Pyrite formation from ferric (Fe+3) and hydrogen sulfide (HS-)

[ahmadreza_shojaee]

Dear all,

I have a question about one of iron reactions. In the PHREEQC database, there is no reaction defined between Fe+3 and sulfide/hydrogen sulfide; however, in some publications, it is mentioned that Fe+3 can react with HS-, and pyrite can precipitate.

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2 Fe+3 + HS- = 2 Fe+2 + S + H+
Fe+2 + HS- = FeS + H+
FeS + S = FeS2

Source: https://doi.org/10.1016/0927-7757(96)03568-6

Is this reaction possible in the point of view of Thermodynamics?

[dlparkhurst]

Fe(3) and S(-2) will not coexist in significant quantities at equilibrium.

It depends on the conditions of the system, but either all the S(-2) in the system will be oxidized or all the Fe(3) will be reduced. You may form a ferric iron oxyhydroxide, pyrite, or something else in the process.

[ahmadreza_shojaee]

Dear David,

Thanks for your response.
I am a bit confused, honestly. The only reaction between Fe(2) and Fe(3) in phreeqc database is:

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Fe+2 = Fe+3 + e-
-log_k -13.02
-delta_h 9.680 kcal
-gamma 9.0 0

I am not sure what the exact mechanism of oxidation/reduction of Fe(2)/F(3) is; in phreeqc database it is assumed that they will be in equilibrium according the above reaction.
In some cases, it is assumed that oxidation/reduction of iron is because of microbial activity (example 9). I was thinking that is reduction of Fe(3) possible with S(-2)? If it is possible, it is needed to add new reactions for that.

Or in another case, iron (III) minerals such as Goethite can be reduced to iron (II) by a microbial reaction in presence of Hydrogen:

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2 FeO(OH) + H2 + 4 H+ = 2 Fe+2 + 4 H2O
or
H2 + 2 Fe(OH)3 + 4H+ = 2 Fe+2 + 6 H2O


[dlparkhurst]

There does not need to be an explicit reaction in SOLUTION_SPECIES or PHASES for redox reactions to occur. You note the reaction for Fe(2)/Fe(3):

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Fe+2 = Fe+3 + e-

but there is also a reaction for S(-2)/S(6):

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SO4-2 + 9 H+ + 8 e- = HS- + 4 H2O

The calculation determines the activity(e-) (or pe), and it  is the same for both equations. If pe is large, HS- must be small, and if pe is small Fe+3 must be small.

The same pe applies to mineral reactions.H2 is replaced with

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2 H+ + 2 e- = H2

so, the effect is to use pe in the equations; same with O2.

[ahmadreza_shojaee]

Thank you David for your response.

The thing here is that can we assume that S(6) reduction to S(-2) and also Fe(III) reduction to Fe(II) are equilibrium reactions?

If in laboratory, we mix an Iron(III) Chloride solution with a Sodium Sulfide solution, can we expect pyrite will form after a while?

[dlparkhurst]

By default PHREEQC calculates thermodynamic equilibrium.

You will have to decide whether you want to calculate some of the reactions kinetically. Example 9 in the version 3 manual gives an example of a kinetic redox reaction.