Colloid Retention In Porous Media Research Articles

Effect of Strain-Specific Biofilm Properties on the Retention of Colloids in Saturated Porous Media under Conditions of Stormwater Biofiltration

Filter performance can be affected by bacterial colonization of the filtration media, yet little is known about how naturally occurring bacteria modify the surface properties of filtration media to affect colloidal removal. We used sand columns and simulated stormwater conditions to study the retention of model colloidal particles, carboxyl-modified-latex (CML) beads, in porous media colonized by naturally occurring bacterial strains. Colloid retention varied substantially across identical columns colonized by different, in some cases closely related, bacterial strains in a cell density independent manner. Atomic force microscopy was applied to quantify the interaction energy between CML beads and each bacterial strain's biofilm surface. We found interaction energy between CML and each strain was significantly different, with adhesive energies between the biofilm and CML, presumed to be associated with polymer-surface bonding, a better predictor of CML retention than other strain characteristics. Overall, the findings suggest that interactions with biopolymers in naturally occurring bacterial biofilms strongly influence colloid retention in porous media. This work highlights the need for more investigation into the role of biofilm microbial community composition on colloid removal in porous media to improve biofilter design and operation.

Colloid deposition in monolithic porous media – Experimental investigations using X-ray computed microtomography and magnetic resonance velocimetry

For experimental investigations of colloid retention in porous media, also denoted as deep bed filtration, X-ray computed microtomography (µCT) has become a basic tool within the last decade. On the one hand, µCT can spatially resolve particle deposition at discrete points of filtration time. On the other hand, the topological information of the porous media including the porosity and the pore size distribution can be obtained. Aside from structural parameters, the velocity field of the fluid within the pores, which cannot be measured by means of µCT, plays an important role in the underlying mechanisms of particle transport and immobilization. In a given structure, a high flow rate will result in increased velocity gradients as well as increased shear forces compared to a lower flow rate. High shear forces are in turn unfavorable for particle deposition. Another imaging modality, magnetic resonance velocimetry (MRV), is capable of quantifying the desired velocity maps. We demonstrate an experimental approach that combines both, MRV and µCT. In contrast to the majority of other investigations about colloid retention, the porous media investigated in this work are monolithic foam-like structures. The evaluation of colloid deposition in those monolithic filters is based on analyzing individual pores. Particle deposition in a pore is expressed by the volumetric fraction of particles while the pore flow is described by the Reynolds number. Results indicate that pores with high Reynolds numbers are not among the pores with the highest or lowest volume fraction of particles for a given time. The particle volume fraction in pores with low Reynolds numbers is mainly a function of the axial position of the pore.

Functional models for colloid retention in porous media at the triple line

Spectral confocal microscope visualizations of microsphere movement in unsaturated porous media showed that attachment at the Air Water Solid (AWS) interface was an important retention mechanism. These visualizations can aid in resolving the functional form of retention rates of colloids at the AWS interface. In this study, soil adsorption isotherm equations were adapted by replacing the chemical concentration in the water as independent variable by the cumulative colloids passing by. In order of increasing number of fitted parameters, the functions tested were the Langmuir adsorption isotherm, the Logistic distribution, and the Weibull distribution. The functions were fitted against colloid concentrations obtained from time series of images acquired with a spectral confocal microscope for three experiments performed where either plain or carboxylated polystyrene latex microspheres were pulsed in a small flow chamber filled with cleaned quartz sand. Both moving and retained colloids were quantified over time. In fitting the models to the data, the agreement improved with increasing number of model parameters. The Weibull distribution gave overall the best fit. The logistic distribution did not fit the initial retention of microspheres well but otherwise the fit was good. The Langmuir isotherm only fitted the longest time series well. The results can be explained that initially when colloids are first introduced the rate of retention is low. Once colloids are at the AWS interface they act as anchor point for other colloids to attach and thereby increasing the retention rate as clusters form. Once the available attachment sites diminish, the retention rate decreases.

Surface Heterogeneity on Hemispheres-in-Cell Model Yields All Experimentally-Observed Non-Straining Colloid Retention Mechanisms in Porous Media in the Presence of Energy Barriers

Many mechanisms of colloid retention in porous media under unfavorable conditions have been identified from experiments or theory, such as attachment at surface heterogeneities, wedging at grain to grain contacts, retention via secondary energy minimum association in zones of low flow drag, and straining in pore throats too small to pass. However, no previously published model is capable of representing all of these mechanisms of colloid retention. In this work, we demonstrate that incorporation of surface heterogeneity into our hemispheres-in-cell model yields all experimentally observed non-straining retention mechanisms in porous media under unfavorable conditions. We also demonstrate that the predominance of any given retention mechanism depends on the coupled colloid-collector-flow interactions that are governed by parameters such as the size and spatial frequency of heterogeneous attractive domains, colloid size, and solution ionic strength. The force/torque balance-simulated retention is shown to decrease gradually with decreasing solution ionic strength, in agreement with experimental observations. This gradual decrease stands in sharp contrast to predictions from mean field theory that does not account for discrete surface heterogeneity.

