# PhreeqcUsers Discussion Forum

## Beginners => PHREEQC Manual Examples => Topic started by: joannadziadkowiec on June 28, 2018, 11:02:27 AM

Title: example 5.9 Appelo Postma, Geochemistry p.213
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

Title: Re: example 5.9 Appelo Postma, Geochemistry p.213
Post by: dlparkhurst on June 28, 2018, 03:34:04 PM
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

Title: Re: example 5.9 Appelo Postma, Geochemistry p.213