decoupling water redox reactions?

[dvlassopoulos]

I am working on a large reactive transport model for fluoride using muFlowRT (PhreeqcRM coupled to OpenFoam) with the phreeqc.dat database. The chemical system includes Ca-Na-Mg-Al-F-C(4)-PO4-O-H and the problem does not involve any redox reactions. Occasionally, the model stops due to non-convergence and I have traced the error to cells where a negative pe is calculated which results in significant reduction of H2O to H2. Since H2 (and O2) are not important for this particular problem, is there a way to decouple them from H2O?

Thanks in advance for any suggestions,
Dimitri

[dlparkhurst]

Hi Dimitri. The simple answer is that you can decrease the log K for H2(aq), which will inhibit the formation of H2(aq).

However, I think the formation of H2 is telling you that you have another problem. Something is causing the reduction of H2O, and it could be an incorrectly formulated reaction. One possibility is adding elements with the wrong valence, either in KINETICS or an incorrectly balanced mineral formula or other chemical species.

Alternatively, if the elements are not transported accurately enough, you could end up with a really reducing situation. The simplest thing would be to crank down on the transport tolerances. You could also try using the PhreeqcRM to transport water, excess H, excess O, and charge. Transporting the excess quantities may make the transport calculation more accurate (or not).

I don't know how to run and debug muFlowRT. If you can reproduce the problem in PHREEQC or PHAST, I could probably debug the problem.

[dvlassopoulos]

Hi David,

Thank you for the quick response!
We have not added any elements and I have run a 1D version of the model in Phreeqc to check the database and inital conditions - no errors were identified. My guess is that in the 3D uFlowRT model the numerical errors build up over time. I will try changing the log K for H2 and cranking doen the tolerances and see if either of those help.

Best regards,
Dimitri