example 5.9 Appelo Postma, Geochemistry p.213

[joannadziadkowiec]

Hello all!

I am trying to reproduce the example 5.9 from the page 213 in the Appelo & Postma Geochmistry book.

The results when using the simplified calcite dissolution kinetics model (simulation 1 with Calcit2 dissolution rate) are in a pefect accordance with the Figure 5.33 on page 214.

However, when I use the calcite dissolution rate from the phreeqc.dat database ("Calcite"), the results are completely different (the amount of dissolved calcite is one order of magnitude less than for the simplified Calcit2 model for the same solution paramters and initial amount of calcite).
I have not modified the RATES for calcite dissolution given in the phreeqc.dat  database.
Also, according to the Figure 5.33 on page 214 the results of these two simulations should be only slightly different.
Does anybody know what might be the issue?

Below I am attaching my phreeqC input files:

# Simulation 1 with Calcit2 dissolution rate

RATES
   Calcit2   #simplified rate cf. Equation (5.45)
   -start
   10 rate = 10^-6.91 - 10^-1.52 *(tot("Ca"))^2 #mmol/cm2/s
   20 save 1*rate*time      # integrate in moles/L
   -end

SOLUTION
   temp 10;
   pH 6 charge;
   C 1 CO2(g) -1.5;

EQUILIBRIUM PHASES
   CO2(g) -1.5

KINETICS
   Calcit2
   formula CaCO3
   -m0 1
   -step 30000 in 20

INCREMENTAL_REACTIONS true

END

# Simulation 2 with Calcite dissolution rate from phreeqc.dat database

SOLUTION 1
   temp 10;
   pH 6 charge
   C 1 CO2(g) -1.5;

EQUILIBRIUM_PHASES 1
    CO2(g)    -1.5

KINETICS 1
   Calcite
   -m0 1
   -parms 10 0.67   
   -step 100000 in 20

INCREMENTAL_REACTIONS true

END

[dlparkhurst]

The RATES definitions were changed in June 2015. Here is the comment from the release notes (ftp://brrftp.cr.usgs.gov/pub/charlton/phreeqc/RELEASE.12927.TXT,
from https://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/)

        --------
        svn 9462
        --------
        RATES definitions were revised in all databases. The area parameter
        was changed to specific area (area per mole of reactant). Also, for aqueous kinetic
        reactions, the solution volume was added explicitly. The changes
        were made to facilitate a new version of PHAST that uses a representative
        volume that does not have one liter of water.

The calcite RATES definition in phreeqc.dat has different surface area parameters than the book example:

########
#Calcite
########
# Example of KINETICS data block for calcite rate,
#   in mmol/cm2/s, Plummer et al., 1978, AJS 278, 179; Appelo et al., AG 13, 257.
# KINETICS 1
# Calcite
# -tol   1e-8
# -m0    3.e-3
# -m     3.e-3
# -parms 1.67e5   0.6  # cm^2/mol calcite, exp factor
# -time  1 day

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

[joannadziadkowiec]

Thank you!

So the best way to specify solution volume is to use PHAST in this case?

Best,
Joanna

[dlparkhurst]

The change was made to simplify definitions for PHAST. With the new rates, when PHAST scales the number of moles, the surface area scales appropriately. With the old RATES definition, the surface area would remain the same when PHAST scaled the number of moles for the representative volume of the cell.

However, the change is in the PHREEQC databases, so if you use the new RATES definitions with PHREEQC or PHAST, you need to use the correct parameters for the new RATES definition.

Note, the calcite RATES definition does not include a solution volume directly. It calculates a rate based on the reactive surface area. The number of moles reacted over a given TIME period are added to the solution, regardless of the solution volume.