I was thinking that you can add enough O2 to the water before it gets to the filter if you plumbed it in front of the column and made an O2 input or sorts to get the iron to react. Make it so the water falls down into the filter container from the top then you can have air holes on the top of the container around the inlet where air would be drawn in by the venturi effect of the falling water. Somewhat similar to my idea of a packed column but the bottom would be your filter media.

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