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Specific Ion Interaction Theory (SIT) Database


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I am working with a series of 50 highly concentrated solutions in SpecE8 and an element of interest is Uranium (up to 80 g/L in solution). All 50 solutions were successfully speciated using thermo_minteq.tdat. Due to a range of "higher than normal" ionic strengths (0.2 Molal - 0.9 Molal) and differences in the aqueous species and solid phases associated with uranium, I am also working with the thermo_nea.tdat database. From the GWB Essentials guide, the Specific Ion Interaction Theory (SIT) activity model is "accurate to higher ionic strengths than the Debye-Hückel equations, but not to the salinities achievable using the Pitzer equations." Given the resulting ionic strengths, the SIT model appears to be an appropriate database to use. Unfortunately, the same solutions that converged using thermo_minteq.tdat will not converge using the NEA database. The error produced is:

       -- Internal error:  calc_sit: Ionic strength is out of range

As an alternative approach, I successfully ran the solutions through PHREEQC using the same NEA database.

Why does the error message reference ionic strength as being "out of range?" Is there a maximum range associated with the SIT activity model? Also, why do solutions speciate without issue in PHREEQC but not in GWB. The NEA database for each software was checked and they are identical.

Any assistance is greatly appreciated.

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Hello,

From your description, it doesn't sound like your fluid is really out of the ionic strength range. I am not sure what might be the exact issue without seeing the input file though. Was there any additional information provided regarding the error in SpecE8?  In some cases, other users have run into this error when they choose an aqueous species in the basis species that's very small in concentration. In that case, it would help the program to converge on a solution if a more abundant species is chosen to represent the component concentration in the basis. 

Hope this helps,
Jia Wang
Aqueous Solutions 

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Hi Jia,

Thank you for the response. Prior to posting a forum message, I attempted basis swapping for several of the major elements. I also removed a few of the minor elements with little to no success. Convergence was achieved by lowering the U concentration to 50 g/L and removing several metals, however, uranium is the element of interest and should not be altered. Unfortunately, convergence achieved by tweaking individual concentrations defeats the purpose of using GSS and SpecE8 to process ~400 solutions. Attached is one of the solutions along with the current NEA database. Any additional thoughts or comment are greatly appreciated.

Thank you!

D-CL1-03.sp8 thermo_nea_Jan2021.tdat

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Hello,

Thank you for providing the input file and your dataset. I noticed that the issues seem to arise with your lower concentration anion components. I think the issue here is that the free concentration of the aqueous species for AsO4-- and SeO4-- is in a very small concentration given the condition of your fluid. When this is the case, the program may have difficulty converging in its Newton-Raphson iterations. For more information on this, please see section 4.3.7 Optimizing the Starting Guess in the Geochemical and Biogeochemical Reaction Modeling textbook. When I removed both components from your basis pane, SpecE8 had no issue converging. I returned to your input file and added both components back into the Basis at very small amounts (1e-5 mg/l) and then ran the calculation to find the most abundant arsenic and selenium species, which is calculated to be NiHAsO4 and FeSeO3+ respectively. Then I returned to the Basis and swapped them in for AsO4-- and SeO4-- with the same concentration you had in your original file. When I ran the calculation this time, SpecE8 was able to converge successfully. 

By the way, your input script was using the default thermo_nea.tdat dataset and not the customized version you attached. I also noticed that you had decoupled the UO2+/U+++ redox couple. This allows you to set the component concentration for U+++ independently from UO2+, but looking at thermo_nea.tdat, there are no species or minerals that are in the redox form of U+++. If uranium speciation is your interest here, I am not sure how useful this would be. If you expect uranium to speciate across different redox state, then you would need to allow UO2+/U+++ remain in equilibrium. For more information on redox disequilibrium and examples, please see section 7.3 in the GWB Essentials User Guide. 

Hope this helps,
Jia 

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Hello,

Just a few more thoughts for your consideration. The SIT dataset accounts for fluids where the dominant electrolytes are Na+, Cl-, K+, ClO4-, and NO3-. Interaction parameters are provided for these major electrolyte ions to work accurately at higher ionic strength. Since your fluid is dominated by sulfates and uranium, this dataset doesn't have the interaction parameters for these species to account for speciation at the higher ionic strength as you were expecting. 

I should clarify that when swapping in As and Se species, the concentration should be constrained with equivalent concentrations. You can do so by finding the elemental concentration matching the original constraint and then using the "as" unit, similar to what you have done for H2(PO4). 

Also note that since there are fewer U species in this model than Minteq's database, so I suspect the ionic strength is going to be relatively different. You might also want to consider the various redox equilibria / disequilibria for each redox component to make sure that couples are decoupled appropriately. When you add a redox species in GSS, the redox species is automatically disabled in your SpecE8 calculations. 

If you can post your PhreeqC script, we can take a look to see how it is setup compared to your GWB input script. 

Best regards,
Jia

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