Deposition and mobilization of viruses in unsaturated porous media: Roles of different interfaces and straining

E. coli cotransport with composite colloid in unsaturated porous media: Multi-risk on migration and biomolecular response

The diversity of colloidal types and the differences in the composite ratios in porous media are important factors governing the migration and biological risk of pathogenic microorganisms in the subsurface environment. In this study, E. coli O157:H7 was subjected to co-migration experiments with different compositions of the composite colloid montmorillonite (MMT)-Fe2O3, and the biomolecular response of E. coli under the action of colloids was analyzed by Raman spectroscopy to quantify the risk of E. coli under the action of composite colloids based on both. The results showed that Fe2O3 colloids inhibited E. coli migration mainly by electrostatic adsorption and reduced E. coli metabolism. MMT colloid inhibited E. coli migration mainly by blockage, and E. coli metabolism increased, and surface macromolecules decreased to reduce E. coli adhesion. MMT-Fe2O3 complex colloids inhibited migration through electrostatic attraction between the two and formation of cohesive colloids, with reduced E. coli metabolism and insignificant biomolecular response. It was briefly assessed that the composite colloids reduced E. coli risk less strongly than single colloids, stemming from the difference in the mechanism of influence and the actual need to consider colloid interactions. This conclusion can inform the management and control of pathogen risk in porous media environments.

E. coli phage transport in porous media: Response to colloid types and water saturation

Because of its long survival time, high migration ability and high pathogenicity, the migration of the virus in the subsurface environment deserves in-depth exploration and research. In this study we investigated the migration behavior of E. coli phage (VI) with organic colloids (HA) or inorganic colloids (SiO2) in the saturated or unsaturated bands and compared the differences in their migration mechanisms.The effects of different colloids on the surface characteristics of VI were analyzed according to particle size and zeta potential. Column experiments were conducted to simulate their migration in the subsurface environment. The results show that HA enhances the stability of VI-HA, promotes VI migration and plays a dominant role in its migration process under both saturated and unsaturated conditions. In contrast, SiO2 puts VI-SiO2 in an unstable state and is easily separated in the unsaturated state, thus promoting VI migration. Based on migration experiments, the extent of influence factors on VI migration was quantified and compared. The effect of colloids on VI migration is greater than that of moisture content, where the effect of HA is greater than that of SiO2. This study provides an experimental research idea to determine the dominant factors affecting virus migration, and provides a general direction and theoretical basis for the biological risk assessment of pathogenic microorganisms in complex underground environments, in order to enable the decision makers to make a response in time, accurately, and efficiently.

Transport and Retention of Fecal Indicator Bacteria in Unsaturated Porous Media: Effect of Transient Water Flow

For production of clean drinking water, the processes governing bacterial remobilization in the unsaturated zone at transient water flow are critical. Although managed aquifer recharge is an effective way to dispose of pathogens, there are concerns about recontamination after heavy precipitation. To better understand how bacteria that were initially retained in porous media can be released to groundwater due to transient water content, transport experiments and modeling for Escherichia coli and Enterococcus moraviensis were conducted at the soil column scale. After inoculating dune sand columns with a bacteria suspension for 4 h, three rainfall events were performed at 24-h intervals. The effluent from sand columns was collected to analyze bacteria breakthrough curves (BTCs). After the rainfall experiments, the bacteria distribution in the sand column was determined. The collected BTCs and profile retentions were modeled with HYDRUS-1D, using different model concepts, including one-site kinetic attachment/detachment (M1), Langmuirian (M2), Langmuirian and blocking (M3), and two-site attachment/detachment (M4). After inoculation, almost 99% of the bacteria remained in the soil. The M1 and M2 bacteria models had a high agreement between observed and modeled concentrations, and attachment and detachment were two significant mechanisms for regulating bacteria movement in a porous medium with fluctuations in water flow. At the end of the experiment, the majority of bacteria were still found within the depth range of 5 cm to 15 cm. Our experiments show that E. coli is more mobile in sandy soils than E. moraviensis. The results of this study also suggest that the unsaturated zone is an important barrier between microbial contamination at the soil surface and groundwater. Follow-up studies are needed to completely understand the variables that regulate bacteria remobilization in the unsaturated zone of dune sands. IMPORTANCE At managed artificial recharge sites in the Netherlands, recontamination of infiltrated water with fecal indicator bacteria has been observed. The results of this study suggest that the unsaturated zone is an important barrier between microbial contamination at the soil surface and groundwater. Bacteria that accumulate in the unsaturated zone, on the other hand, can multiply to such an extent that they can be released into the saturated zone when saturation increases due to major rain events or a rise in groundwater level.

