Using parallel rates for minerals

[Benoit]

Hi,

 

I am unsuccessfully trying to simulate the kinetically-controlled behavior of a mineral using 2 parallel rate laws (React, version 10).

 

I have tried several cases, the simplest being based on the Ch28_Mn react script provided with GWB. This test case simulates the oxidation of Mn(II) into Mn(OH)3(cr) using a 'redox' kinetic rate law. If I simply add 'reactants->kinetic->Mineral->Mn(OH)3(cr), in order to assign a kinetic control of the mineral dissolution, it looks like the redox oxidation reaction now affects the Mn(III) concentration instead of the mineral concentration.

 

So, I understand that once a TST kinetic is turned on, then the mineral concentration can only been altered through this TST rate. Is this correct? If yes, is there a way to add another rate to the TST? Thanks in advance for your responses.

 

Benoit

[Brian Farrell]

Hi Benoit,

 

The kinetic redox transformations describe the transfer of mass from one oxidation state to another. In the example you mentioned, you could replace the Mn(OH)3(cr) in the redox reaction with a dissolved Mn(III) species. For example, instead of “Mn++ + 1/4 O2(aq) + 5/2 H2O -> Mn(OH)3© + 2 H+” you could write “Mn++ + 1/4 O2(aq) + H+ -> Mn+++ + 1/2 H2O”. In either case, the program honors the equilibrium between Mn(OH)3(cr) and Mn+++ (the reaction in this case, accounting for redox decoupling, is Mn(OH)3(cr) + 3 H+ -> 3 H2O + Mn+++) so you end up getting the same results.

 

If you incorporate a rate law for the precipitation or dissolution of Mn(OH)3(cr), though, that reaction will not remain in equilibrium. Even though the Mn(II) is being oxidized to Mn(III), the supersaturated mineral would be slow to precipitate, so you’d end up with the dissolved Mn(III) species accumulating in the fluid. The redox reaction affects the mineral’s precipitation rate by limiting the supply of Mn(III) species, but the rate law for dissolution/ precipitation controls how quickly the Mn(III) species can be utilized in precipitating Mn(OH)3(cr).

 

You can set parallel rate laws for redox transformation reactions (for example, abiotic and biotic oxidation of Fe++) but not for the other types of reactions, at least using the Build-in Rate Law. You could probably prepare a custom rate law for the dissolution/ precipitation of Mn(OH)3(cr), though. There might be two rate equations that you evaluate and sum together to get the overall rate of dissolution/ precipitation. For more info, please see section 5, Custom Rate Laws, in the GWB Reaction Modeling Guide.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC

[Benoit]

Hi Brian,

 

I was suspecting something like that for the kinetic redox so it is clear to me now. Anyway, your suggestion of summing the rates works fine. So, many thanks for your quick and efficient reply.

 

Benoît