Phase transitions in the Mg-CO2-H2O system and the thermal decomposition of dypingite, Mg5(CO3)4(OH)2·5H2O: Implications for geosequestration of carbon dioxide

The application of hydrated Mg-carbonates as CO2 sequestering media is a pressing environmentalchallenge, which requires a deep knowledge of the phase transitions occurring in the Mg-CO2-H2Osystem as well as the thermal and structural stability of these phases. In this paper we investigatethe phase transition of nesquehonite (MgCO3·3H2O) to dypingite (Mg5(CO3)4(OH)2·5H2O),occurring after an incubation of months and years in solution, at ambient conditions. However, asthe kinetics of this process resulted to be slow, the phase transition of dypingite to hydromagnesite(Mg5(CO3)4(OH)2·4H2O) was investigated at non-ambient conditions using in situ real-time highresolutionX-ray powder diffraction. Moreover, the thermal behaviour of dypingite and itsdecomposition have been also investigated with the aim to explore the appropriateness of thiscarbonate and the products of its decomposition as sinks of anthropogenic carbon dioxide. Theresults suggest that the dypingite structure remains unaffected up to 438 K. At temperature abovethis threshold, dypingite transforms into hydromagnesite. A further increase in temperature convertsthe well-ordered hydromagnesite into a “collapsed form” at 528 K. The heating of dypingite doesnot produce a loss of CO2 as the intermediate phases have the same CO2:Mg molar ratio. The finalproduct of the heating is periclase MgO. Its nucleation occurs at temperature ranging from 573 to663 K and it becomes the only phase in the temperature range 633-678 K. These results highlightedthat dypingite assures a stable storage of CO2 in the conditions that prevail at the Earth’s surface.Moreover, the transformation of dypingite in more thermodynamically stable hydromagnesite,occurring without release of CO2, enhances the safety of carbon dioxide disposal in solid form.Furthermore, the observed volume changes during phase transitions, which in turn could affect theporosity and permeability of the geological reservoir, have been evaluated in order to improve theprediction of the safe and permanent storage of CO2 in underground.