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Minerals and natural organic matter are often associated and interact in aquatic and terrestrial systems. Interactions such as adsorption, dissolution and precipitation affect their physical and chemical properties as well as those of the surrounding matrix, ultimately influencing (bio)geochemical cycles. For example, natural organic matter can cover the mineral surface, changing the reactivity, solubility and sorption properties of the mineral towards other substances. In order to investigate the influence of characteristic functional groups of natural organic matter on the reduction of iron minerals, model compounds with corresponding properties were selected. The aim of the work was to investigate the influence of the adsorption of these model compounds on the reducibility of two widely used iron oxides in order to derive process-oriented insights into iron reduction in more complex natural systems.
Firstly, adsorptive redox organic model compounds (quinones, e.g., anthraquinone-2,3-dicarboxylic acid and 2-hydroxy-1,4-naphthoquinone) were utilized to study adsorption on ferrihydrite, and the bio-reduction kinetics by Shewanella oneidensis MR-1 were measured. The comparison of reduction kinetics between adsorptive redox-active s and non-sorbing redox-active organic model compounds (e.g., anthraquinone-2,6-disulfonate and anthraquinone-2-sulfonate) showed that the adsorption of quinones inhibits bio-reduction kinetics of ferrihydrite. However, reduction kinetics and the amount of adsorption showed a complex relation. This may be due to the toxicity of quinones to bacteria, which is consistent with the cell numbers in each experimental sample.
Secondly, to investigate the effect of quinone adsorption on the reduction kinetics of ferrihydrite, electrochemical approaches were applied to simulate bio-reduction process. The results demonstrated that the reduction kinetics of ferrihydrite was not related to the redox potential and electron transfer capacities of quinones, but to the concentration of dissolved quinones. Reduced quinone species were not detected on the mineral surface, suggesting rapid electron transfer upon quinone contact with the minerals.
Thirdly, to study whether covering surface active sites affects the reduction kinetics of ferrihydrite, the electrochemical reduction in the presence of adsorptive organic model compounds (e.g., salicylic acid, 3-hydroxy-2-naphthoic, phthalic acid, and 2,3-naphthalene-dicarboxylic acid) was compared to an inorganic sorbing redox-inert model compound (phosphate). Phosphate exhibited higher adsorption affinity for ferrihydrite than the organic models. The adsorption of inert organic model compounds and phosphate inhibited the reduction kinetics of ferrihydrite. Although phosphate did not desorb during reduction, vivianite was not formed. This may be due to the short experimental time and/or the lack of free phosphate.
Finally, the adsorption and reduction characteristics were compared with goethite, a thermodynamically more stable iron oxide. The results showed that ferrihydrite exhibited higher affinity for adsorptive organic models and phosphate and faster reduction kinetics than goethite. Phosphate desorbed from the goethite surface during reduction in contrast to the organic model compounds. The adsorption of organic and inorganic substances blocked the reactive sites on the iron oxide surface and retarded their reduction kinetics.
Overall, the results of this thesis showed that the adsorption of sorbates on the mineral surface retarded their reduction kinetics, regardless of whether they were redox active or redox-inert. These findings foster our understanding of how the interactions between minerals and natural organic matter affect their (bio)geochemical cycling. The findings of this work have implications for nutrient management, heavy metal removal, organic contaminant degradation, and carbon fixation. To comprehensively understand the electron transfer mechanism during natural organic matter adsorption onto iron oxides, the morphological and mineralogical characteristics of minerals should be studied further. To this end, more complex organic models should be utilized to better represent environmental conditions. |
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