What is Speciation?
In the previous article, we defined “pure water” in PHREEQC and ran a basic calculation. In this session, we will input actual seawater data and dive into a more essential concept: Speciation (Chemical Species Distribution).
Speciation is the calculation of which chemical forms (species) an element takes within a solution.
For instance, even if seawater contains 2712 mg/L of \(\mathrm{SO_4^{2-}}\), not all of it exists as a free \(\mathrm{SO_4^{2-}}\) ion. Some portion forms complexes with \(\mathrm{Mg^{2+}}\) (as \(\mathrm{MgSO_4}\)), and another portion pairs with \(\mathrm{Na^+}\) to form \(\mathrm{NaSO_4^-}\).
Why Speciation Matters
The "Saturation Index (SI)," which determines whether a mineral will dissolve or precipitate, is calculated using the chemical activity of ions in solution. Activity differs from simple concentration because it is influenced by complexation and ionic strength. Speciation is the process of accurately determining these activities.
Chemical Composition of Seawater
Below is the seawater data we will use, representing a typical average seawater composition.
| Component | Formula | Conc. (mg/L) | Notes |
|---|---|---|---|
| Sodium | Na | 10768 | Major cation |
| Potassium | K | 399.1 | |
| Calcium | Ca | 412.3 | Relates to carbonates |
| Magnesium | Mg | 1291.8 | Forms complexes with SO₄ |
| Chloride | Cl | 19353 | Major anion |
| Bicarbonate | HCO₃⁻ (Alkalinity) | 141.682 | Input as Alkalinity |
| Sulfate | SO₄ | 2712 | Forms multiple complexes |
| Nitrate | NO₃ | 0.29 | Trace |
| Iron | Fe | 0.002 | Ultratrace |
| Silicon | Si | 4.28 | Exists as SiO₂ |
The pH is 8.22, temperature is 25°C, and density is 1.023 g/cm³.
Input Procedure in PHREEQC
1. Click the SOLUTION Icon
As before, click the SOLUTION icon in the left icon bar. The SOLUTION property window will open.
2. Enter pH, pe, and Temperature
Input the following values into the fields at the top of the window:
3. Change Units to ppm
The default unit is mmol/kgw, but since our data is in mg/L (approximately ppm), we need to change it. Select ppm from the unit dropdown menu.
One of the most common mistakes in PHREEQC is using the wrong units. Entering mg/L data as mmol/kgw will cause your results to be off by orders of magnitude. Always double-check before proceeding.
4. Select Elements and Enter Concentrations
Check the boxes for the elements you want to include and enter their concentrations in the fields on the right.
Bicarbonate (\(\mathrm{HCO_3^-}\)) should be entered as Alkalinity rather than HCO3. By specifying as HCO3- in the units column, PHREEQC will correctly perform calculations for the carbonate system.
Alkalinity 141.682 as HCO3-PHREEQC Code
The GUI will automatically generate the following code. You can also copy and paste this directly into the text editor.
SOLUTION 1 Seawater
temp 25
pH 8.22
pe 8.451
redox pe
units ppm
density 1.023
Na 10768
K 399.1
Ca 412.3
Mg 1291.8
Cl 19353
Alkalinity 141.682 as HCO3-
S(6) 2712 # Sulfate entered as S(6)
N(5) 0.29 # Nitrate entered as N(5)
Fe 0.002
Si 4.28
-water 1 # kg
ENDS(6) and N(5)
In PHREEQC, sulfate (\(\mathrm{SO_4^{2-}}\)) is written as S(6) to specify the oxidation state of sulfur. Similarly, nitrate (\(\mathrm{NO_3^-}\)) is written as N(5). This distinguishes them from reduced forms like \(\mathrm{S^{2-}}\) (sulfide) or \(\mathrm{NH_4^+}\) (ammonium).
Running the Calculation
After entering the code, ensure you have an END statement, then click Run → OK. Choose your workspace folder, save the file (e.g., seawater_speciation.pqi), and click Start → Dismiss.
Reading the Output
Basic Solution Information
The beginning of the Output file provides a summary of the solution:
----------------------------Description of solution----------------------------
pH = 8.220
pe = 8.451
Specific Conductance (µS/cm, 25°C) = 52634
Density (g/cm³) = 1.02282
Activity of water = 0.981
Ionic strength (mol/kg) = 6.748e-01
Mass of water (kg) = 1.000
...Ionic strength = 0.665 mol/kg: This is quite high. Freshwater typically has an ionic strength below 0.001 mol/kg. At such high values, the activity coefficients (\(\gamma\)) of ions deviate significantly from 1, making speciation calculations essential.
