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Pyrite Oxidation: Batch Reactor


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Hi. I'm trying to model pyrite oxidation in a batch reactor (Liu et al 2008). Both pH and the concentration of dissolved oxygen in the system is held constant (at 3 and 9-12 ppm, respectively).

  • pH is maintained by through discrete additions of HCl or NaOH
  • The concentration of dissolved oxygen was maintained by controlling the oxygen partial pressure in the reactor headspace

 

When I try to fix O2 in the Reactants window, I get a convergence error. As soon as I "unfix the O2", the program is able to arrive at a solution.

 

I'm new to GWB, so any advice is greatly appreciated! Many thanks, in advance, for your time!

PyriteOxidation_8June_v2.rea

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Hello,

 

I'm trying to look at your script, but I notice a few issues/ omissions in the script. What unit are you using to constrain the mass of Pyrite? The time units of your simulation were not saved correctly (it says hou in your script). Perhaps GWB isn't saving your script correctly. Which version are you using?

 

Thanks,

 

Brian Farrell

Aqueous Solutions LLC

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Hello,

 

I'm trying to look at your script, but I notice a few issues/ omissions in the script. What unit are you using to constrain the mass of Pyrite? The time units of your simulation were not saved correctly (it says hou in your script). Perhaps GWB isn't saving your script correctly. Which version are you using?

 

Thanks,

 

Brian Farrell

Aqueous Solutions LLC

 

Hi Brian,

Thanks for your help. I'm trying to model the experimental setup in Liu (2008). I used "grams" to constrain the mass of pyrite. I think the time units are correct: I want to see what Fe species are in solution after 24 hours. I tried to enter one of the rate laws calculated in the manuscript, but am having issues with how to deal with the oxygen.

 

I re-saved the script and attached the file. I also captured screen shots, if that helps. I'm using GWB 8.0 Pro. I checked for updates and the computer says I'm up to date.

 

Thanks so much for your help! I appreciate it.

post-10377-048387400 1339469765_thumb.jpg

post-10377-053628600 1339469772_thumb.jpg

post-10377-082509800 1339469780_thumb.jpg

PyriteOxidation_11June.rea

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Hi,

 

I think I can identify the source of error in your script. First, though, I want to go over how the specified reaction paths (in the Reactants pane) are affecting your run. Pyrite oxidation in a closed system will stop as soon as oxygen runs out. In a system open to the atmosphere, the reaction will likely continue until the Pyrite is completely dissolved. In the absence of a pH buffer, some variant of the reaction Pyrite + 2 H2O + 3.75 O2(aq) = .5 Hematite + 2 SO4-- + 4 H+ will occur, generating acid. In the experiment, however, f O2(g) and pH are manipulated to remain fixed.

 

In the model, pH is fixed by removing H+ from the system. Since SO4-- keeps accumulating as Pyrite dissolves, Cl- (default charge balancing ion) must be adjusted (lowered) to maintain charge balance. Eventually it disappears completely and causes (I believe) the error you are experiencing. Turning charge balance off helps here (although a plot of charge imbalance error vs. time shows quite a large error, which isn't exactly desirable). Since pH is controlled in the experiment by addition of NaOH, you can use Na+ as the charge balancing ion to more accurately simulate the experiment and get a model that runs. Note that if you remove the pH stat, using Cl- for charge balance works just fine.

 

The other (smaller) issue appears to be the value for the rate constant (or rather, the product of the rate constant, Pyrite's surface area, and any promoting/ inhibiting species if included in the rate law). The reaction (with fixed O2(g), charge balanced by Na+) appears to reach completion extremely quickly (about 8 minutes). I don't know how quickly the reaction proceeded in the experiment, but if I assume that 24 h was chosen for the length of the simulation because that's how long it took the Pyrite to completely dissolve, then a smaller rate constant would be more reasonable. You should check to make sure the units are correct for your kinetic parameters, and whether any promoting/ inhibiting species are supposed to be included.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC

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