The US government currently spends significant resources managing the legacies of the Cold War, including 300 million liters of highly radioactive wastes stored in hundreds of tanks at the Hanford (WA) and Savannah River (SC) sites. The materials in these tanks consist of highly radioactive slurries and sludges at very high pH and salt concentrations. The solid particles primarily consist of the aluminum hydroxides and oxyhydroxides gibbsite and boehmite. These form complex aggregates that dramatically affect the rheology of the waste slurry and, therefore, efforts to recover and treat these wastes. In order to better understand the effects of solution chemistry on particle aggregation in the tank wastes we have performed a series of tumbler (U)SANS experiments on 20 weight percent solid solutions with varying cation concentrations and radius. The solutes consisted of H, Li, Na, K and Rb nitrates at 10-5, 10-3, 10-1, 2 and 4 molal concentrations, as well as in pure H2O. Synthetic boehmite solids were used with a size range from ~20 to 30 nm. Tumbler cells were used to keep the solids from settling. Although initial particles form individual rhombohedral platelets, once placed in solution they quickly form well-bonded stacks, primary aggregates, up to ∼1500 Å long, and a second level of aggregates is observed whose concentration and structure varies as a function of cation type and concentration. Aggregation generally increases with increased solute concentration and with cation radius up to a concentration somewhat above10-1m, at which point the trend reverses. Primary aggregates become more rod-like and larger. The Kirkwood-like reversal probably reflects a change from DLVO/Debye behavior controlled by surface chemistry to a frustrated coulombic system controlled by the solution structure. These data suggest that an understanding of the effects of salt concentration and chemistry on aggregate structures provides useful physical insights into the microscopic origin of slurry rheology in the Hanford wastes.

Frustrated coulombic and cation size effects on boehmite aggregation: a tumbler small- and ultra-small angle neutron scattering study

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.