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[OLD] Fe(III) in AMD


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From: Nikolay Sidenko

Subject: Fe(III) in AMD

Please could you help me with modeling acid-mine drainage solutions? I would like to create diagram pH-[so4] diagram in Act2 in presence of Fe(III) and K. I'm a new user and my particular question is about input of Fe(III) activities. I do have two options. The first option is input measured molar concentration of Fe(III), which was analytically measured. Usually when people build such diagrams they refer to concentration of total Fe (III)in the solution. I assume they use it as input for calculations.

Second is run a program like WATEQ4F, MINTEQ .. inputting analyzed Fe(III) and other components of the solution to calculate activity of Fe(III) taking into account formation of complexes, FeSO4-(aq) for example. Molar concentration (the first option) is 2-3 orders of magnitude higher than speciated one (second option). Diagrams for those inputs look quite different. Which way should I use? I will appreciate any advises or references.

 

From: Mark Logsdon

Subject: Re: Fe(III) in AMD

Dear Users, a week or so ago, I responded directly to a request from a new user for some information on approaches to using the GWB models to evaluate some issues in the chemistry of Fe in acid-mine drainage. Some of the matters that arose during our conversation may be of somewhat more general interest, particularly to folks who are new to GWB. In hopes that this may be true, I forward some of the discussion. If you think I'm nuts or have questions, give me a shout.

 

----- Original Message -----

Let me have a quick go at your questions in the same order you ask them.

1. Activities. I take it that when you say "... precipitating from my AMD samples" you have actual waters that you are studying (or perhaps have generated experimentally.) Maybe I am too plodding a geochemist, but I always begin my analysis by running a simple speciation model. You do this in GWB (REACT) by inputting your water analysis and saying "go" (or using the run function from the menu). Actually, before that even I usually do a charge-balance check on my analysis, which I do in an EXCEL spreadsheet that I produced using factors from Hem (1985). If there is an issue, I want to know it before I invoke the real model, and this also allows me to look at my analysis one more time, perhaps to help decide how I want REACT to set the charge balance (e.g., using Cl, SO4 or something else). My prejudice for AMD solutions is that the parameter most likely to be imprecise is SO4, so I usually change the REACT default from Cl to SO4. Of course, you have to be convinced that the problem is on the anion side - if the analysis does not include Al, Fe, Mn and probably some other metals, too (Cu, Zn - depends on the source ore, of course), then the charge balance problem may lie on the cation side, in which case you may have to go back to your ICP data and sort through the metals again. But assuming I have a good match, then I pop the analysis into REACT, and start to work with it. As you know from reading the documentation, REACT is a path-seeking code, using a minimization-of-free-energy routine to do its equilibrium calculations. So, if you just put in your analysis and leave REACT on the default settings, it will give you two sets of outputs. The first, called step 0, is the standard distribution of species calculation for the input chemistry, including activities (and activity coefficients). It also will print out the Saturation Indices for a whole bunch of minerals,but in this step it does not change any solution chemistry. Then it prints out "Step 1", which does the *default* equilibrium calculation for the water you input. For your waters, for Fe it almost surely will go directly to Hematite, because for the LLNL database that is the ferric oxide with the lowest Gibbs Free Energy. Of course, this will hugely deplete the "equilibrium" solution in Fe, and all the other Fe phases will be "undersaturated" (i.e., negative SI). But you know that Hematite will not precipitate directly from your AMD water at say 10 C (or whatever your field T is), so this automatic step is not very useful. Two choices at the preliminary phase. Keep everything in default mode and just pay no attention to the "Step 1"calculations. Or, before you run the distribution, input "precip off". This will then limit the output to the initial distribution of species. Long story short: You can get the activities in GWB, using REACT. There are some advantages to this compared to using WATEQF extrenally as a prior step, most importantly, the thermodynamic databases in GWB are internally consistent from module to module. The database for WATEQF is not entirely compatible with the LLNL database.

2. If you think that all the Fe in your solution is Fe3+, then you can just swap Fe3+ for Fe2+ and enter your value. If you have both, then you need to decouple and control the concentrations of each separately.

3. By "checking the output", I just meant that you can have ACT 2 print out the equations and work through them to be sure that the figure in generates makes sense to you. This is okay as an exercise for students, but I usually count on the University of Illinois programmers as getting the arithmetic right and just work with the figure. [This may or may not be "good for the soul", but what it certainly limits the systems most of us had the strength to analyze.]

4. There is nothing in ACT 2 that prevents you from entering your entire water chemistry, and in fact, I suggest that you do - or at least as much of it as makes any difference to the phases you are concerned with. For example, if you have very much Mg in your real water, then it will be extremely important to the speciation of SO4, and if you don't account for it, you will have too much SO4 "available" for your secondary phases, predicting greater stability than they actually would have. If you have both Fe2+ and Fe3+, then I strongly urge you to include both in your input file. One last thing that you didn't ask about, but will need to address. The default thermo database for GWB does *not* include schwertmannite. You can check the alternative database to see if it is there. If it is not in the GWB databases, then you need to modify the database you want to use to add it. Read the User Guide CAREFULLY about how to do this - always copy the database, then use the copied version for modifications. Obviously, you are going to have a hard time modeling relationships of jarosite and schwertmannite if one of them is not in the database. Modifying the database in GWB is pretty straight forward, assuming you have the basic data to work with

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