Although aluminum species constitute a predominant proportion of tank wastes, the chemistry of aluminum in these alkaline solutions is inadequately understood. This is due, in part, to the extreme conditions of the HLW supernatant tank liquors. In REDOX waste, for example, the concentration of Al in HLW supernatants can exceed 1.5 M. In addition, the pH is often greater than 14, and the individual molar concentrations of Na+, NOi, and NOa- are in excess of 2 M. As a consequence of the high molar concentrations of A13+, Na+, NO; z-, and N03-, the activity of water in these supernatant solutions is greatly decreased. The concentration of water in these solutions, is in the range of 35-45 M, a significant reduction from the pure bulk water value of 55 M. On the molecular scale, there is not enough water present to satisfy the hydration requirements of the Na+ and AI(O) ions in the alkaline aluminum solutions. Thus, most of the water present in the HLW supernatants is chemically 'tied up' as hydrated water around the Na+ and Al(OH)d- ions, as well as structural water present in solid aluminate hydrates. The solution-solid phase behavior of the HLW supernatants can be thought of as a competition for water between ionic hydration and the structural water required for the aluminate hydrates. The role of water in these liquid waste concentrates emerges as a key determinant of which solid phases will form and ultimately control the viscosity and precipitation properties as well as the activities of the radionuclides of interest in HLWs.
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