Dien Li
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Posts posted by Dien Li
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Hi, Brian
I tried to simply use COMMAND to input all data and information from your text book (Bethke 2008, page 410). I used thermo.dat. It looked that I could run the model, but the results for kaolinite and gibbsite were different from your text book Fig. 27.3 for the first 0.1 meter profile, after that, it looked all identical. I attached the new input file. Could you instruct? what datebase you used to get your texk book results. Thanks.
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Hi, Brian
How are you! I am trying to practice a modeling using X1T. The modeling is on weathering of soils, the data were given on page 410-420 of Craig's book (2008). My question is how to set up the entrance data for kailonite, gibbsite and tridymite, specially how to deal with the nucleus = 4000. Attached please see the data file I am trying to work, could you please help looking at it. Thanks.
Dien
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Hi, Brian
Thanks you so much.
Dien
Hi Dien,
Like thermo.dat, thermo.com.v8.r6+.dat uses an extended form of the Debye-Huckel equation called the B-dot model. If thermo_phreeqc.dat is loaded, however, GWB uses the same activity model originally used in PhreeqC. As far as I can tell, this is the Davies equation (at least for charged species). Both of these activity models are described in Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text, Section 8.1.
The B-dot equation is valid to ionic strengths slightly higher than the Davies equation, but both work relatively poorly at high ionic strength. In this case, virial methods like the Pitzer equations may be useful (these have their own limitations, however, notably a lack of data for redox reactions and certain components, and a limited temperature range).
You should take a look at Chapter 8 of that text, or Section 7.4 of the GWB Essentials Guide for more on the implementation of activity coefficients and activity models in GWB.
As for the validity of your models at high pH or low redox potential, I don't think the activity model used is particularly important. Rather, the availability of data for appropriate species and minerals is likely to be your main concern. The various thermo datasets have their various strengths and weaknesses, and some are more complete with respect to certain components than others. Take a look at Section 2.3 (Uncertainty in geochemical modeling) of the GBRM textbook for an example.
Hope this helps,
Brian
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HI, Brian
In this same topic on the solubility colcaulation of Tc solids in cement leachates which have very high ion strength (Na, K and Ca...) and high pH under reducing condition, we used two databases: thermo.com.v8.r6+.com and thermo_phreeqc.dat. My questions are:
1. In general, what are the algebra or math descriptions of activity coefficients in these two models, how differet these descriptions are in the two models.
2. More specific, are the model desciriptions still valid in solutions of very high ion strength, high pH and under reducing conditions?
Thanks you so much for your help.
Dien
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HI, Brian
For the same topic, I used ACT2 to make phase diagram of Tc in pure water, I suppressed all Tc minerals, except for TcO2© or TcO2.1.6H2O, to determine the phase diagram at Eh = -0.4 V. They indicated that the solubility of TcO2.1.6H2O is higher in the magnitude of ~4 orders than that of TcO2©, which is what I believe.
However, when I used REACT to calculate the solubility of TcO2© and TcO2.1.6H2O at pH 8 and Eh -0.4, their solubilities of these two are identical at ~9e-12 M. In fact, in all other aqueous systems, they showed similar solubility, in contrast to the phase diagram. Could you advise, thanks you so much.
Is this is a right way to calculate the solubility of TcO2 or any other minerals? Thanks.
Please see the attached files.
DL
Solubility of TcO2-in pure water.rea
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HI, Brian
Thanks you so much for your reply. I still have the troubleshooting problem for the not convergent runs. Could you have a quick trouble shototing for this data file. Thanks and ave a nice holidays.
DL
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Hi, Brian
How are you! I still need your help in modeling of TcO2 solubility in cement leachate solution of high pH. I input the cement leachate chemistry in BASIC and place TcO2 in REACTANT, and like to determine the aqueous Tc species concentration, and then solubility of TcO2. Is this a right logic to do it? The key problem is that it is not convergent as I ran this type of simple model. What is the general approach to this. Please you can see attached file. Please you can help run it to see how to make it work. This is urgent to me, I wish you can help me, thanks.
DL
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HI, Brian
Per your last email, the time set up is meaningless if no kinetic rate is set. The 2 gram quartz was assumed to be added invrementally over the setting time. Now, if I want to model the case: I have or add 2 gram quartz into the system once in the beginning, I like to model how solubility changes with time and second, with pH. Could you advise on how to set this model.
Also, in my previous sample, I tried again, it looks that addting Na or Cl and keep it as a charge balance, the modeling is not convergent due to high residue. If I add Cl and keep balance off, it works. More comments about this. Thanks.
DL
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HI, Brian
Thanks you so much.
DL
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Hi, Brian
I like to calculate solubility of TcO2 and other compounds in high pH solution. I used REACT to do it. I set the solution chemistry in the basic, and set the solids in reactant. However, it indicated first the charge imbalance, I added Cl for charge balance, but after several hundred runs, it could not converged, and said that the high residue for Cl. Attached please you can see the sample for quartz. It is the same for other solids. Could you help me? You can also reply me at dien.li@srs.gov. Thanks.
DL
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HI, Brian
Thanks you so much. From experimental work it appears that addition of P would reduce the U species solubility and trigger U-P phase precipitation. However, GWB phase disgram showed the opposite way. Yes, it is good to check the database to see if the appropriate U-P phase data were in there. I will check and contact with you again. Thanks.
DL
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HI, Brian
Could you please give a quick shot on this simple question. In a simple U-water system, at 2e-6 M, schopite is precipitated at near neutral pH. However, with addtion of HPO4-- at 2e-4 M into the system, the modeling indicated that U became dorminantly soluble U-phosphate species, rather than conventional concept that U would form U phosphate precipitation. Could you please instruct. Thanks.
DL
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Thanks you so much, Brian.
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, 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 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
Reaction modeling
in X1T and X2T
Posted
Hi, Brian
Thanks. I noticed that the Al+++ issue in the first run. Anyhow, I now found it out. Thanks indeed.
Dien