In situ infrared spectroscopic study of forsterite carbonation in wet supercritical CO2.

Carbonation reactions are central to the prospect of CO(2) trapping by mineralization in geologic reservoirs. In contrast to the relevant aqueous-mediated reactions, little is known about the propensity for carbonation in the key partner fluid: supercritical carbon dioxide containing dissolved water ("wet" scCO(2)). We employed in situ mid-infrared spectroscopy to follow the reaction of a model silicate mineral (forsterite, Mg(2)SiO(4)) for 24 h with wet scCO(2) at 50 °C and 180 atm. The results show a dramatic dependence of reactivity on water concentration and the presence of liquid water on the forsterite particles. Exposure to neat scCO(2) showed no detectable carbonation reaction. At 47% and 81% water saturation, an Ångstrom-thick liquid-like water film was detected on the forsterite particles and less than 1% of the forsterite transformed. Most of the reaction occurred within the first 3 h of exposure to the fluid. In experiments at 95% saturation and with an excess of water (36% above water saturation), a nanometer-thick water film was detected, and the carbonation reaction proceeded continuously with approximately 2% and 10% conversion, respectively. Our collective results suggest constitutive links between water concentration, water film formation, reaction rate and extent, and reaction products in wet scCO(2).

Representative Sampling of High-Level Radioactive Waste Tank Headspaces.

Headspaces of the underground high-level radioactive waste-storage tanks at the U.S. Department of Energy's Hanford Site have been sampled to resolve tank safety and industrial hygiene issues and to estimate regulated air pollutant emissions. Because sampling these tanks is difficult and expensive, samples have been collected from a single location of the headspaces, based on the supposition that this would provide representative samples. In most tanks, mixing of vapors occurs because of thermally driven convection from heat generated by radioactive decay of the waste. However, in some low-temperature tanks, the ground temperature above the tank may be warmer than the waste, minimizing thermally induced convection, and raising the concern that samples from a single location may not be representative. To resolve this issue, six samples at different vertical and horizontal locations were taken from each of three low-temperature tanks and analyzed for ammonia, water, permanent gases, total non-methane organic compound concentration, and selected organic vapors. Statistical analysis of the data indicated that the tanks did not exhibit significant horizontal or vertical concentration gradients.