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Dissolution and Precipitation

Dissolution and Precipitation. Lecture 23 . Carbonate Solubility. Carbonate is the most common kind of chemical sediment and carbonate components ( Ca , Mg, CO 3 ) are often the dominant species in natural waters.

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Dissolution and Precipitation

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  1. Dissolution and Precipitation Lecture 23

  2. Carbonate Solubility • Carbonate is the most common kind of chemical sediment and carbonate components (Ca, Mg, CO3) are often the dominant species in natural waters. • Using equilibrium constants, we can calculate calcite solubility as a function of PCO2: • Thus calcite solubility increases with 1/3 power of PCO2. • One consequence is that calcite shells tend to dissolve in deep ocean water. • A second is that calcite will dissolve out of soils when microbial activity is present.

  3. Carbonate Solubility • Another interesting feature is because of this non-linearity, mixing of two saturated waters can produce an undersaturated water.

  4. Carbonate Solubility • Open system solutions, those in equilibrium with CO2 gas in the atmosphere or soil, can dissolve more calcite than closed systems waters. • In a sense, this is because dissolving CO2 in water lowers pH, resulting in greater dissolution.

  5. Mg Solubility ΣCO2 = 10-2.5 M • Several Mg-bearing minerals can precipitate from solution: • brucite, Mg(OH)2 • dolomite CaMg(CO3)2 • magnesite MgCO3 • We can use equilibrium constant expressions, such as: • to construct a predominance diagram showing which phase will precipitate under a given set of conditions. • We construct these in the same way we constructed pe-pH diagrams, namely manipulate the log equilibrium constant expressions to get [Mg2+] on one side of the equation and pH on the other. • We simplify things by calculating equilibrium with only 1 carbonate species at a time and ignoring the others.

  6. Mg solubility as a function of CO2 & pH [Mg2+] = 10-4 M

  7. Mg solubility as a function of CO2 & Ca/Mg Mg2+ = 10-4 M

  8. Constructing stability diagrams • This diagram shows the stability of ferrous iron minerals as a function of pH and sulfide for fixed total Fe and CO2. • Procedure: manipulate equilibrium constant expressions to obtain and expression for ΣS in terms of pH. For example: • FeCO3 + H+⇋ Fe2+ + HCO3– • FeCO3+ 2H2O ⇋ HCO3- H+ + Fe(OH)2 • FeS + 2H2O ⇋ Fe(OH)2 + H+ + HS- • Trick: simplify by ignoring species present at low conc. (e.g., CO32- at low pH). ΣCO2 = 5x 10-2 M [Fe2+] = 10-6 M (Pyrrhotite) (Siderite) ② ③ ➄

  9. Solubility of SiO2 • Silica forms silicic acid (H4SiO4) in solution, which can then dissociate through a series of reactions, e.g., • H4SiO4 ⇋ H3SiO4– + H+ K1 • H3SiO4–⇋ H2SiO42-+ H+K2 • Solubility can be expressed as: • where [H4SiO4] is controlled by solubility of either quartz or amorphous silica. • As a consequence, its solubility is a function of pH: high only at high pH.

  10. Solubility of Hydroxides • The hydroxide is the least soluble salt of many metals. Therefore, it is the solubility of their hydroxides that controls the solubility of these metals in natural waters. • Since these dissolution reactions involve OH–, they are pH-dependent, and the slope of the solubility curve depends on the valence of the metal (e.g., -3 for Fe3+, -2 for Fe2+, -1 for Ag+).

  11. Solubility of Al(OH)3 • Solubility of gibbsite: • Al(OH)3+ 3H+ ⇋ Al3+ + H2O • However, Al forms hydroxide complexes, e.g.: • Al3+ + H2O ⇋ Al(OH)2+ + H+ • The total dissolved Al will be the sum of all Al species in solution: • A consequence of this is that acid rain leads to Al poisoning.

  12. Solubility of Ferric Iron

  13. Silicate Solubility • We’ve looked at the solubility of Si, Al, Fe and other cations in the isolation of simple laboratory-like systems. • The real world is usually more complex. Silicate rocks predominate at the surface of the Earth, thus Si, Al, and other cations will generally all be present. • In addition to gibbsite and SiO2, some of the more common weathering products of silicate rocks include: • gibbsite: Al(OH)3 • kaolinite: Al2Si2O5(OH)4 • pyrophyllite: Al2Si4O10(OH)2 • illite (muscovite): KAl3Si3O10(OH)2

  14. Silicate Solubility Gibbsite will precipitate from Al-bearing solution only at lowest concentrations of SiO2. Occurrence generally restricted to highly weathered soils where all the SiO2 has washed out.

  15. Silicate Solubility

  16. Clay Minerals • Review section 6.5 to become familiar with clay minerals, but we will not cover it in class.

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