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ABSTRACT: To examine colloid transport in geochemically heterogeneous porous media at a scale comparable to field experiments, we monitored the migration of silica-coated zirconia colloids in a two-dimensional layered porous media containing sand coated to three different extents by ferric oxyhydroxides. Transport of the colloids was measured over 1.65 m and 95 days. Colloid transport was modeled by an advection-dispersion-deposition equation incorporating geochemical heterogeneity and colloid deposition dynamics (blocking). Geochemical heterogeneity was represented as favorable (ferric oxyhydroxide-coated) and unfavorable (uncoated sand) deposition surface areas. Blocking was modeled as random sequential adsorption (RSA). Release of deposited colloids was negligible. The time to colloid breakthrough after the onset of blocking increased with increasing ferric oxyhydroxide-coated surface area. As the ferric oxyhydroxide surface area increased, the concentration of colloids in the breakthrough decreased. Model-fits to the experimental data were made by inverse solutions to determine the fraction of surface area favorable for deposition and the deposition rate coefficients for the favorable (ferric oxyhydroxide-coated) and unfavorable sites. The favorable deposition rate coefficient was also calculated by colloid filtration theory. The model described the time to colloid breakthrough and the blocking effect reasonably well and estimated the favorable surface area fraction very well for the two layers with more than 1% ferric oxyhydroxide coating. If mica edges in the uncoated sand were considered as favorable surface area in addition to the ferric oxyhydroxide coatings, the model predicted the favorable surface area fraction accurately for the layer with less than 1% ferric oxyhydroxide coating.
Journal of Contaminant Hydrology 10/2003; 65(3-4):161-82. · 2.32 Impact Factor
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ABSTRACT: A two-dimensional model for virus transport in physically and geochemically heterogeneous subsurface porous media is presented. The model involves solution of the advection-dispersion equation, which additionally considers virus inactivation in the solution, as well as virus removal at the solid matrix surface due to attachment (deposition), release, and inactivation. Two surface inactivation models for the fate of attached inactive viruses and their subsequent role on virus attachment and release were considered. Geochemical heterogeneity, portrayed as patches of positively charged metal oxyhydroxide coatings on collector grain surfaces, and physical heterogeneity, portrayed as spatial variability of hydraulic conductivity, were incorporated in the model. Both layered and randomly (log-normally) distributed physical and geochemical heterogeneities were considered. The upstream weighted multiple cell balance method was employed to numerically solve the governing equations of groundwater flow and virus transport. Model predictions show that the presence of subsurface layered geochemical and physical heterogeneity results in preferential flow paths and thus significantly affect virus mobility. Random distributions of physical and geochemical heterogeneity have also notable influence on the virus transport behavior. While the solution inactivation rate was found to significantly influence the virus transport behavior, surface inactivation under realistic field conditions has probably a negligible influence on the overall virus transport. It was further demonstrated that large virus release rates result in extended periods of virus breakthrough over significant distances downstream from the injection sites. This behavior suggests that simpler models that account for virus adsorption through a retardation factor may yield a misleading assessment of virus transport in "hydrogeologically sensitive" subsurface environments.
Journal of Contaminant Hydrology 09/2002; 57(3-4):161-87. · 2.32 Impact Factor
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ABSTRACT: A coupled model of concentration polarization and pore transport of multicomponent salt mixtures in crossflow nanofiltration rigorously predicts local variations of ionic concentrations, flux and individual ion rejections along a rectangular crossflow filtration channel by a coupled solution of the convective-diffusion and extended Nernst-Planck equations. Coupling the pore transport model with the multicomponent convective-diffusion equation in the concentration polarization layer provides a comprehensive understanding of the interplay between concentration polarization and salt rejection. The coupled model is used to predict the local variations of ion rejection, permeate flux and mixture composition in a rectangular crossflow filtration channel for three-component salt mixtures. The total membrane surface concentration of the ions and the ratio of different ions in the mixture (salt ratio) can change considerably along a crossflow filtration channel, and, consequently, cause remarkable variations in intrinsic ion rejections with axial position in the channel.
AIChE Journal 11/2001; 47(12):2733 - 2745. · 2.26 Impact Factor
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ABSTRACT: The role of spatial distribution of porous medium patchwise chemical (charge) heterogeneity in colloid transport in packed bed columns is investigated. Colloid transport experiments with carboxyl latex particles flowing through columns packed with chemically heterogeneous sand grains were carried out. Patchwise chemical heterogeneity was introduced to the granular porous medium by modifying the surface chemistry of a fraction of the quartz sand grains via reaction with aminosilane. Colloid transport experiments at various degrees of patchwise charge heterogeneity and several spatial distributions of heterogeneity were conducted at different flow rates and background electrolyte concentrations. Colloid deposition rate coefficients were determined from analysis of particle breakthrough curves as a response to short-pulse colloid injections to the column inlet. Experimental colloid deposition rate coefficients compared well with theoretical predictions based on a colloid transport model that incorporates patchwise chemical heterogeneity. The results revealed the particle deposition rate and transport behavior to be independent of the spatial distribution of porous medium chemical heterogeneity. It is the mean value of chemical heterogeneity rather than its distribution that governs the colloid transport behavior in packed columns. © 2001 Elsevier Science B.V. All rights reserved.
