[OLD] Need help with diagram

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From: Michael R. Schock

Subject: Need help with diagram

I have just recently acquired GWB 3.2 and am really looking forward to applying it to our various drinking water chemical treatment studies. I have run into a snag with a diagram I would like to make, and I think I am probably missing something basic because of my inexperience. My ultimate goal is to construct a solubility vs. Eh diagram for Pb (or copper), at different pH's and for a given total carbonate concentration. I'll do multiple diagrams for different total carbonate concentrations later. I have tried using Act2 with the thermo.com.v8.r6.dat database, to which I have (I think correctly) added some aqueous species and modified some stability constants of Pb solids and complexes to make them consistent with several prior publications and modeling efforts I've done. I can get a line for Pb(II) activity, which starts flat (hydrocerussite or cerussite control) and then drops off dramatically as it passes the Eh about where I'd expect Pb(IV) to start forming and platternite to take over as the solubility control. However, I haven't been able to be positive I'm getting the solubility of Pb, and I don't get any subsequent line for equilibrium with plattnerite. I've expanded the scale to unreasonably low Pb(II) activities and it never flattens out, even at the very low concentrations I'd expect.

 

From: Michael R. Schock

Subject: Need Diagram Help

At Craig's suggestion, I am reposting this help request, with a file attached that is the ACT2 script I am using that is generating this problem and question. I have just recently acquired GWB 3.2 and am really looking forward to applying it to our various drinking water chemical treatment studies. It should be extremely useful. I have run into a snag with a diagram I would like to make, and I think I am probably missing something basic because of my inexperience. My ultimate goal is to construct a solubility vs. Eh diagram for Pb (or copper), at different pH's and for a given total inorganic carbonate concentration. I'll do multiple diagrams for different total carbonate concentrations later. I have run the attached Act2 script with the original LLNL thermo.com.v8.r6.dat database, as furnished with GWB. Later, I hope to add some aqueous species and modified some stability constants of Pb solids and complexes to make them consistent with several prior publications and modeling efforts I've done, but I want to get the fundamental diagram process working first. When the attached script is run, here are the problems I'm running into. For the initial conditions I set the pH to 8, the inorganic C concentration to 18 mg/L as C (local tap water concentration), decoupled C so there aren't organic reaction complications (when I saved the script it saved it as the individual decouple commands), and set HCO3- to speciate across pH for when I do other pH's. I also disallowed several Pb solids and Pb(g) that are not viable for drinking water conditions. I have tried other pH's, and the controlling Pb(II) solid and Pb(II) aqueous species seem to change as expected, but not total Pb solubility.

(1) I can get a line for Pb(II) activity, which starts flat (hydrocerussite or cerussite control) and then drops off dramatically as it passes the Eh about where I'd expect Pb(IV) to start forming and platternite to take over as the solubility control. This makes sense.

(2) However, I can't find a way to produce a line for Pb(IV) activity when it would be controlled by the solubility of plattnerite. I've expanded the scale to unreasonably low Pb(II) activities and it never flattens out, even at the very low concentrations I'd expect.

(3) Would it be possible to alter such a diagram for ionic strength effects?

Could one of you experienced users show me where I've gone wrong, or give me an example of how to set up this kind of problem? I can't really find a comparable example in either the User's Guide or the green book. Is it not possible to do such a diagram with Act2? Or should it be set up somehow in REACT, and then somehow plotted with Gtplot or some other plotting program?