Supercritical CO2 Water Research Articles

Dissolution kinetics of anorthite in a supercritical CO2-water system

The Gibbs free energy change, Δ G r , dependence of the anorthite dissolution rate in a supercritical CO2–water system was measured as part of a geochemical assessment of CO2 geological sequestration. Being bounded at Δ G r crit, the critical Δ G r required for an opening of the etch pit at a screw dislocation, it has been accepted that mineral dissolution follows etch pit formation assisted by dislocations below Δ G r crit, whereas the horizontal step retreats without the etch pit above Δ G r crit. The experiment described herein, however, revealed that another mode of dissolution occurs more distant from equilibrium by spontaneous formation of the etch pit over the entire surface, as observed on calcite. The dissolution rate is higher by more than one order of magnitude than that in the dislocation-assisted mode. Therefore, including the rate gap at Δ G r crit, a nonlinear curve with three steps instead of a sigmoidal curve is proposed for the Δ G r dependence of the anorthite dissolution rate. Extremely slow rates were observed depending on observed points for the same Δ G r condition. Although the reason for such a rate difference remains unknown, it is likely related to the defect density on the crystal surface. It is possible that initial spreading of the dissolved surface attributable to the etch pit formation assisted by defects provides some trigger for subsequent explosive etch pit formation. These findings suggest that the initial transient process can strongly influence the kinetics of geochemical reactions that occur during CO2 geological sequestration.

CO2 地中貯留における灰長石溶解プロセスの速度論的考察

The Gibbs free energy change, ΔGr, dependence of the anorthite dissolution rate under a supercritical CO2-water system was measured as part of a long-term assessment of CO2 geological sequestration. The surface observation on a nanoscale described herein showed that the ΔGr dependency follows to the rate function with three steps (i.e., the horizontal step retreats without the etch pit, the etch pit formation assisted by dislocations, and the spontaneous formation of the etch pit) instead of transient state theory and a traditional sigmoidal curve, where the rate increases abruptly more distant from equilibrium by spontaneous formation of the etch pit over the entire surface. Such a variation of the dissolution rate can affect largely not only the change in porosity or permeability of reservoir rocks and the resultant flow property of formation water, but also the precipitation rate of secondary minerals. Our preliminary numerical simulation revealed that the difference of the rate function form according to ΔGr dependence produces the time gap more than two orders on both the dissolution of anorthite and the precipitation of secondary minerals. On the other hand, extremely slow rates were observed depending on surface conditions for the same ΔGr condition. Therefore, understanding of transient processes before a steady state is also necessary for evaluation of the mineral dissolution rate under natural conditions including CO2 geological sequestration.

Partition coefficients of environmentally important phenols in a supercritical carbon dioxide-water system from cocurrent extraction without analysis of the compressible phase.

Partition coefficients of phenol, salicylic acid, and several environmentally important chloro- and nitrophenols in a supercritical CO2-water system were measured using direct cocurrent extraction of aqueous solutions of the individual solutes with CO2. Partitioning data on the nitrophenols and salicylic acid were obtained for the first time. To bypass the troublesome and error-prone analysis of the CO2-rich phase, the present method employed only the solute concentrations in the aqueous phase before and after extraction to determine the partition coefficient. Unlike most previous engineering studies of phenol partitioning in a CO2-water system, the concentrations of phenolic solutes approached infinite dilution in both phases. This makes the results relevant to analytical-scale SFE of environmental water samples with CO2. Because of effective infinite dilution of the solutes, the partition coefficients provide a direct measure of relative CO2-philicity/hydrophilicity of the individual phenols. Compared to the octanol-water partition coefficients of substituted phenols, the CO2-water partition coefficients are more sensitive to substitution in the position neighboring the hydroxyl group.

Interfacial activity of nitric acid at the water–supercritical CO2 interface: a molecular dynamics investigation

We report a molecular dynamics study on a nitric acid water–supercritical CO2 interface. Three extreme models are compared, where the acid is either all neutral HNO3, completely dissociated into NO3− and H3O+, or represented by a 1 : 1 mixture of both forms. The ionic species are found to be “repelled” by the neat interface, while the neutral HNO3 molecules are highly surface active. Similar features are observed with SC-CO2 or with chloroform as the organic phase, pointing to the generality of interfacial activity of the acid.

Feasibility of on-line supercritical fluid extraction of steroids from aqueous-based matrices with analysis via gas chromatography-mass spectrometry.

The solubility of testosterone, boldenone, androstenone, etiocholanolone, and epitestosterone are measured in pure supercritical CO2. Testosterone exhibited the highest solubility in supercritical CO2. The solubility of all steroids except epitestosterone increased by one order of magnitude with increasing pressure from 100 to 400 atm. Epitestosterone had the lowest solubility in supercritical CO2 and its solubility was not affected by pressure. The extraction efficiency of steroids from an aqueous saline environment exceeded 95%. Because of the partial solubility of water in supercritical CO2, the addition of a moisture trap after the aqueous vessel is necessary to prevent the plugging and deterioration of the gas chromatographic (GC) column. It is demonstrated that on-line supercritical fluid extraction-GC-mass spectrometry is feasible for the quantitative extraction and analysis of steroids from both saline and urine solutions. However, it is determined that the adsorbent vessel filled with Hydromatrix is not sufficient to trap all the moisture, and after 3 to 4 extractions, the GC column efficiency lowered.