A Pitzer model for darapskite solubility in NaOH solutions at 25°C
Résumé
The objective of this work is to define a set of Pitzer [1] interaction parameters for the Na-SO4-NO3-OH-H2O quaternary system, at 25 °C and different pH conditions, up to the solubility of any salt of the chemical system, including mirabilite (Na2SO4:10H2O), thenardite (Na2SO4), nitratine (NaNO3) and darapskite (Na2SO4:NaNO3:H2O). The prediction of this double-salt solubility is of particular interest within the framework of the nuclear waste storage [2]. To our knowledge, only one author proposes experimental data of darapskite solubility in alkaline solutions up to 3 m hydroxide at 25°C [2]. Practically, describing the measured solubility of this mineral is not immediate and requires a step-by-step approach. First, the main work consists of describing accurately the Na2SO4-H2O, NaNO3-H2O and NaOH-H2O binary systems. The interaction parameters for these three systems are taken from literature. They were established in previous studies [3,4] from measurement data of heat capacity and osmotic coefficient. Due to the high solubility of NaOH (about 28 mol·kg-1 at 25°C) the partial dissociation of NaOH is considered. For the NaNO3-H2O binary system, several studies propose a set of Pitzer interaction parameters. In this study, we chose the interaction parameters published by Marion [5], because the brine properties such as the osmotic coefficient are correctly represented with only two interaction parameters. Although the interaction parameters are determined up to the salts solubility, i.e. 1.98 mol·kg-1 for the Na2SO4-H2O binary system and 10.8 mol·kg-1 for the NaNO3-H2O binary system, they can be used beyond this concentration range. Indeed, they are able to correctly represent the osmotic coefficient in supersaturated solutions, up to 6 mol·kg-1 for the Na2SO4-H2O system and to 15 mol·kg-1 for the NaNO3-H2O system, showing the robustness of the models. Then, the fitting work consists of optimizing the ternary interaction parameters on the solubility data of minerals. Finally, the aqueous properties and chemical equilibrium model including dissolution/precipitation mechanisms of the Na-SO4-NO3-OH-H2O quaternary system can be described without supplementary parameters.
References
[1]K.S. Pitzer, Activity coefficients in electrolyte solutions, 2nd ed., CRC Press, Boca Raton, 1991.
[2]R.K. Toghiani, V.A. Phillips, L.T. Smith, J.S. Lindner, Solubility in the Na + SO4 + NO3 and Na + SO4 + NO2 Systems in Water and in Sodium Hydroxide Solutions, J. Chem. Eng. Data. 53 (2008) 798–804.
[3]A. Lach, F. Boulahya, L. André, A. Lassin, M. Azaroual, J.-P. Serin, et al., Thermal and volumetric properties of complex aqueous electrolyte solutions using the Pitzer formalism - The PhreeSCALE code, Comput. Geosci. 92 (2016) 58–69
[4]A. Lach, Modélisation thermodynamique des propriétés d’excѐs des saumures naturelles et industrielles, Pau, 2015. http://www.theses.fr/2015PAUU3021
[5]G.M. Marion, A molal-based model for strong acid chemistry at low temperatures (<200 to 298 K), Geochim. Cosmochim. Acta. 66 (2002) 2499–2516