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

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

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 1e8
# m0 3.e3
# m 3.e3
# 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

Thank you!
So the best way to specify solution volume is to use PHAST in this case?
Best,
Joanna

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.