DIFF_C not working as intended

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khwee:
I tested both phreeqci and iphreeqccom 3.8.7, and saw that the values returned by phreeqc for DIFF_C did not account for the viscosity correction or concentration dependence. In the example given below, phreeqc returns the same value for DIFF_C("Na+") for the two solutions, where the second solution has 2 mole/kgw of NaCl.

I had reviewed an older thread, but didn't find any new info that could help with a fix.
https://phreeqcusers.org/index.php/topic,1853.0.html

--- Code: ---Database file: C:\Program Files (x86)\USGS\Phreeqc Interactive 3.8.7-17149\database\pitzer.dat

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Reading data base.
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SOLUTION_MASTER_SPECIES
SOLUTION_SPECIES
PHASES
PITZER
GAS_BINARY_PARAMETERS
EXCHANGE_MASTER_SPECIES
EXCHANGE_SPECIES
SURFACE_MASTER_SPECIES
SURFACE_SPECIES
MEAN_GAMMAS
END
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Reading input data for simulation 1.
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DATABASE C:\Program Files (x86)\USGS\Phreeqc Interactive 3.8.7-17149\database\pitzer.dat
SOLUTION 1
temp 25
SOLUTION 2
units mole/kgw
temp 25
Na 2
Cl 2
USER_PRINT
10 REM Calculation of concentration dependence of Na+ diffusion coefficient
20 dw = 1.33e-9 # SOLUTION_SPECIES
30 damp = 75 # SOLUTION_SPECIES
40 viscos_0_25 = 0.8900239182946
50 visc = 0.7037
60 dw_temp = dw * EXP(dw_T/TK - dw_T/298.15) * viscos_0_25/VISCOS
61 dw_temp_SC = dw_temp * (VISCOS_0 / VISCOS) ^ visc
70 PRINT "DIFF_C(Na+): ", DIFF_C("Na+")
80 PRINT "Dw(TK): ", dw_temp
90 PRINT "Dw(TK)_SC: ", dw_temp_SC
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Beginning of initial solution calculations.
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Initial solution 1.

----------------------------------User print-----------------------------------

DIFF_C(Na+): 1.3300e-09
Dw(TK): 1.3300e-09
Dw(TK)_SC: 1.3300e-09

-----------------------------Solution composition------------------------------

Elements Molality Moles

Pure water

----------------------------Description of solution----------------------------

pH = 7.000
pe = 4.000
Specific Conductance (uS/cm, 25oC) = 0
Density (g/cm3) = 0.99704
Volume (L) = 1.00297
Viscosity (mPa s) = 0.89002
Activity of water = 1.000
Ionic strength (mol/kgw) = 1.006e-07
Mass of water (kg) = 1.000e+00
Total alkalinity (eq/kg) = 1.216e-09
Temperature (oC) = 25.00
Electrical balance (eq) = -1.216e-09
Percent error, 100*(Cat-|An|)/(Cat+|An|) = -0.60
Iterations = 3
Gamma iterations = 2
Osmotic coefficient = 0.99988
Density of water = 0.99704
Total H = 1.110124e+02
Total O = 5.550622e+01

----------------------------Distribution of species----------------------------

MacInnes MacInnes
MacInnes Log Log Log mole V
Species Molality Activity Molality Activity Gamma cm3/mol

OH- 1.013e-07 1.012e-07 -6.995 -6.995 -0.000 -4.14
H+ 1.000e-07 1.000e-07 -7.000 -7.000 -0.000 0.00
H2O 5.551e+01 1.000e+00 1.744 -0.000 0.000 18.07

------------------------------Saturation indices-------------------------------

Phase SI** log IAP log K(298 K, 1 atm)

H2O(g) -1.50 -0.00 1.50 H2O

**For a gas, SI = log10(fugacity). Fugacity = pressure * phi / 1 atm.
For ideal gases, phi = 1.

Initial solution 2.

----------------------------------User print-----------------------------------

DIFF_C(Na+): 1.3300e-09
Dw(TK): 1.0996e-09
Dw(TK)_SC: 9.6186e-10

-----------------------------Solution composition------------------------------

Elements Molality Moles

Cl 2.000e+00 2.000e+00
Na 2.000e+00 2.000e+00

----------------------------Description of solution----------------------------

pH = 7.000
pe = 4.000
Specific Conductance (uS/cm, 25oC) = 145262
Density (g/cm3) = 1.07204
Volume (L) = 1.04183
Viscosity (mPa s) = 1.07649
Activity of water = 0.932
Ionic strength (mol/kgw) = 2.000e+00
Mass of water (kg) = 1.000e+00
Total alkalinity (eq/kg) = 1.238e-07
Temperature (oC) = 25.00
Electrical balance (eq) = -1.238e-07
Percent error, 100*(Cat-|An|)/(Cat+|An|) = -0.00
Iterations = 6 (9 overall)
Gamma iterations = 4
Osmotic coefficient = 0.98407
Density of water = 0.99704
Total H = 1.110124e+02
Total O = 5.550622e+01

----------------------------Distribution of species----------------------------

MacInnes MacInnes
MacInnes Log Log Log mole V
Species Molality Activity Molality Activity Gamma cm3/mol

OH- 1.980e-07 9.429e-08 -6.703 -7.026 -0.322 -0.62
H+ 7.423e-08 1.000e-07 -7.129 -7.000 0.129 0.00
H2O 5.551e+01 9.315e-01 1.744 -0.031 0.000 18.07
Cl 2.000e+00
Cl- 2.000e+00 1.148e+00 0.301 0.060 -0.241 19.29
Na 2.000e+00
Na+ 2.000e+00 1.558e+00 0.301 0.193 -0.108 0.14

------------------------------Saturation indices-------------------------------

Phase SI** log IAP log K(298 K, 1 atm)

H2O(g) -1.53 -0.03 1.50 H2O
Halite -1.33 0.25 1.58 NaCl

**For a gas, SI = log10(fugacity). Fugacity = pressure * phi / 1 atm.
For ideal gases, phi = 1.

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End of simulation.
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Reading input data for simulation 2.
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------------------------------
End of Run after 0.03 Seconds.
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--- End code ---

dlparkhurst:
To have an ionic strength effect on the diffusion coefficient, it is necessary to define a_v_dif, the 7th parameter of -Dw in SOLUTION_SPECIES for each ion.

So far, I don't think any of the databases have these parameters defined. They range from about 0.5 to 0.9. Tony is working on a paper describing the viscosity model, and I think will add the parameters after it is published.

khwee:
Thank you for clarifying. I look forward to the next update that will complete this feature!

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