Long-Term Release Kinetics of Colloidal Particles from Natural 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.
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.
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.
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
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
Equilibrium and kinetic models for colloid release under transient solution chemistry conditions
Journal of contaminant hydrology, 2015
We present continuum models to describe colloid release in the subsurface during transient physicochemical conditions. Our modeling approach relates the amount of colloid release to changes in the fraction of the solid surface area that contributes to retention. Equilibrium, kinetic, equilibrium and kinetic, and two-site kinetic models were developed to describe various rates of colloid release. These models were subsequently applied to experimental colloid release datasets to investigate the influence of variations in ionic strength (IS), pH, cation exchange, colloid size, and water velocity on release. Various combinations of equilibrium and/or kinetic release models were needed to describe the experimental data depending on the transient conditions and colloid type. Release of Escherichia coli D21g was promoted by a decrease in solution IS and an increase in pH, similar to expected trends for a reduction in the secondary minimum and nanoscale chemical heterogeneity. The retention...
Aggregation and deposition kinetics of mobile colloidal particles in natural porous media
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2001
A novel method for assessing the deposition kinetics of colloidal particles in natural porous media is presented. The method is applied for studying the deposition kinetics of in situ mobilized colloidal particles in a non-calcareous soil in mixed electrolytes containing sodium and calcium as counter ions. Particle deposition rate constants were measured by combining deposition experiments in packed columns and aggregation measurements by dynamic light scattering. The relative deposition and aggregation rate constants follow very similar trends, featuring fast (favorable) and slow (unfavorable) regimes at high and low salt concentrations, respectively. These regimes are separated by the critical coagulation or deposition concentrations (CCC or CDC, respectively), which sensitively depend on the type of counterion. In systems containing a single electrolyte, the CCC and CDC follow the classical Schulze-Hardy rule. In mixed sodium-calcium electrolytes, a gradual transition of the CCC and CDC between the values obtained for the corresponding pure (single) electrolyte systems is observed. The present approach provides a facile route for assessing deposition rates of mobile colloidal particles in natural porous media.
Impact of Particle Size Distribution of Colloidal Particles on Contaminant Transport in Porous Media
Applied Sciences, 2019
The presence of retained colloidal particles causes the retardation of contaminant transport when the contaminant is favorably adsorbed to colloidal particles. Although the particle size distribution affects the retention behavior of colloidal particles, the impact of particle size distribution on contaminant transport has not been reported to date. This study investigates the impact of the particle size distribution of the colloidal particles on contaminant transport through numerical simulation by representing the particle size distribution as a lognormal distribution function. In addition, the bed efficiency and contaminant saturation of simulated breakthrough curves were calculated, and a contaminant transport model with the Langmuir isotherm for the reaction between the contaminant–sand and contaminant–colloidal particle was introduced and validated with experimental data. The simulated breakthrough curves, bed efficiency, and contaminant saturation indicated that an increase i...
Colloid mobilization and transport within unsaturated porous media under transient-flow conditions
Water Resources Research, 2003
Contaminant-scavenging colloids are mobilized in the vadose zone during infiltration events, characterized by transient flow regimes. In the research reported here we develop a mathematical model in order to investigate the influences of flow transients on colloid mobilization in unsaturated media. The model solves coupled equations for unsteady pore water flow, colloid release, advective-dispersive colloid transport, and redeposition of pore water colloids. The immobile-phase colloid population is discretized into a series of compartments, each of which are assigned a value of critical moisture content ( cr) according to a piecewise linear density function. A compartment is activated when the model-calculated moisture content exceeds the  cr for the compartment, whereupon colloids are released at a rate proportional to the product of the pore water velocity and the colloid concentration within the compartment. We fit solutions of the model equations to data on the mobilization and transport of silica colloids within columns of unsaturated quartz sand. The model accurately reproduces measured pulse-type colloid releases induced by successive step-change increases in flow rate and moisture content. This work illuminates the important role of flow transients in colloid mobilization and, through the derivation of a model that couples pore water flow characteristics with colloid mass transfer kinetics, provides a means for quantifying the phenomenon.