kinetics problem in phreeqcRM

[oatteia]

Hello,
I coupled OpenFoam with phreeqcRM to have multiphase transport. I did a lot of tests but until now i did not check kinetics. Something strange appears : I put a simple 1st order rate (Pce -> Tce), and phreeqcRM shows too much degradation (approx. 3 times) compared to Pht3d or looking the direct input and output before and after phreeqcRM calculation at a given cell. I thought that it could come from kinetics units, as it look like it would consider conc in the cell instead of conc in the water, or something like this. However when I change SetUnitsKinetics to 0,1 or 2 there is no change, I got the same rate.
any explanation?
sincerely
Olivier Atteia

[dlparkhurst]

I don't know the problem for sure. I can give two possible reasons:

(1) You may need to account for the solution volume. If your rate is mol/kgw/s or mol/L/sby . When multiplying by TIME you have mol/kgw or mol/L; you need to multiply by the Basic function TOT("water") or solution volume, SOLN_VOL to arrive at a value with units mol that you would use for the SAVE value.

Note the solution volume is the representative volume times the porosity times the saturation for the cell.

(2) It doesn't sound like you are using surface area, but if you are, you need to be careful with the PARMS definition in KINETICS. The number of moles, M and M0, is scaled depending on the choice of SetUnitsKinetics, but the parameters are not scaled. You need to use a parameter of surface area per mole, and then multiply by M (moles) to arrive at a surface area. If you give an absolute surface area, m^2, as a PARM, then the rate will not scale properly if the number of moles of kinetic reaction is scaled.

To debug, I would start by using SELECTED_OUTPUT/USER_PUNCH to see the values for KIN, KIN_DELTA, KIN_TIME, SOLN_VOL, and TOT("water"). You can also use PUT in the RATES definition and GET in USER_PUNCH to add other variables that are used in the RATES definition; however, these values will be the values at the last evaluation of the RATES definition for the time step, and may differ from the average values for the time step and may not be the values at the end of the time step. (If you use SetPrintChemistryOn and SetPrintChemistryMask, you can use USER_PRINT to print values to the output file.)

If you still have issues, I would try to simplify the problem to single phase, saturated flow and simplified reactions that you can compare to hand, batch, or 1D flow calculations.