Pressure dependence of log_k llnl.dat

[Mouadh]

Hi everyone,

I'm trying to understand how the pressure dependence of log_k is calculated using the llnl database.
The equation provided in the 2013 user manual is
log_k_P=log_k_P1-dVr/(2.3RT)(P-1)
where -dVr is is the volume change of the reaction.
How is this volume change of the reaction calculated in the llnl.dat?
It seems no -Vm parameters are provided in the database.
I did test it and I do see a small change in log_k when the pressure is high. as shown in the following example:
##################################################
## Input
DATABASE PATH\Databases\llnl.dat
SOLUTION 1
temp 25
#pressure 500     
units  mol/kgw
EQUILIBRIUM_PHASES
Calcite   0   1
Muscovite   0   1
##################################################
## Output (Phase assemblage)
# Case1 Pressure =1
-------------------------------Phase assemblage--------------------------------

                                                      Moles in assemblage
Phase               SI  log IAP  log K(T, P)   Initial            Final               Delta
Calcite              0.00    1.82      1.82    1.000e+00   9.999e-01    -1.243e-04
Muscovite         0.00    13.45     13.45  1.000e+00   1.000e+00   -4.391e-06

# Case 2  Pressure =500
-------------------------------Phase assemblage--------------------------------

                                                      Moles in assemblage
Phase               SI  log IAP  log K(T, P)   Initial            Final               Delta
Calcite             0.00     1.82      1.82      1.000e+00   9.999e-01   -1.342e-04
Muscovite         0.00    12.51     12.51    1.000e+00   1.000e+00  -4.275e-06
##################################################

Why is log_k Calcite the same at both pressures but logK Muscovite changes when -vm is not defied for either of them in the llnl database?

Thank you!

##################################################
#  $Id: llnl.dat 12776 2017-08-02 20:02:16Z dlpark $
PHASES
:
Calcite
        CaCO3 +1.0000 H+  =  + 1.0000 Ca++ + 1.0000 HCO3-
        log_k           1.8487
   -delta_H   -25.7149   kJ/mol   # Calculated enthalpy of reaction   Calcite
#   Enthalpy of formation:   -288.552 kcal/mol
        -analytic -1.4978e+002 -4.8370e-002 4.8974e+003 6.0458e+001 7.6464e+001
#       -Range:  0-300
:
Muscovite
        KAl3Si3O10(OH)2 +10.0000 H+  =  + 1.0000 K+ + 3.0000 Al+++ + 3.0000 SiO2 + 6.0000 H2O
        log_k           13.5858
   -delta_H   -243.224   kJ/mol   # Calculated enthalpy of reaction   Muscovite
#   Enthalpy of formation:   -1427.41 kcal/mol
        -analytic 3.3085e+001 -1.2425e-002 1.2477e+004 -2.0865e+001 -5.4692e+005
#       -Range:  0-300
##################################################

From the manual:
"Pressure effects can be simulated by entering molar volumes of solids and parameters for defining the specific volume of aqueous species as a function of temperature, pressure, and ionic strength with a Redlich-type equation (for example, Redlich and Meyer, 1964)". (p27)

"The parameters for calculating the specific volume of aqueous species, the Peng-Robinson parameters for gases, and molar volumes of minerals have been added to the databases phreeqc.dat, Amm.dat, and pitzer.dat." (p 27) (NOT llnl.dat)

If -Vm parameters are defined in SOLUTION_SPECIES, specific volumes are calculated relative to the volume of H+ (which is zero by convention at all pressures, temperatures and ionic strengths); otherwise, specific volumes are listed as (0). (p 314)" (NO -Vm is defined in the llnl.dat)



 

[dlparkhurst]

Pressure dependence is calculated explicitly only for the databases phreeqc.dat, Amm.dat, and pitzer.dat. All other databases lack the necessary -Vm parameters.

llnl.dat is formulated to follow the pressure-temperature relation of the steam curve. So, I think pressure is 1 atm to 100 C, and then the log Ks (where there is temperature dependence) for a given temperature are appropriate for the pressure at which water vapor and water are in equilibrium.

[Mouadh]

Thank you, it was also my understanding from reading the manual and the Appelo et al. 2014 and Appelo 2015 papers. Could you please elaborate more on the last part.

I can see now that the minerals  that show 'change' with pressure all have water in the reaction (Muscovite: KAl3Si3O10(OH)2 +10.0000 H+  =  + 1.0000 K+ + 3.0000 Al+++ + 3.0000 SiO2 + 6.0000 H2O) so It makes sense that it is related to the water vapor equilibrium, but I am not sure how  this change is linked to the mineral log_k calculation in PHREEQC. Is log-k adjusted based on the IAP change?
 
I have checked that it is not specific for the llnl database. I tested a similar example using the phreeqc.dat where I deactivated all log_k parameters for K-feldspar (logk (default=0), delta_h, analytical_expression and Vm) and I get a similar change at high pressures.

Best,
Mouadh

[dlparkhurst]

This is actually a bug in the handling of llnl.dat. When the changes related to Appelo and others were coded, the molar volume of H2O was hardwired. So, log Ks for mineral reactions involving water had some pressure dependence that logically should not be included. For llnl.dat, the temperature dependence of log K is supposed to include the pressure dependence as well. For phreeqc.dat, Amm.dat, and pitzer.dat, the Ks should respond to changes with pressure of the molar volume of water.

We have corrected this problem, but have not released new versions yet. If you contact me offline, we can discuss your needs.