Colloids In Porous Media Research Articles

Deposition Dynamics of Rod-Shaped Colloids during Transport in Porous Media under Favorable Conditions.

A three-dimensional computational modeling study of the deposition dynamics of rod-shaped colloids during transport in porous media under favorable conditions (no energy barrier to deposition) is presented. The objective was to explore the influences of the particle shape on colloid transport and retention. During simulation, both translation and rotation of ellipsoidal particles were tracked and evaluated based on an analysis of all forces and torques acting on the particle. We observed that the shape was a key factor affecting colloid transport and attachment. Rod particles exhibited enhanced retention compared with spheres of equivalent volume in the size range greater than ∼200 nm. The shape effect was the most pronounced for particles around 200 nm to 1 μm under simulated conditions. The shape effect was also strongly dependent upon the fluid velocity; it was most significant at high velocity, but not so at very low velocity. The above-described shape effect on retention was directly related to particle rotation dynamics due to the coupled effects from rotational diffusion and flow hydrodynamics. Rotational diffusion changed the particle orientation randomly, which caused the rod particles to drift considerably across flow streamlines for attachment in the size range from 200 nm to 1 μm. The hydrodynamic effect induced periodic particle rotation and oscillation, which rendered large-sized rod particles to behave like "spinning bodies," prescribed by their long axes so as to easily intercept with the collector surface for retention. Our findings demonstrated that the practice of using equivalent spheres to approximate rods is inadequate in predicting the transport fate and adhesion dynamics of rod-shaped colloids in porous media.

Controlled Deposition of Particles in Porous Media for Effective Aquifer Nanoremediation

In this study, a model assisted strategy is developed to control the distribution of colloids in porous media in the framework of nanoremediation, an innovative environmental nanotechnology aimed at reclaiming contaminated aquifers. This approach is exemplified by the delivery of humic acid-stabilized iron oxide nanoparticles (FeOx), a typical reagent for in situ immobilization of heavy metals. By tuned sequential injections of FeOx suspensions and of solutions containing a destabilizing agent (i.e. calcium or magnesium), the two fronts, which advance at different rates, overlap at the target location (i.e., the central portion) of the porous systems. Here, the particles deposit and accumulate irreversibly, creating a reactive zone. An analytical expression predicting the position of the clustering zone in 1D systems is derived from first principles of advective-dispersive transport. Through this equation, the sequence and duration of the injection of the different solutions in the medium is assessed. The model robustness is demonstrated by its successful application to various systems, comprising the use of different sands or immobilizing cations, both in 1D and 2D geometries. The method represents an advancement in the control of nanomaterial fate in the environment, and could enhance nanoremediation making it an effective alternative to more conventional techniques.

Effect of naphthalene on transport and retention of biochar colloids through saturated porous media

In this study, we conducted batch and column experiments to study the effect of naphthalene on retention and transport of biochar colloids in saturated porous media under different solution chemistry. The results of the batch adsorption experiments showed that the ionic strength and pH had negligible effect on naphthalene sorption onto biochar colloids. The results from the column experiments presented that biochar colloid transport decreased with increasing naphthalene concentrations, the presence of naphthalene in suspensions reduced the mobility of biochar colloids in porous media under various ionic strength and pH conditions. The reason was that nonpolar naphthalene was strongly attached on biochar colloid surface, and further lowered the zeta potential of biochar colloids due to charge-shielding. The findings from this study have important implications for biochar application in naphthalene contaminated soils.

Role of air‐water interfaces in colloid transport in porous media: A review

AbstractAir‐water interfaces play an important role in unsaturated porous media, giving rise to phenomena like capillarity. Less recognized and understood are interactions of colloids with the air‐water interface in porous media and the implications of these interactions for fate and transport of colloids. In this review, we discuss how colloids, both suspended in the aqueous phase and attached at pore walls, interact with air‐water interfaces in porous media. We discuss the theory of colloid/air‐water interface interactions, based on the different forces acting between colloids and the air‐water interface (DLVO, hydrophobic, capillary forces) and based on thermodynamic considerations (Gibbs free energy). Subsurface colloids are usually electrostatically repelled from the air‐water interface because most subsurface colloids and the air‐water are negatively charged. However, hydrophobic interactions can lead to attraction to the air‐water interface. When colloids are at the air‐water interface, capillary forces are usually dominant over other forces. Moving air‐water interfaces are effective in mobilizing and transporting colloids from surfaces. Thermodynamic considerations show that, for a colloid, the air‐water interface is the favored state as compared with the suspension phase, except for hydrophilic colloids in the nanometer size range. Experimental evidence indicates that colloid mobilization in soils often occurs through macropores, although matrix transport is also prevalent in absence of macropores. Moving air‐water interfaces, e.g., occurring during infiltration, imbibition, or drainage, have been shown to scour colloids from surfaces and translocate colloids. Colloids can also be pinned to surfaces by thin water films and capillary menisci at the air‐water‐solid interface line, causing colloid retention and immobilization. Air‐water interfaces thus can both mobilize or immobilize colloids in porous media, depending on hydrodynamics and colloid and surface chemistry.