A new workflow for assigning removal credits to assess overall performance of managed aquifer recharge (MAR)

Pathogen removal in managed aquifer recharge (MAR) systems is dependent upon numerous operational, physicochemical water quality, and biological parameters. Due to the site-specific conditions affecting these parameters, guidelines for specifying pathogen removal have historically taken rather precautionary and conservative approaches in order to protect groundwater quality and public health. A literature review of regulated pathogens in MAR applications was conducted and compared to up-and-coming indicators and surrogates for pathogen assessment, all of which can be gathered into a toolbox from which regulators and operators alike can select appropriate pathogens for monitoring and optimization of MAR practices. Combined with improved knowledge of pathogen fate and transport obtained through lab- and pilot-scale studies and supported by modeling, this foundation can be used to select appropriate, site-specific pathogens for regarding a more efficient pathogen retention, ultimately protecting public health and reducing costs. This paper outlines a new 10 step-wise workflow for moving towards determining robust removal credits for pathogens based on risk management principles. This approach is tailored to local conditions while reducing overly conservative regulatory restrictions or insufficient safety contingencies. The workflow is intended to help enable the full potential of MAR as more planned water reuse systems are implemented in the coming years.

Migration risk of Escherichia coli O157:H7 in unsaturated porous media in response to different colloid types and compositions

The vadose zone is a critical zone for microbial entry into the subsurface environment, and various types of inorganic and organic colloids can affect the migration of pathogenic bacteria. In the study, we explored the migration behavior of Escherichia coli O157:H7 with humic acids (HA), iron oxides (Fe₂O₃) or their mixture, uncovering their migration mechanisms in the vadose zone. The effect of complex colloids on the physiological properties of E. coli O157:H7 was analyzed based on the measured particle size, zeta potential and contact angle. HA colloids significantly promoted the migration of E. coli O157:H7, where Fe₂O₃ was opposite. The migration mechanism of E. coli O157:H7 with HA and Fe₂O₃ is obviously different. Multiple colloids dominated by organic colloid will further highlight its promoting effect on E. coli O157:H7 under the guidance of electrostatic repulsion due to the influence of colloidal stability. Multiple colloids dominated by metallic colloid will inhibit the migration of E. coli O157:H7 under the control of capillary force due to the restriction of contact angle. The risk of secondary release of E. coli O157:H7 can be effectively reduced when the ratio of HA/Fe₂O₃ is ≥ 1. Combining this conclusion with the distribution characteristics of soil in China, an attempt was made to analyse the migration risk of E. coli O157:H7 on a national scale. In China, from north to south, the migration capacity of E. coli O157:H7 gradually decreased, and the risk of secondary release gradually increased. These results provide ideas for the subsequent study of the effect of other factors on the migration of pathogenic bacteria on a national scale and provide risk information about soil colloids for the construction of pathogen risk assessment model under comprehensive conditions in the future.

Chromium Transport and Fate in Vadose Zone: Effects of Simulated Acid Rain and Colloidal Types

Chromium (Cr) can enter groundwater through rainfall infiltration and significantly affects human health. However, the mechanisms by which soil colloids affect chromium transport are not well investigated. In this study, column experiments were conducted to simulate the chromium (Cr) transport mechanism in two typical soils (humic acid + cinnamon soil and montmorillonite + silt) in the vadose zone of a contaminated site and the effects of acid rain infiltration conditions. The results showed that Mt colloids have less influence on Cr. The fixation of Cr by colloid mainly occurs in the cinnamon soil layer containing HA colloid. The adsorption efficiency of Cr was increased by 12.8% with the addition of HA. In the HA-Cr system, the introduction of SO4^2- inhibited the adsorption of Cr, reducing the adsorption efficiency from 31.4% to 24.4%. The addition of Mt reduced the adsorption efficiency of Cr by 15%. In the Mt-Cr system, the introduction of SO4^2- had a promoting effect on Cr adsorption, with the adsorption efficiency increasing from 4.4% to 5.1%. Cr release was inhibited by 63.88% when HA colloid was present, but the inhibition owing to changes in acidity was only 14.47%. Mt colloid promotes Cr transport and increases the leaching rate by 2.64% compared to the absence of Mt. However, the effect of acidity change was not significant. Intermittent acid rain will pose a higher risk of pollutant release. Among the influencing factors, the type of colloid had the most significant influence on the efficiency of Cr leaching. This study guides the quantitative assessment of groundwater pollution risk caused by Cr in the vadose zone.

