the presence/absence of DOMs were investigated by breakthrough tests. The test results showed that DOMs facilitated colloid mobility. The measured ζ-potentials showed that DOMs enhanced the electrostatic repulsion between colloids and glass beads and reduced colloid deposition. These findings were further confirmed by calculating the interaction energy using the DLVO theory. Batch tests showed the strong adsorption of Pb2+ and Cu2+ on the colloid, and the adsorption was enhanced by DOMs. The colloid-metal co-transport tests showed that colloids can significantly facilitate the transport of Pb2+ and Cu2+ and that the facilitation was further enhanced by DOMs. By heavy metals, the colloid mobility was retarded, mainly due to the increased deposition. The transport of Cu2+ facilitated by DOM was more obvious than that of Pb2+. Compared to BSA, the effect of HA on enhancing colloid mobility, increasing colloid adsorption to heavy metals, and hence on the facilitation of colloid-associated heavy metals transport was more prominent.

Quantification of two-site kinetic transport parameters of polystyrene nanoplastics in porous media

In this study, a combination of column experiments, interface chemistry theory and transport model with two-site kinetics was used to systematically investigate the effect of pH on the transport of polystyrene nanoparticles (PSNPs) in porous media. The porous media containing quartz sand (QS) and three kinds of clay minerals (CMs)-kaolinite (KL), illite (IL) and montmorillonite (MT), was used in column experiments to simulate the porous media in the soil-groundwater systems. Experimental results showed that the inhibitory effect of CMs on the transport of PSNPs is weakened as pH increases. The two-dimensional (2D) surface of the DLVO interaction energy (2D-pH-DLVO) was built to calculate the interactions between PSNPs and CMs under different conditions of pH. Results suggested the inflection point of PSNP-QS, PSNP-KL, PSNP-IL and PSNP-MT are 2.42, 3.30, 2.84 and 3.69, respectively. Most importantly, there was a significant correlation between the two-site kinetic parameters related to PSNPs transport and the DLVO energy barrier (DB). The contributions of the interactions of PSNPs-PSNPs and PSNPs-minerals were determined for PSNPs transport in porous media. The critical values of pH related to the migration ability of PSNPs in porous media could be determined by a combination of column experiments, 2D-pH-DLVO and PSNPs transport model. The critical values of pH were 2.95-3.01, 3.22-3.51, 2.98-3.02, 3.31-3.33 for the migration ability of PSNPs in QS, QS + KL, QS + IL and QS + MT porous media, respectively. The stronger migration ability of PSNPs under high pH conditions is attributed to the enhanced deprotonation of the media surface and increased negative surface charge, which increases the electrostatic repulsion between PSNPs and porous media (QS, CMs). Moreover, the agglomeration of PSNPs usually is weaker and the average particle size of agglomerates is smaller under the condition of high pH, thus leading to the stronger migration ability of PSNPs under high pH conditions.

Effects of input concentration, media particle size, and flow rate on fate of polystyrene nanoplastics in saturated porous media

Nanoplastics have gradually attracted widespread attention, but the studies in this area are still very scarce. In this research, the adsorption, transport, long-term release, and particle fracture of polystyrene nanoplastics (PS-NPs) in saturated porous media were investigated at different media particle sizes, input concentrations, and flow rates. The increased PS-NPs concentration and sand grain size promoted the adsorption of PS-NPs onto quartz sand. In transport tests, the peak breakthrough amounts of PS-NPs ranged from 0.5761 to 0.8497, demonstrating their high mobility in saturated quartz sand. Transport of PS-NPs in saturated porous media increased with decreasing input concentration and increasing media particle sizes. The effect of input concentration could be predicted by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, in which adsorption played a dominant role. The effect of media particle size was mainly dominated by filtration rather than adsorption. As a result of higher shear force, increasing flow rate might boost transport of PS-NPs. With increasing media particle size and flow rate, more retained PS-NPs were released, which was in line with the findings of the transport tests on the mobility of PS-NPs. Notably, PS-NPs could be broken down into smaller PS-NPs during long-term release and the percentage of released PS-NPs (<100 nm) was gradually increased from 1st to 3rd PV effluent in all media particle sizes and flow rates. The fracture of released PS-NPs from medium quartz sand was the most in relation to fine and coarse and showed a decreased trend with increasing flow rate, which was likely to be governed by the force perpendicular to the contact surface with the media particle. This study showed that PS-NPs have strong mobility in porous media and are easily broken into smaller particles during long-term release. The findings of this research provided fundamental information for clarifying transport laws of nanoplastics in porous media.

