Understanding the reactivity of CO2-rich phases is essential for predicting the fate and transport of CO2 in geologic carbon storage reservoirs. Recent research has demonstrated that the mechanisms and processes of mineral carbonation in wet, or water-bearing, supercritical CO2 (scCO2) fluids are fundamentally different than those occurring in CO2-saturated aqueous phases. This is due to adsorbed thin (nm- to Å-sized) water films (TWF) that develop on the mineral surfaces and allows for CO2 dissolution, solute transport, and coupled mineral dissolution and precipitation. Although the TWF phases approximate bulk aqueous conditions, there is evidence of intriguing differences, such as precipitation of magnesite (MgCO3) at low temperatures (35-50 °C). In order to further investigate TWF parameters that influence magnesite precipitation, we exposed high surface area synthetic forsterite (Mg2SiO4) to water-saturated scCO2 at 35, 50, and 65 °C and 90 bar in a high-pressure X-ray diffraction (HXRD) reactor. This technique allowed for precise control of the scCO2 water activity (aH2O) and in situ identification and quantification of metastable reaction intermediates. At all three temperatures, small scale reductions in the aH2O(scCO2) inhibited the precipitation of transitory hydrated Mg-carbonate phases. For example, at 50 °C and 90 bar, the forsterite carbonated to nesquehonite (MgCO3◦3H2O) prior to conversion to magnesite. However, when the aH2O(scCO2) was decreased by only 4.1%, nesquehonite was not detected in the collected X-ray tracings. These results indicate that the aH2O(TWF) is highly sensitive to the aH2O(scCO2), which buffers the thickness of the water film. The reduced aH2O(TWF) likely facilitated dewatering of the Mg2+ cation and subsequent promotion of anhydrous magnesium carbonate precipitation. Results from ongoing pressurized infrared spectroscopy experiments that measure the TWF thicknesses and probe the growth of amorphous species will be presented. Preliminary results confirm Mg-carbonate and amorphous silica precipitation. Details of the novel methods that allow for precise control of the aH2O and adsorbed water film thicknesses will be presented, and their applicability to other geochemical and biogeochemical investigations will be discussed.

In situ X-ray diffraction and infrared spectroscopic investigation of magnesite precipitation in interfacial water films

249th National Meeting (2015)

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