About the sequence of calculations in Phreeqc for a RT

[Pak]

Dear all,

Imagine a RT with the following blocks in Phreeqc:

Solution 0
Solution 1-Ncells
Equilibrium_Phases 1-Ncells
Rates
Kinetics 1-Ncells
Transport

after running your simulation in Phreeqc if it has been succesfully done, you will get the output file of Phreeqc. There you can read the following sequence of calculations:

(input, output, database)
- Reading data base
- Reading input data for simulation 1 (imagine all is put together, i.e. no END block)
- Beginning of initial solution (speciation of Solution 0 and also solution 1, since solution 1 is equal to the other cells also)
- Beginning of batch-reaction calculations (Reaction step 1. Using solution 0. Using pure phase assemblage 1. Using kinetics 1.)
- Beginning of transport calculations
                         -- Equilibrating initial solution --> Transport step --> Equilibrating --> Transport

I wonder what is the purpose of the Beginning of batch-reaction calculations step.


Thank you

[dlparkhurst]

If you do not use any END statements, then PHREEQC will react the SOLUTION with the EQUILIBRIUM_PHASES and KINETICS. The resulting compositions of the equilibrium_phases and solution are not saved. The amount of kinetic reaction will change, and its change will be the only change to affect the TRANSPORT calculation.

If you place an END statement after the SOLUTION definitions, then there will be no batch reaction calculation.

[Pak]

Ok thanks.

So, the solution that is used in the transport calculations continues to be the same, but the amount of the kinetic and equilibrium species for the first cell, would be different. Why is not the solution saved, but the mineral is?

Also, if I have:


Solution 0
Solution 1-Ncells
END
Equilibrium_Phases 1-Ncells
Rates
Kinetics 1-Ncells
Transport

Where Solution 0 and Solution 1-Ncells has been equilibrated with Cl.

I will get the following calculation sequence:

Beginning of initial solution calculations

Beginning of transport calculations

In the first Beginning of initial solution calculations:

I can see in the solution composition section that the solution is charge with Cl, as I have specified. (image1)

But once the transport start, the initial equilibrium (image2) and after transport steps (image3), the charge balace is related to the pH, instead of Cl. What is the reason for that?

The solution 0 has also been charged to pH, or it is still charged to Cl. And the other solution which also undergoes the process of Beginning of initial solution calculations.

Thank you again.

[dlparkhurst]

I think only the amount of kinetic reactant would change. To keep the reaction calculation results, you would need to use SAVE solution and SAVE equilibrium_phases (or use RUN_CELLS, which automatically saves results).

[Pak]

I have another similar question, if I have:


Solution 0
Solution 1-Ncells
END
Equilibrium_Phases 1-Ncells
Rates
Kinetics 1-Ncells
Transport

Where Solution 0 and Solution 1-Ncells has been equilibrated with Cl.

I will get the following calculation sequence:

Beginning of initial solution calculations

Beginning of transport calculations

In the first Beginning of initial solution calculations:

I can see in the solution composition section that the solution is charge with Cl, as I have specified. (image1)

But once the transport start, the initial equilibrium (image2) and after transport steps (image3), the charge balace is related to the pH, instead of Cl. What is the reason for that?

The solution 0 has also been charged to pH, or it is still charged to Cl. And the other solution which also undergoes the process of Beginning of initial solution calculations.

Thank you again.

[dlparkhurst]

Charge balance adjustment with Cl (in your example) only applies to the initial solution. Once the initial solution has been calculated, with or without charge balancing, the charge of the solution is maintained; consider it an inert ion. For all subsequent reaction calculations, the pH is adjusted to maintain the charge from the initial solution calculation (or mixture of charge from multiple solutions during diffusive transport).