Scott A. Bradford

Physical factors affecting the transport and fate of colloids in saturated porous media

[1] Saturated soil column experiments were conducted to explore the influence of colloid size and soil grain size distribution characteristics on the transport and fate of colloid particles in saturated porous media. Stable monodispersed colloids and porous media that are negatively charged were employed in these studies. Effluent colloid concentration curves and the final spatial distribution of retained colloids by the porous media were found to be highly dependent on the colloid size and soil grain size distribution. Relative peak effluent concentrations decreased and surface mass removal by the soil increased when the colloid size increased and the soil median grain size decreased. These observations were attributed to increased straining of the colloids; i.e., blocked pores act as dead ends for the colloids. When the colloid size is small relative to the soil pore sizes, straining becomes a less significant mechanism of colloid removal and attachment becomes more important. Mathematical modeling of the colloid transport experiments using traditional colloid attachment theory was conducted to highlight differences in colloid attachment and straining behavior and to identify parameter ranges that are applicable for attachment models. Simulated colloid effluent curves using fitted first-order attachment and detachment parameters were able to describe much of the effluent concentration data. The model was, however, less adequate at describing systems which exhibited a gradual approach to the peak effluent concentration and the spatial distribution of colloids when significant mass was retained in the soil. Current colloid xfiltration theory did not adequately predict the fitted first-order attachment coefficients, presumably due to straining in these systems. INDEX TERMS: 1831 Hydrology: Groundwater quality; 1832 Hydrology: Groundwater transport

