Influence of surfactants on siderophore-promoted dissolution of iron oxides
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2007
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Doctoral Thesis
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Abstract
Iron is an essential nutrient for most living organisms and it plays an important role in many biogeochemical processes in soils. Because the solubility of Fe is exceedingly low under oxic conditions, many microorganisms and plants actively modify their chemical environment in order to facilitate iron acquisition. For instance, molecules such as siderophores (Fe(III)-specificligands) are produced by the biota in order to promote the dissolution of iron oxides and therefore enhance the bioavailability of iron. While it is well recognized that siderophores play an important role in ligand-promoted dissolution, little is known about the possible interactions witii other microbial exudates including biosurfactants. Surfactants are amphiphilic molecules that self-assemble, forming aggregates on mineral-water interfaces. These aggregates can strongly modify the physico-chemical properties of the interfaces. Ligand-promoted dissolution of minerals is controlled by reactions at the mineral-waterinterface and may therefore be influencedby the presenceof adsorbed Surfactants. The objective of this thesis was to investigate the influence of anionic Surfactants on the siderophore-promoted dissolution of goethite (a-FeOOH). Two anionic Surfactants were considered: a mixture of mono- and di-rhamnolipids(RhLl and RhL2, respectively) produced by Pseudomonas aeruginosa and the synthetic Surfactant sodium dodecyl Sulfate (SDS). In a first step, the adsorption and aggregationof Surfactantson the goethite surface were characterized by conducting adsorption isotherm experiments and by measuring the electrophoretic mobility of the goethite particles. Then, the adsorption of the two siderophores desferrioxamineB (DFOB) and desferrioxamineD (DFOD) and the ligand EDTAand their iron complexeswas investigated in batch reactors at pH 6 with increasing SDS concentrations. The two siderophores were selected because their only structural difference is the modificationof the terminating group attached to the pentyl chain. From this modificationarises a difference in charge in the pH ränge under investigation:DFOB is positively charged and DFOD is neutral. Thereby, it was possible to identify the importance of electrostatic and hydrophobic interactions. The ligand-promoted dissolution for the same siderophoresand ligand was investigated under similar conditions. The effect of SDS on the inner-sphere and outer-sphere surface speciation of adsorbed ligands was investigated by fluorescence spectroscopy in a system with 8-hydroxyquinoline-5-sulfonate (HQS) as a ligand and boehmite (y-AlOOH) as the solid phase. HQS is a useful probe Compound for surface complexation because the free ligand species are nonfluorescent, whereas the inner-sphere complex of HQS with structural AI is strongly fluorescent, as well as the AI-HQS complex. Finally, the surface protonationof goethite induced by the adsorption of SDS was also investigated in a batch experiment at constant pH. Adsorption of SDS to goethite at pH 6 caused a charge reversal,as was shown by electrophoretic mobility measurements. This indicates the formation of admicelles (bilayered aggregates) at the mineral surface. The adsorption of the siderophores DFOB and DFOD increased similarly with increasing SDS concentrations, despite their difference in molecular charge, while the adsorption of EDTA was not influencedby SDS. This suggests that hydrophobic interactions are more relevant than electrostatic interactions in presence of Surfactants. The ligand-promoted dissolution rate in the presence of SDS and RhL was enhanced by a factor of 2 to 3 at low Surfactant concentrations (< 80 uM SDS, < 6 rag F1 RhL). At higher concentrations, the effect was smaller and the dissolution rates were similar to ligand-promoted dissolution in the absence of Surfactants. By direct comparisonbetween the adsorbed amounts of ligands and the dissolution rates, we showed that the rate expression for ligand-promoted dissolution (R=KL[L]ads) can not be applied in the presence of variable Surfactant concentrations. The experiments with the system HQS-boehmite revealedthat the ratio of inner-sphere to outer-sphere HQS complexes was constant for varying SDS concentrations. Moreover, the inner-sphere complexation was not correlated to the dissolution rates, suggesting that anothermechanism triggered by Surfactantsmust lead to the enhanced dissolution of boehmite in presence of SDS. Although the surface protonation increased due to the adsorption of low SDS concentrations, the resulting proton-promoted dissolution was estimated to be insignificant cornpared to the observed dissolution rates. Because the effects are similar for different ligands and surfaces, the results suggestthat Surfactantsmay have an importanteffecton mineralsurface properties leading to enhancedligand-promoted dissolution rates. Most probably, a synergistic effect between adsorbed Surfactant molecules, protons, siderophoresand the mineral surface lead to the observed increase in siderophores-promoted dissolution rates. However this synergisticeffect vanishes with increasing Surfactant concentrations. In conclusion, this work demonstrates that small concentrations of Surfactants increase the ligand-promoted dissolution rates of Fe and AI hydroxides. This can have importantimplication for the acquisition of iron and other nutrientsby microorganism and plants. The presence of Surfactants in the environment might also influence other important biogeochemical processes such as the adsorption of organic Compounds onto the soil matrix and mineralweathering.
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Examiner: Kretzschmar, Ruben
Examiner : Krämer, Stephan
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AMPHIPHILE MOLEKÜLE UND INTERAKTIONEN (BIOPHYSIKALISCHE CHEMIE); SIDEROPHORE (BIOCHEMIE); GOETHIT (MINERALOGIE); GOETHITE (MINERALOGY); AMPHIPATHIC MOLECULES AND INTERACTIONS (BIOPHYSICAL CHEMISTRY); SOLUTION KINETICS (LIQUIDS); BOEHMITE (MINERALOGY); LÖSUNGSKINETIK (FLÜSSIGKEITEN); BOEHMIT (MINERALOGIE); SIDEROPHORS (BIOCHEMISTRY)
Organisational unit
03541 - Kretzschmar, Ruben / Kretzschmar, Ruben
03621 - Peter, Markus (ehemalig) / Peter, Markus( former)