Brian Farrell

Hi Ellie, No worries. That's what I was going to suggest. It sounds like you're on the right track. Regards, Brian

Hi, Before you dive in too deep with your own data, I think it would be a good idea to practice some of the skills you’ll need using examples from the User’s Guide. I think the most important parts of the problem are: -defining the initial system (Sections 7.1 and 7.2 in the GWB Essentials Guide; Sections 2.1 and 2.3 in the GWB Reaction Modeling Guide) -redox disequilibrium (Section 7.3 in the GWB Essentials Guide) -sliding fugacity paths (Section 3.5 in the GWB Reaction Modeling Guide) -picking up chemical systems (Section 3.9 in the GWB Reaction Modeling Guide) -sliding temperature paths (Section 3.4 in the GWB Reaction Modeling Guide) As a first step, I would define a fluid containing Fe++, HS-, Ni++, H+, and so on, with a negligible amount of HCO3-, then verify that you precipitate your desired mineral. Next, you can fold in your CO2 addition, pick up the results, and then use those as the starting point for a sliding temperature path (the GWB does indeed account for the effects of temperature on pH). Hope this helps, Brian Farrell Aqueous Solutions LLC

Dear Yanlu, Thanks for explaining your situation and for providing the error messages you’ve received. I just sent you a file that should reset your activation. If you need to clean your OS in the future, you can deactivate your license beforehand, then reactivate when you're done. That will be a little easier than running the reset file I sent. Regards, Brian Farrell Aqueous Solutions LLC

Hi Ben, You’re very welcome. One idea would be to add a kinetic rate law for any mineral present but not in equilibrium with the initial system. Of course, you’d need meaningful kinetic parameters. If you prefer the “backwards” configuration, you can do that too. In any titration path (minerals added as simple reactants to a fluid, for example) the reactant mass is added incrementally throughout the calculation (100 steps by default). So if you react 1 kg of a mineral with ~1 kg of solution, you’ll first add 10 g, then another 10 g, and so on, until you’ve reacted the entire mass. In this way, you’re effectively starting at an infinite water:rock ratio and decreasing it to 1 at the end of the reaction path. And as before, you can plot the amount of mass reacted on your x or y axis. This is obviously different conceptually from the previous suggestion, but I think the term leaves a lot of room for interpretation. I'm not positive how EQ3/6 conceptualizes the problem, but after a quick search I found an excerpt from Low-Grade Metamorphism of Mafic Rocks edited by Peter Schiffman and Howard W. Day. The description on page 90 seems pretty similar to what I've described in this reply. Hope this helps, Brian

Dear Avery, The thermo_ymp.R2.tdat dataset was first made available for the GWB12 release. As such, it follows the “jul17” dataset format introduced with GWB12. The format adds support for including formulae for aqueous species; includes factors for calculating fugacity coefficients for the gas species; and designates a default method for the calculation of fugacity coefficients and partial pressure. The dataset will not work with GWB releases before 12. For more information on thermo dataset formats, please see the “Thermo Datasets” chapter of the GWB Reference Manual. The “Legacy formats” section should be especially relevant. Regards, Brian Farrell Aqueous Solutions LLC

Dear Benjamin, There are probably a variety of ways to conceptualize the problem, but combining React’s “flush” configuration with the “reactants times” feature is probably one of the more elegant ways to do so. A flush model is traced from the reference frame of the rock through which a fluid migrates. The migrating fluid displaces existing fluid from the system. Please note there is no “water-to-rock ratio” variable in the plot output. However, the “H2O reacted” (solvent) or “Mass reacted” (solvent + solutes) variables available under Reactants properties (or Chemical parameters for GWB releases before 12) are probably the best ways to describe the progress of reaction. If you export the plot as an enhanced metafile to PowerPoint, it’s a simple matter to adjust the label. For more information and an example of the flush configuration, please see section 3.3, Flush model, in the GWB Reaction Modeling Guide. More good examples can be found in section 3.4, Polythermal reaction paths, of the Reaction Modeling Guide, as well as section 19.4, Dolomitization of a limestone, in Craig Bethke’s Geochemical and Biogeochemical Reaction Modeling text. Hope this helps, Brian Farrell Aqueous Solutions LLC

You are very welcome. I hope it goes well. Brian

You should be able to plot user defined analytes on XY plots, but the special plots only accept true basis species (e.g. B(OH)3, Cl-, Li+) and in some cases chemical parameters (e.g. pH and TDS in the Durov diagram). Thanks for your suggestions to improve our software. We receive many requests from our users (some more feasible than others), so it's hard to keep everyone happy, but we can certainly add your request to the list to consider as we plan the next release. Regards, Brian