Straining and Attachment of Colloids in Physically Heterogeneous Porous Media

Many soil column and batch experiments have been conducted to quantify fundamental properties and proColloid transport studies were conducted in water-saturated physicesses that control colloid transport and fate in the subcally heterogeneous systems to gain insight into the processes controlling transport in natural aquifer and vadose zone (variably saturated) surface, including sedimentation (Wan et al., 1995), hysystems. Stable monodispersed colloids (carboxyl latex microspheres) drodynamics (Wang et al., 1981; Tan et al., 1994), ionic and porous media (Ottawa quartz sands) that are negatively charged strength (Abu-Sharar et al., 1987), pH (Suarez et al., were employed in these studies. The physically heterogeneous systems 1984), chemical heterogeneity (Song and Elimelech, consisted of various combinations of a cylindrical sand lens embedded 1994; Song et al., 1994), hydrophobicity (Bales et al., in the center of a larger cylinder of matrix sand. Colloid migration was 1993), and surfactants (Ryan and Gschwend, 1994). The found to strongly depend on colloid size and physical heterogeneity. A primary mechanism of colloid mass removal by the soil decrease in the peak effluent concentration and an increase in the is typically ascribed to colloid attachment. Attachment colloid mass removal in the sand near the column inlet occurred when is the removal of colloids from solution via collision the median grain size of the matrix sand decreased or the size of the with and fixation to the solid phase, and is dependent colloid increased. These observations and numerical modeling of the transport data indicated that straining was sometimes an important on colloid–colloid, colloid–solvent, and colloid–porous mechanism of colloid retention. Experimental and simulation results media interactions. According to traditional clean-bed suggest that attachment was more important when the colloid size attachment theory (first-order attachment), colloid rewas small relative to the sand pore size. Transport differences between moval by a filter bed decreases exponentially with conservative tracers and colloids were attributed to flow bypassing depth. Colloid attachment theory also predicts an optiof finer-textured sands, colloid retention at interfaces of soil textural mum particle size for transport for a given aqueouscontrasts, and exclusion of colloids from smaller pore spaces. Colloid porous medium system (Yao et al., 1971; Rajagopalan retention in the heterogeneous systems was also influenced by spatial and Tien, 1976). Smaller particles are predicted to be variations in the pore water velocity. Parameters in straining and removed more efficiently by diffusive transport, and attachment models were successfully optimized to the colloid translarger particles by sedimentation and interception. port data. The straining model typically provided a better description of the effluent and retention data than the attachment model, espeExperimental observations of colloid transport are cially for larger colloids and finer-textured sands. Consistent with not always in agreement with colloid attachment theory previously reported findings, straining occurred when the ratio of the (Tufenkji et al., 2003). For example, researchers have colloid and median grain diameters was 0.5%. reported enhanced colloid retention at the soil surface (Camesano and Logan, 1998) and that the spatial distribution of retained colloids does not follow a simple I organic, and microbiological colloids exist exponential decrease with depth (Bolster et al., 1999; in natural and contaminated aquifer and vadose zone Redman et al., 2001; Bradford et al., 2002). Some of environments. These colloid particles can be released these discrepancies have been attributed to soil surface into soil solution and groundwater through a variety of roughness (Kretzschmar et al., 1997; Redman et al., hydrologic, geochemical, and microbiological processes 2001), charge heterogeneity (Johnson and Elimelech, (MacCarthy and Zachara, 1989; Ryan and Elimelech, 1995), and variability in colloid characteristics (Bolster 1996). Knowledge of the processes that control colloid et al., 1999). A time-dependent attachment rate has also transport and fate is required to assess the contaminabeen reported to occur as a result of differences in the tion potential and to protect drinking water supplies attachment behavior of colloids on clean porous media from pathogenic microorganisms (Bitton and Harvey, and on media already containing attached colloids 1992), to develop engineered bioaugmentation and bior(Johnson and Elimelech, 1995). Blocking and ripening emediation strategies (Wilson et al., 1986), and to devise refer to decreasing and increasing attachment with microbially enhanced oil recovery systems (MacLeod et time, respectively. al., 1988). Furthermore, the high surface area of colloids Some of the discrepancies between colloid transport facilitates the sorption of many organic and inorganic data and attachment theory may also be due to the fact contaminants. Colloids can then act as a mobile solid that attachment theory does not account for straining. phase that accelerates the transport of sorbed contamiStraining is the trapping of colloid particles in downnants (Kretzschmar et al., 1999; Ouyang et al., 1996). gradient pore throats that are too small to allow particle passage (McDowell-Boyer et al., 1986). The magnitude of colloid retention by straining depends on both colloid S.A. Bradford, M. Bettahar, J. Simunek, and M.Th. van Genuchten, George E. Brown, Jr. Salinity Laboratory, USDA, ARS, 450 West Big and porous medium properties. Straining occurs when Springs Road, Riverside CA 92507-4617. Received 1 May 2003. Special colloids are retained in dead-end pores that are smaller Section: Colloids and Colloid-Facilitated Transport of Contaminants than some critical size. Colloid transport may still occur in Soils. *Corresponding author (sbradford@ussl.ars.usda.gov). in pores that are larger than this critical size. SakthivadiPublished in Vadose Zone Journal 3:384–394 (2004).  Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: PV, pore volume.

Estimating interfacial areas for multi-fluid soil systems

Abstract Knowledge of the fluid-fluid and fluid-solid interfacial areas is important to better understand and quantify many flow and transport processes in porous media. This paper presents estimates for interfacial areas of porous media containing two or three fluids from measured capillary pressure (Pc)-saturation (S) relations. The thermodynamic treatment of two-fluid Pc−S relations presented by Morrow (1970) served as the basis for the predictions. In media containing two fluids (air-oil, air-water, oil-water), the solid-nonwetting interfacial area ( A sN ∗ ) equaled zero when the solid was completely wetted by the wetting fluid. The area under the Pc−S curve was directly proportional to the nonwetting-wetting interfacial area ( A NW ∗ ). If the solid surface was not completely wetted by one fluid, A NW ∗ and A sN ∗ were estimated by weighed partitioning of the area under the Pc−S curve. For porous media with fractional wettability, the procedure was applied separately to water- and oil-wet regions. The values of A NW ∗ and A sN ∗ were highest and lowest, respectively, in systems that were strongly wetted. In three-fluid media the wetting and spreading behavior of the liquids greatly affected the estimated interfacial areas. For a water-wet medium with a continuous intermediate oil phase, the interfacial areas were predicted from Pc−S data in a similar manner as for two-fluid media. The oil-water and oil-solid interfacial areas were estimated from the oil-water Pc−S curve, while the air-oil interfacial area was obtained from the air-oil Pc−S curve. For a fractional wettability or oil-wet medium there may be as many as six interfaces. These interfacial areas were estimated from three-fluid Pc−S relations based on previously developed methods for predicting three-fluid Pc−S relations from two-fluid data.

