HR: 09:15h
AN: B11B-05    [Abstracts]
TI: Siderophore Promoted Dissolution of a Series of Mn-Substituted Goethites
AU: * Holmstrom, S J
EM: sholm@nature.berkeley.edu
AF: Division of Ecosystem Sciences, University of California, 137 Mulford Hall #3114 , Berkeley, CA 94720-3114 United States
AU: Sposito, G
EM: gsposito@nature.berkeley.edu
AF: Division of Ecosystem Sciences, University of California, 137 Mulford Hall #3114 , Berkeley, CA 94720-3114 United States
AB: The presence of organic ligands, like siderophores, can strongly influence mineral dissolution. Recent research suggests that at least some siderophores enhance mineral dissolution by formation of surface complexes with Fe and Mn. The impact of biogeochemical weathering caused by exudates of plants, fungi and bacteria containing siderophores has been discussed. We have studied the dissolution kinetics of Mn-substituted goethites (mol % Mn < 11) in the presences of 80 μM desferrioxamine B (DFO-B), a common and well-studied hydroxamate siderophore that has been identified in both terrestrial and marine environments and which forms very stable 1:1 complexes with Fe(III) or Mn(III). (The stability constants at I = 0.1 are 10^30.6 and 10^28.3, respectively.) A series of Mn-substituted goethites (α-Mn_xFe_1-xOOH) were synthesized from ferrihydrite in the presence of Mn(II) in alkaline media. The Fe(III) in octahedral positions in the mineral structure was partially replaced by Mn, which was confirmed visually by the change to darker color when the content of Mn increased and proved by infra-red spectroscopy and X-ray diffraction studies of the samples. Substitution of Fe in the goethite by Mn caused a change in the cell dimensions. The calculated unit cell edge lengths a and c decreased, while b increased, for the Mn-goethites compared to pure goethite. The difference of the unit cell parameters between the pure goethite and the Mn-substituted goethites increased with increased Mn content, providing further confirmation that Fe had been substituted by Mn incorporated into the goethite structure. X-ray absorption near-edge structure spectroscopy analysis of the Mn-substituted goethites showed that the oxidation state of Mn in the samples was, as expected, Mn(III), even when Mn-goethites were prepared from Mn(II) solutions. Both SEM and TEM micrographs showed that the Mn-substituted goethite crystals had the same acicular shape as pure goethite. The specific surface area σ of the particles was also affected by their Mn content, with increased Mn-substitution causing higher σ. We determined the adsorption of 80 μM DFO-B for the Mn-substituted goethites using batch experiments conducted at pH 7. The results showed that the surface excess of DFO-B, determined as mmolkg^-1 mineral and mmolm^-2 mineral, increased with Mn content. Continuous-flow stirred reactors were used for the dissolution experiments and were performed under the same conditions as the adsorption experiments. Increasing Mn content led to higher dissolution rates of the Mn-substituted goethites. For example, our investigation showed that the dissolution rate of 11% Mn-substituted goethite is about 23 times higher than unsubstituted goethite. The results indicate that the changes of the goethite structure by low rates of Mn(III)substitution have a significant affect on the dissolution rate and thereby may increase the bioavailability of essential nutrients (or adsorbed toxicants) for plants and microorganisms under physiological conditions.
DE: 0412 Biogeochemical kinetics and reaction modeling (0414, 0793, 1615, 4805, 4912)
DE: 0461 Metals
SC: Biogeosciences [B]
MN: Fall Meeting 2005