Co-transport of polystyrene microplastics and kaolinite colloids in goethite-coated quartz sand: Joint effects of heteropolymerization and surface charge modification

This study investigated the transport behavior of polystyrene microplastics (MPs) in saturated quartz sand and goethite-coated sand in the presence of coexisting kaolinite colloids. Column experiments were conducted under a wide range of solution chemistry conditions, including pH levels of 6.0, 7.0, and 9.0, as well as background Na⁺ concentrations of 5 mM and 25 mM. We found that: (1) The individual transport of MPs in porous media diminished both with increasing background ion strength and decreasing pH, and its transport ability was significantly dominated by the interactions between MPs and porous media rather than the interplay between MPs, which has been further corroborated by the aggregation stability experiments of MPs particles. (2) MPs had a much lower ability to move through goethite-coated sand columns than quartz sand columns. This is because goethite coating reduces the repulsion energy barriers between porous media and MPs. The increased specific surface area and surface complexity of sand columns after goethite coating should also account for this difference. (3) MPs transport would be subjected to the differentiated impact of co-transported kaolinite colloids in the two types of porous media. The promotion effect of kaolinite colloid on MPs' transport capacity is not significantly affected by background ionic strength changes when quartz sand is served as the porous medium; however, the promotion effect is highly correlated with the background ionic strength when goethite-coated sand is served as the porous medium. In comparison with low background ionic strength conditions, kaolinite colloids under high background ionic strength conditions significantly facilitated MPs transport. This is mainly because under high background ionic conditions, kaolinite colloids are more likely to be deposited on the surface of goethite-covered sand, competing with MPs for the limited deposition sites. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory is applicable to describe the transport behavior of MPs.

Analysis and study of the migration pattern of microplastic particles in saturated porous media pavement

This work aims to establish an analytical and comparative model of pavement stormwater runoff and determine how to solve water pollution in saturated porous media pavements. Heavy metal element particles in the stormwater runoff due to the rainfall will cause inevitable environmental pollution. First, the pavement runoff and materials of saturated porous media are analyzed. Besides, particle migration laws and separation effects of different materials are compared. Based on this, microplastics are selected as the primary material for pavement filling. Then, the adsorption effect of microplastics and the parameters of rainwater infiltration rate and infiltration ratio are analyzed to propose a multi-level ecological integrated treatment system for pavement runoff. Specifically, the environmental resource pollution and saturated porous media materials are analyzed. In addition, the adsorption effect of microplastic particles is analyzed to establish a model to study the selection process of the optimal adsorption material. The main contribution of the research is to analyze the migration process of metal particles in the soil in combination with the internal particle migration rules of plastic granular materials. The research results demonstrate that the rain runoff coefficient gradually increases with the expansion of the permeable area of the pavement. The rain runoff coefficient reaches the maximum value under the pavement of 120 square meters. In addition, a comparative analysis of three street pavements is conducted on the residential street pavement (RSP), commercial street pavement (CSP), and active street pavement (ASP). When comparing the two sets of data, the overall average permeability of the RSP is better than CSP and ASP. The research materials are compared under isothermal conditions. The particle adsorption effect of the same material at 50 °C is significantly better than that at 30 °C. Therefore, it is feasible to resolve the pavement runoff water pollution through technical schemes.

Two-Dimensional Materials Applied in Membranes of Redox Flow Battery

Redox flow batteries (RFBs) are one of the most promising techniques to store and convert green and renewable energy, benefiting from their advantages of high safety, flexible design and long lifespan. Membranes with fast and selective ions transport are required for the advances of RFBs. Remarkably, two-dimensional (2D) materials with high mechanical and chemical stability, strict size exclusion and abundantly modifiable functional groups, have attracted extensive attentions in the applications of energy fields. Herein, the improvements and perspectives of 2D materials working for ionic transportation and sieving in RFBs membranes are presented. The characteristics of various materials and their advantages and disadvantages in the applications of RFBs membranes particularly are focused. This review is expected to provide a guidance for the design of membranes based on 2D materials for RFBs.

