REVISED THERMODYNAMIC PROPERTIES OF BRUCITE DETERMINED BY SOLUBILITY STUDIES AND ITS SIGNIFICANCE TO NUCLEAR WASTE ISOLATION

MgO is the only engineered barrier certified by EPA for the Waste Isolation Pilot Plant (WIPP) in USA. The German Asse repository will also employ an Mg(OH)₂ (brucite)-based engineered barrier. The chemical function of the engineered barrier is to consume CO₂ that may be generated by the microbial degradation of organic materials in waste packages. Experimental results at SNL indicate that MgO is first hydrated as brucite, and then brucite is carbonated as hydromagnesite (5424) (Mg₅(CO₃)₄(OH)_2· 4H₂O). As MgO is in excess relative to CO₂ that may be produced, the brucite–hydromagnesite (5424) assemblage would buffer f_CO2 in the repository. Consequently, the thermodynamic properties of this assemblage is of great significance to the performance assessment (PA) as actinide solubility is strongly affected by f_CO2. In turn, PA is important to the demonstration of the long-term safety of nuclear waste repositories, as assessed by the use of probabilistic performance calculations.

There is a substantial discrepancy for D_fG^o_brucite in recent publications, ranging from -830.4 (Harvie et al., 1984; Geochim. Cosmochim. Acta, 723–751), through -831.9 (Brown et al., 1996; J. Chem. Soc., Dalton Trans., 3071–3075), through –833.5 (Robie and Hemingway, 1995; USGS Bull., 2131), and to –835.9 kJ mol^–1 (Konigsberger et al., 1999; Geochim. Cosmochim. Acta, 3105–3119). Using the D_fG^o_{hydromagnesite (5424)} from Konigsberger et al., the predicted log f_CO2 for this assemblage would range from –5.96 (D_fG^o_brucite from Harvie et al.) to –4.84 (D_fG^o_brucite from Konigsberger et al.). Therefore, it is desirable to better constrain the D_fG^o_brucite. For this reason, a series of solubility experiments involving brucite in NaCl solutions ranging from 0.01 M to 4.0 M have being conducted at SNL. The derived D_fG^o_brucite from this study by extrapolation to infinite dilution via Pitzer formalism is –830.8 kJ mol^–1, which is in excellent agreement with recommended values of Harvie et al. and Brown et al.

ACKNOWLEDGEMENTS: This research is funded by WIPP programs administered by U.S. Department of Energy.

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.