Cotransport of Graphene Oxide Nanoparticles and Kaolinite Colloids in Porous Media

This study examines the influence of pH and ionic strength ( $$I_{\mathrm{S}}$$ ) on the cotransport of graphene oxide (GO) nanoparticles and kaolinite (KGa-1b) colloids. Several flowthrough experiments were conducted in water-saturated columns, packed with either glass beads or quartz sand, in order to determine the transport behavior of GO and KGa-1b independently, as well as the cotransport behavior of GO together with KGa-1b. Various water chemistry conditions ( $$\hbox {pH}=4, 7, 10$$ and $$I_{\mathrm{S}}=7, 12, 27\,\hbox {mM}$$ ) were considered. Collision efficiencies were calculated using the classical colloid filtration theory. Interaction energy profiles between GO nanoparticles or KGa-1b colloids and glass beads or quartz sand were constructed for the various experimental conditions, by using measured zeta potentials and applying the classical Derjaguin–Landau–Verwey–Overbeek theory. The cotransport experimental breakthrough data suggested that by lowering the pH, the retention of GO nanoparticles is enhanced, due to a possible increase in heteroaggregation between GO nanoparticles and KGa-1b colloids. Also, by increasing the $$I_{\mathrm{S}}$$ values, the retention of GO nanoparticles was slightly increased. The mass recovery of GO nanoparticles was reduced, and the transport of GO nanoparticles was retarded in the presence of KGa-1b colloids. Furthermore, the retention of GO nanoparticles was greater for columns packed with quartz sand than glass beads.

Influence of graphene oxide on the transport and deposition behaviors of colloids in saturated porous media

The effects of graphene oxide (GO) on the transport and deposition behaviors of colloids with different sizes in packed quartz sand were investigated in both NaCl (10 and 50 mM) and CaCl2 solutions (1 and 5 mM) at pH 6. Fluorescent carboxylate-modified polystyrene latex microspheres (CMLs) with size ranging from 0.2 to 2 μm were utilized as model colloids. Both breakthrough curves and retained profiles of colloids in the presence and absence of GO in suspensions under all examined solution conditions were analyzed. The breakthrough curves of all three different-sized CMLs with GO were higher yet the retained profiles were lower than those without GO at both examined ionic strengths in NaCl solutions. The observation showed that GO increased the transport and decreased the deposition of all three different-sized CMLs in NaCl solutions. However, in CaCl2 solutions, opposite observation was achieved at two different ionic strength conditions. Specifically, the presence of GO increased the transport and decreased the deposition of all three different-sized CMLs in 1 mM CaCl2 solutions, whereas, it decreased the transport and increased the deposition of all three different-sized CMLs in 5 mM CaCl2 solutions. Comparison the breakthrough curves and retained profiles of CMLs versus those of GO yielded that the overall transport and deposition behaviors of all three different-sized CMLs with GO copresent in suspensions agreed well with the transport and deposition behaviors of GO under all examined conditions. The transport and deposition behaviors of CMLs in packed porous media clearly were controlled by those of GO under the conditions investigated in present study due to the adsorption of CMLs onto GO surfaces. Our study showed that once released into natural environment, GO would adsorb (interact with) different types of colloids and thus have significant influence on the fate and transport of colloids in porous media.

