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Speciation

Speciation. Any element exists in a solution, solid, or gas as 1 to n ions, molecules, or solids Example: Ca 2+ can exist in solution as: Ca ++ CaCl + CaNO 3 + Ca(H 3 SiO 4 ) 2 CaF + CaOH +

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Speciation

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  1. Speciation • Any element exists in a solution, solid, or gas as 1 to n ions, molecules, or solids • Example: Ca2+ can exist in solution as: Ca++ CaCl+ CaNO3+ Ca(H3SiO4)2 CaF+ CaOH+ Ca(O-phth) CaH2SiO4 CaPO4- CaB(OH)4+ CaH3SiO4+ CaSO4 CaCH3COO+ CaHCO3+ CaHPO40 CaCO30 • Plus more species  gases and minerals!!

  2. How do we know about all those species?? • Based on complexation  how any ion interacts with another ion to form a molecule, or complex (many of these are still in solution) • Yet we do not measure how much CaNO3+, CaF+, or CaPO4- there is in a particular water sample • We measure Ca2+  But is that Ca2+ really how the Ca exists in a water??

  3. Aqueous Complexes • Why do we care?? • Complexation of an ion also occurring in a mineral increases solubility • Some elements occur as complexes more commonly than as free ions • Adsorption of elements greatly determined by the complex it resides in • Toxicity/ bioavailability of elements depends on the complexation

  4. Defining Complexes • Use equilibrium expressions: • cC + lHL  CL + lH+ • Where B is just like Keq!

  5. Mass Action & Mass Balance • mCa2+=mCa2++MCaCl+ + mCaCl20 + CaCL3- + CaHCO3+ + CaCO30 + CaF+ + CaSO40 + CaHSO4+ + CaOH+ +… • Final equation to solve the problem sees the mass action for each complex substituted into the mass balance equation

  6. Mineral dissolution/precipitation • To determine whether or not a water is saturated with an aluminosilicate such as K-feldspar, we could write a dissolution reaction such as: • KAlSi3O8 + 4H+ + 4H2O  K+ + Al3+ + 3H4SiO40 • We could then determine the equilibrium constant: • from Gibbs free energies of formation. The IAP could then be determined from a water analysis, and the saturation index calculated.

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