PNNL

Abstract

Despite its widespread importance, predicting metal solubilities in highly concentrated multicomponent aqueous solutions is often difficult due to the poorly understood balance of ion-ion and ion-solvent interactions. Aluminum solubility in concentrated sodium hydroxide solutions is one such case, with major consequences in ore refining as well as processing of legacy radioactive waste stored at U.S. Department of Energy sites such Hanford. In particular, the solubility of gibbsite (a-Al(OH)3) is often not well predicted because other ions interact with and affect the activity coefficients of the OH- and Al(OH)4- in a manner that has not yet been carefully scrutinized. In the present study we systematically examine the influence of key anions, NO2- and NO3- on the solubility of a-Al(OH)3 in NaOH solutions across a range of temperatures, taking care to establish equilibrium values by approaching from both under- and oversaturation. Rapid equilibration was enabled by use of a nano-sized, highly pure, and crystalline synthetic a-Al(OH)3 phase of well-defined particle size and shape. Measured dissolved aluminum concentrations were compared with those predicted by the Misra equation, and specific anion effects were expressed as an enhancement factor, Alenhc, conveying the multiplicity of excess soluble aluminum. For NaNO2 and NaNO3-containing systems, respective Alenhc values for NaNO2 or NaNO3 containing systems were 2.25 and 1.56 at 27°C, 2.44 and 1.69 at 45°C, and 3.28 and 2.17 at 80°C, consistently indicating significant enhancements and a larger effect from NO2-, likely due to stronger nature of anion-anion interactions with the aluminate anion, Al(OH)4-. Terminal solids characterization suggested a dynamic evolution of the a-Al(OH)3, including particle coarsening and agglomeration, revealing the likely importance of dissolution/reprecipitation in establishing equilibrium.