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Modeling birnessite (Mn-oxide) precipitation and the thermo.dat's use of disproportionation reaction


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Hello,

 

We are interested in modeling the precipitation (SI values) of birnessite from Mn^2+ that is released to solution by dissolution of primary Mn(II)-bearing silicates. In our system, this Mn(II) reacts with dissolved O2 to form birnessite.

 

However, when we model birnessite precipitation, aqueous species include manganate(VI). The birnessite precipitation reaction in thermo.dat is the disproportionation reaction:

 

Mn8O19H10 + 4H+ = 5Mn^2+ + 3MnO4^2- + 7H2O

 

Therefore, we are wondering if there is another way to model the precipitation of birnessite from Mn2+ solutions that do not require the disproportionation reaction. Or is there another way to otherwise suppress the unlikely manganate(VI) aqueous species for our system. No doubt a new equation would require a new set of LogK values...

 

[e.g, Mn7O13(5H2O which balances using only Mn^2+, H+, and H20 might be a better fit for our system]

 

Pyrolusite (another Mn-oxide) also has a similar disproportionation reaction in the database.

 

Further, in our soil solution system, we have a pH range 8-9.5 and Temp 15-35 C so we need Log K values at 0, 25 and 60. I used the van't Hoff approximation to calculate Log K at 60 for birnessite for the default database. Then, for SpecE8 to give birnessite results, we had to peg O2 concentrations at some value (1ppm for instance). In other words: no oxygen GSS entry = no birnessite. Why is that?

 

Thoughts?

 

Thanks in advance for your insight.

 

Michael

 

 

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Hi Michael,



You can prevent the formation of certain species or minerals in your simulations by suppressing them. Just go to Config -> Suppress or use the suppress command to do so.



Some of the reactions involving Manganese species in the thermo dataset are a little weird. You can certainly rewrite reactions in a thermo dataset, however. Obviously you'll need a new log K for a new reaction, but you can calculate this by simply combing reactions and log Ks. It would be a good idea to create new Redox species of the same oxidation state as the minerals you're interested in (like birnessite and pyrolusite), even if no aqueous forms actually exist.



For example, you could define a fictive Mn(IV) species with an activity of 10^-9 in equilibrium with pyrolusite:



(1) Pyrolusite + 2H+ = Mn++ + 1/2 O2(aq) + H2O log K = -1.4378

and

(2) Pyrolusite = MnO2(aq) log K = -9

so

(3) MnO2(aq) + 2H+ = Mn++ + 1/2 O2(aq) + H2O log K = 7.5622


Reaction (1) is the original mineral entry for pyrolusite (rebalanced in Rxn to be in terms of O2(aq) - you can get to the disproportionation reaction by swapping O2(aq) out for MnO4--), (2) is the new mineral entry for pyrolusite, and (3) is a new redox couple. For some explanation of rebalancing reactions and creating fictive species, take a look at sections 17.5 and 28.2 in the Geochemical and Biogeochemical Reaction Modeling text.


If a reaction's log K is unknown at a certain temperature, you could certainly use the van't Hoff approximation. Another method, if you get a reaction the way you like in Rxn, is to use the polynomial fit of the log K as a function of temperature reported by Rxn. For the specific case you mentioned where you only have log Ks at two temperatures, however, it's only going to be a straight line.



As for the questions of requiring O2(aq) in your GSS spreadsheet (or SpecE8, etc.), this won't be obvious unless you use Rxn to load the default reaction for birnessite. If you do, you'll find it's been recast in terms of the default set of basis species (which includes O2(aq)) rather than the disproportionation reaction involving multiple redox states of Mn. Since this is still a redox reaction you're considering you'll need some measure of oxidation state.


Hope this helps,


Brian Farrell

Aqueous Solutions LLC

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