Formation of Fe(OH)2

[JEntwistle_10]

Hi all (firstly a disclaimer, I have no experience with phreeqc at all),

I have some experiments planned involving the use of a Fe-bearing clay mineral SWy-2 and the influence of aqueous Fe(II) - essentially we want to know whether degradation of PCE and TCE is enhanced when a Fe(OH)2 phase is present or not.

I've been asked to model what concentration of aqueous Fe(II) I would need in order to precipitate a Fe(OH)2 - the experimental setup we use is pH 7.5, with 10mM MOPS buffer and 50mM NaCl for ionic strength.

For reactors that contain the clay mineral, we use a mineral concentration of 5g/L.

So yeah, I've no idea where to start really in order to determine how much aqueous Fe(II) I need, so any help would be greatly appreciated (this is all assuming phreeqc is the right thing to be using for this sort of problem!).

Many thanks,
Jim

[dlparkhurst]

Here is a starting point for you. It touches on several features of PHREEQC. The first data data blocks define thermodynamic data for phases and aqueous species. I like to use phreeqc.dat because it is simpler, but it does not have a definition for Fe(OH)2(s). PHASES defines Fe(OH)2 based on a definition in llnl.dat.

The SOLUTION_MASTER_SPECIES and SOLUTION species are necessary to define a MOP buffer, which is not present in any database. This may be a simplified version of how the real MOP buffer works.

Then there is a solution definition, which contains an arbitrary 1 mole of buffer plus the ionic strength. I added a lot of buffer because Fe(OH)2 is relatively soluble, and could easily make a big change in pH.

Finally, an EQUILIBRIUM_PHASES containing Fe(OH)2 is defined and then brought together with the solution to determine how much dissolves to equilibrium. It says 0.081 mol of Fe(OH)2 dissolves, bringing the pH up to 7.8.

As you add the clay, you will be adding exchange capacity. It will be necessary to include EXCHANGE, and consider what the initial composition of the exchanger is. If Fe+2 exchanges, then that will drive the dissolution of more Fe(OH)2.

If you want to add FeCl2 to reach equilibrium with Fe(OH)2, look at the option in EQUILIBRIUM_PHASES to add an alternative reaction ("Fe(OH)2 0 FeCl2 10").


PHASES
Fe(OH)2  # from llnl.dat
    Fe(OH)2 + 2H+ = Fe+2 + 2H2O
    log_k     13.9045
    delta_h   -95.4089 kJ
    -analytical_expression -86.666 -0.01844 7572.3 32.597 118.18 0
SOLUTION_MASTER_SPECIES
    Mop           Mop-             1     1               1
SOLUTION_SPECIES
Mop- = Mop-
    log_k     0
H+ + Mop- = MopH
    log_k     7.5
END
SOLUTION 1
    temp      25
    pH        7.5
    pe        4
    redox     pe
    units     mmol/l
    density   1
    Cl        50
    Mop       1000
    Na        50
    -water    1 # kg
END
EQUILIBRIUM_PHASES 1
    Fe(OH)2   0 10
END
USE equilibrium_phases 1
USE solution 1
END

[SlippinJimmy]

You might also want to take note of the fact that the result is quite dependent on the amount of MOPS you have used. The equilibrium attained after addition of Fe(OH)2 will alter the pH by as much as 2 pH units depending on the amount of MOPS buffer you start with which will give about 3 orders of magnitude variation in your answer, e.g:

Mops(mol/l)  mu(mol/kgw)   pH            Fe(2)(mol/kgw)
0.0              0.0502021       9.51583    2.5092e-05
0.001          0.0513209       8.96395    2.3313e-04
0.01            0.0611307       8.49494    1.9118e-03
0.1              0.148145         8.1076      1.4053e-02
1.0              0.90883           7.84053    8.3841e-02

For increasing concentrations of MOPS you might also want to titrate with NaOH to get the specified initial pH of 7.5 which will raise the ionic strength (you can do this by using the "charge" option on the Na entry in the SOLUTION block to give electroneutrality at the specified pH). If you want to be really fancy you could also play around with having an additional Fe(3) equilibrium phase as well such as ferrihydrite or FeOOH since reductive degradation of PCE/TCE will result in oxidation of dissolved Fe(2)  (Wyoming montmorillonite also typically contains some FeOOH as an impurity), but this will likely have little impact on the equilibrium Fe(II) concentration you calculate for the setup you describe here.

[JEntwistle_10]

Hi David and James,

Thank you very much for you swift reply and for the help, it is very much appreciated!

Many thanks,

Jim