Page 608 from Appelo & Postma Geochemistry, groundwater & pollution 2nd edition

[HFlower]

This is not a PHREEQC manual question, but I thought maybe this would be the right place to post it.

Surface complexation example, page 608, from Appelo & Postma Geochemistry, groundwater & pollution 2nd edition

Example 2
I put in the "input script" as follows:

TITLE A&P p. 608 ex 2
SOLUTION 2
pe 14 O2(g) -0.68
Na 1.; Cl 1.; Zn 1e-3
EQUILIBRIUM_PHASES 2
    Fe(OH)3(a) 0 0.001
SAVE solution 2
END
SURFACE 2
    -equilibrate with solution 2
    Hfo_s   Fe(OH)3(a)      equilibrium_phase 0.005  53400
    Hfo_w   Fe(OH)3(a)      equilibrium_phase 0.2
-equil 2
END

I get the error message:

WARNING: Suggest using master species formula in SURFACE
>>for surface related to equilibrium_phase Hfo_sOH

and so on for all surfaces.

I'm not sure how important this is, but I'm trying to learn as I go.
Thoughts?
Thanks

[dlparkhurst]

It means you should define the following:

    Hfo_sOH  Fe(OH)3(a)      equilibrium_phase 0.005  53400
    Hfo_wOH   Fe(OH)3(a)      equilibrium_phase 0.2

to make sure there is mole balance on O and H.

[eeftekhari]

Hi David,

What if the solid phase does not contain OH? For instance, I have defined chalk in the master species as

Chalk   ChalkOH

But chalk is made of calcite, and I receive a warning about the hydrogen mass balance when I define a surface as

ChalkOH      calcite    equilibrium_phase    2.0

Is there any way to solve this problem?

[dlparkhurst]

My recollection is that the mass balance will not work out exactly if the elements are not in the mineral.

You can add H2O to the calcite formula to account for some H. Unless the activity of water is very different from 1, it should not make much difference. If it is a problem, you can use a smaller coefficient for the associated water, but there will need to be enough water to satisfy the H in  ChalkOH  that is formed as a mole of MyCalcite precipitates.

PHASES
MyCalcite
   CaCO3:H2O = CO3-2 + Ca+2 +H2O
   -log_k   -8.48
   -delta_h -2.297 kcal
   -analytic   -171.9065   -0.077993   2839.319   71.595
   -Vm 36.9 cm3/mol # MW (100.09 g/mol) / rho (2.71 g/cm3)

[HFlower]

Mr Parkhurst- I really appreciate your response. I am glad to understand what was missing and why. Could come in handy. Thank you.

[eeftekhari]

Thank you very much David. This is very helpful.