Characterizing and controlling trivalent aluminum (Al3+) metal ion speciation in highly concentrated NaOH solutions is a challenge in the treatment of the nation’s high-level legacy radioactive waste streams, and in industrial hydro-metallurgical processes, such as the Bayer process for Al extraction. Due to chemical decladding of Al-clad defense fuels, and addition of Al(NO3)3 as a salting agent, Al3+ hydroxides and oxyhydroxides, such as Al(OH)3, are the primary solids in radioactive wastes stored in large tanks at the Hanford Site, WA. These solids are in contact with supersaturated solutions of dissolved Al3+ in NaOH, added to increase pH for storage. The speciation of Al3+ in these solutions, and how it influences dissolution and precipitation, is not well understood, making it a challenge to predict Al solubility as a function of concentration, temperature and the presence of other anions and counter-cations. A technical basis for this prediction is required so that the tank waste can be diluted and stored for subsequent processing into a vitrified wasteform, without risk of Al precipitation. Research by the IDREAM EFRC has focused on unravelling Al3+ ion coordination, solvation, pairing, clustering, and extended networks of connectivity in these complex chemical environments. The goal of this research is to understand changes in speciation by developing a definitive guide to multi-metal centered Al species signatures from multiple spectroscopic techniques. A combined experimental and theoretical interrogation will be presented to predict dynamic solute and solvent behavior in these concentrated systems where ions must share water molecules with each other. Specific examples include: (i) demonstrating the significance of medium-range (4−6 Å) solvent-separated Al(OH)4−…Na+ pairs under conditions incipient to and concurrent with Al(OH)3 crystallization; and (ii) the existence of multiple dimeric species that are thermodynamically close in energy, but whose relative stability can be reversed by ion pairing in saturated salt solutions.

Relating structure to spectroscopic signatures: Experimental analysis and theoretical interpretation of different aluminum coordination environments

2019 Spring National Meeting

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