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# Brian Farrell

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1. ## Bulk quantities and phase/activity diagrams

Hi Silvain, As long as you don’t specify “free” or choose a unit that implies a free constraint (e.g. activity or pH), your constraint is for the bulk concentration of a component. To test this out, fire up SpecE8 and enter HCO3- = .001 molal pH = 6 Na+ = 1 molal balance on Cl- go You can look in the text output file to see the concentrations of several carbon species: CO2(aq) has the highest concentration at .0005060 molal, then in decreasing abundance HCO3-, NaHCO3, CO3--, and NaCO3-. Add them up and you’ll find the molal concentrations will sum to .001 molal, which is the bulk constraint you supplied. Scroll down to “Original basis total moles” in the text file and you’ll see HCO3- = .001 moles, which matches your bulk constraint. You can of course look in the plot file as well. The “species concentration” variable type refers to all the individual aqueous species, and the “components in fluid” refers to bulk composition. You could alternatively use the input below and you’d get the same results. swap CO2(aq) for HCO3- CO2(aq) = .001 molal pH = 6 Na+ = 1 molal balance on Cl- go Of course, if you use units like mg/kg you’ll have to account for the mole weight of whatever species is in your basis, or use the “as” setting to specify concentration in terms of mg Carbon, mg CO2, etc. Since we calculated the complete distribution of species, we can take our results from before and test out the “free” constraint setting: swap CO2(aq) for HCO3- CO2(aq) = .0005060 free molal pH = 6 Na+ = 1 molal balance on Cl- go Running this latest version should give equivalent results. For your second question: Act2 calculates the simple type of diagram that geochemists have traditionally drawn by hand. By design, a number of simplifications make the calculation straightforward (but still laborious). For example, you can only have logarithmic axes (log activity, pH, pe or Eh, etc.). There’s no mass balance. You have to work in terms of activity, rather than concentration. The diagrams are in many cases fairly similar, though. If you want a general picture of how chemistry of a particular system works, a traditional calculation might be fine. Certain applications might demand a more rigorous solution. Beyond recreating activity or redox-pH diagrams, though, Phase2 calculates other diagrams that Act2 cannot even contemplate. You can include surface complexes in your calculations, for example. Or, you can diagram how various properties (solubilities, saturation indices, gas pressures, and so on) vary across the diagram using color maps or contours. You can account for isotope fractionation or kinetic reactions. You can plot assemblages (combinations) of stable minerals under different geochemical conditions. Basically, it has all the capabilities of React, so it's almost endlessly configurable. Hope this helps, Brian Farrell Aqueous Solutions LLC
2. ## Chemical compositions of pore water in bentonite using React module

Hi Polly, For the two-layer surface complexation dataset, you need to specify a site density or densities for each sorbing mineral. If you don’t, the program can’t account for the existence of the surface. If Ferrihydrite contains both of the sites you defined, >(w1)SOH and >(w2)SOH, your entry might look like this: Ferrihydrite surface area= 600.0000 m2/g 2 sorption sites >(w1)SOH site density= .0050 mol/mol mineral >(w2)SOH site density= .2000 mol/mol mineral You should, of course, supply values appropriate for your sorbing mineral. In GWB14, by the way, you can alternatively specify site density in sites/nm2, as you sometimes see in the literature. You should also remove the entry for >(w) in the surface species section. I’m not sure if it was intentionally put in the dataset like this, or it was a stub of an entry that you didn’t finish, but there’s no reaction, stability, or mole weight and that’s causing a problem. Finally, the surface dataset has a field where the user specifies the thermo dataset to use with it. The aqueous species that are included in the surface reactions, as well as the sorbing minerals, are drawn from the thermo dataset you specify. If you plan to use this surface dataset with your custom thermo dataset, it might be best to specify that custom thermo dataset within the surface dataset. Note that the sorbing mineral you’ve chosen, Ferrihydite, is not in your custom thermo dataset, so you may need to make further modifications to either your thermo or surface dataset. You’ll probably have to make these modifications in a text editor, like Notepad, since TEdit can’t open the unproperly formatted dataset. As for your thermo dataset, I don’t think you should include SiO2, H4SiO4, and Si(OH)4(aq) as separate basis entries. They all represent essentially the same thing. Pick one and write all reactions in terms of that species. Hope this helps, Brian Farrell Aqueous Solutions
3. ## Features of act2 and tact

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
4. ## Regarding solubility calculation using REACT module of GWB

Hi Polly, Thanks for providing the thermo dataset. I'm taking a look to see if I can offer any suggestions. Regards, Brian
5. ## Chemplugin: Pore Volume (PV) and Extendrun()

Johan, Some time ago you tried to report the pore volumes displaced from a ChemPlugin instance in GWB12. I’m writing to let you know that GWB14 is now available, and ChemPlugin instances now plot pore volumes displaced and have the value available in the report command. Additionally, React now plots pore volumes displaced from flush and flash models. I hope you enjoy using the software. Cheers, Brian

