Effects of Ionic Strength, Salt, and pH on Aggregation of Boehmite Nanocrystals: Tumbler Small-Angle Neutron and X-ray Scattering and Imaging Analysis

Abstract

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 aluminum hydroxides and oxyhydroxides (gibbsite and boehmite), although many other materials are present. These form complex aggregates that dramatically affect the rheology of the solutions and, therefore, efforts to recover and treat these wastes. In this paper, we have used a combination of transmission and cryo-transmission electron microscopy, dynamic light scattering, and X-ray and neutron small and ultrasmall-angle scattering to study the aggregation of synthetic nanoboehmite particles at pH 9 (approximately the point of zero charge) and 12, and sodium nitrate and calcium nitrate concentrations up to 1 m. Although the initial particles form individual rhombohedral platelets, once placed in solution they quickly form well-bonded stacks, primary aggregates, up to ∼1500 Å long. These are more prevalent at pH = 12. Addition of calcium nitrate or sodium nitrate has a similar effect as lowering pH, but approximately 100 times less calcium than sodium is needed to observe this effect. These aggregates have fractal dimension between 2.5 and 2.6 that are relatively unaffected by salt concentration for calcium nitrate at high pH. Larger aggregates (>∼4000 Å) are also formed, but their size distributions are discrete rather than continuous. The fractal dimensions of these aggregates are strongly pH-dependent, but only become dependent on solute at high concentrations.

Applications of Kinetic Methods in Thermal Analysis: A Review

“…[72,73] Moreover, gibbsite is one of the major components in nuclear wastes stored at the Hanford site (WA, USA) and at the Savannah River site (SC, USA). [74][75][76][77][78][79] Understanding the solid-state phase transformation of the gibbsite could help to improve the processes related to both nuclear waste treatment and alumina materials synthesis. As shown in Fig 3A, there are three steps for the solid phase transformation of gibbsite to alumina based on the DTA and TG/DTG curves.…”

Influences and Mechanisms of Nano-C-S-H Gel Addition on Fresh Properties of the Cement-Based Materials with Sucrose as Retarder

“…Even the monodispersed nanoparticles in colloidal silica sol (CSS) coagulate immediately to form loose flocs due to the high ionic strength in cement pastes when cement is mixed with water containing CSS [51,59]. While incorporating colloidal boehmite, the nanoparticles may also aggregate or form clusters under the high ionic strength in fresh cement pastes [60,61]. Therefore, it should be the behavior of the final agglomerates, aggregates, flocs or clusters, rather than those individual nanoparticles, that controls the effects of them on properties of the fresh and hardened cement-based materials [51].…”

Mass fractal dimension from 2D microscopy images via an aggregation model with variable compactness

“…In the context of -alumina catalyst carriers synthesis, the porosity of the final solid depends on the size and shape of the aggregates (meso-porosity ≃ 10 nm) and agglomerates (macro-porosity ≃ 100 nm) resulting from colloidal inter-particle interactions during different stages of preparation. By varying the chemical parameters of the colloidal suspension, it is possible to change the aggregation and agglomeration processes, 14 leading to different size distribution and fractal dimension. Our interest is to use microscopy-image analysis to study this behaviour, in order to determine the physical-chemical parameters that lead to specific structure change within boehmite suspensions.…”