Bruce C. Bunker

Immobilization of High-Level Radioactive Sludges in Iron Phosphate Glass

Borosilicate glasses used for vitrifying radioactive high-level wastes are not optimized for disposing of sludges rich in phosphorus, fluorine, and heavy metals. Low melt solubilities of these problem constituents require the production of large quantities of waste glass which is expensive to make and to store in waste repositories. Existing disposal problems could be alleviated if a host glass could be found that could incorporate higher concentrations of problem constituents while retaining the chemical durability of borosilicate glass.

The structure and properties of two new silicotitanate zeolites

Two new zeolitic crystalline phases with stoichiometry, CSTiSi 2 O 6.5 and CS 2 TiSi 6 O 15 , have been discovered. CSTiSi 2 O 6.5 has a crystal structure isomorphous to the mineral pollucite, CsAlSi 2 O 6 , with Ti +4 replacing Al +3 . This replacement requires a mechanism for charge compensation. A combination of techniques including neutron diffraction, single crystal x-ray diffraction and x-ray absorption spectroscopy have revealed that eight extra oxygens are present per unit cell Cs 2 TiSi 2 O 6.5 as compared to pollucite. As a result of the extra oxygen, the titanium coordination geometry is five-fold. Pentacoordinate titanium and tetrahedral silicon form a network structure with Cs residing in cages formed by the network. The crystal structure of Cs 2 TiSi 6 O 15 is unique, with titanium octahedra and silicon tetrahedra forming an open framework structure with the Cs residing in large cavities. The largest covalently bonded ring opening to the Cs cavities in both compounds are smaller than a Cs ion, revealing that the Cs ion has minimal mobility in the structure. Cesium leach rates for both compounds are lower than or comparable to borosilicate glass.

Colloidal agglomerates in tank sludge and their impact on waste processing

Disposal of millions of gallons of existing radioactive wastes in underground storage tanks is a major remediation activity for the United States Department of Energy. These wastes include a substantial volume of insoluble sludges consisting of submicron colloidal particles. Processing these sludges under the proposed processing conditions presents unique challenges in retrieval transport, separation, and solidification of these waste streams. Depending on processing conditions, these colloidal particles can form agglomerated networks having high viscosities that could clog transfer lines or produce high volumes of low-density sediments that interfere with solid-liquid separations. Under different conditions, these particles can be dispersed to form very fine suspended particles that do not settle. Given the wide range of waste chemistries present at Department of Energy sites, it is impractical to measure the properties of all treatment procedures. Under the current research activities, the underlying principles of colloid chemistry and physics are being studied to predict and eventually control the physical properties of sludge suspensions and sediment layers in tank wastes and other waste processing streams. Proposed tank processing strategies include retrieval transport, and solid-liquid separations in basic (pH 10 to 14), high ionic strength (0.1 to 1.0 M) salt solutions. The effect of salt concentration, ionic strength, and salt composition on the physical properties such as viscosity, agglomerate size, and sedimentation of model suspensions containing mixtures of one or two of the major components found in actual wastes have been measured to understand how agglomeration influences processing. Property models developed from theory and experiment on these simple suspensions are then applied to explain the results obtained on actual wastes.

Effect of hydrolysis on the colloidal stability of fine alumina suspensions

Abstract This paper investigates the effect of hydrolysis and the formation of hydrated polycations on the colloidal stability and rheological properties of fine alumina suspensions. This is an important phenomenon in colloidal processing of advanced ceramic materials and nanocomposite materials from fine particles. The aging process and the formation of large polycations in the solution are monitored by nuclear magnetic resonance, and the aggregation rate and viscosity of the suspension are measured under similar conditions. It is observed that aging under acidic conditions increases colloidal stability against aggregation in dispersed suspensions, and reduces the viscosity in flocculated suspensions. This behavior corresponds well to the formation of large polynuclear species through hydrolysis. It is suggested that the hydrated polycations significantly modify the total interaction energy between two particles at short separation distances.

Heterogeneous nucleation of ordered mesoporous materials

Recently we proposed that heterogenous nucleation is an important phenomenon for the preparation of ordered mesoporous materials. In this paper we further investigate the effect of colloidal particles on the nucleation process of mesoporous materials. Based on the change of the electrical mobilities of the particles in the surfactant solution, we suggest that the adsorption and co-adsorption of surfactant and ceramic precursors changes local structural and chemistry on the particle surfaces, and favors the nucleation events within these regions.

Calcium Oxalate Nucleation and Growth on Oxide Surfaces

The heterogeneous nucleation of calcium oxalate onto colloidal oxides from aqueous solution has been studied as a model system for understanding the role of surface chemistry in biomimetic processes. The Constant Composition technique was used for measuring nucleation induction times. Results show that the dependence of nucleation on supersaturation fit well with classical nucleation theories. Surfaces which are anionic appear to promote calcium oxalate nucleation better that neutral or cationic surfaces. Modifications to positive surfaces via the adsorption of anionic surfactants, lower the effective energy barrier for nucleation, and stimulates the heterogeneous nucleation of calcium oxalate

Heterogeneous nucleation of calcium oxalate on native oxide surfaces

The aqueous deposition of calcium oxalate onto colloidal oxides has been studied as a model system for understanding heterogeneous nucleation processes of importance in biomimetic synthesis of ceramic thin films. Calcium oxalate nucleation has been monitored by measuring induction times for nucleation using Constant Composition techniques and by measuring nucleation densities on extended oxide surfaces using an atomic force microscope. Results show that the dependence of calcium oxalate nucleation on solution supersaturation fits the functional form predicted by classical nucleation theories. Anionic surfaces appear to promote nucleation better than cationic surfaces, lowering the effective energy barrier to heterogeneous nucleation.