Colloids in Porous Media

Research objective

Over the past few years, our research group has been studying the fundamental processes that govern the fate and transport of colloids and biocolloids (e.g. viruses, bacteria, spores and other microorganisms) in subsurface environments. Our work aims to improve upon current knowledge of migration and removal of colloidal particles in both saturated, unsaturated and transient conditions.

A comprehensive understanding of these mechanisms is required not only to assess potential pathogenic contamination of water supplies but also to develop bioremediation strategies, to quantify colloid-facilitated transport of contaminants and to improve water filtration treatment systems.

Our research, which includes both experimental investigations and numerical modeling, is carried out following two complementary approaches: studies at the pore scale (micromodels) to visualize the processes and work at the macroscopic scale (columns and tanks) to provide quantitative data.

We examine the effect of particle and pore size on colloid mobilization in particular on advection, dispersion and filtration. To achieve this goal we are performing experiments in PDMS and glass micrmodels as well as in sand and soil columns.

Micromodels are transparent networks of pores and constrictions that simulate some of the complexities of natural porous media. Their use allows optical microscopy observations of the migration of colloidal particles within the pores. Particle trajectories, residence times and dispersion coefficients are determined from image analysis.

Detail of 2-µm colloid trajectories in the micromodel analyzed using IDL®. The colors indicate different particles, tracked at different times but superimposed here to illustrate the different pathways.

Exclusion from detouring flow streamlines. Due to their size, larger colloids travel in the center streamlines, leading to faster velocities, less dispersion and thus lower range of transit times.

Relative pore water velocity calculated by solving the Navier-Stokes equations in a 2-D finite element grid using FEMLAB®.s

Column breakthrough curves for dissolved tracer and different sized particles (0.05 and 3 µm) showing the early arrival of colloids or size exclusion effect.

We investigate the dynamics of the removal process (filtration) in terms of the dependence on throat size/colloid diameter ratio (T/C), electrolyte solution, flow velocity, hydrophobicity of the surfaces as well as grain surface morphology.

Publications

Auset M, Keller AA. Pore-scale processes that control dispersion of colloids in saturated porous media. 2004. Water Resources Research, 40 (3): Art. No. W03503.

Keller, AA, S Sirivithayapakorn, C Chrysikopoulos. 2004. Early breakthrough of Colloids and Bacteriophage MS2 in a water saturated sand column. Water Resources Research 40(8):W08304, doi:10.1029/2003WR002676.

S Sirivithayapakorn, AA Keller. 2003. Transport of colloids in saturated porous media: A pore scale observation of the size exclusion effect and colloid acceleration, Water Resources Research, 39(4): 1109, doi:10.1029/2002WR001583.

Oral presentations

Auset, M. and A.A. Keller, Colloidal acceleration and dispersion in micromodels, Eos Trans. AGU, 84 (46), Fall Meet. Suppl., Abstract H12K-08, 2003. Download presentation.

Auset, M. and A.A. Keller. Colloid transport in micromodels of saturated porous media. 78th American Chemistry Society, Colloid and Surface Symposium. Yale University. June 2004. Download presentation.

Influence of the Air Water Interface

Observations from micromodels experiments show that colloid immobilization in unsaturated conditions is controlled by attachment to air-water interfaces, straining within thin-water films that surround mineral grains and entrapment within stagnant-water zones. Colloids may be released during transient-flow events characterized by temporal increases in flow rate and volumetric moisture content.

Colloids accumulate at the AWI almost irreversibly, until the dissolution of the air bubble reduces or eliminates the AWI. Once the air bubbles are near the end of the dissolution process, the colloids trapped at the AWI form a cluster that detaches and moves along with the flow:

Publications

Auset, M., Keller, A. A., Brissaud, F. and V. Lazarova, 2005. Intermittent filtration of bacteria and colloids in porous media. Accepted in Water Resources Research. Vol 41, doi:10.1029/2004WR003611.

Keller, AA & S Sirivithayapakorn, 2004. Transport of colloids in unsaturated porous media: Explaining arge scale behavior based on pore scale mechanisms. Water Resources Research, 40:W12403, doi:10.1029/2004WR003315.

Sirivithayapakorn, S. & AA Keller, 2003. Transport of colloids in unsaturated porous media: A pore scale observation of processes during the dissolution of air-water interface. Water Resources Research, 39(12):1346, doi:10.1029/2003WR002487.

Oral presentations

Keller, A., Sirivithayapakorn, S., Auset, M. Transport of colloids in unsaturated porous media: Explaining large scale behavior based on pore scale mechanisms. European Geophysical Union General Assembly 2005. Viena. Geophysical Research Abstracts, Vol. 7, 02466, 2005. Download presentation.

Auset, M., Keller, A., Brissaud, F. and Lazarova, V.Transport of bacteria in intermittent sand filters. Abstracts of Papers of the American Chemical Society 229 : U639-U639 064-COLL Part 1, MAR 13 2005. Download presentation.

Posters

Auset, M., Keller, A., Brissaud, F. and Lazarova, V., Intermittent filtration of bacteria and colloids at pore and column scales, Eos Trans. AGU, 85(47), Fall Meet. Suppl., Abstract H33B-0467, 2004. Download poster.

Auset, M., Keller, A., Brissaud, F. and Lazarova, V. Transport of bacteria and colloids in intermittent sand filters. 4th Intl. Slow Sand and Alternative Biological Filtration Conference. 3-5 May 2006 at IWW Water Centre, Mülheim an der Ruhr, Germany. Download poster.

Different pore geometries of model porous media, realized using soft photolithography.

An optical epi-fluorescent microscope (Nikon Optiphot-M) and a Charged-Coupled Device (CCD) camera, mounted directly onto the eye-piece (Optronics Engineering), are used for the visualization of the migration of colloidal particles within the micromodels.