Analytic Studies of Colloid Transport in Fractured Porous Media

Three-dimensional modeling for colloid-facilitated contaminant transport with the effect of mobile and immobile sorbents

A conceptual equilibrium-based mathematical model for colloid-associated contaminant transport has been developed to study the impact of the subsurface environment on contaminant transport through a three-dimensional, saturated, and homogeneous groundwater flow system with uniform flow. The kinetic model's critical limitation is dealing with the more significant number of parameters utilized upon application to larger scales in three-dimensional regions when a series of transport mechanisms are incorporated. Therefore, the present study is the first attempt to study the equilibrium approach in three-dimensional regions to avoid complexities in the model. The current study, however, shows that the mere existence of colloids does not indicate that contaminants will move more quickly; rather, it also depends on how the aqueous phase interacts with the static solid matrix, captured colloid particles, and mobile colloids as well as how colloids interact with stationary solid matrix phase. We noticed that the affinity of contaminants to immobile sorbents (stationary solid matrix and captured colloids) can reduce the transport even in the presence of colloids. Three-dimensional numerical experiments reveal that contaminants infiltrate more in the downward direction in the absence of colloids and can be distributed more in the longitudinal direction and less in the downward direction when colloids are present. The dual nature of colloids is espied here: first, colloids can remove pollutants from a specific area more quickly, and second, in a similar manner, colloids can pollute a specific region more quickly.

Model analysis of the colloid and radionuclide retardation experiment at the Grimsel Test Site

The colloid and radionuclide retardation experiments performed at NAGRA's Grimsel Test Site in Switzerland are part of an international collaboration program designed to collect in situ data on the impacts of colloids on radionuclide transport. In this work, breakthrough behaviors of trivalent americium (i.e., 241Am and 243Am) both in the absence and presence of bentonite colloids are analyzed with COLFRAC--a code that models colloid-facilitated solute transport in discretely-fractured, porous media. Model fits to the experimental results indicate that Am sorbed onto mobile colloids, which enhance Am transport relative to a non-sorbing tracer, 131I. Modelling results suggest that Am is kinetically sorbed onto both naturally occurring and exogenous bentonite colloids. Results also indicate that desorption of Am from colloids is slow with respect to the duration of the experiment. In addition, early colloid breakthrough compared to a conservative tracer suggests the effects of hydrodynamic chromatography. Overall, Am breakthrough curves suggest enhanced mobility due to co-transport with both naturally occurring and bentonite colloids.

Observation and characterization of colloids derived from leached cement hydrates

The possibility of colloid generation from cement hydrates in a cementitious repository environment has been investigated through leaching experiments. Pulverized samples of High Flyash and Silica fume-content Cement (HFSC) and 1:9 ordinary portland cement/blast furnace slag (1:9 OPC/BFS) hydrate were leached in low-salinity groundwater at three solid-to-liquid (S/L) mass ratios (1:5, 1:50 and 1:100), and two temperatures (20 and 60 degrees C) for durations of nearly 2 and 8 months. Detailed characterization of colloid populations has been undertaken by TEM coupled with X-ray analysis. In addition, the surface charge and stability behavior of colloids have been investigated. The colloid concentrations in HFSC hydrate leachates generated at 20 and 60 degrees C show similar trends with S/L ratio. The colloid concentrations of leachates with the lower S/L ratio (1:50 and 1:100) are in the range of 10(11)-10(12) particles per liter. The majority of these particles are composed predominantly of Si, Ca, and Al; the mean particle size is less than 100 nm. The lowest colloid concentrations are found in the leachates with the highest S/L ratios, and the colloid populations tend to be dominated by larger particles. HFSC-derived colloid stability is due to a high negative zeta potential at alkaline pH values, combined with a calcium concentration that is below the critical coagulation concentration (CCC) for the colloids. A preliminary interpretation of HFSC-derived colloid stability based on classical DLVO theory provides a semi-quantitative explanation of the dependence of colloid populations on the S/L ratio in the leaching experiments.

Migration of radionuclides in porous rock in the presence of colloids: effects of kinetic interactions

This work investigates the colloid-facilitated migration of radionuclides with radioactive decay in porous media. The sorption processes for radionuclides with both the solid matrix and colloids are treated as equilibrium or nonequilibrium. An analytical solution is obtained from a simplified linear equilibrium interaction mechanism. In addition, the adsorption processes for radionuclides with colloids and porous rock can be assumed as nonequilibrium and modeled by the linear kinetic adsorption. The numerical method is employed to solve the coupled colloid and radionuclide transport equations under nonequilibrium sorption assumption. Moreover, the reaction rates of the adsorption processes for radionuclides with the solid matrix and colloids affect the transport characteristics of radionuclides. The fast reaction rate of radionuclides with colloids causes a higher concentration of radionuclides adsorbed on colloids in a dispersed phase and enlarges acceleration caused by colloids. However, the fast reaction rate for radionuclides with solid matrix increases the retardation effect caused by the solid matrix. This work developed a predictive model for the transport of colloid-facilitated radionuclides in porous medium and to assess the importance of various phenomenological coefficients, particularly parameters for the adsorption interactions.

Effects of hydrodynamic chromatography on colloid-facilitated migration of radionuclides in the fractured rock

The performance assessment of high level radioactive waste disposal has emphasized the role of colloids in the migration of radionuclides in the geosphere. Previous literature [Nagasaki S, Tanaka S, Suzuki A. Fast transport of colloidal particles through quartz-packed columns. J. Nucl. Sci. Technol. 1975;30(11):1136] indicates that owing to hydrodynamic chromatography the colloid velocity may not be equal to that of groundwater. Using hydrodynamic chromatography, this work investigates the effects of the size of colloidal particles on the radionuclide migration facilitated by colloids in a single fractured porous rock. Also, a methodology is proposed to develop a predictive model to assess transport within the fracture rock as well as various other phenomenological coefficients, particularly the size of colloidal particles. In addition, a fully developed concentration profile for non-reactive colloids in the fracture is developed to elucidate hydrodynamic chromatography of colloids in geological media. The external forces acting on colloidal particles hypothesized in the model proposed herein include inertial force, van der Waals attractive force, double layer force as well as gravitational force. The dispersion coefficient of colloids and the distribution coefficient for radionuclides with colloids are also considered as they pertain to the size of the colloid. In addition, the size distributions of colloids are utilized to investigate the effects of polydispersed colloids.