Gibbsite [Al(OH)3] dissolves in high pH aqueous solutions via the reaction: The increase in gibbsite solubility with increase in hydroxide concentration is relatively modest below 8 molal NaOH. Above 9 molal, however, the solubility of gibbsite drastically increases with increasing NaOH addition. This drastic increase in the solubility occurs where there is insufficient water molecules to provide sodium, hydroxide and aluminate their full hydration sphere without sharing water molecules. An 8 molal NaOH solution has only about 3.5 water molecules per dissolved ion. There is a transition in the coordination number for the hydroxide ion from 4 to 3 hydrogen bonded water molecules as the NaOH concentration goes above 8 molal. There is also a noted shift in the Al27 NMR spectra and aluminate Raman bands in these solutions caused by ion pairing. The present study ask the question “if these shifts in gibbsite solubility and aluminate ion-pairing occur because there is a water deficits in highly concentrated NaOH solutions, can the same shift in gibbsite solubility and aluminate ion-pairing at lower hydroxide concentrations by simply replacing water with some other highly soluble salt like NaNO2 or NaNO3?”. The NaNO2 and NaNO3 were chosen because alkaline nuclear waste at the Hanford Site has high concentration of these ions and has elevated aluminum solubility that has not been explained in the past. The present study equilibrated gibbsite with modest quantities of NaOH dissolved in highly concentrated solutions of NaNO2 or NaNO3 concentrations high enough that the water content was below 4 moles per dissolved ion. The solubility of gibbsite was indeed found to be considerably higher than in dilute solutions with the same hydroxide molality. Raman and Al27 NMR spectra showed that sodium and aluminate formed contact ion-pairs. Thus, these results confirm that the high solubility of gibbsite observed at low water contents is caused by change in speciation that occurs because there is insufficient water molecules to hydrate all of the ions. This change in speciation occurs whether the low water content is caused by high NaOH concentrations or by replacing water with other electrolytes like NaNO2 or NaNO3.

Gibbsite solubility in alkaline electrolyte solutions with insufficient water to completely hydrate all ions

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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