Hello, Currently there is no way to add multipliers on a ternary diagram, apart from manually adjusting the concentration values in your GSS spreadsheet. Regards, Brian Farrell Aqueous Solutions LLC

Hi Yongtae Kim, The Dzombak and Morel two-layer model is a very thorough compilation of cation and anion sorption reactions involving hydrous ferric oxide. It is in no way complete, however. The FeOH.sdat dataset included with the GWB contains reactions and log Ks that Dzombak and Morel derived from actual experiments, while the FeOH+.sdat includes some estimates for additional sorbing species. I believe there is a reaction for SiO2 sorption in the FeOH+ dataset, but not in the FeOH dataset, for example, because they didn’t have any experimental data available. You should keep in mind that the Dzombak and Morel datasets are examples. The mineral they studied (and its surface properties) might be close to the iron mineral in your experiments, but they are not necessarily the same. In your models, the SiO2(aq) and HCO3- components do not significantly decrease the efficiency of arsenic removal. As I mentioned above, the FeOH.sdat dataset contains no sorption reactions for SiO2, while neither FeOH.sdat nor FeOH+.sdat include any reactions for HCO3- species. They do, however, have reactions for As(III) species and PO4---, which is why you observe both of those components to sorb. You can view the datasets yourself using the TEdit app. TEdit can also be used to add reactions that you need. You’ll need to supply a reaction for each surface complex along with an equilibrium constant, expressed as a log K. There should be information for sorption of HCO3- to iron oxyhydroxides in the literature, and there may be additional information for SiO2 as well. Or, you can perform sorption experiments and derive log Ks yourself. For more information on TEdit, please see chapter 9 Using TEdit in the GWB Essentials Guide. If you have a redox equilibrium model enabled (the default), the arsenic will be present as both As(V) and As(III), but mostly As(V), reflecting the high DO concentration you’ve specified. If you believe that the arsenic should be present in only one valence, you should decouple the redox reactions involving arsenic and add only the appropriate basis entry. For more information, please see 2.4 Redox couples and 7.3 Redox disequilibrium in the GWB Essentials Guide. By the way, if your reactor remains in contact with the atmosphere throughout the experiment, you should fix the fugacity of O2(g) in your model (and CO2(g) as well, when you include the HCO3- component). If it is closed to the atmosphere, then you most likely should not fix the fugacity of these gases. Please see section 3.5 Buffered paths in the GWB Reaction Modeling Guide for more information. Hope this helps, Brian Farrell Aqueous Solutions LLC

I'm glad to hear that you can draw the water stability limits now. I hope you enjoy using the software. Brian

Hi Xinyu, Act2 needs reactions for O2(g) and H2(g) to draw the water stability limits. The thermo_minteq.tdat dataset does not contain a reaction for H2(g), however, so you’ll have to add it. Because the reaction for H2(g) is typically written in terms of H2(aq), there are actually two reactions you need to add: H2(aq) + .5 O2(aq) = H2O (in the redox couples section) H2(g) = H2(aq) (in the gases section) These are the reactions you’ll find in the GWB’s default dataset, thermo.tdat. Each of these reactions has equilibrium constants (expressed as log Ks) at multiple principal temperatures. The principal temperatures differ between the two datasets, so you’ll need to make sure to specify the log Ks for the correct temperatures. In older versions of the GWB, you had to edit the datasets by hand in a text editor like Notepad, so this was easy to overlook. Now you can simply copy and paste (or drag and drop) an entry from one dataset to another using the TEdit app. Best of all, TEdit will automatically find matching principal temperatures and insert the log Ks in the correct locations. For more on using TEdit to copy reactions, please see 9.2.6 Transferring dataset entries in the GWB Essentials Guide. For an older discussion of a similar topic, please see this thread. Hope this helps, Brian Farrell Aqueous Solutions LLC

Hi Mauricio, The GWB does not currently support surface precipitation as developed in Dzombak and Morel’s generalized two-layer model. Regards, Brian Farrell Aqueous Solutions LLC

Hi Alero, After thinking about this some more, you might be able to put together something like you described with a modified thermo dataset. I'd start by adding h+ as a redox couple, using the reaction h+ = H+ + .25 O2(aq) - .5 H2O. Set the charge of h+ to 1, its mass to that of an electron, and make it really unstable. In React, set up the chloride oxidation problem with a kinetic rate law. Decouple the h+ as well as the ClO4-, set very small initial amounts of h+ and ClO4-, then enter the reaction above for the kinetic redox reaction and assume a rate constant. If you add the h+ as a simple reactant, you should observe the Cl- oxidizing to ClO4- and pH decreasing with time, like in the reaction you described. Hope this helps, Brian

