SOLID_SOLUTIONS in pH dependency tests

[mbrandely]

Hi everyone,

I am currently working on the modelling of Acid Neutralization Capacity (ANC) tests carried out on excavated earths samples.
 
To better describe the mineral assemblage of these earths, it seems that I need to take into account a strontianite/aragonite or strontianite/calcite solid solution. Acid addition (HNO3) are simulated through the REACTION keyword and allow carbonates mineral to dissolve progressively. However, when complete dissolution is achieved (pH = 5.2), PHREEQ-c send an error message and no further acid addition are done. I am struggling to determine the reason of such an error but it seems that I should have missed something in the conceptual definition of solid solutions under PHREEQ-c.

I would be honor if you could have a look on the script below :

Code: [Select]

PHASES
Quartz(alpha)
SiO2 + 2.000H2O = 1.000H4SiO4
     log_k    -3.737

Aragonite
CaCO3 + 1.000H+ = 1.000HCO3- + 1.000Ca+2
     log_k     2.014

Calcite
CaCO3 + 1.000H+ = 1.000HCO3- + 1.000Ca+2
     log_k     1.847
END

SOLID_SOLUTIONS 1
Ca(x)Sr(1-x)CO3 
     -comp1   Aragonite       0.759 
     -comp2   Strontianite    0.001
     -Gugg_nondim   3.43    -1.82
END

SOLUTION 1
-ph 7 charge
-redox pe
-units mol/kgw
-temp 25
-water 1
Sr 7e-6
END

EQUILIBRIUM_PHASES 1
Aragonite 0 0
Strontianite 0 0
Quartz(alpha) 0 0.0666
END

SELECTED_OUTPUT
-file                  Solid_solution.txt
-reaction true
-totals Ca Sr Si
-solid_solutions Aragonite Strontianite
-equilibrium_phases Aragonite Quartz(alpha)


####            ANC modelling             ####

####         HNO3 addition from 0 to 0.02mol/L     ####

USE SOLUTION 1
USE SOLID_SOLUTIONS 1
USE EQUILIBRIUM_PHASES 1
REACTION 1
HNO3 1
0.02 mole in 100 steps
END

####         HNO3 addition from 0 to 2.5mol/L     ####

USE SOLUTION 1
USE SOLID_SOLUTIONS 1
USE EQUILIBRIUM_PHASES 1
REACTION 2
HNO3 1
2.5 mole in 125 steps

END
Thanks in advance.
Maxime

[dlparkhurst]

PHREEQC can have trouble determining when a solid solution should exist or not. I think there is trouble determining when the solid solution has completely dissolved.

First, you should not include aragonite and strontianite in EQUILIBRIUM_PHASES; just in SOLID_SOLUTIONS.

Then, if you add the following, it will take smaller internal steps to try to approach the numerical solution.

KNOBS
-step 5
-pe 2

Finally, I am not sure you want to use SOLID_SOLUTIONS. If you are simply dissolving the solid, then I think a fixed composition EQUILIBRIUM_PHASE might be more realistic. SOLID_SOLUTION allows the solid composition to adjust to equilibrium, but I doubt that happens. My guess is that the solid simply dissolves at the original composition. That leaves the problem of trying to estimate the log K for the fixed composition solid, but that might be a better approach than allowing the solid composition to vary.

[mbrandely]

Thanks a lot for your answer.
KNOBS helps a lot to go through calculation and your conceptual guess is probably right. I simply assumed that SOLID_SOLUTION could be a way to define the solid and let it dissolve along with the pH changes.
Best regards.
Maxime