Conceptual Models > Equilibrium assumptions

Alkalinity balance

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GeeqC:
Hi everyone, dear David Parkhurst,

I am coming back with a problem of alkalinity:
In some solutions (e.g. the one below), the alkalinity of the output is changed. In other contributions to the forum it says that an alert appears if not enough weak acids are available that could act as carrier of the alkalinity.

But how is the alkalinity actually modified? Based on what assumptions? At the same time, C(4) and charge balance remained more or less unchanged.

Besides, also in solutions with sufficient weak acids, alkalinity is sometimes changed, although not as strongly. Apparently, there is a command to display all the species contributing to alkalinity (-alk to PRINT?) but it didn't work so far.

SOLUTION_SPREAD
    -temp     10
    -units    mmol/l

Alkalinity  13.612
C(4)         6.17
N(-3)        0.68
S(6)         0.00
S(-2)        4.57
Si             0.65
P              0.44
Ca            3.02
Mg           11.25
Na            91.58
K              1.95
Cl             109.14    charge

SELECTED_OUTPUT 1
    -file                 Porewater_solution.xls
    -simulation           true
    -solution             true
    -pH                   true
    -reaction             true
    -alkalinity           true
    -ionic_strength       true
    -charge_balance       true
    -totals               Cl  C(4)
    -saturation_indices   Calcite  CO2(g)

Best wishes
Patrick

dlparkhurst:
To print the contributions to alkalinity, use the following:


--- Code: ---PRINT
-alk

--- End code ---

The alkalinity is different because of the conversion from mmol/L to mol/kgw (actually eq/L and eq/kgw). I assume that is the answer, but post again if you still have questions.

GeeqC:
The PRINT -alk function works now. A list with all species contributing to alkalinity is included in the output file.

I am using mmol/l because the data are given in this unit. Adding:
    -density 1 calc
    -water 1 # kg
returns totals close to the input values.

A message appears: "pH will be adjusted to obtain desired alkalinity".
Indeed, the alkalinities in the unreacted output solutions are fine. Only after the reaction step, alkalinities are by up to 2 meq/l lower:

REACTION_PRESSURE 1-36
    2.0000
REACTION_TEMPERATURE 1-36
    10
END

RUN_CELLS
    -cells 1-36
END

Something seems to modify the alkalinity there.

dlparkhurst:
I did not notice that you have defined both C(4) and Alkalinity in the SOLUTION definition. PHREEQC treats that as a special case. Of pH, Alkalinity, and C(4), only two of these items can be defined independently; the third can be calculated from the other two. So, in your case, the pH is adjusted to be consistent with the Alkalinity and C(4) that you have defined.

Usually, you do not want to make this calculation if you have a reliable pH. The most common situation is pH and Alkalinity, which you should probably use if you have these two measurements; TDIC will be calculated from the two values.

The change in alkalinity is due to redox reactions when a "reaction" calculation occurs. For all calculations other than the SOLUTION (initial solution) calculation, redox equilibrium will be calculated. Initial solution calculations allow for redox disequilibrium; for example, O2(aq) can coexist with S(-2) in the SOLUTION definition. If you run the following, you will find that the reaction solution contains C(-4), N(0), and S(6), which were not present in the initial solution. The result of these reactions changes the alkalinity. You can see the change in alkalinity species in the detailed print from PRINT; -alk.


--- Code: ---SOLUTION 1
    -temp     10
    -units    mmol/l

Alkalinity  13.612
C(4)         6.17
N(-3)        0.68
S(6)         0.00
S(-2)        4.57
Si             0.65
P              0.44
Ca            3.02
Mg           11.25
Na            91.58
K              1.95
Cl             109.14    charge
END
RUN_CELLS
-cell 1
END

--- End code ---

GeeqC:
... I ran your example and compared the species contributing to alkalinity before and after reaction. Further sulphur species appear, as you say. I assume this is because sulphur is the only element for which two redox states are given with the input data.

The question is now, what can we do to bring back the alkalinity to the original value?
- Option 1: not running the reaction step.
- Option 2: is there a way to keep the alkalinity constant or to re-adjust the alkalinity after reaction (and change the pH instead)?

P.S. The reason why I use alkalinity is because it was measured. The measured pH may not be reliable due to escape of CO2.

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