Colloid Transport In Porous Media Research Articles

Mechanisms of Pore-Clogging Using a High-Resolution CFD-DEM Colloid Transport Model

Colloidal transport and clogging in porous media is a phenomenon of critical importance in many branches of applied sciences and engineering. It involves multiple types of interactions that span from the sub-colloid scale (electrochemical interactions) up to the pore-scale (bridging), thus challenging the development of representative modelling. So far published simulation results of colloidal or particulate transport are based on either reduced set of forces or spatial dimensions. Here we present an approach enabling to overcome both computational and physical limitations posed by a problem of 3D colloidal transport in porous media. An adaptive octree mesh is introduced to a coupled CFD and DEM method while enabling tracking of individual colloids. Flow fields are calculated at a coarser scale throughout the domain, and at fine-scale around colloids. The approach accounts for all major interactions in such a system: elastic, electrostatic, and hydrodynamic forces acting between colloids, as well as colloids and the collector surface. The method is demonstrated for a single throat model made of four spherical segments, and the impact of clogging is reported in terms of the evolution of the critical path diameter for percolation and permeability. We identified four stages of clogging development depending on position and time of individual colloid entrapment, which in turn correlates to a cluster evolution and local transport.

Detachment of inclined spheroidal particles from flat substrates

The detachment of fine particles attached to a solid substrate or rock surface significantly affects colloidal transport in porous media. The work is devoted to quantifying the detachment conditions for inclined oblate spheroids subjected to creeping viscous flow. A numerical and a laboratory study were conducted to systematically evaluate the impact of the orientation (pitch) angle towards the hydrodynamic moment, adhesive DLVO force, contact area, and critical detachment velocity. The methodology developed to produce stretched latex polystyrene particles reduces the geometric uncertainty of nonspherical fines. The geometry of the produced oblate spheroids is fully determined by the semi-major and semi-minor axes. For inclined attached particles, an explicit formula has been derived for the detaching velocity, where the detaching drag torque exceeds the attaching adhesion torque. The probabilistic distribution of the model parameters for the detachment velocity of the inclined particles attached to the plane substrate provides upscaling for fines detachment from the particle to porous media scale.

Pore-scale morphology effects on colloid deposition by trajectory tracking simulations

The migration and deposition of colloids in natural porous media is a phenomenon of high importance in hydrocarbon recovery, wastewater treatment and contaminant hydrogeology. Commonly described by colloid filtration theory (CFT), and using a single representative collector element, the theoretical framework is not accounting for morphological effects and heterogeneity of realistic porous media, including the local variations of balance of forces or presence of hydraulically stagnant regions in the vicinity of grain-to-grain contacts. So far developed numerical models of colloidal transport in porous media accounting for all major interactions and pore structure were applied to 2D systems. Known 3D simulations are limited to particulate flow with a reduced set of interactions.We propose a coupled colloid transport model combining computational fluid dynamics (CFD) and discrete element model (DEM) approaches, where the former provides the velocity field at a given time step, while the second calculates the updated colloid positions accounting for a set of forces acting on each colloid. Relationships between colloid retention and morphological characteristics, such as the pore coordination number and surface to volume ratio of a random sphere pack are investigated by pore partitioning of a digitized representation. The role of stagnation zones is quantified by expressing the capture probability of colloids as a function of distance to the nearest grain-to-grain contact measured along the curved surface.The analysis of pore space morphology of the collector and the simulated filtration process reveals a strong relation between colloid retention and pore connectivity. The simulations successfully mimic the expected effects of ionic strength of the carrier fluid, such as the increasing role of grain-to-grain contacts in colloid retention at low ionic strength.

