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Speciation in aqueous solutions


Dien Li
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Hi, Brian

 

I am a scientist at SRNL. I have an aqueous solution system that may contain Fe, Ca, Cu, Mn, Zn, NH4, Cl, NO3, PO4, BO3. It is Ca, NO3 and NH4 dominant. I like to model the speciation of Fe (either soluble species or precipitates) vs (1) the solution pH; (2) the initial Fe concentration; (3) the initial P concentration. I like to know which module (e.g., SPEC8 or REACT...) I need to use to achieve this goal. If SPEC8 is supposedly to calculate the species in the equilibruim state systems, SPEC8 can not calculate the species change against the pH or initial concentrations, is it right? I also tried REACT, but it looked that no data was obtained after RUN. Please instruct me on how I can apporach this project, thanks.

 

DL

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Hi DL,

 

SpecE8 solves for the equilibrium distribution of aqueous species in a fluid and on surfaces, as well as mineral saturation state and the fugacity of dissolved gases. If you want to know how pH affects your equilibrium system, just change the pH value a small amount, rerun your SpecE8 model, and repeat again and again. This is where React makes things easy, as it is a program for modeling reaction paths. What you need to do is start with an initial system just like in SpecE8, then go to the reactants pane. You can titrate in aqueous species like HCl or NaOH (as simple reactants) to vary the pH, or you can use a sliding pH path. You will do the same thing for Fe and P (sliding paths are calculated in terms of activity, but if you're interested in concentation just add in Fe and P as simple reactants.

 

The GWB Reaction Modeling Guide has sections on Titration paths (section 3.1 in Release 8.0) and Sliding activity and fugacity paths (3.6) with examples that should help you figure out how to set up your calculations.

 

You might also try diagramming Fe in Act2, choosing pH and Fe or P activity as axes. This will give you a simple predominance diagram, rather than a complete analysis that you would get from a SpecE8 or React simulation.

 

Hope this helps,

 

Brian Farrell

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Hi, Brian

 

Thanks you so much for your quick response. I believed that I tried REACT, by adding Fe as a simple reactant, but it indciated that no reaction paths or somehting like that. I may try again to see what is going to happen. Thanks and I will keep in touch with you.

 

DL

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Hi, Brian

 

I sent a message to you yesterday, but I am not sure if you have received it, because it is not shown in this topic. My question is when we need to couple Fe2+/Fe3+, and when we need to decouple Fe2+ /Fe3+ in our modeling. At a constant Eh, would to couple or decouple Fe2+/Fe3+ make any difference in modeling results, thanks.

 

DL

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Hi DL,

 

I didn't get your message. As for your question, you should decouple redox pairs when you believe your system is not in redox equilibrium (low temperature natural waters, perhaps), or when you have analytical data for the same element in several redox states. If you set Eh (or pe, f O2(g), O2(aq)) and leave Fe2+/Fe3+ coupled then the equilibrium between Fe2+/Fe3+ is determined by that Eh value. However, if you decouple Fe2+/Fe3+, then the redox equilibrium between them is set by constraining each component separately (Fe2+ and Fe3+). In this case, the system Eh has no effect on Fe speciation. If you decouple the pair and only specify Fe2+, then no Fe3+ species will form (similarly decoupling the pair and only constraining Fe3+ will prevent Fe2+ from forming), so be careful with this.

 

You should check out Section 7.3 of the GWB Essentials Guide (Release 8) or Chapter 7 of Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text for more on redox disequilibrium.

 

Hope this helps,

 

Brian

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