Re: redox modeling

["David L Parkhurst" <dlpark@xxxxxxxx>]

Jeroen,

> I would like to model the addition (by 'Mix') of acetate to lower the pe
and
to invoke sulphate reducing circumstances.

MIX is used to mix solutions together and would be appropriate if you are
adding an acetate-containing water to your system. Alternatively, if you
are simply adding acetate to a solution, you might consider adding it
through the REACTION data block.

> The Acetate reactions from the
minteq database do not change pe.

That is correct, the way Acetate- is defined in both llnl.dat and
minteq.dat, there is no connection with the inorganic carbon system. The
original llnl data has acetate as a carbon species that would react to
equilibrium with water and inorganic CO2, which is I think the way your are
headed.

> Therefore I took a reaction given by Lindsay:
#taken from Lindsay, W.L., 'chemical equilibria in soils' 1979, p376

2 H2O + CH3COO- = 2 CO2 + 8 e- + 7 H+
             log_k                         -9.64
             delta_h           0           kcal

CH3COO- = 2 CO2 + 4 e- + 3 H+
             log_k                         -16.66
             delta_h           0           kcal

> This to involve pe in the reactions.

Right, you need acetate to affect the inorganic carbon/water system.

> Also is replaced all 'Acetate-' from minteq.dat by 'CH3COO-'.
I am getting the following messages:

ERROR: Coefficient of first species on rhs is not equal to 1.0.
ERROR: Equation for species CO2 does not balance.
ERROR: Could not reduce equation to secondary master species, CO2.
ERROR: Calculations terminating due to input errors.
Stopping.
No memory leaks

> maybe you can help me, I have attached the input-file to this message,

The second equation does not balance with respect to oxygen. Both reactions
define the acetate species, and you only need one definition. The reactions
should be "association" reactions, meaning the species defined should be
the first on the right hand side (and its coefficient should be 1.0; error
message referred to the 2 for CO2 as the first species on the right hand
side in your definition). The following reaction should work.

2 CO2 + 8 e- + 7 H+ = CH3COO- + 2 H2O
             log_k      9.64

However, thermodynamics (the log K of the reaction) will cause the
equilibrium concentration of CH3COO- to be very small. In other words,
there will not be any way to generate appreciable CH3COO- in any solution
that you define. So, you can model the pe effect without adding CH3COO- as
an aqueous species, simply by adding NaCH3COO in a REACTION data block. The
carbon will end up as CO2 and CH4 in solution and the pe will be quite low.

SOLUTION
REACTION
      NaCH3COO 1.0
      1 mole
END

Results should be almost identical to the following:

SOLUTION_SPECIES
2 CO2 + 8 e- + 7 H+ = CH3COO- + 2 H2O
             log_k      9.64
SOLUTION
REACTION
      NaCH3COO 1.0
      1 mole
END


A more complicated approach is to keep Acetate as a SOLUTION_MASTER_SPECIES
and use a KINETICS definition to transform Acetate into CH3COO; the CH3COO
will react as shown in the examples above. This probably is closer to
reality, where acetate will remain in solution, but be consumed at some
rate by biological activity or perhaps by inorganic processes.

David


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled