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THE GEOCHEMISTRY OF NATURAL WATERS

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THE GEOCHEMISTRY OF NATURAL WATERS

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    1. 1 THE GEOCHEMISTRY OF NATURAL WATERS REDOX REACTIONS AND PROCESSES - II CHAPTER 5 - Kehew (2001) Cr-O2-H2O

    2. 2 Cr-O2-H2O SYSTEM The last example calculation of a pe-pH diagram we will perform in this lecture is one for the Cr-O2-H2O system. In most natural, uncontaminated waters, Cr is not usually present in measurable concentrations. However, at a number of sites around the world, Cr is present in elevated concentrations owing to contamination from industrial processes such as tanning of leather and chrome plating. As we will see, in the 6+ state Cr is highly soluble, and therefore mobile and bioavailable (i.e., readily absorbed by living organisms). This is of environmental concern because Cr(VI) is highly toxic and carcinogenic. On the other hand, Cr(III) is considerably less toxic, and as we shall see, also considerably less soluble. The last example calculation of a pe-pH diagram we will perform in this lecture is one for the Cr-O2-H2O system. In most natural, uncontaminated waters, Cr is not usually present in measurable concentrations. However, at a number of sites around the world, Cr is present in elevated concentrations owing to contamination from industrial processes such as tanning of leather and chrome plating. As we will see, in the 6+ state Cr is highly soluble, and therefore mobile and bioavailable (i.e., readily absorbed by living organisms). This is of environmental concern because Cr(VI) is highly toxic and carcinogenic. On the other hand, Cr(III) is considerably less toxic, and as we shall see, also considerably less soluble.

    3. 3 In the species HCrO4- and CrO42-, we have Cr(VI), whereas in Cr(OH)2+, Cr2O3(s) and CrO2- we have Cr(III). Among the oxidized species, the order of decreasing acidity is HCrO4- > CrO42-, whereas among the reduced species, it is Cr(OH)2+ > Cr2O3(s) > CrO2-. In the species HCrO4- and CrO42-, we have Cr(VI), whereas in Cr(OH)2+, Cr2O3(s) and CrO2- we have Cr(III). Among the oxidized species, the order of decreasing acidity is HCrO4- > CrO42-, whereas among the reduced species, it is Cr(OH)2+ > Cr2O3(s) > CrO2-.

    4. 4 HCrO4-/CrO42- BOUNDARY HCrO4- ? CrO42- + H+ ?Gr° = = ?Gf°CrO42- - ?Gf°HCrO4- ?Gr° = (-727.8) - (-764.7) = 36.9 kJ mol-1 By now the procedure should be thoroughly familiar. For the first three boundaries in this pe-pH diagram, redox is not involved and so the boundaries depend on pH only.By now the procedure should be thoroughly familiar. For the first three boundaries in this pe-pH diagram, redox is not involved and so the boundaries depend on pH only.

    5. 5

    6. 6 Cr(OH)2+/Cr2O3(s) BOUNDARY ½Cr2O3(s) + 2H+ ? Cr(OH)2+ + ½H2O(l)

    7. 7 ?Gr° = = ?Gf°Cr(OH)2+ + ½?Gf°H2O - ½?Gf°Cr2O3 ?Gr° = (-431.0) + ½(-237.1) - ½(-1058.1) = -20.5 kJ mol-1 We set ?Craq ? m Cr(OH)2+ = 10-6 mol L-1.

    8. 8

    9. 9 CrO2-/Cr2O3(s) BOUNDARY 1/2Cr2O3(s) + 1/2H2O(l) ? CrO2- + H+ ?Gr° = ?Gf°CrO2- - ½?Gf°H2O - ½?Gf°Cr2O3 ?Gr° = (-535.6) - ½(-237.1) - ½(-1058.1) = 112.0 kJ mol-1

    10. 10

    11. 11 Cr(OH)2+/HCrO4- BOUNDARY HCrO4- + 6H+ + 3e- ? Cr(OH)2+ + 3H2O(l) ?Gr° = ?Gf°Cr(OH)2+ + 3?Gf°H2O - ?Gf°HCrO4- ?Gr° = (-431.0) + 3(-237.1) - (-764.7) = -377.6 kJ mol-1

    12. 12

    13. 13 Cr2O3(s)/HCrO4- BOUNDARY HCrO4- + 4H+ + 3e- ? 1/2Cr2O3(s) + 5/2H2O(l) ?Gr° = 1/2?Gf°Cr2O3 + 5/2?Gf°H2O - ?Gf°HCrO4- ?Gr° = 1/2(-1058.1) + 5/2(-237.1) - (-764.7) = -357.1 kJ mol-1

    14. 14

    15. 15 Cr2O3(s)/CrO42- BOUNDARY CrO42- + 5H+ + 3e- ? 1/2Cr2O3(s) + 5/2H2O(l) ?Gr° = 1/2?Gf°Cr2O3 + 5/2?Gf°H2O - ?Gf°CrO42- ?Gr° = 1/2(-1058.1) + 5/2(-237.1) - (-727.8) = -394.0 kJ mol-1

    16. 16

    17. 17 CrO2-/CrO42- BOUNDARY CrO42- + 4H+ + 3e- ? CrO2- + 2H2O(l) ?Gr° = ?Gf°CrO2- + 2?Gf°H2O - ?Gf°CrO42- ?Gr° = (-535.6) + 2(-237.1) - (-727.8) = -282.0 kJ mol-1

    18. 18 The pe-pH diagram above shows that, if we wish to keep Cr from migrating from contaminated sites, we need to keep conditions reducing, and maintain the pH above 5. If we can accomplish this, then Cr should be relatively immobile.The pe-pH diagram above shows that, if we wish to keep Cr from migrating from contaminated sites, we need to keep conditions reducing, and maintain the pH above 5. If we can accomplish this, then Cr should be relatively immobile.

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