Gas-phase Si and Al oxyanions were formed by particle bombardment, isolated by mass, and then reacted with H2S in an ion trap secondary ion mass spectrometer (IT-SIMS). The reactions proceeded by different reaction pathways depending on whether the oxyanions were even- or odd-electron species. The radical anion SiO2 •- reacted with H2S by abstracting a •SH radical to form the even-electron SiO2SH-. Once formed, the even electron SiO2SH- reacted with a second H2S molecule by O-for-S exchange to form SiOS2H-. The radical anion SiO3 •- abstracted an •H radical from H2S to form even-electron SiO3H-, which then underwent two consecutive O-for-S exchange reactions with H2S to form SiO2SH- and SiOS2H-. For the reactions of the even-electron anion AlO2 -, the products of two consecutive O-for-S exchange reactions with H2S were AlOS- and AlS2 -. The radical abstraction reactions and the O-for-S exchange reactions of SiO3H-, AlO2 -, and AlOS- were efficient in the 30−50% range. The efficiency of the O-for-S exchange reaction of SiO2SH- (producing SiOS2H-) was substantially less efficient at 8%.

Water was reacted with gas-phase oxyanions having the general composition Si x O y H z - that were formed and isolated in an ion trap-secondary ion mass spectrometer (IT-SIMS). The radical SiO2 •- reacted slowly with H2O to abstract HO•, forming SiO3H-, at a rate of 8 × 10-13 cm3 molecule-1 s-1, corresponding to an efficiency of about 0.03% compared with the theoretical collision rate constant (average dipole orientation). The product ion SiO3H- underwent a consecutive condensation reaction with H2O to form SiO4H3 - at a rate that was approximately 0.4−0.7% efficient. SiO4H3 - did not undergo further reaction with water. The multiple reaction pathways by which radical SiO3 •- reacted with H2O were kinetically modeled using a stochastic approach. SiO3 •- reacted with water by three parallel reaction pathways:  (1) abstraction of a radical H• to form SiO3H-, which then reacted with a second H2O to form SiO4H3 -; (2) abstraction of a radical OH• to form SiO4H-, which further reacted by consecutive H• abstractions to form SiO4H2 •- and then SiO4H3 -; and (3) condensation with H2O to form SiO4H2 •-, which subsequently abstracted a radical H• from a second H2O to form SiO4H3 -. In all of these reactions, the rate constants were determined to be very slow, as determined by both direct measurement and stochastic modeling. For comparison, the even electron ion Si2O5H- was also investigated:  it underwent condensation with H2O to form Si2O6H3 -, with a rate constant corresponding to 50% efficiency. The reactions were also modeled using ab initio calculations at the UB3LYP/6-311+G(2d,p) level. Addition of H2O to SiO3 •-, SiO3H-, and Si2O5H- was calculated to be approximately 42, 45, and 55 kcal mol-1 exothermic, respectively, and encountered low activation barriers. Modeling of SiO2 •- and SiO3 •- reactions with H2O failed to produce radical abstraction reaction pathways observed in the IT-SIMS, possibly indicating that alternative reaction mechanisms are operative.

Researchers at the Savannah River Technology Center (SRTC) successfully demonstrated the Caustic-Side Solvent Extraction (CSSX) process flow sheet using a 33-stage, 2-cm centrifugal contactor apparatus in two 24-hour tests using actual high level waste. The CSSX process for removal of cesium from alkaline solutions is the reference process for decontamination of high level waste (HLW) at the Savannah River Site (SRS). The solvent consists of a calix [4]arene-crown-6 extractant (BOBCalix), an alkylphenoxy alcohol modifier, and trioctylamine (TOA) dissolved in an inert hydrocarbon matrix (Isopar ® L). Previously, we demonstrated the solvent extraction process with actual SRS HLW supernatant solution using a non-optimized solvent formulation. Following that test, the solvent system was optimized to enhance extractant solubility in the diluent by increasing the modifier concentration. We now report results of two tests with the new and optimized solvent. The first test used a composite of supernatant solutions from two waste tanks and the second test used a solution derived from dissolved salt cake. Test results showed that the CSSX process using the optimized solvent reduces 137 Cs in HLW supernatant solutions to concentrations below the waste acceptance criterion (WAC) of 45 nCi/g for disposal as low-level waste (called "Saltstone"). Waste decontamination factors as high as three million were achieved during testing. Test durations exceeded 24 hours of uninterrupted operation and demonstrated hydraulic stability of the contactor array while operating with the optimized solvent. Carryover of organic solvent in aqueous streams (and aqueous in organic streams) was found to be less than 1%. The concentration factor (i.e., the ratio of the cesium concentration in the strip raffinate to the concentration in the waste) averaged approximately 13 during both tests, slightly below the process requirement of 15. Uncertainties in process flow rate measurement and control prevented the test from achieving the target of 15.

Oligomeric oxyanions were observed in the secondary ion mass spectra (SIMS) of zeolite materials. The oxyanions have the general composition AlmSinO2(m+n)H(m-1)(-)(m+n = 2 to 8) and are termed dehydrates. For a given mass, multiple elemental compositions are possible because (Al + H) is an isovalent and isobaric substitute for Si. Using 18 keV Ga+ as a projectile, oligomer abundances are low relative to the monomers. Oligomer abundance can be increased by using the polyatomic projectile ReO4- (approximately 5 keV). Oligomer abundance can be further increased using an ion trap (IT-) SIMS; in this instrument, long ion lifetimes (tens of ms) and relatively high He pressure result in significant collisional stabilization and increased high-mass abundance. The dehydrates rapidly react with adventitious H2O present in the IT-SIMS to form mono-, di-, and trihydrates. The rapidity of the reaction and comparison to aluminum oxyanion hydration suggest that H2O adds to the aluminosilicate oxyanions in a dissociative fashion, forming covalently bound product ions. In addition to these findings, it was noted that production of abundant oligomeric aluminosilicates could be significantly increased by substituting the countercation (NH4+) with the larger alkali ions Rb+ and Cs+. This constitutes a useful tactic for generating large aluminosilicate oligomers for surface characterization and ion-molecule reactivity studies.

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