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Date of Award
Doctor of Philosophy (PhD)
Geologic sequestration of anthropogenic carbon dioxide can greatly reduce CO2 emission from coal-fired power plants. However, the injection of CO2 into deep subsurface rock formations for long-term storage may disturb the thermodynamic equilibrium of the original subsurface system and induce a series of water-rock interactions driven by a significant decrease in the pH of the related aquatic systems. The consequences of these induced multiphase interactions need to be accounted for when assessing the storage capacity of CO2 and the risks associated with the sequestration practice. In addition to the heterogeneity of mineral composition, the presence of various aqueous species in subsurface formation water can also contribute to the complexity of the system.
Effects of representative organic ligands on the chemical weathering of feldspar, the most abundant silicate mineral in the earth's crust, were investigated in the context of geologic carbon sequestration (GCS). The effect of initially present acetate, the most abundant organic compound in many formation waters, on the dissolution of anorthite (CaAl2Si2O8) and on subsequent secondary mineral precipitation under GCS conditions (35oC and 74.8 atm) was studied. Acetate was found to decrease the cumulative aqueous concentrations of Al, Si, and Ca upon CO2 injection by inhibiting anorthite dissolution and increasing the amount of secondary mineral precipitates. The extent of the effect of acetate on metal concentration changes was element-specific (Al > Si > Ca), and the effect was found to be more significant in systems with lower salinity and lower pH. The presence of kaolinite as a secondary mineral was confirmed using electron diffraction data from high resolution transmission electron microscopy. An increase in the relative amount of precipitation due to the initial presence of acetate was suggested by mass balancing and verified on the cleaved anorthite surfaces by atomic force microscopy. The effects of acetate and oxalate on alkali feldspar-brine interactions in a simulated geologic carbon sequestration (GCS) environment at 100 atm of CO2 and 90 °C were also investigated. Oxalate was chosen for its high effectiveness in altering the rate of chemical weathering of silicate minerals under non-GCS conditions. We showed that the increased reactivity of Al-O-Si linkages due to the presence of oxalate resulted in the promotion of both Al and Si release from feldspars. As a consequence, the degree of Al/Si ordering may affect the effect of oxalate on feldspar dissolution: a promotion of ~500% in terms of cumulative Si concentration was observed after 75 hours of dissolution for sanidine (a highly disordered feldspar) owing to oxalate, while the corresponding increase for albite (a highly ordered feldspar) was ~90%.
The experimental investigation of feldspar-brine interactions under GCS conditions led us to believe that the effects of solution chemistry are mineral structure-specific. Hence, a quantitative description of how the bulk properties of aluminosilicates affected their dissolution kinetics is important in helping people understand the regulation of atmospheric CO2 concentration by silicate weathering, and predict the fate and transport of geologically sequestered CO2 through brine-rock interactions. The effects of a mineral's bulk structure on its dissolution rate were not reflected explicitly in the current Transition State Theory-Surface Complexation Models (TST-SCM) formalism. We thus proposed a new formalism for feldspar dissolution which, while compatible with the current TST-SCM conceptual framework, incorporates the effects of essential crystallographic properties into the rate law of feldspar dissolution. With the new formalism, we predicted that the dissolution stoichiometry of feldspar was affected by the degree of Al/Si ordering in the feldspar's polymerized framework, and that the effects of water chemistry on feldspar dissolution were sensitive not only to the composition of a specimen, but also to how different types of chemical bonds were distributed in the solid matrix. The outcome of this study provides new insights into the mechanism of chemical weathering of silicate minerals abundant in subsurface formations, and helps improve the rate laws that can be used in large scale transport-reactive modeling of geologic sequestration of carbon dioxide.
Richard Axelbaum, David Cole, John Fortner, Daniel Giammar, Sophia Hayes
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