Setting Solution Volume for Dissolution/Precipitation Reactions

[ZYWTDaEO6]

Hi,

I'm working on modeling mineral dissolution and precipitation using Phreeqc, with a focus on the SOLUTION and EQUILIBRIUM_PHASES keywords. My goal is to define both a specific amount of mineral and a specific volume of solution for the reaction. However, I've noticed that the SOLUTION keyword doesn?t directly allow input of solution volume?only solution mass through the -water parameter (in kg).

To work around this, I'm considering calculating the mass by multiplying my target solution volume by my input density, and then using this calculated mass as the input for the -water parameter. So my question is: can I use the mass obtained by multiplying the target solution volume by my specified input density directly as the -water parameter? Will this approach could it introduce discrepancies?

Thank you for any insights on this setup!

Code: [Select]

SOLUTION 0
        water         "my target volume * density"
        units          mol/L
        temp          15.0
        pH             7.0
        density       1.2 calc
        K 0.814897719
        Na             1.02525979
        Ca             0.00280945690
        Mg             0.739249069
        Li             0.00905169597
        Cl             12.9469969
        S(6)           0.0221635841
        C(4)           0.0000804767662
EQUILIBRIUM_PHASES 0
        Halite     0.0   1.0
        Halite     0.0   1.0
        Carnallite 0.0   1.0
        Polyhalite 0.0   1.0
        Sylvite    0.0   1.0
        Gypsum     0.0   1.0
        Calcite    0.0   1.0
        Dolomite   0.0   1.0
END


[dlparkhurst]

If you are using PhreeqcRM, then concentrations are converted by the call to GetConcentrations. You can choose one of the following units: mg/L, mol/L; or mass fraction, kg/kgs, with the method SetUnitsSolution.

If you are not doing transport, then you could probably ignore the difference between mol/kgw and mol/L. You are introducing a few percent error, which is probably small relative to other uncertainties.

To consider density, you will need to use one of the databases--phreeqc.dat, Amm.dat, or pitzer.dat. With these databases, you can calculate concentrations as mol/L as follows:

Code: [Select]

USER_PRINT
10 PRINT "Ca concentration, mol/L: ", TOTMOL("Ca") / SOLN_VOL


With every reaction, the volume of solution will change, so creating a liter of the initial solution will not result in a liter of the reacted solution. In that case, I don't see the point of creating a liter of any particular solution.

If you really, really need to do the calculation, here is the way I would do it.

Code: [Select]

SOLUTION 1
-density 1 calc
-units mol/L
Na 2
Cl 2
USER_PRINT
10 PRINT "Cl, mol/L: ", TOTMOL("Cl") / SOLN_VOL
20 PRINT "Volume, L: ", SOLN_VOL
SELECTED_OUTPUT 100
-file scale_1L.pqi
USER_PUNCH 100
10 s$ = "MIX 1" + EOL$
20 s$ = s$ + "1 " + STR_F$(1/SOLN_VOL, 10, 5) + EOL$
30 s$ = s$ + "END" + EOL$
40 PUNCH s$
50 end
END
SELECTED_OUTPUT 100
-active false
END
INCLUDE$ scale_1L.pqi
END