Experimental study (batch test) showed the the effectiveness of GAC in removal of Fe2+ and Mn2+ from groundwater.
Read the attached paper. Good work, published in 'Desalination (Elsevier)' in 2005. (I found this paper from my archive).
There are other proved methods of Fe-Mn removal. Our institute has developed iron removal plant and domestic iron removal filler which are in use in many villages.