Colloid release and transport processes in natural and model porous media (original) (raw)
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Release and transport of colloidal particles in natural porous media: 1. Modeling
Water Resources Research, 2001
We present an extensive experimental data set of particle release from natural porous media saturated with monovalent cations. The generation process of mobile colloidal particles is studied by means of leaching of saturated laboratory columns packed with a noncalcareous soil material with various monovalent electrolytes and by analyzing the colloids in the effluent over typically 1000 pore volumes. The concentration of released particles cannot be modeled with simple first-order kinetics but can be rationalized in terms of a distribution of release rate coefficients k. The experimentally observed effluent concentration often decays with time as a power law c cr t-(s+ 1) for long times, suggesting a distribution of release rate coefficients p(k) cr k •-1 for small k. The observed values of exponent a range between 0.01 and 0.8. The composition of the pore water is found to have a profound influence on the particle release characteristics. With decreasing salt concentration the rate for particle release increases. Anionic organic and inorganic ligands have a major effect on the release process. For the ligands studied, the amount of released particles decreases in the sequence malonate, chloride, phtalate, and azide.
Water Resources Research, 2001
We present an extensive experimental data set of particle release from natural porous media saturated with monovalent cations. The generation process of mobile colloidal particles is studied by means of leaching of saturated laboratory columns packed with a noncalcareous soil material with various monovalent electrolytes and by analyzing the colloids in the effluent over typically 1000 pore volumes. The concentration of released particles cannot be modeled with simple first-order kinetics but can be rationalized in terms of a distribution of release rate coefficients k. The experimentally observed effluent concentration often decays with time as a power law c cr t-(s+ 1) for long times, suggesting a distribution of release rate coefficients p(k) cr k •-1 for small k. The observed values of exponent a range between 0.01 and 0.8. The composition of the pore water is found to have a profound influence on the particle release characteristics. With decreasing salt concentration the rate for particle release increases. Anionic organic and inorganic ligands have a major effect on the release process. For the ligands studied, the amount of released particles decreases in the sequence malonate, chloride, phtalate, and azide.
Long-Term Release Kinetics of Colloidal Particles from Natural Porous Media
Environmental Science & Technology, 1999
Release of colloidal particles from natural porous media saturated with monovalent cations is studied with saturated laboratory column experiments packed with a noncalcareous soil material. The column outflow was monitored over 1000 pore volumes. The composition of released particles remains constant over the course of the experiment. The major finding of the present study is that the release process cannot be modeled with simple first-order kinetics, but the effluent concentration decays in a nonexponential fashion. The experimentally observed effluent concentration decays as t-(1+R) where the exponent R typically has values between of 0.01 and 0.3. Such a powerlaw decay can be rationalized in terms of an exponential distribution of the activation energies of release rate coefficients, and the exponent R turns out to be inversely proportional to the average activation energy. We observe a strong dependence of the release on the ionic strength, which can be interpreted as a systematic increase of the average activation energy for particle release with increasing ionic strength. In the present system pH was accurately controlled, and the release pattern was surprisingly insensitive to pH variations.
Release of colloidal particles in natural porous media by monovalent and divalent cations
Journal of Contaminant Hydrology, 2006
We study mobilization of colloidal particles from natural porous media, such as soils and groundwater aquifers. Extensive laboratory scale column experiments of particle release from four different subsurface materials are presented. The important characteristics of the release process are (i) its non-exponential kinetics, (ii) the finite supply of colloidal particles and (iii) the strong dependence of the release kinetic on the nature of the adsorbed cations. Particle release depends most sensitively on the relative saturation of the medium with divalent cations. We propose a mathematic model, which captures all these aspects quantitatively, and can be used to describe the coupling between transport of major cations and the release of colloidal particles. The present experimental investigations as well as the developed modeling framework represent an important step towards the understanding of colloid-facilitated transport phenomena in natural porous media.
2009
1] We directly observed pore-scale attachment of fluorescent synthetic polystyrene colloids (1.0 mm diameter) in a partially saturated sand pack (pore space saturation ranging from 0.7 to 0.9) at four solution ionic strengths (0, 1, 100, 200 mmol NaCl). Sequential confocal laser microscope images were analyzed to quantify colloid retention, particularly at air-water meniscus-solid (AW m S) interfaces. We concurrently measured effluent colloid concentrations to determine overall matrix retention. Ionic strength had no effect on meniscus contact angles (26.7 ± 3.7 degrees) or surface tension (63-67 mN/m), both important components of the capillary forces thought to play the primary role in retention at the AW m S interfaces. AW m S interfaces attachment was greatest at 1 mmol, with the 0 mmol ionic strength reducing attachment by half. Increasing ionic strength to 100 and 200 mmol markedly decreased colloid retention at the AW m S interfaces due to observed increased competing attachment at grain surfaces (solid/water interface) that reduced the number of colloids available for AW m S interface attachment. (2009), Transport and retention of colloidal particles in partially saturated porous media: Effect of ionic strength, Water Resour. Res., 45, W12403,
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
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
Straining and Attachment of Colloids in Physically Heterogeneous Porous Media
Vadose Zone Journal, 2004
Colloid transport studies were conducted in water-saturated physicesses that control colloid transport and fate in the subcally heterogeneous systems to gain insight into the processes controlling transport in natural aquifer and vadose zone (variably saturated) surface, including sedimentation , hysystems. Stable monodispersed colloids (carboxyl latex microspheres) drodynamics (Wang et al., 1981;, ionic and porous media (Ottawa quartz sands) that are negatively charged
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.