katezat

Hello Alida, I'm sorry to hear you're having trouble using Xtplot. It sounds like there might be a problem with your config file(s). To test this out, please try creating a new folder (for example, make a GWB test folder on the desktop), set that to be the working directory in X1t or X2t, rerun your model, then see if you can open Xtplot. Or, you can just search for the config file(s) in your current working directory (.gtc or .xtc extensions for Gtplot and Xtplot config files, respectively) and delete them, then rerun your model and try to plot. If that doesn't help, can you please attach a screen shot of the error message you receive when you try to plot. This will help us understand your problem and troubleshoot your issue more effectively. Thank you, Katelyn

There are a variety of methods to constrain the oxidation state, including setting the pe, Eh, oxygen fugacity, dissolved oxygen concentration, equilibrium with a mineral, or the ratio of oxidized to reduced species. Rather than setting a specific pe value, though, I think you first need to conceptualize your problem. You may want to see how different conditions affect your system by setting up a reducing or oxidizing environment. If the fluid is open to the atmosphere then you could set the oxygen fugacity and you’d end up with most of your sulfur present as sulfate. Or if you’re working in an anaerobic chamber and you know that most of the sulfur is present as sulfide, you could decouple HS- from SO4—and set the sulfide concentration directly. Try taking a look at sections 7.2 (Equilibrium models) and 7.3 (Redox disequilibrium) in the GWB Essentials Modeling Guide. For even more information, Chapters 6 and 7 in the Geochemical and Biogeochemical Reaction Modeling text should be useful. A potentially interesting calculation would be to set up a reaction path model (you’d need React for this) to simulate the addition of Na2S to an initially oxygenated fluid. You could even control whether the fluid remains in contact with the atmosphere or if it’s isolated at the beginning of the path. In this way, you could see how the addition of Na2S would affect the oxidation state of the fluid, and as a result, the saturation state of galena.

Hi Spring2015, You can use SpecE8 to determine the distribution of mass among aqueous species at equilibrium. SpecE8 will figure the concentration, activity, and activity coefficient of aqueous species based on the fluid composition you define. For more info, see Section 7.1 in the GWB Essentials Guide. Kind regards, Katelyn

You can create a pourbaix diagram using the GWB program Act2. First, you will need to choose a main species, either Pb or S and then set your axis, pH (H+) and Eh (e-). You can learn how to set up an Eh-pH diagram using the GWB Essentials Guide (under the Doc tab on the GWB dashboard) in the "Using Act2" section. To incorporate additional species (i.e. Na+ or NO3-) in the diagram, you can add them under the "in the presence of" section of the Basis pane in Act2. Additionally, you can choose to decouple redox pairs (i.e. NO2-from NO3-, etc.) if you want to allow different species to coexist. You can read more on Redox couples in the Essentials Guide, Section 7.3 Redox disequilibrium. Spring2015 I noticed you found another topic helpful. For everyone else: http://forum.gwb.com/index.php?showtopic=2125&do=findComment&comment=5074

Hello Chris, I have manually reset your license. Please try activating it again and let me know if you have any trouble. Kind regards, Katelyn

Hi Tarang, You are correct, the suppress feature is a function of stability. If you suppress a mineral phase, the next most stable species will have a chance to form in its place, as in your attached example. The reason the other iron sulfide compounds (greigite, mackinawite, and marcasite) are not included in the mineral species is because those minerals are not included in the thermodynamic dataset loaded by the program (thermo.tdat by default). Thermo datasets are fully editable, though. To add a new mineral, for example, you just need an equilibrium constant corresponding to a reaction to form the mineral from species in the dataset. You can use the TEdit program (under the support tab on the GWB dashboard) to modify a dataset in this way. You can find a tutorial on how to edit thermo datasets here. Also, in the Geochemical and Biogeochemical Reaction Modeling text there is an example calculation that specifically shows how to add mackinawite to a thermo dataset (Chapter 18 Example Calculation).

Hello Tarang, Thank you for attaching your Act2 diagram, it was very helpful. There are two options for you to include sulfate and sulfide depending if you are assuming they are in equilibrium or not. If you are assuming sulfate and sulfide are in equilibrium (and that the form of the complexing sulfur species depends on the Eh and pH conditions), then you should set SO42- to speciate over x-y. This is what you have done. It’s called a mosaic diagram. You can include labels for the sub-fields of the plot where the different sulfur species (HSO4-, SO42-, H2S, HS-, S2-,etc.) are predominant. To do so, right click on your diagram, select view and check the mosaic labels box. Alternatively, you can choose to decouple H2S from SO42- to allow both species to coexist since they will no longer be in equilibrium. To decouple the redox couples go to Config > Redox Couples > select HS-/SO42-. Then you can independently add HS- to the “in the presence of field” and swap in H2S(aq). Kind regards, Katelyn

Methanol is listed as an aqueous species in the thermo.com.V8.R6+ dataset. It maybe fine for you to use as is, or you may want to modify the thermo dataset to make methanol a redox couple. A redox couple can be in terms of other redox couples (Ethanol) or in terms of other basis species (bicarbonate). If you want it in terms of bicarbonate, Rxn might be helpful for balancing the reaction. You can modify a thermo dataset using the TEdit program (under the support tab on the dashboard). You can find a tutorial on how to edit thermo datasets here. At the bottom of the page, there is a section on "Editing thermo data".

There are a few ways you can look at speciation as a function of pH in GWB Student. The easiest and simplest is to set up an activity diagram for your system of interest using Act2. The diagram should include pH as one of the axes. Here are some examples and tutorials. To determine the abundances of various species under a given condition (i.e. at a given pH) you can use the program SpecE8. In the GWB Standard Edition, the program React is useful to see how the distribution of species varies as a function of pH. You should keep in mind that the species considered in a diagram or model will depend on the thermo dataset loaded by the programs. You can use TEdit to see which species are included in the various thermo datasets distributed with the GWB. The default thermo dataset, thermo.tdat, does not include any thiosulfate species, but thermo_minteq.tdat on the other hand does.