Humidity, Temperature and Exposure to Amorphous Silica Control the Stability of CO2 Storage in Hydrated Mg-Carbonate Minerals | AIChE

Humidity, Temperature and Exposure to Amorphous Silica Control the Stability of CO2 Storage in Hydrated Mg-Carbonate Minerals

Authors 

Wilson, S. A. - Presenter, Monash University
Morgan, B., CSIRO Mineral Resources
Power, I. M., The University of British Columbia

Increasing attention is being paid to technologies that employ hydrated Mg-carbonate minerals as traps for CO2 pollution. However, relatively little is known about the environmental behaviour and long-term stability of these minerals under conditions relevant to CO2storage. Thus, we have undertaken a long-term experiment to monitor and map the relative stability of four hydrated Mg-carbonate minerals over the range of atmospheric conditions experienced at Earth’s surface.

Aliquots of synthetic lansfordite (MgCO3·5H2O), nesquehonite (MgCO3·3H2O), dypingite [Mg5(CO3)4(OH)2·~5H2O] and hydromagnesite [Mg5(CO3)4(OH)2·4H2O] were maintained under set conditions of temperature (-25 °C ≤ T ≤ 75 °C) and relative humidity (2% ≤ RH ≤ 100%) for a period of approximately 20 months. Changes in mineralogy were assessed using powder X-ray diffraction data and scanning electron microscopy after approximately 2, 7 and 20 months.

Our experiments demonstrate irreversible decomposition of more hydrous Mg-carbonate minerals to less hydrous phases with increasing atmospheric relative humidity (RH) and/or temperature. Highly hydrated minerals such as lansfordite and nesquehonite decompose to relatively stable hydromagnesite at high RH. However, these phases and dypingite persist to unexpectedly high temperatures at low RH – even after 20 months. Therefore, transformation to more stable, less hydrous Mg-carbonate minerals appears to require a supply of H2O to mediate reactions. Because Mg-carbonate minerals are hygroscopic this provides a mechanism by which atmospheric H2O vapour may become sorbed at grain surfaces to facilitate reactions. Scanning electron microscope imaging reveals (1) pristine, unreacted crystals at low RH and (2) abundant evidence of decomposition reactions at grain surfaces in high RH experiments. These observations support the conclusion that Mg-carbonate phase transitions occur via vapour-mediated dissolution–reprecipitation reactions.

Phase transitions amongst hydrated Mg-carbonate minerals also appear to be dependant on reaction path. For instance, formation of hydromagnesite, the most stable of the hydrated Mg-carbonates, is more sluggish via decomposition of structurally and compositionally related dypingite than via decomposition of the more hydrated phases, lansfordite and nesquehonite. Thus, energy barriers to ensuring more stable storage of CO2 within hydromagnesite appear to be lower for decomposition of lansfordite and nesquehonite to this phase.

Within four months of storage at high RH (~100%) and T ≥ 23 °C, all hydrated Mg-carbonate minerals begin to decompose to an X-ray amorphous phase that exhibits short range order on the nanoscale. Our scanning transmission electron microscopy and energy dispersive X-ray spectroscopy results indicate that this phase is Mg- and Si-bearing with a 15-Å layered structure. This likely indicates formation of a smectite-like phase, formed by reaction of the hydrated Mg-carbonates with the silica glass vessels in which they were stored. Because partial dissolution of Mg-carbonates produces high-pH solutions, this would lead to dissolution of the amorphous silica vessels. Indeed, the glass vessels used in several of the 100% RH, 75 °C experiments showed evidence of dissolution.

Our experimental results suggest that decomposition of hydrated Mg-carbonate minerals toward hydromagnesite, and ultimately magnesite, is likely mediated by dissolution–reprecipitation reactions. These may be driven by formation of thin films of H2O on mineral surfaces at or below the deliquescence RH (~100%) of these hygroscopic minerals. Thus, atmospheric conditions will be a major control on decomposition of hydrated Mg-carbonate minerals to more stable phases in both industrial carbonation processes and geoengineered landscapes. Another important implication of this work is that long-term storage of hydrated Mg-carbonate minerals with amorphous silica may pose a risk for CO2 release in environments that are subject to high humidity and precipitation. Both of these phases result from mineral carbonation reactions; thus, it may become important to separate these products prior to storage or to ensure storage under appropriate climatic conditions.