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

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) (Mg5(CO3)4(OH)2 · 4H2O) 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 (Mg3(OH)5Cl · 4H2O) also forms in addition to hydromagnesite (5424).