Colloids In Porous Media Research Articles

Role of Grain-to-Grain Contacts on Profiles of Retained Colloids in Porous Media in the Presence of an Energy Barrier to Deposition

Deposition of 36-microm gold-coated hollow microspheres in two porous media (glass beads and quartz sand, 710-850 microm) was examined using X-ray microtomography (XMT) in the presence of an energy barrier to deposition under fluid velocity conditions representative of engineered filtration systems. XMT allowed examination of the deposition at different locations at the grain surfaces (deposition at grain-to-grain contacts versus single-contact deposition). We demonstrate that in the presence of an energy barrier to deposition, grain-to-grain contacts strongly influence colloid deposition and the spatial distribution of retained colloids in porous media. This result contrasts drastically with observations in the absence of an energy barrier to deposition, where consistency with filtration theory was observed. In the presence of an energy barrier, colloids were dominantly retained at grain-to-grain contacts, and the concentration of retained particles varied nonmonotonically with transport distance. It is proposed that the nonmonotonic profiles resulted from translation of surface-associated microspheres and subsequent immobilization at grain-to-grain contacts. This hypothesis is demonstrated using a conceptual model. The mutability and sensitivity of retained profiles to system conditions (from hyper-exponential to nonmonotonic) may reflect the interplay of different deposition mechanisms under different conditions.

Intermittent filtration of bacteria and colloids in porous media

Intermittent filtration through porous media used for water and wastewater treatment can achieve high pathogen and colloid removal efficiencies. To predict the removal of bacteria, the effects of cyclic infiltration and draining events (transient unsaturated flow) were investigated. Using physical micromodels, we visualized the intermittent transport of bacteria and other colloids in unsaturated porous media. Column experiments provided quantitative measurements of the phenomena observed at the pore scale. Tagged Escherichia coli and a conservative tracer (NaI) were introduced in an initial pulse into a 1.5 m sand column. Subsequent hydraulic flushes without tagged bacteria or tracer were repeated every 4 hours for the next 4 days, during which outflow concentrations were monitored. Breakthrough behavior between colloids and dissolved tracer differed significantly, reflecting the differences in transport processes. Advancement of the wetting front remobilized bacteria which were held in thin water films, attached to the air‐water interface (AWI), or entrapped in stagnant pore water between gas bubbles. In contrast, the tracer was only remobilized by diffusion from immobile to mobile water. Remobilization led to successive concentration peaks of bacteria and tracer in the effluent but with significant temporal differences. Observations at the pore‐scale indicated that the colloids were essentially irreversibly attached to the solid‐water interface, which explained to some extent the high removal efficiency of microbes in the porous media. Straining, cluster filtration, cell lysis, protozoa grazing, and bacteriophage parasitism could also contribute to the removal efficiency of bacteria.

Effects of natural organic matter and solution chemistry on the deposition and reentrainment of colloids in porous media.

The role of humic acid in the transport of negatively charged colloids through porous media was examined. Adsorption of humic acid on latex colloids and silica collectors reduced the deposition of suspended particles and enhanced the reentrainment of deposited particles in porous media. These effects are considered to arise from additional electrostatic and steric contributions to the repulsive interaction energy due to the adsorption of negatively charged humic acid on both the suspended particles and the media collectors. At low ionic strength reversible deposition in shallow secondary minima is hypothesized to be the principal attachment mechanism, independent of the presence of humic acid. It is proposed that under these solution conditions, particle deposition and reentrainment are the result of a dynamic process, in which particles are continuously captured and released from secondary minima. At higher ionic strengths, deposition may be regarded as a combination of two mechanisms: capture in the primary well and capture in the secondary minimum. Theoretical calculations of the attachment efficiency were conducted using two existing mathematical models. The first model is based on deposition in the primary well (interaction force boundary layer, IFBL), and the second model is based on the Maxwell kinetic theory and deposition in the secondary minimum (Maxwell model). Simulations conducted with the Maxwell model provide significantly better fits of the experimental results than those conducted with the IFBL model.

