[Speciate different components over x-y]

I am having exactly the same problem with calculating Eh/pH diagrams for Fe-Se sulfides with act2. In fact, I am not titally convinced yet that there is no way to derive a unique solution. In my case, the Se concentration is 5 orders of magnitude below that of Fe and S, hence the influence of Se equilibrium on Fe and S speciation might be negligible. Hence in principle one should be able to derive the Fe-S stability fields first, and then speciate the Fe-S output with Se. Has somebody found a solution to this problem (which is rather common, I assume)?

 

Andreas

 

 

 

From: "Brugger, Joel (SAM)"

Subject: Speciate different components over x-y

I'm trying to calculate a log f O2(g) vs pH diagram to show relationships between Fe-Ni sulfides. I therefore speciate SO4-- and Fe++ over x-y and make the diagram for Ni++. I get a nice diagram, but something disturbs me.

The dashed boundaries correspond to the overlap of the boundaries for the pure Fe-O-H and S-O-H systems. It seems more appropiate that the boundaries for Fe should take into account Fe sulfides (e.g. pyrite), i.e. the Fe should be speciated in the Fe-S-O-H system.

So my question is: is there any way to tell GWB to speciate S in the S-O-H system, and then Fe in the Fe-S-O-H system, and then to calculate the nickel speciate over these two diagrams?

 

From: Craig Bethke

Subject: Re: Speciate different components over x-y

I think you'll find that the scheme you propose is not internally consistent and will not result in a unique solution. To convince yourself, try drawing such a diagram by hand. I think you will find that you end up with more than one way to balance the reactions among the Ni species.