Experimental Investigations of the Reaction Pathway in the MgOH2OCO2 System in Concentrated Brines at Ambient Temperature and Ambient Laboratory Atmospheric CO2: Applications to the Waste Isolation Pilot Plant
Yongliang Xiong and Anna Snider
Sandia National Laboratories 1
Carlsbad Programs Group
4100 National Parks Highway, Carlsbad, NM 88220, USA
The hydration-carbonation or hydration-and-carbonation reaction path in the MgOCO2H2O system at ambient temperature and atmospheric CO2 is of both geological interest and practical significance. From the geological point of view, the knowledge of the reaction path in this system would lead us to a better understanding of the low temperature alteration and weathering of mafic and ultramafic rocks, and consequently a better understanding of the global budgets of Mg and CO2. From a practical point of view, the exact knowledge of the reaction path in this system is of great significance to the performance assessment (PA). In turn, PA is extremely important to the demonstration of the long-term safety of nuclear waste repositories, as assessed by the use of probabilistic performance calculations. This is because in the United States, MgO is the only engineered barrier certified by EPA for the Waste Isolation Pilot Plant (WIPP). In addition, the German Asse repository will employ an Mg(OH)2-based engineered barrier.
The portion of the reaction path involving carbonation of brucite (Mg(OH)2) is particularly complex, as Mg has a strong tendency to form a series of metastable hydrous carbonates. These metastable hydrous carbonates include hydromagnesite (Mg5(CO3)4(OH)24H2O, or Mg4(CO3)3(OH)23H2O), artinite (Mg2CO3(OH)23H2O), nesquehonite (MgCO33H2O), and lansfordite (MgCO35H2O). The free energy of formation (DfG) for these metastable hydrous carbonates differ from one another. The fugacity of CO2 gas (fCO2) of the repository will be buffered by the assemblage of brucite and any one of these metastable hydrous carbonates. The carbonation product will affect the fCO2, which would affect the performance assessment for the WIPP and similar geological repositories, for fCO2 has a significant effect on the actinide solubilities.
Therefore, this presentation focuses on the portion of the reaction path from brucite to the resulting magnesium carbonate. Our experimental results demonstrate that hydromagnesite (Mg5(CO3)4(OH)24H2O) forms during the carbonation of brucite in a series of solutions with different ionic strengths at ambient temperature and ambient laboratory atmospheric CO2.
ACKNOWLEDGEMENTS: This research is funded by WIPP programs administered by the Department of Energy.
1. Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energys National Nuclear Security Administration under Contract DE-AC04-94AL85000.