3, Washington State University, Pullman, Washington, United States
2, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Aluminum oxyhydroxide (boehmite, AlOOH) and aluminum hydroxide (gibbsite, Al(OH)3) are prominent components in high-level nuclear waste stored in large quantities at the Hanford Site, Washington, U.S.A., and at the Savannah River Site, South Carolina, U.S.A, with future processing plans dependent on developing a predictive understanding of the growth and dissolution behavior of these two materials in highly alkaline solution. However, mechanisms of crystal growth, dissolution, 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, the dissolution of gibbsite/boehmite, 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 reliable techniques to manage the aluminum based minerals in nuclear waste.