6246lect08_S10

6246lect08_S10 - Advanced Environmental Geochemistry, GLY...

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Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 1 LECTURE 8 - CARBON CHEMISTRY Carbonate Chemistry Carbon, in many different forms, plays a critical role in the geochemistry and geological processes of the earth. We are concentrating on water in this course. The fact that water exists as a liquid over much of the earth's surface is critically dependent on the behavior of carbon. Carbon plays a large role in organic chemistry. It also plays a very important role in inorganic and biochemical precipitates; meaning carbonates of organic origin such as fossiliferous limestones, etc. Carbonate chemistry involves the species H 2 CO 3 , HCO 3 - (bicarbonate ion), CO 3 2- (carbonate ion), CaCO 3 , and other carbonates. Carbonate chemistry is important for several reasons. A. Buffer - The various reactions of carbonate chemistry in seawater act to buffer the pH of seawater at about 8.2. This provides a stable medium in which much of the world's life exists. It prevents the problems associated with acid rains in lakes. B. Carbon sink - The carbonate chemistry serves as a reservoir for carbon in the oceans and on land (as limestone, dolomite, etc.) and regulates CO 2 in the atmosphere, thus helping to prevent a runaway greenhouse effect. C. Weathering reaction control - As a secondary effect of controlling carbon dioxide in the atmosphere, the pH of natural rainfall is maintained between 5-6. If the atmosphere were richer in carbon dioxide, natural rainfall would have a lower pH. The problems associated with acid rain would be greatly increased and far more widespread.
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Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 2 8-1 8-2 8-3 8-4 Carbonate Equilibria Several reactions are important in the carbonate system. The reactions that occur depend on the pH of the environment, and serve to help regulate that pH. Lect08, slide 2 here Weak acid: Lect08, slide 3 here Strong acid: Lect08, slide 4 here Alkaline: Lect08, slide 5 here These may be summarized in the overall reaction: The direction of this reaction will depend on the conditions given above. At low pH (acid) the forward reaction proceeds. At high pH the back reaction dominates.
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Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 3 8-5 In addition the presence of CO 2 in the atmosphere in contact with the solution will affect the equilibrium. Lect08, slides 6 here As CO 2 pressure increases more H 2 CO 3 is formed. This leads to the dissolution of CaCO 3 . When CO 2 pressure decreases, H 2 CO 3 is destroyed and more calcium carbonate precipitates. Pressure increases the solubility of CaCO 3 . In the deep ocean calcium carbonate should be more soluble than at shallower depths. Calcium carbonate is also more soluble at lower temperatures, again favoring dissolution at depth. Calcium carbonate is also more likely to precipitate in warm tropical waters than
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This note was uploaded on 05/04/2011 for the course GLY 6246 taught by Professor Warburton during the Spring '11 term at FAU.

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6246lect08_S10 - Advanced Environmental Geochemistry, GLY...

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