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General questions about react- 


Bea
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- When I want to react a fluid with oxides that are not in the basis panel I add them to the basis in very small amounts just so the reaction will happen. Is there a better way of doing this? 
How does it affect the reaction if you do that with oxygen and hydrogen? I want the oxidation state and pH to be controlled by the reaction without fixing them to a specific pH and oxidation state. 

- What is the difference between the "reaction times" on the bottom of the reactants panel to delxi under stepping? I am trying to see the mass reacted at 0.5,1, 1.5,2 etc, but I keep getting a jump from 0.1 g to 18 g of mass reacted in the results. This only happens with lower temperatures. 

- How do I add a mineral to my fluid in the basis panel without swapping an element? I want to have gypsum in my fluid without needing to swap Ca or SO2  to gypsum as the fluid has both in the composition. 

Cheers.

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Hello Bea,

 
The Basis pane in React and other apps in the GWB is used to set the initial fluid composition. If you swap in a mineral in the basis pane for a component (e.g. Quartz for SiO2(aq), this would ask React to set the fluid in equilibrium with that mineral and calculate the dissolved concentration accordingly. You can also swap in a gas fugacity if that is assumed to constrain that component concentration in the fluid. If you want to react a mineral with a fluid, then you should set the mineral as a reactant instead. 
 
If you swap in a mineral for O2(aq) and H+, React will calculate the concentration of dissolved O2(aq) and H+ component in equilibrium with the minerals respectively. You can set the initial oxidation state of your fluid by swapping in the redox species for the O2(aq) component and then add the basis species component concentration. For example, I can set the oxidation state of a fluid in the basis pane by swapping in Fe3+ for O2(aq), add in Fe++, and set the concentration for both. The software will calculate the Eh for the overall system based on Fe3+/Fe2+ component concentrations. You can also swap in a mineral for O2(aq) as well to set the oxidation state. Note that the initial oxidation state and pH is not fixed throughout the simulation unless you choose to do so specifically in the Reactants pane. 
 
Reactants times is a factor the program multiples the mass of reactants set to react into the system. For example, if you set 100 milligrams of Quartz with reactants times 2, then the program will react a total of 200 milligrams of Quartz into the system. Delxi is used to set the reaction interval the maximum length of the reaction program over the course of the simulation. By default, this is set at 0.01 so that the program takes 100 steps throughout the simulation 0.01, 0.02, 0.03.. etc. There are two other commands that control the output of the simulation results to the output text file and Gtplot. Please see the GWB Command Reference for more information on these settings.
 
You can add minerals to your system through the Reactants pane as a simple reactant or as a kinetic reactant. This depends on the type of reaction that you are trying to model for gypsum in your system. Is gypsum reacting quickly in your system over the time scale that you're interested in? If so, then you might consider adding it as a simple reactant that simply dissolves or precipitates according to thermodynamics. If gypsum is reacting slowly, then you might consider a kinetic approach. If it barely reacts at all, then you might consider ignoring it. To see more on how to add various types of reactants, please see section 3 Tracing Reaction Paths and 4 Kinetic Reaction Paths in the GWB Reaction Modeling user guide.
 
Hope this helps,
Jia Wang
Aqueous Solutions LLC
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Hi Jia, 

Thanks for the answer. 

So, if I understood correctly I can't have a fluid with a free mineral in the basis and react it with a different rock composition in the reactants panel? for my model, I need to have a fluid (seawater composition with a set amount of Ca and SO4) that has gypsum in it (in wt%). The reactants have a completely different rock composition (basalt). Do you have a suggestion on how to do this?

Regarding O2 and H, I don't want to set my oxidation state nor my pH, but the model will not run without them. I add them in very small amounts as I did with all other elements that are in the reactants panel but not in the basis panel. My question is if there is a better way of doing this and how will this affect the results of my oxidation stat and pH?

Regarding the data set used in the new version of GWB. I have run the same models with the new version and got different results. I am not sure what changed and how this would affect my results. Why would this change? is there a way of seeing it in my previous version of GWB?

Thanks for all you help. 

Cheers, 

Bea

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 Hello Bea,

You're welcome. You can only include a mineral in your basis by swapping it in for a component, which sets the mineral in equilibrium with that component. You can try swapping in Gypsum for SO4-- and run a Go initial calculation in React to see if the concentration of SO4-- calculated is close to what you fluid analysis indicates. If so, you can probably assume Gypsum is in equilibrium and swap it into your Basis. If you don't want to do that, or if your mineral is not in equilibrium in with your initial fluid, then it should be added as a reactant. It is not possible to include a mineral that doesn't constrain the initial fluid composition in the Basis pane. 

I am not sure that it is a good idea to use small arbitrary values for O2(aq) and H+. If you are receiving an error about setting O2(aq) concentration, then you are probably including reactions of species with various redox states. React will use the values set for O2(aq) to set the initial oxidation state and this will control the distribution of mass for all redox reactions unless they have been decoupled. Do you have any clues in your fluid analysis or environment that you can use to set the redox state of your system? Perhaps a HS- / SO4-- or another redox pair measurement? Is your fluid open to the atmosphere? If so, you may swap into the basis species on of these measurements to set the initial oxidation state. pH of a system is important too. Like any other component, the pH will affect the overall mass distribution of species that has H+ in its reaction. 

I am sorry to hear about the issues you're having with the update. Can you please attach an example input file and any custom thermo dataset so we can take a closer look? Also, what are the differences observed before the update and post update? Any details you can provide would be help us track down any issues. If you would like to revert to a previous version of the GWB for the time being, you can install the original installer for your software. 

Hope this helps,
Jia

 
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