HR: 09:00h
AN: B11B-04 INVITED     [Abstracts]
TI: Self Assembly of Biogenic Surfactants at Mineral Surfaces and Their Effect on Biological Iron Acquisition
AU: * Kraemer, S M
EM: kraemer@env.ethz.ch
AF: Department of Environmental Science, ETH Zurich, ETH Zentrum, CHN F 21.1 Universitaetstr. 16, Zurich, 8092 Switzerland
AB: Microorganisms exude biogenic surfactants to modify the physical and chemical properties of mineral-water interfaces. Surfactants with negatively charged hydrophilic head groups interact strongly with oppositely charged mineral surfaces such as iron or aluminum oxides. Surfactant self assembly at mineral surfaces can result in the formation of admicelles that have a significant effect on the surface charge and hydrophobicity. These effects are exploited by microorganisms to facilitate attachment to mineral surfaces. Similarly, plants exude surfactants into the rhizosphere and change the surface tension and flow of soil water. Other surface active compounds that are typically found in soils and surface waters are humic substances and fatty acids that are produced by degradation of biomass. In general, surface active compounds are ubiquitous in natural systems. In this study we investigated how surfactants influence bio-mineral interactions using the example of siderophore promoted iron acquisition. Siderophore promoted iron acquisition involves the adsorption of a biogenic iron specific ligand (i.e. the siderophore) to iron oxides and the subsequent siderophore promoted iron oxide dissolution. The hypothesis of this project is that the modification of the iron oxide surface charge and hydrophobicity by adsorbed surfactants will have an important effect on siderophore adsorption and dissolution kinetics. We approached this subject by investigating the adsorption of a natural surfactant (rhamnolipids: RhL) and the synthetic surfactant (sodium dodecyl sulfate: SDS) on goethite (α-FeOOH, a common pedogenic iron oxide) and observing the effect of surfactant self assembly on the properties of the mineral water interface. We observed fast adsorption kinetics at pH 3 and slow adsorption at pH 6. The adsorbed surfactants reversed the surface potential of goethite (as evidenced by electrophoretic mobility measurements) at soluble surfactant concentrations below 10 μM (SDS) and 4 mg/l (RhL). Surfactant self assembly at the mineral surface influences the adsorption of organic ligands. We studied the adsorption of two siderophores, Desferrioxamine B (DFO-B) and Desferrioxamine D (DFO-D). These siderophores have identical ligating groups and backbone, but DFO-B is positively charged while DFO-D has no charge at pH 6 due to structural modifications of a terminal amine group. Finally, we studied the adsorption of EDTA, which is negatively charged at the experimental pH. We found increasing adsorption of DFO-B and DFO-D with increasing surfactant concentrations, indicating that hydrophobic interactions between the ligand and the adsorbed surfactant are more important than electrostatic attraction. This interpretation is consistent with the observation that the surfactant had no effect on EDTA adsorption which is twice negatively charged and hydrophilic. Siderophore controlled iron oxide dissolution rates increased with increasing surfactant concentrations up to 10 μM but slightly decreased at higher surfactant concentrations, despite a further increase of adsorbed siderophore concentrations. This indicates that the presence of low surfactant concentrations facilitates inner sphere surface complexation of siderophores. However, at high surfactant concentrations siderophores adsorb as outer sphere complexes, presumably by adsorption to the surfactant admicelle. Our results demonstrate that surfactants have an important effect on bio-mineral interactions.
DE: 0412 Biogeochemical kinetics and reaction modeling (0414, 0793, 1615, 4805, 4912)
DE: 0448 Geomicrobiology
DE: 0463 Microbe/mineral interactions
DE: 0470 Nutrients and nutrient cycling (4845, 4850)
DE: 0486 Soils/pedology (1865)
SC: Biogeosciences [B]
MN: Fall Meeting 2005