Straining and Attachment of Colloids in Physically Heterogeneous Porous Media (original) (raw)
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Physical factors affecting the transport and fate of colloids in saturated porous media
Water Resources Research, 2002
1] Saturated soil column experiments were conducted to explore the influence of colloid size and soil grain size distribution characteristics on the transport and fate of colloid particles in saturated porous media. Stable monodispersed colloids and porous media that are negatively charged were employed in these studies. Effluent colloid concentration curves and the final spatial distribution of retained colloids by the porous media were found to be highly dependent on the colloid size and soil grain size distribution. Relative peak effluent concentrations decreased and surface mass removal by the soil increased when the colloid size increased and the soil median grain size decreased. These observations were attributed to increased straining of the colloids; i.e., blocked pores act as dead ends for the colloids. When the colloid size is small relative to the soil pore sizes, straining becomes a less significant mechanism of colloid removal and attachment becomes more important. Mathematical modeling of the colloid transport experiments using traditional colloid attachment theory was conducted to highlight differences in colloid attachment and straining behavior and to identify parameter ranges that are applicable for attachment models. Simulated colloid effluent curves using fitted first-order attachment and detachment parameters were able to describe much of the effluent concentration data. The model was, however, less adequate at describing systems which exhibited a gradual approach to the peak effluent concentration and the spatial distribution of colloids when significant mass was retained in the soil. Current colloid xfiltration theory did not adequately predict the fitted first-order attachment coefficients, presumably due to straining in these systems. INDEX TERMS: 1831 Hydrology: Groundwater quality; 1832 Hydrology: Groundwater transport Citation: Bradford, S. A., S. R. Yates, M. Bettahar, and J. Simunek, Physical factors affecting the transport and fate of colloids in saturated porous media, Water Resour.
Concentration dependent transport of colloids in saturated porous media
Journal of Contaminant Hydrology, 2006
A series of column experiments was undertaken to explore the influence of colloid input concentration (2, 1, 0.5, and 0.25 times a reference concentration), colloid size (negatively charged 3.2 and 1.0 Am carboxyl latex), and sand grain size (360, 240, and 150 Am quartz sands) on transport and deposition. A similar mass of stable mono-dispersed colloids was added to each column. For a given input concentration, decreasing the sand size and increasing the colloid size resulted in increased mass retention in the sand near the column inlet and lower relative concentrations in the effluent. For a given sand and colloid, increasing the input concentration produced less deposition and higher mass recovery in the effluent, especially for coarser sands and smaller colloids. Results of a time dependent attachment (blocking) and detachment model were not consistent with this behavior because the simulations predicted much less retention near the column inlet and a decreasing number of favorable attachment sites (mass of deposited colloids) with increasing input concentration in a given system (colloid and sand). A time dependent straining model (filling of straining sites) provided a better description of the effluent and deposition data, but still could not account for the observed concentration dependent mass recovery. Alternatively, the straining model was refined to include a liberation term that assumed that straining was hindered at higher concentrations (collision frequencies) due to repulsive colloid (aqueous phase)-colloid (strained) interactions. Simulations that included straining, liberation, attachment, and detachment significantly improved the description of the experimental data. Published by Elsevier B.V.
Transport and retention mechanisms of colloids in partially saturated porous media
Vadose Zone …, 2005
tion and deposition has improved in the past 10 yr, scientific reviews emphasize the need for more research on The transport, retention, and release of hydrophobic and hydrothe mechanisms controlling transport in the unsaturated philic polystyrene latex microsphere colloids were examined in 0.5cm-thick, 26-cm-long slab chambers filled with either regular (hydro-zone (Ouyang et al., 1996; Kretzschmar et al., 1999). philic) or weakly water-repellent sand. The water-repellent sand In a review of colloid transport in the vadose zone, consisted of a mixture of 0.4% strongly water-repellent grains with Lenhart and Saiers (2002) described the transport and unmodified regular sand for the remainder. The concentration of coldistribution of colloids in the vadose zone as advection loids in the outflow water was measured at the same time as the poreand dispersion, together with a sink-source term. The scale distribution of colloids was recorded in still and video images. advection-dispersion is relatively well understood and Although the type of sand affected the flow pattern in the top of the could be modified to include the effects of preferential chamber, it did not affect the breakthrough for the same type of flow (Steenhuis et al., 2001). Difficulties arise from size colloids. More hydrophilic colloids were eluted in the drainage water exclusion that limits the accessibility of colloids to porthan hydrophobic colloids. Images showed that there was a greater tions of the pore space. Determining the sink-source retention of the hydrophobic colloids due to strongly attractive hydrophobic interaction forces between colloids and subsequent filtering term is even more uncertain and currently an area of acof colloidal aggregates in the narrow passages between grains. Once tive research. As shown below, one of the major limitafiltered, these aggregates then served as preferred sites for attachment tions in understanding and modeling this term is the of other hydrophobic colloids. The hydrophilic colloids were retained difficulty of visualizing the processes in a medium where primarily in a thin film of water at the edge of the menisci, the airthe location and extent of the AW interface is a function water-solid (AWS) interface. Centrifugal motion within the pendular of many factors, such as matric potential and the previrings observed in the videos contributed to movement of the colloids ous wetting history (Lenhart and Saiers, 2002). Visualiztoward the AWS interface, where colloids were retained due to both ation is difficult; therefore, most studies have been limlow laminar flow velocities near the grain surface and straining in the ited to colloid breakthrough experiments, as well as thin water film at the edge of the meniscus. Except near the solid conceptual, analytical, and/or computer models. Colloid interface, sorption at the air-water (AW) interface was not observed and appeared unimportant to the retention of colloids. The findings breakthrough experiments in partly saturated media form an essential link between colloid retention and transport pro