Colloids and Surfaces A Physicochemical and Engineering Aspects 01/2001; 191:3-15. · 2.24 Impact Factor
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ABSTRACT: The influence of hydrodynamic shear on the shape of deformable molecular assemblages is studied using Monte Carlo simulations. Metropolis Monte Carlo simulations are performed to generate spherical assemblages of amphiphilic molecules by considering coarse statistical mechanical models for the various constituent subunits of the amphiphiles and by avoiding explicit consideration of the solvent molecules. The resulting assemblage is subjected to a unidirectional hydrodynamic shear, and the transition of the system to a secondary equilibrium is studied using a modified Monte Carlo simulation technique that accounts for the systematic force on the assemblage due to the hydrodynamic shear. The influence of hydrodynamic shear on the shape of the assemblage is described qualitatively through several simulation snapshots. The quantitative analysis suggests that the aggregate size, intermolecular interactions between various subunits, and shear rate govern the extent of shear-induced deformation. The three parameters can be combined using a single dimensionless group, the Peclet number. Results indicate that the Peclet number can provide considerable insight into the nature and extent of structural deformation of molecular assemblages in the presence of hydrodynamic shear. There exists a critical Peclet number below which hydrodynamic shear has no effect on the aggregate structure. Above the critical Peclet number, the aspect ratio (a measure of deformation) of the assemblage increases almost linearly with logarithm of the Peclet number.
12/2000;
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ABSTRACT: The orientation dependent interaction energy between a spheroidal particle and an infinite planar surface is determined using the surface element integration (SEI) technique. The interaction energy predictions of SEI are shown to be considerably more accurate than the corresponding predictions based on Derjaguin's approximation (DA). Comparison with the Hamaker approach for evaluating the non-retarded van der Waals interaction energy reveals that SEI predicts the orientation dependent interaction energy for spheroidal particles with remarkable accuracy. It is further shown that both SEI and DA give nearly identical predictions of the electrostatic double layer interaction energy between a spheroidal particle and a flat plate at high electrolyte concentrations. However, at low electrolyte concentrations, considerable deviations are noted between the predictions of SEI and DA, particularly for very small aspect ratios of the particle (aspect ratio=length of minor axis/length of major axis). It is also noted that when the spheroidal particle is oriented with its major axis parallel to the planar surface, DA incorrectly predicts the interaction energy as that of a spherical particle with a radius equal to the semi-major axis of the spheroid. This limitation of DA is avoided in SEI, which accounts for the dependence of the interaction energy on the actual shape (aspect ratio) of the particle at any orientation. Predictions of the DLVO interaction energy based on SEI indicate that, at high electrolyte concentrations, the orientation dependence of the interaction energy is not significant at large separation distances, and assumption of an equivalent spherical particle may be sufficient. However, significant deviation of the interaction energy from that of a spherical particle is observed at small separation distances, particularly at low electrolyte concentrations. At these small separation distances, where the correct orientation dependence of the interaction energy must be considered for proper calculations of particle interaction phenomena with flat surfaces (e.g. particle deposition), SEI provides a facile route to perform such calculations. © 2000 Elsevier Science B.V. All rights reserved.
Colloids and Surfaces A Physicochemical and Engineering Aspects 01/2000; 165:143-156. · 2.24 Impact Factor
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ABSTRACT: The interaction energy between microscopic bodies is almost exclusively determined assuming perfectly smooth and geometrically regular surfaces. Quite often, such interactions fail to explain several colloidal phenomena. These inexplicable behaviors of colloidal systems are generally ascribed to surface chemical and morphological heterogeneities. Here, we employ the surface element integration technique to determine the interaction energy between surfaces containing morphological heterogeneity. Random asperities are generated to represent surface morphological heterogeneity (roughness), and their influence on the DLVO interaction potential is investigated. Incorporation of surface roughness causes a significant reduction in the repulsive interaction energy, the extent of which depends on the size of the asperities and their densities on the surface. Predictions of interaction energy indicate that the DLVO interaction energy profiles for rough surfaces deviate significantly from those derived assuming perfectly smooth surfaces, particularly at very short separation distances.
