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Thermodynamic Properties of Brucite Determined by Solubility Studies and Their Significance to Nuclear Waste Isolation

Authors:	Yongliang Xiong

Institution:	(1) Sandia National Laboratories (SNL), Carlsbad Programs Group, 4100 National Parks Highway, Carlsbad, NM 88220, USA

Abstract:	Solubility experiments were conducted for the dissolution reaction of brucite, Mg(OH)₂ (cr): ${\text{Mg}}\left( {{\text{OH}}} \right)_{ 2} \left( {{\text{cr}}} \right) + 2 {\text{H}}^{ + } = {\text{Mg}}^{{ 2+ }} + 2 {\text{H}}_{ 2} {\text{O.}}$ Experiments were conducted from undersaturation in deionized (DI) water and 0.010–4.4 m NaCl solutions at 22.5°C. In addition, brucite solubility was measured from supersaturation in an experiment in which brucite was precipitated via dropwise addition of 0.10 m NaOH into a 0.10 m MgCl₂ solution also at 22.5°C. The attainment of the reversal in equilibrium was demonstrated in this study. The solubility constant at 22.5°C at infinite dilution calculated from the experimental results from the direction of supersaturation by using the specific interaction theory (SIT) is: $\log K_{s}^\circ = 1 7. 2\pm 0. 2 {\text{ }}\left( { 2\sigma } \right)$ with a corresponding value of 17.0 ± 0.2 (2σ) when extrapolated to 25°C. The dimensionless standard chemical potential (μ°/RT) of brucite derived from the solubility data in 0.010 m to 4.4 m NaCl solutions from undersaturation extrapolated to 25°C is −335.76 ± 0.45 (2σ), with the corresponding Gibbs free energy of formation of brucite, $\Updelta _{f} G_{298.15,{\text{brucite}}}^{\circ}$ , being −832.3 ± 1.1 (2σ) kJ mol⁻¹. In combination with the auxiliary thermodynamic data, the $\log K_{s}^{\circ}$ is calculated to be 17.1 ± 0.2 (2σ), based on the above Gibbs free energy of formation for brucite. This study recommends an average value of 17.05 ± 0.2 in logarithmic unit as solubility constant of brucite at 25°C, according to the values from both supersaturation and undersaturation. Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.

Keywords:	Solubility studies Specific interaction theory (SIT) Pitzer equations Solubility constants Hydromagnesite Nuclear waste isolation
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