William Paul Johnson, PhD, University of Utah
Microbial transport and retention in porous media is a process relevant to many engineered processes, including but not limited to microbial enhanced oil recovery, water treatment and filtration, bio-remediation. Bacteria and other microbes differ from well-studied non-biological spherical colloids in two main aspects: i) bacteria often possess complex surface properties or surface structures (e.g., fimbriae, pili, flagellae) that may blur the distinction between favorable and unfavorable conditions (i.e., the absence versus presence of energy barriers); and ii) bacterial cells are typically non-spherical in shape (e.g., rod-like). To elucidate the influences of bacterial cell shape and surface properties on their transport and retention, we have taken two innovative approaches: 1) isolating influences of cell shape from surface properties or surface structures; and 2) isolating bacterial transport and retention behaviors under favorable conditions from those under unfavorable conditions. Below our findings on bacterial retention in porous media obtained from the above two approaches are briefly described.
To determine the influence of bacterial shape on their retention, rod-shaped carboxylate-modified polystyrene latex colloids were used as bacterium surrogates because these colloids were negatively charged just as bacteria, however they were devoid of other complex bacterial surface structures. The transport and retention behaviors of rod-shaped colloids with two different aspect ratios (2:1 and 6:1) in porous media were examined and compared to those of spheres of equal volume under both favorable and unfavorable conditions. Relative to spheres, the observed retention for rod-shaped particles was less under favorable conditions but greater under unfavorable conditions, indicating that for both conditions, treating rod-shaped colloids as effective spheres was inadequate and the shape (or orientation) of rods needed to be considered to describe their retention during transport. Under both favorable and unfavorable conditions, rod-shaped colloids were observed to undergo tumbling motions near the collector surfaces; which suggested their attachment onto the surfaces involves an intermediate step in which an initial end-on contact of rods with the surface was established, and subsequently the end-on orientation may change to side-on under the coupled influences of colloidal and hydrodynamic interactions. This is a possible explanation for the observed two modes of attachment (end-on and side-on) for rods on the surfaces, but the predominant mode being the side-on orientation, for both favorable and unfavorable conditions. The effect of shape on retention was less pronounced at high fluid velocity because hydrodynamic interactions may overshadow near-surface colloidal interactions. Throughout our column experiments, these two types of rods (1:2 and 1:6 aspect ratios) exhibited very similar breakthrough and retention behaviors, and both rods showed strongly contrasting behavior to spheres, indicating low sensitivity to particle aspect ratio in the range from 1:2 to 1:6.
The heterogeneous surface structures or properties have not been extensively studied for their influences on bacterial retention, partly due to lack of an easily accessible theory to incorporate these influences. As such, most studies to date still use the mean-field (or averaged) properties to describe bacterial surfaces. To determine the influence of bacterial surface structures or properties on their transport and retention, bacteria were modeled as spherical colloids that would be covered with heterogeneous charge features resembling bacterial surfaces; this way, the complex effect due to bacterial non-spherical shape on their retention could be avoided. Two main challenges exist for incorporating heterogeneity onto colloid surfaces: i) new method needs to be developed to account for colloidal interactions between heterogeneous surfaces because the conventional mean-field DLVO theories were not applicable here; and ii) both the translational and rotational motions of colloids in the flow field need to be tracked. The first challenge has been resolved; we have modified the grid surface integration (GSI) technique, originally developed by other researchers, and have tested the effectiveness of this technique in computing colloidal interactions between a heterogeneously charged collector surface and a uniformly charged sphere. The second challenge is beyond the scope of this present project, but will form part of our future research projects.
With the support of this ACS PRF grant (50790ND8), we have already published one peer-reviewed research article on surface heterogeneity in Langmuir (Ma et al., 2011), and another manuscript is nearly ready for submission regarding particle shape effects on retention. We presented our research from this project at the ACS Colloids and Surface Science Symposium in 2011 and 2012. These research findings not only help elucidate some of the influences of particle shape or surface heterogeneity on bacterial transport and retention, but greatly contribute towards development of a new theory that will predict non-spherical colloid transport in granular porous media under environmentally relevant conditions (i.e., presence of colloid-collector repulsion).
This PRF grant mainly sponsored my postdoctoral associate, Dr. Huilian Ma, who was recently appointed as a Research Assistant Professor in our department. Based on the above described accomplishments, she is highly motivated to continue working on this topic; specifically, to develop a computational method capable of tracking the orientation of colloidal particles in response to hydrodynamic flow as well as to colloidal interactions resulted from their surface heterogeneities. Eventually, the coupled effects due to particle shape and surface heterogeneous properties on retention will be integrated to better elucidate bacterial transport and retention behaviors under both favorable and unfavorable conditions. Additionally, our research on surface heterogeneity, which was partially sponsored by this grant, also provided important resources and guidance to another Ph.D. candidate in our research group, Mr. Eddy Pazmino, to pursue his doctoral research on simulation and characterization of collector surface charge heterogeneity. Finally, this PRF project has also greatly enhanced the collaborative research work between our research group and other researchers, e.g., Dr. Carl Bolster from USDA-ARS in Bowling Green, KY, and Dr. Samir Mitrogotri's lab in Chemical Engineering at University of California, Santa Barbara, who are co-authors on the developing manuscript regarding the influence of colloid non-sphericity on retention.