How much FeCl2 can be dissolved in solution before Fe-minerals are precipitated.

[Phreak]

Hi, I've not used PHREEQC for a long time and struggling to remember how to do the following, so any help would be greatly appreciated.
I would like to model the stepwise addition of FeCl2 up to 1700 umol/L into water at pH 8 to assess at what concentration Fe mineral precipitates start to form. Ideally I'd like PHREEQC to plot a graph of this with the SI of all the minerals on the y axis and the FeCl2 concentration on the x axis. My poor attempt so far is:
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 1
    temp      25
    pH        8
    pe        4
    redox     pe
    units     umol/kgw
    density   1
    Fe(2)     1700
    -water    1 # kg
EQUILIBRIUM_PHASES 1
    Fe(OH)3(a) 0 10
    Goethite  0 10
    Hematite  0 10
    Fe(OH)2   0 10
REACTION 1
    1700 micromoles in 100 steps
USER_GRAPH 1 Example 2
        -chart_title "Fe precipitation"
        -axis_scale x_axis 0 1700 100 0
        -axis_scale y_axis auto 0.05 0.1
        -axis_titles "Concentration of Fe(2), in umol per litre" "Saturation index"
        -initial_solutions false
  -start
  10 graph_x TOT Fe(2)
  20 graph_y SI("Fe(OH)3") SI("Goethite") SI("Hematite") SI("Fe(OH)2")
  -end
END

[dlparkhurst]

This code seems closer to what your describe; however, hematite precipitates even at the first addition, so there is no change in the water composition as more FeCl2 is added. Hematite is not likely to be the phase that forms in fast-reacting systems; that would be goethite or amorphous iron hydroxide. But, more important, you are adding Fe(2); goethite or Fe(OH)3 are going to form when Fe(2) oxidizes to Fe(3), so I think you need to consider more generally what is the source of the iron and what oxidants are available.

Code: [Select]

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 1
    temp      25
    pH        8
    pe        4
    redox     pe
    units     umol/kgw
    density   1
    #Fe(2)     1700
    -water    1 # kg
EQUILIBRIUM_PHASES 1
    Fe(OH)3(a) 0 0
    Goethite  0 0
    Hematite  0 0
    Fe(OH)2   0 0
REACTION 1
    FeCl2 1
    1700 micromoles in 100 steps
USER_GRAPH 1 Example 2
    -headings               Rxn Fe(OH)3 Goethite Hematite Fe(OH)2
    -axis_titles            "Concentration of Fe(2), in umol per litre" "Saturation index" ""
    -chart_title            "Fe precipitation"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x rxn
20 graph_y SI("Fe(OH)3(a)"), SI("Goethite"), SI("Hematite"), SI("Fe(OH)2")
  -end
    -active                 true
END