Michael P Posted June 5, 2014 Share Posted June 5, 2014 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 Quote Link to comment Share on other sites More sharing options...
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