RESUMO
Nitrite (NO2-) is a prevalent nitrogen oxyanion in environmental and industrial processes, but its behavior in solution, including ion pair formation, is complex. This solution phase complexity impacts industries such as nuclear waste treatment, where NO2- significantly affects the solubility of other constituents present in sodium hydroxide (NaOH)-rich nuclear waste. This work provides molecular scale information into sodium nitrite (NaNO2) and NaOH ion-pairing processes to provide a physical basis for later development of thermodynamic models. Solubility isotherms of NaNO2 in aqueous mixtures with NaOH and total alkalinity were also measured. Spectroscopic characterization of these solutions utilized high-field nuclear magnetic resonance spectroscopy (NMR) and Raman spectroscopy, with additional solution structure detailed by X-ray total scattering pairwise distribution function analysis (X-ray PDF). Despite the NO2- deformation Raman band's insensitivity to added NaOH in saturated NaNO2 solutions, 23Na and 15N NMR studies indicated the Na+ and NO2- chemical environments change likely due to ion pairing. The ion pairing correlates with a decrease in diffusion coefficient of solution species as measured by pulsed field gradient 23Na and 1H NMR. Two-dimensional correlation analyses of the 2800-4000 cm-1 Raman region and X-ray PDF indicated that saturated NaNO2 and NaOH mixtures disrupt the hydrogen network of water into a new structure where the length of the OO correlations is contracted relative to the typical H2O structure. Beyond describing the solubility of NaNO2 in a multicomponent electrolyte mixture, these results also indicate that nitrite exhibits greater ion pairing in mixtures of concentrated NaNO2 and NaOH than in comparable solutions with only NaNO2.
RESUMO
Despite widespread industrial importance, predicting metal solubilities in highly concentrated, multicomponent aqueous solutions is difficult due to poorly understood ion-ion and ion-solvent interactions. Aluminum hydroxide solid phase solubility in concentrated sodium hydroxide (NaOH) solutions is one such case, with major implications for ore refining, as well as processing of radioactive waste stored at U.S. Department of Energy legacy sites, such as the Hanford Site, Washington State. The solubility of gibbsite (α-Al(OH)3) is often not well predicted because other ions affect the activity of hydroxide (OH-) and aluminate (Al(OH)4-) anions. In the present study, we systematically examined the influence of key anions, nitrite (NO2-) and nitrate (NO3-), as sodium salts on the solubility of α-Al(OH)3 in NaOH solutions taking care to establish equilibrium from both under- and oversaturation. Rapid equilibration was enabled by use of a highly pure and crystalline synthetic nano-gibbsite of well-defined particle size and shape. Measured dissolved aluminum concentrations were compared with those predicted by an α-Al(OH)3 solubility model derived for simple Al(OH)4-/OH- systems. Specific anion effects were expressed as an enhancement factor (Alenhc) conveying the excess of dissolved aluminum. At 45 °C, NaNO2 and NaNO3-containing systems exhibited Alenhc values of 2.70 and 1.88, respectively, indicating significant enhancement. The solutions were examined by Raman and high-field 27Al NMR spectroscopy, indicating specific interactions including Al(OH)4--Na+ contact ion pairing and Al(OH)4--NO2-/NO3- ion-ion interactions. Dynamic evolution of the α-Al(OH)3 particles including growth and agglomeration was observed revealing the importance of dissolution/reprecipitation in establishing equilibrium. These studies indicate that incomplete ion hydration, as a result of the low water activity in these concentrated electrolytes, results in: (i) enhanced reactivity of the hydroxide ion with respect to α-Al(OH)3; (ii) increased concentrations of Al(OH)4- in solution; and (iii) stronger ion-ion interactions that act to stabilize the supersaturated solutions. This information on the mechanisms by which α-Al(OH)3 becomes supersaturated is essential for more energy-efficient aluminum processing technologies, including the treatment of millions of gallons of Al(OH)4--rich high-level radioactive waste.