Prediction of collector contact efficiency for colloid transport in porous media using Pore-Network and Neural-Network models

Conventional colloid filtration theory (CFT) uses the single collector contact efficiency (η) to describe the mass transfer of colloids to a collector surface. However, this approach neglects the full complexity of the pore structure and flow field of real porous media. In this study, the porous medium geometry, flow field, and colloid mass transfer are quantified using a pore-network model (PNM). A database of pore scale η is established by finite-element method to train a Neural-network model (NNM). The reasonable prediction of η indicates the potential of using the developed NNMs as an alternative to correlation equations, which can free the users from repeated numerical simulation. In contrast to the prediction by conventional CFT, the value of η in the PNM occurs as a distribution, which is dependent upon the geometry parameters of the PNM. The mean value of η increases with the standard deviation of pore radius and decreases with the curvature number, but the dependency on coordination number is more complex. Upscaled values of the deposition rate coefficient (kd) corresponding to the distribution of η are calculated by the breakthrough curves by PNMs. The prediction of kd by PNM is then compared with that by CFT. Results show that kd predicted by PNM shows more significant response to velocity change, and less remarkable response to colloid density change than kd predicted by CFT. The comparison between the flow velocity distribution between PNM and CFT shows that the high-velocity region of the flow field in the porous media has been neglected in CFT, which can lead to insufficient consideration of convection. The results of this work imply that it is necessary to consider the influence of the complex pore structure of porous media on the collection of colloids.

Fluid flow and colloid transport experiment in single-porosity sample; tracking of colloid transport behavior in a saturated micromodel

Release of colloids and their subsequent transport into the subsurface environments takes place during a wide range of applications such as industrial, energy storage, and agricultural activities. Therefore, processes contributing to transport, attachment, and re-mobilization of colloids in porous media are attracting attention. A fraction of the released colloids may cross the soil vadose zone to reach the saturated groundwater. In this study, we explored colloid transport in a micromodel with high repulsion energy barrier where colloid retention is assumed to be low. Three major shortcomings were improved: pore space domain size, imaging resolution, and speed of imaging. The flow path of 1357 colloids with a size of 4 µm were tracked and these enabled precise determination of individual colloid transport mechanism as well as the integrated behavior of the system.Our direct observations have shown that even under unfavorable attachment conditions (defined based on the DLVO theory) colloids deposition occurred which was mainly due to the local flow velocity fluctuations and grain surface heterogeneity. Using the information from collective trajectories, we have quantified the contribution of differently behaved colloids in the observed breakthrough curve which show an integrated, macroscopic, behavior of the system and is often the only available information when performing column or field scale experiments to explore colloid transport in porous media. Furthermore, we have shown that attachment and remobilization of colloids increased the dispersion coefficient, and consequently the dispersivity value of the media.

Probabilistic averaging in kinetic theory for colloidal transport in porous media

This paper develops a modified version of the Boltzmann’s equation for micro-scale particulate flow with capture and diffusion that describes the colloidal-suspension-nano transport in porous media. An equivalent sink term is introduced into the kinetic equation instead of non-zero initial data, resulting in the solution of an operator equation in the Fourier space and an exact homogenization. The upper scale equation is obtained in closed form together with explicit formulae for the large-scale model coefficients in terms of the micro-scale parameters. The upscaling reveals the delay in particle transport if compared with the carrier water velocity, which is a collective effect of the particle capture and diffusion. The derived governing equation generalizes the current models for suspension-colloidal-nano transport in porous media.

Interpolation for the lattice-Boltzmann method to simulate colloid transport in porous media.

The lattice-Boltzmann method is convenient for simulating flow fields in porous media. However, due to its lattice characteristics, the velocity near a solid surface is not accurate, which results in significant errors when simulating colloid transport in porous media. Based on the general properties of a flow field close to a solid surface, we propose an alternative velocity interpolation method in which the velocity at a solid surface is strictly zero. Numerical simulation results show that the proposed method can give more accurate results than the usual bilinear interpolation. In addition, we use this method to simulate the contact efficiency of colloids in porous media and obtain a new power-law form of the contact efficiency.

Boltzmann's colloidal transport in porous media with velocity-dependent capture probability

Mathematical modeling of suspension-colloidal-nano transport in porous media at different scales has long been a fascinating topic of fluid mechanics. In this study, we discuss the multi-pore scale, where Boltzmann's approach of distributed velocities is valid, and average (homogenize) the micro-scale equation up to the core scale. The focus is on the filtration function (particle capture probability per unity trajectory length) that highly depends on the carrier fluid velocity. We develop a modified form of the Boltzmann equation for micro-scale particle capture and diffusion. An equivalent sink term is introduced into the kinetic equation instead of non-zero initial data, resulting in the solution of an operator equation in the Fourier space and an exact homogenization. The upper scale transport equation is obtained in closed form. The upscaled model contains the dimensionless delay number and large-scale dispersion and filtration coefficients. The explicit formulas for the large-scale model coefficients are derived in terms of the micro-scale parameters for any arbitrary velocity-dependent filtration function. We focus on three micro-scale models for the velocity-dependent particle capture rate corresponding to various retention mechanisms, i.e., straining, attachment, and inertial capture. The explicit formulas for large-scale transport coefficients reveal their typical dependencies of velocity and the micro-scale parameters. Treatment of several laboratory tests reveals close match with the modeling-based predictions.

Averaged Boltzmann’s Kinetics for Colloidal Transport in Porous Media

AbstractDue to the stochastic nature of pore space geometry, particle velocities in a colloidal‐suspension flux are also stochastically distributed. This phenomenon is captured by Boltzmann's kinetic equation. We formulate a BGK‐form of Boltzmann's equation for particulate flow with a particle capture rate proportional to the particle speed and derive an exact method for the model's averaging. The averaged equation is of the form of a reaction‐advection‐diffusion equation with the average particle speed lower than the carrier fluid velocity. This delay, reported in numerous laboratory studies, is explained by preferential capture of fast particles. The large‐scale model coefficients of delay, dispersion, and filtration are explicitly expressed via the microscale mixing length, equilibrium velocity distribution, and filtration coefficient. The properties of the averaged coefficients are discussed, and emerging dependencies between them are presented. The derived large‐scale equation resolves two paradoxes of the traditional model for suspension‐colloidal transport in porous media. The large‐scale equation closely matches the laboratory breakthrough curves.

Well-defined poly(ethylene glycol) polymers as non-conventional reactive tracers of colloidal transport in porous media

HypothesisA prominent fraction of mobile organic matter in natural aqueous soil solutions is formed by molecules in sizes that seamlessly exceed the lower end of what is defined as a colloid. The hydrodynamics and the functional diversity of these molecules result in a transport behavior that is fundamentally different from smaller compounds. However, there is a lack of “reactive tracers” that allow for the study of colloidal transport phenomena appropriately. We hypothesize that tailor-made and well-defined synthetic polymers can overcome this limitation. ExperimentsWe prepared and characterized the hydrodynamic properties of water-soluble poly(ethylene glycol)s (PEG) and studied their adsorption to mixtures of quartz, illite, and goethite in batch and column experiments. FindingsWe used this information to independently predict the transport of PEG with striking agreement to the observed mean breakthrough times in all porous media. As PEG transport can be comprehensively and quantitatively reconstructed, we conclude that functionalized PEGs are promising candidates to be used as tailorable and non-toxic tracers available in the size range of natural organic (macro-)molecules.

Coupled influences of particle shape, surface property and flow hydrodynamics on rod-shaped colloid transport in porous media

HypothesisNatural or engineered colloidal particles are often non-spherical in shape. In contrast to the widely-used “homogeneous sphere” assumption, the non-spherical particle shape is expected to alter particle–fluid-surface interactions, which in turn affect particle transport and retention. Experiments and SimulationsPolystyrene microspheres were stretched to rod-shaped particles of two aspect ratios (2:1, 6:1). The transport and retention behaviors of rods versus spheres were investigated in packed quartz sand columns and impinging jet systems. In parallel, a 3D trajectory model was employed to simulate particle translation and rotation, and to elucidate the role and underlying mechanisms of particle shape impact on transport. FindingsRods were observed to undergo rotating and tumbling motions in response to fluid shear from experiments and simulations. However, no distinct retention trends between rods and spheres were observed from column studies, despite BSA-coating on particles, Fe-coating on sand or velocity change. This was primarily due to the super-hydrophobic nature of colloid surfaces acquired from stretching process, which in hydrophilic sand columns, dominated particle–surface charge interactions. Simulations using colloids with randomly distributed charge patches qualitatively produced the observed insensitivity in retention respecting aspect ratio under low charge coverage (<30%). Hence, particle shape influences were strongly coupled with colloid surface properties and flow hydrodynamics.

Colloid transport through soil and other porous media under transient flow conditions—A review

AbstractUnderstanding colloid transport in porous media under transient‐flow conditions is crucial in understanding contaminant transport in soil or the vadose zone where flow conditions vary constantly. In this article, we provide a review of experimental studies, numerical approaches, and new technologies available to determine the transport of colloids in transient flow. Experiments indicate that soil structure and preferential flow are primary factors. In undisturbed soils with preferential flow pathways, macropores serve as main conduits for colloid transport. In homogeneously packed soil, the soil matrix often serves as filter. At the macroscale, transient flow facilitates colloid transport by frequently disturbing the force balance that retains colloids in the soil as indicated by the offset between colloid breakthrough peaks and discharge peaks. At the pore‐scale and under saturated condition, straining, and attachment at solid–water interfaces are the main mechanisms for colloid retention. Variably saturated conditions add more complexity, such as immobile water zones, film straining, attachment to air–water interfaces, and air–water–solid contact lines. Filter ripening, size exclusion, ionic strength, and hydrophobicity are identified as the most influential factors. Our review indicates that microscale and continuum‐scale models for colloid transport under transient‐flow conditions are rare, compared to the numerous steady‐state models. The few transient flow models that do exist are highly parameterized and suffer from a lack of a priori information of required pore‐scale parameters. However, new techniques are becoming available to measure colloid transport in real‐time and in a nondestructive way that might help to better understand transient flow colloid transport.This article is categorized under: Science of Water &gt; Hydrological Processes Science of Water &gt; Water Quality

Coupled influences of particle shape, surface property and flow hydrodynamics on rod-shaped colloid transport in porous media

Coupled influences of particle shape, surface property and flow hydrodynamics on rod-shaped colloid transport in porous media

Colloid Transport in Porous Media: A Review of Classical Mechanisms and Emerging Topics

To celebrate the tenth anniversary of InterPore, we present an interdisciplinary review of colloid transport through porous media. This review aims to explore both classical colloid transport and topics that fall outside that purview and thus offer transformative insights into the physics governing transport behavior. First, we discuss the unique colloid characteristics relative to molecules and larger particles. Then, the classical advection–dispersion–filtration models (both conceptual and mathematical) of colloid transport are introduced as well as anomalous transport behaviors. Next, the forces of interaction between colloids and porous media surfaces are discussed. Fourth, applications that are interested in maximizing the transport of colloids through porous media are considered. Then the concept of motile, active biocolloids is introduced, and finally, colloid swarming as a newly recognized mode of transport is summarized.

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.

On the failure of upscaling the single‐collector efficiency to the transport of colloids in an array of collectors

AbstractDefining the removal efficiency of a filter is a key aspect for colloid transport in porous media. Several efforts were devoted to derive accurate correlations for the single‐collector removal efficiency, but its upscaling to the entire porous medium is still a challenging topic. A common approach involves the assumption of deposition being independent of the history of transport, that is, the collector efficiency is uniform along the porous medium. However, this approach was shown inadequate under unfavorable deposition conditions. In this work, the authors demonstrate that it is not adequate even in the simplest case of favorable deposition. Computational Fluid Dynamics (CFD) simulations were run in a vertical array of 50 identical spherical collectors. Particle transport was numerically solved by analyzing a broad range of parameters. The results evidenced that when particle deposition is not controlled by Brownian diffusion, nonexponential concentration profiles are retrieved, in contrast with the assumption of uniform efficiency. If sedimentation and interception dominate, the efficiency of the first sphere is significantly higher compared to the others, and then declines along the array down to an asymptotic value. Finally, a correlation for the upscaled removal efficiency of the entire array was derived.

