FY04 Annual Report for Environmental Management Science Program - Strategic Design and Optimization of Inorganic Sorbents for Cesium, Strontium and Actinides Page: 3 of 11

                                
WSRC-RP-2004-00516

high-level nuclear wastes. The general approach can likewise be applied to non-
radioactive separations.
Research Progress
During the third year of the project our studies focused along the following paths: (1)
identifying the structure/affinity relationships for crystalline silicotitanate (CST) and
polyoxoniobate (Keggin chain) materials, (2) synthesis of new sorbents that include
a titanosilicate analog of the mineral pharmacosiderite, sodium titanates and
polyoxoniobates, (3) in-situ crystal growth and ion exchange studies with CST
materials and (4) computational models of the CST and polyoxoniobate materials.
CST serves as the baseline or alternative technology sorbent for the separation of
137CS from waste solutions at several of the DOE sites. This material also exhibits a
fairly high affinity for strontium, but no affinity for plutonium, neptunium and
uranium. A sodium titanate material, monosodium titanate (MST), serves as the
baseline material at the Savannah River Site for strontium and actinide separations
from HLW solutions. Performance testing at SRNL demonstrated that
titanosilicates, sodium nonatitanates and polyoxometalates effectively adsorb
strontium and actinides from high ionic strength salt solutions.
Project members at Texas A&M completed two detailed structural characterizations
of CST materials and studied the in-situ crystallization process during the synthesis
of CST. Structural studies included CST and niobium-substituted CST (Nb-CST)

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Figure 1. A polyhedral representation (with green TiO6 octahedra
and yellow SiO4 tetrahedra) of Nb-CST along [001], strontium
(purple spheres) and water (blue spheres) were located in the 8-
atom ring tunnels. In Nb-CST strontium is seven coordinate, but
in CST strontium is ten coordinate.

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Figure 2. A polyhedral perspective representation of TSP
along the [001] plane. Water (blue spheres) and cesium
(pink spheres) are located in the 8-atom ring tunnels. TiO6
octahedra are shown in green and SiO4 tetrahedra in brown.
The coordination of Cs was found to be directly correlated
to the observed exchange capacity.

sorbents before and after exchange with strontium and another closely related
titanosilicate material with the mineral pharmacosiderite topology (TSP). A complete
or partial substitution of elements Ge or Nb was carried out in the framework and the
consequent change in the ion exchange properties towards Cs were studied for
Ti/Ge/Si, Ti/Si, Nb/Ti/Si and pure Ge phases. This research found that substitution
of Ge and Nb into the framework modifies the structure of CST and TSP materials,

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