PNNL

The Science                                

The millions of gallons of legacy radioactive tank wastes at the Hanford and Savannah River Sites contain a complex mixture of highly concentrated salts. Aluminum is one of the most common metallic elements in the mixture and exists partly as boehmite, a type of aluminum oxide hydroxide. Researchers combined experimental and modeling efforts to show that ion-specific effects control the aggregation of nanoparticles of boehmite in water-based solutions. They measured the level of boehmite aggregation in solutions with different ions present and found that both surface interactions between the boehmite and the ions, and changes in the solution structure led to the ion specific effects.

The Impact

The waste generated during plutonium production represents some of the most complicated and toxic materials ever studied. Currently, researchers lack a detailed understanding of how ions at high concentrations interact with solid particles like boehmite. This work identifies the fundamental role that ions play in the aggregation of solids. This is important for understanding the behavior of complex wastes and learning how to safely process them.

Summary

Legacy radioactive tank wastes found at the Hanford Site in Washington and the Savannah River Site in South Carolina consist of complex mixtures with high ion concentrations. Researchers combined experimental work and modeling to better understand the effects of solution chemistry on particle aggregation in highly concentrated water-based solutions. They performed a series of tumbler small and ultra-small angle neutron scattering experiments on boehmite nanoparticles suspended as a slurry in solutions of nitrate salts. They studied nitrate paired with cations of different sizes (H, Li, Na, K, and Rb) over a range of concentrations from pure water to 4 molal.

The boehmite synthesis was tailored to produce nanoparticles in a narrow size range (~20 to 30 nm) for the aggregation experiments. Once placed in the solution, the individual boehmite platelets quickly formed well-bonded stacks, or primary aggregates, about 150 nm long. The boehmite nanoparticles also formed a second level of aggregates, with concentrations and structures varying based on cation type and concentration. Aggregation generally increased with increasing salt concentration and cation radius from ~0 -0.3 molal, after which the trend reversed. The reversal likely reflects a change in the factors influencing aggregation, from a boehmite surface-controlled effect at low concentrations to solution structure effects at high concentrations. As the properties of a slurry depend on factors such as solids concentration, solids density, liquid density, particle size, etc., these data suggest that understanding how salt concentration and chemistry affect nanoparticle aggregate structures can provide useful physical insights into the microscopic origins of slurry behavior in legacy radioactive wastes.

PNNL Contact

Carolyn Pearce, Pacific Northwest National Laboratory, Carolyn.Pearce@pnnl.gov

Lawrence Anovitz, Oak Ridge National Laboratory, anovitzlm@ornl.gov

Funding

This research was supported by the IDREAM (Interfacial Dynamics in Radioactive Environments and Materials) Energy Frontier Research Center. The research was conducted at Oak Ridge National Laboratory, Pacific Northwest National Laboratory, and University of Notre Dame. (U)SANS measurements were performed at Oak Ridge National Laboratory’s Spallation Neutron Source.