Chemical mechanism of flocculation and deposition of clay colloids in coastal aquifers

Seawater intrusion has become one of serious environmental problems in coastal areas. During the replacement of saline water by fresh water in the aquifers, in-situ clay could be released, transport and deposit in the porous media due to the change of hydrodynamic and geochemical conditions, which resulted in the increasing of particle size, plugging of pores and reduction of the permeability. Batch experiments and sand column experiments were explored to study the relationships between the flocculation of in-situ clay and geochemical conditions, by changing ionic strength and ionic type of clay suspension. Column outflow was analyzed for suspended particles and electrical conductivity. The total percentage of colloid straining and interception distribution in porous media was calculated. The results indicate that porous media had an effect on the interception of clay colloid particles with about 10 percent clay colloids captured due to the rough surfaces and spatial structure of porous media. Ionic strength played a key role on the permeability reductions. The higher ionic strength is, the greater the amount of colloidal particles trapped. Ionic type also had a significant effect on the interception of clay colloid particles. Ripening was the main mechanism for the interception within porous media when the bulk solution was potassium chloride while blocking happened when the bulk solution was sodium chloride. The distribution of clay colloids in porous media was heterogeneous. The closer to the sand column inlet was the less interception of clay colloids was. The results can provide the scientific basis for preventing the water sensitivity during the process of salty aquifer restoration.

Transport of barrel and spherical shaped colloids in unsaturated porous media

Model colloids are usually spherical, but natural colloids have irregular geometries. Transport experiments of spherical colloids may not reflect the transport characteristics of natural colloids in porous media. We investigated saturated and unsaturated transport of colloids with spherical and angular shapes under steady-state, flow conditions. A pulse of negatively-charged colloids was introduced into a silica sand column at three different effective water saturations (Se=0.31, 0.45, and 1.0). Colloids were introduced under high ionic strength of [106]mM to cause attachment to the secondary energy minimum and later released by changing the pore water to low ionic strength. After the experiment, sand was sampled from different depths (0, −4, and −11cm) for scanning electron microscopy (SEM) analysis and colloid extraction. Water saturation affected colloid transport with more retention under low than under high saturation. Colloids were retained and released from a secondary energy minimum with more angular-shaped colloids being retained and released. Colloids extracted from the sand revealed highest colloid deposition in the top layer and decreasing deposition with depth. Pore straining and grain–grain wedging dominated colloid retention.

Plant Root Exudates Decrease Mobility of Smectite Colloids in Porous Media in Contrast to Humic Acid

Plant root exudates are primarily composed of carbohydrates (CHs), amino acids (AAs), and organic acids (OAs). Little is known about how plant root exudates influence the stability and mobility of clay colloids in the soil profile. In this study, transport behaviors of K+–saturated smectite colloids dispersed in maize (Zea mays L.) artificial root exudate (ARE) solution, humic acid (HA) solution, and deionized water through water-saturated sand columns were investigated with solution ionic strengths of 0.1 and 10 mmol L−1 KCl and pH of 5, 7, and 9. Results showed that smectite colloids were more aggregated and transported less in ARE solution, followed by water and HA solution. This trend became less apparent with increasing pH but more pronounced with increasing ionic strength, suggesting that enhanced stability and mobility of HA-dispersed smectite colloids probably resulted from increased electrostatic and/or steric repulsions. The results for CH-, AA-, or OA-dispersed smectite colloids revealed that the AA fraction was primarily responsible for enhanced colloid retention because positively charged amine groups in AAs (especially lysine) might neutralize the negative surface charge of colloids and promote intersurface bridging. The residual colloids, after flushing with deionized water (thus eliminating secondary energy minima), decreased in the order of ARE-, water-, and HA-dispersed colloids, suggesting greater retention by primary energy minima and pore straining for the ARE-dispersed smectite colloids. Overall, in contrast to HA, plant root exudates decreased the stability and mobility of smectite colloids, thus facilitating the retention of clay colloids in root zones during water percolation events.

Modeling the co-transport of viruses and colloids in unsaturated porous media

A mathematical model is developed to simulate the co-transport of viruses and colloids in unsaturated porous media under steady-state flow conditions. The virus attachment to the mobile and immobile colloids is described using a linear reversible kinetic model. Colloid transport is assumed to be decoupled from virus transport; that is, we assume that colloids are not affected by the presence of attached viruses on their surface. The governing equations are solved numerically using an alternating three-step operator splitting approach. The model is verified by fitting three sets of experimental data published in the literature: (1) Syngouna and Chrysikopoulos (2013) and (2) Walshe et al. (2010), both on the co-transport of viruses and clay colloids under saturated conditions, and (3) Syngouna and Chrysikopoulos (2015) for the co-transport of viruses and clay colloids under unsaturated conditions. We found a good agreement between observed and fitted breakthrough curves (BTCs) under both saturated and unsaturated conditions. Then, the developed model was used to simulate the co-transport of viruses and colloids in porous media under unsaturated conditions, with the aim of understanding the relative importance of various processes on the co-transport of viruses and colloids in unsaturated porous media. The virus retention in porous media in the presence of colloids is greater during unsaturated conditions as compared to the saturated conditions due to: (1) virus attachment to the air–water interface (AWI), and (2) co-deposition of colloids with attached viruses on its surface to the AWI. A sensitivity analysis of the model to various parameters showed that the virus attachment to AWI is the most sensitive parameter affecting the BTCs of both free viruses and total mobile viruses and has a significant effect on all parts of the BTC. The free and the total mobile viruses BTCs are mainly influenced by parameters describing virus attachment to the AWI, virus interaction with mobile and immobile colloids, virus attachment to solid–water interface (SWI), and colloid interaction with SWI and AWI. The virus BTC is relatively insensitive to parameters describing the maximum adsorption capacity of the AWI for colloids, inlet colloid concentration, virus detachment rate coefficient from the SWI, maximum adsorption capacity of the AWI for viruses and inlet virus concentration.

