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Mechanisms of carbon mineralization from the pore to field scale : implications for carbon dioxide sequestration Harrison, Anna Lee


Innovative technologies to stabilize atmospheric CO₂ concentrations are essential in order to mitigate the harmful effects of anthropogenic greenhouse gas (GHG) emissions on the global climate system. Mineralization of carbon in solid, stable carbonate minerals through reaction of CO₂ with Mg-rich mining wastes is a promising CO₂ sequestration strategy that offers the potential to render certain mines GHG neutral. Here, the physical and chemical controls on rates of and capacity for CO₂ sequestration in systems representative of mine tailings are examined from the mineral-fluid interface to field scale using a combination of experimental techniques. These experimental data and existing field data are used to develop a comprehensive reactive transport model that captures the processes governing carbon mineralization in the shallow subsurface. Stirred batch reactor, microfluidic pore scale, and decimeter to meter scale column carbonation experiments using brucite [Mg(OH)2] revealed that the primary controls on carbonation include the rate of CO₂ supply, the distribution of the reactive phase, the mineral grain size/surface area, and the availability and distribution of water. The rate-limiting step during carbonation varied from CO₂ supply to mineral dissolution depending on the experimental variables. Surface passivation and water-limited reaction resulted in a highly non-geometric evolution of reactive surface area. The extent of reaction was also limited at high water content because viscous fingering of the gas streams supplied to the columns resulted in narrow zones of highly carbonated material, but left a large proportion of brucite unreacted. More robust predictions of the CO₂ sequestration rate and capacity that can be expected at the field scale are possible due to the incorporation of water consumption, water-limited reactivity, and surface passivation functions into the reactive transport code, MIN3P. This research imparts a better understanding of fundamental mechanisms and chemical processes relevant to CO₂ sequestration in mine tailings, with implications for mineral carbonation in other settings that have greater CO₂ sequestration capacity, such as shallow subsurface formations with similar mineralogy. Aspects of this research, such as water-limited reactivity, have broader implications for reactive transport processes in the vadose zone in general, including mineral weathering and groundwater remediation. Supplementary video material is available at: http://hdl.handle.net/2429/51487

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