Difference between -equilibrate and USE

[FlouretAlex]

Hello PHREEQC user’s
I would like to know the differences between the function, “-equilibrate” and “use”.
Indeed, with the same code but by using the two functions I do not have the same result exactly, thus I would like to know what are the differences between those two codes.

PHREEQC code with the function USE

Code: [Select]


SELECTED_OUTPUT
-reset false
-file C:\Users\International\Desktop\VF_model\batch\RHIZOsoil_batch.sel
-high_precision false
USER_PUNCH
-headings logCsaq   logKd   Csini   Csaq Kaq Naaq Caaq Mgaq   Kd
-start
10 Msoil = 0.35/1000 # soil_masse(Kg)
20 Vwater = 7/1000 # volume batch (L)
30 MVw = Msoil/Vwater #Kg/L
40 VwM = Vwater/Msoil #L/Kg
50 Csaq = tot("Cs") #(mol/L)
60 surfCs = (surf("Cs","Illitex")+ surf("Cs","Montx")+ surf("Cs","Kaolinitex")) #(mol/L)
70 exchCs = (mol("CsIlli")+ mol("CsMont")+ mol ("CsKao")) #(mol/L)
80 Csfix = surfCs + exchCs #(mol/L)
90 Kd = (Csfix/Csaq)*VwM #(L/Kg)


200 punch log10(Csaq), log10(Kd), Csini, Csaq, Kaq, Naaq, Caaq, Mgaq, Kd,
-end
 
SURFACE 1
 -sites_units density
#Site density SSA        Mass echantillion
#(site/nm²) (m²/g)      (g)==> (ratio g/L)
Illitex  3.91E-04 97 50 # 14.5%
Montx 1.53E-06 800 50 # 4.2%
Kaolinitex 1.15E-05 10 50 # 7.6%

EXCHANGE 1
Illi 1.63E-03
Mont 1.84E-03
Kao 7.64E-05

Solution 1
units mol/L
pH 5.5
Na 0.05
Cl 0.05 charge
Ca 0.05
C(4) 0.05
use surface 1
use exchange 1
Equilibrium_phases 1
Fix_H+ -5.5   HCl
save surface 1
save exchange 1
end

Solution 2 ; units mol/L ; pH 5.5 ; Na 0.05 ; Cl 0.05 charge ; Ca 0.05 ; C(4) 0.05 ; Cs 1.08E-08 ; use surface 1 ; use Exchange 1 ; EQUILIBRIUM_PHASES 2 ; Fix_H+ -5.5 HCl 1 ; end
Solution 3 ; units mol/L ; pH 5.5 ; Na 0.05 ; Cl 0.05 charge ; Ca 0.05 ; C(4) 0.05 ; Cs 1.18E-07 ; use surface 1 ; use Exchange 1 ; EQUILIBRIUM_PHASES 3 ; Fix_H+ -5.5 HCl 1 ; end
Solution 4 ; units mol/L ; pH 5.5 ; Na 0.05 ; Cl 0.05 charge ; Ca 0.05 ; C(4) 0.05 ; Cs 1.22E-06 ; use surface 1 ; use Exchange 1 ; EQUILIBRIUM_PHASES 4 ; Fix_H+ -5.5 HCl 1 ; end
Solution 5 ; units mol/L ; pH 5.5 ; Na 0.05 ; Cl 0.05 charge ; Ca 0.05 ; C(4) 0.05 ; Cs 1.18E-05 ; use surface 1 ; use Exchange 1 ; EQUILIBRIUM_PHASES 5 ; Fix_H+ -5.5 HCl 1 ; end
Solution 6 ; units mol/L ; pH 5.5 ; Na 0.05 ; Cl 0.05 charge ; Ca 0.05 ; C(4) 0.05 ; Cs 1.21E-04 ; use surface 1 ; use Exchange 1 ; EQUILIBRIUM_PHASES 6 ; Fix_H+ -5.5 HCl 1 ; end
Solution 7 ; units mol/L ; pH 5.5 ; Na 0.05 ; Cl 0.05 charge ; Ca 0.05 ; C(4) 0.05 ; Cs 1.09E-03 ; use surface 1 ; use Exchange 1 ; EQUILIBRIUM_PHASES 7 ; Fix_H+ -5.5 HCl 1 ; end
Solution 8 ; units mol/L ; pH 5.5 ; Na 0.05 ; Cl 0.05 charge ; Ca 0.05 ; C(4) 0.05 ; Cs 1.01E-02 ; use surface 1 ; use Exchange 1 ; EQUILIBRIUM_PHASES 8 ; Fix_H+ -5.5 HCl 1 ; end

PHREEQC code with the function equilibrate

Code: [Select]

SELECTED_OUTPUT
-reset false
-file C:\Users\International\Desktop\VF_model\batch\RHIZOsoil_batch_equi.sel
-high_precision false
USER_PUNCH
-headings logCsaq   logKd   Csini   Csaq Kaq Naaq Caaq Mgaq   Kd
-start
10 Msoil = 0.35/1000 # soil_masse(Kg)
20 Vwater = 7/1000 # volume batch (L)
30 MVw = Msoil/Vwater #Kg/L
40 VwM = Vwater/Msoil #L/Kg
50 Csaq = tot("Cs") #(mol/L)
60 surfCs = (surf("Cs","Illitex")+ surf("Cs","Montx")+ surf("Cs","Kaolinitex")) #(mol/L)
70 exchCs = (mol("CsIlli")+ mol("CsMont")+ mol ("CsKao")) #(mol/L)
80 Csfix = surfCs + exchCs #(mol/L)
90 Kd = (Csfix/Csaq)*VwM #(L/Kg)


