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Dissolution and Solubility Processes

Dissolution and Solubility Processes. Dissolution-precipitation equilibria affect many soil processes, plant growth, etc Dissolution is the disintegration or dissolving of a mineral or compound into solution Dissolution occurs when the soil solution is undersaturated .

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Dissolution and Solubility Processes

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  1. Dissolution and Solubility Processes • Dissolution-precipitation equilibria affect many soil processes, plant growth, etc • Dissolution is the disintegration or dissolving of a mineral or compound into solution • Dissolution occurs when the soil solution is undersaturated. "Most soil solutions are undersaturated with respect to the inorganic mineral components in the soil, which means that these soils are in a continuous state of dissolution."

  2. Congruent dissolution • complete dissolution of a mineral without subsequent reprecipitation of other minerals • stoichiometric (one mole of an element in the mineral dissolves to one mole of the element in solution) Ex’s: Gypsum: CaSO4.2H2O  Ca+2 + SO4-2 + 2H2O Gibbsite: Al(OH)3(s) Al+3(aq) + 3OH-(aq)

  3. Incongruent dissolution • Some of the solutes combine to form secondary minerals • all of the ions from the original mineral don’t stay in solution • nonstoichiometric 4KAlSi3O8 + 0.5 Mg+2(aq) + 2 H+(aq) + 10H2O  (orthoclase) K[Si7.5Al0.5][Al3.5Mg0.5]O20(OH)4 + 4.5Si(OH)4 +3K+(aq) (2:1 clay mineral)

  4. Incongruent dissolution http://darkwing.uoregon.edu/~jrice/geol_311/PyroxeneWeather.GIF

  5. Silicate weathering characteristics • Loss of tetrahedrally coordinated Al • Oxidation of Fe(II) to Fe(III) • Consumption of protons (H+) • Release of silica and the metal cations Na+, K+, Mg+2, and Ca+2 Ex: feldspars + CO2 + H2O  clay minerals + ions + bicarbonate

  6. Dissolution in nature Water, CO2, and carbonic acid (CO2 + H2O  H2CO3 H+ + HCO3-) dissolve rocks and soil minerals and release ions to soil solution (nutrients, salts, toxins, metals, etc.) Ex: feldspars + CO2 + H2O  clay minerals + ions

  7. http://darkwing.uoregon.edu/~jrice/geol_311/CalciteWeather.GIFhttp://darkwing.uoregon.edu/~jrice/geol_311/CalciteWeather.GIF

  8. A saturated solution has the same rate of precipitation and dissolution. http://itl.chem.ufl.edu/2041_f97/lectures/lec_i.html

  9. Thermodynamic dissolution constantsCaSO4.2H2O  Ca+2 + SO4-2 + 2H2O • Account for activities, not just concentrations • If solid is ‘pure’, its activity = 1; • The activity of water = 1 • E.g., Kdis for gypsum: Kdis = (Ca+2)(SO4-2)(H2O)2 (gypsum) Kdis = (Ca+2)(SO4-2) = Kso Kso = 2.4 x 10-5 for gypsum (~10-4.62) 1 1

  10. Chemical Equilibrium Consider the reaction aA + bB cC+dD The equilibrium constant is But, incorporating activities, the correct expression for K is

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