Fe2+ with a lower ionic potential than Fe3+ is much more soluble because it exerts a weaker interaction force with OH− ion present in water and exhibits a less pronounced trend to hydrolysis and precipitation.
Under reducing conditions Fe(II) can be present at relatively high concentration in anoxic water, similar to these encountered for other divalent species such as Ca2+ and Mg2+.
However, once anoxic ground water is pumped from a deep well and is discharged to the surface, it enters in contact with atmospheric oxygen.
Millot (1970) also illustrated the importance of the ionic potential of cations to explain the high, or the low, solubility of minerals and the expansive behaviour (swelling/shrinking) of clay materials.
[4] The ionic potential of the different cations (Na+, K+, Mg2+ and Ca2+) present in the interlayer of clay minerals also contribute to explain their swelling/shrinking properties.