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Calcite (CaCO3) precipitation and dissolution is strongly affected by carbon dioxide (CO2) concentrations in water. As CO2 concentrations increase in water, a weak carbonic acid (H2CO3) is formed, thereby enhancing dissolution of calcite and, subsequently increasing the concentration of calcium (Ca2+) in solution. Conversely, as CO2 concentrations decrease, calcite precipitates from solution. Previous studies have shown that metals co-precipitate and dissolve with the calcite. Other studies have shown that nonpolar organic compounds can bind to calcite, but the influence of H2CO3 on co-precipitation during calcite precipitation was not studied. The objective of this study was to determine if carbonate-induced calcite dissolution and precipitation influenced perchloroethylene (PCE) sorption to CaCO3. The experiments were conducted using 12-milliliter screw-top test tubes containing 2 grams of calcite (grain size = 0.062 to 0.500 millimeters) and distilled water, along with varying amounts of PCE and H2CO3. We used a minimum of three replicates per treatment. The first set of experiments established that CaCO3 dissolved and precipitated as expected under different concentrations of H2CO3. The second set of experiments established a linear sorption isotherm for PCE to calcite without the addition of H2CO3 (Kd = 1.35). The third set of experiments used calcite that was precontaminated with a known amount of PCE. Clean water containing no PCE and 0.00, 0.01, or 0.10 molar H2CO3 was added to the precontaminated calcite and desorption was allowed to occur for 4 hours. Desorption of PCE into solution occurred in all treatments. Approximately 12 percent of the sorbed PCE desorbed into the distilled water. Approximately 28 percent of the PCE desorbed from the calcite when it was placed in the 0.01 and 0.1 molar H2CO3 solutions. Thus, H2CO3 appears to enhance desorption of PCE 16 percent as it dissolves the calcite. The Ca2+, CO2, and pH were also measured to establish geochemical conditions in each test.
Friday, 13-Apr-01 14:34:11 CST
This abstract can be cited as follows:
Saha, Lipi, and Byl, T.D., 2001, The interaction between calcite precipitation and dissolution, carbon dioxide, and perchloroethylene sorption [abs.], in Tennessee Water Resources Symposium, 11th, Burns, Tenn., 2001, Proceedings: Tennessee Section of the American Water Resources Association, p. P-11.
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