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Chrome VI Reduction to Chrome III with Ferrous Sulfate Reactant


bclement2142
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Hi,

 

I'm a new user to the software and am trying to simulate reduction of hexavalent chromium to chromium III by reacting the defined system with a ferrous sulfate amendment. As the ferrous sulfate concentration is increased in the reactants pane, more and more hematite is precipitated but very little to no chromium is converted from Cr VI to CrIII. Am I missing a step, or is this just what is most likely to happen under these conditions? I noticed also that Eh hardly seems to change from iteration 0 to 100 when the solution converged, no matter what concentration of the amendment is used. Is there a way to add your field measured Eh data in the Basis pane? Or can you only add it on a sliding scale in the Reactant pane? When I adjust the Eh in the reactants pane, the chromium does seem to precipitate out in the simulation, but it seems arbitrary for me to manually alter the Eh - I want to make sure that's what would be occurring in the real system as a result of the amendment. Does GWB automatically account for Eh changes as the result of a reactant with the system or does that need to be added as it's own reactant? I've attached the txt file script for the solution and the .rea file.

 

Thanks,

 

Ben

 

 

React_output-React 25 mgL Ferrous Sulfate.txt

MW-5S React_1-123016.rea

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Hi Ben,

 

I noticed a few things in your input file. First, you're actually adding a ferric sulfate, Fe(SO4)2-, instead of ferrous sulfate, FeSO4. As a result the Eh was increasing slightly instead of going down. Second, there are several redox sensitive basis entries that are playing an important role in the simulation as you've set it up. Looking at Manganese, for example, Pyrolusite is supersaturated in the initial system so it precipitates (consuming most of the free O2(aq) in the process) before the reaction path begins. The Fe++ in the ferrous sulfate at first goes into reducing the Pyrolusite to Mn++ (which you can see in plots of mineral mass or the concentration of Mn species vs. added FeSO4). You'll need to add a little more FeSO4 to start Cr(VI) reduction. If you expect all Manganese to be present as Mn(II), you can disable all of the redox coupling reactions involving Manganese. If you do this, the O2(aq) initially present will be the main consumer of the added ferrous iron, but once it's used up you'll eventually get into Cr(VI) reduction.

 

If you'd like to specify Eh instead of the dissolved oxygen concentration, just swap the e- into the Basis and set Eh units. Either way, that is the variable controlling the initial oxidation state of all enabled redox couples.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC

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  • 2 weeks later...

Thanks Brian. Those recommendations did help. I ran into another issue when trying to decouple redox pairs for MnO4- and Mn2+ and Cr04-- and Cr3+. We have both liquid and solid phase data which I would like to be able to enter separately into the same basis file. For example if I assume the dissolved phase data for chromium is present as CrO4-- in units of ug/L and my solid phase is present as Cr 3+ in units of mg/kg, when I go to the results pane to rerun the solution. I get an error that reads "Exit: React stop: set_basindex: bad basis type React is ending." and the program closes. Am I going about this the right way or am I doing something wrong? I just want to make sure I am considering both what we know to be present in solid and dissolved phases within the aquifer at each location and am not underestimating the mass/dosage of ferrous sulfate we will need.

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Hi Ben,

 

I’m glad to hear the suggestions were helpful. If you send me an input file that causes React to crash I’ll take a look.

 

You can decouple the CrO4--/ Cr+++ redox pair, but doing so by definition eliminates the provision for equilibrium between the two oxidation states. They’ll be treated as completely separate entities. Your example as it’s currently set up, with FeSO4 being titrated into the system, will no longer cause chromium reduction, unless you incorporate redox kinetics.

 

There are a variety of organic species included in thermo.tdat (the default GWB thermodynamic dataset) and thermo.com.V8.R6+.tdat (an expanded version) such as acetate, formate, lactate, etc. There is no generic dissolved organic carbon, though. Is this something that’s already present in the system, or another reductant that’s added to the system like the FeSO4?

 

Cheers,

Brian

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