Contributions of Nanoscale Roughness to Anomalous Colloid Retention and Stability Behavior

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

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

Colloid Transport in Bicontinuous Nanoporous Media

Colloid transport and retention in porous media are critical processes influencing various Earth science applications, from groundwater remediation to enhanced oil recovery. These phenomena become particularly complex in the confined spaces of nanoporous media, where strong boundary layer effects and nanoconfinement significantly alter colloid behavior. In this work, we use particle dynamics models to simulate colloid transport and retention processes in bicontinuous nanoporous (BNP) media under pressure gradients. By utilizing particle-based models, we track the movement of each colloid and elucidate the underlying colloid retention mechanisms. Under unfavorable attachment conditions, the results reveal two colloid retention mechanisms: physical straining and trapping in low-flow zone. Furthermore, we investigate the effects of critical factors including colloid volume fraction, d, pressure difference, ΔP, interaction between colloids and BNP media, Ec-p, and among colloids, Ec-c, on colloid transport. Analysis of breakthrough curves and colloid displacements demonstrates that higher values of d, lower values of ΔP, and strong Ec-p attractions significantly increase colloid retention, which further lead to colloid clogging and jamming. In contrast, Ec-c has minimal impact on colloid transport due to the limited colloid-colloid interaction in nanoporous channels. This work provides critical insights into the fundamental factors governing colloid transport and retention within stochastic nanoporous materials.

Dual roles of goethite coating on the transport of plastic nanoparticles in heterogeneous porous media: The significance of collector surface roughness

This study systematically examines the roles of positive goethite on the retention and release of negative plastic nanoparticles (PSNPs) with different surface functional groups (Blank, -COOH, and -NH2). It provides the first evidence for the dual roles of goethite coatings on colloid transport; e.g., increased transport caused by surface morphology modification or decreased transport due to increased surface roughness and charge heterogeneity. Although previous work has shown that goethite-coated sand increases the retention of negative colloids, this work demonstrates that collector surface roughness can also reduce the retention of PSNPs due to increased interaction energy profiles. Nonmonotonic retention of all the different functionalized PSNPs was observed in goethite-coated rough sand, and the magnitude of variations was contingent on the PSNP functionalization, the solution ionic strength (IS), and the goethite coating. The release of PSNPs with IS decrease (phase I) and pH increase (phase II) varied significantly due to differences in energy barriers to detachment, e.g., release in phase I was inhibited in both goethite-coated sands, whereas release in phase II was enhanced in coated smooth sand but completely inhibited in rough sand. The findings of this study provide innovative insight into transport mechanisms for colloidal and colloid-associated contaminants.

Properties of biochar colloids and behaviors in the soil environment: Influencing the migration of heavy metals

Biochar pyrolyzed by biomass shows excellent application prospects for heavy metal (HM) remediation, but a part of biochar can be inevitably broken into micro- and nano-sized biochar colloids (BCs) under biological and physicochemical actions in soil. BCs derived in the process of remediation have rough surface, rich elemental species and contents, and multiple functional groups, which are similar to biochar. However, BCs have some unique colloidal properties because of their micro and nano scale size. Due to these properties, BCs exhibit strong mobilities in the soil environment, and the mobilities may be influenced by a combination of colloidal properties of BCs and environmental factors including soil colloids and other soil environmental conditions. In addition, BCs may have affinity effects on HMs through electrostatic adsorption, ion exchange, surface complexation, precipitation/co-precipitation, and redox because of the properties such as large specific surface area, and rich oxygen-containing functional groups and minerals on the surface. This review summarizes the physicochemical and migratory properties of BCs, and the internal and external factors affecting the migration of BCs in the soil environment, and the possible effects of BCs on HMs are high-lighted. This review provides a theoretical basis for the optimization of soil contaminated with HMs after remediation using biochar. Notably, the innovative idea that BCs may influence the presence of HMs in soil needs to be further confirmed by more targeted detection and analysis methods in future studies to prevent the possible environmental toxicities of the lateral and vertical diffusion of HM caused by BCs in soil.

Surfactant-Aided Stabilization of Individual Carbon Nanotubes in Water around the Critical Micelle Concentration

Surfactants are widely used to disperse single-walled carbon nanotubes (SWCNTs) and other nanomaterials for liquid-phase processing and characterization. Traditional techniques, however, demand high surfactant concentrations, often in the range of 1-2 wt/v% of the solution. Here, we show that optimal dispersion efficiency can be attained at substantially lower surfactant concentrations of approximately 0.08 wt/v%, near the critical micelle concentration. This unexpected observation is achieved by introducing "bare" nanotubes into water containing the anionic surfactant sodium deoxycholate (DOC) through a superacid-surfactant exchange process that eliminates the need for ultrasonication. Among the diverse ionic surfactants and charged biopolymers explored, DOC exhibits the highest dispersion efficiency, outperforming sodium cholate, a structurally similar bile salt surfactant containing just one additional oxygen atom compared to DOC. Employing all-atomistic molecular dynamics simulations, we unravel that the greater stabilization by DOC arises from its higher binding affinity to nanotubes and a substantially larger free energy barrier that resists nanotube rebundling. Further, we find that this barrier is nonelectrostatic in nature and does not obey the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal stability, underscoring the important role of nonelectrostatic dispersion and hydration interactions at the nanoscale, even in the case of ionic surfactants like DOC. These molecular insights advance our understanding of surfactant chemistry at the bare nanotube limit and suggest low-energy, surfactant-efficient solution processing of SWCNTs and potentially other nanomaterials.

Retention and release of black phosphorus nanoparticles in porous media under various physicochemical conditions

Black phosphorus nanosheets/nanoparticles (BPNs) are widely applied in many fields. However, the transport of BPNs in the subsurface still has not yet been reported and there is increasing concern about potential adverse impacts on ecosystems. Roles of median grain size and surface roughness, BPN concentration, and solution chemistries (pH, ionic strength, and cation types) on the retention and release of BPNs in column experiments were therefore investigated. The mobility of BPNs significantly increased with increasing grain size and decreasing surface roughness due to their influence on the mass transfer rate, number of deposition sites and retention capacity, and straining processes. Transport of BPNs was enhanced with an increase in pH and a decrease in ionic strength because of surface deprotonation and stronger repulsion that tends to reduce aggregation. The BPN transport was significantly sensitive to ionic strength, compared with other engineered nanoparticles. Additionally, charge heterogeneity and cation-bridging played a critical role in the retention of BPNs in the presence of divalent cations. Higher input concentrations increased the retention of BPNs, probably because collisions, aggregation at pore throat locations, and hydrodynamic bridging were more pronounced. Small fractions of BPNs can be released under decreasing IS and increasing pH due to the expansion of the electrical double layer and increased repulsion at convex roughness locations. A mathematical model that includes provisions for advective dispersive transport and time-dependent retention with blocking or ripening terms well described the retention and release of BPNs. These findings provide fundamental information that helps to understand the transport of BPNs in the subsurface environments.

