Experiments were carried out at 100 bar pressure and 60 or 150 degrees C in 0.7 m NaCl brine to characterize the reactivity of two Frio quartzofeldspathic sandstone compositions and to elucidate the fate of metals (Ba, Cr, Cu, Fe, Mn, Ni, Pb and Zn) in subsurface reservoirs targeted for carbon dioxide sequestration and storage. The solutions were either acidic (pH similar to 3) or near-neutral (pH similar to 8). In the former, acidity resulted from saturation with carbon dioxide (CO2) or by addition of HCl, and in the latter, the pH was attained by addition of NaHCO3. A pair of experiments was conducted without CO2 to trace the behavior of four dissolved metals (Cr, Ni, Pb and Zn) in circum-neutral solutions. The experiments were conducted in rocking autoclave reactors up to 67 days' time with solutions drawn periodically. Solution analyses indicated modest release of major elements from the starting materials to solution, even at 150 degrees C. Geochemical modeling indicated supersaturation of the solutions with respect to a variety of Fe-and Mn-bearing phases. Scanning electron microscope (SEM) analyses of the powders both before and after experiments showed evidence for minor dissolution of alkali feldspar, quartz, plagioclase and clay minerals. No evidence for precipitated carbonate phases was found in the CO2-bearing experiments. In general, the concentrations of the metals were below their respective maximum contaminant levels (MCLs) by the end of the experiment, except for Ba, and for Cr and Pb in the experiment in which near-neutral conditions were imposed from the beginning. The data are consistent with metal removal from solution as the pH changes from acidic to neutral and SEM results identified Fe-oxides and sulfides as the likely sinks for Cu, Cr and Zn. The data indicate that even under extreme conditions the likelihood of metal concentrations in drinking water exceeding MCLs through accidental mixing with CO2-bearing solution is very low.
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