Straining Credibility: A General Comment Regarding Common Arguments Used to Infer Straining As the Mechanism of Colloid Retention in Porous Media

Straining Credibility: A General Comment Regarding Common Arguments Used to Infer Straining As the Mechanism of Colloid Retention in Porous Media

Correction to Colloid Retention in Porous Media of Various Porosities: Predictions by the Hemispheres-in-Cell Model

Correction to Colloid Retention in Porous Media of Various Porosities: Predictions by the Hemispheres-in-Cell Model

Colloid Retention in Porous Media of Various Porosities: Predictions by the Hemispheres-in-Cell Model

The hemispheres-in-cell model for colloid transport and deposition in simple granular filtration media preserves the utilities provided in the Happel sphere-in-cell but also incorporates features (e.g., grain-to-grain contact) that are shown to drive colloid deposition from experiments and simulations when colloid-surface repulsion exists (Ma, H.; Pedel, J.; Fife, P.; Johnson, W. P. Environ. Sci. Technol., 2009, in press). The capability of the hemispheres-in-cell model to predict colloid deposition in the absence of repulsive energy barriers for different particle sizes and fluid flow velocities was previously examined (Ma, H.; Pedel, J.; Fife, P.; Johnson, W. P. Environ. Sci. Technol., 2009, in press). In this article, we examine the influence of porosity on colloid deposition in terms of theoretical (simulated) collector efficiencies from the hemispheres-in-cell model, as well as existing models, to examine whether expected trends in porosity are similar among the different models. The need for experimentally determined collector efficiencies for porosities outside the range of 0.33-0.42 is highlighted. We demonstrate agreement between existing experimental results and our model predictions, indicating that the hemispheres-in-cell model can potentially serve as a new model geometry to develop a predictive theory of colloid filtration in the presence of energy barriers, a condition that is typical in environmental contexts.

Colloid Retention in Unsaturated Porous Media as Impacted by Colloid Size

This research investigated the impact of particle size on colloid retention in water unsaturated porous media using column experiments. Three different-sized silica sols were used as the model colloids in this research. Colloid column transport experiments were conducted under steady-state water flow conditions with effective water saturation ranging from 0.2 to 1.0 in two different-sized porous media. Colloid transport was described by the mobile-immobile two-region transport model. Under saturated conditions, minimal colloids were retained in the porous media. Under water unsaturated conditions, the observations were closely related to system water saturation. At high water saturation (i.e., effective water saturation of 0.80), colloid retention decreased with the increase of colloid size. At intermediate water saturation (i.e., effective water saturation of 0.50), the effect of particle size on colloid retention was not obvious. At low water saturation (i.e., effective water saturation of 0.20), colloid retention in the porous media increased with the increase of particle size. Under constant chemical conditions, colloid retention in the unsaturated porous media was believed to be determined by a balanced effect of shear forces and physical constraints, which was a function of both particle size and water saturation.

On colloid retention in saturated porous media in the presence of energy barriers: The failure of α, and opportunities to predict η

This contribution reviews recent findings that illuminate the processes governing colloid retention in porous media under environmentally relevant conditions. In the environment, colloids act as conveyors of contaminants, or even as contaminants themselves; however, despite decades of research, we are unable to accurately predict the retention of colloids in granular aquifer media under environmental conditions, where repulsion exists between colloids and surfaces. This failure cannot be blamed solely on the complexities of the subsurface, since colloid filtration theory (CFT) works well in the absence of colloid‐collector repulsion despite its idealization of porous media as consisting of spherical grains completely surrounded by fluid envelopes. Rather, the failure of CFT stems from failure to incorporate the correct mechanisms of retention when repulsion exists. Recent observations implicate wedging in grain‐to‐grain contacts and retention in secondary energy minima as dominant mechanisms of colloid retention in the presence of an energy barrier. Mechanistic simulations in unit cells containing grain‐to‐grain contacts corroborate these mechanisms of colloid retention. The resulting concept for colloid retention in the presence of an energy barrier involves translation of colloids across the collector surfaces until they become wedged within grain‐to‐grain contacts, or are retained via secondary energy minima (without attachment) in zones where the balance of fluid drag, diffusion, gravitational, and colloid‐collector interaction forces allow retention. The above findings highlight the pore domain geometry as a dominant governor of colloid retention in so far as the geometry gives rise to grain‐to‐grain contacts and zones of relatively low fluid drag.

