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[OLD] Sulfur species and Organic Carbon

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From: Henry Kerfoot

Subject: Sulfur species and Organic Carbon

My pH 12.9 water has 1960 mg/l SO4--, 45.4 mg/l S-- and 92 mg/l SO3-- , and 121 mg/l Dissolved Organic Carbon at Eh = -0.44 V. I do not have data on other S species. I was able to swap S—for HS- and add that species, but cannot figure out how to get sulfite in. In order to get in the organic carbon (as a potential reducing agent) I had to swap out O2(aq) for acetate. Then I could not use the measured Eh value. I have used it as some sort of a measure of the accuracy of the description of the system by the inputs. The Eh value resulting is approx. -0.65 V (as opposed to the measured -0.44 V). I have a couple of questions:

1. How can I get the sulfite concentration into the model? Is sulfite present in the database (and if not does anyone have thermo data on 30+ S species?)

2. Is it appropriate to use acetate as a surrogate for organic carbon? (assuming it is reduced organic carbon amenable to aerobic degradation or anaerobic fermentation).

From: Craig Bethke

Subject: Re: Sulfur species and Organic Carbon

First of all, you need to select a database such as thermo.com.v8.r6+.dat that contains sulfite species. Second, instead of swapping the basis, you need to decouple redox reactions. Specifically, you should decouple sulfite and sulfide from sulfate, and acetate from carbonate. In this way, you can calculate a model at redox disequilibrium. You will notice that each redox couple has a Nernst Eh that will almost certainly differ from the measured value. You can read about redox disequilibrium in the “green book� and the references cited there. As to whether acetate is an appropriate proxy for dissolved organic carbon, that depends on the goal of your modeling. But if I were to make such a simplification, I would certainly proceed with caution!

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