The speciation and precipitation of aluminum phases in highly caustic solutions, such as those in radioactive wastes stored at the Hanford and Savannah River sites, is often based on coupled changes in both Al coordination and local structure, with intermediate states too short lived to resolve. As an alternative approach to probing nucleation and dissolution in solution, we have been exploring melting-recrystallization phase transitions of sodium aluminate salt hydrates, which enables us to follow transitions in aluminum coordination between highly ordered initial and final product states. Using monosodium aluminate hydrate (MSA; Na2Al2O3OH*1.5H2O) and nonasodium aluminate hydrate (NSA, Na9[Al(OH)6]2*3(OH)*6H2O), we have been able to follow changes in aluminum coordination using time-resolved XRD, Raman, and nuclear magnetic resonance (NMR) spectroscopies. For the first time, NMR reveals a resonance consistent with the mu-oxo aluminate dimer, Al2O(OH)62-, that is occluded in or sorbed on NSA crystals in a highly alkaline (25 m) NaOH solution at 40 °C. Partnering these experimental results with reactive molecular dynamics simulations and enhanced sampling has revealed intermediate aluminate structures during thermally induced aluminate dihydroxylation, as well as the subsequent coordination change during dissolution. Work is underway to detect the hypothesized temporally and spatially disordered structures of Al in solution by following the crystallization of sodium aluminates from amorphous Al intermediates using in situ NMR and cluster calculations. These novel proxy studies using salt hydrate crystals may ultimately provide the molecular-scale insights needed to understand the stability and solubility of aluminum in highly caustic solutions.

Understanding the local structure and coordination changes of aluminate in caustic solutions using phase transitions in crystalline salt hydrates

ACS Spring 2022

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