Precipitation of Carbonates from Seawater

Up to this point, we have focused on aqueous equilibria involving proton transfer. Now we apply the same principles to the equilibrium that exists between a solid salt and its dissolved ions in a saturated solution. We can use the equilibrium constant for the dissolution of a substance to predict the solubility of a salt and to control precipitate formation. These methods are used in the laboratory to separate and analyze mixtures of salts. They also have important practical applications in municipal wastewater treatment, the extraction of minerals from seawater, the formation and loss of bones and teeth, and the global carbon cycle. 

The latter two assumptions are simplistic, considering the number of factors that affect pH and oxidation state in the oceans (e.g., Sillen, 1967 Holland, 1978 McDuff and Morel, 1980). Consumption and production of CO2 and O2 by plant and animal life, reactions among silicate minerals, dissolution and precipitation of carbonate minerals, solute fluxes from rivers, and reaction between convecting seawater and oceanic crust all affect these variables. Nonetheless, it will be interesting to compare the results of this simple calculation to observation. 

McCallum, M.F. and Guhathakurta, K., 1970. The precipitation of calcium carbonate from seawater by bacteria isolated from Bahama Bank sediments. J. Appl. Bacterial., 33 649—655. 

It is believed that most limestone formed as a precipitate of calcium carbonate (and other carbonates) from seawater.The photograph shows limestone formations at Bryce Point, Bryce Canyon National Park,Utah.More than sixty million years ago, this area was covered by seawater. 

Preparation and Manufacture. Magnesium chloride can be produced in large quantities from (/) camalhte or the end brines of the potash industry (see Potassium compounds) (2) magnesium hydroxide precipitated from seawater (7) by chlorination of magnesium oxide from various sources in the presence of carbon or carbonaceous materials and (4) as a by-product in the manufacture of titanium (see Titaniumand titanium alloys). 

The results are shown in Figures 6-5 and 6-6. Figure 6-5 depicts how the system evolves from its initial conditions to a repeatable oscillation about annual average conditions. This evolution is clearest for the calcium ion concentration, which rises toward twice the seawater value. Calcium does not quite reach twice the seawater value because it is removed from the system by the precipitation of calcium carbonate. The rise in calcium is a consequence of the evaporative concentration of the water s dissolved constituents. 

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