Sensitivity of the transport and retention of stabilized silver nanoparticles to physicochemical factors

Saturated sand-packed column experiments were conducted to investigate the influence of physicochemical factors on the transport and retention of surfactant stabilized silver nanoparticles (AgNPs). The normalized concentration in breakthrough curves (BTCs) of AgNPs increased with a decrease in solution ionic strength (IS), and an increase in water velocity, sand grain size, and input concentration (Co). In contrast to conventional filtration theory, retention profiles (RPs) for AgNPs exhibited uniform, nonmonotonic, or hyperexponential shapes that were sensitive to physicochemical conditions. The experimental BTCs and RPs with uniform or hyperexponential shape were well described using a numerical model that considers time- and depth-dependent retention. The simulated maximum retained concentration on the solid phase (Smax) and the retention rate coefficient (k1) increased with IS and as the grain size and/or Co decreased. The RPs were more hyperexponential in finer textured sand and at lower Co because of their higher values of Smax. Conversely, RPs were nonmonotonic or uniform at higher Co and in coarser sand that had lower values of Smax, and tended to exhibit higher peak concentrations in the RPs at lower velocities and at higher solution IS. These observations indicate that uniform and nonmonotonic RPs occurred under conditions when Smax was approaching filled conditions. Nonmonotonic RPs had peak concentrations at greater distances in the presence of excess amounts of surfactant, suggesting that competition between AgNPs and surfactant diminished Smax close to the column inlet. The sensitivity of the nonmonotonic RPs to IS and velocity in coarser textured sand indicates that AgNPs were partially interacting in a secondary minimum. However, elimination of the secondary minimum only produced recovery of a small portion (<10%) of the retained AgNPs. These results imply that AgNPs were largely irreversibly interacting in a primary minimum associated with microscopic heterogeneity.

Transport and Fate of Microbial Pathogens in Agricultural Settings

An understanding of the transport and survival of microbial pathogens (pathogens hereafter) in agricultural settings is needed to assess the risk of pathogen contamination to water and food resources, and to develop control strategies and treatment options. However, many knowledge gaps still remain in predicting the fate and transport of pathogens in runoff water, and then through the shallow vadose zone and groundwater. A number of transport pathways, processes, factors, and mathematical models often are needed to describe pathogen fate in agricultural settings. The level of complexity is dramatically enhanced by soil heterogeneity, as well as by temporal variability in temperature, water inputs, and pathogen sources. There is substantial variability in pathogen migration pathways, leading to changes in the dominant processes that control pathogen transport over different spatial and temporal scales. For example, intense rainfall events can generate runoff and preferential flow that can rapidly transport pathogens. Pathogens that survive for extended periods of time have a greatly enhanced probability of remaining viable when subjected to such rapid-transport events. Conversely, in dry seasons, pathogen transport depends more strongly on retention at diverse environmental surfaces controlled by a multitude of coupled physical, chemical, and microbiological factors. These interactions are incompletely characterized, leading to a lack of consensus on the proper mathematical framework to model pathogen transport even at the column scale. In addition, little is known about how to quantify transport and survival parameters at the scale of agricultural fields or watersheds. This review summarizes current conceptual and quantitative models for pathogen transport and fate in agricultural settings over a wide range of spatial and temporal scales. The authors also discuss the benefits that can be realized by improved modeling, and potential treatments to mitigate the risk of waterborne disease transmission.