Modeling microbial processes in porous media

The incorporation of microbial processes into reactive transport models has generally proceeded along two separate lines of investigation: (1) transport of bacteria as inert colloids in porous media, and (2) the biodegradation of dissolved contaminants by a stationary phase of bacteria. Research over the last decade has indicated that these processes are closely linked. This linkage may occur when a change in metabolic activity alters the attachment/detachment rates of bacteria to surfaces, either promoting or retarding bacterial transport in a groundwater-contaminant plume. Changes in metabolic activity, in turn, are controlled by the time of exposure of the microbes to electron acceptors/donor and other components affecting activity. Similarly, metabolic activity can affect the reversibility of attachment, depending on the residence time of active microbes. Thus, improvements in quantitative analysis of active subsurface biota necessitate direct linkages between substrate availability, metabolic activity, growth, and attachment/detachment rates. This linkage requires both a detailed understanding of the biological processes and robust quantitative representations of these processes that can be tested experimentally. This paper presents an overview of current approaches used to represent physicochemical and biological processes in porous media, along with new conceptual approaches that link metabolic activity with partitioning of the microorganism between the aqueous and solid phases.

Blocking and ripening of colloids in porous media and their implications for bacterial transport

A model accounting for the dynamics of colloid deposition in porous media was developed and applied to systems containing similarly charged particles and collectors. Colloid breakthrough and intracolumn retention data confirmed that blocking reduced overall colloidal adhesion to soil. The surface coverage at which blocking occurred varied for the type of colloid, as shown by changes in the clean-bed collision efficiency, α 0, and the excluded area parameter, β. Excluded area parameters were relatively high due to unfavorable interactions between particles and collectors, and ranged from 11.5 for one bacterium ( Pseudomonas putida KT2442) to 13.7 and 24.1 for carboxylated latex microspheres with differing degrees of charged groups on their surfaces. Differences in β values for the three colloids were correlated with electrophoretic mobility, with the most negatively charged colloid (carboxylated latex; CL microspheres) having the highest β. No correlation between hydrophobicity and α 0 or β was found. Besides using colloidal particles capable of blocking, the addition of chemical additives to the soil has been suggested as a means for reducing attachment of colloids to porous media. Dextran addition caused an order-of-magnitude reduction in the overall α (for carboxylated-modified latex; CMLs). This reduction was not attributed to blocking, but to the sorption of dextran to the soil which lowered α 0. The filtration-based numerical model used to fit the α 0 and β parameters was used to demonstrate that blocking could result in significantly enhanced bacterial transport in field situations.

Sorption nonequilibrium effects on colloid-enhanced transport of hydrophobic organic compounds in porous media

This paper presents the results of modelling and experimental investigations of factors affecting enhanced transport of hydrophobic organic compounds (HOCs) on colloids in porous media, especially effects of nonequilibrium sorption/desorption. Simulations were performed with a model that considers equilibrium and rate-limited exchange of contaminant between the true dissolved phase, the suspended and attached colloids, and the fixed solid phase as well as the advection, dispersion, deposition, and release of colloidal particles from the porous medium. The model was also applied to laboratory column data for colloid-facilitated transport of a common HOC, phenanthrene. Simulations of colloid-enhanced transport and fitting of experimental data indicated that colloid-enhancement of hydrophobic organic contaminant transport can be significant for sufficiently high colloid concentrations, a high partition coefficient of contaminant on colloids with respect to the fixed porous medium, a low deposition efficiency of the colloids on the fixed porous medium surfaces, and a slow desorption rate of contaminant from the colloids. The last of these conditions was identified as probably the most important in enhancing transport distances of contaminants in natural systems with mobile colloids which usually occur at concentrations not exceeding a few mg/l.

The role of surface conformations in the deposition kinetics of humic matter-coated colloids in porous media

The effects of humic matter surface conformations on the deposition rates of polystyrene latex colloids in packed beds of quartz sand were investigated. Positively- and negatively-charged latex spheres were coated with two well-characterized humic substances and their attachment efficiencies to quartz collectors were determined for varying concentrations of NaCl between 0.02 and 0.8 M, at pH 7.4. The deposition behavior of colloids was highly dependent upon the surface conformations of the adsorbed humic matter. Latex colloids coated with the larger molecular size peat humic acid possessed smaller attachment efficiencies than the same colloids in the presence of a smaller molecular size but more acidic surface water fulvic acid. The negatively-charged sulfate latex particles were more stable than the positively-charged amidine latex particles when coated with the same humic matter at most solution conditions used here. The stability of peat humic acid-coated particles was highly susceptible to changes in salt concentrations up to about [NaCl] = 0.6 M, which may be attributed to the presence of electrosteric interactions, and/or the reduction in the solvency and the resulting collapse of the adsorbed layer of the humic matter. Our results suggest that the differences in the steric interaction forces brought about by the surface conformations are primarily responsible for the observed differences in the deposition behavior of the coated colloids for the ionic strength conditions used here.

