(425e) Crystal Growth and Transformation of Gibbsite and Boehmite

Boehmite (γ-AlOOH) and gibbsite (α-Al(OH)₃) are important archetype (oxy)hydroxides of aluminum in nature that also play diverse roles across a plethora of industrial applications. As a consequence, highly precise synthesis of these pure phases with controlled particle size, shape and properties towards an end of tunable chemical behavior is an important ongoing research and development enterprise. In addition, they are prominent components in high-level nuclear waste stored in large quantities at the Hanford Site, Washington and at the Savannah River Site, South Carolina with future processing plans dependent on developing a predictive understanding of the growth and transformation of these two materials in highly alkaline solution. However, mechanisms of crystal growth and transformation of these minerals still remain poorly understood, particularly in the complex environment of concentrated sodium hydroxide at low water activity. In this work, magic angle spinning nuclear magnetic resonance (MAS-NMR), high resolution atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), high resolution powder X-ray diffraction (XRD), and X-ray Pair Distribution Function (PDF) techniques were conducted to investigate the crystallization of gibbsite/boehmite from amorphous aluminum hydroxide gel precursors and the transformation of gibbsite to boehmite. By focusing on understanding the role of aluminum coordination change dynamics from tetrahedral in solution to octahedral in solids and vice versa, and the intermediate pentacoordinate state, some unifying principles governing these transformation emerge, which are of importance for developing new methods to morphology and size controlled synthesis of boehmite and gibbsite and also are critical to proposal reliable techniques to manage the aluminum based minerals in nuclear waste.