Influence of biochar incorporation on the collector surface properties and the transport of silver nanoparticles in porous media

Biochar is widely used as a soil amendment due to its environmental friendliness and convenient availability. It is believed that the presence of biochar in porous media can influence the transport of colloidal and solute contaminants. In this study, different mass ratios of biochar were added to packed sand with a rough or smooth surface to determine the significance of biochar on the retention and release of silver nanoparticles (AgNPs). The results showed that biochar reduced the transport of AgNPs in rough and smooth sands under different solution conditions. A small amount of biochar (0.1-1% in mass percentage) can significantly enhance the retention of AgNPs due to the alteration in collector surface roughness and chemical heterogeneity that potentially reduce the energy barrier for retention. Furthermore, the retention of AgNPs in rough sand was always higher than that in smooth sand under the same experimental conditions. The presence of biochar also produced nonmonotonic retention of AgNPs mainly due to the changes in collector surface roughness. Additionally, the AgNPs retention associated with biochar tended to be irreversible due to the charge heterogeneity, while the reversible retention could mainly occur on a rough sand surface via shallow primary minima. This work highlights the significance of collector surface roughness that needs to be considered in the process of biochar amendment for practical applications to effectively immobilize colloidal contaminants in soil or groundwater.

Transport of biochar colloids under unsaturated flow condition: Roles of chemical aging and cation type

Biochar colloids released from biochar materials are ubiquitous in the environment and undergo environmental transformation processes that may alter their properties. Natural subsurface environments are usually under unsaturated conditions, which could affect the transport of biochar colloids. This study investigated the transport of pristine and aged biochar colloids under unsaturated conditions by aggregation test, bubble column experiment, and sand column experiment. After aging, the biochar showed a more negative, hydrophilic, and rougher surface. Compared with pristine biochar colloids, aged biochar colloids in NaCl solution were not retained at the air-water interface (AWI) due to their more hydrophilic and rougher surface. In CaCl₂ solution, more pristine and aged biochar colloids were retained at the AWI because Ca²⁺ weakened the electrostatic repulsion between biochar colloids and the AWI. With the decrease in saturation, the transport of pristine and aged biochar colloids decreased by 17 %‑67 % through the retention at AWI and air-water-solid (AWS) interface. The transport of biochar colloids in NaCl solution was increased by 10 %‑20 % after aging as the aged biochar was not retained at the AWI. The difference of transport between pristine and aged biochar colloids in CaCl₂ solution (<8 %) was lower than that in NaCl solution due to the enhanced retention of aggregated biochar colloids at the AWI and AWS interfaces. These results highlight the importance of the surface structure of biochar on its behavior in the environment, which is essential for assessing the potential of biochar application for carbon sequestration and environmental protection.

Effects of the polymer glass transition on the stability of nanoparticle dispersions

In addition to the repulsive and attractive interaction forces described by Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, many charged colloid systems are stabilized by non-DLVO contributions stemming from specific material attributes. Here, we investigate non-DLVO contributions to the stability of polymer colloids stemming from the intra-particle glass transition temperature (T_g). Flash nanoprecipitation is used to fabricate nanoparticles (NPs) from a library of polymers and dispersion stability is studied in the presence of both hydrophilic and hydrophobic salts. When adding KCl, stability undergoes a discontinuous decrease as T_g increases above room temperature, indicating greater stability of rubbery NPs over glassy NPs. Glassy NPs are also found to interact strongly with hydrophobic phosphonium cations (PR₄⁺), yielding charge inversion and intermediate aggregation while rubbery NPs resist ion adsorption. Differences in the lifetime of ionic structuration within mobile surface layers is presented as a potential mechanism underlying the observed phenomenon.

The roles of Fe oxyhydroxide coating and chemical aging in pyrogenic carbon nanoparticle transport in unsaturated porous media

Pyrogenic carbon (PyC) nanoparticles are widespread in the environment, which is important to global carbon cycle. PyC can exist for millions of years and undergo various environmental aging processes. To better understand the roles of Fe oxyhydroxides and water content on the pristine and aged PyC transport, adsorption and column experiments were conducted under three saturations (100%, 70%, and 40%) and three pH (5, 7, and 9) in both clean and Fe oxyhydroxide-coated sand. At high water saturations (100% and 70%), the mobility of both the pristine and aged PyC was enhanced at high pH due to strong electrostatic repulsion, and the aged PyC showed higher mobility than the pristine PyC because of its more negative charge and hydrophilic surface. The coating of Fe oxyhydroxides on sand decreased the mobility of both the pristine and aged PyC due to weak electrostatic repulsion, large specific surface area, and high roughness. At low saturation (40%), solution pH showed little effect on both the pristine and aged PyC mobility, and water saturation became the main factor affecting PyC mobility. Almost no pristine or aged PyC transported out from the Fe oxyhydroxide-coated sand column because Fe oxide increased the roughness of the sand surface, which led to a sharp increase in the air-water-solid interface and retention sites. This study demonstrates that water content, environmental aging, and Fe oxyhydroxides are significant in the fate and transport of PyC nanoparticles in environments, which provides a good fundamental understanding for the assessment of pyrogenic carbon application in environmental protection and carbon sequestration.

Two-Dimensional Modeling of Nano Zero-Valent Iron Transport and Retention before and after Phosphate Adsorption

The mobility of nano zero-valent iron (nZVI) will greatly affect its practical application as a remediation material for contaminated groundwater. One-dimensional (1D) column tests are commonly used in previous work to study its migration behavior, but the two-dimensional (2D) test is still very limited. This study reports a novel research system to study the 2D transport and retention behavior of colloids and solutes, which includes a 2D model test setup and the corresponding image analysis method. The transport behaviors of methyl orange (MO), nZVI, and phosphate-loaded nZVI (PnZVI) are studied using this system. The results show that the research system can reasonably describe the tempo-spatial concentration distribution of colloids and solutes. After phosphate adsorption, the mobility of nZVI is enhanced due to the increase in negative surface charge, which implies a potential environmental risk of nZVI to facilitate contaminant transport. The migration of PnZVI is not significantly influenced by its density, which is faster than MO in the longitudinal direction. The range of the plume of PnZVI in the longitudinal direction is larger than that of MO, which implies that PnZVI has a stronger longitudinal dispersion than MO.

