Physical Properties of Hanford Transuranic Waste Sludge Page: 1 of 2
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Physical Properties of Hanford Transuranic Waste Sludge
Berg, John C.
University of Washington
RESULTS TO DATE: Equipment that was purchased in the abbreviated year 1 of this project has been
used during year 2 to study the fundamental behavior of materials that simulate the behavior of the Hanford
transuranic waste sludge. Two significant results have been found, and each has been submitted for
publication. Both studies found non-DLVO behavior in simulant systems. These separate but related
studies were performed concurrently. It was previously shown in Rassat et al.'s report Physical and Liquid
Chemical Simulant Formulations for Transuranic Wastes in Hanford Single-Shell Tanks that colloidal clays
behave similarly to transuranic waste sludge (PNNL-14333, National Technical Information Service, U.S.
Dept. of Commerce). Rassat et al. also discussed the pH and salt content of actual waste materials. It was
shown that these materials exist at high pHs, generally above 10, and at high salt content, approximately
1.5 M from a mixture of different salts. A type of clay commonly studied, due to its uniformity, is a
synthetic hectorite, Laponite. Therefore the work performed over the course of the last year was done
mainly using suspensions of Laponite at high pH and involving high salt concentrations. One study was
titled "Relating Clay Rheology to Colloidal Parameters." It has been submitted to the Journal of Colloid
and INterface Science and is currently in the review process. The idea was to gain the ability to use
measurable quantities to predict the flow behavior of clay systems, which should be similar to transuranic
waste sludge. Leong et al. had previously shown that the yield stress of colloidal slurries of titania and
alumina could be predicted, given the measurement of the accessible parameter zeta potential (Leong YK et
al. J Chem Soc Faraday Trans, 19 (1993) 2473). Colloidal clays have a fundamentally different
morphology and surface charge distribution than the spheroidal, uniformly charged colloids previously
studied. This study was therefore performed in order to determine the applicability of the previous findings
to the systems of interest. The yield stress of clay slurries was measured using the Physica MCR 300
purchased in year 1 of this project. The zeta potential of these systems was then measured using the
Brookhaven Zeta PALS, also purchased in year 1. These two parameters were then plotted and compared
with the Leong result. It was found that this system behaved in a non-DLVO manner. Leong found that
colloidal slurry yield stress decreases with increased zeta potential which is consistent with the DLVO
theory's assertion that particle attractions decrease as their electrostatic repulsion increases. Clay systems,
however, show an increase in yield stress as zeta potential is increased. This is due to the nature of the
charge distribution on the surface of clay particles. Clay particles are in the form of platelets. It is
generally accepted that the charge on the face of the platelet is negative and the charge on the edge of the
platelet is positive. This can lead to electrostatic attractions between particles which would increase in
strength as the magnitude of the charge on the face or edge is increased. In this case the van der Waals and
electrostatic forces are additive unlike the DLVO case where these forces compete. Leong et al. wrote an
equation that related slurry yield stress to the number and strength of particle interactions based on DLVO
where electrostatic forces were subtracted from van der Waals. In this work Leong et al.'s equation was
modified to make these two forces additive. The resulting equation was compared to laboratory data and
showed good agreement. A system of naturally occurring clays, kaolinites and bentonites, was used to help
confirm the result. The experimental data from this system also fit the model well. These results are thought
to be a significant contribution to the field and have been submitted for publication. Beyond being
interesting academically, these results have implications for designing processes for the removal of
transuranic wastes from storage tanks. In particular it may not be intuitive to think that dilution of sludge
may actually make moving the sludge more difficult. The yield stress of these systems increases as zeta
potential increases. Zeta potential increases as salt concentration decreases. Therefore, as water is added to
the system, thereby decreasing salt concentration, the yield stress of transuranic wastes may actually
increase. Fortunately, there is a significant competing effect. The yield stress is directly related to the
square of the volume fraction of solids available, which would tend to reduce yield stress as water is added.
The proposed relation accounts for this volume fraction dependence and could potentially be used to
determine an optimum dilution for waste removal. The second study, titled "Re-stabilization of Laponite
Clay Particles in High Salt Media," also showed a non-DLVO phenomenon. The paper was submitted for
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Berg, John C. Physical Properties of Hanford Transuranic Waste Sludge, report, June 1, 2005; United States. (digital.library.unt.edu/ark:/67531/metadc883751/m1/1/: accessed April 26, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.