Colloid Transport in Aggregated Porous Media with Intra- and Interaggregate Porosities
Industrial & Engineering Chemistry Research, 2018
Column tracer and colloid transport experiments were performed under both saturated and unsaturated steady state flow conditions in an aggregated porous medium with bimodal pore size distribution (PSD): intra-aggregate porosity with a pore radius between 10-2 and 10-1 m and inter-aggregate porosity ranged between 10 1-10 3 m (inter-porosity). All experiments were carried out under unfavorable conditions for physicochemical attachment to solid-water interfaces, using negatively charged porous media and latex microspheres (1 µm). Both porous media and colloids used in this work were hydrophobic. The results obtained through experimental observations and numerical simulations in the aggregated medium were confronted with those obtained for a sandy medium, characterized by a narrow unimodal PSD with a pore radius ranged between 10 1-10 2 m (inter-porosity), to explore the relative importance of the PSD on water flow, colloid transport and deposition. Physicochemical interactions between colloids and porous media, calculated according to the DLVO theory, showed no primary minimum and low secondary minimum depth, suggesting reversible colloid retention and possibility for colloid detachment by hydrodynamics drag for both sand and aggregated media. Hydrodynamic drag forces were slightly greater than the resisting adhesive DLVO forces in the secondary minimum, indicating a possibility for colloid detachment. For the same flow rate, more non-uniform transport of colloids were obtained in the bimodal aggregated medium compared to the unimodal sand. If the non-uniform and preferential transport of colloids should contribute in decreasing of colloids retention,
Colloid Transport in a Heterogeneous Partially Saturated Sand Column
Environmental Science & Technology, 2008
Colloid transport was studied in heterogeneous sand columns under unsaturated steady-state conditions, using two sizes of acid-cleaned sand to pack the column. Heterogeneity was created by placing three continuous tubes of fine sand (3.6% of the total volume) within a column of coarse sand (mean grain diameters 0.36 and 1.2 mm, respectively). Experiments were performed under three flow rates (0.1, 0.2, and 0.4 cm/ min) applied by a rain simulator at the top of the column. Constant water-content profile in the coarse sand was achieved by applying corresponding suction at the column bottom. Three sizes of latex microspheres (1, 0.2, and 0.02 µm) and soluble tracers (LiBr), diluted in a weak base (pH 7.3, ionic strength 0.0023 M) solution, were used simultaneously. Introduction of preferential pathways reduced front-arrival time about 2-fold and increased colloid recovery which, at the 0.2 cm/min flow rate, was higher than at 0.4 and 0.1 cm/min. Maximum solution flux from coarse to fine sand (due to differences in matric pressure) at 0.2 cm/min, verified by hydrodynamic modeling, could explain this phenomenon. Results suggest that in heterogeneous soil, maximum colloid recovery does not necessarily occur at maximum water content. This has clear implications for colloid transport in natural soils, many of which are heterogeneous.
Environmental Science & Technology, 2013
The prediction of colloid transport in unsaturated porous media in the presence of large energy barrier is hampered by scant information of the proportional retention by straining and attractive interactions at surface energy minima. This study aims to fill this gap by performing saturated and unsaturated column experiments in which colloid pulses were added at various ionic strengths (ISs) from 0.1 to 50 mM. Subsequent flushing with deionized water released colloids held at the secondary minimum. Next, destruction of the column freed colloids held by straining. Colloids not recovered at the end of the experiment were quantified as retained at the primary minimum. Results showed that net colloid retention increased with IS and was independent of saturation degree under identical IS and Darcian velocity. Attachment rates were greater in unsaturated columns, despite an over 3-fold increase in pore water velocity relative to saturated columns, because additional retention at the readily available air-associated interfaces (e.g., the air−water− solid [AWS] interfaces) is highly efficient. Complementary visual data showed heavy retention at the AWS interfaces. Retention by secondary minima ranged between 8% and 46% as IS increased, and was greater for saturated conditions. Straining accounted for an average of 57% of the retained colloids with insignificant differences among the treatments. Finally, retention by primary minima ranged between 14% and 35% with increasing IS, and was greater for unsaturated conditions due to capillary pinning. ■ INTRODUCTION Colloidal transport in porous media has gained notable attention over the past few decades as facilitated contaminant transport in groundwater aquifers, reduced permeability of oil and gas reservoirs, and in situ remediation strategies have become topics of concern in various fields. 1,2 A large body of experimental and theoretical work has been carried out to understand key physicochemical factors affecting colloid transport and deposition in natural porous media. Solution composition has been extensively investigated with particular emphasis on ionic strength (IS) 3−5 and pH 5−8 for affecting electrostatic interactions, natural organic matter, 5,8 and surfactants as electrosteric stabilizers, and cation valence 8 in terms of neutralizing surface charge. Each of these factors affects surface interactions between colloids and between colloids and the porous medium's surface. In addition, unsaturated conditions significantly increase colloid retention by the presence of a gas phase, which restricts the geometry of the fluid phase and partitions deposited colloids toward interfaces that are absent in fully saturated media (e.g., air− water and air−water−solid interfaces). 9−15 Flow rate affects low-flow or flow-stagnation zones where colloids can become trapped. 3,15 Moreover, high flow velocities have been related to increased applied torque from strong hydrodynamic drag forces
Colloid release and transport processes in natural and model porous media
Colloids and Surfaces A-physicochemical and Engineering Aspects, 1996
Colloid release was observed from packed columns for two natural porous media (sands) and one model system (glass beads with deposited latex colloids). Colloid release was found to occur in all cases when the ionic strength was reduced in columns that were in equilibrium with Na+ ions. Most of the released colloids from the natural porous media were smaller than