Sorption of Iron from Siderophore Complexes by Mn Oxides

Siderophores are chelating agents produced by terrestrial biota to increase the bioavailablity of ferric iron in oxic environments. Recent work has suggested that Mn(III) may affect siderophore-mediated iron transport, but little is known about the effects of manganese(III,IV) oxides. To probe the intectaions of layer type manganese oxides with aqueous Fe-siderophore complexes, the sorption of ferrioxamine B [Fe(III)HDFOB⁺] to synthetic and biogenic birnessite was examined. The Mn(III,IV) oxides greatly reduced the aqueous concentration of Fe(III)HDFOB⁺. EXAFS spectra suggest that the dominant fraction of Fe(III) associated with the Mn(IV) oxides is specifically adsorbed to the mineral structure at multiple sites, thus indicating that the Mn(IV) oxides displaced Fe(III) from the siderophore complex. These results indicate manganese oxides, including biominerals, may sequester iron from soluble ferric complexes, and thus we conclude that the sorption of iron-siderophore complexes may play a significant role in the biogeochemical cycling of iron in diverse environments.