Yucca Mountain Project - Science & Technology Radionuclide Absorbers Development Program Overview Page: 4 of 15
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accorded to the scavenging ability of conventional hydrotalcite minerals (mixed metal
hydroxides that act as anion exchangers), as well as the performance of hydrotalcites
where non-traditional metals have been substituted into the normal hydrotalcite
formulation. Optimizing the performance of materials developed along the lines
established by previous studies will provide the basis for the proposed research.
Past SNL studies have identified several materials as sorbents for iodine that are
based on 1) layered double hydroxides (LDH) with rather exotic chemistries
(incorporating Bi and Zr in addition to the traditional Mg-Al normally used in
hydrotalcite formulations) and 2) the ability of copper oxides (and possibly hydroxides)
to retain iodine. In this task, these leads will be followed and the scope of the
investigation broadened by synthesizing and evaluating other materials with related
chemistries and structures. Various candidate materials will be tested as iodine sorbents
through kinetic (flow-through) and batch (static) equilibrium experiments.
Structures of these materials will be determined by scanning electron microscopy
(SEM) and TEM (particle morphology), BET (surface area), XRD (crystal structure) and
Zeta potential (polarity of particle surface charges in different fluids). ICP-MS will be
used to determine the solubility of substrate materials and their sorption capacity for
iodine. For layered materials, the impact of altering interlayer environments through the
use of polyoxometalate ions will also be explored. Iodine uptake rates and longer-term
equilibration experiments will be carried out as a function of temperature, iodine
concentration, iodine to sorbent ratio, and in the presence of potentially competing ions
that may exist in YMR relevant fluids. Iodine speciation (e.g., I~ vs. IO3) will be
assessed by comparing total dissolved iodine (from ICP-MS) with iodide (I) analyses
obtained using an iodide specific ion electrode. As with the preceding task, this work
will continue into FY05/06. FY07/08 studies will be more directed to developing an in-
depth understanding of the retention mechanisms of the best candidate materials and
toward quantifying the extent (or lack thereof) of desorption expected in different
environments. Throughout the study, a limited number of ReO4 batch sorption tests will
also be carried out to screen for materials that could possibly contribute to other tasks
directed at immobilizing pertechnetate, TcO-4.
Layered Double Hydroxides
LDH materials show significant promise for the uptake of anions such as
pertechnetate, TcO4 and iodate, IO3, from water. Their sequestration potential is
attributed, in part, to their high point of zero charge (PZC), the pH at which the particles
exhibit no net surface charge. The PZC for many LDH materials is greater than pH 12.
LDH materials also exhibit high interlayer anion exchange capability. In addition, past
studies have amply demonstrated that a wide variety of responses to radionuclide uptake
may be obtained depending on the synthesis process and the bulk chemistry of the
materials. At present, however, no systematic approach is available that allows a
researcher to determine, in advance, optimal compositions and formulation strategies.
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Jow, Hong-Nian; Moore, Robert C.; Helean, Katheryn B.; Mattigod, Shas; Hochella, Michael, Jr.; Felmy, Andrew R. et al. Yucca Mountain Project - Science & Technology Radionuclide Absorbers Development Program Overview, report, January 14, 2005; Las Vegas, Nevada. (https://digital.library.unt.edu/ark:/67531/metadc779912/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.