This is not my subject, but from scanning the introductions to a couple of reviews on the general topic of iron metabolism, the following factors seem to be important:

• Ferric iron is less soluble than ferric iron [1].

• Ferric iron is potentially toxic in the cell because it can be reduced by oxygen, giving reactive oxygen species which are dangerous to the cell [2].

[1] Srai et al. (2002)

[2] Kaplan (2002)

This is not my subject, but from scanning the introductions to a couple of reviews on the general topic of iron metabolism, the following factors seem to be important:

• Ferric iron is less soluble than ferrous iron [1].

• Ferric iron is potentially toxic in the cell because it can be reduced by oxygen, giving reactive oxygen species which are dangerous to the cell [2].

[1] Srai et al. (2002)

[2] Kaplan (2002)