changes in calcite SI as CO2 is added (dissolved)

[deso]

Hi PHREEQC users,

I have a number of solutions with positive calcite SI in a treatment train. I want to simply demonstrate how much CO2 would need to dissolve to bring the calcite SI down to negative (i.e. calcite will no longer precipitate). By modifying example 2, I think I managed to do a similar exercise for changes in temp, but I am getting something wrong with the CO2 scenario (below). I keep getting error messages: AH2O Activity of water has not converged.

TITLE Example 2.--CO2 needed to maintain calcite SI < 0
SOLUTION_SPREAD
   units mg/l
   number   pH   temp   S(6)   Alkalinity   Cl   Ca   Mg   Na   K   Si   F   pressure
               mg/L as HCO3                     
   1   8.76   15.98   0   2574.2   2610   20   4   2640   11   31.7   4.8   1.1             
       
REACTION_CO2 1
C(4)         
100 moles in 50 steps
SELECTED_OUTPUT
        -file   ex2.sel
        -temperature
        -si     calcite
USER_GRAPH 1 Example 2
        -headings Temperature Calcite
        -chart_title "Calcite SI"
        -axis_titles "CO2 mM" "Saturation index"
        -initial_solutions false
  -start
  10 graph_x TC
  20 graph_y SI("calcite")
  -end

[dlparkhurst]

I don't quite follow your input file, with temperature on the X axis, but from your description, I think the following input file will work. The error was probably caused by the large amounts of CO2 that you were adding.

TITLE Example 2.--CO2 needed to maintain calcite SI < 0
SOLUTION 1
units mg/L
pH    8.76
temp   15.98
pressure   1.1             
S(6)   0
Alkalinity   2574.2  mg/L as HCO3
Cl   2610
Ca   20
Mg   4
Na   2640
K   11
Si   31.7
F   4.8
END
USE solution 1
REACTION 1
CO2 1         
0.01 moles in 10 steps
USER_GRAPH 1 Example 2
        -headings Temperature Calcite
        -chart_title "Calcite SI"
        -axis_titles "CO2 mM" "Saturation index"
        -initial_solutions false
  -start
  10 graph_x RXN
  20 graph_y SI("calcite")
  -end

A more elegant approach would be

USE solution 1
EQUILIBRIUM_PHASES
Calcite 0 CO2(g) 10
END

However, there are two ways to reach calcite equilibrium, (1) addition of CO2 to lower the pH and CO3-2 activity, and (2) removal of CO2 to increase the pH, but also lower the CO3-2 activity. Unfortunately, which of the two is calculated is somewhat random. When I did it, the CO2 was removed and pH went up to 12.

[deso]

Thanks a lot dlparkhurst. I am assuming that the RXN command in the graphing code is stating that the reaction with CO2 is to be used on the x axis. Is this correct...I hadn't come across RXN command before.
I think I follow your corrected code. Was a fundamental error of mine that I did not have a positive "1" after CO2? As I understand now the code:

CO2 1

states CO2 is to be added...but if it was

CO2 -1

CO2 would be removed..?


I also tried a few examples with EQUILIBRIUM PHASE but. i can see from your example it chooses its own pathway (adds CO2, not remove).

Appreciate the swift reply

deso

[dlparkhurst]

I think a coefficient is required after the reactant. The reactant can be a chemical formula or a mineral name. The coefficient is used to specify the relative amounts of different reactants, and is most useful in the case of multiple reactants.

So, there is a coefficient and, on a separate line, there is an amount of reaction in moles. If the product of the coefficient and the moles of reaction is positive, the reactant increases the concentrations in solution. If the product of the coefficient and the moles of reaction is negative, concentrations in solution decrease.

[deso]

many thanks dlparkhurst. I understand now
BTW - what is the RXN command? I can't seem to find information on that in manual or web

I want to run the same simulation at different pressures and temperatures. Using the same code you provided, but adding
REACTION_PRESSURE 4
10 100 200 400
I can get the output to run the same simulation at the four different pressures, but I can't get the user graph to show the results of the simulations at the four different pressures....hence my questions about what RXN means in the USER GRAPH commands

[deso]

BTW - my problem is not knowing how to recall each simulation for the user graph curve. There should be four curves on the graph, one for each pressure or temperature simulation. When I look at the USER GRAPH examples that have multiple curves, they all plot different simulations or results of different elements. I want to plot the same simulation at a different temp/pressure.

[dlparkhurst]

RXN is a Basic function described in "The Basic Interpreter" section of the manual.

You need to repeat the REACTION calculation at different temperature or pressure. This sequence calculates
the reaction for 10 and 100 atm.

TITLE Example 2.--CO2 needed to maintain calcite SI < 0
SOLUTION 1
units mg/L
pH    8.76
temp   15.98
pressure   1.1             
S(6)   0
Alkalinity   2574.2  mg/L as HCO3
Cl   2610
Ca   20
Mg   4
Na   2640
K   11
Si   31.7
F   4.8
END
USER_GRAPH 1 Example 2
    -headings               Temperature 10_atm
    -axis_titles            "CO2 mM" "Saturation index" ""
    -chart_title            "Calcite SI"
    -initial_solutions      false
    -connect_simulations    false
    -plot_concentration_vs  x
  -start
10 graph_x RXN
20 graph_y SI("calcite")
  -end
    -active                 true
END
USE solution 1
REACTION_PRESSURE
10
REACTION 1
CO2 1         
0.01 moles in 10 steps

END
USER_GRAPH 1 Example 2
    -headings               Temperature 100_atm
USE solution 1
USE reaction 1
REACTION_PRESSURE
100
END