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

Microplastics (MPs) are susceptible to aging in the environment, and aged MPs are highly migratory in soil due to their smaller particle size and more negative surface charge, but the effects of soil environmental factors on the fate and transport of aged MPs are still unclear. In this study, the transport behavior of pristine/aged MPs in unsaturated sandy porous media was examined under different ionic strength (IS), cationic type (Na⁺, Ca²⁺) and humic acid (HA) conditions. The results indicated that the surface charge, surface oxygen-containing functional groups and surface morphology of MPs changed significantly after aging, and that the mobility of aged MPs was significantly enhanced than the pristine MPs under all test conditions. The retention amounts of pristine/aged MPs in unsaturated porous media increased with IS, and IS had a less inhibitory effect on the transport of aged MPs than pristine MPs. The mobility of pristine/aged MPs in Ca²⁺ solutions was significantly weaker than that in Na⁺ solutions due to enhanced straining and electrostatic adsorption. HA promoted the mobility of pristine/aged MPs in unsaturated porous media under all IS Na⁺ (1, 10, and 25 mM) solutions and lower IS (1 mM) Ca²⁺ solutions, and the ability of HA to promote the transport of aged MPs was significantly stronger than that of pristine MPs due to the higher adsorption of HA on the surface of aged MPs. However, at higher IS (10 mM) Ca²⁺ solution conditions, the bridging effect of Ca²⁺ led to the formation of HA-MPs complexes, which altered the hydrophobicity of the pristine/aged MPs surface and the pristine/aged MPs were mainly retained on the air-water interface (AWI). CFT theory and two-site kinetic retention models indicated that the retention of pristine/aged MPs in unsaturated media was dominated by monolayer adsorption, straining and clogging effects. The current research findings may provide insights into the fate and transport of aged MPs in soil and their potential risk of groundwater contamination.

Physiological characteristics, geochemical properties and hydrological variables influencing pathogen migration in subsurface system: What we know or not?

The global outbreak of coronavirus infectious disease-2019 (COVID-19) draws attentions in the transport and spread of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) in aerosols, wastewater, surface water and solid wastes. As pathogens eventually enter the subsurface system, e.g., soils in the vadose zone and groundwater in the aquifers, they might survive for a prolonged period of time owing to the uniqueness of subsurface environment. In addition, pathogens can transport in groundwater and contaminate surrounding drinking water sources, possessing long-term and concealed risks to human society. This work critically reviews the influential factors of pathogen migration, unravelling the impacts of pathogenic characteristics, vadose zone physiochemical properties and hydrological variables on the migration of typical pathogens in subsurface system. An assessment algorithm and two rating/weighting schemes are proposed to evaluate the migration abilities and risks of pathogens in subsurface environment. As there is still no evidence about the presence and distribution of SARS-CoV-2 in the vadose zones and aquifers, this study also discusses the migration potential and behavior of SARS-CoV-2 viruses in subsurface environment, offering prospective clues and suggestions for its potential risks in drinking water and effective prevention and control from hydrogeological points of view.

Interactive removal of bacterial and viral particles during transport through low-cost filtering materials

Pathogen filtration is critically important for water sanitation. However, it is a big challenge to balance removal efficiency and filtering material cost. In this study, we quantified the removal processes of a bacterial strain Escherichia coli 652T7 and a model bacteriophage MS2 (ATCC 15597-B1) during their transport through columns containing iron filings (IF), calcined magnesite (CM), natural ore limestone (OL) or corn stalk biochar (BC) under saturated flow conditions. Experimental results showed that 99.98, 79.55, 63.79, and 62.59% of injected E. coli 652T7 and 98.78, 92.26, 68.79, and 69.82% of injected MS2 were removed by IF, CM, OL, and BC, respectively. The differences in removal percentage were attributed to the disparities of the microorganisms and filtering materials in surface function groups, surface charges, and surface morphology. Transport modeling with advection-dispersion equation (ADE) and interaction energy calculation with extended Derjaguin, Landau, Verwey, and Overbeek (XDLVO) model indicated that E. coli 652T7 and MS2 were mostly removed via irreversible attachment. In IF columns, E. coli 652T7 promoted the transport of MS2 but not vice versa. In CM columns, MS2 facilitated the transport of E. coli 652T7 and vice versa at a less extent. Such changes were a combined result of attachment site competition, steric effect, and mechanical straining. We found that the sum of the removal percentages of the two microorganisms in their respective transport experiments were similar to those calculated from their co-transport experiments. This result suggests that the removals were mainly limited by the attachment sites in the filtering materials.