Aging of colloidal contaminants and pathogens in the soil environment: Implications for nanoplastic and COVID-19 risk mitigation

Colloidal contaminants and pathogens are widely distributed in soil, whose tiny sizes and distinct surface properties render unique environmental behaviours. Because of aging, colloids can undergo dramatic changes in their physicochemical properties once in the soil environment, thus leading to diverse or even unpredictable environmental behaviour and fate. Herein, we provide a state-of-art review of colloid aging mechanisms and characteristics and implications for risk mitigation. First, we review aging-induced formation of colloidal contaminants and aging-associated changes. We place a special focus on emerging nanoplastic (NP) contaminants and associated physical, chemical, and biological aging processes in soil environments. Second, we assess aging and survival features of colloidal pathogens, especially viruses. Viruses in soils may survive from several days to months, or even several years in groundwater, depending on their rates of inactivation and the reversibility of attachment. Furthermore, we identify implications for risk mitigation based on aging mechanisms. Hotspots of (photo)chemical aging of NPs, including plastic gauzes at construction sites and randomly discarded plastic waste in rural areas, are identified as area requiring greater research attention. For COVID-19, we suggest taking greater care in regions where viruses are persist for long periods, such as cold climate regions. Soil amendment with quicklime (CaO) may act as an effective means for pathogen disinfection. Future risk mitigation of colloidal contaminants and pathogens relies on a better understanding of aging mechanisms and more sophisticated models accurately depicting processes in real soil environments.

Aggregation kinetics of biochar nanoparticles in aqueous environment: Interplays of anion type and bovine serum albumin

The colloidal particles, especially those at the nanoscale, are the most active part of the pyrogenic carbon (biochar). Increasingly applied biochar has resulted in a large number of biochar nanoparticles (NPs) being released into the environment. The aggregation of biochar NPs affects their environmental behavior and fate. The complex effects of anion type (Cl^-, SO₄^2-) and protein (bovine serum albumin, BSA) on the aggregation of wheat straw biochar (WB) and pinewood biochar (PB) NPs in solutions were investigated by the time-resolved dynamic light scattering method. The critical coagulation concentration (CCC) of WB and PB NPs in Na₂SO₄ solution was higher than their CCCs in NaCl solution, which was consistent with the Hofmeister series that SO₄^2-, a kosmotrope anion, increased the interaction between water molecules, thus enhancing the hydrophobic interactions between biochar NPs in solution and promoting their aggregation, while Cl^-, a chaotropic agent, exhibited the opposite effect. When BSA was added into the solution, BSA was adsorbed on the surface of biochar NPs and BSA corona was formed, which inhibited the aggregation of biochar NPs by inducing steric force. The enhanced stability of biochar NPs by BSA was more significant in NaCl than in Na₂SO₄ solution because BSA corona had a more negatively charged surface and a more steric structure in NaCl solution, thus generating stronger electrical repulsion and steric hindrance. The classical DLVO theory and the XDLVO theory incorporating the steric repulsion (in the presence of BSA) were used to interpret the aggregation and dispersion of biochar NPs. Through this study, we found that anion type indirectly affected the aggregation of biochar NPs by influencing the interaction between water molecules, while the aggregation of BSA-biochar NPs conjugates is mainly influenced by the surface charge and structure of BSA corona.

Significance of Non-DLVO Interactions on the Co-Transport of Functionalized Multiwalled Carbon Nanotubes and Soil Nanoparticles in Porous Media

Derjaguin-Landau-Verwey-Overbeek (DLVO) theory is typically used to quantify surface interactions between engineered nanoparticles (ENPs), soil nanoparticles (SNPs), and/or porous media, which are used to assess environmental risk and fate of ENPs. This study investigates the co-transport behavior of functionalized multiwalled carbon nanotubes (MWCNTs) with positively (goethite nanoparticles, GNPs) and negatively (bentonite nanoparticles, BNPs) charged SNPs in quartz sand (QS). The presence of BNPs increased the transport of MWCNTs, but GNPs inhibited the transport of MWCNTs. In addition, we, for the first time, observed that the transport of negatively (BNPs) and positively (GNPs) charged SNPs was facilitated by the presence of MWCNTs. Traditional mechanisms associated with competitive blocking, heteroaggregation, and classic DLVO calculations cannot explain such phenomena. Direct examination using batch experiments and Fourier transform infrared (FTIR) spectroscopy, asymmetric flow field flow fractionation (AF4) coupled to UV and inductively coupled plasma mass spectrometry (AF4-UV-ICP-MS), and molecular dynamics (MD) simulations demonstrated that MWCNTs-BNPs or MWCNT-GNPs complexes or aggregates can be formed during co-transport. Non-DLVO interactions (e.g., H-bonding and Lewis acid-base interaction) helped to explain observed MWCNT deposition, associations between MWCNTs and both SNPs (positively or negatively), and co-transport. This research sheds novel insight into the transport of MWCNTs and SNPs in porous media and suggests that (i) mutual effects between colloids (e.g., heteroaggregation, co-transport, and competitive blocking) need to be considered in natural soil; and (ii) non-DLVO interactions should be comprehensively considered when evaluating the environmental risk and fate of ENPs.

Investigating transport kinetics of polystyrene nanoplastics in saturated porous media

Understanding the fate and transport of polystyrene nanoparticles (PSNPs) in porous media under various conditions is necessary for evaluating and predicting environmental risks caused by microplastics. The transport kinetics of PSNPs are investigated by column experiment and numerical model. The surface of DLVO interaction energy is calculated to analyze and predict the adsorption and aggregation of PSNPs in porous media, which the critical ionic strength of PSNPs can be accurately investigated. The results of the DLVO energy surface suggest that when the concentration of Na⁺ increases from 1 mM to 50 mM, the DLVO energy barrier of PSNPs-silica sand (SS) decreases from 78.37 kT to 5.46 kT. As a result, PSNPs are easily adsorbed on the surface of SS and the mobility of PSNPs is reduced under the condition of a high concentration of Na⁺ (PSNPs recovery rate decreases from 62.16% to 3.65%). When the concentration of Ca²⁺ increases from 0.1 mM to 5 mM, the DLVO energy barrier of PSNPs-SS decreases from 12.10 kT to 1.90 kT, and PSNPs recovery rate decreases from 82.46% to 4.27%. Experimental and model results showed that PSNPs mobility is enhanced by increasing initial concentration, flow velocity and grain size of SS, while the mobility of PSNPs with larger particle diameter is lower. Regression analysis suggests that kinetic parameters related to PSNPs mobility are correlated with DLVO energy barriers. The environmental behavior and mechanism of PSNPs transport in porous media are further investigated in this study, which provides a scientific basis for the systematic and comprehensive evaluation of the environmental risk and ecological safety of nano-plastic particles 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.

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

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

Review on physical and chemical factors affecting fines migration in porous media

Permeability reduction and formation damage in porous media caused by fines (defined as unconfined solid particles present in the pore spaces) migration is one of the major reasons for productivity decline. It is well accepted that particle detachment occurs under imbalanced torques arising from hydrodynamic and adhesive forces exerted on attached particles. This paper reviewed current understanding on primary factors influencing fines migration as well as mathematical formulations for quantification. We also introduced salinity-related experimental observations that contradict theoretical predictions based on torque balance criteria, such as delayed particle release and attachment-detachment hysteresis. The delay of particle release during low-salinity water injection was successfully explained and formulated by the Nernst-Planck diffusion of ions in a narrow contact area. In addition to the widely recognized explanation by surface heterogeneity and the presence of low-velocity regions, we proposed a hypothesis that accounts for the shifting of equilibrium positions, providing new insight into the interpretation of elusive attachment-detachment hysteresis both physically and mathematically. The review was finalized by discussing the quantification of anomalous salinity effect on adhesion force at low- and high-salinity conditions.

