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[OLD] Stability of pyrite

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From: Gregg Jones

Subject: stability of pyrite

I'm trying to determine if pyrite in limestone in the Floridan aquifer is stable under the set of ground-water conditions described below. I used ACT 2 and plotted the log of the activity of the sulfate/sulfide ratio vs pH. Pyrite falls comfortably within the pyrite stability field and I want to verify that I've set up the input correctly. Here's my input data for ACT 2:

 

Diagram Fe++ on O2(aq) vs pH

Swap SO4--/S- for O2(aq)

Log a SO4-- = -2

Log a Fe++ = -7

Speciate SO4-- over X-Y

x from 0 to 14; y from -25 to 30.

go

 

Some additional info:

 

pH = 6.99

Sulfate Conc. = 1,526 mg/l

Sulfate Activity = 1.59 X 10 -2

Sulfide Conc. = 14.5 mg/l

Sulfide Activity = 1.51 X 10-4

Iron Conc. = 0.065 mg/l

Iron Activity = 6.7 X 10-7

 

Using release 3.0

 

From: Craig Bethke

Subject: Re: stability of pyrite

The good news is that it's a safe bet that pyrite is thermodynamically stable in a water of neutral pH containing ferric iron, sulfate, and sulfide. I suspect the diagram you propose is not exactly what you have in mind (it's hard to know: there is no species S-; perhaps you mean HS-?). To determine if a diagram is appropriate for a purpose, first consider what reactions you wish to consider. After you've made the diagram, select Run -> View -> Act_output.txt and verify that those reactions were used to construct the diagram.

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