Sulfate reduction

[Chard121]

Hi,

 

I'm attempting to use React to model the effect of reducing sulfate to hydrogen sulfide on pE. I understand how to graph the equilibrium between sulfate and hydrogen sulfide as I shift pE, but not how to do it the other way around.

 

More details:

 

I'm modeling sulfate reducing bacteria in culture. They're using acetate as an electron donor and sulfate as an electron acceptor.

 

Thanks for any advice.

[Brian Farrell]

Hello,

 

Instead of swapping HS- in for SO4--, I think you'll want to decouple the HS-/ SO4-- and CH3COO-/ HCO3- redox pairs. That way, you can add both the reactants and products (likely a small, but nonzero concentration) to your Basis.

 

Depending on your interests, you might set a kinetic rate law which describes the rate of sulfate reduction. The GWB User's Guides describe redox/ microbial kinetics and include some simple examples. For more on the theory, you should take a look at Chapter 7 (Redox disequilibrium), Chapter 17 (Redox kinetics), and Chapter 18 (Microbial kinetics) in Craig Bethke's Geochemical and Biogeochmical Reaction Modeling text.

 

If you're not particularly interested in how fast the reaction occurs, but just how the pe changes with different activities of SO4-- and HS-, you might try a parametric model, in which reactant species are titrated out of the system and product species are titrated in. An example for microbial iron reduction is described here.

 

In either case, React should include in its output a calculated Nernst Eh/ pe for the decoupled redox pairs (HS-/ SO4-- and CH3COO-/ HCO3-).

 

Hope this helps.

 

Brian Farrell

Aqueous Solutions LLC

[Chard121]

Hello,

 

Instead of swapping HS- in for SO4--, I think you'll want to decouple the HS-/ SO4-- and CH3COO-/ HCO3- redox pairs. That way, you can add both the reactants and products (likely a small, but nonzero concentration) to your Basis.

 

Depending on your interests, you might set a kinetic rate law which describes the rate of sulfate reduction. The GWB User's Guides describe redox/ microbial kinetics and include some simple examples. For more on the theory, you should take a look at Chapter 7 (Redox disequilibrium), Chapter 17 (Redox kinetics), and Chapter 18 (Microbial kinetics) in Craig Bethke's Geochemical and Biogeochmical Reaction Modeling text.

 

If you're not particularly interested in how fast the reaction occurs, but just how the pe changes with different activities of SO4-- and HS-, you might try a parametric model, in which reactant species are titrated out of the system and product species are titrated in. An example for microbial iron reduction is described here.

 

In either case, React should include in its output a calculated Nernst Eh/ pe for the decoupled redox pairs (HS-/ SO4-- and CH3COO-/ HCO3-).

 

Hope this helps.

 

Brian Farrell

Aqueous Solutions LLC

 

Thanks for the quick reply,

 

I may be interested in the rate of the reaction in the future, but for now the parametric model should be sufficient. I'm pretty new to the program and was unaware of the decoupling option, it certainly helped.