Important Role of Concave Surfaces in Deposition of Colloids under Favorable Conditions as Revealed by Microscale Visualization

This study conducted saturated column experiments to systematically investigate deposition of 1 μm positively charged polystyrene latex micro-colloids (representing microplastic particles) on negatively charged rough sand, glass beads, and soil with pore water velocities (PWV) from 4.9 × 10^-5 to 8.8 × 10^-4 m/s. A critical value of PWV was found below which colloidal attachment efficiency (AE) increased with increasing PWV. The increase in AE with PWV was attributed to enhanced delivery of the colloids and subsequent attachment at concave locations of rough collector surfaces. The AE decreased with further increasing PWV beyond the threshold because the convex sites became unavailable for colloid attachment. By simulating the rough surfaces using the Weierstrass-Mandelbrot equation, the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) interaction energy calculations and torque analysis revealed that the adhesive torques could be reduced to be comparable or smaller than hydrodynamic torques even under the favorable conditions. Interestingly, scanning electron microscopic experiments showed that blocking occurred at convex sites at all ionic strengths (ISs) (e.g., even when the colloid-colloid interaction was attractive), whereas at concave sites, blocking and ripening (i.e., attached colloids favor subsequent attachment) occurred at low and high ISs, respectively. To our knowledge, our work was the first to show coexistence of blocking and ripening at high ISs due to variation of the collector surface morphology.

Effect of Agricultural Organic Inputs on Nanoplastics Transport in Saturated Goethite-Coated Porous Media: Particle Size Selectivity and Role of Dissolved Organic Matter

The transport of nanoplastics (NPs) through porous media is influenced by dissolved organic matter (DOM) released from agricultural organic inputs. Here, cotransport of NPs with three types of DOM (biochar_DOM (BC_DOM), wheat straw_DOM (WS_DOM), and swine manure_DOM (SM_DOM)) was investigated in saturated goethite (GT)-coated sand columns. The results showed that codeposition of 50 nm NPs (50NPs) with DOM occurred due to the formation of a GT-DOM-50NPs complex, while DOM loaded on GT-coated sand and 400 nm NPs (400NPs) aided 400NPs transport due to electrostatic repulsion. According to the quantum chemical calculation, humic acid and cellulose played a significant role in 50NPs retardation. Owing to its high concentration, moderate humification index (HIX), and cellulose content, SM_DOM exhibited the highest retardation of 50NPs transport and promoting effect on 400NPs transport. Owing to a high HIX, the effect of BC_DOM on the mobility of 400NPs was higher than that of WS_DOM. However, high cellulose content in WS_DOM caused it to exhibit a 50NPs retardation ability that was similar to that of BC_DOM. Our results highlight the particle size selectivity and significant influence of DOM type on the transport of NPs and elucidate their quantum and colloidal chemical-interface mechanisms in a typical agricultural environment.

Cotransport of micro- and nano-plastics with chlortetracycline hydrochloride in saturated porous media: Effects of physicochemical heterogeneities and ionic strength

Global production and use of plastics have resulted in the wide dissemination of micro- and nano-plastics (MNPs) to the natural environment. Potentially acting as a vector, the role of MNPs on the fate and transport of environmental pollutants (e.g., antibiotics such as chlortetracycline hydrochloride; CTC) has garnered global concern recently. Herein, the cotransport of MNPs and CTC in columns packed with uncoated sand or soil colloid-coated sand (SCCS) under different degrees of physicochemical heterogeneity and ionic strength was systematically explored. Our results show that MNPs and CTC inhibit the transport of each other when they coexist. The adsorption of CTC onto sand grains, soil colloids, and MNPs, as well as the aggregation of MNPs in the presence of CTC could be the major contributors to the enhanced retention of CTC and MNPs. In SCCS with different degrees of soil colloid coating, the adsorption of CTC on soil colloids is critical to influence the transport of CTC, and the nonlinear retention of MNPs to soil colloids is mainly attributed to the alteration of collector surface roughness by soil colloids. High ionic strength slightly facilitates CTC transport due to the competition for adsorption sites and the formation of CTC macromolecules, but significantly inhibits MNPs transport by suppressing the electrostatic double layers based on colloid stability theory. Consequently, the cotransport of MNPs and CTC is governed by the coupled interplay of collector surface roughness and chemical heterogeneity, due to the soil colloid coatings and the adsorbed CTC on the surfaces associated with solution chemistries such as ionic strength. Increased cotransport of MNPs and CTC occurred under a higher concentration of MNPs due to a larger number of adsorption sites for CTC. Our findings advance the current understanding of the complex cotransport of MNPs and antibiotics in the environment. This information is valuable for understanding contaminant fate and formulating strategies for environmental remediation due to the contamination of MNPs and co-occurring contaminants.

Effects of selected functional groups on nanoplastics transport in saturated media under diethylhexyl phthalate co-contamination conditions

The production and degradation of plastic remains can result in nanoplastics (NPs) formation. However, insufficient information regarding the environmental behaviors of NPs impedes comprehensive assessment of their significant threats. In this study, the transport behavior of unmodified NPs (PSNPs), carboxyl-modified NPs (PSNPs-COOH), and amino-modified NPs (PSNPs-NH₂) was investigated using column experiments in the presence and absence of goethite (GT) and diethylhexyl phthalate (DEHP). Quantum chemical computation was performed to reveal the transport mechanisms. The results showed that GT decreased the transport of NPs and the presence of DEHP decreased it further. Van der Waals forces and small electrostatic interactions coexisted between the PSNPs and GT and caused deposition. Ligand exchange caused greater deposition of PSNPs-COOH on GT-coated sand than that of PSNPs. Although hydrogen bonding existed between the DEHP and NPs with functional groups, an increase in the positive charge and chemical heterogeneity of the collector was the main reason for DEHP promoting the deposition of NPs. Because of low absolute negative zeta potential values, PSNPs-NH₂ was sensitive to chemical heterogeneity, and thus fully deposited (over 96.9%) in GT and GT-DEHP-coated columns. Generally, the deposition of NPs due to chemical heterogeneity was more significant than that due to the formation of chemical bonds and van der Waals, electrostatic, and hydrogen interactions. Our results highlight that the surface charge and functional groups significantly influence the transport behaviors of NPs and elucidate the fate of NPs in the terrestrial environment.

