Introduction
Aqueous electrolyte solutions are one of the most common chemical systems in both nature and industry. Despite extensive investigation for more than 100 years, salt solubility in multicomponent aqueous electrolyte solutions are still not well described theoretically except in the most dilute solutions.1 The cause of this difficulty is in part because of what are called “specific interactions”,2 which are interactions between different constituents in the solution. All ions create an electric field in solution because they are charged. Specific interactions are all of those thermodynamic impacts of a specific ion that are above and beyond the impact of the electric field.
Specific interactions are captured experimentally in popular models of electrolyte solution thermodynamics through the activity coefficients of the ions.3−5 Unfortunately, the current models must be parameterized with the experimental data for concentrated solutions, which can require substantial experimentation to capture all of the specific interactions between all ions in systems with many components.1 While it is certainly possible to determine a large number of interaction parameters experimentally, it is cost prohibitive in many cases. Voigt emphasized this point with seawater.6,7 Seawater has been studied for more than 100 years, yet there is still insufficient data to develop all of the important interaction parameters over the whole range of salinities, temperatures, and pressures important to chemical oceanographers.
Xem thêm : CO2 + NaOH → NaHCO3
Alkaline nuclear waste in underground storage tanks at the Hanford and Savannah River Sites in the United States of America is another example of a many component aqueous system. The aqueous phase has high concentrations of nitrate, nitrite, aluminate, hydroxide, carbonate, phosphate, formate, acetate, chloride, fluoride, and oxalate anions counterbalanced with sodium and lesser amounts of potassium. Sodium, nitrate, nitrite, hydroxide, and aluminate are the most prevalent ions.8 Thus, the other ions in the waste possess a liquid phase with high concentrations of these electrolytes. There are hundreds of less prevalent species in the waste. The liquid phase is commonly saturated with four or more salts simultaneously, and the waste solution temperatures range from approximately 15-40 °C.9 There has been some work performed to quantify the specific interactions between ions in Hanford waste from the available data (for instance10−13), but the data is simply unavailable for most ions over the concentration and temperature range relevant to Hanford waste processing.
Given the complexity of the Hanford waste, the question is asked: are there some trends that can be exploited to explain Hanford waste behavior? One of the most ubiquitously used trends in electrolyte chemistry is the Hofmeister series. In the 1880s, Hofmeister and students (translated by ref (14)) determined that the solubility of proteins in aqueous solution depends on the identity of other dissolved ions, and that the order of that impact for the background ions was the same regardless of what protein was investigated. They went on to show that water activity and colloid stability in aqueous solution follow this same series.14
The Hofmeister series has been used extensively in the fields of colloid science and biophysics over the last 130 years to explain many different phenomena in aqueous electrolyte solutions.15,16 There was also much early work on the solubility of nonelectrolytes, showing that their solubilities in aqueous solutions follow the Hofmeister series.17,18 These early studies showed that the solubility of gases and organic nonelectrolytes depended on the concentration and identity of background electrolytes; but at a given molality of electrolytes, the relative solubility of each nonelectrolyte in each electrolyte solution followed the trend NaNO3 > NaCl > NaOH.18 Those studies did not include NaNO2, but a recent study determined that the solubility of nonelectrolyte chlorouridine 1 followed the solubility trend NaNO3 > NaNO2 > NaCl > NaOH.19
The Hofmeister series has been studied more thoroughly for nonelectrolyte solubilities than salts, even though Brønsted noted nearly a hundred years ago that the salting-out effect is much larger for salts than most nonelectrolytes.20 Extending the Hofmeister series to salt solubilities may provide qualitative trends that can be used to evaluate solubilities in systems where empirical data is unavailable. This study shows that the solubilities of key salts in the Hanford radioactive waste follow the Hofmeister series, at least for the electrolytes NaNO3, NaNO2, NaCl, and NaOH. This trend can be used to constrain the magnitude of specific interactions between other ions in the waste and these four ions.
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