Characterization of Uranium Solids Precipitated with Aluminosilicates Page: 4 of 20
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WM'04 CONFERENCE, FEBRUARY 29-MARCH 4, 2004, TUSCON, AZ.
Ion Exchange in a more restrictive sense as used in this study is an electrostatic process involving the replacement of
one readily exchangeable hydrated ion by another similarly exchangeable ion (Fig. 1B). This type of sorption is
also referred to in the literature as Outer Sphere sorption. It does not involve the formation of bonds with the
participating surface because the sorbed ion is only present in the diffuse double layer (DDL). This type of outer
sphere sorption is normally reversible (b) and is a function of ionic strength (i.e., such as Na* ion concentration).
Ion exchange sorption is often associated with materials that have constant surface charge and exhibit no change in
overall surface charge upon ion exchange sorption. An example of this process would be exchange of two hydrated
Na* ions for one hydrated U022' ion (also called the uranyl ion) in the DDL.
Specific Adsorption (often referred to as Chemisorption or Inner Sphere sorption) involves the formation of
predominantly covalent bonds with the surface, but the bonds can have some ionic behavior. These adsorbed metals
typically have one or more atoms from the participating surface in the second coordination shell (Fig. 1C). This
type of sorption involves the release of H' or structural surface ions (such as Na in monosodium titanate or MST)
upon sorption. For example, specific sorption of U022+ to NAS could result in the presence of Si and Al atoms in
the second coordination shell of the U022+. Specific adsorption is usually irreversible [Error! Bookmark not
defined.]. However, in the literature, specific adsorption is not always differentiated from structural incorporation
or surface precipitation. Specific adsorption may involve mononuclear complexes or polymeric species. It may
occur with metals and their associated ligands [such as a U(VI)-carbonate ion] and it is influenced by other solution-
and surface-related variables (c)..
Surface Precipitation occurs by nucleation of new solid phase on a host surface (Fig. 1D). For example, when the
concentration of a dissolved metal such as U022+ is high enough to result in the super-saturation of one or more
U022-containing phases [such as schoepite, which has a formula of U(VI)O3-2H20(s)] in the presence of another
solid, the other solid may facilitate the nucleation of a new solid UO22*-rich phase. This U-rich material would have
numerous U atoms in the second or third coordination shell of the U. The formation of colloidal (polymeric) U
species on surfaces could resemble the same local environment (on the atom scale) as observed for surface
precipitation. When atoms from a potential host surface are absent and polymeric species are present, the
mechanism of U uptake from solution is likely to be direct homogeneous (solid phase) precipitation.
In HLW, U may be concentrated by sorption to the surfaces of the NAS, precipitation within NAS structures and
precipitation as U phases. Sorption can be divided into two types of molecular scale processes (outer sphere and
specific adsorption) that involve the uptake of atoms near or at a participating sorptive surface. An element such as
U could co-precipitate with the NAS and related solids. [For zeolites, the term co-precipitation could be further
divided to include uptake into zeolite channels and any isomorphic substitution (i.e., of U for Si or Al) in the zeolite
structure . The U could also deposit by precipitation via surface nucleation (often referred to as chemical
seeding) on NAS minerals. It is also possible that U solids could seed the growth of NAS solids. The U could
potentially precipitate as an oxide [such as U03(s)], a hydrous oxide [such as Na2(U02)303(OH)2s)], and a silicate
[e.g., (U02)2SiO 2H20s)] (d) . Precipitation of U could occur simultaneously with the precipitation of NAS
solids. This process is referred to as solid phase nucleation . The U precipitation could occur independent of
NAS formation. A crystalline U form that is not associated with another separate mineral surface would be
identifiable based on the positions and identities of the atoms in that structure (i.e., its crystallographic structure).
This information can be obtained from structural refinements of X-ray diffraction (XRD) data and from XAFS data.
Uranium may also interact with silica sols, which have no defined crystal structure because of their amorphous
nature. At an atom- or molecular- scale basis, this type of interaction with U may be best be described by structural
incorporation in Fig. lA, which shows U in a crystalline structural-type environment. However, from a XAFS point
of reference, the local environment of U that is associated U with amorphous silica would not resemble that of a
crystalline U silicate structure.
Review of U(VI) Chemistry and Uptake Studies with U(VI) and Zeolites
In oxidized systems, dissolved U exists as the highly soluble uranyl [U(VI)02 2] species with two axial U=O double
bonds at -1.8 A. This form of U(VI) can exist in U solids. However, U(VI) can also exist in solids as the less
common uranate form, which has at least three single U-O bonds and no short axial double bonds. This form of
U(VI) is very small in size (-0.72-0.8 A) relative to the large uranyl ion group (-3.6 A).
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DUFF, MC. Characterization of Uranium Solids Precipitated with Aluminosilicates, article, January 9, 2004; South Carolina. (digital.library.unt.edu/ark:/67531/metadc739181/m1/4/: accessed January 23, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.