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CO2 in AMD fluids

Tom Meuzelaar

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[admin notice: the below is from the former GWB users group email distribution list. This message was originally posted 12/13/2005]


Posted by: Johan Fourie


Dear Craig


In my current study I am working with acid solutions in a coal mine AMD environment.


Typically I let pyrite kinetically react with calcite and dolomite. The pH decreases in my model from neutral (pH 7.5) to acidic (pH 3).


I also fix the CO2 log fugacity to atmospheric conditions -3.5 in order that the model is not a closed system (throughout the model).


Is this correct?


In your book Geochemical Reaction Modeling you state that one must let the CO2(aq) reach an activity of one and then correct (?) the CO2 content.


However I am not sure how to practically apply this.


I appreciate your help.





Posted by: Craig Bethke


Hi Johan,


I think there are two ideas here. If you have a fluid in contact with the atmosphere and it is acidified slowly enough for CO2 to diffuse from the fluid easily, you would likely want to assume equilibrium with atmospheric CO2, as you have done.


Alternatively, the fluid may be isolated from the atmosphere, or the acidification may proceed too quickly to maintain equilibrium with atmospheric CO2. In this case, the CO2 content of the fluid and hence its CO2 fugacity will build up.


Once the fugacity exceeds the confining pressure on the fluid (e.g., 1 atm at the surface), it will start to effervesce, like when you open a can of soda or put a drop of acid on limestone. In this case, you would want to run the build-up leg of the simulation as a closed system, then the remaining leg with CO2 fugacity set to one.


Hope this helps,



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