Effects of solution chemistry and humic acid on transport and deposition of aged microplastics in unsaturated porous media

Escherichia coli and phosphate mediated the distinct retention of small-sized nano-plastic particles in seawater-saturated porous sands

Small nano-plastics (NPs, < 30 nm) with a high accumulation in biological organisms in coastal areas might react with widely presented bacteria and phosphate, which remains unclear. Therefore, the mechanisms governing the transport of two-sized NPs with Escherichia coli (E. coli) and phosphate were investigated in hyper-saline water-saturated sand porous media. The results showed that 20 nm NPs exhibited more hetero-aggregation with E. coli than 80 nm NPs, associated with lower k1d/k1 values (0.268 vs. 0.412) and more substantially suppressed depth of φmax (17.83 KBT vs. 23.44 KBT), based on two-site kinetic attachment retention model fitting and extended-Derjaguin-Landau-Verwey-Overbeek theory. Accordingly, even though the mass recovery percentage of both sized NPs alone was similar, the irreversible deposition of 20 nm NPs doubled by E. coli, increasing the coastal environmental risks. In contrast, 80 nm NPs reversibly attached to the sands with less effect by E. coli, causing secondary pollution. The copresence of phosphate pronouncedly enhanced the transportability of two-sized NPs with E. coli, especially increasing 20 nm NP mobility from 17.7 % to 39.2 % in 200 mM NaCl by preferentially adsorbing onto E. coli to avoid agglomeration with NPs. This study highlights the potential risk of small NPs in complicated coastal ecosystems.

Escherichia coli migration in saturated porous media: Mechanisms of humic acid regulation

The regulatory behavior of humic acid (HA) on the migration of Escherichia coli (E.coli) in saturated porous media has garnered considerable research interest. Although prior studies have confirmed that HA indeed facilitates the migration of E. coli in saturated porous media, investigating the migration process and regulatory mechanisms at the microscale remains challenging. This study compared the differences in the migration behavior of E. coli in saturated porous media under conditions with and without HA, revealing the dynamic mechanism by which HA regulates microbial migration through the "bacterium-medium-solution" triple interface interaction. The results indicated that E. coli achieves the transition of the "run-tumble" movement pattern (run ≈ 1 s, tumble ≈ 0.1 s) through flagellar morphological regulation, thus completing directed migration in a complex pore network. The addition of HA significantly enhanced the migration rate of E. coli, with an increase of at least 5 %. For the bacteria, HA induced the restructuring of lipopolysaccharides on the bacterial surface, altered the surface Zeta potential of the bacteria, and promoted the formation of stable hetero-aggregates between bacteria and HA. At the medium interface, HA modifies the surface charge of the medium, regulates pore structure, and increases hydrophilicity through the adsorption-desorption mechanism. In the solution system, the dissociation characteristics of HA's carboxyl and phenolic hydroxyl groups dynamically regulated the solution's ionic strength and pH value, creating a chemical microenvironment suitable for bacterial migration. This study systematically revealed the multi-dimensional mechanisms by which HA regulates microbial transport through molecular interface engineering. It provides theoretical support for establishing predictive models of pathogen migration in groundwater systems and offers important guidance for optimizing microbial control strategies in water treatment processes.

Surfactant-mediated transport of polyvinyl chloride nanoplastics in porous media: Influence of natural organic matter, natural inorganic ligands and electrolytes

This study investigates the transport behavior of polyvinyl chloride nanoplastics (PVC-NPs) in porous media under surfactant-mediated conditions through a combination of column experiments, numerical simulations, and extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) interaction energy analysis. The effects of different surfactant types, ionic species, ionic strength, humic acid (HA), and phosphate were examined. Results indicate that surfactants enhance the transport of PVC-NPs, with anionic surfactants exhibiting a stronger enhancement effect than cationic ones. Generally, the addition of cations inhibited PVC-NPs transport, with divalent Ca2+ exhibiting a stronger inhibitory effect than monovalent Na+. Interestingly, at low ionic strengths, Na+ had a stronger inhibitory effect than Ca2+. In the presence of anionic surfactants, higher Na+ concentrations promoted PVC-NPs transport. In contrast, both HA and phosphate inhibited PVC-NPs transport under cationic surfactants, with the degree of inhibition positively correlated with their concentrations. However, under anionic surfactants, high concentrations of HA inhibited PVC-NPs transport, while lower concentrations had no significant impact. Phosphate, under anionic surfactant conditions, initially inhibited but subsequently promoted PVC-NPs transport. This study provides a comprehensive understanding of the natural transport and transformation mechanisms of PVC-NPs in the environment under surfactant influence, offering a solid data foundation and theoretical framework for accurately assessing the potential ecological and human health risks posed by nanomaterials.