Favorable and unfavorable attachment of colloids in a discrete sandstone fracture

The transport of cationic amine-modified latex (AML) and anionic carboxylate-modified latex (CML) microspheres through a discrete sandstone fracture with mineralogical heterogeneity and roughness was studied. Two microsphere sizes (200 nm and 1000 nm), two ionic strengths (5 mM and 10 mM), and two specific discharges (0.35 mm.s-1 and 0.70 mm.s-1) were tested to observe the impact on transport under favorable and unfavorable conditions. The difference in retention between AML (net favorable) and CML (net unfavorable) microsphere attachment was 25% for the 200 nm microspheres and 13% for the 1000 nm microspheres. Less than 50% of the AML microspheres were retained in the fracture, postulated to be due to the effects of mineralogical heterogeneity and fracture surface roughness. The effect of an increase in ionic strength in increasing retention was significant for unfavorable attachment, but was not significant for favorable attachment conditions. The effect of specific discharge was minor for all but the 200 nm CML microspheres at 10 mM ionic strength. When flushing the fracture first with cationic microspheres, then with anionic microspheres, the recovery of anionic microspheres resembled favorable attachment presumably due to interaction with cationic microspheres that remained attached to the sandstone surface. Colloid breakthrough curves could be fit well with a two site attachment model, with reversible and irreversible sites.

Insights into the transport of pristine and photoaged graphene oxide-hematite nanohybrids in saturated porous media: Impacts of XDLVO interactions and surface roughness

The transport behaviors of nanomaterials, in especial multifunctional nanohybrids have not been well disclosed until now. In this study, environmentally relevant conditions, including cation types, ionic strength and pH, were selected to investigate the transport and retention of graphene oxide-hematite (GO-Fe₂O₃) nanohybrids and a photoaged product in saturated sandy columns. Results show that more hybridization of hematite led to decreased negative surface charge, while increased particle size and hydrophobicity of the nanohybrids, which depressed their transport according to extented Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. However, the inhibitory transport of photoaged nanohybrids was attributed to their distinct surface roughness caused by relatively high hybridization and photoirradiation. Notably the restrained transport was alleviated in the CaCl₂ saturated media, since the less surface O-functional groups of the corresponding nanohybrids reduced the cation bridging effect caused by Ca²⁺. Similarly, increasing pH promoted the transport of the nanohybrids in NaCl saturated media, particularly for the nanohybrids that contained rich O-functional groups, but exerted inconspicuous effect on mobility of the nanohybrids in CaCl₂ saturated media. These observations highlight that both XDLVO interactions and surface roughness may work together to impact the transport and fate of the burgeoning, versatile nanohybrids in the environment.

Bacteria have different effects on the transport behaviors of positively and negatively charged microplastics in porous media

Bacteria, biological colloids with wide presence in natural environments, would interact with plastic particles (emerging colloids with great concern recently) and thus would influence the fate and distribution of plastics in environment. In present research, the impacts of bacteria (both Gram (-) E. coli and Gram (+) B. subtilis) on the transport/deposition of model microplastics (MPs) in porous media were examined in NaCl salt solutions (5 and 25 mM, pH = 6). Both negative carboxylate-modified MPs (CMPs) and positive amine-modified MPs (AMPs) were concerned. We found that under both solution conditions, the presence of both types of bacteria decreased CMPs transport and enhanced retention of CMPs in sand columns. In contrast, the presence of bacteria (regardless of cell type) yet increased AMPs transport and decreased their deposition in sand columns under both ionic strength conditions. The mechanisms leading to the altered transport of CMPs and AMPs by bacteria were different. The formation of larger sized CMPs-bacteria clusters and the extra deposition sites resulted from bacteria adsorbed on quartz sand contributed to the decreased CMPs transport and enhanced their deposition in sand columns. Whereas, the formation of AMPs-bacteria clusters with overall negatively surface charge improved AMPs transport in quartz sand.

Upscaling Transport of Bacillus subtilis Endospores and Coliphage phiX174 in Heterogeneous Porous Media from the Column to the Field Scale

Groundwater contamination and transport of viruses and bacteria in aquifers are a major concern worldwide. To ascertain the ability of these aquifers to remove pathogens, tracer tests with microbial surrogates are carried out. These tests are laborious and may require special permits, and therefore, column tests are often done instead. Unfortunately, results from column tests tend to grossly overestimate removal rates when compared to the field scale, which can lead to an underestimation of groundwater contamination risks. Scale is an important consideration when examining pathogen transport through porous media, as pathogen removal is rarely a linear process. In this study, field tests were carried out with endospores of Bacillus subtilis and coliphage phiX174 over a distance of 25 m in an alluvial gravel aquifer near Vienna, Austria. The sandy gravel material from the field site was also used in column tests with the same tracers. Both attachment-detachment and colloid filtration theory were used to model these tests, as well as log-removal rates per meter. The results show that the spatial removal rate (log/m) is approximately 2 orders of magnitude higher on the column scale, when compared to the field. A comparison with the literature showed a correlation between the heterogeneity of the porous media and the difference in removal rates between the column and field scale.

Evidence on enhanced transport and release of silver nanoparticles by colloids in soil due to modification of grain surface morphology and co-transport

Natural soils have frequently been considered to decrease the mobility of engineered nanoparticles (NPs) in comparison to quartz sand due to the presence of colloids that provide additional retention sites. In contrast, this study demonstrates that the transport and release of silver nanoparticles (AgNPs) in sandy clay loam and loamy sand soils were enhanced in the presence of soil colloids that altered soil grain surface roughness. In particular, we found that the retention of AgNPs in purified soils (colloid-free and acid-treated) was more pronounced than in raw (untreated) soils or soils treated to remove organic matter (H₂O₂ or 600 °C treated). Chemical analysis and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy demonstrated that the grain surfaces of raw and organic matter-removed soils were abundant with metal oxides and colloids compared to purified soil. Column transport and release experimental results, SEM images, and interaction energy calculations revealed that a significant amount of concave locations on purified soils hindered AgNP release by diffusion or ionic strength (IS) reduction due to deep primary energy minima. Conversely, AgNPs that were retained in soils in the presence of soil colloids were more susceptible to release under IS reduction because the primary minimum was shallow on the tops of convex locations created by attached soil colloids. Additionally, a considerable fraction of retained AgNPs in raw soil was released after cation exchange followed by IS reduction, while no release occurred for purified soil under the same conditions. The AgNP release was highly associated with soil colloids and co-transport of AgNPs and soil colloids was observed. Our work is the first to show that the presence of soil colloids can inhibit deposition and facilitate the release and co-transport of NPs in soil by alteration of the soil grain surface morphology and shallow primary minimum interactions.

