pH goes backwards when goethite precipitates

[afisch80]

I'm trying to simulate the precipitation of Fe from solution as goethite by the addition of calcite.  When the amount of calcite added is sufficient to raise the pH to the point at which goethite begins to precipitate, the next addition of calcite results in a decrease in pH.  Does this make sense?

Code: [Select]

TITLE Acidic Fe/Mg sulfate solution reaction with calcite
INCREMENTAL_REACTIONS true
SOLUTION 1    Pure water
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mmol/kgs
    density   1
    -water    1 # kg

REACTION 1    Add metal sulfates and acid
    H2SO4      0.10
    Fe2(SO4)3  0.05
    MgSO4      0.285
    1 moles in 1 steps

SAVE solution 1

END

REACTION 2 Add limestone slurry
    Calcite    1
    H2O        10.957   
    0.20 moles in 40 steps

EQUILIBRIUM_PHASES 1
     Gypsum        0 0
     Calcite       0 0
     CO2(g)        -1.4
     O2(g)         -0.7
     Goethite      0 0

USE solution 1

END

[dlparkhurst]

pH will tend to decrease when goethite precipitates.

Fe+3 + 2H2O = FeOOH + 3H+

Your solution after the first reaction is supersaturated with goethite,so it precipitates. pH goes down even though some calcite has been added. As more calcite is added, pH increases. Seems OK. The key is the supersaturated condition of you initial solution. If you equilibrate that, then pH will only increase as you add the calcite/water slurry.