How can I get the activitiy of each element before I start a pH-Eh diagram

[Shaofeng]

Hi Brian,

I am new for GWB9. I am trying to construct pH-Eh diagram for arsenic, sulfur and Fe in a system containing 0.01333mM As, 3mM Fe and 3mM S. But in GWB, it requires activity rather than concentrations of each element. So, how can I get the correct activity in this system. Use some software to calculate them? I will greatly appreciate to your help if you can show me step by step.

 

 

Regards,

 

Shaofeng

[Shaofeng]

By the way I used system in the attachement to construct the diagram. But I got a diagram as shown in the attachment. I think this diagram is not correct because scorodite is unstable under neutral to alkaline conditions.

Please see the attachements.

Shaofeng

As-Fe-S.ac2

As-Fe-S.pdf

[Brian Farrell]

Hi Shaofeng,

 

People sometimes assume that activity is equivalent to molality in dilute fluids. If this assumption is not appropriate, however, it is easy enough to use GSS or SpecE8 to calculate the activity of individual species in solution. Just pick an appropriate pH and oxidation state to use as constraints with the information you provided. The chapters on GSS and SpecE8 in the GWB Essentials Modeling Guide describe how to calculate the activity of free species.

 

I can't comment specifically on where scorodite should be stable, but if I create a mosaic diagram using your input the stability field changes quite a bit. A mosaic diagram accounts for the speciation of complexing ligands in addition to the main species being diagramed. In your case, the SO4-- component can be present as H2S, HS-, and S-- in addition to SO4--, depending on the pH and Eh. The Fe++ component can similarly be present as Fe+++, Hematite, Fe++, Magnetite, etc. You should take a look at section 5.3 of the GWB Essentials Modeling Guide for more on mosaic diagrams.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC