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.

Comparison of recharge from drywells and infiltration basins: A modeling study.

Drywells (DWs) and infiltration basins (IBs) are widely used as managed aquifer recharge (MAR) devices to capture stormwater runoff and recharge groundwater. However, no published research has compared the performance of these two engineered systems under shared conditions. Numerical experiments were conducted on an idealized 2D-axisymmetric domain using the HYDRUS (2D/3D) software to systematically study the performance of a circular IB design (diameter and area) and partially penetrating DW (38 m length with water table > 60 m). The effects of subsurface heterogeneity on infiltration, recharge, and storage from the DW and IB under constant head conditions were investigated. The mean cumulative infiltration (µI) and recharge (µR) volumes increased, and the arrival time of recharge decreased with the IB area. Values of µI were higher for a 70 m diameter IB than an DW, whereas the value of µR was higher for a DW after 1-year of a constant head simulation under selected subsurface heterogeneity conditions. A comparison between mean µI, µR, and mean vadose zone storage (µS) values for all DW and IB stochastic simulations (70 for each MAR scenario) under steady-state conditions demonstrated that five DWs can replace a 70 m diameter IB to achieve significantly higher infiltration and recharge over 20 years of operation. Additional numerical experiments were conducted to study the influence of a shallow clay layer by considering an IB, DW, and a DW integrated into an IB. The presence of such a low permeable layer delayed groundwater recharge from an IB. In contrast, a DW can penetrate tight clay layers and release water below them and facilitate rapid infiltration and recharge. The potential benefits of a DW compared to an IB include a smaller footprint, the potential for pre-treatments to remove contaminants, less evaporation, less mobilization of in-situ contaminants, and potentially lower maintenance costs. Besides, this study demonstrates that combining both IB and DW helps to get the best out of both MAR techniques.

Groundwater Recharge from Drywells Under Constant Head Conditions.

Drywells are widely used as managed aquifer recharge devices to capture stormwater runoff and recharge groundwater, but little research has examined the role of subsurface heterogeneity in hydraulic properties on drywell recharge efficiency. Numerical experiments were therefore conducted on a 2D-axisymmetric domain using the HYDRUS (2D/3D) software to systematically study the influence of various homogenous soil types and subsurface heterogeneity on recharge from drywells under constant head conditions. The mean cumulative infiltration (µI) and recharge (µR) volumes increased with an increase in the saturated hydraulic conductivity (Ks ) for various homogeneous soils. Subsurface heterogeneity was described by generating ten stochastic realizations of soil hydraulic properties with selected standard deviation (σ), and horizontal (X) and vertical (Z) correlation lengths. After 365 days, values of µI, µR, and the radius of the recharge area increased with σ and X but decreased with Z. The value of µR was always smaller for a homogeneous than a heterogeneous domain. This indicates that recharge for a heterogeneous profile cannot be estimated with an equivalent homogeneous profile. The value of µR was always smaller than µI and correlations were highly non-linear due to vadose zone storage. Knowledge of only infiltration volume can, therefore, lead to misinterpretation of recharge efficiency, especially at earlier times. The arrival time of the wetting front at the bottom boundary (60 m) ranged from 21-317 days, with earlier times occurring for increasing σ and Z. The corresponding first arrival location can be 0.1-44 m away from the bottom releasing point of a drywell in the horizontal direction, with greater distances occurring for increasing σ and X. This knowledge is important to accurately assess drywell recharged performance, water quantity, and water quality.

Drywell infiltration and hydraulic properties in heterogeneous soil profiles.

Drywells are increasingly used to capture stormwater runoff for surface infiltration and aquifer recharge, but little research has examined the role of ubiquitous subsurface heterogeneity in hydraulic properties on drywell performance. Numerical experiments were therefore conducted using the HYDRUS (2D/3D) software to systematically study the influence of subsurface heterogeneity on drywell infiltration. Subsurface heterogeneity was described deterministically by defining soil layers or lenses, or by generating stochastic realizations of soil hydraulic properties with selected variance (σ) and horizontal (X) and vertical (Z) correlation lengths. The infiltration rate increased when a high permeability layer/lens was located at the bottom of the drywell, and had larger vertical and especially horizontal dimensions. Furthermore, the average cumulative infiltration (I) for 100 stochastic realizations of a given subsurface heterogeneity increased with σ and X, but decreased with Z. This indicates that the presence of many highly permeable, laterally extending lenses provides a larger surface area for enhanced infiltration than the presence of isolated, highly permeable lenses. The ability to inversely determine soil hydraulic properties from numerical drywell infiltration results was also investigated. The hydraulic properties and the lateral extension of a highly permeable lens could be accurately determined for certain idealized situations (e.g., simple layered profiles) using constant head tests. However, variability in soil hydraulic properties could not be accurately determined for systems that exhibited more realistic stochastic heterogeneity. In this case, the heterogeneous profile could be replaced with an equivalent homogeneous profile and values of an effective isotropic saturated conductivity (Ks) and the shape parameter in the soil water retention function (α) could be inversely determined. The average value of Ks for 100 stochastic realizations showed a similar dependency to I on σ, X, and Z. Whereas, the average value of α had large confidence interval for soil heterogeneity parameters and played a secondary role in drywell infiltration. This research provides valuable insight on the selection of site, design, installation, and long-term performance of a drywell.

Minimizing Virus Transport in Porous Media by Optimizing Solid Phase Inactivation.

The influence of virus type (PRD1 and ΦX174), temperature (flow at 4 and 20°C), a no-flow storage duration (0, 36, 46, and 70 d), and temperature cycling (flow at 20°C and storage at 4°C) on virus transport and fate were investigated in saturated sand-packed columns. The vast majority (84-99.5%) of viruses were irreversibly retained on the sand, even in the presence of deionized water and beef extract at pH = 11. The reversibly retained virus fraction () was small (1.6 × 10 to 0.047) but poses a risk of long-term virus contamination. The value of and associated transport risk was lower at a higher temperature and for increases in the no-flow storage period due to the temperature dependency of the solid phase inactivation. A model that considered advective-dispersive transport, attachment (), detachment (), solid phase inactivation (µ), and liquid phase inactivation (µ) coefficients, and a Langmuirian blocking function provided a good description of the early portion of the breakthrough curve. The removal parameters were found to be in the order of > µ >> µ. Furthermore, µ was an order of magnitude higher than µ for PRD1, whereas µ was two and three orders of magnitude higher than µ for ΦX174 at 4 and 20°C, respectively. Transport modeling with two retention, release, and inactivation sites demonstrated that a small fraction of viruses exhibited a much slower release and solid phase inactivation rate, presumably because variations in the sand and virus surface roughness caused differences in the strength of adhesion. These findings demonstrate the importance of solid phase inactivation, temperature, and storage periods in eliminating virus transport in porous media. This research has potential implications for managed aquifer recharge applications and guidelines to enhance the virus removal by controlling the temperature and aquifer residence time.

Evaluating drywells for stormwater management and enhanced aquifer recharge.

Drywells are increasingly used for stormwater management and enhanced aquifer recharge, but only limited research has quantitatively determined the performance of drywells. Numerical and field scale experiments were, therefore, conducted to improve our understanding and ability to characterize the drywell behavior. In particular, HYDRUS (2D/3D) was modified to simulate transient head boundary conditions for the complex geometry of the Maxwell Type IV drywell; i.e., a sediment chamber, an overflow pipe, and the variable geometry and storage of the drywell system with depth. Falling-head infiltration experiments were conducted on drywells located at the National Training Center in Fort Irwin, California (CA) and a commercial complex in Torrance, CA to determine in situ soil hydraulic properties (the saturated hydraulic conductivity, Ks , and the retention curve shape parameter, α) for an equivalent uniform soil profile by inverse parameter optimization. A good agreement between the observed and simulated water heights in wells was obtained for both sites as indicated by the coefficient of determination 0.95-0.99-%, unique parameter fits, and small standard errors. Fort Irwin and Torrance drywells had very distinctive soil hydraulic characteristics. The fitted value of Ks =1.01 × 10-3 m min-1 at the Torrance drywell was consistent with the sandy soil texture at this site and the default value for sand in the HYDRUS soil catalog. The drywell with this Ks = 1.01 × 10-3 m min-1 could easily infiltrate predicted surface runoff from a design rain event (∼51.3 m3) within 5760 min (4 d). In contrast, the fitted value of Ks=2.25 × 10-6 m min-1 at Fort Irwin was very low compared to the Torrance drywell and more than an order of magnitude smaller than the default value reported in the HYDRUS soil catalog for sandy clay loam at this site, likely due to clogging. These experiments and simulations provide useful information to characterize effective soil hydraulic properties in situ, and to improve the design of drywells for enhanced recharge.

Contributions of Nanoscale Roughness to Anomalous Colloid Retention and Stability Behavior.

All natural surfaces exhibit nanoscale roughness (NR) and chemical heterogeneity (CH) to some extent. Expressions were developed to determine the mean interaction energy between a colloid and a solid-water interface, as well as for colloid-colloid interactions, when both surfaces contain binary NR and CH. The influence of heterogeneity type, roughness parameters, solution ionic strength (IS), mean zeta potential, and colloid size on predicted interaction energy profiles was then investigated. The role of CH was enhanced on smooth surfaces with larger amounts of CH, especially for smaller colloids and higher IS. However, predicted interaction energy profiles were mainly dominated by NR, which tended to lower the energy barrier height and the magnitudes of both the secondary and primary minima, especially when the roughness fraction was small. This dramatically increased the relative importance of primary to secondary minima interactions on net electrostatically unfavorable surfaces, especially when roughness occurred on both surfaces and for conditions that produced small energy barriers (e.g., higher IS, lower pH, lower magnitudes in the zeta potential, and for smaller colloid sizes) on smooth surfaces. The combined influence of roughness and Born repulsion frequently produced a shallow primary minimum that was susceptible to diffusive removal by random variations in kinetic energy, even under electrostatically favorable conditions. Calculations using measured zeta potentials and hypothetical roughness properties demonstrated that roughness provided a viable alternative explanation for many experimental deviations that have previously been attributed to electrosteric repulsion (e.g., a decrease in colloid retention with an increase in solution IS; reversible colloid retention under favorable conditions; and diminished colloid retention and enhanced colloid stability due to adsorbed surfactants, polymers, and/or humic materials).

The extent to which soil hydraulics can explain ecohydrological separation.

Field measurements of hydrologic tracers indicate varying magnitudes of geochemical separation between subsurface pore waters. The potential for conventional soil physics alone to explain isotopic differences between preferential flow and tightly-bound water remains unclear. Here, we explore physical drivers of isotopic separations using 650 different model configurations of soil, climate, and mobile/immobile soil-water domain characteristics, without confounding fractionation or plant uptake effects. We find simulations with coarser soils and less precipitation led to reduced separation between pore spaces and drainage. Amplified separations are found with larger immobile domains and, to a lesser extent, higher mobile-immobile transfer rates. Nonetheless, isotopic separations remained small (<4‰ for δ2H) across simulations, indicating that contrasting transport dynamics generate limited geochemical differences. Therefore, conventional soil physics alone are unlikely to explain large ecohydrological separations observed elsewhere, and further efforts aimed at reducing methodological artifacts, refining understanding of fractionation processes, and investigating new physiochemical mechanisms are needed.

Virus transport from drywells under constant head conditions: A modeling study.

Many arid and semi-arid regions of the world face challenges in maintaining the water quantity and quality needs of growing populations. A drywell is an engineered vadose zone infiltration device widely used for stormwater capture and managed aquifer recharge. To our knowledge, no prior studies have quantitatively examined virus transport from a drywell, especially in the presence of subsurface heterogeneity. Axisymmetric numerical experiments were conducted to systematically study virus fate from a drywell for various virus removal and subsurface heterogeneity scenarios under steady-state flow conditions from a constant head reservoir. Subsurface domains were homogeneous or had stochastic heterogeneity with selected standard deviation (σ) of lognormal distribution in saturated hydraulic conductivity and horizontal (X) and vertical (Z) correlation lengths. Low levels of virus concentration tailing can occur even at a separation distance of 22 m from the bottom of the drywell, and 6-log10 virus removal was not achieved when a small detachment rate (kd1=1 × 10⁻5 min⁻¹) is present in a homogeneous domain. Improved virus removal was achieved at a depth of 22 m in the presence of horizontal lenses (e.g., X=10 m, Z=0.1 m, σ=1) that enhanced the lateral movement and distribution of the virus. In contrast, faster downward movement of the virus with an early arrival time at a depth of 22 m occurred when considering a vertical correlation in permeability (X=1 m, Z=2 m, σ=1). Therefore, the general assumption of a 1.5-12 m separation distance to protect water quality may not be adequate in some instances, and site-specific microbial risk assessment is essential to minimize risk. Microbial water quality can potentially be improved by using an in situ soil treatment with iron oxides to increase irreversible attachment and solid-phase inactivation.

Biomaterial Surface Hydrophobicity-Mediated Serum Protein Adsorption and Immune Responses.

The nature of the protein corona forming on biomaterial surfaces can affect the performance of implanted devices. This study investigated the role of surface chemistry and wettability on human serum-derived protein corona formation on biomaterial surfaces and the subsequent effects on the cellular innate immune response. Plasma polymerization, a substrate-independent technique, was employed to create nanothin coatings with four specific chemical functionalities and a spectrum of surface charges and wettability. The amount and type of protein adsorbed was strongly influenced by surface chemistry and wettability but did not show any dependence on surface charge. An enhanced adsorption of the dysopsonin albumin was observed on hydrophilic carboxyl surfaces while high opsonin IgG2 adsorption was seen on hydrophobic hydrocarbon surfaces. This in turn led to a distinct immune response from macrophages; hydrophilic surfaces drove greater expression of anti-inflammatory cytokines by macrophages, whilst surface hydrophobicity caused increased production of proinflammatory signaling molecules. These findings map out a unique relationship between surface chemistry, hydrophobicity, protein corona formation, and subsequent cellular innate immune responses; the potential outcomes of these studies may be employed to tailor biomaterial surface modifications, to modulate serum protein adsorption and to achieve the desirable innate immune response to implanted biomaterials and devices.

