Activities of Seawater constituents

[Kaushik Mitra]

Hi everyone,
I am trying to find out the ACTIVITY OF CONSTITUENTS in normal sea-water concentration and then find out the activities for progressive evaporation of seawater.

I have used the following concentration (mol/kg)
Al           3.7E-08
Alkalinity   0.0023
Ca(+2)      0.0104
Cl(-)           0.5460
Fe           6.1E-08
K(+)           0.00996
Mg(+2)      0.05330
Na(+)        0.46796
SO4(-2)   0.02810 at 25 degree C, pH=8, pe=4.

I would like to know the activities of various components like Cl(-), Fe(+2) etc.

The result that I am getting is like:
Species       Concentration    Activity
Cl               5.460e-01
Cl-              5.460e-01        3.415e-01   
FeCl+          1.536e-10        1.022e-10     
FeCl+2        1.090e-17        2.135e-18   
FeCl2+        4.375e-18        3.257e-18   
FeCl3          9.575e-20        1.112e-19   

I am facing the following problems:

1. How to get the activity of Cl(-) ? The concentration assigned in the input is Cl(-) i.e. 5.460e-01  but the software is giving out different other complex species of Cl(-). Can I add the other activities to get total activity of Cl(-)?

2. I also want to carry out the same for different stages of evaporation of sea-water. I can calculate the molalities of the constituents but how do I take into account the pH and pe?

Thank you

P.S. Attached is the PHREEQC Interactive file of my simulation

 

[dlparkhurst]

The activity of Cl- is given in the output next to the molality of the bare Cl- ion, along with the activity coefficient. Assuming you are using an ion-association model (phreeqc.dat, wateq4f.dat, minteq, llnl) the activity coefficient is a function solely of the ionic strength. The distribution of species is determined by mass action equations for each ion pair or complex (for Cl-, FeCl+, FeCl+2 ...). So the activity of Cl- is given, perhaps you are thinking of a total activity coefficient such that Cl(total) * G(total) = aCl-, in which case you can calculate the total activity coefficient from the total concentration and the calculated activity.

The Pitzer approach has many fewer ion pairs, and in fact, may provide a total activity coefficient in many cases. However, the Pitzer approach has few, if any, redox species, which means that redox reactions cannot be simulated. Also, the Pitzer approach does not pretend that individual ion activity coefficients are meaningful. You can calculate individual ion activity coefficients with Pitzer in Phreeqc, either with or without MacInnis scaling, but only charge-balanced combinations of activity coefficients are intended to be useful. Individual-ion activity coefficients are, in fact, not measureable, so the Pitzer approach is simply acknowledging reality. All ion-association model activity coefficients are simply a model.

As for evaporating, you can remove water with REACTION or sometimes by specifying an activity of water with EQUILIBRIUM_PHASES and allowing water to be removed. The pH and pe will be calculated as a result of reaction.