Solid-liquid interface adsorption of antibiotic resistance plasmids induced by nanoplastics aggravates gene pollution in aquatic ecosystems

Antibiotic resistance genes (ARGs) and nanoplastics (NPs) have been identified as emerging pollutants in water environment; the interactions between antibiotic resistance plasmids (ARPs) and NPs will influence ARG transport in sediments. Herein, the adsorption experiments of a typical ARP onto polystyrene nanoplastics (PS-NPs) in river and lake sediments were conducted to elucidate the adsorption mechanisms and the effects of environmental factors. Results indicated that the adsorption amounts of PS-NPs increased with the dosages while decreased with the particle size of sediments. Multi-layer adsorption of PS-NPs was found to exist mainly in sand and silt sediments, whereas the filling adsorption dominated in the clay. Moreover, the adsorbed PS-NPs enhanced the physisorption of ARPs in sediments through stimulating the intraparticle diffusion of ARPs induced by electrostatic force. Besides, the adsorption amounts of ARPs onto the PS-NPs decreased with the increasing pH and dissolve organic matter due to the enhanced electrostatic repulsion and competitive adsorption. The ion strength played catalytic roles by increasing the electrostatic attraction and adsorption sites of ARPs on PS-NPs. The adsorbed ARPs in sediments were closely related with the ARGs in extra/intracellular DNA of biofilms, influencing the distribution and proliferation of ARGs largely. The findings indicate that ARG-associated pollution might be enhanced by the solid-liquid interface adsorption induced by NPs, which was controlled by pH, ion strength and dissolve organic matter. This study provides supplementary insights into the roles of NPs as carriers of ARP in sediments, and advances our understanding on the risks of NP-ARG co-occurring contamination in aquatic ecosystems.

Using colloidal AFM probe technique and XDLVO theory to predict the transport of nanoplastics in porous media

The plastic concentration in terrestrial systems is orders of magnitude higher than that found in marine ecosystems, which has raised global concerns about their potential risk to agricultural sustainability. Previous research on the transport of nanoplastics in soil relied heavily on the qualitative prediction of the mean-field extended Derjaguin-Landau-Verwey-Overbeek theory (XDLVO), but direct and quantitative measurements of the interfacial forces between single nanoplastics and porous media are lacking. In this study, we conducted multiscale investigations ranging from column transport experiments to single particle measurements. The maximum effluent concentration (C/C0) of amino-modified nanoplastics (PS-NH₂) was 0.94, whereas that of the carboxyl-modified nanoplastics (PS-COOH) was only 0.33, indicating PS-NH₂ were more mobile than PS-COOH at different ionic strengths (1-50 mM) and pH values (5-9). This phenomenon was mainly attributed to the homogeneous aggregation of PS-COOH. In addition, the transport of PS-NH₂ in the quartz sand column was inhibited with the increase of ionic strength and pH, and pH was the major factor governing their mobility. The transport of PS-COOH was inhibited with increasing ionic strength and decreasing pH. Hydrophilicity/hydrophobicity-mediated interactions and particle heterogeneity strongly interfered with interfacial forces, leading to the qualitative prediction of XDLVO, contrary to experimental observations. Through the combination of XDLVO and colloidal atomic force microscopy, accurate and quantitative interfacial forces can provide compelling insight into the fate of nanoparticles in the soil 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.

Effects of clay minerals on the transport of polystyrene nanoplastic in groundwater

Microplastics are widely detected in the soil-groundwater environment, which has attracted more and more attention. Clay mineral is an important component of the porous media contained in aquifers. The transport experiments of polystyrene nanoparticles (PSNPs) in quartz sand (QS) mixed with three kinds of clay minerals are conducted to investigate the effects of kaolinite (KL), montmorillonite (MT) and illite (IL) on the mobility of PSNPs in groundwater. Two-dimensional (2D) distributions of DLVO interaction energy are calculated to quantify the interactions between PSNPs and three kinds of clay minerals. The critical ionic strengths (CIS) of PSNPs-KL, PSNPs-MT and PSNPs-IL are 17.0 mM, 19.3 mM and 21.0 mM, respectively. Experimental results suggest KL has the strongest inhibition effect on the mobility of PSNPs, followed by MT and IL. Simultaneously, the change of ionic strength can alter the surface charge of PSNPs and clay minerals, thus affecting the interaction energy. Experimental and model results indicate both the deposition rate coefficient (k) and maximum deposition (S_max) linearly decrease with the logarithm of the DLVO energy barrier, while the mass recovery rate of PSNPs (R_m) exponentially increases with the logarithm of the DLVO energy barrier. Therefore, the mobility and associated kinetic parameters of PSNPs in complex porous media containing clay minerals can be predicted by 2D distributions of DLVO interaction energy. These findings could help to gain insight into understanding the environmental behavior and transport mechanism of microplastics in the multicomponent porous media, and provide a scientific basis for the accurate simulation and prediction of microplastic contamination in the groundwater system.

