Frustrated Coulombic and Cation Size Effects on Nanoscale Boehmite Aggregation: A Tumbler Small- and Ultra-Small-Angle Neutron Scattering Study

Abstract

To better understand the effects of solution chemistry on particle aggregation in the complex legacy tank wastes at the Hanford (WA) and Savannah River (SC) sites, we have performed a series of tumbler small- and ultra-small-angle neutron scattering experiments on 20 wt % solid slurries of nanoparticulate aluminum oxyhydroxide (boehmite) with M1+ nitrates of various concentrations and radii. The solutes consisted of H, Li, Na, K, and Rb nitrates at 10–5, 10–3, 10–1, 2, and 4 molal (m) concentrations, as well as in pure H2O. Synthetic boehmite nanoparticles were used with a size range from ∼20 to 30 nm. Tumbler cells were used to keep the solids from settling. Although particles initially 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 whose concentration and structure vary as a function of cation type and concentration. Aggregation generally increases with increased solute concentration and with cation radius up to a concentration somewhat above 10–1 m, at which point the trend reverses. Primary aggregates become more rodlike and larger. The Kirkwood-like reversal probably reflects a change from Derjaguin–Landau–Verwey–Overbeek (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 nanoparticle aggregate structures provides useful physical insights into the microscopic origin of slurry rheology in the Hanford and Savannah River legacy wastes.

Atomistic Mismatch Defines Energy–Structure Relationships during Oriented Attachment of Nanoparticles

“…The conclusions in this work are applicable to other mineral–water systems where hydrogen bonding occurs between the mineral surfaces and water (i.e., boehmite, kaolinite (gibbsite layers)), and the minerals are characterized by large basal surfaces and narrow edges. The HB network between water and the surfaces maintains the solvent-separated configuration, enabling rotating and sliding motions.…”

Effects of γ-Irradiation, Cation Size, and Salt Concentration on the Aggregation of Boehmite Nanoplatelets

“…(U)­SANS experiments were performed using boehmite synthesized in accordance with the hydrothermal approach of Zhang et al − Transmission electron microscopy (TEM) analysis shows that the particles are 19.4 (±7.6) nm across the face and with a height of 5.3 (±1.4) nm . Measurements of pH were performed by adding 200 mg of boehmite irradiated to 2 MGy to 1 mL of a solution and sonicating the mixture to disperse the particles.…”

“…In addition, there are also significant amounts of solids present, including boehmite (γ-AlOOH) and gibbsite (α- or γ-Al­(OH) 3 ). , The presence of these solid aluminum hydroxide and oxyhydroxide particles adds to the challenge inherent in the transport and treatment of the waste because aggregation of the particles dramatically affects their rheology. − In addition to the effects of concentrated salts, the aluminum particles in the tanks have been subjected to high radiation fields for long periods of time. Thus, it is reasonable to question whether aggregation and rheological data obtained for unirradiated materials apply to the tank waste solids . A fundamental understanding of how radiation affects the particles and subsequent particle–particle interactions in highly concentrated salts is, therefore, critically needed to facilitate waste cleanup.…”

“…Nominally, these interactions should be largely cation independent. However, recent work using small-angle and ultra-small-angle neutron scattering [(U)­SANS] to measure aggregation of nonirradiated (pristine) boehmite over a wide concentration range of various nitrate salts showed that both the salt concentration and cation effects were significant. For example, LiNO 3 aggregation did not change very much for concentrations of up to about 10 –3 molal ( m ), but on further increasing it to 0.1 m , there was a sharp increase in particle aggregation due to the increased screening effects of the electric double layer.…”

“…In parallel to the previous work, (U)­SANS techniques were used to determine aggregate distributions as a function of the ionic concentration from 10 –5 to 4 m and as a function of the cation size for hydrogen, lithium, sodium, potassium, and rubidium nitrate solutions. Comparison of the results with analogous slurries of pristine boehmite allows for the elucidation of the effects of changes in the surface charge on the electric double layer and to better evaluate the predictions of current models.…”