Freeze-thaw cycles induce diverse bacteria release behaviors from quartz sand columns with different water saturations

Bacteria present in natural environment especially those in cold regions would experience freeze-thaw (FT) process during day-night and season turns. However, knowledge about the influence of FT on bacteria release behaviors in porous media was limited. In present study, the bacteria release behaviors from quartz sand columns without and with 1 and 3 FT treatment cycles under three water saturations (θ=100%, 90%, and 60%) were investigated. We found that for all three water saturated columns without FT treatment, negligible bacteria released from columns via background salt solution elution, while the subsequent release of bacteria from sand columns via low ionic strength (IS) solution elution decreased with decreasing column water saturations. More importantly, we found unlike the negligible bacteria release in columns without FT treatment, for columns with high saturations (θ=100% and 90%), FT treatment could promote bacteria release with background salt solution elution. Moreover, for high saturated columns, FT treatment would decrease subsequent bacteria release with low IS solution elution. This phenomenon was more obvious with increasing FT treatment cycles. In contrast, FT treatment had negligible influence on bacteria release from columns with lower saturation (θ=60%). The decreased bacterial sizes, the loss of bacterial flagella, as well as the change of local configuration of porous media (via changing water into ice and ice back into water) during the FT processes contributed to increased bacteria release via background salt solution elution from high saturated sand columns. While, the reduced amount of bacteria being retained at secondary energy minima drove to the subsequently decreased bacteria release via low IS solution elution. The results of this study clearly showed that for porous media with high saturations, FT cycles would increase the risk of bacteria detaching from porous media with flushing by the background solution.

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

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

Transport of ZIF-8 in porous media under the influence of surfactant type and nanoparticle concentration

Knowledge of the fate and transport of metal-organic frameworks (MOFs) in porous media is essential to understanding their environmental impacts. However, to date, the transport mechanisms of MOFs are not fully revealed. Meanwhile, surfactants can promote MOFs dispersion by forming a stable suspension. They also allow MOFs to migrate in the aqueous environment, which would increase the risks of MOFs being exposed to human health and the ecological environment. In this study, the effect of surfactants type and nanoparticle (NP) concentrations (50, 100, and 200 mg/L) were investigated using a sand column to study the transportability of ZIF-8 NPs in saturated porous media. Surfactants used were categorized into three groups, including cationic surfactants (CTAB, DTAB), anionic surfactants (SDBS, SDS), and nonionic surfactants (Tween 80, Tween 20). Experimental results showed that the ionic surfactants significantly increased the transportability of ZIF-8 NPs. Furthermore, a low concentration of NPs tended to break through the column under ionic surfactant conditions, and the maximum effluent recovery of ZIF-8 NPs (50 mg/L) was 87.4% in the presence of SDS. Nevertheless, ZIF-8 NPs tended to deposit in the inlet of the sand column in the presence of nonionic surfactants due to hydrodynamic bridging and straining. This research provides a comprehensive understanding of the deposition mechanism of ZIF-8 NPs as affected by surfactant types and NP concentrations. Most importantly, the study highlights those ionic surfactants had a significant impact on the mobility of ZIF-8 NPs, which arouses attention to the ecological and human health risk assessment related to the manufacturing of MOFs with the aid of various dispersing agents.

Novel analytical expressions for determining van der Waals interaction between a particle and air-water interface: Unexpected stronger van der Waals force than capillary force

HYPOTHESIS

Analytical expressions for calculating Hamaker constant (HC) and van der Waals (VDW) energy/force for interaction of a particle with a solid water interface has been reported for over eighty years. This work further developed novel analytical expressions and numerical approaches for determining HC and VDW interaction energy/force for the particle approaching and penetrating air-water interface (AWI), respectively.

METHODS

The expressions of HC and VDW interaction energy/force before penetrating were developed through analysis of the variation in free energy of the interaction system with bringing the particle from infinity to the vicinity of the AWI. The surface element integration (SEI) technique was modified to calculate VDW energy/force after penetrating.

FINDINGS

We explain why repulsive VDW energy exists inhibiting the particle from approaching the AWI. We found very significant VDW repulsion for a particle at a concave AWI after penetration, which can even exceed the capillary force and cause strong retention in water films on a solid surface and at air-water-solid interface line. The methods and findings of this work are critical to quantification and understanding of a variety of engineered processes such as particle manipulation (e.g., bubble flotation, Pickering emulsion, and particle laden interfaces).