Hi all,
I want to calculate the speciation and activity coefficient of the solution below by Phreeqc 3.1 but as far as know as the ionic strength of the solution is more than 0.1M then instead of Debye-Huckel I should use Davies equation. But I do not know how to change the settings in Phreeqc to do it. Would you please guide me?
The solution contains: 0.05M CaCl2.2H2O, 0.05M Na2SO4, 0.05M MgCl2.6H2O
My calculations in PhREEQC starts with
SOLUTION 1
temp 22
pH 7
pe 4
redox pe
units mmol/kgw
density 1
water 1 # kg
REACTION 1
CaCl2:2H2O 0.05
Na2SO4 0.05
MgCl2 0.05
1 moles in 1 teps
END
The aqueous speciation model in phreeqc is set by the database you use. You can change that in the code or in the pop-up window you get before you run a file.
I would use the llnl.dat database for your problem. That uses the Truesdell-Jones equations which are supposed to be good for activity calculations in chloride dominated brines up to ionic strengths of 2 mol/kgw.
I don't really know if one of the databases that come with phreeqc uses Davies. That should be explained in the manual somewhere.
regards,
Pieter
Thanks for your complete explanation. But if I want to use PITZER equation, can I use the Pitzer database?
yes, you can, but you should have a look in pitzer.dat, to check if it has all the species you need.
The Davies equation is used to describe the activity of charged species by default. (pg 11).
Thanks Geoffrey, I was not really aware of that. The manual says Davies is used unless the b-dot of extended D-H (T-J) is specified. Do you know where those b are specified? are those the ones in the beginning of the llnl.dat file? I would have expected them to be specified for each species?
Pieter, As I remember the addition of the -gamma in the SOLUTION_SPECIES command block is the key. There is an example described on page 157. I believe the LLNL database incorporates this directly for some species. As to why it isn't available for all species? - my guess is the LL guys only included ones with data. Those without revert to the Davies formulation as I understand it.
This page gives more details for llnl.dat:
http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/phreeqc3-html/phreeqc3-26.htm
only the hard core diameter is specific to each aqueous species, and that is the one given by the gamma identifier in the solution_species block. This is different from the Truesdell-Jones model as explained in many texts: e.g. Appelo and Postma give a table with the b-dot for some species in T-J. Those vary quite a bit, like for sulfate b-dot is negative. I'll have to adapt my course ...
The Pitzer speciation model uses quite a lot of experimentally determined parameters which aren't trivial to measure. They simply haven't been measured for all species. If you do a literature search, you might be able to find some more than in the database.
this is one of the papers containing extra parameters: doi:10.1021/je7006499
Hi Pieter,
Thanks for your useful article suggestion. I am trying to make Fraction diagram for Rare earth elements in the PO4(3-) - SO4(2-) - F-Cl-Fe-H2O system. Finding all Pitzer equation parameters is kind of impossible work for mentioned system. Because of this I am trying to use Helgeson model (1981) for this system which has Ionic strength of about 3!.
H.C.Helgeson, D.H.Kirkham and G.C.Flowers, Amer. Jour. Sci., 281 (1981) 1249-1516