Colloid Interaction Energies for Surfaces with Steric Effects and Incompressible and/or Compressible Roughness

Colloid aggregation and retention in the presence of macromolecular coatings (e.g., adsorbed polymers, surfactants, proteins, biological exudates, and humic materials) have previously been correlated with electric double layer interactions or repulsive steric interactions, but the underlying causes are not fully resolved. An interaction energy model that accounts for double layer, van der Waals, Born, and steric interactions as well as nanoscale roughness and charge heterogeneity on both surfaces was extended, and theoretical calculations were conducted to address this gap in knowledge. Macromolecular coatings may produce steric interactions in the model, but non-uniform or incomplete surface coverage may also create compressible nanoscale roughness with a charge that is different from the underlying surface. Model results reveal that compressible nanoscale roughness reduces the energy barrier height and the magnitude of the primary minimum at separation distances exterior to the adsorbed organic layer. The depth of the primary minimum initially alters (e.g., increases or decreases) at separation distances smaller than the adsorbed organic coating because of a decrease in the compressible roughness height and an increase in the roughness fraction. However, further decreases in the separation distance create strong steric repulsion that dominates the interaction energy profile and limits the colloid approach distance. Consequently, adsorbed organic coatings on colloids can create shallow primary minimum interactions adjacent to organic coatings that can explain enhanced stability and limited amounts of aggregation and retention that have commonly been observed. The approach outlined in this manuscript provides an improved tool that can be used to design adsorbed organic coatings for specific colloid applications or interpret experimental observations.

Interaction of metal oxide nanoparticles with microplastics: Impact of weathering under riverine conditions

The ubiquitous distribution of microplastics (MPs) leads to inevitable interactions with the toxic pollutants present in the environment including metal-oxide nanoparticles. This study investigates the interaction of CeO₂ nanoparticles (CeNPs) with MPs generated from a disposable plastic container. Further, rough MPs (R-MPs), generated through mechanical abrasion of MPs with sand, were used to probe the impact of roughness. To understand the sorption kinetics and underlying interaction processes, batch experiments were carried out. The results distinctly indicate that CeNPs sorption occurred on MPs surfaces and was consistent with the pseudo-second order kinetics model. For pristine MPs, the sorption capacity was as high as 12.9 mg/g while for R-MPs kinetic equilibrium was achieved faster and an enhanced sorption capacity (13.4 mg/g) was identified. A rise in sorption with an increase in salinity was noted while pH and humic acid exhibited a negative correlation. The observed interactions were attributed to the aggregation profile and surface charge of CeNPs and MPs. Surprisingly, CeNPs also got loaded onto MPs in non-agitated and undisturbed conditions. The sorption process was influenced by the type of aqueous matrix and the sorption capacity at equilibrium followed the trend: distilled water> synthetic freshwater> river water. FTIR spectra, zeta potential, SEM imaging, and elemental mapping revealed electrostatic interaction as the dominant mechanism. This work contributes towards the knowledge gap on the environmental risk of MPs.

Importance of surface roughness on perfluorooctanoic acid (PFOA) transport in unsaturated porous media

Understanding the subsurface transport of perfluorooctanoic acid (PFOA) is of considerable interest for evaluating its potential risks to humans and ecosystems. In this study, packed-column experiments were conducted to examine the influence of surface roughness on PFOA transport in unsaturated glass beads, quartz sand and limestone porous media. Results showed decreasing moisture content significantly increased the air-water interfacial adsorption of PFOA and led to greater retardation in all three types of porous media. Particularly, rougher surface (limestone > quartz sand > glass beads) and smaller grain size (i.e. a larger solid specific surface area, SSSA) significantly enhanced PFOA retardation under unsaturated conditions. These results were further supported by bubble column experiments and SSSA analysis of porous media, which demonstrate that except for the factors affecting PFOA transport in solid-water interface (e.g. surface charge and chemical heterogeneity), the greater retardation of PFOA during transport is attributed to the larger air-water interfacial areas associated with rougher surface and smaller grain size and hence greater interfacial adsorption of PFOA. Our results indicated the importance of surface roughness on the retention and transport of PFOA in the unsaturated zone.

Synergies of surface roughness and hydration on colloid detachment in saturated porous media: Column and atomic force microscopy studies

Saturated column experiments were conducted to systematically examine the influence of hydration on the detachment of nano- and micro-sized latex colloids (35 nm and 1 μm, respectively) from sand. The colloids were attached on the sand in primary minima (PM) using high ionic strength (IS) NaCl solutions. The PM were predicted to be shallower and located farther from sand surfaces with increasing IS due to the hydration force. Consequently, a greater amount of colloid detachment occurred in deionized water when the colloids were initially deposited at a higher IS. Atomic force microscopy (AFM) examinations showed that both nanoscale protruding asperities and large wedge-like valleys existed on the sand surface. The influence of these surface features on the interaction energies/forces was modeled by approximating the roughness as cosinoidal waves and two intersecting half planes, respectively. The PM were deep and attachment was irreversible at concave regions for all ISs, even if the hydration force was included. Conversely, colloids were weakly attached at protruding asperities due to a reduced PM depth, and thus were responsible for the detachment upon IS reduction. The AFM examinations confirmed that the adhesive forces were enhanced and reduced (or even completely eliminated) at concave and convex locations of sand surfaces, respectively. These results have important implications for surface cleaning and prediction of the transport and fate of hazardous colloids and colloid-associated contaminants in subsurface environments.

Interactions between functionalised silica nanoparticles and Pseudomonas fluorescens biofilm matrix: A focus on the protein corona

Biofilms are microbial communities embedded in an extracellular polymeric matrix and display an enhanced tolerance to the action of antimicrobials. The emergence of novel functionalised nanoparticles is considered a promising avenue for the development of biofilm-specific antimicrobial technologies. However, there is a gap in the understanding of interactions between nanoparticles and the biofilm matrix. Particularly, questions are raised on how nanoparticle charge and surface groups play a role in aggregation when in contact with biofilm components. Herein we present the synthesis of four types of silica nanoparticles and undertake an analysis of their interactions with Pseudomonas fluorescens biofilm matrix. The effect of the biofilm matrix components on the charge and aggregation of the nanoparticles was assessed. Additionally, the study focused on the role of matrix proteins, with the in-depth characterisation of the protein corona of each nanoparticle by Liquid Chromatography with Tandem Mass Spectrometry experiments. The protein corona composition is dependent on the nanoparticle type; non-functionalised nanoparticles show less protein selectivity, whereas carboxylate-functionalised nanoparticles prefer proteins with a higher isoelectric point. These outcomes provide insights into the field of biofilm-nanoparticle interactions that can be valuable for the design of new nano-based targeting systems in future anti-biofilm applications.

Effect of particle surface corrugation on colloidal interactions

HYPOTHESIS

Production of corrugated particles generally introduces several morphological heterogeneities, such as surface roughness and local variations in the corrugation pattern, which are known from model system studies to significantly alter the colloidal interaction energy. Thus, realistic particle morphologies need to be investigated and compared to simple model shapes to yield insights into how interactions are influenced by such morphological heterogeneities.

EXPERIMENTS

We applied the surface element integration method to study the colloidal interactions of electron tomography-based, realistic, corrugated colloidal particles and their symmetric, concave polyhedral analogs by differentiating local surface features to vertices, ridges and ridge networks. We applied molecular modelling to assess the surface access of these features.

