Equilibrum_Phases_modify (Non saturated)

[Pak]

I have read in some sources (e.g. Souhila Sabit. "Les méthodes numériques de transport réactif" These, Université de Rennes 1) that in an unsaturated media, the mass balance precipitation equation instead of being a equality becomes a inferior/equality (sorry I do not know the name in english =</=>) (look at the attached image please, equation (1.5)).

I was wonder if there is a way to change the equilibrium constant regarding the saturation in Phreeqc.  So far, I think it would be possible to change the value at the solution_species datablock after every iteration, although since this data block is always define in the database or in the initial input, I am not sure if it can be changed after a transport step without using an extremely intrusive approach.

Thanks

[Pak]

Briefly, I wonder how to affect the reactivity of the dissolution respect the saturation of the system.

[dlparkhurst]

In EQUILIBRIUM_PHASES, for example

Calcite -0.5 10  dissolve_only

the first number is the target saturation index. In this case, calcite will react until it reaches a saturation index of -0.5.

You can also use KINETICS instead of EQUILIBRIUM_PHASES, which allows you a wide variety of options. You can devise rate expressions that depend on solution composition, amount of reactant present, and (or) time.

One last option, for the advanced users, you can adjust the target saturation index with EQUILIBRIUM_PHASES_MODIFY.

[Pak]

Thanks for the answer, just to be clear there is no way to change the equilibrium constant for an equilibrium approach during a RT simulation. If I desired to do so, I should aim for a nonequilibrium approach and use a kinetic formulation, should I not.

[dlparkhurst]

Using the batch version of PHREEQC, there is no way to change the log K for calcite in the middle of a TRANSPORT calculation.

However, the log K can be redefined with a PHASES data block at the beginning of any simulation (keyword data block finished with an END statement). So you could do TRANSPORT for, say, half the desired time steps, redefine the log K (or target saturation index with EQUILIBRIUM_PHASES modify), and run the second half of the time steps with the new K.

KINETICS, as opposed to EQUILIBRIUM_PHASES, would provide the most flexible approach, and it sound like you are describing a kinetic approach.

[Pak]

Thanks, that is exactly what I was looking for.

Actually, the model should be made kinetically (or mixed), but I would like to try first an equilibrium approach.