Enhancing Groundwater Quality through Computational Modeling and Simulation to Optimize Transport and Interaction Parameters in Porous Media

There is a shortage of high quality drinking water caused by the introduction of contaminants into aquifers from various sources including industrial processes and uncontrolled sewage. Studies have shown that colloids, collections of nanoparticles, have the potential to remediate polluted groundwater. For such applications of nanoparticles, it is important to understand the movement of colloids. This study aims to enhance the previously developed MNM1D (Micro- and Nanoparticle transport Model in porous media in one-dimensional geometry) by making more realistic assumptions about physical properties of the groundwater-porous medium system by accounting for a non-constant flow velocity and the presence of electromagnetic interactions. This was accomplished by coupling the original model with the Darcy-Forchheimer fluid model, which is specific to transport in porous media, coupled with electromagnetic effects. The resulting model also accounts for attachment and detachment phenomena, both of the linear and Langmuirian type, as well as changes to hydrochemical parameters such as maximum colloidal particle concentration in the porous medium. The system of partial-differential equations that make up the model was solved using an implicit finite-difference discretization along with the iterative Newton’s method. A parameter estimation study was also conducted to quantify parameters of interest. This more realistic model of colloid transport in porous media will contribute to the production of a more efficient method to counteract contaminants in groundwater and ultimately increase availability of clean drinking water.

Effect of gravity on colloid transport through water-saturated columns packed with glass beads: modeling and experiments.

The role of gravitational force on colloid transport in water-saturated columns packed with glass beads was investigated. Transport experiments were performed with colloids (clays: kaolinite KGa-1b, montmorillonite STx-1b). The packed columns were placed in various orientations (horizontal, vertical, and diagonal) and a steady flow rate of Q = 1.5 mL/min was applied in both up-flow and down-flow modes. All experiments were conducted under electrostatically unfavorable conditions. The experimental data were fitted with a newly developed, analytical, one-dimensional, colloid transport model. The effect of gravity is incorporated in the mathematical model by combining the interstitial velocity (advection) with the settling velocity (gravity effect). The results revealed that flow direction influences colloid transport in porous media. The rate of particle deposition was shown to be greater for up-flow than for down-flow direction, suggesting that gravity was a significant driving force for colloid deposition.

The Effects of Surface Charging Properties on Colloid Transport in Porous Media

The Effects of Surface Charging Properties on Colloid Transport in Porous Media

Comparison of enhanced microsphere transport in an iron-oxide-coated porous medium by pre-adsorbed and co-depositing organic matter

The effects of two types of organic matter (OM) (humic acid and protein) on colloid transport in porous media were investigated with column experiments and numerical modeling. Colloid deposition was explored in double-pulse experiments (DPEs) by injecting a pulse of OM to allow its pre-adsorption to iron-oxide-coated sand, followed by a pulse of latex microspheres. These experiments were compared with single-pulse experiments (SPEs), where one mixed pulse of OM and microspheres was introduced to the column. The experimental results show that pre-adsorbed and co-depositing OM may, respectively, limit initial and transient colloid deposition. The DPE results can be interpreted with an existing colloid transport model (random sequential adsorption: RSA). SPE results were interpreted with an extended RSA model (RSAE) that couples the process of OM co-deposition and blocking to colloid transport by introducing a conversion factor (Kconv) that reflects the blocking efficiency of OM. The calculations with the verified model suggest a comparable site blocking capacity of pre-adsorbed and co-depositing OM with a blocking capacity that differs for different sources of OM. Traditionally described as a modification of the chemistry of the overall deposition surface for the suspended particles, OM adsorption can also be interpreted as blocking local colloid attachment sites by the OM. This second interpretation may be more useful in describing nanoparticle deposition where the particle size is comparable to the length scale where a molecule of OM modifies the deposition surface.