IE-911 is a bound form of cystalline silicotitanate (CST) that was extensively tested for removing 137Cs from the Savannah River Site (SRS) tank wastes. In some simulant tests, column plugging incidents were observed, which led to thorough investigations to determine the causes and to develop protocols to avoid future plugging incidents. A related problem was the apparent decrease in Cs scavenging capability in some long-term tests. Our studies revealed that the interaction of IE-911 with the highly basic, high ionic strength, SRS average salt simulant could result in precipitation of; 1) poorly crystalline Nb-oxide, or 2) aluminosilicate zeolitic phases. The source for the Nb-oxide precipitate was determined to be a minor impurity phase that is a byproduct of CST manufacturing. The mechanisms of dissolution and re-precipitation of this phase in column pretreatment solution were investigated, and a protocol to rid IE-911 of this impurity was devised. The source material for the aluminosilicate zeolite precipitate was determined to be predominantly from the waste solution rather than the IE-911. Solubility experiments coupled with a thermodynamic analysis provided a protocol to predict when aluminosilicate precipitation will and will not occur. Finally, it was also established that aluminosilicate precipitation on the surfaces of the IE-911 granules could also account for an apparent decrease in equilibrium Kd and decrease in kinetics of Cs sorption.

Oxides of lanthanide dissolution in a Na2O-Al2O3-B2O3-SiO2 glass using Gd as a probe was studied in detail using Raman, fluorescence, and phonon sideband (PSB) spectroscopic techniques. A mechanism of Gd cation dissolution, via partitioning, in the borate-rich and silicate-rich environments was confirmed by examining characteristics of the Raman band evolution, fluorescence emission and lifetime, and PSBs. The study concluded that i) Gd(III) cations in the borate-rich environment resembles Gd-metaborate local structure, i.e., 1BO4:1Gd:2BO3, ii) Gd cations partitioning in the borate-rich environment at low Gd2O3 concentration, [Gd2O3] -1/3[B2O3] < 0, and iii) excess Gd(III) cations, [Gd2O3]–1/3[B2O3], in the silicate-rich environment at high Gd2O3 concentration, [Gd2O3]–1/3[B2O3]>0.

A Gd-rich crystalline phase precipitated in a sodium gadolinium alumino-borosilicate glass during synthesis. The glass has a chemical composition of 45.4-31.1 wt% Gd2O3, 28.8-34.0 wt% SiO2, 10.8-14.0 wt% Na2O, 4.3-5.9 wt% Al2O3, and 10.8-14.9 wt% B2O3. Backscattered electron images revealed that the crystals are hexagonal, elongated, acicular, prismatic, skeletal or dendritic, tens of μm in size, some reaching 200 μm in length. Electron microprobe analysis confirmed that the crystals are chemically homogeneous and have a formula of NaGd9(SiO4)6O2 with minor B substitution for Si. The X-ray diffraction pattern of this phase is similar to that of lithium gadolinium silicate apatite. Thus, this hexagonal phase is a rare earth silicate with the apatite structure. We suggest that this Gd-silicate apatite in a Gd-borosilicate glass is a potential glass-ceramic nuclear waste form for actinide disposition. Am, Cm and other actinides can easily occupy the Gd-sites. The potential advantages of this glass-ceramic waste form include: 1) both the glass and apatite can be used to immobilize actinides, 2) silicate apatite is thermodynamically more stable than the glass, 3) borosilicate glass-bonded Gd-silicate apatite is easily fabricated, and 4) the Gd is an effective neutron absorber.

The solubilities of Hf and Gd in sodium alumino-borosilicate glasses based on the target compositions were examined and confirmed by electron microprobe analysis. The measured compositions of essentially crystal-free glasses are generally homogeneous and close to the target compositions. Therefore, the solubilities of Gd and Hf in sodium alumino-borosilicate glasses based on the target glass compositions are valid. However, for glasses containing precipitates (crystals grown from the melt) and undissolved HfO2 with overgrowths, the chemical compositions are often heterogeneous and may be significantly different from the target compositions. Precipitated crystalline phases include a rare earth silicate with the apatite sructure (NaGd9(Si5.25B)O26) in a gadolinium sodium alumino-borosilicate glass and a HfO2 phase in hafnium sodium alumino-borosilicate glasses.