Micro- and nanoplastics retention in porous media exhibits different dependence on grain surface roughness and clay coating with particle size

The presence and/or coating of natural colloids (e.g., clays and metal oxides or hydroxides) on collector surfaces has frequently been demonstrated to enhance the retention of engineered colloids that are negatively charged due to favorable electrostatic interactions. However, this work demonstrates that the presence of natural clay coating can lead to reduced or nonmonotonic retention of micro- and nanoplastics (MNPs). Column experiments were carried out to systematically investigate the transport of MNPs with different sizes in relatively smooth and rough sands that had various clay coating fractions. These coating fractions on the collector were found to significantly influence MNP retention in a complex manner that changed with the colloid size and the roughness properties of the sand. This observation was attributed to the impact of clay coatings on the roughness and morphology properties of collector surfaces that were dominant over surface charge. Scanning electron microscopy and interaction energy calculations on surfaces with pillars or valleys indicate that mechanisms that contributed to MNP retention changed with the colloid size. In particular, retention of nanosized plastics was mainly controlled by interactions on convex/concave locations that changed with the solution chemistry, whereas microsized plastics were also strongly influenced by the applied hydrodynamic torque and straining processes. Additionally, the significant sensitivity of MNP retention under a low-level ionic strength also reflects the importance of roughness and charge heterogeneities. These observations are important for investigating the mechanisms of colloid transport in natural systems that ubiquitously exhibit clay coating on their surfaces.

Nanoplastics interaction with feldspar and weathering originated secondary minerals (kaolinite and gibbsite) in the riverine environment

Despite the massive accumulation of nanoplastics (NPs) in the freshwater system, research so far has highly focused on the marine environment. NPs interaction with mineral surfaces can influence their fate in freshwater, which will further impact their bioavailability and transport to the oceans. Current work focuses on understanding NPs interaction with weathering sequence of minerals in freshwater under varying geochemical conditions. Primary mineral feldspar and weathering originated secondary minerals, i.e., kaolinite and gibbsite, were investigated for interaction with NPs under batch mode under relevant environmental conditions. Minerals-NPs interaction was also investigated in natural water samples. Results showed that the amorphous nature, small particle size, and positive surface charge of gibbsite resulted in multi-fold sorption of NPs (108.1 mg/g) compared to feldspar (7.7 mg/g) and kaolinite (11.9 mg/g). FTIR spectroscopy revealed hydrogen bonding and complexation as major players in gibbsite-NPs interaction suggesting the possibility of their co-precipitation. The continuous adsorption-desorption and limited sorption capacity of feldspar and kaolinite can be attributed to their negative surface charge, larger size, crystalline nature, and physical sorption. Therefore, both minerals may co-transport and enhance the mobility of NPs.

Transport and deposition behaviors of microplastics in porous media: Co-impacts of N fertilizers and humic acid

Due to the interaction of fertilizers with microplastics (MPs) and porous media, fertilization process would influence MPs transport and distributions in soil. The co-impacts of N fertilizers (both inorganic and organic N fertilizers) and humic substance on MPs transport/retention behaviors in porous media were examined in 10 mM KCl solutions at pH 6. NH₄Cl and CO(NH₂)₂ were employed as inorganic and organic N fertilizers, respectively, while humic acid (HA) was used as model humic substance. We found that for all three sized MPs (0.2, 1 and 2 µm) without HA, both types of N fertilizers decreased their transport/increased their retention in porous media (both quartz sand and soil). N fertilizers adsorbed onto surfaces of MPs and sand/soil, lowering the electrostatic repulsion between MPs and porous media, thus contributed to the enhanced MPs deposition. MPs with N fertilizers in solutions more tightly attached onto porous media and thus were more difficult to be re-mobilized by low ionic strength solution elution. Via steric repulsion and increasing electrostatic repulsion between MPs and porous media due to adsorption onto their surfaces, HA could increase MPs transport with N fertilizers in solutions.