Bacterial Transport in Saturated,
Unsaturated, and Air-Sparged Porous Media
A research project conducted in cooperation with
the University of Arizona NIEHS Superfund Center.
For more information on the NIEHS program, click
For information on the University of Arizona Superfund center, click
Need: There are over 217,000 contaminated
waste sites in the United States awaiting remedial actions, with 547
designated as Superfund National Priority Lists (NPL) sites. The
remedial costs for the cleanup of the contaminants emanating from
solvents, petroleum products, and metals is projected to exceed $187
billion. Due to the need for less costly remedial treatments, there
is a great interest in contaminant-specific treatment techniques
such as bioremediation.
In-situ soil bioremediation is a promising method of aquifer
remediation. In many cases pollutant biodegradation can be
accelerated through bioaugmentation, or the delivery of
pollutant-degrading microbes into the contaminated soils.
Researchers have isolated, identified, and cultured in the
laboratory microbes capable of degrading toxic pollutants such as
benzene, toluene, and carbon tetrachloride. Unfortunately, most of
these laboratory-grown bacteria readily stick to soil grains,
limiting their use for bioaugmentation. There is little benefit to
digging a well and injecting bacteria, for example, if the bacteria
travel only a few centimeters from the well casing. From an economic
standpoint, it would be cheaper to dig up the soil than to attempt
in-situ remediation. Fortunately, there are several methods that can
be used to enhance bacterial transport in soils.
Transport Studies: As a Part of the NIEHS Superfund
Program(web site), The University of Arizona was designated as a
Superfund Research Center. Professor Logan and graduate students
Terri Camesano and Amanda DeSantis are working with The University
of Arizona to investigate methods to increase the transport of
bacteria in porous media. The approach used by the research team to
identify factors that reduce attachment was to study bacterial
retention in short soil columns. By incorporating a radiolabeled
amino acid into the cells, they were able to detect low cell
concentrations sorbed to soil. This made it possible to measure
changes in retention, in columns only centimeters long, that would
enhance the transport cells over distances of tens of meters in the
field. The success of adhesion modifying treatments was quantified
in terms of a sticking coefficient (the fraction of collisions of
cells with soil grains that are successful) and mathematical models.
Transport was judged to be successful when sticking coefficients
were reduced to below 10-2 to 10-3.
It was discovered that by suspending cells in demineralized water it
was possible to transport cells over much longer distances than that
possible using groundwater. Increased transport resulted from an
increase in the electrostatic repulsive forces between the
negatively charged soil particles and bacteria. Although soils will
slowly weather and increase the conductivity of the injected water,
low conductivities in the soils were maintained over the brief
periods necessary to inject the bacterial pulse. Similar
enhancements in bacterial transport were obtained using non-ionic
surfactants (such as Tween 20), although surfactants had to be added
at concentrations that were too high (0.01%) to make this approach
practical at actual sites.
Other factors are being investigated that can be used to increase
the effectiveness of bioaugmentation-- although the success of these
approaches was not anticipated from simulations using current
particle transport models. Critical factors include fluid injection
rate (fluid velocity), cell concentration, and whether or not the
cells are motile. Current models, based on a set of equations known
as clean-bed filtration theory, suggest that high fluid velocities
can increase cell transport. Furthermore, these models suggest that
soil retention would be larger for motile than non-motile cells and
that cell concentrations do not alter overall deposition rates.
However, much different results have been observed in laboratory
column experiments. The transport of several motile strains of
bacteria (Pseudomonas stutzeri KC, Pseudomonas fluorescens P17, and
Pseudomonas putida KT2442) have been found to be much greater at low
fluid velocities (<1 m/d) than at a high fluid velocities (>100
The distance bacteria can move in the soil increases for some
strains as cells are increasingly deposited on the soil, indicating
there is an effect of particle concentration (or total injected cell
mass). Deposited cells can either reduce the subsequent attachment
rates of other cells, via a phenomenon known as blocking, or
increase rates (ripening). The outcome is strain specific. A
trichloroethylene (TCE) degrader Burkholderia cepacia G4 exhibited a
blocking effect while Pseudomonas fluorescens P17 has been observed
to promote ripening. Blocking can substantially reduce deposition
rates because deposited bacterium creates a "shadow zone", or
blocked surface area, that can be greater than the size of the
bacterium itself. The size of this shadow zone is a function of
solution chemistry and hydrodynamics (flow velocity).