Transport and retention of multi-walled carbon nanotubes in saturated porous media: Effects of input concentration and grain size

Water-saturated column experiments were conducted to investigate the effect of input concentration (C₀) and sand grain size on the transport and retention of low concentrations (1, 0.01, and 0.005 mg L⁻¹) of functionalized ¹⁴C-labeled multi-walled carbon nanotubes (MWCNT) under repulsive electrostatic conditions that were unfavorable for attachment. The breakthrough curves (BTCs) for MWCNT typically did not reach a plateau, but had an asymmetric shape that slowly increased during breakthrough. The retention profiles (RPs) were not exponential with distance, but rather exhibited a hyper-exponential shape with greater retention near the column inlet. The collected BTCs and RPs were simulated using a numerical model that accounted for both time- and depth-dependent blocking functions on the retention coefficient. For a given C₀, the depth-dependent retention coefficient and the maximum solid phase concentration of MWCNT were both found to increase with decreasing grain size. These trends reflect greater MWCNT retention rates and a greater number of retention locations in the finer textured sand. The fraction of the injected MWCNT mass that was recovered in the effluent increased and the RPs became less hyper-exponential in shape with higher C₀ due to enhanced blocking/filling of retention locations. This concentration dependency of MWCNT transport increased with smaller grain size because of the effect of pore structure and MWCNT shape on MWCNT retention. In particular, MWCNT have a high aspect ratio and we hypothesize that solid phase MWCNT may create a porous network with enhanced ability to retain particles in smaller grain sized sand, especially at higher C₀. Results demonstrate that model simulations of MWCNT transport and fate need to accurately account for observed behavior of both BTCs and RPs.

Retention and Remobilization of Stabilized Silver Nanoparticles in an Undisturbed Loamy Sand Soil

Column experiments were conducted with undisturbed loamy sand soil under unsaturated conditions (around 90% saturation degree) to investigate the retention of surfactant stabilized silver nanoparticles (AgNPs) with various input concentration (Co), flow velocity, and ionic strength (IS), and the remobilization of AgNPs by changing the cation type and IS. The mobility of AgNPs in soil was enhanced with decreasing solution IS, increasing flow rate and input concentration. Significant retardation of AgNP breakthrough and hyperexponential retention profiles (RPs) were observed in almost all the transport experiments. The retention of AgNPs was successfully analyzed using a numerical model that accounted for time- and depth-dependent retention. The simulated retention rate coefficient (k1) and maximum retained concentration on the solid phase (Smax) increased with increasing IS and decreasing Co. The high k1 resulted in retarded breakthrough curves (BTCs) until Smax was filled and then high effluent concentrations were obtained. Hyperexponential RPs were likely caused by the hydrodynamics at the column inlet which produced a concentrated AgNP flux to the solid surface. Higher IS and lower Co produced more hyperexponential RPs because of larger values of Smax. Retention of AgNPs was much more pronounced in the presence of Ca(2+) than K(+) at the same IS, and the amount of AgNP released with a reduction in IS was larger for K(+) than Ca(2+) systems. These stronger AgNP interactions in the presence of Ca(2+) were attributed to cation bridging. Further release of AgNPs and clay from the soil was induced by cation exchange (K(+) for Ca(2+)) that reduced the bridging interaction and IS reduction that expanded the electrical double layer. Transmission electron microscopy, energy-dispersive X-ray spectroscopy, and correlations between released soil colloids and AgNPs indicated that some of the released AgNPs were associated with the released clay fraction.

Humic Acid Facilitates the Transport of ARS-Labeled Hydroxyapatite Nanoparticles in Iron Oxyhydroxide-Coated Sand

Hydroxyapatite nanoparticles (nHAP) have been widely used to remediate soil and wastewater contaminated with metals and radionuclides. However, our understanding of nHAP transport and fate is limited in natural environments that exhibit significant variability in solid and solution chemistry. The transport and retention kinetics of Alizarin red S (ARS)-labeled nHAP were investigated in water-saturated packed columns that encompassed a range of humic acid concentrations (HA, 0-10 mg L(-1)), fractional surface coverage of iron oxyhydroxide coatings on sand grains (λ, 0-0.75), and pH (6.0-10.5). HA was found to have a marked effect on the electrokinetic properties of ARS-nHAP, and on the transport and retention of ARS-nHAP in granular media. The transport of ARS-nHAP was found to increase with increasing HA concentration because of enhanced colloidal stability and the reduced aggregate size. When HA = 10 mg L(-1), greater ARS-nHAP attachment occurred with increasing λ because of increased electrostatic attraction between negatively charged nanoparticles and positively charged iron oxyhydroxides, although alkaline conditions (pH 8.0 and 10.5) reversed the surface charge of the iron oxyhydroxides and therefore decreased deposition. The retention profiles of ARS-nHAP exhibited a hyperexponential shape for all test conditions, suggesting some unfavorable attachment conditions. Retarded breakthrough curves occurred in sands with iron oxyhydroxide coatings because of time-dependent occupation of favorable deposition sites. Consideration of the above effects is necessary to improve remediation efficiency of nHAP for metals and actinides in soils and subsurface environments.

