Sulphate reduction in mine pit lake

[kristeng]

Hi there,

 

I am learning how to model sulphate reduction in hypolimnetic waters of a circumneutral pit lake. I have used the sulphate reduction set up from the Bethke textbook example for the aqufier (33.2). I have added acetate and balanced with Na, and decoupled CH3COO-/HCO3- and HS-/SO4--. I have the Fe++ concentration swapped for pyrite (ideally mackinawite but I have not yet updated my database) as I thought this would control Fe++ concentration under sulphate reducing conditions.

 

I am perplexed as to why the sulphate concentration decreases only by ~400 mg/L (initial 1400 mg/L, final 1000 mg/L). In the Bethke textbook it indicates that sulphide in solution will inhibit the SRBs, so perhaps this is why. However I would assume this would be buffered by sulphide minerals. In addition I do not have a huge amount of sulphides precipitating.

 

Is there an error in the way I have set up the basis or the reactants?

 

If I want to indicate that pyrite or mackinawite controls the Fe concentration, but also specify a starting Fe concentration (total Fe, I do not know spp) - can I do this? Do I just specify Fe (Fe++ as Fe) in the basis and then add a small amount of FeS in reactants?

 

I look forward to responses if anyone has insight here.

 

Regards,

Kristen

 

Sulphate reduction example.rea

[Brian Farrell]

Hi Kristen,

 

Are you sure that the script you've attached corresponds to the results you've described? After 60 hours, I see only about 2 mg/l of SO4-- being reduced (1480 to 1478 mg/l), not 400 mg/l.

 

To try to figure out what's going on with the sulfate reducer, I can look in the System Parameters and plot the Reaction rate. I see that it's pretty small, although it's not decreasing at all. In fact, it's increasing with time. Next, I plot the kinetic factors corresponding to electron donation and acceptance (KD and KA) as well as the thermodynamic potential factor (TPF). All of these are very close to 1, indicating that none of these are limiting the reaction rate. Finally, if I plot the biomass, I see that it looks to be roughly proportional to the reaction rate. As time increases, the biomass concentration increases. This leads me to believe that there is simply not enough biomass in the simulation to catalyze the reaction to any significant extent. If I increase the simulation time (or increase the initial biomass concentration), the microbes reproduce enough to have a significant catalytic capacity and they end up reducing quite a bit of SO4-- before the reaction eventually stops.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions

[kristeng]

Hi Brian,

 

Thanks for your response. I increased the biomass and it dramatically changed my results!

 

I do believe that was the correct file - I attached the output. I had misread the results. I was looking at spp totals for SO4-- which was 1018 mg/L, leading me to believe a drop of 400 mg/L had occurred in the first step of the reaction; however, I realize now that I had not accounted for other SO4 spp like CaSO4 and MgSO4. From the original basis totals I can see that the SO4 conc did not decrease. However, I see the change now that I increased the biomass concentration.

 

Can I constrain the amount of Fe (Fe2+ + Fe3+) in the system and have it equilibrate with FeS? Would I do that as I described above in my first post?

 

Thanks for your help.

[Brian Farrell]

Okay, glad the biomass increase worked for you and that you noticed the difference between the SO4-- component and the SO4-- species.

 

The "as Fe" option enables you to set concentration in terms of elemental equivalents. It is unrelated to how you control the distribution of mass between different oxidation states. All it controls is what molecular weight to use in conjunction with the concentration that you specify. Fe++ and Fe both have the same molecular weight, so the "as Fe" setting would have no effect. The molecular weight of SO4--, on the other hand, is about three times that of S. Depending on how the chemical analysis is performed, the mass of the entire sulfate oxyanion could be determined or only that of the sulfur itself. You would choose the "as S" option if only the mass of sulfur was determined. As an example, 1 mg/l SO4-- is equivalent to 0.334 mg/l SO4-- as S. See section 7.1, Example calculation, in the GWB Essentials Modeling Guide for more on elemental equivalents.

 

If you want to consider Fe++ and Fe+++ in equilibrium with each other, then the redox pair must remain coupled (the pair is decoupled in your example). Furthermore, you need some measure of oxidation state (DO, O2(g) fugacity, Eh, etc.) in order to distribute iron between the ferrous and ferric iron redox states. Otherwise, it will all be ferrous iron. For more info, see Chapter 7 in the Geochemical and Biogeochemical Reaction Modeling text.

 

If you want to constrain the amount of iron in your initial system and have it be in equilibrium with FeS, you would need to set the concentration of Fe++ and swap Pyrite (or FeS when you edit your thermo dataset) for HS-. In that case, you wouldn't be able to specify the dissolved sulfide concentration. Or, if you set very low concentrations of Fe++ and HS-, the fluid would quickly reach equilibrium with Pyrire (or FeS) after a small amount of SO4-- reduction. Adding FeS in the Reactants pane would titrate FeS into the system, which doesn't seem like what you want to do.

 

Hope this helps,

Brian

[kristeng]

Got it. Thanks very much for clarifying.