Dissolution of a solid solution in alkaline water

[JakubV]

Hello,

I am new to PhreeqC modelling, I would like to model the possible dissolution of Bentonite by alkaline leaching aqueous solution. I started with Bentonite modelled as solid solution and used this code (further I plan to study the pH and liquid/solid phase ratio dependence)

DATABASE C:\Program Files\Phreeqc21\Databases\PCHatches.dat
PHASES
      Montmor-ca
             Ca0.165Mg0.33Al1.67Si4O10(OH)2 + 6H+ + 4H2O = 1.67Al+3 + 0.165Ca+2 + 4H4SiO4 + 0.33Mg+2
             log_k  3.06
             delta_h -39.4 kcal
      Montmor-na
             Na0.33Mg0.33Al1.67Si4O10(OH)2 + 6H+ + 4H2O = 1.67Al+3 + 4H4SiO4 + 0.33Mg+2 + 0.33Na+
             log_k  3.06
             delta_h -37.7 kcal
      Montmor-k
             K0.33Mg0.33Al1.67Si4O10(OH)2 + 6H+ + 4H2O = 1.67Al+3 + 4H4SiO4 + 0.33K+ + 0.33Mg+2
             log_k  2.72
             delta_h -36.5 kcal
      Montmor-mg
             Mg0.495Al1.67Si4O10(OH)2 + 6H+ + 4H2O = 1.67Al+3 + 4H4SiO4 + 0.495Mg+2
             log_k  2.98
             delta_h -40 kcal
END

SOLUTION 1 # leaching water
units mol/L
pH 9.5
temp 25.0
Ca 1.082   charge # is fixing pH
Mg .097
Na .39
K .003
Cl .73
C .00018
S(6) .017

SOLID_SOLUTIONS 2
   MontmoriloniteTry
-comp Montmor-na  500.00 #moles
-comp Montmor-ca  15.00
-comp Montmor-k   0.05
-comp Montmor-mg  0.03
END
EQUILIBRIUM_PHASES 1 2

#USER_PUNCH
#10 PUNCH SUM_S_S(MontmoriloniteTry, Montmor-na)
#END

SELECTED_OUTPUT
-file tran81.sel
-totals Cs Ca montmor-na montmor-ca montmor-k montmor-mg # concentration in aqueous phase, mol/kgw
-molalities Ca+2 OH- # concentration in solid phase, mol/kgw
-ionic_strength
-equilibrium_phases anhydrite
-saturation_indices montmor-na montmor-ca

USE solution 1
USE solid_solution 2
KNOBS
-iterations 400
END

within PhreeqC 3.2.2. I am not sure if it really mixes the solid and liquid phases together (??) and how to obtain the total dissolved mass (or contributions from different elements to this mass). Please can you help me?

Thank you in advance,
JV

[dlparkhurst]

Here is your file revised to show the sequence of calculations more clearly. The SOLUTION definition results in an initial solution calculation and the USE statements result in a second calculation where the solid solution is reacted with the solution. The mole transfer of the solid solution components is given in the output.

Solid solution                Component        Moles  Delta moles   Mole fract

MontmoriloniteTry                             5.15e+02
                             Montmor-na     4.99e+02    -1.24e+00     9.68e-01
                             Montmor-ca     1.56e+01     5.87e-01     3.03e-02
                              Montmor-k     5.99e-02     9.91e-03     1.16e-04
                             Montmor-mg     6.71e-01     6.41e-01     1.30e-03


Input:

PHASES
Montmor-ca
    Ca0.165Mg0.33Al1.67Si4O10(OH)2 + 6H+ + 4H2O = 1.67Al+3 + 0.165Ca+2 + 4H4SiO4 + 0.33Mg+2
    log_k     3.06
    delta_h   -39.4 kcal
Montmor-na
    Na0.33Mg0.33Al1.67Si4O10(OH)2 + 6H+ + 4H2O = 1.67Al+3 + 4H4SiO4 + 0.33Mg+2 + 0.33Na+
    log_k     3.06
    delta_h   -37.7 kcal
Montmor-k
    K0.33Mg0.33Al1.67Si4O10(OH)2 + 6H+ + 4H2O = 1.67Al+3 + 4H4SiO4 + 0.33K+ + 0.33Mg+2
    log_k     2.72
    delta_h   -36.5 kcal
Montmor-mg
    Mg0.495Al1.67Si4O10(OH)2 + 6H+ + 4H2O = 1.67Al+3 + 4H4SiO4 + 0.495Mg+2
    log_k     2.98
    delta_h   -40 kcal
END

SOLUTION 1
    temp      25
    pH        9.5
    pe        4
    redox     pe
    units     mol/l
    density   1
    C         0.00018
    Ca        1.082 charge
    Cl        0.73
    K         0.003
    Mg        0.097
    Na        0.39
    S(6)      0.017
    -water    1 # kg
END
SOLID_SOLUTIONS 2
    MontmoriloniteTry
        -comp Montmor-na 500
        -comp Montmor-ca 15
        -comp Montmor-k 0.05
        -comp Montmor-mg 0.03
END
USE solution 1
USE solid_solution 2
END