Gibbsite Solubility in Hanford Nuclear Waste Approached from above and below Saturation

Abstract

Liquid Hanford nuclear wastes are highly concentrated caustic aqueous electrolyte solutions. They contain elevated dissolved aluminum concentrations compared to simple NaOH(aq) solutions in equilibrium with gibbsite [γ-Al(OH)3]. The reason for this elevated solubility has been debated for years, with slow gibbsite precipitation kinetics or various unverified thermodynamic factors being historically offered as explanations. The present study determines whether there is a kinetic or thermodynamic explanation. Here, dissolved aluminum in real tank waste was equilibrated with excess gibbsite, approached from both above and below saturation at 40 °C. In both cases, the samples equilibrated to an aluminum concentration up to four times higher than in pure NaOH(aq) solutions of the same hydroxide concentration. However, in one case, when all of the gibbsite dissolved during heating, no measurable precipitation was observed from the supersaturation direction when gibbsite seed was unavailable for nucleation. These results indicate that there is a real (and, as of yet, unknown) thermodynamic effect that accounts for the elevated solubility of aluminum exhibited by the waste. There is also a kinetic effect superimposed under some conditions.