Retention mechanisms of microplastics in soil environments during saturation-desaturation cycles: Impact of hydrophobicity and pore geometry

Forming ubiquitous contaminants in sediments, microplastics (MPs) are of growing concern due to their rapid infiltration into the environment and detrimental effects on ecosystems and human health. Understanding MP transport dynamics in pore networks is essential for predicting their mobility in sediments and soils and developing strategies to mitigate their spread. This study examines how pore geometry and MP hydrophobicity affect retention mechanisms within porous media during saturation-desaturation cycles. Microfluidic experiments were conducted using micromodels representing porous media with varied pore characteristics. MPs with hydrophilic, hydrophobic, and mixed hydrophobicity properties were introduced into these micromodels, and high-resolution imaging analyzed their retention patterns. The results reveal distinct retention behaviors based on MP hydrophobicity and pore geometry. Hydrophilic MPs were retained through clustering and sieving within smaller throats, particularly in low-connectivity geometries, with retention reaching 25 %. Hydrophobic MPs attached strongly to the solid-water interface (SWI) during saturation and shifted to the air-water interface (AWI) during desaturation, achieving retention rates up to 40 % in high-connectivity geometries. Mixed MPs exhibited combined behaviors, with early SWI attachment and subsequent clustering and sieving, resulting in retention rates as high as 50 % in geometries with high specific surface areas. These findings highlight the role of pore geometry and MP surface properties in determining retention and mobility. Hydrophilic MPs form contamination hotspots in fine-grained sediments, while hydrophobic MPs are more mobile in high-connectivity environments. Mixed MPs persist due to multiple retention mechanisms, posing challenges for remediation. This study informs strategies to manage MP contamination in subsurface environments.

Microplastics inhibit lead binding to sediment components: Influence of surface functional groups and charge environment

The coexistence of heavy metals and microplastics in sediments is well recognized, yet the interactions within ternary systems remain underexplored, and comprehensive studies addressing the diverse sequences of sediment-microplastic-heavy metal coexistence are lacking. In this study, we systematically investigated the interactions among lead (Pb), polystyrene (PS) microplastics, and sediments (using goethite (Goe) and goethite-humic acid composite (GH) as examples) under different coexistence orders. The presence of PS significantly inhibited Pb adsorption by both Goe and GH. For Goe, adsorption kinetics and hydrochemical condition effects showed that PS reduced the electrostatic repulsion between Goe and Pb, leading to a fourfold increase in the mass transfer rate of Pb to the Goe surface. However, Pb 4f deconvolution indicated competition between PS and Pb for hydroxyl groups on Goe, resulting in a 7.4% reduction in Pb adsorption. In the GH system, hydrophobic interactions and coordination complexes between PS and humic acid on GH inhibited the electrostatic adsorption and mass transfer processes between Pb and GH. Pb adsorption behavior and changes in Pb-O content under different coexistence orders further verified that competition between PS and Pb for carboxyl and hydroxyl groups on GH led to a 28.0% reduction in Pb adsorption. This study highlights the inhibitory effect of PS on Pb adsorption by Goe and GH, providing a theoretical basis for understanding the migration and transformation patterns of microplastics and heavy metals in sediments.

Interpretable machine learning reveals transport of aged microplastics in porous media: Multiple factors co-effect

Microplastics (MPs) easily migrate into deeper soil layers, posing potential risks to subterranean habitats and groundwater. However, the mechanisms governing the vertical migration of MPs in soil, particularly aged MPs, remain unclear. In this study, we investigate the transport of MPs under varying MPs properties, soil texture and hydrology conditions. Under nearly all controlled conditions, aged MPs demonstrated a higher vertical mobility compared to virgin MPs. By employing interpretable machine learning models (IML), we not only identified the dominant role of individual parameters in the vertical migration of MPs but also discovered that the increased contribution of carbonyl index and O/C ratio in aged MPs, along with the enhanced interaction with other feature parameters, collectively promotes the elevated vertical mobility of aged MPs. The varying contributions of different feature parameters under individual control variables revealed the mechanisms of MPs vertical migration under different gradients and highlighted the dual constraints of physical obstruction and chemical retention between MPs and soil particles. The established machine learning model was also utilized to predict the differences in vertical mobilities of MPs with varying degrees of aging. The nonlinear increasing relationship between MPs vertical mobility and simulated aging time suggests that MPs can migrate to deeper soil layers shortly after entering the soil environment. The integration of laboratory experiment with IML elucidates the key drivers of vertical MP migration. It also provides a theoretical basis for the timely removal of MPs from soil and the assessment of their potential risks.

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.

Experimental and mathematical investigation of cotransport of clay and microplastics in saturated porous media

Microplastics in the subsurface cause groundwater contamination, thereby posing potential risks to human health and the ecosystem. Clay particles are ubiquitous in the subsurface and can interact and alter the transport behavior of microplastics. Hence, it is essential to understand the effect of clays on the transport behavior of microplastics to estimate the groundwater contamination potential. This study investigated the individual transport and cotransport of clay and microplastics under different pore-water velocities and sand types in saturated porous media through column experiments and mathematical modeling. Copresence of suspended microplastics retarded the transport of clay due to the preferential attachment of clay over microplastics on grain surfaces and the formation of clay-microplastic heteroaggregates which have a greater retention in sand than free clay and free microplastics. However, in contrast, cotransport with clay enhanced the transport of microplastics due to the lower affinity of microplastics than clay for deposition on grain surfaces and the lesser mass fraction of microplastics than clay in the heteroaggregates. The cotransport of clay and microplastics was successfully simulated using a two-way coupled model, which accounted for the retention of free clay and free microplastics in the sand, kinetics of clay-microplastics heteroaggregation, and heteroaggregate retention in the sand. The rates of heteroaggregation and heteroaggregate retention in sand decreased with increasing velocity and grain size, resulting in increased transport of clay and microplastics.

Impact of diatomit on the transport behavior of unmodified and carboxyl-modified nanoplastics in saturated porous media

Due to the extensive use of plastic products and unreasonable disposal, nanoplastics contamination has become one of the important environmental problems that mankind must face. The composition and structure of porous media can determine the complexity and diversity of the transport behavior of nanoplastics. In this study, the influence of diatomite (DIA) on the nanoplastics transport in porous media is investigated by column experiments combined with XDLVO interaction energy and transport model. Results suggest that the recovery rates of unmodified polystyrene nanoparticles (PSNPs) and carboxyl-modified polystyrene nanoparticles (PSNPs-COOH) in the porous media containing DIA decreases compared with that in the pure quartz sand (QS), and the BTCs showed a "blocking" pattern. The presence of DIA inhibits the transport of both PSNPs and PSNPs-COOH, but the inhibition is not significant. This may be because the presence of DIA provides more favorable deposition sites for PSNPs and PSNPs-COOH to some extent. However, since DIA itself carries a certain negative charge, this can only play a role in compressing the double electric layer for PSNPs and PSNPs-COOH with the same negative charge, and cannot destabilize them. The migration capacity of PSNPs and PSNPs-COOH is strongest in the DIA-QS porous media at pH = 7, and is weak at pH = 9 and pH = 5. The inhibition of migration at pH = 9 can be attributed to the dissolution of the DIA surface under alkaline conditions and the formation of pore and defect structures, which provide more deposition sites for PSNPs and PSNPs-COOH. The presence of humic acid (HA) leads to an increase in the mobility of PSNPs and PSNPs-COOH, and the mobility is enhanced with HA concentration. The mobility of PSNPs and PSNPs-COOH in DIA-QS decreases with ionic valence and ionic strength, and PSNPs-COOH is more significantly inhibited compared to PSNPs.

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

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

Enhanced sinks of polystyrene nanoplastics (PSNPs) in marine sediment compared to freshwater sediment: Influencing factors and mechanisms

The difference in the transport behaviors of nanoplastics consistently assistant with their toxicities to benthic and other aquatic organisms is still unclear between freshwater and marine sediments. Here, the mobilities of polystyrene nanoplastics (PSNPs) and key environmental factors including salinity and humic acid (HA) were systematically studied. In the sand column experiments, both tested PSNPs in the freshwater system (100 nm NPs (100NPs): 90.15 %; 500 nm NPs (500NPs): 54.22 %) presented much higher penetration ratio than in the marine system (100NPs: 8.09 %; 500NPs: 19.04 %). The addition of marine sediment with a smaller median grain diameter caused a much more apparent decline in NPs mobility (100NPs: from 8.09 % to 1.85 %; 500NPs: from 19.04 % to 3.51 %) than that containing freshwater sediment (100NPs: from 90.15 % to 83.56 %; 500NPs: from 54.22 % to 41.63 %). Interestingly, adding HA obviously led to decreased and slightly increased mobilities for NPs in freshwater systems, but dramatically improved performance for NPs in marine systems. Electrostatic and steric repulsions, corresponding to alteration of zeta potential and hydrodynamic diameter of NPs and sands, as well as minerals owing to adsorption of dissolved organic matter (DOM) and aggregations from varied salinity, are responsible for the mobility difference.

