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Calcite (CaCO3) precipitation and dissolution is strongly affected by carbon dioxide (CO2) concentration in the water. As CO2 concentrations rise in water, a weak carbonic acid (H2CO3) is formed, enhancing dissolution of CaCO3, subsequently raising the concentration of calcium in the water. Conversely, as CO2 concentrations drop, CaCO3 precipitates from solution. Previous studies have shown that metals co-precipitate with the calcite. Other studies have shown that non-polar organic compounds will bind to calcite, but the influence of carbonic acid on organic binding was not studied. This could play a role in a semi-closed system, such as a karst conduit, where CO2 levels can change abruptly over a short distance, leading to calcite dissolution or precipitation. This study is designed to examine the role of carbonate-induced calcite dissolution and precipitation on PCE sorption. Batch-sorption experiments are being conducted to establish a sorption isotherm for PCE to pure calcite of grain size 0.062 to 0.500 millimeters (fine to medium sand). Further PCE-sorption experiments are run with varying concentrations of carbonic acid. The levels of PCE and calcium ions in solution are measured to quantify PCE sorption and calcite dissolution processes. The carbonate levels and pH are also measured to establish geochemical conditions in the batch tests. The results are intended to increase understanding of PCE sorption to calcite as a function of carbonate-induced precipitation and dissolution.
U.S. Department of the Interior, U.S. Geological Survey
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