Activity of water = 0.981: While pure water has an activity of 1.000, solutes in seawater lower this value. This decreases the vapor pressure, which is one reason why seawater evaporates more slowly than freshwater.
Species Distribution
The middle section of the Output shows the distribution for each element. Let’s look at Sulfate (\(S(6)\)):
Log Log Log mole V
Species Molality Activity Molality Activity Gamma cm³/mol
S(6) 2.926e-02
SO4-2 1.432e-02 2.604e-03 -1.844 -2.584 -0.740 17.49
MgSO4 7.170e-03 8.375e-03 -2.144 -2.077 0.067 5.84
NaSO4- 6.637e-03 4.482e-03 -2.178 -2.349 -0.171 21.21
CaSO4 9.548e-04 1.115e-03 -3.020 -2.953 0.067 7.50
KSO4- 1.756e-04 1.186e-04 -3.755 -3.926 -0.171 34.85SO₄²⁻ Species Distribution (Calculated)
Based on SO₄²⁻ Molality (1.432e-2) and Activity (2.60e-3), the activity coefficient (γ) is approximately 0.18. This indicates a significant reduction in the chemical effectiveness of free SO₄²⁻. About 51% of total sulfate exists as ion pairs.
Saturation Index (SI)
The latter half of the Output contains the Saturation Indices section:
-------------------------------Saturation indices-------------------------------
Phase SI log IAP log K(T, P)
Anhydrite -0.93 -5.20 -4.28 CaSO4
Aragonite 0.61 -7.73 -8.34 CaCO3
Calcite 0.75 -7.73 -8.48 CaCO3
Chalcedony -0.52 -4.07 -3.55 SiO2
Chrysotile 3.36 35.56 32.20 Mg3Si2O5(OH)4
CO2(g) -3.39 -4.86 -1.47 CO2
Dolomite 2.39 -14.70 -17.08 CaMg(CO3)2
Fe(OH)3(a) 0.18 5.07 4.89 Fe(OH)3The Saturation Index (SI) is defined as:
\[SI = \log \frac{IAP}{K_{sp}}\]
- \(SI\): Saturation Index \([-]\)
- \(IAP\): Ion Activity Product \([-]\)
- \(K_{sp}\): Thermodynamic Solubility Product at specific \(T\) and \(P\) \([-]\)
| SI Value | Meaning | Mineral Behavior |
|---|---|---|
| \(SI > 0\) | Supersaturated | Tendency to precipitate |
| \(SI = 0\) | Equilibrium | No net dissolution/precipitation |
| \(SI < 0\) | Undersaturated | Tendency to dissolve |
Discussion: Geochemical Implications for Seawater
Our speciation results reveal several important aspects of seawater geochemistry:
1. Calcite is Supersaturated (SI = 0.75)
Seawater is supersaturated with respect to calcite, meaning it is thermodynamically favored to precipitate. This explains the abundance of biogenic carbonate (calcite and aragonite) in tropical shallow waters.
2. Anhydrite and Gypsum are Undersaturated
With SI values of -0.93 and -0.59 respectively, sulfate minerals do not precipitate in normal seawater. If evaporation occurs and the solution concentrates, the SI will rise, eventually leading to the precipitation of evaporites like gypsum.
3. Dolomite is Highly Supersaturated (SI = 2.39)
While thermodynamically favored, dolomite rarely precipitates directly from modern seawater. This is due to kinetic constraints (a kinetic barrier), a phenomenon known in geochemistry as the “Dolomite Problem.” It is important to remember that PHREEQC calculates thermodynamic equilibrium but does not account for reaction kinetics.
- Verify Units — Don’t confuse ppm with mmol/kgw.
- Inputting Alkalinity — Remember to use
as HCO3-. - Interpreting SI — Positive values indicate precipitation; negative values indicate dissolution.
Next Time: Mixing and EQUILIBRIUM_PHASES
In the next tutorial, we will combine Mixing (merging two solutions) and EQUILIBRIUM_PHASES (calculating equilibrium with minerals). We will investigate what happens when gypsum and anhydrite are added to pure water as temperature increases from 25°C to 75°C—a fascinating example of how stability can flip between two minerals.
References
Other articles in this series:
- #1 Installation and Initial Calculation
- #2 Analyzing Seawater with Speciation (This article)
- #3 Mixing and EQUILIBRIUM_PHASES
DeepFlow | Science beneath the surface