Predominantly ferric iron in low pH solution in my output - kinetics?

[Gaulty]

Hi there,

I run the following simulation, but in the output my pH 3.3 solution that is entirely Fe(3). This seems unrealistic. Is this because I equilibrated with O2? I would still assume that given the slow kinetics of O2 oxidation at low pH I would still have predominantly Fe2+. Perhaps I have to decouple the Fe2/Fe3 redox couple since Phreeqc doesn't take kinetics into account in my simulation? Thank you for your assistance.

SOLUTION_SPREAD
    -units    mg/l
  pH    Alkalinity     Cl    S(6)      Al       Ba      Ca        Mg        K      Na    Fe(2)    Fe(3)    Number
                                                             charge                                             
 2.6             0     19    4000     119    0.003     370       375     0.76     8.8      178                  1
 8.4           179    1.7     616    0.02    0.005     207      62.4     3.14    2.69        0     0.01         2
 6.4          70.5    0.1     3.1    0.15     0.01    24.2       3.9      0.6    1.09              0.31         3
 5.6                          0.6                     0.17     0.025    0.017    0.02                           4
 3.5             0    1.7    1428    13.7    0.007     247       125      3.1     6.2       48                  5
 4.3             0      4    2000    13.9     0.13     446       181     1.99    4.25              9.58         6
MIX 1 2014 scenario 1
    1    0.1
    2    0.02
    3    0.09
    4    0.79
END
USE mix 1
EQUILIBRIUM_PHASES 1 atmosphere
    Al(OH)3(a) 0 0
    Barite    0 0
    Calcite   0 0
    Co2(g)    -2.5 10
    Fe(OH)3(a) 0 0
    Gypsum    0 0
    O2(g)     -0.68 10
SELECTED_OUTPUT 1
    -file                 xxxxxxxxxxxxxxx.xls
    -reset                true
    -distance             false
    -time                 false
    -reaction             false
    -totals               Alkalinity  Cl  S(6)  Al  Ba  Ca  Mg
                          K  Na  Fe(2)  Fe(3)
    -equilibrium_phases   Al(OH)3(a)  Barite  Calcite  Fe(OH)3(a)
                          Gypsum
    -gases                CO2(g)  O2(g)
END

[dlparkhurst]

At low pH and oxygenated water, ferric iron will predominate. Hence, the iron ferric oxyhydroxide coating of   Cement Creek and the red Animas River after last month's mine blowout in Colorado. You can look at the kinetics of iron oxidation in the literature. There is an example in the PHREEQC manual.