Precipitation of jarosite

[Dejavu]

Hello, Phreeqc users and experts,
I am trying to calculate precipitation of possible minerals in river water affected by acid mine drainage. The pH value of water is neutral.  The PHREEQC results show that SI values for jarosite are positive (1.4 for example). But according to many other scientific researches, jarosite should be dissolved at pH more than 4. Does it mean I should ignore the results for jarosite or it precipitates anyway?
I attach an example:

SOLUTION 1
    temp      16
    pH        6.8
    pe        6.9
    redox     N(-3)/N(5)
    units     mg/l
    density   1
    Al        0.1
    Alkalinity 20 as HCO3
    As        0.005
    Ca        30.7
    Cl        20
    Fe        10.7
    K         1.1
    Mg        7.41
    Mn        0.3
    N(-3)     0.04 as NH4
    N(5)      0.1 as NO3
    Na        4.24
    S(6)      71
 

[John Mahoney]

At your pH of 6.8, Ferrihydrite is more stable and would be the preferred phase to precipitate. 

Using just those two Possible Equilibrium_Phases, I get:   

-------------------------------Phase assemblage--------------------------------

                                                      Moles in assemblage
Phase               SI  log IAP  log K(T, P)   Initial       Final       Delta

Ferrihydrite      0.00     4.86      4.86    0.000e+00   1.830e-04   1.830e-04
Jarosite         -6.75   -15.14     -8.38    0.000e+00           0   0.000e+00


AND the pH drops to about 6.1. You have to look at the whole system and not just jarosite. 

I played around with your starting  pH and you need to be at about 3.8 or lower to get the jarosite to form. then tried to generate an Eh pH diagram using PhreePlot but I did not get the jarosite to show up on the diagram, so it is more complicated than originally expected

[dlparkhurst]

You will also see that goethite SI is over 9; so, I think some ferric phase would precipitate. Given the following choice between goethite and jarosite-K (wateq4f.dat), goethite is calculated to precipitate rather than jarosite, although kinetics, redox, and other considerations make this an over-simplification.

EQUILIBRIUM_PHASES
Goethite 0 0
Jarosite-K 0 0

I think you need to consider the redox state of this solution. If it really contains mostly ferric iron, then a ferric phase should readily precipitate. Perhaps the concentration of iron is so high because it is ferrous iron. If oxygen is available, it will oxidize to ferric iron and precipitate, but that may be a kinetic process that depends on the availability and exchange rate of oxygen, bacteria, temperature, and other factors.

[Dejavu]

Thank you very much for the answers. I understood that ferrohydrite would be preferred phase, and as ferrohydrite precipitates the degree of saturation of jarosite would decrease until it become undersaturated. But, is jarosite still stable in the presented solution?
Suspended jarosite comes into the river water with acid mine drainage (with pH about 3) upstream.

[dlparkhurst]

I think thermodynamics says that jarosite should dissolve and another ferric phase should precipitate. Jarosite could be stable in a microenvironment or because of slow kinetics.

You can specify an amount of jarosite that is available in an EQUILIBRIUM_PHASES definition. If enough is present to react with your solution, jarosite dissolves, goethite/ferrihydrite precipitates, and some jarosite remains, but the pH is again less than 3.

[Dejavu]

Thanks a lot. All comments are very helpful!