Arc2: Sb speciation over pH as a function of sulfide

[Apus]

I am trying to plot Sb(III) speciation over pH as a function of sulfide, which itself speciates over pH (using Arc2). The x axis is pH and the y axis should be the sum of HS- and H2S concentrations. I have added the thermodynamc data for Sb sulfide complexes to the database, but sulfide does not speciate as HS-/H2S.  I managed to plot Sb speciation with either HS- or H2S as the y axis and I have attached these files.

Could you please advise me if (and how) it is possible that sulfide speciates over pH (no redox reactions required) to merge the two  diagrams into one?

Thanks a lot

KH

[Brian Farrell]

Hi KH,

This is an interesting problem. It’s similar to the “mosaic diagram” feature in Act2, but it’s the axis variable that needs to speciate. There’s no way to account for this in the simple calculations that Act2 performs, apart from drawing separate diagrams for the regions in which HS- or H2S predominate. 

An actual multicomponent chemical model, like a SpecE8 calculation, allows you to handle more complexity. For a single condition, you can calculate the predominant Sb species or stable Sb minerals. You can set the composition of a fluid in terms of total (or component) concentrations, and the program will account for the complete set of equations describing the distribution of mass. 

You can take this analysis a step further in React by looking at the effect of one variable at a time. By titrating HS- into a fluid at a fixed pH (preferably using log steps), or by sliding the pH in fluids of specified total HS- concentration, you can look at the effects of total HS- or pH on species predominance and mineral stability. Essentially, these calculations are like transects plotted through the 2D diagram you’re envisioning. 

The best solution to make the diagram you want is to use the new Phase2 program in GWB12. It was created for problems like this. It’s essentially a two-stage calculation in which a series of React paths are stacked together to form a 2D grid. Some diagrams may resemble what you can calculate in Act2, but you have many more capabilities and fewer simplifying assumptions. I've attached an example of the diagram I think you're trying to make, using Phase2 and the thermo.com.V8.R6+.tdat dataset.

Hope this helps,

Brian Farrell
Aqueous Solutions LLC