Colloid Transport and Retention in Unsaturated Porous Media: A Review of Interface-, Collector-, and Pore-Scale Processes and Models (original) (raw)
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Coupling of physical and chemical mechanisms of colloid straining in saturated porous media
Water Research, 2007
Transport Straining Attachment Filtration theory Modeling a b s t r a c t Filtration theory does not include the potential influence of pore structure on colloid removal by straining. Conversely, previous research on straining has not considered the possible influence of chemical interactions. Experimental and theoretical studies were therefore undertaken to explore the coupling of physical and chemical mechanisms of colloid straining under unfavorable attachment conditions (pH ¼ 10). Negatively charged latex microspheres (1.1 and 3 mm) and quartz sands (360, 240, and 150 mm) were used in packed column studies that encompassed a range in suspension ionic strengths (6-106 mM) and Darcy water velocities (0.1-0.45 cm min À1 ). Derjaguin-Landau-Verwey-O-
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
Colloid transport in unsaturated porous media
International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 1994
This paper explores the significance of the gas-water interface on colloid sorption and transport. Three types of common saturation conditions were simulated in packed sand columns: (I) a completely water-saturated condition, (2) gas bubbles trapped by capillary forces as a nonwetting residual phase (15% gas), and (3) gas present as a continuous phase (46% gas), or in other words, a vadose zone situation. Different saturations provided different interfacial conditions. Two types of polystyrene latex particles (0.2/am), hydrophilic and hydrophobic, were used in each of the three saturations. Relative surface hydrophobicity of latex particles was characterized by contact angle measurements. Each experiment was repeated five times. A total of 30 columns with good reproducible packing and gas content gave reproducible particle breakthrough curves. The retention of both hydrophilic and hydrophobic colloids increased with gas content of the porous medium. Colloids preferentially sorbed onto the gas-water interface relative to the matrix surface. The degree of sorption increased with the increase of colloid surface hydrophobicity. The results validate and quantify direct microscopic scale visualization observations made in two-dimensional pore network glass micromodels (Wan and Wilson, 1994). These findings suggest an additional mechanism for filtration and for particulate transport in the subsurface environment, whenever more than one fluid phase is present. The results also give clear evidence that the presence of inadvertently trapped residual gas could help explain the discrepancy between water-saturated laboratory column experiments and related theory. lutants from suspensions [e.g., Ives and Gregory, !966; Yao et al., 1971; O'Metia, 1989]. Historically, research on colloid transport has focused on water-saturated porous media. Quantitative models for predicting particle transport are presented in the water filtration literature [Yao et al., 1971; Spielman and FitzPatrick, 1973; FitzPatrick and Spielman, 1973; Tien and Payatakes, 1979; Tien, 1989]. These models account for the deposition mechanisms of particle-medium collisions and the conditions for particle deposition. For Brownian particles, diffusion and advection control the collision rate. Deposition is determined by the interfacial forces between the mobile particles and fixed solid surfaces (collectors). These forces include electrostatic, van der Waals, hydration, and hydrodynamic forces. A particle immersed in a solvent often perturbs the local ordering of the
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,
Water Resources Research, 2013
A colloid transport model is introduced that is conceptually simple yet captures the essential features of colloid transport and retention in saturated porous media when colloid retention is dominated by the secondary minimum because an electrostatic barrier inhibits substantial deposition in the primary minimum. This model is based on conventional colloid filtration theory (CFT) but eliminates the empirical concept of attachment efficiency. The colloid deposition rate is computed directly from CFT by assuming all predicted interceptions of colloids by collectors result in at least temporary deposition in the secondary minimum. Also, a new paradigm for colloid re-entrainment based on colloid population heterogeneity is introduced. To accomplish this, the initial colloid population is divided into two fractions. One fraction, by virtue of physiochemical characteristics (e.g., size and charge), will always be re-entrained after capture in a secondary minimum. The remaining fraction of colloids, again as a result of physiochemical characteristics, will be retained “irreversibly” when captured by a secondary minimum. Assuming the dispersion coefficient can be estimated from tracer behavior, this model has only two fitting parameters: (1) the fraction of the initial colloid population that will be retained “irreversibly” upon interception by a secondary minimum, and (2) the rate at which reversibly retained colloids leave the secondary minimum. These two parameters were correlated to the depth of the Derjaguin-Landau-Verwey-Overbeek (DLVO) secondary energy minimum and pore water velocity, two physical forces that influence colloid transport. Given this correlation, the model serves as a heuristic tool for exploring the influence of physical parameters such as surface potential and fluid velocity on colloid transport.
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.
Water Resources Research, 2003
We present experimental evidence of the effect of colloid exclusion from areas of small aperture sizes, using direct observations at the pore‐scale using a realistic micromodel of porous media. Four sizes of hydrophobic latex spheres in aqueous suspension, from 0.05 to 3 μm, were introduced into the micromodel at three different pressure gradients. We observed the frequency of occurrence of the size exclusion effect and the influence of relative size of pore throats and colloids (T/C ratio) and flow velocity. From our observations the smallest T/C ratio entered by these different colloids was 1.5. We also observed certain preferential pathways through the pore space for different colloid sizes, such that size exclusion eventually results in distinct pathways. These preferential paths become more important for larger colloids and for greater pressure gradients. Measured colloid velocities were 4–5.5 times greater than estimated pore water velocities. Acceleration factors (ratio of co...