Experimental investigations of the reaction path in the MgO-CO2-H2O system in solutions with various ionic strengths, and their applications to nuclear waste isolation

摘要

The reaction path in the MgO-CO2-H2O system at ambient temperatures and atmospheric CO2 partial pressure(s), especially in high-ionic-strength brines, is of both geological interest and practical significance. Its practical importance lies mainly in the field of nuclear waste isolation. In the USA, industrial-grade MgO, consisting mainly of the mineral periclase, is the only engineered barrier certified by the Environmental Protection Agency (EPA) for emplacement in the Waste Isolation Pilot Plant (WIPP) for defense-related transuranic waste. The German Asse repository will employ a Mg(OH)(2)-based engineered barrier consisting mainly of the mineral brucite. Therefore, the reaction of periclase or brucite with carbonated brines with high-ionic-strength is an important process likely to occur in nuclear waste repositories in salt formations where bulk MgO or Mg(OH)(2) will be employed as an engineered barrier. The reaction path in the system MgO-CO2-H2O in solutions with a wide range of ionic strengths was investigated experimentally in this study. The experimental results at ambient laboratory temperature and ambient laboratory atmospheric CO2 partial pressure demonstrate that hydromagnesite (5424) (Mg-5(CO3)(4)(OH)(2) center dot 4H(2)O) forms during the carbonation of brucite in a series of solutions with different ionic strengths. In Na-Mg-Cl-dominated brines such as Generic Weep Brine (GWB), a synthetic WIPP Salado Formation brine, Mg chloride hydroxide hydrate (Mg-3(OH)(5)Cl center dot 4H(2)O) also forms in addition to hydromagnesite (5424). The observation of nesquehonite (MgCO3 center dot H2O) and subsequent appearance of hydromagnesite (5424) in the experiments in a Na-Cl-dominated brine (ERDA-6) at room temperature and P-CO2 = 5 x 10(-2) atm allows estimation of the equilibrium constant (log K) for the following reaction: Mg-5(CO3)(4)(OH)(2) center dot 4H(2)O + CO2(g) + 10H(2)O = 5MgCO(3) center dot 3H(2)O as similar to 2.5 at 25 degrees C. The logK for the above reaction at 5 degrees C is calculated to be similar to 4.0 by using the Van't Hoff equation. By using these equilibrium constants, the co-existence of hydromagnesite (5424) with nesquehonite in various, natural occurrences such as in weathering products of the meteorites from the Antarctic and serpentine-rich mine tailings, can be well explained. Since the stoichiometric ratio of Mg to C is higher in hydromagnesite (5424) than in nesquehonite, this finding could have important implications for the sequestration of anthropogenic CO2 in mafic and ultramafic rocks, suggesting that the sequestration of anthropogenic CO2 is optimal in the stability field of nesquehonite. (C) 2008 Elsevier Ltd. All rights reserved.