Reuse of Concentrated Animal Feeding Operation Wastewater on Agricultural Lands

Concentrated animal feeding operations (CAFOs) generate large volumes of manure and manure-contaminated wash and runoff water. When applied to land at agronomic rates, CAFO wastewater has the potential to be a valuable fertilizer and soil amendment that can improve the physical condition of the soil for plant growth and reduce the demand for high quality water resources. However, excess amounts of nutrients, heavy metals, salts, pathogenic microorganisms, and pharmaceutically active compounds (antibiotics and hormones) in CAFO wastewater can adversely impact soil and water quality. The USEPA currently requires that application of CAFO wastes to agricultural lands follow an approved nutrient management plan (NMP). A NMP is a design document that sets rates for waste application to meet the water and nutrient requirements of the selected crops and soil types, and is typically written so as to be protective of surface water resources. The tacit assumption is that a well-designed and executed NMP ensures that all lagoon water contaminants are taken up or degraded in the root zone, so that ground water is inherently protected. The validity of this assumption for all lagoon water contaminants has not yet been thoroughly studied. This review paper discusses our current level of understanding on the environmental impact and sustainability of CAFO wastewater reuse. Specifically, we address the source, composition, application practices, environmental issues, transport pathways, and potential treatments that are associated with the reuse of CAFO wastewater on agricultural lands.

Surface characteristics and adhesion behavior of Escherichia coli O157:H7: role of extracellular macromolecules.

Experiments were conducted using enterohemorrhagic Escherichia coli O157:H7 cells to investigate the influence of extracellular macromolecules on cell surface properties and adhesion behavior to quartz sand. Partial removal of the extracellular macromolecules on cells by a proteolytic enzyme (proteinase K) was confirmed using Fourier transform infrared spectroscopy analyses. The proteinase K treated cells exhibited more negative electrophoretic mobility (EPM) at an ionic strength (IS) < or = 1 mM, a slightly lower isoelectric point, and were less hydrophobic as compared to the untreated cells. Potentiometric titration results indicated that the total site concentration (i.e., the total amount of exposed functional groups per cell) on the treated cells was approximately 22% smaller than the untreated cells, while the dissociation constants were almost identical. Analysis of the EPM data using soft particle theory showed that the removal of extracellular macromolecules resulted in polymeric layers outside the cell surface that were less electrophoretically soft. The more negative mobility for the treated cells was likely due to the combined effects of a change in the distribution of functional groups and an increase in the charges per unit volume after enzyme treatment and not just removal of extracellular macromolecules. The proteolytic digestion of extracellular macromolecules led to a significant difference in the cell adhesion to quartz sand. The adhesion behavior for treated cells was consistent with DLVO theory and increased with IS due to less negativity in the EPM. In contrast, the adhesion behavior of untreated cells was much more complex and exhibited a maximum at IS = 1 mM. The treated cells exhibited less adhesion than the untreated cells when the IS < or = 1 mM due to their more negative EPM. However, when the IS > or = 10 mM, a sudden decrease in the removal efficiency was observed only for the untreated cells even through EPM values were similar for both treated and untreated cells. This result suggested that an additional non-DLVO type interaction, electrosteric repulsion, occurred at higher IS (> or =10 mM in this study) for the untreated cells due to the presence of extracellular macromolecules that hindered cell adhesion to the quartz surface. This finding provides important insight into the role of macromolecule-induced E. coli O157:H7 interactions in aquatic environments.