FINDINGS

Significant mixing of the interaction energy was found between the different surface features. Larger and smaller energy barrier heights and secondary minimum depths were observed compared to the concave polyhedral models with similar volume or surface area depending on the contacting surface feature. Analysis of surface area distributions suggests that the deviations originate from the altered effective contact distance as a result of surface roughness and other morphological heterogeneities. We also found that the surface access of nanoparticles is greatly impaired at the crevices between the surface corrugations.

Influence of agricultural organic inputs and their aging on the transport of ferrihydrite nanoparticles: From enhancement to inhibition

Organic matter effectively regulates nanoparticles transport. However, little is known about the effect of agricultural organic inputs on the transport of ferrihydrite nanoparticles (FHNPs) during aging. In this study, columns were filled with sand mixed with varying proportions of pristine, water-processing, or alkali-processing biochar or swine manure and used to simulate the release of organic matter and changes in surface roughness of sand grains during field aging. The influence of these factors on FHNPs transport was investigated using column experiments. The dissolved organic matter (DOM) (0.008-24.8 mg L^-1) released from agricultural organic inputs decreased the zeta potential of the FHNPs from 30.8 mV to 14.6--48.9 mV and further caused electrostatic repulsion, osmotic repulsion, and elastic-steric repulsion between FHNPs and mixed sand, thus enhancing FHNPs transport. Ferrihydrite nanoparticles transport increased with increasing content of biochar and swine manure due to the increased amount of DOM. However, with the presence of organic inputs, surface roughness up to a certain degree (the increase in specific surface area up to 4.6 m²) became the dominant inhibition factor affecting FHNPs transport. After DOM release, agricultural organic inputs decreased the enhancement of FHNPs transport; with the increase input, their rougher surface gradually increased inhibition of FHNPs transport. The strongest FHNPs retention in the alkali-processing biochar (0.2-10%) or swine manure (1-2%) mixed sand columns indicated that fully aged agricultural organic inputs strongly inhibited FHNPs transport. Our findings provided novel insights into the critical influence of agricultural organic inputs and their aging on FHNPs transport, which changed gradually from enhancement to inhibition gradually.

Release of colloidal biochar during transient chemical conditions: The humic acid effect

Our understanding of colloidal biochar (CB) transport and release is largely unknown in environments with transient chemical conditions, e.g., ionic strength (IS), pH, and especially humic acid (HA). In this study, column experiments were conducted to investigate CB transport and retention in the presence and absence of HA, and CB release under transient IS and pH conditions in saturated sand. Step reductions in solution IS from 25 to 0.01 mM produced significant release peaks of CB due to a reduction in the depth of the primary minima on rough surfaces with small energy barriers. In contrast, step increases of solution pH from 4 to 10 only slightly increased CB release presumably due to the strong buffering capacity of CB. The CB retention was diminished by HA during the deposition phase. However, the release of CB with transients in IS and pH was not influenced much when deposition occurred in the presence of HA. These observations indicate that HA increased the energy barrier during deposition but did not have a large influence on the depth of the interacting minimum during transient release. Potential explanations for these effects of HA on CB retention and transient release include enhanced repulsive electrostatic interactions and/or altering of surface roughness properties. Our findings indicated that the release of retained CB is sensitive to transient IS conditions, but less dependent on pH increases and CB deposition in the presence of HA. This information is needed to quantify potential benefits and/or adverse risks of mobile CB in natural environments.

Evidence for the critical role of nanoscale surface roughness on the retention and release of silver nanoparticles in porous media

Although nanoscale surface roughness has been theoretically demonstrated to be a crucial factor in the interaction of colloids and surfaces, little experimental research has investigated the influence of roughness on colloid or silver nanoparticle (AgNP) retention and release in porous media. This study experimentally examined AgNP retention and release using two sands with very different surface roughness properties over a range of solution pH and/or ionic strength (IS). AgNP transport was greatly enhanced on the relatively smooth sand in comparison to the rougher sand, at higher pH, and lower IS and fitted model parameters showed systematic changes with these physicochemical factors. Complete release of the retained AgNPs was observed from the relatively smooth sand when the solution IS was decreased from 40 mM NaCl to deionized (DI) water and then the solution pH was increased from 6.5 to 10. Conversely, less than 40% of the retained AgNPs was released in similar processes from the rougher sand. These observations were explained by differences in the surface roughness of the two sands which altered the energy barrier height and the depth of the primary minimum with solution chemistry. Limited numbers of AgNPs apparently interacted in reversible, shallow primary minima on the smoother sand, which is consistent with the predicted influence of a small roughness fraction (e.g., pillar) on interaction energies. Conversely, larger numbers of AgNPs interacted in deeper primary minima on the rougher sand, which is consistent with the predicted influence at concave locations. These findings highlight the importance of surface roughness and indicate that variations in sand surface roughness can greatly change the sensitivity of nanoparticle transport to physicochemical factors such as IS and pH due to the alteration of interaction energy and thus can strongly influence nanoparticle mobility in the environment.

Transport of a PAH-degrading bacterium in saturated limestone media under various physicochemical conditions: Common and unexpected retention and remobilization behaviors

Laboratory saturated columns packed with granular limestone grains were used to explore the retention and remobilization of functional bacteria FA1 under various physicochemical conditions. The unique surface properties of limestone and FA1 caused some unexpected phenomena. Solution IS, cation type, temperature and surface biological property all affected FA1 retention in the columns. The IS effect was temperature dependent and initial solution pH showed little influence due to the strong buffering ability of limestone. Perturbations of solution IS caused slight release of previously retained bacteria in some columns with NaCl as the background electrolyte, while increase in flow rate caused no release at all. When CaCl₂ was the background, bacterial remobilization only occurred following both cation exchange and IS reduction. DLVO forces incorporating with surface roughness calculation were determined to assist with interpretation of interaction mechanisms. All the experimental evidences suggest the importance of cation bridging, cation exchange, surface roughness, and hydrophobic interaction in controlling bacterium transport in saturated limestone porous media.

Transport and retention of engineered silver nanoparticles in carbonate-rich sediments in the presence and absence of soil organic matter

The transport and retention behavior of polymer- (PVP-AgNP) and surfactant-stabilized (AgPURE) silver nanoparticles in carbonate-dominated saturated and unconsolidated porous media was studied at the laboratory scale. Initial column experiments were conducted to investigate the influence of chemical heterogeneity (CH) and nano-scale surface roughness (NR) arising from mixtures of clean, positively charged calcium carbonate sand (CCS), and negatively charged quartz sands. Additional column experiments were performed to elucidate the impact of CH and NR arising from the presence and absence of soil organic matter (SOM) on a natural carbonate-dominated aquifer material. The role of the nanoparticle capping agent was examined under all conditions tested in the column experiments. Nanoparticle transport was well described using a numerical model that facilitated blocking on one or two retention sites. Results demonstrate that an increase in CCS content in the artificially mixed porous medium leads to delayed breakthrough of the AgNPs, although AgPURE was much less affected by the CCS content than PVP-AgNPs. Interestingly, only a small portion of the solid surface area contributed to AgNP retention, even on positively charged CCS, due to the presence of NR which weakened the adhesive interaction. The presence of SOM enhanced the retention of AgPURE on the natural carbonate-dominated aquifer material, which can be a result of hydrophobic or hydrophilic interactions or due to cation bridging. Surprisingly, SOM had no significant impact on PVP-AgNP retention, which suggests that a reduction in electrostatic repulsion due to the presence of SOM outweighs the relative importance of other binding mechanisms. Our findings are important for future studies related to AgNP transport in shallow unconsolidated calcareous and siliceous sands.

