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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.