Solid solutions in non equilibrium conditions

[srohmen]

I was working on a model where I have to control the target SI appropriately to allow a certain amount of super saturation to prevent early precipitation. Now I had to change the model to a solid solution approach. Unfortunately, I figured out that the solid solution "phase" is always in equilibrium (SI = 0), independently of the target SI values I set  for the components of the solid solution defined.

I was wondering, if I am just to blind to find the right settings or is there no possibility to achieve this with PhreeqC (or even mathematically....)?

[dlparkhurst]

The "target SI" in EQUILIBRIUM_PHASES simply adjusts the log K of the mineral. Not sure if this is what you want, but, for a solid solution you could simply redefine the minerals (with PHASES) with higher log Ks.

[srohmen]

Thanks for your quick reply.
I am a little confused about your first sentence: As far as I understood the target SI is the difference between log IAP and log K. The solution is modified by dissolution/precipitation of minerals until the log IAP matches the difference to the log K value defined by "target SI" value of the mineral in EQUILIBRIUM_PHASE.

My needs are a little different than just redefining the log K once, since my system must redefine the target supersaturation dynamically. For this is I am using a similar concept like:

Code: [Select]

EQUILIBRIUM_PHASES_MODIFY 0
Calcite
-si 1.234In fact to improve performance I changed the PhreeqcRM interface code for this purposes, but this is another story...

Lets take the following example:

Code: [Select]

SOLUTION 0
        -units mol/kgw
        -pH 7 charge
        Ba 1.0
        Sr 1.0
        S(6) 2.0

SOLID_SOLUTIONS 0
BaSrSO4
        -comp Barite    0
        -comp Celestite 0
Result:

Code: [Select]

Solid solution                Component        Moles  Delta moles   Mole fract

BaSrSO4                                     2.00e+00
                                 Barite     1.00e+00     1.00e+00     5.00e-01
                              Celestite     1.00e+00     1.00e+00     5.00e-01

-----------------------------Solution composition------------------------------

        Elements           Molality       Moles

        Ba                2.939e-07   2.939e-07
        S                 4.164e-04   4.164e-04
        Sr                4.161e-04   4.161e-04

Code: [Select]

  Phase               SI** log IAP   log K(298 K,   1 atm)

  Barite           -0.30    -10.14   -9.84  BaSO4
  Celestite        -0.30     -6.96   -6.66  SrSO4
  H2(g)           -35.32    -38.42   -3.10  H2
  H2O(g)           -1.50     -0.00    1.50  H2O
  H2S(g)         -116.94   -124.93   -7.99  H2S
  O2(g)           -12.64    -15.53   -2.89  O2
  Sulfur          -87.60    -82.72    4.88  S

As far as I understand is that the solid solution itself is in equilibrium with the solution (SI=0) while the end-members itself are under-saturated (at least for an ideal solid solution like in the example) due to the activity correction of the solid phase. I could not find any possibility to reduce the amount of precipitation amount of BaSrSO4 (and letting Ba + Sr + S(6) dissolved). There is no possibility to set a target SI value for BaSrSO4 (which I found).

After I wrote the forum entry this morning, I was thinking about tinkering with some non-ideality parameters of the solid solution to achieve the effect I want to have. But this is sounding a little overcomplicated for me, on first sight.

I hope you can understand my concern now more clearly.
Best regards!

[dlparkhurst]

I was trying to say that if you have a target saturation of 1.0, it is equivalent to defining the equilibrium constant as one unit higher.

All of the following create the same solution that has a IAP = 8.97. The first uses the target saturation index, the second redefines the equilibrium constant, and the third redefines the equilibrium constant with an undocumented option -add_constant.

If you are using EQUILIBRIUM_PHASES_MODIFY to change the target saturation index with a pure phase, you can equally easily redefine a mineral with PHASES to have a different log K. If you use -add_constant, it is very similar to adjusting the target saturation index, except you do it with PHASES using the same base definition of the equilibrium constant, but the target saturation index goes into -add_constant.

SOLUTION
EQUILIBRIUM_PHASES
Barite  1.0 10
END
PHASES
Barite             
        BaSO4 = Ba+2 + SO4-2
        log_k           -8.97
END
SOLUTION
EQUILIBRIUM_PHASES
Barite  0 10
END
PHASES
Barite             
        BaSO4 = Ba+2 + SO4-2
        log_k           -9.97
        -add_constant   1.0
END
SOLUTION
EQUILIBRIUM_PHASES
Barite  0 10
END