Nitrate and Nitrite Incompatibility with Hydroxide Ions in Concentrated NaOH Solutions: Implications for Hydroxide and Gibbsite Reactivity in Alkaline Nuclear Waste

Abstract

Electrolyte solutions in alkaline nuclear waste contain aluminate, hydroxide, nitrate and nitrite with sodium as the predominant counterion. The salts of these ions are highly soluble, so the liquids are highly concentrated. This study found that there is a substantial incompatibility between the hydroxide and nitrate and/or nitrate ions. This was determined by the observations that adding just one molal of NaNO2 or NaNO3 to saturated NaOH solution precipitation of 12 moles of NaOH·H2O salt, whereas the common ion effect would have expected only about 1 mole to precipitate. Further analysis indicates that the presence of nitrate and nitrite drastically increases the reactivity of sodium hydroxide ions in solution, which likely influences the reactivity of other hydroxide-mediated reactions. This enhanced reactivity is likely because it disrupts large Na+-OH- ion networks because nitrate and nitrite do not fit in those ion networks similarly to how some ions cannot substitute into a foreign crystal lattice. In contrast, the aluminate ion did not have the same large incompatibility with hydroxide.