automated redox adjustment (Ferrihydrite)

[Felix]

Hello PHREEQC-Users,

I recently encountered an interesting behaviour of PHREEQC accounting the Redox: In the water sample below, Ferrihydrite is oversaturated. I tried to precipitate it with Equilibrium_Phases but instead of precipitation PHREEQC is dissolving additional Ferrihydrite. This happens possibly due to the automated redox adjustment. Look the script below.

1) What is "Adjusted to redox equilibrium" in detail? How is it possible to prevent/work with it?
2) In addition there is a "Charge balance" for the pH. What does it mean?
3) With phreeqc.dat the programm calculates two redox couples:
   Redox couple             pe  Eh (volts)

   C(-4)/C(4)          -2.9279     -0.1668
   Fe(2)/Fe(3)          5.6715      0.3231

Which couple should I choose as pe for the Solution 1?

Best regards!
Felix

Example script:

SOLUTION 1
temp 14
pH 6.5
units mg/l
Na 1400
K 75
Ca 96
Mg 65
Fe(+2) 9.4
Fe(+3) 0.63
Mn 0.28
C(-4) 0.24 as CH4
Cl 1600 charge
S(6) 650 as SO4-2
Alkalinity 650 as HCO3

EQUILIBRIUM_Phases 1
Fe(OH)3(a) 0.0
Siderite 0.0

END