Experiments on CO 2 -water-rock interaction at hydrothermal temperatures have been performed to investigate dissolution and precipitation phenomena, including Ca extraction from rocks that might occur during CO 2 sequestration into geothermal fields. Distilled water samples were exposed to a CO 2 atmosphere at a temperature of 25°C and pressures up to 6 MPa. The resulting solutions were then reacted with granodiorite samples from the Ogachi hot/dry rock field and labradorite, at 200°C and 120°C respectively. The calcium concentrations in the solutions that had reacted with CO 2 were twice those with N 2 instead of CO 2 . Combined with the results of thermodynamic calculations, these observations indicate that calcium can be released from rocks (silicates) easily and might be removed as CaCO 3 and/or CaSO 4 during CO 2 sequestration into geothermal fields.
The solubility of UO2 (s) was examined in dilute NaCl solutions at room temperature and in the pH range from 2 to 12. Dissolution equilibrium showed good agreements between the data obtained by oversaturation and undersaturation experiments.The dissolution reactions that control the solubility of U[IV] are estimated as UO2 (s) + 4H+ → U4+ + 2H2O and UO2 (s) + 2H2O → U(OH)4°. The log K at zero ionic strength are yielded 0.34 ± 0.4 and −8.7 ± 0. 4, respectively. The hydrolysis constant (log β14) is calculated −9.0 ± 0.5 for the hydrolysis reaction of U4+ + 4H2) → U(OH)4° + 4H+. Contribution of the other uranium species to the solubility are of minor importance.The crystallization of the precipitations of U02 (s) progressed gradually in the oversaturation experiments. Nevertheless the progress of crystallization had little effect on the solubility.
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