Unraveling the complexities of the velocity dependency of E. coli retention and release parameters in saturated porous media.

Escherichia coli transport and release experiments were conducted to investigate the pore-water velocity (v) dependency of the sticking efficiency (α), the fraction of the solid surface area that contributed to retention (Sf), the percentage of injected cells that were irreversibly retained (Mirr), and cell release under different (10-300mM) ionic strength (IS) conditions. Values of α, Sf, and Mirr increased with increasing IS and decreasing v, but the dependency on v was greatest at intermediate IS (30 and 50mM). Following the retention phase, successive increases in v up to 100 or 150mday-1 and flow interruption of 24h produced negligible amounts of cell release. However, excavation of the sand from the columns in excess electrolyte solution resulted in the release of >80% of the retained bacteria. These observations were explained by: (i) extended interaction energy calculations on a heterogeneous sand collector; (ii) an increase in adhesive strength with the residence time; and (iii) torque balance consideration on rough surfaces. In particular, α, Sf, and Mirr increased with IS due to lower energy barriers and stronger primary minima. The values of α, Sf, and Mirr also increased with decreasing v because the adhesive strength increased with the residence time (e.g., an increased probability to diffuse over the energy barrier) and lower hydrodynamic forces diminished cell removal. The controlling influence of lever arms at microscopic roughness locations and grain-grain contacts were used to explain negligible cell removal with large increases in v and large amounts of cell recovery following sand excavation. Results reveal the underlying causes (interaction energy, torque balance, and residence time) of the velocity dependency of E. coli retention and release parameters (ksw, α, and Sf) that are not accounted for in colloid filtration theory.

Temperature dependency of virus and nanoparticle transport and retention in saturated porous media.

The influence of temperature on virus (PRD1 and ΦX174) and carboxyl-modified latex nanoparticle (50 and 100nm) attachment was examined in sand-packed columns under various physiochemical conditions. When the solution ionic strength (IS) equaled 10 and 30mM, the attachment rate coefficient (katt) increased up to 109% (p<0.0002) and the percentage of the sand surface area that contributed to attachment (Sf) increased up to 160% (p<0.002) when the temperature was increased from 4 to 20°C. Temperature effects at IS=10 and 30mM were also dependent on the system hydrodynamics; i.e., enhanced retention at a lower pore water velocity (0.1m/day). Conversely, this same temperature increase had a negligible influence on katt and Sf values when IS was 1mM or >50mM. An explanation for these observations was obtained from extended interaction energy calculations that considered nanoscale roughness and chemical heterogeneity on the sand surface. Interaction energy calculations demonstrated that the energy barrier to attachment in the primary minimum (∆Φa) decreased with increasing IS, chemical heterogeneity, and temperature, especially in the presence of small amounts of nanoscale roughness (e.g., roughness fraction of 0.05 and height of 20nm in the zone of influence). Temperature had a negligible effect on katt and Sf when the IS=1mM because of the large energy barrier, and at IS=50mM because of the absence of an energy barrier. Conversely, temperature had a large influence on katt and Sf when the IS was 10 and 30mM because of the presence of a small ∆Φa on sand with nanoscale roughness and a chemical (positive zeta potential) heterogeneity. This has large implications for setting parameters for the accurate modeling and transport prediction of virus and nanoparticle contaminants in ground water systems.

Transport and retention of bacteria and viruses in biochar-amended sand.

The transport and retention of Escherichia coli and bacteriophages (PRD1, MS2 and ФX174), as surrogates for human pathogenic bacteria and viruses, respectively, were studied in the sand that was amended with several types of biochar produced from various feedstocks. Batch and column studies were conducted to distinguish between the role of attachment and straining in microbe retention during transport. Batch experiments conducted at various solution chemistries showed negligible attachment of viruses and bacteria to biochar before or after chemical activation. At any given solution ionic strength, the attachment of viruses to sand was significantly higher than that of biochar, whereas bacteria showed no attachment to either sand or biochar. Consistent with batch results, biochar addition (10% w/w) to sand reduced virus retention in the column experiments, suggesting a potential negative impact of biochar application to soil on virus removal. In contrast, the retention of bacteria was enhanced in biochar-amended sand columns. However, elimination of the fine fraction (<60µm) of biochar particles in biochar-amended sand columns significantly reduced bacteria retention. Results from batch and column experiments suggest that land application of biochar may only play a role in microbe retention via straining, by alteration of pore size distribution, and not via attachment. Consequently, the particle size distribution of biochar and sediments is a more important factor than type of biochar in determining whether land application of biochar enhances or diminishes microbial retention.