Kinetic Dissolution of Calcite

[Renmori]

Hi there,

I am super new to PHREEQC, and having a very hard time following manual examples.

Really struggling to set up a kinetic process.

Would anyone care to give an example with something I can relate to and understand?

For instance:

Pure water
Calcite
CO2 partial pressure in air

I want to see how the Ca concentration and pH evolve overtime in the solution until equilibrium is attained.

This would give me some clues on how all of this works.

I would really appreciate any help or pointers.

Many thanks in advance.

[dlparkhurst]

There was a recent discussion of calcite kinetics:

http://phreeqcusers.org/index.php/topic,1131.msg3392.html#msg3392

[Renmori]

There was a recent discussion of calcite kinetics:

http://phreeqcusers.org/index.php/topic,1131.msg3392.html#msg3392

Thank you.  So I am trying to follow this, and I'm trying to run this for my example in the post:

Code: [Select]

Calcite
   -start
1   REM   PARM(1) = specific surface area of calcite, cm^2/mol calcite
2   REM   PARM(2) = exponent for M/M0
10  si_cc = SI("Calcite")
20  IF (M <= 0  and si_cc < 0) THEN GOTO 200
30  k1 = 10^(0.198 - 444.0 / TK )
40  k2 = 10^(2.84 - 2177.0 /TK )
50  IF TC <= 25 THEN k3 = 10^(-5.86 - 317.0 / TK)
60  IF TC > 25 THEN k3 = 10^(-1.1 - 1737.0 / TK )
80  IF M0 > 0 THEN area = PARM(1)*M0*(M/M0)^PARM(2) ELSE area = PARM(1)*M
110 rate = area * (k1 * ACT("H+") + k2 * ACT("CO2") + k3 * ACT("H2O"))
120 rate = rate * (1 - 10^(2/3*si_cc))
130 moles = rate * 0.001 * TIME # convert from mmol to mol
200 SAVE moles
   -end
END



SOLUTION 1 Pure Water
    temp      25
    pH        7
    pe        4
END



KINETICS 1
Calcite
 -tol   1e-8
 -m0    10
 -m     10
 -parms 1.67e5   0.6  # cm^2/mol calcite, exp factor
 -time  1 day

USE solution 1

EQUILIBRIUM_PHASES 1
    CO2(g)    -3.58 10
No idea how to plot, but it doesn't run anyway.  Why?

[dlparkhurst]

First, you need the keyword RATES for the calcite definition.
Looks like the default integrator has problems because your rate is so fast, about 10 s to equilibrium.

Why didn't you start with the code from the other example, which would have worked and modified from there?

Code: [Select]

RATES
Calcite
   -start
1   REM   PARM(1) = specific surface area of calcite, cm^2/mol calcite
2   REM   PARM(2) = exponent for M/M0
10  si_cc = SI("Calcite")
20  IF (M <= 0  and si_cc < 0) THEN GOTO 200
30  k1 = 10^(0.198 - 444.0 / TK )
40  k2 = 10^(2.84 - 2177.0 /TK )
50  IF TC <= 25 THEN k3 = 10^(-5.86 - 317.0 / TK)
60  IF TC > 25 THEN k3 = 10^(-1.1 - 1737.0 / TK )
80  IF M0 > 0 THEN area = PARM(1)*M0*(M/M0)^PARM(2) ELSE area = PARM(1)*M
110 rate = area * (k1 * ACT("H+") + k2 * ACT("CO2") + k3 * ACT("H2O"))
120 rate = rate * (1 - 10^(2/3*si_cc))
130 moles = rate * 0.001 * TIME # convert from mmol to mol
200 SAVE moles
   -end
END



SOLUTION 1 Pure Water
    temp      25
    pH        7
    pe        4
END



KINETICS 1
Calcite
 -tol   1e-8
 -m0    10
 -m     10
 -parms 1.67e5   0.6  # cm^2/mol calcite, exp factor
 -time  10 s
 -cvode
USE solution 1

EQUILIBRIUM_PHASES 1
    CO2(g)    -3.58 10


[Renmori]

Apologies, I can be very slow.  I think I understand how this works now.  Hope I can pick up PHREEQC very quickly.  Thank you so much for your help!!!

Wanted to ask you before I waste time looking into this, is it at all possible to model dissolution kinetics of CO2 from the gas phase into water using PHREEQC?

[dlparkhurst]

You will have to write add a rate expression for the dissolution of CO2 in the RATES data block. Then you will need to include CO2 dissolution (using the name from the RATES definition) in the KINETICS data block. You can include multiple kinetic reactions in KINETICS, so you can have both Calcite and CO2.