27Al Pulsed Field Gradient, Diffusion–NMR Spectroscopy of Solvation Dynamics and Ion Pairing in Alkaline Aluminate Solutions

Pulsed field gradient nuclear magnetic resonance (PFG-NMR) measurements were successfully applied to the 27Al (I = 5/2) nucleus in concentrated electrolytes to investigate the diffusion of aluminate ions [Al­(OH)4 –] in simulant high-level nuclear waste (3 M NaOH) between 25 and 85 °C. The temperature-dependent diffusion coefficients obtained from 1H, 23Na, and 27Al PFG-NMR were well fit by a Vogel–Fulcher–Tammann model and a power law equation. Comparison of 27Al diffusion coefficients of 0.1 M Al­(OH)4 – in ∼3 M MOH (where M = Na+, K+, (CH3)4N+) at room temperature varied in agreement with the expected changes in solution viscosity via Stokes–Einstein relationship, confirming that the dominant Al species at these conditions are Al­(OH)4 – monomers. This 27Al PFG-NMR study extends an established methodology to a previously unexplored nucleus enabling this experimental technique to be leverage for exploring ion transport, speciation, and solution structure in concentrated electrolytes.