Concentration dependent transport of colloids in saturated porous media (original) (raw)
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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.
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.
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
Modeling Colloid Attachment, Straining, and Exclusion in Saturated Porous Media
Environmental Science & Technology, 2003
A conceptual model for colloid transport is developed that accounts for colloid attachment, straining, and exclusion. Colloid attachment and detachment is modeled using firstorder rate expressions, whereas straining is described using an irreversible first-order straining term that is depth dependent. Exclusion is modeled by adjusting transport parameters for colloid-accessible pore space. Fitting attachment and detachment model parameters to colloid transport data provided a reasonable description of effluent concentration curves, but the spatial distribution of retained colloids at the column inlet was severely underestimated for systems that exhibited significant colloid mass removal. A more physically realistic description of the colloid transport data was obtained by simulating both colloid attachment and straining. Fitted straining coefficients were found to systematically increase with increasing colloid size and decreasing median grain size. A correlation was developed to predict the straining coefficient from colloid and porous medium information. Numerical experiments indicated that increasing the colloid excluded volume of the pore space resulted in earlier breakthrough and higher peak effluent concentrations as a result of higher pore water velocities and lower residence times, respectively. Velocity enhancement due to colloid exclusion was predicted to increase with increasing exclusion volume and increasing soil gradation.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2008
Colloid transport experiments are often conducted in cleaned sand, and the results are then projected onto mechanisms in natural porous media. We conducted tracer experiments to investigate the effect of the sand-cleaning preparations on physical and chemical surface properties and to compare retention mechanisms in natural sand of three levels of cleanliness under saturated conditions. Negatively charged fluorescent microspheres (0.02, 0.1 and 1 m) together with a soluble tracer (LiBr) were applied at the top of vertical sand columns, while effluent samples were taken at the column outlets. At the end of the experiment, columns were dissected and samples were collected at different depths. For each sand, the grain size distribution and surface chemistry were examined. The electrostatic interaction free energies between grain surface and colloid were calculated based on their thermodynamic surface properties according to the traditional and extended DLVO theory. Colloid retention was much higher in natural sand than in its washed counterparts. A comparison of the physical and chemical surface properties of the differently cleaned sands implied that a dust-like particle fraction present in the natural sand had relatively greater impact on colloid retention than changes in grain surface charge resulting from the cleaning procedures.
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
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.
Water Resources Research, 2011
A mathematical model is presented for colloid transport and retention in saturated porous media under unfavorable attachment conditions. The model accounts for colloid transport in the bulk aqueous phase and adjacent to the solid surface, and rates of colloid collision, interaction, release, and immobilization on the solid phase. Model parameters were estimated using (1) filtration theory; (2) calculated interaction energies in conjunction with the Maxwellian kinetic energy model of diffusion; (3) information about the velocity magnitude and distribution adjacent to the solid phase that was obtained from pore scale water flow simulations; (4) colloid and collector sizes ; (5) the balance of applied hydrodynamic and resisting adhesive torques; and (6) time dependent filling of retention locations using a Langmuirian approach. The presented theory constrains the model parameters and output to physically realistic values in many instances, and minimizes the need for parameter optimization. Example simulations demonstrate that our modeling formulation is qualitatively consistent with observed trends for retention with colloid size and concentration, grain size, and velocity for many systems. The model provides a clear conceptual explanation for the causes of hyperexponential, exponential, uniform, and nonmonotonic retention profiles without invoking hypotheses with regard to colloid heterogeneity, aggregation, or multiple deposition rates. Furthermore, the model formulation and research presented herein helps to identify areas where additional research and theory development are still needed.