05/1998;
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ABSTRACT: A theoretical model for prediction of permeate flux during crossflow membrane filtration of rigid hard spherical solute particles is developed. The model utilizes the equivalence of the hydrodynamic and thermodynamic principles governing the equilibrium in a concentration polarization layer. A combination of the two approaches yields an analytical expression for the permeate flux. The model predicts the local variation of permeate flux in a filtration channel, as well as provides a simple expression for the channel-averaged flux. A criterion for the formation of a filter cake is presented and is used to predict the downstream position in the filtration channel where cake layer build-up initiates. The predictions of permeate flux using the model compare remarkably well with a detailed numerical solution of the convective diffusion equation coupled with the osmotic pressure model. Based on the model, a novel graphical technique for prediction of the local permeate flux in a crossflow filtration channel has also been presented.
Journal of Membrane Science.
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ABSTRACT: Recent experimental investigations suggest that interaction of colloidal particles with polymeric membrane surfaces is influenced by membrane surface morphology (roughness). To better understand the consequences of surface roughness on colloid deposition and fouling, it is imperative that models for predicting the Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy between colloidal particles and rough membrane surfaces be developed. We present a technique of reconstructing the mathematical topology of polymeric membrane surfaces using statistical parameters derived from atomic force microscopy roughness analyses. The surface element integration technique is used to calculate the DLVO interactions between spherical colloidal particles and the simulated (reconstructed) membrane surfaces. Predictions show that the repulsive interaction energy barrier between a colloidal particle and a rough membrane is lower than the corresponding barrier for a smooth membrane. The reduction in the energy barrier is strongly correlated with the magnitude of surface roughness. It is further suggested that the valleys created by the membrane surface roughness produce wells of low interaction energy in which colloidal particles may preferentially deposit.
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ABSTRACT: A novel asymmetric clamping cell is used to measure the potential of macroscopic sample surfaces. The unique design of the cell allows in-situ measurement of potentials of any flat surfaces, either solids (such as glass, plastics, metals, and ceramics) or flexible sheets (like polymer films, papers, foils, and membranes), without having to cut or shape the test surfaces to fit the measuring cell dimensions. The cell enables measurement of the streaming potential through several parallel rectangular channels formed by firmly pressing a grooved spacer against the test surface. The term "asymmetric" specifically refers to the unique configuration of the channels, where one of the walls (the test surface) bears a different charging property (or, more specifically, potential), compared to the other three surfaces of the channel (formed by the spacer). A mathematical formulation on the basis of Smoluchowski-Helmholtz approach reveals that the measured potential in such a cell represents the average potential of the test surface and the spacer material. Several surfaces, including clean glass disk, aminosilane modified glass, polymeric membrane, and poly(methyl methacrylate) (PMMA) plate, were tested using the cell. The potentials of the polymeric membrane obtained using the present cell were compared with the corresponding potentials measured using the traditional rectangular cell, and good agreement between the two measurements was observed. The asymmetric cell can be used in conjunction with standard commercially available streaming potential analyzers.
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ABSTRACT: Van der Waals and electrostatic double layer interactions between two colloidal particles are evaluated from the corresponding inter-action energies per unit area between two infinite flat plates using a recently developed technique, the surface element integration. Application of the technique to two interacting spheres results in predictions of interaction energies that are substantially more ac-curate compared to the predictions based on conventional Der-jaguin's approximation. The superior results of the technique com-pared to Derjaguin's approximation are attributed to the more ri-gorous consideration of particle curvature effects in the surface ele-ment integration technique.
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ABSTRACT: A theoretical approach for predicting the influence of interparticle interactions on concentration polarization and the ensuing permeate flux decline during cross-flow membrane filtration of charged solute particles is presented. The Ornstein–Zernike integral equation is solved using appropriate closures corresponding to hard-spherical and long-range solute–solute interactions to predict the radial distribution function of the solute particles in a concentrated solution (dispersion). Two properties of the solution, namely the osmotic pressure and the diffusion coefficient, are determined on the basis of the radial distribution function at different solute concentrations. Incorporation of the concentration dependence of these two properties in the concentration polarization model comprising the convective-diffusion equation and the osmotic-pressure governed permeate flux equation leads to the coupled prediction of the solute concentration profile and the local permeate flux. The approach leads to a direct quantitative incorporation of solute–solute interactions in the framework of a standard theory of concentration polarization. The developed model is used to study the effects of ionic strength and electrostatic potential on the variations of solute diffusivity and osmotic pressure. Finally, the combined influence of these two properties on the permeate flux decline behavior during cross-flow membrane filtration of charged solute particles is predicted.
Journal of Colloid and Interface Science.