Transport and Retention of Colloids in Porous Media: Does Shape Really Matter?

The effect of particle shape on its transport and retention in porous media was evaluated by stretching carboxylate-modified fluorescent polystyrene spheres into rod shapes with aspect ratios of 2:1 and 4:1. Quartz crystal microbalance with dissipation (QCM-D) experiments were conducted to measure the deposition rates of spherical and rod-shaped nanoparticles to the collector (poly-l-lysine coated silica sensor) surface under favorable conditions. The spherical particles displayed a significantly higher deposition rate compared with that of the rod-shaped particles. Theoretical analysis based on Smoluchowski-Levich approximation indicated that the rod-shaped particles largely counterbalance the attractive energies due to higher hydrodynamic forces and torques experienced during their transport and rotation. Under unfavorable conditions, the retention of nanoparticles in a microfluidic flow cell packed with glass beads was studied with the use of laser scanning cytometry (LSC). Significantly more attachment was observed for rod-shaped particles than spherical particles, and the attachment rate of the rod-shaped particles showed an increasing trend with the increase in injection volume. Rod-shaped particles were found to be less sensitive to the surface charge heterogeneity change than spherical particles. Increased attachment rate of rod-shaped particles was attributed to surface heterogeneity and possibly enhanced hydrophobicity during the stretching process.

Numerical simulation of retention and release of colloids in porous media at the pore scale

Transport of a solid colloidal particle was simulated at the pore scale in presence of surface roughness and particle/pore physicochemical interaction by adopting a “one fluid” approach. A code developed in our laboratory was used to solve equations of motion, while implementing additional modules in order to take into account lubrication and physicochemical forces. Particles were recognized through a phase indicator function and the particle/fluid interface position at each instant was obtained by solving a transport equation. Roughnesses of different shapes were considered and the magnitude of the particle/pore physicochemical interaction was monitored through the change of the ionic strength of the suspending fluid. We first show that if pore surface is smooth no retention of the transported particle occurs whether the particle/pore surface is attractive or repulsive. However for shape roughnesses of “peak” or “valley”, particles may be retained inside pores or not depending on the considered ionic strength. In absence of particle retention, the residence time (the time needed for a particle to travel a characteristic pore distance) is finite and was found to be an increasing function of ionic strength for every considered roughness at fixed hydrodynamic conditions.

Transport in Porous Media

Transport through porous medium occurs in numerous processes of environmental, chemical, petroleum and civil engineering. A lot of investigations have been done in order to understand the mechanisms of the transport of particulate suspension flow through porous medium. Transport of particulate suspensions and colloids in porous media is accompanied by particle capture and consequent permeability decline. In general, deep bed filtration studies have been conducted to analyse the mechanism involved in the processes of capturing and retaining particles occurs throughout the entire depth of the filter and not just on the filter surface. In this work, the steady-state transport equation is presented and the solution to the complete advective-dispersion equation for particulate suspension flow has been derived for the case of a constant filter coefficient. This model includes transport parameters which are particle advective velocity and particle longitudinal dispersion coefficient. This theoretical investigation of the transport of particles flowing in porous media is limited to flows with low Reynolds number (linear and laminar flow) and high Peclet number.

Hysteresis of Colloid Retention and Release in Saturated Porous Media During Transients in Solution Chemistry

Saturated packed column and micromodel transport studies were conducted to gain insight on mechanisms of colloid retention and release under unfavorable attachment conditions. The initial deposition of colloids in porous media was found to be a strongly coupled process that depended on solution chemistry and pore space geometry. During steady state chemical conditions, colloid deposition was not a readily reversible process, and micromodel photos indicated that colloids were immobilized in the presence of fluid drag. Upon stepwise reduction in eluting solution ionic strength (IS), a sharp release of colloids occurred in each step which indicates that colloid retention depends on a balance of applied (hydrodynamic) and resisting (adhesive) torques which varied with pore space geometry, surface roughness, and interaction energy. When the eluting fluid IS was reduced to deionized water, the final retention locations occurred near grain-grain contacts, and colloid aggregation was sometimes observed in micromodel experiments. Significant amounts of colloid retention hysteresis with IS were observed in the column experiments, and it depended on the porous medium (glass beads compared with sand), the colloid size (1.1 and 0.5 mum), and on the initial deposition IS. These observations were attributed to weak adhesive interactions that depended on the double layer thickness (e.g., the depth of the secondary minimum and/or nanoscale heterogeneity), colloid mass transfer on the solid phase to regions where the torque and force balances were favorable for retention, the number and extent of grain-grain contacts, and surface roughness.

Combined physical and chemical nonequilibrium transport model: Analytical solution, moments, and application to colloids

The transport of solutes and colloids in porous media is influenced by a variety of physical and chemical nonequilibrium processes. A combined physical–chemical nonequilibrium (PCNE) model was therefore used to describe general mass transport. The model partitions the pore space into “mobile” and “immobile” flow regions with first-order mass transfer between these two regions (i.e, “physical” nonequilibrium or PNE). Partitioning between the aqueous and solid phases can either proceed as an equilibrium or a first-order process (i.e, “chemical” nonequilibrium or CNE) for both the mobile and immobile regions. An analytical solution for the PCNE model is obtained using iterated Laplace transforms. This solution complements earlier semi-analytical and numerical approaches to model solute transport with the PCNE model. The impact of selected model parameters on solute breakthrough curves is illustrated. As is well known, nonequilibrium results in earlier solute breakthrough with increased tailing. The PCNE model allows greater flexibility to describe this trend; for example, a closer resemblance between solute input and effluent pulse. Expressions for moments and transfer functions are presented to facilitate the analytical use of the PCNE model. Contours of mean breakthrough time, variance, and spread of the colloid breakthrough curves as a function of PNE and CNE parameters demonstrate the utility of a model that accounts for both physical and chemical nonequilibrium processes. The model is applied to describe representative colloid breakthrough curves in Ottawa sands reported by Bradford et al. (2002). An equilibrium model provided a good description of breakthrough curves for the bromide tracer but could not adequately describe the colloid data. A considerably better description was provide by the simple CNE model but the best description, especially for the larger 3.2-µm colloids, was provided by the PCNE model.

Effects of solution chemistry on straining of colloids in porous media under unfavorable conditions

This study examines effects of solution chemistry on straining of colloids in porous media under unfavorable conditions via theoretical analysis and laboratory column experiments. Geometric analysis was used to estimate the effects of solution chemistry on the critical straining ratio of colloid diameter (dp) to collector diameter (dc), above which straining occurs. Column experiments were conducted with various sizes of latex particles and glass beads at different solution ionic strengths. A two‐ or three‐step procedure was used to measure colloid rentention and release by monitoring effluent colloid concentrations, and the columns were dissected at the end of experiments to determine distribution profiles of retained colloids. Measured effluent breakthrough curves and spatial deposition profiles were modeled with the convection‐diffusion equation including terms of attachment and straining. The experimental results confirm our theoretical analysis that the critical straining ratio is significantly reduced at higher ionic strength due to additional straining induced by attachment, predominantly at the grain‐to‐grain contacts. Through systematic evaluation of contributions of physical and physiochemical factors to colloid straining, this study provides explanations for the inconsistent and wide range of dp/dc ratios reported in the literature as well as some of the reported limitations of filtration theory.

A Stochastic Model for Colloid Transport and Deposition

Profiles of retained colloids in porous media have frequently been observed to be hyper-exponential or non-monotonic with transport depth under unfavorable attachment conditions, whereas filtration theory predicts an exponential profile. In this work we present a stochastic model for colloid transport and deposition that allows various hypotheses for such deviations to be tested. The model is based on the conventional advective dispersion equation that accounts for first-order kinetic deposition and release of colloids. One or two stochastic parameters can be considered in this model, including the deposition coefficient, the release coefficient, and the average pore water velocity. In the case of one stochastic parameter, the probability density function (PDF) is characterized using log-normal, bimodal log-normal, or a simple two species/region formulation. When two stochastic parameters are considered, then a joint log-normal PDF is employed. Simulation results indicated that variations in the deposition coefficient and the average pore water velocity can both produce hyper-exponential deposition profiles. Bimodal formulations for the PDF were also able to produce hyper-exponential profiles, but with much lower variances in the deposition coefficient. The shape of the deposition profile was found to be very sensitive to the correlation of deposition and release coefficients, and to the correlation of pore water velocity and deposition coefficient. Application of the developed stochastic model to a particular set of colloid transport and deposition data indicated that chemical heterogeneity of the colloid population could not fully explain the observed behavior. Alternative interpretations were therefore proposed based on variability of the pore size and the water velocity distributions.

Influence of biofilms on the movement of colloids in porous media. Implications for colloid facilitated transport in subsurface environments

Colloid transport through porous media can be influenced by the presence of biofilms. Sterile and non-sterile sand columns were investigated using Laponite RD as model colloid and a highly mucoid strain of Pseudomonas aeruginosa as model biofilm former. Laponite RD was marked specifically by fluorescent complexes with rhodamine 6G. Breakthrough curves (BTCs) were used as parameters for determination of colloid transport characteristics. In the sterile columns, the colloid was mobile (collision efficiencies from 0.05 to 0.08) both after the presence of Na + and Ca 2+ ions followed by deionised water influent. In the biofilm-grown column, the same treatment did not result in colloid retention in the case of Na + exposure, but in altered or enhanced colloid transport. In the case of Ca 2+ ions exposure, colloid retention increased with biofilm age. After 3 weeks, almost complete retention was observed. Similar observations were made in columns packed with material from slow sand filtration units. These data reveal the complex interactions between biofilms, cations and colloid transport. Changes in the electrolyte composition of water percolating the subsurface can frequently occur and will result in different colloid transport characteristics with regard to the dominating species of ions and the relative abundance of microbial biofilms. This has to be considered when modelling colloid transport through the subsurface.

Deposition and re-entrainment dynamics of microbes and non-biological colloids during non-perturbed transport in porous media in the presence of an energy barrier to deposition

This paper examines the non-perturbed deposition and re-entrainment dynamics of biological and non-biological colloids in porous media in the presence of an energy barrier to deposition at the grain surface. Deposition and re-entrainment rate coefficients were determined from numerical simulation of breakthrough–elution behavior and the profiles of retained colloids. We present composite trends from original and previously published data for biological and non-biological colloids which demonstrate that hydrodynamic drag mitigates deposition and drives re-entrainment of both biological and non-biological colloids in the presence of an energy barrier under non-perturbed conditions. Original data is presented for two sizes of colloids (1.1 and 5.7 μm microspheres) under a variety of ionic strength and fluid velocity conditions to examine the torque balance governing re-entrainment of colloids attached to the grain surfaces. The analysis indicates that in the presence of an energy barrier to deposition, hydrodynamic drag may influence deposition and re-entrainment of colloids associated directly with the grain surface via primary energy minima. However, the hydrodynamic field would also be expected to influence deposition and re-entrainment of colloids associated with the surface via secondary energy minima. Hence, the observed influences of fluid velocity are consistent with colloid association via either mechanism. These results call for the development of colloid transport theories that explicitly account for the influence of the hydrodynamic field at the grain surface.

Role of Grain-to-Grain Contacts on Profiles of Retained Colloids in Porous Media in the Presence of an Energy Barrier to Deposition

Deposition of 36-microm gold-coated hollow microspheres in two porous media (glass beads and quartz sand, 710-850 microm) was examined using X-ray microtomography (XMT) in the presence of an energy barrier to deposition under fluid velocity conditions representative of engineered filtration systems. XMT allowed examination of the deposition at different locations at the grain surfaces (deposition at grain-to-grain contacts versus single-contact deposition). We demonstrate that in the presence of an energy barrier to deposition, grain-to-grain contacts strongly influence colloid deposition and the spatial distribution of retained colloids in porous media. This result contrasts drastically with observations in the absence of an energy barrier to deposition, where consistency with filtration theory was observed. In the presence of an energy barrier, colloids were dominantly retained at grain-to-grain contacts, and the concentration of retained particles varied nonmonotonically with transport distance. It is proposed that the nonmonotonic profiles resulted from translation of surface-associated microspheres and subsequent immobilization at grain-to-grain contacts. This hypothesis is demonstrated using a conceptual model. The mutability and sensitivity of retained profiles to system conditions (from hyper-exponential to nonmonotonic) may reflect the interplay of different deposition mechanisms under different conditions.