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 filtration theory did not adequately predict the fitted first‐order attachment coefficients, presumably due to straining in these systems.

A conceptual model for colloid transport is developed that accounts for colloid attachment straining, and exclusion. Colloid attachment and detachment is modeled using first-order 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.

Transport of colloidal particles in porous media is governed by the rate at which the colloids strike and stick to collector surfaces. Classic filtration theory has considered the influence of system hydrodynamics on determining the rate at which colloids strike collector surfaces, but has neglected the influence of hydrodynamic forces in the calculation of the collision efficiency. Computational simulations based on the sphere-in-cell model were conducted that considered the influence of hydrodynamic and Derjaguin-Landau-Verwey-Overbeek (DLVO) forces on colloid attachment to collectors of various shape and size. Our analysis indicated that hydrodynamic and DLVO forces and collector shape and size significantly influenced the colloid collision efficiency. Colloid attachment was only possible on regions of the collector where the torque from hydrodynamic shear acting on colloids adjacent to collector surfaces was less than the adhesive (DLVO) torque that resists detachment. The fraction of the collector surface area on which attachment was possible increased with solution ionic strength, collector size, and decreasing flow velocity. Simulations demonstrated that quantitative evaluation of colloid transport through porous media will require nontraditional approaches that account for hydrodynamic and DLVO forces as well as collector shape and size.

A: AWI, air-water interface; DLVO, Derjaguin-Landau-Verwey-Overbeek; SWI, solid-water interface.S S : V Z M Our ability to accurately simulate the transport and reten on of colloids in the vadose zone is currently limited by our lack of basic understanding of colloid reten on processes that occur at the pore scale. This review discusses our current knowledge of physical and chemical mechanisms, factors, and models of colloid transport and reten on at the interface, collector, and pore scales. The interface scale is well suited for studying the interac on energy and hydrodynamic forces and torques that act on colloids near interfaces. Solid surface roughness is reported to have a signifi cant infl uence on both adhesive and applied hydrodynamic forces and torques, whereas non-Derjaguin-Landau-Verwey-Overbeek (DLVO) forces such as hydrophobic and capillary forces are likely to play a signifi cant role in colloid interac ons with the air-water interface. The fl ow fi eld can be solved and mass transfer processes can be quan fi ed at the collector scale. Here the potenal for colloid a achment in the presence of hydrodynamic forces is determined from a balance of applied and adhesive torques. The frac on of the collector surface that contributes to a achment has been demonstrated to depend on both physical and chemical condi ons. Processes of colloid mass transfer and reten on can also be calculated at the pore scale. Diff erences in collector-and pore-scale studies occur as a result of the presence of small pore spaces that are associated with mul ple interfaces and zones of rela ve fl ow stagna on. Here a variety of straining processes may occur in saturated and unsaturated systems, as well as colloid size exclusion. Our current knowledge of straining processes is s ll incomplete, but recent research indicates a strong coupling of hydrodynamics, solu on chemistry, and colloid concentra on on these processes, as well as a dependency on the size of the colloid, the solid grain, and the water content.

[1] Considerable research suggests that colloid deposition in porous media is frequently not consistent with filtration theory predictions under unfavorable attachment conditions. Filtration theory was developed from an analysis of colloid attachment to the solid-water interface of a single spherical grain collector and therefore does not include the potential influence of pore structure, grain-grain junctions, and surface roughness on straining deposition. This work highlights recent experimental evidence that indicates that straining can play an important role in colloid deposition under unfavorable attachment conditions and may explain many of the reported limitations of filtration theory. This conclusion is based upon pore size distribution information, size exclusion, time-and concentration-dependent deposition behavior, colloid size distribution information, hyperexponential deposition profiles, the dependence of deposition on colloid and porous medium size, batch release rates, micromodel observations, and deposition at textural interfaces. The implications of straining in unsaturated and heterogeneous systems are also discussed, as well as the potential influence of system solution chemistry and hydrodynamics. The inability of attachment theory predictions to describe experimental colloid transport data under unfavorable conditions is demonstrated. Specific tests to identify the occurrence and/or absence of straining and attachment are proposed.

[1] The transport and deposition behavior of pathogenic Escherichia coli O157:H7 was studied under unfavorable electrostatic conditions in saturated quartz sands of various sizes (710, 360, 240, and 150 mm) and at several flow rates. At a given velocity, column effluent breakthrough values for E. coli tended to decrease in magnitude, and concentration curves became more asymmetric with decreasing sand size. In a given sand, experiments conducted at a higher velocity tended to produce higher effluent concentrations, especially for finer (240 and 150 mm) textured sands. The shape of the deposition profiles for E. coli was also highly dependent on the sand size and velocity. Coarser-textured sands and higher flow rates were associated with less deposition and gradually decreasing concentrations with depth. Conversely, finer-textured sands and lower flow rates tended to produce greater deposition and nonmonotonic deposition profiles that exhibited a peak in retained concentration. This deposition peak occurred nearer to the column inlet for finer-textured sands and at low flow rates. Microscopic observations of E. coli retention in these finer-textured sands (micromodel experiments) clearly indicated that straining was the dominant mechanism of deposition. Batch experiments also indicated minor amounts of E. coli attachment for the selected sands and solution chemistry. A conceptual and numerical model was developed and successfully used to describe the observed E. coli transport and deposition data. Our conceptual model assumes that E. coli can aggregate when large numbers of monodispersed E. coli are deposited at pore constrictions or straining sites. When the deposited E. coli reach a critical concentration at the straining site, the aggregated E. coli O157:H7 can be released into aqueous solution as a result of hydrodynamic shearing forces.

[1] Experimental and theoretical studies were undertaken to explore the coupled effects of chemical conditions and pore space geometry on bacteria transport in porous media. The retention of Escherichia coli D21g was investigated in a series of batch and column experiments with solutions of different ionic strength (IS) and ultrapure quartz sand. Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations and results from batch experiments suggested that bacteria attachment to the sand surface was negligible when the IS was less than or equal to 50 mM. Breakthrough data from column experiments showed significant cell retention and was strongly dependent on the IS. This finding indicates that cell retention was dependent on the depth of the secondary energy minimum which increases with IS. When the IS of the influent bacteria-free solution was decreased to 1 mM, only a small fraction of the retained bacteria was released from the column. The remaining retained bacteria, however, were recovered from the sand, which was excavated from the column and suspended in a cell-free electrolyte having the original IS. These observations suggest that the solution chemistry is not the only parameter controlling bacteria retention in the porous media. Computational simulations of flow around several collector grains revealed the retention mechanism, which is dependent on both the solution chemistry and the pore space geometry. Simulations demonstrate that the pore space geometry creates hydrodynamically disconnected regions. The number of bacterial cells that may be transported to these relatively ''immobile'' regions will theoretically be dependent on the depth of the secondary energy minimum (i.e., the IS). Once bacteria are trapped in these immobile regions, reduction of the secondary energy minimum does not necessarily release the cells owing to hydrodynamic constraints.

No abstract