Convergence issues perhaps due to alkalinity?



--- Code: ---Hi all,
I am having issues with convergence while trying to simply calculate saturation indices. My water samples are from column experiments treating AMD so I initially made these calculations using WATEQ4f database. These solutions had really high percent errors. I switched over to the minteq.v4 database as it has volatile organic acids which are very high in some of my samples. I am having issues with convergence which I suspect is due to differences in calculated alkalinity and measured total alkalinity. Total alkalinity was measured at the time of sampling through titration and reported in mg/l as CaCO3. any advice on how to make this sample converge would be appreciated.
[DATABASE C:\Program Files (x86)\USGS\Phreeqc Interactive 3.6.2-15100\database\minteq.v4.dat
    -units    mg/l
 Temperature    pH      Acetate     Fe         S(6) S(-2)     Ca     Mn           Br           Cl     Formate     K     Na     Mg      P     Si   Propionate            pe        Alkalinity
                                                                                                                          ug/l                                 as Ca0.5(CO3)0.5
          21 5.59 657.3452458 163.9 2652.729722   4.06 858.2 12.32 52.58985847 4.504965294 8.05079465 48.8 17.07 329.1 28680 13.12 362.4737729 -1.380772192              638.46
--- End code ---

Yes, I think it is related to alkalinity. The sum of Acetate, Propionate, and other contributors to the alkalinity of the solution is 1.457e-02 eq/kgw. Your specified alkalinity is 638.46/50 = 12.7 eq/kgw. PHREEQC uses the Alkalinity definition to calculate the concentration of TDIC (total dissolved inorganic carbon); however, it fails because Alkalinity would need to be a negative contribution. The calculation can only succeed if the Alkalinity defined is greater than the sum of the non-carbonate alkalinity.

I renamed Alkalinity to AlkalinityX and added PRINT; -alk to produce the following table, which accounts for the alkalinity of the solution with no contribution from carbon.

At pH less than 6.3, most TDIC is CO2(aq). You have a few options.

(1) You can define TDIC directly as C(4) if you have measure TDIC (if you acidify and measure CO2 produced).

(2) You can specify a partial pressure of CO2(g) to define TDIC [C(4)], the following defines your solution to be in equilibrium with the atmosphere:

--- Code: ---C(4) 1 CO2(g) -3.4 10

--- End code ---

(3) You can ignore C(4), or potentially there are other options.

--- Code: -----------------------------Distribution of alkalinity---------------------------

Total alkalinity (eq/kgw)  =   1.467e-02

Species          Alkalinity    Molality   Alk/Mol

Acetate-          8.684e-03   8.684e-03      1.00
Propionate-       4.039e-03   4.039e-03      1.00
Ca(Acetate)+      7.446e-04   7.446e-04      1.00
Mg(Acetate)+      6.248e-04   6.248e-04      1.00
Ca(Propionate)+   1.950e-04   1.950e-04      1.00
Mg(Propionate)+   1.419e-04   1.419e-04      1.00
Fe(Acetate)+      1.185e-04   1.185e-04      1.00
HPO4-2            2.786e-05   2.786e-05      1.00
CaHPO4            2.410e-05   2.410e-05      1.00
MgHPO4            2.220e-05   2.220e-05      1.00
FeHPO4            1.938e-05   1.938e-05      1.00
Fe(HS)2           1.148e-05   5.741e-06      2.00
Mn(Acetate)+      1.026e-05   1.026e-05      1.00
HS-               5.172e-06   5.172e-06      1.00
K(Acetate)        4.032e-06   4.032e-06      1.00
H+               -3.261e-06   3.261e-06     -1.00
Na(Acetate)       2.404e-06   2.404e-06      1.00
HSO4-            -1.762e-06   1.762e-06     -1.00
H(Formate)       -1.596e-06   1.596e-06     -1.00
H3PO4            -1.553e-07   1.553e-07     -1.00
KHPO4-            9.039e-08   9.039e-08      1.00
NaHPO4-           8.406e-08   8.406e-08      1.00
FeOH+             7.042e-08   7.042e-08      1.00
CaPO4-            5.747e-08   2.873e-08      2.00
H3SiO4-           1.452e-08   1.452e-08      1.00
Fe(HS)3-          9.700e-09   3.233e-09      3.00
MgOH+             4.590e-09   4.590e-09      1.00
S5-2              4.045e-09   2.023e-09      2.00
OH-               3.739e-09   3.739e-09      1.00
S6-2              1.516e-09   7.579e-10      2.00
S4-2              1.017e-09   5.084e-10      2.00
MgPO4-            5.994e-10   2.997e-10      2.00
MnOH+             3.847e-10   3.847e-10      1.00
CaOH+             3.542e-10   3.542e-10      1.00
S3-2              1.565e-10   7.823e-11      2.00
PO4-3             2.801e-11   1.401e-11      2.00
S2-2              1.473e-11   7.366e-12      2.00
Fe(OH)2           2.439e-13   1.219e-13      2.00
FeHPO4+          -1.828e-13   1.828e-13     -1.00
Fe(OH)3           1.358e-14   1.358e-14      1.00
FeOH+2           -7.632e-15   7.632e-15     -1.00
Fe(Acetate)3      1.838e-15   1.838e-15      1.00
H2SiO4-2          1.106e-15   5.530e-16      2.00
Fe(OH)3-          5.515e-16   1.838e-16      3.00
Fe(Acetate)+2    -4.362e-16   4.362e-16     -1.00
FeSO4+           -2.280e-16   1.140e-16     -2.00
Fe(Propionate)+2  -1.976e-16   1.976e-16     -1.00
FeH2PO4+2        -5.727e-17   2.863e-17     -2.00
Fe(SO4)2-        -3.507e-17   1.753e-17     -2.00
S-2               2.772e-17   1.386e-17      2.00
Fe+3             -2.529e-17   1.265e-17     -2.00
Fe(OH)4-          2.198e-17   1.099e-17      2.00
FeCl+2           -2.037e-20   1.018e-20     -2.00
Mn(OH)3-          1.483e-22   4.943e-23      3.00
FeCl2+           -5.136e-24   2.568e-24     -2.00
Fe2(OH)2+4       -1.857e-25   9.287e-26     -2.00
FeCl3            -4.088e-29   2.044e-29     -2.00
Mn(OH)4-2         5.054e-30   1.263e-30      4.00
Fe3(OH)4+5       -3.452e-34   1.726e-34     -2.00

--- End code ---


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