1. Aqueous Speciation
in Hydrochemistry
aqion.de

2. Parameter Value
T °C 25
pH 7.0
Ca mM 3.818
Mg mM 0.864
Na mM 0.957
K mM 0.072
Alkalinity mM 4.960
Chloride mM 1.805
Sulfate mM 1.520
Nitrate mM 1.100
Start: Measured data (water analysis)
 cations
 anions
Example

3. The Naïve Approach (no speciation)
each measured element/parameter  one ion
in aqueous solution
Element Ion z
Ca Ca+2 2
Mg Mg+2 2
Na Na+ 1
K K+ 1
Alkalinity HCO3
- -1
Chloride Cl- -1
Sulfate SO4
-2 -2
Nitrate NO3
- -1
1-to-1
correspondence
measured
charge

4. But, in reality:
Each measured concentration
comprises “free ions” and complexes.
Ca = Ca+2 + CaHCO3
+ + CaCO3(aq) + CaSO4(aq) + ...
total concentration = free ion + complexes
in the solutionmeasured
Example:
naïve approach

5. The naïve approach ignores at least
three things:
formation of
aqueous complexes
the role of alkalinity
(which is not an element)
activity corrections
(for non-ideal solutions)
2
3
1

6. Aqueous Complexation is described
by the Law-of-Mass Action (LMA):
ba
ba
}B{}A{
}BA{
K aA + bB = AaBb
equilibrium constant
curly braces denote activities as
“effective concentrations”
1

7. Alkalinity should be replaced by DIC
as dissolved inorganic carbon:
DIC = CO2 + HCO3
- + CO3
-2 + ...
The partitioning is described by LMA
(plus 3 equilibrium constants).
(bi)carbonate
complexes
2

8. Ib
IBa1
IzA
lg i
i
2
i
i 



]i[z
2
1
I
i
2
iionic strength
activity = γ ∙ concentration
Activity Correction (DEBYE-HÜCKEL)
ion concentration
iterative
3

9. Speciation calculations can be performed
by PhreeqC or any other
hydrochemistry program (aqion, ...)
Results for the example water

10. Speciation model
no yes
Ca+2 3.818 3.388
Mg+2 0.864 0.758
Na+ 0.975 0.952
K+ 0.072 0.072
Fe+2 0.052 0.037
Mn+2 0.019 0.013
CaHCO3
+ 0.128
MgHCO3
+ 0.027
MnHCO3
+ 0.004
HCO3
- 4.960 4.762
SO4
-2 1.520 1.139
Cl- 1.805 1.805
NO3
- 1.100 1.100
CO3
-2 0.003
NaSO4
- 0.003
KSO4
- 0.0004
Speciation model
no yes
CO2 0.948
CaCO3 0.007
CaSO4 0.295
MgCO3 0.001
MgSO4 0.078
NaHCO3 0.002
MnCO3 0.001
MnSO4 0.001
FeCO3 0.001
FeSO4 0.003cations
Aqueous Species [mM]
anions
neutralspecies
the Naïve Approach

11. [mM] free ions HCO3
-
CO3
-2
SO4
-2
total
free ions
HCO3
-
CO3
-2
SO4
-2
5.904
4.762 0.003 1.139
Na
Na+
NaHCO3 NaSO4
-
0.957
0.952 0.002 0.003
K
K+
KSO4
-
0.072
0.072 0.0004
Ca
Ca+2
CaHCO3
+
CaCO3 CaSO4
3.818
3.388 0.128 0.007 0.295
Mg
Mg+2
MgHCO3
+
MgCO3 MgSO4
0.864
0.758 0.027 0.001 0.078
Mn
Mn+2
MnHCO3
+
MnCO3 MnSO4
0.019
0.013 0.004 0.001 0.001
Fe
Fe+2
FeHCO3
+
FeCO3 FeSO4
0.052
0.037 0.011 0.001 0.003
total 5.220 4.933 0.013 1.520 11.686
Aqueous Speciation [mM]
aqueous complexes

12. [mM] free ions HCO3
-
CO3
-2
SO4
-2
total
free ions
HCO3
-
CO3
-2
SO4
-2
5.904
4.762 0.003 1.139
Na
Na+
NaHCO3 NaSO4
-
0.957
0.952 0.002 0.003
K
K+
KSO4
-
0.072
0.072 0.0004
Ca
Ca+2
CaHCO3
+
CaCO3 CaSO4
3.818
3.388 0.128 0.007 0.295
Mg
Mg+2
MgHCO3
+
MgCO3 MgSO4
0.864
0.758 0.027 0.001 0.078
Mn
Mn+2
MnHCO3
+
MnCO3 MnSO4
0.019
0.013 0.004 0.001 0.001
Fe
Fe+2
FeHCO3
+
FeCO3 FeSO4
0.052
0.037 0.011 0.001 0.003
total 5.220 4.933 0.013 1.520 11.686
Mass Balance
DIC = CO2 + 4.933 + 0.013 = 5.894 mM

13. DIC = CO2 + HCO3
- + CO3
-2 + ...
Note: CO2 = CO2(aq) + H2CO3
Dissolved Inorganic Carbon
CO2 0.948 mM
HCO3
- 4.762 mM
CO3
-2 0.003 mM
C complexes 0.181 mM
DIC 5.894 mM

14. Cation Speciation model
no yes
Ca+2 7.636 6.776
Mg+2 1.728 1.516
Na+ 0.975 0.952
K+ 0.072 0.072
Fe+2 0.104 0.074
Mn+2 0.038 0.026
CaHCO3
+ 0.128
MgHCO3
+ 0.027
MnHCO3
+ 0.004
Total 10.553 9.575
Aqueous Species [meq/L]
Anion Speciation model
no yes
HCO3
- 4.960 4.762
SO4
-2 3.040 2.278
Cl- 1.805 1.805
NO3
- 1.100 1.100
CO3
-2 0.006
NaSO4
- 0.003
KSO4
- 0.0004
Total 10.905 9.954
%94.1100
L/meq529.19
L/meq379.0
100
anionscations
anionscations
[%]CBE 







Charge Balance Error