Siderophore-manganese(III) interactions II. Manganite dissolution promoted by desferrioxamine B

Recent laboratory and field studies suggest that Mn(III) forms persistent aqueous complexes with high-affinity ligands. Aqueous Mn(Ill) species thus may play a significant but largely unexplored role in biogeochemical processes. One formation mechanism for these species is the dissolution of Mn(Ill)-bearing minerals. To investigate this mechanism, we measured the steady-state dissolution rates of manganite (gamma-MnOOH) in the presence of desferrioxamine B (DFOB), a common trihydroxamate siderophore. We find that DFOB dissolves manganite by both reductive and nonreductive reaction pathways. For PH > 6.5, a nonreductive ligand-promoted reaction is the dominant dissolution pathway, with a steady-state dissolution rate proportional to the surface concentration of DFOB. In the absence of reductants, the aqueous Mn(III)HDFOB+ complex resulting from dissolution is stable for at least several weeks at circumneutral to alkaline PH and at 25 degrees C. For PH < 6.5, Mn2+ is the dominant aqueous species resulting from manganite dissolution, implicating a reductive dissolution pathway. These results have important implications for the biogeochemical cycling of both manganese and siderophores-as well as Fe(III)-in natural waters and soils.