Dear GWB users, We are pleased to announce our latest maintenance release, GWB 12.0.2. The 12.0.2 update features improvements to special plots; addresses issues with Xtplot contours and color maps; updates and fixes GWB Plugin feature and documentation, including support for Visual Studio 2017; adds fixes for ChemPlugin wrappers for Fortran, Java, and MatLab; and provides fixes for all known issues. Update from 12.0.1 at no charge to ensure you have all the newest features and bug fixes. Existing installations should automatically update to this release, unless auto-update is disabled. In that case, users should update their installations from the Help menu of any GWB app. Regards, Brian Farrell Aqueous Solutions

Hi Coralie, You can overlay contours of a single variable on a predominance map in P2plot. Currently you can plot a predominance map of one basis entry or element at a time, however. If you want to overlay the predominance bounds from other basis entries or elements you will need to do so in another program like PowerPoint or Illustrator. Regards, Brian

Hi Alero, The software does not currently support photoelectrochemical reactions like that you've described. Regards, Brian Farrell Aqueous Solutions LLC

I'm happy to hear it helped. Cheers, Brian

Hi Coralie, There are various ways to interact with P2plot and the other plotting apps. You can double-click on a specific plot aspect, like the "Rxn progress (x)" axis variable, to open the "Axis Range and Variables" dialog. Or, you can right-click on "Rxn progress (x)" and pick "X Axis Range" to open the same dialog. Finally, you can use the menubar along the top of P2plot to access the dialogs. Under Format, choose "Axis Range and Variables...". From that dialog, you can pick your axis variables, set linear or log scales, tick increments, units, and so on. You can add contours from Format -> "Contour...". I recommend reading the Phase2 and P2plot chapters in the GWB Reaction Modeling Guide, which you can access from the Help menu of any GWB app. At the very least, you should check out the first couple examples (7.8 Example: Speciation diagram and 7.9 Example: Mineral solubility) in the guide to see the process of calculating and rendering the diagrams. The thermo dataset that you set in the Preferences dialog will become the default dataset every subsequent time you open a GWB app, but it won't load that dataset into an instance of the app that is already open. To do that, you would go to File -> Open -> Thermo Data... You can verify at any time the dataset that is currently loaded by going to File -> File Properties -> Thermo Data. Hope this helps, Brian

Hi Coralie, It sounds like you're using Act2 and Tact for your diagram construction, but I think what you need is Phase2, a new app we developed for GWB12. From the GWB Reaction Modeling Guide: A Phase2 diagram differs from a simple diagram of the type constructed by Act2 and Tact in that each point in the diagram represents the complete solution to the equations describing the system’s distribution of mass. If you were to use Phase2 to calculate an Eh-pH diagram, for example, you would find that, unlike the result from Act2, the boundary lines are curved, rather than linear. Because of the calculation’s completeness, some Phase2 diagrams differ qualitatively from their Act2 counterparts. For this reason, the diagrams are sometimes referred to as “true” Eh-pH or “true” activity diagrams. Phase2 diagrams, furthermore, can be plotted over a wide range of variable choices, and in a variety of ways. You can plot the results of Phase2 calculations as maps of species predominance or mineral assemblages, and you can color map, mask, or contour any variable. You can additionally render cross-sections through the diagrams. Because a Phase2 calculation is conceptually similar to React, you can use any units to constrain your chemical system, and you can choose a variety of axis variables not possible in Act2 or Tact. For more information, please see our Phase2 webpage and the documentation on Phase2 and P2plot in the GWB Reaction Modeling Guide. Regards, Brian Farrell Aqueous Solutions LLC

Hi Mauricio, It took me a while, but I thing I figured out your problem. There are various conventions for modeling bidendate surface complexation reactions. For a helpful review, please see "Mass action expressions for bidendate adsorption in surface complexation modeling: theory and practice" by Wang and Giammar, 2013. As far as I can tell, the CHESS program and FITEQL (by default) use "model 1", in which the molar or molal concentration of surfaces complexes are carried in the mass action equation (see Table 2 in the reference). This convention is problematic in certain ways, as explained in the text. Starting with release 9, the GWB apps use "model 3", in which the mole faction of surface sites is carried in the mass action equation. Section 4.4 Practical Suggestions for SCM Practitioners provides some guidance for converting data collected under "model 1" to a form for use with "model 3" (equation 26). When I did this, I found a log K of 7.945 for the bidentate complexation reaction seemed to match up with the curves in Figure 3a pretty well. As for combining the updated bidentate model with the ion exchange model for H+ (but not Cu++), I don't observe a visual difference either. I think this is okay, though. Comparing Figure 3a with Figure 5a in the paper, I can't observe a difference between those two. And in the text, the authors state "...The goodness-of-fit for this model is not statistically different from the 1-site bidentate model...". Hope this helps, Brian

