Surface chemistry investigation of colloid transport in packed beds. Final report, August 1, 1989--July 31, 1996 Page: 4 of 10
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The second major objective of the project was to investigate the influence of humic
matter and other selected polyelectrolyte coatings on deposition rates in porous media,
using colloids of environmental interest - iron oxide particles. Specifically we sought to
determine the nature of the repulsive interaction forces that stabilize humic-coated
particles and thereby identify the most important general features of humic molecules and
solution chemistry variables that influence particle transport. This phase of the project
was also designed to complement other DOE field projects, specifically the Georgetown,
S.C. colloid transport studies conducted by John McCarthy's group at ORNL. For
example, many of the same materials used in the ORNL field study, including natural
organic matter isolates and iron oxide particles, were used in our project. These efforts
and accomplishments are described under "Task 2" below.
Results and Accomplishments
Task 1. Experimentally determine the importance of collector surface heterogeneity in
controlling particle deposition rates.
Several model systems, including spherical glass beads and ultrapure quartz sand,
were utilized as filter media to assess whether filtration rates in the presence of
electrostatically repulsive interactions were also a function of non-electrostatic factors.
Particular attention was given to the importance of collector cleaning and preparation
methods. Particle filtration rates were extremely sensitive to bed media preparation
methods, despite the similar electrophoretic mobilities of the bed media. The results of
these filtration experiments, which are described in Papers #7 and #8 of the Project
Bibliography, illustrate the importance of non-electrostatic factors. It was postulated that
microscale surface heterogeneity is an important overlooked variable that affects filtration
rates when overall particle-surface interactions are apparently unfavorable.
Additional experimental evidence that collector surface heterogeneities serve as
favorable deposition sites for particles was obtained by masking favorable sites with
surfactants. With the addition of anionic surfactants, colloid filtration rates were much
more sensitive to solution ionic strength, as DLVO theory suggests they should be (see
Paper #4). Particle deposition rates in the presence of surfactants were also found to be
particle-size dependent (see Paper #1), unlike previous studies in the absence of
surfactants in which filtration was insensitive to particle size . Gary Litton's
presentation of these findings at a National ACS Meeting in San Diego was awarded an
ACS Certificate of Merit.
Other investigators have since shown using theoretical approaches that collector
surface heterogeneity could explain the differences between observed colloid filtration
rates and the predictions based on DLVO theory . The data from our experimental
studies (Paper #7) was in fact modeled in this theoretical study and used as further
evidence that the heterogeneity explanation is likely to be the correct one.
In addition to exploring reasons for the well-known underestimation of particle
attachment rates by DLVO theory, we have also determined that there are conditions in
which these models overestimate colloid deposition rates. Anomalous repulsive
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Olson, T.M. Surface chemistry investigation of colloid transport in packed beds. Final report, August 1, 1989--July 31, 1996, report, December 31, 1996; United States. (digital.library.unt.edu/ark:/67531/metadc710782/m1/4/: accessed January 16, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.