Speciation

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

• mCa2mCa2MCaCl 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.