Error convergence modele

[m.gamrani]

hello
I find a problem that manifests in the inability of the phreeqc to converge my solutions.
for example :
 Error: Al has not converged. Total: 3.622960e-04 Calculated: 7.714500e-04 Residual: -4.091540e-04.
          Alkalinity has not converged. Total: 3.703026e-04 Calculated: 1.912204e-03 Residual: -1.541901e-03.

I want to know how I can solve this problem.

[dlparkhurst]

If you attach your input file and database, I will look at it.

[m.gamrani]

I use the data.base llnl

My input file  :


SOLUTION_MASTER_SPECIES
Si SiO2 0 SiO2 28.0843

SOLUTION_SPECIES
SiO2 + 2H2O = H3SiO4- + H+
      -log_k  -9.83; -delta_h 6.12 kcal
      -analytic   -302.3724   -0.050698   15669.69    108.18466   -1119669.0
      -Vm  7.94  1.0881  5.3224  -2.8240  1.4767 # supcrt + H2O in a1

SiO2 + 2H2O  = H2SiO4-2 + 2 H+
      -log_k  -23.0;  -delta_h 17.6 kcal
      -analytic   -294.0184   -0.072650   11204.49    108.18466   -1119669.0

SOLUTION 1 # eau de nappe en sol acide: différentes phases minérales du sol
   Units mmol/L
   pH 6.34
   Alkalinity 0.37
   F 0.00
   Cl 0.08
   Br 0.00
   S(6) 0.03
   N(5) 0.07
   N(-3) 0.05
   Ca 0.06
   Mg 0.02
   Na 0.20
   K 0.17
   Si 12.44
   Al 0.362
   Fe 0.358


EQUILIBRIUM_PHASES 1
CO2(g) -2

SAVE Solution 1

PHASES
ferribeidelite
(K0.05 Ca0.05 Mg0.07)(Si3.18Al0.82 )O10 (Al1.69 Fe0.46 Mg0.04)(OH)2  + 9.28H+ = \
2.51Al+3 + 0.05Ca+2 + 0.46Fe+3 + 0.05K+ + 0.11Mg+2 + 3.18SiO2 + 5.64H2O           
    log_k 7.581

Fe-Kaolinite
Al1.9Fe0.1Si2O5(OH)4 + 6H+ = 1.9Al+3 + 0.1Fe+3 + 2SiO2 + 5H2O
    log_k 6.471 
   

END

thank you sir for your return.

[dlparkhurst]

First, the cause of your problem is that non-carbonate alkalinity in SOLUTION 1 is greater than the specified alkalinity.

If you remove the Alkalinity definition from SOLUTION 1 and add PRINT; -alkalinity, you get the following "Distribution of alkalinity" following the "Distribution of species" (first few lines):

Code: [Select]

Total alkalinity (eq/kgw)  =   9.042e-04

Species          Alkalinity    Molality   Alk/Mol

Al13O4(OH)24+7    8.840e-04   2.763e-05     32.00
AlO2-             4.535e-06   1.134e-06      4.00
HAlO2             4.325e-06   1.442e-06      3.00
H3SiO4-           4.210e-06   4.210e-06      1.00

This output shows that the alkalinity due to Al is about 0.9 mol/L, whereas you specified an alkalinity of 0.3. PHREEQC adjusts the carbon concentration to attain the desired alkalinity, but if non-carbonate alkalinity is already more than the specified alkalinity, there is no concentration of carbon that can be found, and convergence fails.

Now, I have always been skeptical of the llnl.dat species Al13O4(OH)24+7. Seems physically unlikely, but even if you remove it, aluminum alkalinity is almost the same due to AlO2-. So, as long as you use the specified Al and pH, the Alkalinity must be somewhat greater than 0.9 meq/kgw for PHREEQC to succeed with the initial SOLUTION calculation. Alternatively, you can remove the Alkalinity definition, or define total dissolved inorganic carbon (C (4)).

A final note, llnl.dat has aqueous species SiO2, HSiO3-, and H2SiO4--, plus some additional silica complexes. I prefer the way most databases have H4SiO4, H3SiO4-, and H2SiO4-2, but the key is that H3SiO4- is equivalent to HSiO3- + H2O. PHREEQC recognizes that H2SiO4-2 and H2SiO4-- are the same species, so your definition replaces the one in the llnl.dat database. However, PHREEQC thinks that HSiO3- and H3SiO4- are two separate species, so you are including this species twice. To remove llnl's HSiO3-, you should define it in SOLUTION_SPECIES in the input file with a much smaller log K (making its concentration irrelevant) or remove it from llnl.dat. Alternatively, you can omit your definitions for SOLUTION_MASTER_SPECIES and SOLUTION_SPECIES in the input file and simply use the llnl.dat definitions.

[m.gamrani]

Thank you, sir,
I want to thank you for your time and your relevant response.
I was really stuck in this problem even though I consulted the 1995 user guide, and the documentation.
I thank you for your time and your support.