Colloid Retention in Porous Media: Mechanistic Confirmation of Wedging and Retention in Zones of Flow Stagnation

A three-dimensional particle tracking model for colloid transport in porous media was developed that predicts colloid retention in porous media in the presence of an energy barrier via two mechanisms: (1) wedging of colloids within grain to grain contacts; (2) retention of colloids (without attachment) in flow stagnation zones. The model integrates forces experienced by colloids during transport in porous media, i.e., fluid drag, gravity, diffusion, and colloid-surface Derjaguin-Landau-Verwey-Overbeek interactions. The model was implemented for a fluid flow field that explicitly represented grain to grain contacts. The model utilized a variable time stepping routine to allow finer time steps in zones of rapid change in fluid velocity and colloid-surface interaction forces. Wedging was favored by colloid: collector ratios greater than about 0.005, with this threshold ratio increasing with decreasing fluid velocity. Retention in flow stagnation zones was demonstrated for colloid: collector ratios less than about 0.005, with this threshold decreasing with increasing fluid velocity. Both wedging and retention in flow stagnation zones were sensitive to colloid-surface interaction forces (energy barrier height and secondary energy minimum depth). The model provides a mechanistic basis for colloid retention in the presence of an energy barrier via processes that were recently hypothesized to explain experimental observations.

Excess Colloid Retention in Porous Media as a Function of Colloid Size, Fluid Velocity, and Grain Angularity

The deposition and re-entrainment behaviors of five sizes of carboxylate-modified microspheres (ranging from 0.1 to 2.0 microm) were examined both in porous media and impinging jet systems under a variety of environmentally relevant pore fluid velocities (2-8 m day-'), and in both the absence and the presence of an energy barrier to deposition. The magnitudes of the deposition efficiencies were compared among the porous media and impinging jet systems under equivalent fluid velocities, solution chemistries, and surface chemistries. The observed deposition efficiencies were factors of about 5 to 50 greater in the porous media relative to the impinging jet across the entire size range of microspheres examined, demonstrating that this excess deposition in porous media is relevant to a wide range of colloid sizes. The magnitude of excess deposition increased with increasing fluid velocity, and was greatest for the smallest colloids (0.1 microm). A range between 15% and 40% of the excess retained colloids were released upon introduction of low ionic strength solution, indicating that they were retained via secondary energy minima without direct contact with the grain surfaces. The observations indicate that pore geometry is a critical governor of colloid deposition in the presence of an energy barrier, even in porous media composed of spherical collectors. A portion of this excess deposition results from retention in flow stagnation zones.

Influence of Fluid Velocity and Cell Concentration on the Transport of Motile and Nonmotile Bacteria in Porous Media

The effect of fluid velocity on the transport of motile and nonmotile bacteria was studied in saturated soil columns using radiolabeled cells. According to colloid filtration theory, decreasing the bulk fluid velocity in a porous medium increases the number of collisions of passive colloids with particles and, therefore, should result in increased colloid retention in porous media. However, for motile cells, there was a variation in cell retention significantly different from that predicted by filtration theory at low fluid velocities, leading to the conclusion that filtration theory is not applicable for this motile bacterial strain at low fluid velocities. As the pore velocity was decreased from 120 to 0.56 m/day, the fractional retention of motile cells (Pseudomonas florescens P17) decreased by 65%, and the collision efficiency (α) defined as the ratio of particles that attach to soil grains to particles that collide with the soil (calculated using a filtration equation) decreased from 0.37 (120 m/day) to 0.003 (0.56 m/day). For passive colloids, the fractional retention (if α is a constant equal to 0.01) would increase by more than 800% over this same velocity range. To support our conclusion that cell motility was the factor producing this change from filtration theory, we rendered P17 cells nonmotile and tested this strain and a second nonmotile strain [Burkholderia (Pseudomonas) cepacia G4] under the same conditions. Collision efficiencies for both nonmotile suspensions were constant. For nonmotile P17, α was equal to 0.018 ± 0.003 (0.56−590 m/day). Over a wide velocity range for nonmotile G4, α was equal to 0.22 ± 0.067 (11−560 m/day). Swimming cells were presumably able to avoid sticking to soil grains at low fluid velocities, but at high fluid velocities, cell motility did not reduce attachment. Two additional factors known to affect cell transport (solution ionic strength and cell concentration) were also examined with these two strains in porous media. Decreasing the ionic strength from 4.14 to 0.0011 mM (at a constant pH) decreased cell retention for motile P17 by 39 ± 12%, but this is less of a reduction than is typically observed for nonmotile strains. Increasing the cell concentrations of motile P17 increased the overall retention of cells, suggesting that previously deposited cells provided a more favorable surface for adhesion than the native soil (ripening). In contrast, increasing the cell concentrations of G4 resulted in lower retention, suggesting that deposited cells provided a less favorable collector surface (blocking). These results need to be further investigated with other motile and nonmotile species. However, our results do suggest that wider dispersal of cells during bioaugmentation than previously thought possible may be achieved by using a combination of motile cells, low pumping velocities, and low ionic strength solutions. Optimal cell concentrations to use for in situ bioaugmentation of contaminated soil will depend on the adhesion of the bacterial strains for soil grains and with each other, but in general blocking-type cells are capable of greater dispersal at higher concentration than ripening-type cells.