EAS446lec15 - EAS 44600 Groundwater Hydrology Lecture 15...

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EAS 44600 Groundwater Hydrology Lecture 15: Water Chemistry 2 Dr. Pengfei Zhang Carbonate Chemistry In non-urban, non-industrial areas, the pH of rain and melted snow falls in the range between 5 and 6. This pH results from equilibration of precipitation with atmospheric carbon dioxide (CO 2 ) as it falls to the Earth’s surface. Everything strives towards some kind of equilibrium. Therefore, raindrops forming in the atmosphere must take on some CO 2 , since CO 2 is prevalent in the atmosphere. Obviously, this is also true for nitrogen, oxygen, and anything else that is present in the atmosphere. The amount of the gas present in the atmosphere and the solubility of the gas ultimately determine how much of the gas will dissolve in the raindrop. In the case of carbon dioxide solution, the combination of CO 2 and H 2 O is thought of as a complex H 2 CO 3 . Hence: ) ( 3 2 ) ( 2 2 aq g CO H CO O H + (15-1) where the arrow shows that the process goes one way or the other depending on the concentration of H 2 O and CO 2 in the atmosphere relative to the concentration of H 2 CO 3 in the raindrop. For example, if the concentrations of CO 2 and H 2 O are sufficiently high relative to the concentration of H 2 CO 3 , they will react to form H 2 CO 3 . Likewise, if the concentration of H 2 CO 3 is sufficiently high relative to the concentrations of CO 2 and H 2 O, the H 2 CO 3 will dissociate to form CO 2 and H 2 O. At equilibrium, the following relationship exists according to the law of mass action: ] [ ] [ 2 3 2 2 2 O H P CO H K CO CO = (15-2) where is the equilibrium constant for CO 2 CO K 2 dissolution. Carbonic acid is a very important acid in natural waters. Carbonic acid, being an acid, dissociates by transferring hydrogen ions through the reactions: (15-3) + + 3 3 2 HCO H CO H (15-4) + + 2 3 3 CO H HCO where HCO 3 - is bicarbonate ion, and CO 3 2- is carbonate ion. Notice that these reactions release hydrogen ions ( protons ) to the water. The respective equilibrium constants for reactions 15-2, 15-3, and 15-4 at 25 °C are: ] [ ] [ 10 2 3 2 47 . 1 2 2 O H P CO H K CO CO = = (15-5) 15-1
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] [ ] ][ [ 10 3 2 3 35 . 6 3 CO H HCO H K HCO + = = (15-6) ] [ ] ][ [ 10 3 2 3 33 . 10 2 3 + = = HCO CO H K CO (15-7) Now let us return to our raindrop and recall that as it falls through the atmosphere it equilibrates
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