Unknown source of Na concentration during Batch reaction

[c.Jefferies]

Hi,

I am trying to model determine how varying levels of NaCl in solution will influence the amount degradation of Illite. Since I am not worried about reaction rates, I can just let the system reach equilibrium and determine the delta_illite. I don't have very much experience with PHREEQC, however, I believe this is a pretty simple model. I have developed a model that can successfully run, and the results seem reasonable except for one thing; there is an increase in the amount of sodium present in the final solution (on the order of magnitude of 10^-5 mol/kg). Since PHREEQC uses an Illite chemical formula of K0.6Mg0.25Al2.3Si3.5O10(OH)2 + 11.2H2O (no sodium present) and I have a defined amount of sodium in the initial solution, I don't know where the increase in sodium is coming from. I am running the model with an initial amount of water of 1 kg and it finished at 0.999998 kg, so that is not enough to cause the increase in molality.

Any suggestions or help on figuring out where the additional sodium may be coming from would be much appreciated!

Thanks,

c.Jefferies

[dlparkhurst]

The change in molality is either a units conversion, for example if you defined SOLUTION with mol/L, or it is the result of consumption of water by the reactions. I'd have to see the input file, but I think the calculation is correct and is accounting for a minor effect.

[c.Jefferies]

Thank you very much for your response. I have believe I have used molality throughout the script, but I could be mistaken.

TITLE IX with Illite


PHASES
Fix_PH
H+ = H+
log_k 0.0
delta_h 0.0



SOLUTION 2 Min Solution
    temp      25
    pH        7
    pe        8
    redox     pe
    units     mol/kgw
    density   1.001
    Ca        0.0002884
    Cl        0.00085616
    Fe        3.58e-07
    K         0.0001054
    Mg        0
    N(5)      2.177e-05
    Na        0.00085616
    S(6)      0.00016
    -water    1 # kg


EQUILIBRIUM_PHASES 1
    Illite    0
    Fix_PH   -7.0   HCl   10.0
EXCHANGE 1
    KX      Illite               equilibrium_phase   0.6
    -equilibrate with solution 2
    -pitzer_exchange_gammas false


SAVE equilibrium_phases 2
SAVE exchange 1



SELECTED_OUTPUT 2
    -file                 testIX.xls
    -reset                true
    -state                false
    -distance             false
    -step                 false
    -pe                   false
    -temperature          false
    -alkalinity           false
    -ionic_strength       false
    -totals               Na  Cl  K  Ca  Fe(3)  Mg  Al  Si
                          Fe
    -molalities           Na+  Cl-  K+  Ca+2
                          Mg+2  Fe+2  Fe+3  NO3-
                          SO4-2
    -equilibrium_phases   Illite

end

In this scenario the output for both Na and Cl have slight increases. When the initial concentrations of these are increased then the changes are not noticed, so I am not sure if it is just a rounding in the calculations that becomes evident at lower concentrations.

Thanks for your help!

c.Jefferies

[dlparkhurst]

I am sure the calculation is accurate.

For Na, you have an exchanger initially in equilibrium with solution 2. During the reaction, the exchanger composition changes and the dissolved Na changes.

For Cl, you are adding some HCl during the reaction, which increases the Cl in solution.

Here is a slight revision of your calculations using USER_PRINT to account for the moles (not molality) of Na and Cl in the system. Compare the results of the last two simulations to find mole balance for Na; for Cl, you need to consider the Cl transfer for fix_pH. There will also be a small difference between moles and molality between the last two calculations because of the reaction of water during the reaction of illite.

Code: [Select]

TITLE IX with Illite
PHASES
Fix_PH
H+ = H+
log_k 0.0
delta_h 0.0

USER_PRINT
10 PRINT "Na, mol, solution: ", STR_F$(TOTMOL("Na"),20,10)
20 PRINT "Na, mol, exchange: ", STR_F$(MOL("NaX")*TOT("Water"), 20, 10)
30 PRINT "Na, mol, system:   ", STR_F$(SYS("Na"), 20, 10)
40 PRINT
50 PRINT "Cl, mol, solution: ", STR_F$(TOTMOL("Cl"),20,10)

SOLUTION 2 Min Solution
    temp      25
    pH        7
    pe        8
    redox     pe
    units     mol/kgw
    density   1.001
    Ca        0.0002884
    Cl        0.00085616
    Fe        3.58e-07
    K         0.0001054
    Mg        0
    N(5)      2.177e-05
    Na        0.00085616
    S(6)      0.00016
    -water    1 # kg

END
EQUILIBRIUM_PHASES 1
    Illite    0
    Fix_PH   -7.0   HCl   10.0
END
EXCHANGE 1
    KX      Illite               equilibrium_phase   0.6
    -equilibrate with solution 2
    -pitzer_exchange_gammas false
USE solution 2
END
USE solution 2
USE exchange 1
USE equilibrium_phases 1
SAVE equilibrium_phases 2
SAVE exchange 1
end


[c.Jefferies]

Awesome, Thank you very much for your help!
So if I understand correctly, the substrate is initially in equilibrium with the solution (which causes some Na to change the composition of the exchanger). And then that "Na altered exchanger" undergoes the ion exchange reaction with the specified solution. The increase in total Na from the first simulation to the last two is from the change in exchanger composition.

Thanks again!

c.Jefferies