8. ## mass problem with Pitzer model

Hi Bill, I’m writing to let you know about a new feature available in GWB14. You can control species loading in a calculation by specifying a temperature range over which thermodynamic data should be available. You can additionally force the program to always get log K values by evaluating a polynomial, even when the calculation is isothermal at a principal temperature. In this case, the program would normally use the value directly from the thermo dataset. The difference is small, but it’s why the endpoint of your polythermal path and your isothermal calculation post-pickup were slightly different. You can read about the feature in section 6.92 span in the GWB Command Reference. The new release has many more new features as well. Please let us know if you’re interested in trying GWB14. I’m happy to send you a demo. Cheers, Brian

10. ## Changing the units and legend on a piper plot

Hi again Dave, Upon closer inspection, you can include a user-defined analyte in a radial plot as long as you put it in the "Components in fluid" category, since this is where the normal basis species plot in Gtplot. If you instead choose a category like "Chemical parameters" and pick Concentration for the dimension, it won't be recognized by the water chemistry plots. Still, if you want to do any thermodynamic calculations including Be, you'll need to use a dataset that includes the relevant reactions. Hope this helps, Brian
11. ## Changing the units and legend on a piper plot

Hi Dave, The radial diagram can plot uncharged species when you choose units like mg/l, as you've done. However, the special plots, including the radial plot, only diagram basis and redox species in your spreadsheet. They do not include user-defined analytes. User analytes can only be plotted in xy plots. You might try using a different thermo dataset, like thermo.com.V8.R6+.tdat. That has Be++ as a basis species. Or, you can modify the thermo dataset you're using by adding the element Be and any species that you need, then load that dataset into GSS. Hope this helps, Brian Farrell Aqueous Solutions LLC

13. ## potential carbonate alkalinity units typo

Hi, Thanks for catching that. This issue has been fixed. It will be available in the next maintenance release. Regards, Brian Farrell Aqueous Solutions

15. ## Pourbaix diagram using Phase2

Hi Frank and Andrew, Act2 uses a simple analytical method to calculate equilibrium lines and assemble them into Pourbaix diagrams, so there's no difficulty in creating a diagram that spans large Eh and pH ranges. If you wanted, you could set the Eh and pH to span the range -100 to 100, and it would draw the diagram. With Phase2, however, you're setting up numerical reaction path models. In a model of the aqueous phase, working far outside the stability limits of water (i.e., the bottom left and top right corners of an Eh-pH diagram) can be difficult. To investigate your calculations, try overlaying some contour plots on your 2D diagram, or better yet, just look at a simple xy plot of the diagram’s left edge. You can select the left-most vertical cross-section through the entire diagram, use the “Go Y” option in Phase2 to calculate only the left edge of the diagram, or set up a React script to reproduce the left edge of the calculation. Whatever you choose, start out by plotting the Mass of Solution or Fluid volume vs. Eh. You'll see they're initially enormous. If you make a plot of species concentration, you’ll see it’s due to the extremely high concentration of H2(aq) that would exist under your initial conditions, which are far outside the stability range of water. Next, try plotting the concentration of your metal component of interest (U++++ or Al+++ in the system) and you should see that your initial conditions are honored. In the U example, U++++ was set to 1e-10 mol/l, but under the initial conditions that’s equivalent to .19 molal! That’s why so many minerals are stable throughout the diagram, compared to the Act2 calculation. As for the Al example, Al+++ was set to 1e-5 mol, and there’s still 1 kg of solvent in the system, so the initial Al+++ molal concentration isn’t skewed the way the U++++ was. You didn’t attach an Act2 script, Andrew, but presumably it’s not as different as the U calculations were. As for the failures you’re encountering, unfortunately it’s likely to happen when you’re so far outside water’s stability region, as in the corners of an Eh-pH diagram with a traditional range. In a log f O2(g)-pH diagram, by contrast, you don’t have to include such extreme conditions. Since a large portion of the area in an Eh-pH diagram is likely to be masked, log f O2(g)-pH diagrams can be a nice alternative in that they fill more of the plot area with useful information, rather than blank space. If you need to know the Eh, you can always plot contours on the log f O2(g) diagram. Whatever diagram you choose, it’s common to use a narrower range in these types of diagrams. I think when you do that you’ll find that you can reproduce an Act2 diagram much more closely. There will still be differences, of course. Boundary lines can be somewhat curved, rather than straight, reflecting the fact that Phase2 is solving a complete multicomponent calculation at each point in the grid instead of drawing equilibrium lines. It includes mass balance and activity calculations, unlike Act2. And whereas an Act2 diagram shows the stability of minerals and predominance of aqueous species (in terms of highest activity), Phase2/P2plot’s predominance map strictly shows the species accounting for the most mass at each point in the diagram. In other words, a mineral’s stability field (which you can see with an assemblage map) can be slightly larger than the area in which that mineral predominates all other species. Sorry for the delay in responding. It looks like we missed the original post. Hope this helps, Brian Farrell Aqueous Solutions
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