200 punch log10(Csaq), log10(Kd), Csini, Csaq, Kaq, Naaq, Caaq, Mgaq, Kd,
-end
 
SURFACE 1
 -sites_units density
 -equilibrate with solution 1
#Site density SSA        Mass echantillion
#(site/nm²) (m²/g)      (g)==> (ratio g/L)
Illitex  3.91E-04 97 50 # 14.5%
Montx 1.53E-06 800 50 # 4.2%
Kaolinitex 1.15E-05 10 50 # 7.6%

EXCHANGE 1
 -equilibrate with solution 1
Illi 1.63E-03
Mont 1.84E-03
Kao 7.64E-05


Solution 1
units mol/L
pH 5.5
Na 0.05
Cl 0.05 charge
Ca 0.05
C(4) 0.05
Equilibrium_phases 1
Fix_H+ -5.5   HCl
save surface 1
save exchange 1
END

Solution 2 ; units mol/L ; pH 5.5 ; Na 0.05 ; Cl 0.05 charge ; Ca 0.05 ; C(4) 0.05 ; Cs 1.08E-08 ; use surface 1 ; use Exchange 1 ; EQUILIBRIUM_PHASES 2 ; Fix_H+ -5.5 HCl 1 ; end
Solution 3 ; units mol/L ; pH 5.5 ; Na 0.05 ; Cl 0.05 charge ; Ca 0.05 ; C(4) 0.05 ; Cs 1.18E-07 ; use surface 1 ; use Exchange 1 ; EQUILIBRIUM_PHASES 3 ; Fix_H+ -5.5 HCl 1 ; end
Solution 4 ; units mol/L ; pH 5.5 ; Na 0.05 ; Cl 0.05 charge ; Ca 0.05 ; C(4) 0.05 ; Cs 1.22E-06 ; use surface 1 ; use Exchange 1 ; EQUILIBRIUM_PHASES 4 ; Fix_H+ -5.5 HCl 1 ; end
Solution 5 ; units mol/L ; pH 5.5 ; Na 0.05 ; Cl 0.05 charge ; Ca 0.05 ; C(4) 0.05 ; Cs 1.18E-05 ; use surface 1 ; use Exchange 1 ; EQUILIBRIUM_PHASES 5 ; Fix_H+ -5.5 HCl 1 ; end
Solution 6 ; units mol/L ; pH 5.5 ; Na 0.05 ; Cl 0.05 charge ; Ca 0.05 ; C(4) 0.05 ; Cs 1.21E-04 ; use surface 1 ; use Exchange 1 ; EQUILIBRIUM_PHASES 6 ; Fix_H+ -5.5 HCl 1 ; end
Solution 7 ; units mol/L ; pH 5.5 ; Na 0.05 ; Cl 0.05 charge ; Ca 0.05 ; C(4) 0.05 ; Cs 1.09E-03 ; use surface 1 ; use Exchange 1 ; EQUILIBRIUM_PHASES 7 ; Fix_H+ -5.5 HCl 1 ; end
Solution 8 ; units mol/L ; pH 5.5 ; Na 0.05 ; Cl 0.05 charge ; Ca 0.05 ; C(4) 0.05 ; Cs 1.01E-02 ; use surface 1 ; use Exchange 1 ; EQUILIBRIUM_PHASES 8 ; Fix_H+ -5.5 HCl 1 ; end


[dlparkhurst]

The difference is caused by the conceptual model of the initial state of the SURFACE and EXCHANGE sites. I'll talk initially about SURFACE, but the same applies to EXCHANGE.

When you use -eq 1, you are assuming the initial state of the surface is that which is in equilibrium with solution 1. The calculation adjusts the surface composition to be in equilibrium with solution 1, without any change to the composition of solution 1.

When you do not use -eq, then the initial surface composition is as you define it. For Hfo_w, you might choose to define SURFACE as

Code: [Select]

SURFACE 1
Hfo_wOH ...

This would define no sorbed elements other than O and H. When you then use USE solution and USE surface, the solution and the surface will react, changing both the composition of the solution and the surface.

The two calculations can be quite different, and it could lead to a completely erroneous simulation. If you define the following:

Code: [Select]

SOLUTION
EXCHANGE
X 1
END

The simulation will fail because there are no cations in the initial exchange composition, and there are no exchangeable cations in solution (no definition of HX is present in most databases).  Even if HX were defined, the following reaction would occur:

X- + H2O = HX + OH-

So, if there were a mole of exchange sites, the reaction would generate a mole of OH- or pH 14. A more physically reasonable definition would be to use CaX2, NaX, etc in the EXCHANGE definition so that you specify explicitly the cations of the initial exchange composition.

[FlouretAlex]

Hello,
Thank you for your reply this is really helpful, now I see the differences!
Have a good day.
Alex