Hi John, I tried to run the model as is. I think one problem is that the Al2O3 mineral that’s swapped into the basis is really unstable at the specified pH conditions. I also can’t understand why the Quartz and Al2O3 are both swapped into the basis and set as simple reactants. I’ve attached my interpretation of the model that is discussed in the paper. It runs fine, but I’m not getting the same answers as the paper, so obviously my interpretation isn’t quite right either. There are a few details that I’m pretty confident in, though. I think they are using the water chemistry data from Sample 1. The reported analysis is an average from several sampling periods, but in their modeling they state they use pH and Eh values from the second sampling period, which are 4.44 and 450 mV, respectively. They specify the concentration of nitrate (NO3-) as nitrogen (N). In other words, the mass of the oxygens is not counted. You can set concentrations as elemental equivalents in React by clicking the pulldown next to the unit for NO3- and selecting “as -> N”. I think all redox coupling reactions should be enabled here (specifically between Fe++ and Fe+++) so that Magnetite can form in response to the specified Eh. They listed the stability of Kaolinite, Quartz, Magnetite, and Al2O3. The values for Magnetite and Al2O3 were slightly different from those in the thermo dataset, so I modified the 25 C values from the Config -> Alter log Ks dialog. The original model was done in PHREEQC, which by default does not allow supersaturated minerals to precipitate. You can replicate this by going to Config -> Suppress, changing the “list” pulldown from “All” to “Minerals”, then hitting “suppress all”. Then, unsuppress the minerals they reported (Kaolinite, Quartz, Magnetite, and potentially Al2O3). The paper calls the mineral reactants, so instead of swapping them into the Basis, I set them as simple reactants on the Reactants pane. I needed to set a nonzero but trivial amount of H4SiO4 and Al+++ in the Basis pane. Hope this helps, Brian teat-modified.rea

Hi John, Thanks for the additional information. I read through the section of the paper entitled “Geochemical simulation in the granitic aquifer”. It’s really not clear from the paper how the model was constructed. It’s especially difficult to check because they only report the end-point values for a few parameters like Eh and pH. I would recommend writing to the corresponding author to ask for a more detailed explanation of the model, along with an input file for the PHREEQC calculations. Perhaps with this information we can help your client get the model up and running. Regards, Brian Farrell Aqueous Solutions LLC

Hi John, Can you please explain the conceptual model for this input file? What type of process are you trying to simulate? What is the extent (size or volume) of the system? I'm especially curious about the Al2O3 and Quartz constraints in the Basis. Also, are the constraints for Fe++ and Fe+++ real (both are the same value)? Thanks, Brian Farrell Aqueous Solutions LLC

Hi Mauricio, I think your surface dataset looks okay. It's possible that your thermo dataset is slightly different from the one used in the original calculations, but I'm not sure how important that is in this case. The paper lists several Cu++ species, including carbonate complexes, that are accounted for in the calculation, but I'm not sure your dataset includes them all. Furthermore, the paper lists a specific Cu++ species, CuOH+, with a log K of formation of -7.29. The dissociation reaction for that species is listed in your thermo dataset with a log K of 7.497. You might try using the "alter" command to set its value to 7.29. If you track down the carbonate complexes, you might consider adding a CO2 buffer to your calculation, but I think the paper mentioned those complexes weren't too important in this calculation, so you might not worry about it. I think the most important factor is that the "sorbate include" setting has not been applied. In the experiments, the Cu concentration refers to the total amount of Cu in solution and sorbed to the surfaces. In the GWB's default state, the Cu concentration you set on the Basis pane refers only to that in solution (think of sampling water from a well). You can use the "sorbate include" option from the Config -> Iteration dialog to specify that the Cu concentration set on the Basis pane includes sorbed as well as dissolved mass. When I do that, and disable charge balancing and delete the Cl- basis entry, I get results that look very much like those in Figure 2. For more information, please see React's "sorbate" command in the GWB Command Reference/ Reference Manual. Hope this helps, Brian Farrell Aqueous Solutions LLC