Visualization of the role of the gas‐water interface on the fate and transport of colloids in porous media

This paper exposes the significant role played by the gas‐water interface in the fate and transport of colloids in porous media and also introduces a micromodel method to allow direct visualization of colloid behavior in pore networks. The gas‐water interface was created by trapping air in the pore space. Various types of latex and clay particles, as well as bacteria, were studied. The results suggest that the gas‐water interface sorbs not only hydrophobic but also hydrophilic particles. The degree of sorption is controlled by particle surface hydrophobicity, solution ionic strength, and particle charge sign. Sorption increases with increasing particle hydrophobicity and solution ionic strength, while positively charged particles have a very strong affinity for the gas‐water interface. The sorption on the gas‐water interface is essentially irreversible, in that the capillary free energy provides a large attractive force to hold particles on the gas‐water interface after its rupture. These findings reveal a mechanism of vadose zone transport: A static gas‐water interface behaves as a sorbent phase retarding the transport of particulate contaminants. The visualization method developed in this research is very useful for the investigation of particulate contaminant behavior and interface‐related transport, especially in the context of bioremediation.

A hybrid equilibrium model of solute transport in porous media in the presence of colloids

Groundwater contaminants adhered to colloid surfaces may migrate to greater distances than predicted by using the conventional advective—dispersive transport equation. Owing to various factors such as changes in soil properties or pumping, colloids are released and aqueous phase contaminants are transported by mobile solid particles. A mathematical hybrid equilibrium model is developed to describe the simultaneous transport and fate of a contaminant and colloids in porous media. The problem is represented by a three-phase model with two solid phases, i.e. colloids and a stationary solid matrix, and an aqueous phase. Sorption and migration processes of the contaminant and colloids are formulated by mass balance equations of respective phases and sorption relations. The interaction of colloids with the contaminant and porous matrix is included in the three-phase transport model by employing the local equilibrium assumption for colloid—contaminant mass partitioning and first-order kinetic relations for colloid—matrix mass partition mechanisms. Numerical solutions are obtained by use of a finite difference scheme to provide estimations of the contaminant and colloid concentrations. A significant sensitivity to model parameters, particularly the contaminant equilibrium distribution coefficients with mobile and immobile colloids, is found. This conclusion has practical implications in fine-grained aquifers with low permeability where the flow rate is slower than the rate of sorption.

An experimental study of the transport and capture of colloids in porous media by a chromatographic technique

We present an experimental investigation of the transport and capture of model colloids in a well-defined porous medium. The experiments were performed using standard liquid chromatography equipment and involve detecting the passage of small impulses of monodisperse latex particles passing through a bed of monosize glass beads. Experiments were performed with three samples of monodisperse latex particles of diameters 226, 550 and 790 nm in carrier solutions with ionic strengths ranging from pure water up to 0.1 M at a pH of either 5.2 or 8 at flow velocities varying from 0.01 to 0.1 cm s −1. The results are presented as capture efficiency as a function of the various parameters and also in the form of Sherwood number vs Reynolds number for comparison with existing correlations of mass transfer. It is shown that capture is strongly influenced by hydrodynamics and the repulsive potential energy barrier.

Formation and transport of radioactive colloids in porous media

This paper deals with the effect of the presence of colloids in natural groundwater on radionuclide transport. The system considered here treats groundwater as a dispersing medium and colloid or finely divided solid material resulting from several different repository sources as a dispersed phase. Evaluation of the radionuclides adsorption on colloid, concepts of effective transport velocity and migration distance, and mathematical formulation of the filtration equation were driven, along with the case studies using typical parameter values of a conceptual radioactive waste repository and concentration on the effect of polydispersed colloids on radionuclide transport. This paper also introduces the three phase analysis to treat the radionuclide transport more practically. When we compared with the previously published experimental data, the modified filtration equation gives a satisfactory result. Results of the case studies show that the reduction of colloidal size enhances the corresponding colloid concentration when colloidal transport is only affected by diffusion phenomena. However, the three phase analysis shows that this trend can be reversed if the colloidal filtration becomes a dominant mechanism in the colloidal transport. Consequently, these results show that colloid could play a very important role in radionuclide transport under a repository environment.

Chemical mobilization of micron-sized particles in saturated porous media under steady flow conditions

Many waterborne contaminants are particulate in na ture or are associated with small particles or colloids. Thus, consideration of the mobility of colloids in porous media is necessary for an understanding of contaminant transport in groundwater. Immobilization of colloids in porous media has been studied extensively for water filtration applications, but the reverse process of mobilization is seldom considered. This study addresses the latter process