GSS - How to enter Iron without defining ionic species

[ZTwatson]

I am having trouble running a solution through GSS when I enter iron as Fe instead of Fe++ or Fe+++ (this doesn't happen if I leave the species as Fe++ or Fe+++). The calculated error report shows up. I do not know the speciation of iron in my solution. Related articles containing solution composition give iron as Fe(total) and make no remarks on the speciation. If it helps, sulfate dominates the solution for sulfur, so I assume the solution is oxidized.

 

Here is my input...

 

Input:

 

data "C:\Program Files\Gwb\Gtdata\thermo.dat" verify

 

1.000000 free kg H2O

6.590000 "pH" H+

temperature 16.500000

1056 "mg/l" Ca++

237 "mg/l" Mg++

4269 "mg/l" Na+

347 "mg/l" K+

4698 "mg/l" HCO3-

1921 "mg/l" SO4--

4046 "mg/l" Cl-

4.1 "mg/l" Li+

13.8 "mg/l" Sr++

1.4 "mg/l" Mn++

2.1 "ug/l" V+++

7.5 "ug/l" Ni++

11.4 "mg/l" Fe++ as Fe

 

 

 

Results:

 

 

Any help is greatly appreciated.

 

Thanks,

Zach W.

[Brian Farrell]

Hi Zach,

 

Are you trying to launch SpecE8 or React from GSS? It sounds like in GSS you have Fe++ in mg/l (as Fe). The (as __) designation does not refer to oxidation state (or the lack of one, rather) but is useful for defining the concentration of compounds like SO4-- or SiO2 when analytical concentration is reported as the mass of the element S or Si, but not the molecule as a whole. For basis species like Fe++ the (as Fe) designation is not necessary, and SpecE8 does not like this. You should use the default setting (as Fe++).

 

Making this change will allow you to launch SpecE8, but you'll notice that only ferrous iron species are loaded, since there is no oxygen in the Basis from which to form ferric iron species. Do you have any measure of redox state like DO or Eh? Can you assume an O2(g) fugacity corresponding to equilibrium with the atmosphere? Equilibrium with a mineral? Perhaps if you have measures of SO4-- and HS- you can swap in that ratio for O2(aq)?

 

You should check out the Redox Couples section of the GWB Essentials Guide, or consult Chapter 7 of the Geochemical and Biogeochemical Reaction Modeling textbook.

 

Hope this helps.

 

Brian Farrell

Aqueous Solutions LLC

[ZTwatson]

Brian,

 

Thanks for the response.

 

I assume I am launching SpecE8, though I am not entirely sure. I just enter the data and then go to calculate and determine things like saturation indices or CO₂ fugacity.

 

We do not have any measured DO or Eh. O₂ fugacity is not at equilibrium with the atmosphere. The water comes from depths below the surface, would that mean O₂ fugacity is lower? I don't believe we can assume equilibrium with any minerals as well.

 

When I am running solutions using GSS, are there important variables that I need to set up before I run the program? I only have pH, temperature and ionic compositions.

[Brian Farrell]

Hi Zach,

 

If you're just calculating things like SI within GSS, SpecE8 handles the actual calculations. You can also launch SpecE8 from GSS for a more complete analysis (for example, SI of every possible mineral, not just what you specify, etc.) Actually launching SpecE8 gives you a little more flexibility in constraining your system, as you can swap equilibrium minerals, gases, or activity ratios (like SO42-/ HS- or Fe+++/ Fe++) into the basis when you have missing analytical data.

 

Redox state is an important variable in speciation calculations. Without any mention of redox state in your input, all iron is assumed to be Fe(II). You might try adding Eh to your spreadsheet (or O2(aq)) and just rerun the simulation with a range of oxidation states.

 

If your water samples comes from deep below the surface, then it's probably a good bet that the O2(g) fugacity is lower than a water sample in equilibrium with the atmosphere. This of course depends on how long it's been since the water was recharged into the subsurface and what sort of reductants (and catalysts) are present in the water or along the flowpath.

 

Cheers,

Brian

[ZTwatson]

Brian,

 

So if I do not specify the redox condition SpecE8 will assume the water is fully oxidized?

 

Also, what values are fugacities reported in? I can't seem to find any specifications on what the unit is.

 

I assume it is the value of pressure released by the gas, so a value of 1 would relate to 1 atm, or 1 bar (roughly). and .54 would relate to .54 atm?

 

thanks,

zach w.

[Brian Farrell]

Hi Zach,

 

If you do not specify redox state, SpecE8 will not consider any of the redox species (redox species are basis species in alternative oxidation states). Section 2.4 (Redox couples) of the GWB Essentials Guide lists the oxidation state of all basis and redox species. In thermo.dat, Fe++ and SO4-- are the basis species, and Fe+++ and HS- are the redox species. Fe++ is reduced, and SO4-- oxidized, so there is no assumption about water being fully oxidized or fully reduced. By omitting Eh, pe, O2(aq), etc., you are instructing SpecE8 to only consider species in the same oxidation state as the basis species (Fe as Fe(II) and S as S(VI)). No Hematite can form, nor will H2S(g), for example.

 

Omitting oxidation state is similar to assuming redox disequilibrium for all couples, except you are only constraining one of the couples.

 

The fugacity is in effect the partial pressure in atm.

 

Hope this helps,

Brian