Iron is normally absorbed in the duodenum & jejunum. You don't absorb the metallic iron or its oxide salt; you just absorb ionized iron.
Ferric = Fe +++ Ferrous = Fe++
Ferrous is absorbed from the GI tract & oxidized to ferric in the enterocyte. From there Ferric gets picked up by transferrin & carried around the plasma. Ferric is the kind in methemoglobin.
The rate limiting step is GI absorption which is carrier mediated. The problem when you OD is that the carrier system is overloaded. Once the carrier system is overloaded, GI absorption of iron is passive & concentration dependent. Transferrin is saturated & you have free iron running around in the plasma.
GI - iron salts are corrosive & cause hemorrhagic necrosis of the GI lining while in the tract (which is especially bad if an iron containing pill gets adhered to the mucosal surface.)
* By the way, iron supplement pills are radio-opaque
Cardiovascular - iron causes fatty necrosis of the myocardium, post arteriolar dilation & increased capillary permeability all of which serve to drop cardiac output. Iron also causes serotonin & histamine release which further drop blood pressure.
Liver - iron causes hepatic necrosis as the liver takes it up
Activated charcoal doesn't adsorb iron very well. At one point it was advocated to try lavaging the stomach with sodium bicarb or sodium phosphate with the idea that iron would precipitate into a non-absorbable form but this hasn't panned out in reality.
The best chelator of iron is deferoxamine (desferal). It is infused as an IV drip (rapid admin can cause arrhythmias & in iron tox your cardiac output is low enough as it is). Expect a few days treatment. You want a serum iron level < 300 ul/dl