HR: 1340h
AN: B53C-1011    [Abstracts]
TI: Formation and Stability of Manganese-Desferrioxamine B Complexes
AU: * Duckworth, O W
EM: owend@nature.berkeley.edu
AF: University of California , Division of Ecosystem Sciences Hilgard Hall #3110 , Berkeley, CA 94720
AU: Sposito, G
EM: gsposito@nature.berkeley.edu
AF: University of California , Division of Ecosystem Sciences Hilgard Hall #3110 , Berkeley, CA 94720
AB: Recent laboratory and field studies suggest that Mn(III) forms persistent aqueous complexes with high-affinity ligands, particularly those produced by microbes. Aqueous Mn(III) species thus may play a significant, as-yet largely unexplored role in biogeochemical processes. We determined stability constants for both Mn(II) and Mn(III) complexes with the common tri-hydroxamate siderophore, desferrioxamine B (DFOB). We found the thermodynamic stability constants of the species, MHDFOB$^{{it\x-2}}$ [M = Mn(II), {\it x} = 2; M = Mn(III), {\it x} = 3] to be K$_{Mn(II)}$ = 10$^{6.8 \pm 0.1}$ and K$_{Mn(III)}$ = 10$^{29.2 \pm 0.2}$ at 25$\deg$C. The Mn(III)HDFOB$^{+}$ complex is stable for pH in the range 7.0 - 11.3, but at pH $<$ 7.0, Mn(III)HDFOB$^{+}$ decays by internal electron transfer, yielding oxidized DFOB products and Mn$^{2+}$. For pH $>$ 11.3, the complex decays by disproportionation, yielding Mn$^{2+}$ and solid MnO$_{2}$. The Mn(III)HDFOB$^{+}$ complex may be formed by either the oxidation of aqueous Mn(II)-HDFOB complexes or the DFOB-promoted dissolution of solid manganese(III) oxides. The DFOB-promoted Mn(II) air-oxidation rate was found to be proportional to the concentration of Mn(II)-DFOB complexes. At pH $>$ 6.5, the dissolution of manganite ($\gamma$-MnOOH) in the presence of DFOB is predominantly a non-reductive ligand-promoted reaction whose rate is proportional to the adsorbed surface concentration of DFOB. At pH $<$ 6.5, Mn$^{2+}$ is the dominant species resulting from manganite dissolution, thus implicating a reductive dissolution pathway. The results of this study have broad implications for the biogeochemical cycling of manganese, redox-active elements, and siderophores in natural waters and soils.
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1055 Organic geochemistry
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting