The precipitation of calcium carbonate in artificial seawater at sustained supersaturation

Precipitation and dissolution mechanisms of calcium carbonate are important processes for the regulation of pH in natural waters and for the transport of pollutants to and from the sediments. In the present work the precipitation of calcium carbonate in supersaturated solutions made from synthetic seawater was investigated by the introduction of calcite seed crystals at conditions of sustained solution supersaturations. Aragonite was found to form on the calcite seed crystals when the magnesium concentration was 53.3mM, while below this concentration calcite was the only phase forming. The kinetics of formation of both calcite and aragonite showed strong dependence on the solution supersaturation and the apparent order was found to be 7, suggesting a polynuclear mechanism. Throughout the crystal growth process the number of seed crystals remained constant, while their size increased.