Dissolution Experiments of Serpentinite and Kinetic Modeling of CO 2 Behavior for Underground Sequestration of CO 2 in Deep Serpentinite Aquifers

It is widely accepted that recent global warming is being caused by an increase in greenhouse gases such as CO2 emitted by anthropogenic activities (e.g., fossil fuel burning). Therefore, it is mandatory that humans reduce these anthropogenic CO2 emissions. One potentially useful method to reduce CO2 emissions and the greenhouse effect is underground sequestration of CO2 into groundwater aquifers. When CO2 is injected into an aquifer, the CO2 dissolves into the groundwater, reacts with the host rocks in the aquifer and, in the long-term, may precipitate as secondary minerals. If the reaction produces carbonate minerals, most of the CO2 will be trapped permanently in deep underground. So, over the long-term, CO2 injected into aquifers could be trapped in groundwater (solubility trapping) and in carbonate minerals (mineral trapping). Thus, this method is generally thought to have great potential as a highly useful and efficient method for long-term fixing of CO2 deep underground 6, 7. However, one of this method’s problems is that the longterm behavior of CO2 underground is not well understood. The trapping mechanism and the amount of carbon fixed underground depend on geologic environmental factors such as the properties of the host rocks, such as their constituent minerals, grain size, permeability, and porosity, and of the groundwater, such as its chemical composition and f low rate. Among the various kinds of rocks, ultramafic rocks dissolve at higher rates than other igneous rocks such as granite and basalt, and sedimentary rocks such as sandstone and mudstone. Therefore, we expected that ultramafic rocks would react quickly with injected groundwater containing high concentrations of CO2, resulting in efficient CO2 fixation by solubility and mineral trapping in a relatively short period of time compared with other rocks. This is due to the high dissolution rate of the constituent minerals in ultramafic rocks (olivine and pyroxene) and high concentrations of Ca, Mg, and Fe (the main elements in carbonate minerals) in groundwater that has reacted with ultramafic rocks. Although a large number of experimental studies on the dissolution of the minerals which are contained in ultramafic rocks (e.g. olivine, pyroxene, and serpentine) have been performed, there are few experimental studies on the dissolution of ultramafic rocks (Marini, 2007). Therefore, in this study themselves, serpentinite, a common ultramafic rock, was selected for the dissolution experiment. The serpentinite samples collected from Mt. Iwanai, Hokkaido, northern Japan were analyzed and experimentally studied. The dissolution rate constants of the serpentinite samples were obtained by experiment. Based on those dissolution rate constants, kinetic calculations were performed to estimate the Journal of MMIJ Vol.130 p.396 − 403 (2014) ©2014 The Mining and Materials Processing Institute of Japan