Transcript Speciation

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++
Ca(H3SiO4)2
Ca(O-phth)
CaB(OH)4+
CaCH3COO+
CaCO30
CaCl+
CaF+
CaH2SiO4
CaH3SiO4+
CaHCO3+
CaNO3+
CaOH+
CaPO4CaSO4
CaHPO40
• Plus more species  gases and
minerals!!
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??
Aqueous Complexes
•
Why do we care??
1. Complexation of an ion also occurring in a
mineral increases solubility
2. Some elements occur as complexes more
commonly than as free ions
3. Adsorption of elements greatly determined
by the complex it resides in
4. Toxicity/ bioavailability of elements depends
on the complexation
Defining Complexes
• Use equilibrium expressions:
• cC + lHL  CL + lH+
c
 n
[CL] [ H ]
i 
c
l
[C ] [ HL ]
• Where B is just like Keq!
Mass Action & Mass Balance
c
 n
[CL] [ H ]
i 
c
l
[C ] [ HL ]
mCa   mCa L
2
2 n
x
• 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
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:
aK  a Al3 aH3 4 SiO4
K
4
aH 
• from Gibbs free energies of formation. The IAP could
then be determined from a water analysis, and the
saturation index calculated.