surface complexation model

[sharmilan]

Dear Phreeqc experts,

for my surface complexation, I am defining the surface site of CSH-llss phase through the function of
surface binding-site formula, name, [(equilibrium_phase or kinetic_reactant)], sites_per_mole, specific_area_per_mole.

     SURFACE 1
     -equilibrate with solution 1
     Surf_sOH Tob-II solid solution 5.233440000000E-01 6.541800000000E+04
     Surf_sOH Jennite solid solution 7.655160000000E-01 9.568950000000E+04
     -no_edl

here, I am using the "solid solution" phrase for "equilibrium_phase" because I have included the CSH-llss phase as the solid solution phase (see script below).


-------------------------------Phase assemblage--------------------------------

                                                      Moles in assemblage
Phase               SI  log IAP  log K(T, P)   Initial       Final       Delta

Amor-Sl          -6.20    -8.96     -2.76    0.000e+00           0   0.000e+00
CO2(g)          -13.49   -31.64    -18.16    0.000e+00           0   0.000e+00
Cal               0.00    -8.45     -8.45    2.919e-01   2.919e-01  -1.821e-14
Gbs              -2.68   -18.03    -15.35    0.000e+00           0   0.000e+00
Gp               -2.84    -7.42     -4.58    0.000e+00           0   0.000e+00
Portlandite       0.00    23.19     23.19    7.806e+00   7.806e+00   5.329e-14
hydrotalcite      0.00    28.69     28.69    1.183e-01   1.183e-01   0.000e+00
monocarbonate     0.00    25.03     25.03    3.365e-01   3.365e-01   1.826e-14
natrolite        -8.22   -38.90    -30.68    0.000e+00           0   0.000e+00
straetlingite    -2.88     1.35      4.23    0.000e+00           0   0.000e+00
zeoliteX        -13.98   -34.45    -20.47    0.000e+00           0   0.000e+00

--------------------------------Solid solutions--------------------------------


Solid solution                Component        Moles  Delta moles   Mole fract

CSH-llss                                    6.09e+00
                                 Tob-II     7.60e-01    -4.66e-15     1.25e-01
                                Jennite     5.33e+00     4.44e-15     8.75e-01
MSHss                                       0.00e+00
                                 M075SH     0.00e+00     0.00e+00     0.00e+00
                                  M15SH     0.00e+00     0.00e+00     0.00e+00
Si&Fe_HGss                                  4.15e-02
                          C3AS0.84H4.32     1.88e-02     6.14e-16     4.53e-01
                          C3FS0.84H4.32     2.27e-02     3.47e-18     5.47e-01
hys_Aftss                                   1.64e-01
                             ettringite     1.33e-01    -1.55e-14     8.10e-01
                           ettringite30     3.12e-02    -3.38e-15     1.90e-01


but, the defined surface complexation does not work. could you advise me on how to define the solid solution phase (CSH-llss ) in the surface complexation model?

Thank you
S.Sharmilan

[dlparkhurst]

"Solid_solution" is not an option in SURFACE definitions. A surface can only be related to a phase in EQUILIBRIUM_PHASES or a kinetic reaction in KINETICS. It also is not possible to relate the surface to a single component of a solid solution.

You may be able to use KINETICS to define a reaction that generates an amount of kinetic reactant equal to the moles of components in a solid solution. The script below works by defining a surface in SURFACE related to a kinetic reaction in KINETICS. The -formula of KINETICS has no effect on the solution composition, however the moles of kinetic reaction should adjust to the the moles of components in one of the solid solutions. The moles and surface area of the surface adjust according to the number of moles in the kinetic reaction.

The rate of the reaction is based on the difference between the moles of kinetic reaction and the moles of solid solution. This kind of reaction involving M can have numerical issues. I have arbitrarily set the rate to approach equilibrium about 10 times faster than the time step. It may be fairly stable if the number of moles of kinetic reaction does not have to change a lot. -cvode may have more problems than the default Runge-Kutta. You will just have to test what works.

Code: [Select]

SURFACE_MASTER_SPECIES
    Surf_w        Surf_w       
SURFACE_SPECIES
Surf_w = Surf_w
    log_k     0
Na+ + Surf_w = Surf_wNa+
    log_k     0.1
Ca+2 + Surf_w = Surf_wCa+2
    log_k     0.1
Mg+2 + Surf_w = Surf_wMg+2
    log_k     0.2

RATES
    CSH-llss_Surface
-start
10 tot_ss = S_S("Jennite") + S_S("Tob-II")
20 moles = (M - tot_ss)/10
30 SAVE moles*TIME
-end
SOLUTION 1
    temp      25
    pH        10 charge
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    Ca        1
    Mg        1
    Na        1
    S(6)      3
    -water    1 # kg
END
SOLID_SOLUTIONS 1
    CSH-llss
        -comp Jennite 6
        -comp Tob-II 0
END
KINETICS 1
CSH-llss_Surface
    -formula  H  0
    -m        5.9
-steps       100
END
SURFACE 1
    Surf_w  CSH-llss_Surface kinetic           0.5    100000
END
RUN_CELLS
    -cells 1
END


[pawanagra0@gmail.com]

Hi,

It might be a silly question but can anyone please tell me if we can use the equation written below:

S_sOH2+ + Na+ = S_sOH2Na2+
log_k ....

[dlparkhurst]

You can if you have additional definitions. The following is an example with arbitrary definitions of surface species, log Ks, surface sites, and surface areas.

Code: [Select]

SURFACE_MASTER_SPECIES
    S_s           S_sOH   
SURFACE_SPECIES
S_sOH = S_sOH
log_k 0

S_sOH + H+ = S_sOH2+
-log_k 7.29

S_sOH2+ + Na+ = S_sOH2Na+2   
-log_k 1
END
SOLUTION 1
Na 1
Cl 1
END
SURFACE
S_sOH 1 600 1
-eq 1
END



[pawanagra0@gmail.com]

Hello Dr.

Thank you so much for your answer.

But can you please explain a bit about how the protonated positive site (S_sOH2+) can attract positively charged Na+ ions? Since they both have positive charges.

S_sOH2+ + Na+ = S_sOH2Na+2

Thank you
pawan

[dlparkhurst]

You only asked whether it could be done. It is up to you to decide on realistic species and log Ks.

[pawanagra0@gmail.com]

I am writing this equation exactly in the same way as you have explained here.

SURFACE_MASTER_SPECIES
    S_s           S_sOH   
SURFACE_SPECIES
S_sOH = S_sOH
log_k 0
.
.
.

And it's fitting my data as well. but I do not have any logical explanation for this (S_sOH2+ + Na+ = S_sOH2Na+2) equation. As u said it is not a realistic species.

Thank you for your time.
Pawan