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Author Topic: how to measure total precipitate?  (Read 5884 times)

csoeder

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how to measure total precipitate?
« on: 11/11/15 20:51 »
Hi all,
I'm working on a project involving the precipitation of iron compounds under varying ocean chemistries. I've made a pass through the PHREEQC user's manual and skimmed some of the posts here, but I'm still having trouble getting a toehold on the software, so I thought I would begin at the beginning (basically, test out a couple of points on a Pourbaix diagram) and then add more realism in layers as I get a better understanding of how PHREEQC works (incorporating kinetics, other ionic species, phosphate adsorption, etc.)

Here's where I've gotten to on my own. Based upon the Pourbaix diagram of iron (fig. 13.2, here: http://www2.nau.edu/~doetqp-p/courses/env440/env440_2/lectures/lec13/lec13.html ) I would expect iron to be very soluble as Fe(+2) at pH=2 and pE=5, and very insoluble as Fe(OH)3  at pH=12 and pE=5.

I thus created an input file for each condition (here's the acidic one)
TITLE test1
SOLUTION 1  Acidic
        units   ppm
        pH   2.0
        pe   5.0
        temp   25.0
   Fe   100

END

These both ran without error. Here's what looks like the relevant part of the output:
  Phase               SI** log IAP   log K(298 K,   1 atm)
(acid)
  Fe(OH)3(a)       -9.82     -4.93    4.89  Fe(OH)3
  Goethite         -3.93     -4.93   -1.00  FeOOH
  H2(g)           -14.05    -17.15   -3.10  H2
  H2O(g)           -1.50     -0.00    1.50  H2O
  Hematite         -5.85     -9.86   -4.01  Fe2O3
  O2(g)           -55.19    -58.08   -2.89  O2
(base)
  Fe(OH)3(a)        1.93      6.82    4.89  Fe(OH)3
  Goethite          7.82      6.82   -1.00  FeOOH
  H2(g)           -34.05    -37.15   -3.10  H2
  H2O(g)           -1.50     -0.00    1.50  H2O
  Hematite         17.64     13.63   -4.01  Fe2O3
  O2(g)           -15.19    -18.08   -2.89  O2

I guess my first question is, how do I calculate from these figures the total amount of precipitated iron compounds in each situation?

Thanks!
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dlparkhurst

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Re: how to measure total precipitate?
« Reply #1 on: 11/11/15 21:57 »
If you want to generate a pe/pH diagram, you may want to look at PHREEPLOT, which can generate the predominance diagram that you are looking for (http://www.phreeplot.org/).

Sticking with PHREEQC, you have calculated saturation indices for your solutions. The first indicates that ferric oxyhydroxide phases all have negative saturation indices, and thus, no ferric minerals would precipitate from this solution.

The second speciation calculation has positive saturation indices. Each mineral with a positive saturation index would tend to precipitate from this solution. It sometimes depends on the stoichiometry of the minerals and the suite of minerals considered, but in this case the mineral hematite with the greatest saturation index is the most stable phase. Virtually all of the dissolved ferric iron will precipitate.

If you want to do the calculation to find the stable phase, add at the end of the input file:

USE solution 1
EQUILIBRIUM_PHASES 1
Fe(OH)3(a) 0 0
Goethite 0 0
Hematite 0 0
END


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csoeder

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  • Posts: 2
Re: how to measure total precipitate?
« Reply #2 on: 12/11/15 19:59 »
Thanks for your reply. I'm not actually trying to create Pourbaix diagrams; I only mention them as a starting point for test cases. My interest is in the amount of solid iron compounds which are deposited under different conditions.

I added the EQUILIBRIUM PHASES lines as you suggested, and the output now gives two sets of calculation results, before and after equilibration presumably. Right now, I'm only interested in iron flux, and looking at the output, I think the relevant information is here:

-----------------------------Solution composition------------------------------
   Elements           Molality       Moles
   Fe                1.791e-03   1.791e-03 (pH=2)
   Fe                3.152e-12   3.152e-12 (pH=12)

I assume I can just subtract these figures from the starting concentration to get the amount precipitated. Down the road I might be interested in the speciation of the precipitate, at which point I would look at the SI for each of the solid phases.
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dlparkhurst

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Re: how to measure total precipitate?
« Reply #3 on: 12/11/15 22:15 »
The numbers you cite are the concentrations in solution under "Solution composition". If you have multiple interacting solids forming, the solution concentrations are the net, which could be the difference between the amount of one mineral that dissolved and another mineral that precipitated. You would also have to sort out the stoichiometry of the element in each mineral.

If you want to know the amounts of a mineral (in EQUILIBRIUM_PHASES) that reacted, then you should look at the "Phase assemblage" part of the output, or write the amounts using SELECTED_OUTPUT, or the Basic functions EQUI or EQUI_DELTA in USER_PRINT, USER_PUNCH, or USER_GRAPH.

The SIs will not give you any information about the amounts precipitated.
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