Simulate the carbonation of Mg(OH)2 with NaHCO3

[lrossi]

Hello everyone,

I am a beginner on PhreeqC and I would like to simulate a Brucite carbonation experiment done in the lab. I have two solutions: (1) 300ml of 0.5M Mg(OH)2, to which I add 300ml of 1M NaHCO3 at a flow rate of 1ml/min. After 5 hours of titration, I let the mixture equilibrate for several days.
Here's the script I made:

Code: [Select]


SOLUTION 1
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mol/L
    density   1
    -water    1 # kg

EQUILIBRIUM_PHASES 1
    CO2(g)    -3.5 1


REACTION 1
    Brucite    1
    0.5 moles in 1 steps

SAVE SOLUTION 1
END


SOLUTION 2
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mol/L
    density   1
    -water    1 # kg

EQUILIBRIUM_PHASES 2
    CO2(g)    -3.5 1

REACTION 2
    Nahcolite    1
    1 moles in 1 steps

SAVE SOLUTION 2
END

MIX 1
    1    1
    2    1
END

SELECTED_OUTPUT 1
    -file                 selected_output_1.xls
    -simulation           true
    -solution             true
    -pH                   true
    -totals               Mg
    -saturation_indices   Brucite
END

However, I have a question: is it possible to use PhreeqC to obtain results as a function of time?
I'd like to have the results of Mg concentrations over time, but my script won't let me.
Perhaps with the Keywords KYNETICS/ RATES? But I don't see how I can simulate the gradual addition of NaHCO3 if I use the KYNETICS/RATES keywords for Brucite.

Can you help me?
Thank you in advance!
Best regards,
LR

[dlparkhurst]

Here is a script that does the conservative mixing of the two solutions. It is not easy to do a series of MIX calculations, so the NaHCO3 solution was added with REACTION. This approach may not work in more complicated situations.

If you want to precipitate a solid, you will have to add EQUILIBRIUM_PHASES or KINETICS to account for additional reactions.

Code: [Select]

SOLUTION 1
EQUILIBRIUM_PHASES 1
    CO2(g)    -3.5 1
REACTION 1
    Brucite    1
    0.5 moles in 1 steps
SAVE SOLUTION 1
END

MIX 1
    1    0.3
REACTION
H2O 55.5
NaHCO3 1
0.300 in 30
USER_GRAPH 1
    -headings               rxn Mg Na
    -axis_titles            "mL 1M NaHCO3 added" "Molality" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X RXN*1000
20 GRAPH_Y TOT("Mg"), TOT("Na")
  -end
    -active                 true
END

[lrossi]

Great, thank you. That's what I was trying to do and the script works.
I have a question though: After the 5 hour Mg(OH)2 titration with NaHCO3 is complete, I continue to measure Mg and Cu concentrations over time for several days. Is there any way with PhreeqC to model this equilibration over time ? Or perhaps an alternative?


Thank you in advance,
Best regards,
LR

[dlparkhurst]

If concentrations are changing, I assume that solids are precipitating. You can start by adding EQUILIBRIUM_PHASES to estimate concentrations in equilibrium with the solids you think are precipitating. You my be precipitating a solid solution, in which case, you would use SOLID_SOLUTIONS for some of the phases.

If your concentrations continue to change with time, you are dealing with kinetic reactions. You can use KINETICS and RATES to model the time-dependent changes if it is worthwhile. You can certainly fit the rate with some rate expression, but I do not know how much transfer value the exercise would have.