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Heavy Element Photophysics and Photochemistry
Comparison
of As-received and Thermally Densified Diphosil
Shown below are reflected light photomicrographs of Diphosil ion exchange beads.
The image on the left shows as-received Diphosil (irregularly shaped granules
that are highly light scattering and therefore appear white). The image on the
right shows Diphosil that was fully metal ion-loaded by immersion in dilute nitric
acid that contained heavy element ions. The metal-ion loaded Diphosil was then
heated to1273 K and cooled back to ambient temperature before the image was recorded.
Diphosil is high surface area, porous silica that has been functionalized with
diphosphonic acid groups that strongly bind heavy metal ions.
The heated material shown in the image on the right is nearly colorless to
the eye and scatters very little light in comparison with the as-received material
(left image). On average, the heated particles are smaller, have edges that are
smoother, and exhibit striated surfaces. These observations are all consistent,
as are our laser-induced fluorescence studies, with having achieved full thermal
densification. Such densification encapsulates the sorbed heavy metal ions in
phosphate-based nanophase regions within fused silica.
Fused silica is a highly radiation resistant glassy material. Because many
ground waters are saturated with respect to silica, very low leaching rates for
metal ions embedded in fused silica would be expected. However, fused silica has
not been a strong contender for use as a nuclear waste storage medium due to the
poor solubility of most heavy metal ions in it and processing temperatures that
exceed 2000 K. Our method of encapsulating heavy metal ions in fused silica promises
good heavy metal loading. Required temperatures are not only much below those
needed to create bulk fused silica,but, more importantly, are ~300 K lower than
those needed to make heavy metal loaded borosilicate glass.Borosilicate glass
is the current reference nuclear waste storage form for high level waste at several
Department of Energy facilities. In addition, Diphosil's ability to effectively
remove heavy metal ions from aqueous acid solutions provides additional waste
minimization and cost avoidance opportunities in treating high level waste streams.
The Diphosil used in these studies was from a developmental lot and was furnished
by EIChroM Industries Inc. (Darien, IL).
Our work is providing significant contributions to the scientific knowledge base
that is essential to achieve sorption from solution
to final waste form in a single material.
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