Using GetConcentrations() for redox reaction case

[SaiP]

Hi David,

I have a question regarding the use of 'GetConcentrations()' function in phreeqcRM. I am running a redox reaction with transport (transport solved by a CFD solver). When I have Fe+2, Fe+3 in the system, normal phreeqc classifies these as 2 different components. But in phreeqcRM, using GetConcentrations() returns me a single set of data which is a mix of Fe+2 and Fe+3 data (like the boundary concentration data of Fe+2 and internal concentration data of Fe+3).

My question:
1. Considering the scenario I want to model (redox reactions in a clogging system), do I need to consider transport of both Fe+2 and Fe+3 seperately (which I think shall be the case) or is the concentration data of Fe only is sufficent [in such a scenario I am not sure how can phreeqc distinguish between Fe+2 and Fe+3]?

2. If I need both the concetrations of Fe+2 and Fe+3, how can I get them. It looks like GetConcentrations() cant return the concentrations of both Fe+2 and Fe+3?

Thanks. 

[dlparkhurst]

You do not need to separate Fe into Fe(II) and Fe(III). A solution composition is defined uniquely by moles of H, total O, charge, and total moles of other elements (including total Fe).

There is an option to transport H2O, excess H (H not in water), excess O in place of total H and total O. The idea was that it might be numerically easier to transport because most H and O are in water.

[adityain2003]

Hello David,

I read in another post that in addition to species, we should transport components such as charge, H and O as solute in order to maintain the mass and charge balance. But what should be the solute transport properties (like dispersity) of these components when they are considered as solutes?

 I guess it should be a high value to simulate instantaneous transfer.

[dlparkhurst]

Diffusion is usually small relative to advection and dispersion, so you could consider advection and dispersion as one transport case, and diffusion of solutes as another.

If you are simulating advection and dispersion, generally, all solutes are assigned the same dispersivity. You can use total concentrations of elements, as well as total H, O, and charge imbalance for the transported concentrations. A rule-of-thumb is that the dispersivity (units of meters) is 10% of the length of the domain, but you are free to define a value appropriate to your system.

For diffusion, you can assign a common diffusion coefficient, which simplifies the transport calculations, but multicomponent diffusion is better, where each ion and species diffuses at a rate dependent on an individual diffusion coefficient with the provision that there is no net electrical current.  Here you must transport each aqueous species rather than total element concentrations. See the documentation for multicomponent diffusion in the TRANSPORT section of the Phreeqc version 3 manual if you are considering multicomponent diffusion.

[adityain2003]

Thank you David,

Your insights helped a lot