Transport and retention of Shewanella oneidensis strain MR1 in water-saturated porous media with different grain-surface properties

Bacterium Shewanella oneidensis strain (MR1), a facultative microorganism that plays critical role in contaminant transformation and degradation, was used as an example to decipher the transport and retention of microorganisms in water-saturated porous media with different grain-surface properties and under different ionic compositions (i.e. Na⁺, Ca²⁺, and Mg²⁺). Dolomite and quartz sands, which contained different surface charge properties, were used as the representative minerals. Dolomite was selected because its surface charges are significantly affected by solution composition. The mobility of MR1 in the dolomite column was lower than that in the quartz column, because the lower energy barrier between MR1 and dolomite than that between MR1 and quartz, resulting in the larger retention of MR1 in the dolomite column. The breakthrough curves were well simulated by the two sites kinetic model with HYDRUS-1D. The maximum concentration of attached bacteria (S_max) were positively correlated to the ionic strength regardless of mineral types. The values of S_max were about 1.1-4.0 times larger in the MR1-dolomite system than that in the MR1-quartz system under different ionic strength conditions. The retention of the MR1 on dolomite surfaces in the presence of divalent cations Ca²⁺ is significantly higher than that on quartz surfaces primarily due to the larger electrostatic attraction energy between the MR1 and dolomite grains. The findings demonstrate that the porous media with the lower negative charge has the higher capacity for the retention and deposition of MR1, potentially affecting the transport of MR1 and other bacteria in the subsurface.

Transport and retention of Microcystis aeruginosa in porous media: Impacts of ionic strength, flow rate, media size and pre-oxidization

Due to the climate change and human activity, the frequency and intensity of algal blooms have increased significantly. Recent studies have shown that during the bloom event, evaluated levels of cyanobacteria could infiltrate the drinking water treatment process and emerge in the filtered and disinfected water, thus threatening the safety of the drinking water supply. Among these concerned cyanobacteria, Microcystis aeruginosa is one of the most commonly detected species that cause algal bloom in a fresh water body. The present work was designed to investigate the transport and retention behaviour of Microcystis aeruginosa in a packed column to resolve the mechanisms that drive the transport behaviour of Microcystis under various operational conditions. The results showed that lab-cultured Microcystis aeruginosa could effectively break through the packed column regardless of ionic strength, media size or flow rate, as well as the presence of dissolved organic matter in the water under the conditions investigated. Such behaviour significantly contradicts those of fluorescent microspheres, which are commonly considered as ideal colloids. In addition, the combined impacts of pre-oxidation technologies and filtration on Microcystis aeruginosa removal were tested systematically. It was found that even the cells have been lysed/oxidized, no significant improvement of cell removals were observed in packed column. This paper provides a significant and comprehensive record of transport and retention behaviour of Microcystis aeruginosa in porous media. The results found herein suggest that in addition to the effort preventing toxin release/exposure during bloom events in source water, engineers and researchers should also pay attention to the transport and retention of Microcystis aeruginosa and other algal cells in filters to minimize the risk of breakthrough of cyanobacteria cells in the drinking water treatment process.

Influence of titanium dioxide nanoparticles on the transport and deposition of microplastics in quartz sand

The influence of titanium dioxide nanoparticles (nTiO₂) on the transport and deposition of polystyrene microplastics (MPs) in saturated quartz sand was investigated in NaCl solutions with ionic strengths from 0.1 to 10 mM at two pH conditions (pH 5 and 7). Three different-sized polystyrene (PS) MPs (diameter of 0.2, 1, and 2 μm) were concerned in present study. We found that for all three different-sized MPs in NaCl solutions (0.1, 1 and 10 mM) at both pH 5 and 7, lower breakthrough curves and higher retained profiles of MPs with nTiO₂ copresent in suspensions relative to those without nTiO₂ were obtained, demonstrating that the copresence of nTiO₂ in MPs suspensions decreased MPs transport and increased their deposition in quartz sand under all examined conditions. The mechanisms contributing to the increased MPs deposition with nTiO₂ in suspensions at two pH conditions were different. The formation of MPs-nTiO₂ heteroaggregates and additional deposition sites provided by previously deposited nTiO₂ were found to drive to the increased MPs deposition with nTiO₂ in suspensions at pH 5, while the formation of MPs-nTiO₂ aggregates, additional deposition sites and increased surface roughness induced by the pre-deposited nTiO₂ on quartz sand surfaces were responsible for the enhanced MPs deposition at pH 7. The results give insights to predict the fate and transport of different-sized MPs in porous media in the copresence of engineered nanoparticles.

Effects of myo-inositol hexakisphosphate, ferrihydrite coating, ionic strength and pH on the transport of TiO2 nanoparticles in quartz sand

Evaluating the fate and transport of nanoparticles (NPs) in the subsurface environment is critical for predicting the potential risks to both of the human health and environmental safety. It is believed that numerous environmental factors conspire to control the transport dynamics of nanoparticles, yet the effects of organic phosphates on nanoparticles transport remain largely unknown. In this work, we quantified the transport process of TiO₂ nanoparticle (nTiO₂) and their retention patterns in water-saturated sand columns under various myo-inositol hexakisphosphate (IHP) or phosphate (Pi) concentrations (0-180 μM P), ferrihydrite coating fractions (λ, 0-30%), ionic strengths (1-50 mM KCl), and pH values (4-8). The transport of nTiO₂ was enhanced at increased P concentration due to the enhanced colloidal stability. As compared with Pi at the equivalent P level, IHP showed stronger effect on the electrokinetic properties of nTiO₂ particles due to its relatively more negative charge and higher adsorption affinity, thereby facilitating the nTiO₂ transport (and thus reduced retention) in porous media. At the IHP concentration of 5 μM, the retention of nTiO₂ increased with increasing λ and ionic strength, while decreased with pH. In addition, the retention profiles of nTiO₂ showed a typical hyperexponential pattern for most scenarios mainly due to the unfavorable attachment, and can be well described by a hybrid mathematical model that coupled convection dispersion equations with a two-site kinetic model and DLVO theory. These quantitative estimations revealed the importance of IHP on affecting the transport of nTiO₂ typically in phosphorus-enriched environments. It provides new insights into advanced understanding of the co-transport of nanoparticles and phosphorus in natural systems, essential for both nanoparticle exposure and water eutrophication.