Starvation Process Would Induce Different Bacterial Mobilities and Attachment Performances in Porous Media without and with Nutrients on Surfaces

The influence and mechanisms of starvation on the bacterial mobile performance in porous media with different nutrition conditions are not well understood. The present study systematically investigated the impacts of starvation on the mobility and attachment of both Gram-negative and Gram-positive strains in porous media without and with nutrients on surfaces in both simulated and real water samples. We found that regardless of strain types and water chemistries, starvation would greatly inhibit bacterial attachment onto bare porous media without nutrients yet could significantly enhance cell attachment onto porous media with nutrients on their surfaces. The mechanisms driving the opposite transport behaviors induced by starvation in porous media without and with nutrients were totally different. We found that the starvation process decreased cell motility and increased repulsive force between bacteria and porous media via decreasing cell sizes and zeta potentials, reducing EPS secretion and cell hydrophobicity, thus increasing transport/inhibiting attachment of bacteria in porous media without nutrients on sand surfaces. In contrast, through strengthening the positive chemotactic response of bacteria to nutrients, the starvation process greatly enhanced bacterial attachment onto porous media with nutrients on sand surfaces. Clearly, via modification of the nutrient conditions in porous media, the mobility/attachment performance of bacteria could be regulated.

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.

Co-impacts of cation type and humic acid on migration of polystyrene microplastics in saturated porous media

The aging process of microplastics (MPs) could significantly change their physical and chemical characteristics and impact their migration behavior in soil. However, the complex effects of different cations and humic acids (HA) on the migration of aged MPs through saturated media are not clear. In this research, the migration and retention of pristine/aged PSMPs (polystyrene microplastics) under combined effects of cations (Na+, Ca2+) (ionic strength = 10 mM) and HA (0, 5, 15 mg/L) were investigated and analyzed in conjunction with the two-site kinetic retention model and DLVO theory. The findings showed that the aging process accelerated PSMPs migration under all tested conditions. Aged PSMPs were less susceptible to Ca2+ than pristine PSMPs. Under Ca2+ conditions, pristine/aged PSMPs showed higher retention than under Na+ conditions in the absence of HA. Furthermore, under Na+ conditions, the migration of aged PSMPs significantly increased at higher concentrations of HA. However, under Ca2+ conditions, the migration of aged PSMPs decreased significantly at higher concentrations of HA. In higher HA conditions, HA, Ca2+, and PSMPs interact to cause larger aggregations, resulting in the sedimentation of aged PSMPs. The DLVO calculations and two-site kinetic retention models' results showed the detention of PSMPs was irreversible under higher HA conditions (15 mg/L) with Ca2+, and aged PSMPs were more susceptible to clogging. These findings may help to understand the potential risk of migration behavior of PSMPs in the soil-groundwater environment.

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.

Interaction between microplastics and humic acid and its effect on their properties as revealed by molecular dynamics simulations

The characteristics and fates of microplastics (MPs) and humic acid (HA) in the environment are significantly influenced by their interactions. Thus, the influence of the MP-HA interaction on their dynamic characteristics was explored. Upon MP-HA interaction, the number of hydrogen bonds established in the HA domains decreased significantly, and the water molecules bridging the hydrogen bonds shifted to the exterior regions of the MP-HA aggregates. The distribution intensity of Ca²⁺ located at ∼0.21 nm around HA deceased, indicating that the coordination of Ca²⁺ with the carboxyl on HA was impaired in the presence of MPs. Additionally, the Ca²⁺-HA electrostatic interaction was suppressed because of the steric hindrance of the MPs. However, the MP-HA interaction improved the distribution of water molecules and metal cations around the MPs. The diffusion coefficient of HA decreased from 0.34 × 10^-5 cm²/s to 0.20-0.28 × 10^-5 cm²/s in the presence of MPs, implying that the diffusion of HA was retarded. The diffusion coefficients of polyethylene and polystyrene increased from 0.29 × 10^-5 cm²/s and 0.18 × 10^-5 cm²/s to 0.32 × 10^-5 cm²/s and 0.22 × 10^-5 cm²/s, respectively, indicating that the interaction with HA accelerated the migration of polyethylene and polystyrene. These findings highlight the potential environmental hazards posed by MPs in aquatic environments.