New Molecular-level Approaches: Ongoing
research is aimed at examining the importance of cell motility and
factors that affect the extent of blocking for a number of different
bacteria in two ways. First, researchers are examining how
deposition rates (and blocking) are altered by the addition of
surface modifying chemical additives as a function of cell
concentrations and fluid velocities. Second, the factors that
inhibit adhesion (increase repulsion) and promote blocking are being
probed at nano-scales using an atomic force microscope (AFM). This
instrument can be used to generate 3-D images of surface topography
and maps of surface elasticity. By studying the response of the tip
to the treated and untreated cells, it is possible to precisely
measure electrostatic forces that are thought to govern cell
attachment to surfaces. The researchers are studying the effects of
various chemical treatments on the morphologies of Burkholderia
cepacia G4 and Pseudomonas stutzeri KC as well as the repulsive
response of the AFM silica tip to see if it can be correlated to
bacterial sticking coefficients in a sandy soil.
It is hoped that these experiments will lead to a better
understanding of the factors that control cell adhesion to soil
particles as well as to other surfaces. Ultimately, this research
will contribute to the increased success of in situ soil remediation
by improving the dispersion of laboratory cultivated bacteria and
increasing the overall efficiency of bioaugmentation processes.
|Ms. Terri Camesano
(email@example.com), shown working on the Atomic Force
Microscope (AFM), is a Ph.D. candidate in Environmental
Engineering. She has shown that cell motility can increase
the transport of one bacterial strain at low groundwater
velocities. Using the AFM, she is examining the topography
and surfaces forces of bacteria in order to improve
bacterial transport for bioaugmentation.
|Ms. Amanda DeSantis
(firstname.lastname@example.org) is examining the generality of the finding
that motile bacterial transport is enhanced at low
groundwater velocities compared to non-motile cells. She is
running soil column experiments to calculate bacterial
adehsion to soil grains and to determine the effect of
blocking on microbe transport.
|Mr. Karl Shellenberger
using the AFM to examine surface forces important in
controlling cell attachment. It has been hypothesized by
others that localized regions of positive charge create
favorable adhesion sites on mineral surfaces. He is using
the AFM to scan surfaces to test this hypothesis by
measuring surface-tip interactions over large regions of the
For more information please contact:
Bruce Logan, Ph.D.
Phone: 814-863-7908, Fax: 814-863-7304, Email:
To learn more about this area of research please refer to the
Rogers, B. and B.E. Logan. 2000. Bacterial
transport in NAPL-contaminated porous media. J. Environ. Engrg.
126(7): In press.
Camesano, T.A. M.J. Natan, and B.E. Logan. 2000. Observation of
changes in bacterial cell morphology using tapping mode atomic
force microscopy. Langmuir 16(10):4563-4572.
Unice, K.M., and B.E. Logan. 2000. The insignificant role of
hydrodynamic dispersion on bacterial transport. J. Environ.
Engin. 126(6): 491-500.
Camesano, T.A., K.M. Unice and B.E. Logan. 1999. Modeling
dynamic blocking of colloids in porous media using intracolumn
deposition patterns and breakthrough curves. Colloids Surf. A.
Physicochem. Engin. Aspects. 160(3):291-307.
Fang, Y. and B.E. Logan. 1999. Bacterial transport in gas
sparged porous media. J. Environ. Engng. 125(7):668-673.
Logan, B.E., T.A. Camesano, A.A. DeSantis, and J.C. Baygents.
1999. Comment on "A method for calculating bacterial deposition
coefficients using the fraction of bacteria recovered from
laboratory columns" by Bolster et al. Environ. Sci. Technol. In
Jewett, D.G., B.E. Logan, R.G. Arnold, and R.C. Bales. 1999.
Transport of Pseudomonas fluorescens strain P17 through porous
media as a function of water content. J. Contam. Hydrol.
Li, Q. and B.E. Logan. 1999. Enhancing bacterial transport for
bioaugmentation of aquifers using low ionic strength solutions
and surfactants. Wat. Res., 33(4):1090-1100.
Camesano, T.A. and B.E. Logan. 1998. Influence of fluid velocity
and cell concentration on the transport of motile and non-motile
bacteria in porous media. Environ. Sci. Technol. 32:1699-1708.
Gross, M.J., Albinger, O., Jewett, D.G., Logan, B.E., Bales, R.C.,
and R.G. Arnold. 1995. Measurement of Bacterial Collision
Efficiencies in Porous Media. Wat. Res. 29:1151-1158.
Gross, M.J. and B.E. Logan 1995. Influence of Different Chemical
Treatments on Transport of Alcaligenes paradoxus in Porous
Media. Appl. Environ. Microbiol. 61:1750-1756.
Johnson, W.P., Martin, M.J., Gross, M.J., and B.E. Logan. 1996.
Facilitation of Bacterial Transport through Porous Media by
Changes in Solution and Surface Properties. Colloid Surf. A:
Physiocochem. Eng. Aspects 107:263-271.