Effects of Anions on Phase Transformation and Crystallization of Aluminum Hydroxide

Gibbsite (α-Al(OH)₃) is naturally abundant and plays a vital role as a raw material in various industrial applications. Furthermore, it constitutes a significant portion of the solid components in caustic nuclear waste at the Hanford Site (WA, USA). While extensive research has been conducted to understand gibbsite's behavior in processes such as crystallization, dissolution, phase transformation, solution chemistry, and irradiation-induced changes, the effects of co-existing high concentrations of salts on gibbsite's nucleation and growth remain a less-explored area. In this study, we investigated gibbsite crystallization from the solutions containing six sodium-based salts, specifically sodium salts of fluoride, chlorine, bromide, nitrate, phosphate, and sulfate. Our findings illuminate the diverse ways in which different anions can influence the crystal phase, structure, morphology, and growth rate of gibbsite as determined with various advanced techniques, including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman, nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and scanning/transmission electron microscopy (S/TEM). These results were further substantiated through the calculation of molecular cluster energies using density functional theory. In summary, our exploration of gibbsite crystallization in complex, multicomponent electrolytes simulates the compositional complexity encountered in nuclear waste, shedding light on the intricate interplay between gibbsite and co-existing salts.