PNNL

Abstract

Although it is well known that aqueous electrolyte solutions behave non-ideally, few are so non-ideal that adding an electrolyte to a saturated solution with a common ion enhances rather than depresses solubility. Nonetheless, this apparent reversal of the Law of Mass Action (LMA) has been observed in simple nitrate solutions at high concentrations. A leading hypothesis is that ion clusters are formed and stabilized by ions having different charge densities. The present study examines this concept, in part by reviewing relevant data from multicomponent aqueous solutions containing sodium nitrate (NaNO3), sodium nitrite (NaNO2), sodium hydroxide (NaOH), and sodium aluminate (NaAl(OH)4) - the major constituents in alkaline nuclear waste. Here, NaOH and NaAl(OH)4 did not enhance the solubility of NaNO3 or NaNO2, whereas NaNO2 and NaNO3 enhanced rather than depressed the solubility of each other despite each having the sodium cation (Na+) in common. Solutions evaluated in this study have more than 20 molal total Na+ concentration, and most have less than one mole of water per mole of ion. Thus, this reversal of the LMA occurs in solutions where there is not enough water to fully hydrate the ions, pointing to the importance of ion cluster formation. Within the composition range of the solutions analyzed here, this reversal of the LMA occurs regardless of NaAl(OH)4 and NaOH concentrations. Elevated temperatures also result in the reversal of the LMA in the subsystems NaNO2-NaNO3-H2O and NaOH-NaNO3-H2O, consistent with spectroscopic and computational studies showing enhanced interactions at higher temperatures. Although speciation in these highly concentrated electrolyte solutions is not well understood, the stabilization of ion clusters by: (i) optimizing charge density around the ions through different combinations of mixed cations and anions in solution; and (ii) high temperatures where ions experience prolonged contact, offers important clues for future research into how these species control solubility.