Pyrite dissolution in batch reactor

[Aquawolfe]

Hi,

I am trying to modeling pyrite dissolution in a batch reactor, but I'm having issues getting reasonable results. I think part of the issue is how I'm entering the rate law (?). I’ve tried multiple configurations, but have been unsuccessful. I’ve looked over Sxn 31.1 in the text book, as well as Ch.5 in the Reaction Modeling Guide.

I’ve attached the experimental parameters, GWB input and results of the simulations. The experimental runs were over a 24 hour period; at the conclusion of the experiments, <2% of the total pyrite had dissolved.

Any insight is appreciated!

Many thanks, in advance, for your time!

Amy

PyriteDiss-GWB_23Dec2015_forumv2.pdf

React_output_GSS_1_23Dec2015.txt

React_output_GSS_3_23Dec2015.txt

React_output_GSS_5_23Dec2015.txt

[Brian Farrell]

Hi Amy,

 

I think your problem arises from the way you've set the oxidation state of the system. You mentioned that you maintain a DO concentration of 9-12 ppm by controlling the O2 partial pressure in the headspace. You also measure an Eh of about .36 mV, or pe of 6.1. When you use the pe as your constraint, you end up with an extremely small concentration for the O2(aq) species (nowhere near 9-12 mg/kg). Because O2(aq) is a promoting species in your rate law, the pyrite dissolution rate ends up being extremely slow. I set up a model much like yours with 12 free mg/kg of O2(aq) in the Basis pane, and I fix O2(g) fugacity. That seems to give me reasonable results. Eh probes can be unreliable, so I think setting DO directly is acceptable.

 

I also noticed that your reactor is 1.5 L. You should increase the mass of solvent water in the system or reduce the amount of Pyrite so that their relative proportions match up.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions

[Aquawolfe]

Hi Brian,

Thanks for your help!

 

When I set up the basis using the inputs above (pH = 3; DO = 12 mg/kg free), the system becomes saturated with pyrite in <3 hours, when I don't think it should be - there is only 5.355 gr of pyrite in the system. I've tried entering a couple of different rate laws, with mixed results.

 

The main issue is that when the system is unsaturated with respect to pyrite, the amount of pyrite that reacts is much too low.

 

Is there anyway to constrain the pe of the system, if pH is fixed and DO is maintained?

 

Best,

Amy

[Brian Farrell]

Hi Amy,

 

Can you attach your React script? I think the first custom rate law should work just fine. If you can, though, you might use the "soln_volume" internal parameter instead of "Wmass". You'll have to check whether the GWB release you're using accepts it. Take a look at Table 5.1, Internal parameters, in the Custom Rate Laws section of the GWB Reaction Modeling Guide.

 

You can only set a single oxidation state for the system as a whole, but by disabling various redox coupling reactions you can account for the redox disequilibrium characteristic of most natural waters. Take a look at Chapter 7, Redox disequilibrium, in the GBRM text for more information.

 

Regards,

Brian

[Aquawolfe]

Hi Brian,
I've attached two react files. The first, FeS2_Kin_7Jan2016#1.rea, uses the custom rate law you suggested in your previous post; unfortunately, I wasn't able to use "soln_vol" in my GWB version.

 

In the second file, FeS2_Kin_5Jan2016#11c.rea, I entered a constant value for O2(aq) and set the rate constant equal to k * A/V. In this run, the amount of pyrite that reacts more closely resembles what was observed/measured in the experimental system; however, the system is still saturated with pyrite.

 

In all of the simulations, I decoupled all relevant redox pairs.

 

Thanks for your help; I really appreciate it!

 

Amy

FeS2_Kin_7Jan2016#1.rea

FeS2_Kin_5Jan2016#11c.rea