THE EFFECTS OF THE ADSORPTION OF BIOGENIC LIGANDS ON THE DISSOLUTION OF MANGANESE OXIDES

Siderophores are biogenic chelating agents exuded in terrestrial and marine environments to increase the bioavailablity of ferric iron. Recent work suggests that both solid and aqueous manganese may affect siderophore speciation and thus siderophore-mediated iron transport. We studied the interaction of the trihydroxamate siderophore desferrioxamine B with several environmentally relevant manganese (hydr)oxides: hausmannite and manganite, lower-valence minerals typically produced by abiotic Mn(II) oxidation; two synthetic layer-type Mn(IV) oxides; and a biogenic oxide produced by a model organism, Pseudomonas putida MnB1. For pH 5-7, we find that all minerals studied dissolve by “reductive” dissolution, yielding Mn(II); for pH 7-9, “ligand-promoted” dissolution, which yields Mn(III)-siderophore complexes, is the predominant reaction pathway. In the case of manganite, the ligand-promoted dissolution rate is demonstrated to be proportional to the sorbed concentration of DFOB. Additionally, for all manganese oxides the dissolution rates increase with decreasing average manganese oxidation number. Because reductive dissolution results in siderophore oxidation, Mn-oxide dissolution at acidic pH may provide a significant abiotic sink for siderophores in natural waters. At alkaline pH, the complexes produced may profoundly affect the aqueous speciation of siderophores as well as provide a source of reactive Mn(III)-complexes.