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Features of act2 and tact


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Dear all,

 

I have been calculating activity diagrams by hand for many years, and now I have started to work with act2 (and also in tact) to save time. The basic stuff is straightforward and clear, but in addition I would need some special calculation types and plot options and I am not sure where to find them or if this can be done at all with the programs. These would be the following:

 

1) Solubility contours

Is it possible to plot simultaneously (i.e., in one diagram) aqueous species - mineral boundaries (e.g. some metal solubility) for different values of this metal (i.e., the diagram species)? I know that one can easily change the value of the diagram species and this way contour the solubility using a series of different individual diagrams and then combine this in a graphics package like Adobe Illustrator. But can this also be done directly in act or gtplot?

 

2) Total activity of an element as independent variable (either x or y axis)

For some illustrative purposes it is very useful to show mineral stabilities (or solubility boundaries) as a function of the total activity of an element. Consider for example that one wants to portray the stability of Fe minerals in the Fe-S-O-H system, then the main variables that control the stability are f(O2), pH and the sulfur activity. The latter one can express as activity of a selected species like a(H2S), but alternatively one could also select a(total sulfur). The topology of that system would be best portrayed by two complementary diagrams, one with f(O2) vs. pH and a second one with f(O2) vs. a(total S) or any other combination of the 3 independent variables. For the f(O2) vs. a(total S) the pH would be fixed, but the speciation of aqueous sulfur would need to change at some redox boundary, e.g. at the H2S - SO4-2 boundardy depending on the pH. Can this be set up in act2? So far I have only found that speciation is possible for any variables but not for the linearly independent variables that define the x and y axis of the diagram.

 

3) Is it possible to define different line colors for boundaries between aqueous species and minerals (solubility lines) and for predominance boundaries between aqueous species?

 

Best regards,

 

Thomas

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Hi Thomas:

 

Thank you for your questions and suggestions.

 

Regarding items 1 and 3, neither are currently possible in Act2 (item 1 has been requested on several occasions). I will add both to the feature request list. Note, however, that since we don't have anywhere the developer resources of Adobe or MicroSoft at our disposal, our first priorities have always been to add more geochemical modeling tools to GWB, rather than try to create the perfect graphics utilities.

 

Regarding item 2- I'm not sure that I understand the meaning of 'a(total S)' - can you explain this to me? According to the Debye-Huckel and related models, the activity of an aqueous species is calculated by multiplying its concentration times the activity coefficient. If we were going to generate a diagram for 'a(total S)', it would need to be quantified.

 

Regards,

 

Tom Meuzelaar

RockWare, Inc.

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Hello Tom,

 

thanks for your response, and especially for taking into account to add the feature of plotting several solubility contours into one diagram. As said, this task can of course be accomplished by creating several diagrams for different values of the diagram species, and then assembling them in a graphics package. But of course this is much less convenient than doing this right away in act2 before exporting to the graphics package.

 

Regarding my question number (2), I am adding some act2 scripts to illustrate this point. Diagram pyrite1 is a log(fO2) versus pH diagram that shows stability of Fe sulfides and oxides. In order to contruct that diagram, I have to fix the activity of aqueous sulfur, set for example as activity of H2S(aq) and then allow to speciate over x and y. You would certainly agree that the stability of Fe phases in the Fe-S-O-H system will not only depend on log(fO2) and pH, but will change substatially with changing the preset value for a(H2S). For example, the pyrite stability field will enlarge when increasing a(H2S) and get smaller when decreasing a(H2S).

 

To illustrate the combined effect of log(fO2), pH and a(sulfur), it is pretty useful to complement a diagram with log(fO2) versus pH with one that shows the stability of Fe minerals as function of sulfur activity in the fluid. Consider for example a section at a pH value of 6 in the log(fO2) vs. pH diagram, and then construct a diagram of log(fO2) versus log(aH2S). Now in this diagram at a log(fO2) value of about -35.5 (at pH of 6) the sulfur speciation will change from H2S(aq) to HSO4-, consequently the pyrite-hematite reaction will change slope at this point. The continuation of the pyrite-hematite reaction that involves H2S(aq) beyond that point is in effect a metastable reaction, because H2S(aq) is not stable any more (because the reaction between H2S and HSO4- has been crossed). This you can see clearly when looking at the two act2 scripts pyrite2a and pyrite2b that I attach (just superpose them).

 

I hope that it becomes clear now what my question was trying to address. As said before, I have not found a direct way of doing the composite calculation that allows for change in speciation of sulfur when using H2S(aq) as axis variable. A workaround will of course be to calculate two diagrams for the two different species that are stable at pH of 6 and then mount them together in a graphics package. This is somehwat inconvenient, and it would be preferebale to account for the change in speciation directly in one diagram.

 

Best regards,

 

Thomas

pyrite1.ac2

pyrite2a.ac2

pyrite2b.ac2

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Hi Thomas:

 

I believe I understand- correct me if I'm wrong. Ideally, you'd want a 3D activity diagram utility with fO2(g), pH and aH2S (or HSO4-) as x, y and z axes. Essentially, you'd see sulfur speciation changing according to sulfur activity in the z-axis plane (ie. coming out of the diagram).

 

There is no way to do this, nor is there an easy way to composite diagrams for separate sulfur activities. As you suggest, you'd need to put these together in a third party application.

 

I've always thought that a 3D activity diagram app would be very interesting - it's not too far removed from solid modeling. However, it is well beyond the scope of the direction we're currently going in with GWB.

 

I hope that helps,

 

Tom

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Hello Tom,

 

yes ideally I would like a 3D diagram with pH, log(fO2) and log(aS) as the three axes. Because 3D diagrams are generally difficult to read for people in publications, I am also happy with 2D sections through that 3D space. Two such sections will usually be sufficient to illustrate the effect the three variables have relative to each other. The only point I wanted to make is that if a(H2S,aq) is selected as independent variable along with f(O2,g), then it appears to me that H2S,aq cannnot be speciated. But if you look at the pyrite example scripts that I posted, you will see that beyond a certain value of f(O2,g) the species H2S,aq is not predominant, because for eaxmple at a pH of 6 we cross the predominance boundary between H2S,aq and HSO4- at a log(fO2,g) at about -34.5. The projection of the that reaction between H2S,aq and HSO4- is a simple horizontal line in the log(fO2) versus log(aS) diagram. Hence, any reactions between Fe minerals that involve the predominant aqueous sulfur species should change slope at this predominance boundary. Currently, the only way to construct a diagram that to my opinion considers the true stable reactions is to plot two diagrams, one of them in terms of f(O2) versus a(H2S,aq) and the second one in terms of f(O2) versus a(HSO4-) and then superpose them in a graphics package. Doing so is not too big a problem, but of course it would be more convenient and efficient if total activities could be used as independent variables and then speciation taken into account. The S example is rather simple and straightforward, but there are systems with multiple predominance boundaries where tracking the change in speciation by hand can become more difficult.

 

Best regards,

 

Thomas

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  • 9 years later...

Hi Thomas,

I hope you're doing well. I happened to come across this old post and thought you might be interested to know about Phase2, an app introduced with GWB12. The program essentially traces a stacked series of reaction paths, as you'd run in React, to traverse two geochemical variables of interest.

You can set up a diagram with sliding log f O2(g) and fixed pH along the y axis, then sliding pH with fixed f O2(g) along the x axis. The basis fluid is defined in terms of total concentrations, as in React, and mass is conserved throughout the calculation. You can also titrate a species into a fluid initially devoid of it to consider a range of total concentrations. By titrating SO4-- in log steps you can make a diagram much like you've envisioned, with the sulfur speciation depending on the y axis variable, log f O2(g). 

You can render the calculation results in various types of 2D diagrams or in horizontal or vertical cross-sections through the diagram. For the 2D diagrams, you can plot "true predominance" for any basis species or element (the species accounting for the most mass predominates, not the species with the highest activity), mineral assemblage diagrams (which show every stable mineral or combinations of minerals), and render any variable as a color map, mask, or contour. In a log f O2-pH diagram, for example, you can diagram the stable iron minerals under various conditions and contour the concentration of dissolved Fe. 

Please visit GWB.com/phase2.php to learn more. I'm happy to send a demo if you'd like to try it out.

Cheers,

Brian Farrell
Aqueous Solutions LLC

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