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Redox Reactions and Phase Transformation Processes at Iron Mineral Surfaces Studied by Compound Specific Isotope Analysis

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Redox Reactions and Phase Transformation Processes at Iron Mineral Surfaces Studied by Compound Specific Isotope Analysis Redox reactions and phase transformation processes at iron mineral surfaces studied by compound specific isotope analysis Dissertation zur Erlangung des Grades eines Doktors der Naturwissenschaften der Geowissenschaftlichen Fakultät der Eberhard Karls Universität Tübingen vorgelegt von Anke Buchholz aus Bad Saulgau 2009 Tag der mündlichen Prüfung: 05. Oktober 2009 Dekan: Prof. Dr. Peter Grathwohl 1. Berichterstatter: Prof. Dr. Stefan Haderlein 2. Berichterstatter: Prof. Dr. Stefan Peiffer Erklärung Hiermit versichere ich wahrheitsgemäß, dass ich die vorliegende Arbeit selbständig verfasst, keine anderen als die angegebenen Quellen und Hilfsmittel benutzt und wörtlich oder inhaltlich übernommene Stellen als solche gekennzeichnet habe. iv Danksagung Danksagung Mein besonderer Dank geht an Prof. Stefan Haderlein, der es mir ermöglichte, auf einem für mich neuen Gebiet zu arbeiten. Zahlreiche Diskussionen, Treffen und seine ständige Erreichbarkeit bei auftauchenden Problemen ermöglichten es, diese Arbeit weiterzuentwickeln. Auch die Teilnahmen an nationalen und internationalen Konferenzen und Workshops haben sehr zu einem besseren Überblick über die gesamte Thematik beigetragen. Herzlichen Dank! Ein weiterer Dank geht an meinen zweiten Betreuer Prof. Stefan Peiffer (Universität Bayreuth) für die Begutachtung dieser Arbeit und die vielen Diskussionen während unserer Forschergruppentreffen. Auch den anderen Mitgliedern der Forschergruppe ein Dankeschön für die halbjährlichen Treffen an schönen Orten, die Diskussionen und lustigen Abende: Prof. Andreas Kappler, Prof. Rainer Meckenstock, PD Dr. Christian Zwiener, Dr. habil. Hans-Hermann Richnow, Dr. Christine Laskov, Iris Bauer, Katrin Hellige, Rita Kleemann, Carsten Jobelius und Stefan Feisthauer. Besonders Iris möchte ich für den gemeinsamen Start in Tübingen und die dreijährige abwechslungsreiche Laborzeit und Katrin für die gute Zusammenarbeit bei der Synthese und Charakterisierung der Minerale danken. Des Weiteren gilt mein Dank Dr. Thomas Wendel für unzählige Helium- Flaschenwechsel und seine Hilfsbereitschaft bei allen analytischen Problemen, Prof. Marcus Nowak für die Möglichkeit am ATR-FTIR Messungen durchzuführen und Daniel Russ für BET-Messungen. Auch ein Dankeschön an Katja Amstätter und Detlef Dierksen für µ-XRD und BET-Messungen, Phil Larese-Casanova, PhD, für Mössbauer-Messungen Danksagung v und viele Diskussionen und an Ellen Struwe und Julia Sauter für DOC- und AAS-Messungen. Allen ehemaligen und gegenwärtigen Mitgliedern der Arbeitsgruppen Umweltmineralogie und -chemie sowie der Geomikrobiologie danke ich für eine gute Zusammenarbeit und zahlreiche Hilfestellungen im Labor. Mein besonderer Dank geht an Michaela Blessing für die Einführung in das GC-IRMS und die gemeinsamen und leider zahlreichen Reparaturen desselben. Durch deine gute Laune und Entschlossenheit haben wir das „Biest“ immer wieder zum Laufen bekommen. Auch an Katharina Porsch ein spezieller Dank für die verschiedenen sportlichen Aktivitäten neben dem ganzen Laboralltag. Für zahlreiche weitere Aktivitäten neben der Arbeit danke ich: Florian Hegler, Iris Bauer, Katja Amstätter, Katharina Porsch, Lihua Liu, Michaela Blessing, Nicole Posth und Satoshi Endo. Ich denke immer gerne daran zurück. Björn, danke für deine ausgeglichene Art, dein Zuhören nach stressigen Labortagen und deine liebevolle Unterstützung. Zuletzt möchte ich mich bei meinen Eltern und Großeltern bedanken, die mich während der gesamten Studien- und Promotionszeit nicht nur finanziell sondern auch menschlich immer unterstützt haben. vi Abstract Abstract Iron minerals are widely distributed in the subsurface due to the oxidation of aqueous ferrous iron released by weathering processes. They are known to support electron cycles and facilitate redox reactions in the anoxic subsurface. The high affinity of iron oxides for dissolved ferrous iron leads to accelerated oxidation of Fe(II) upon sorption which involves changes in the mineral structure. These highly reactive Fe(II) phases mostly present as mineral coatings, may act as electron donors and acceptors for chemical and microbial processes. During the oxidation of surface bound Fe(II) by the simultaneously transformation of contaminants, several environmental conditions, i.e., the mineral structure, the available amount of dissolved iron, pH and the occurrence of organic material, may affect the properties of the sorbed Fe(II) and the dynamics of the iron mineral surface. So far the in situ characterization of the properties of reactive Fe(II) species and the dynamics of mineral phases is very limited due to a lack of spectroscopic methods. Therefore this work focused on an indirect method, the reactive isotope tracer approach, which based on changes in the isotopic composition of probe compounds reacting with surface bound Fe(II) species. In the second chapter, the iron minerals were characterized by different surface sensitive methods, i.e., µ-XRD, BET and SEM, and selected for the further experiments. This work was essential for the further experiments to guarantee well defined reaction conditions and the comparability with former studies. The effect of different geochemical factors on the oxidation of surface bound Fe(II) were investigated in chapter 3 and 4. Therefore batch experiments with goethite, lepidocrocite, magnetite or hematite were performed under well defined conditions (50 m2/L mineral, 1 mM aqueous Fe(II)final, CCl4 as Abstract vii contaminant) and the effect of various pH values (5-8) was investigated in chapter 3. Initially the amount of sorbed Fe(II) decreased in the order: goethite > magnetite > hematite > lepidocrocite. The subsequent transformation of CCl4 on the mineral/Fe(II) surface showed the fastest reaction rate constants for the goethite system for all pH values. Whereas in the goethite system the reductive dehalogenation of CCl4 forms chloroform and carbon monoxide as products, only the formation of CO was observed in the hematite system at pH 7. Additionally sorbed Fe(II) on hematite was less oxidized (< 10 times) compared to goethite with different product distribution. All batch experiments showed no reactivity towards CCl4 at pH 5 and 6 as well as the magnetite and lepidocrocite systems at pH 7. At pH 8 secondary iron mineral formation occurred in the lepidocrocite system during the addition of Fe(II) which was identified as magnetite by µ-XRD and Mössbauer spectroscopy. Only batch experiments with magnetite showed no reactivity towards CCl4 at pH 8. A Mössbauer study demonstrated the non-stoichiometric nature of magnetite also in the presence of aqueous Fe(II) which could be responsible for the missing reactivity of this iron oxide. In chapter 4 the influence of several environmental factors like different pH values, amounts of sorbed Fe(II) and sorption times of Fe(II) were investigated at the goethite surface. In general, the results indicated that the surface speciation of Fe(II) on goethite changed significantly already after a small fraction of Fe(II)sorb was oxidized by CCl4. Thus, changes (i) in the product distribution monitored by the formation of chloroform, (ii) in reaction rate constants for CCl4 and (iii) in the isotope fraction of CCl4 could be observed. At the mineral surface two different “types” of reactive Fe(II) species could be identified as shown by the isotope fractionation factors: highly reactive Fe(II) species (ε = -10 to -15‰) and with the depletion of theses species less reactive Fe(II) sites (ε ~ -25‰), formed by readsorption of Fe(II) and/or further surface remodeling processes. The hypothesis of two different Fe(II) species could be further proven by Mössbauer spectroscopy. viii Abstract Finally, the potential effect of the organic buffers MOPS and HEPES on reaction conditions in a goethite/Fe(II) system was examined in chapter 5. Although the application of these buffers is discussed controversially, many studies include organic buffers to control reaction conditions in batch experiments. Desorption of previously sorbed Fe(II) from the goethite surface in the presence of organic buffers occurred simultaneously with significant sorption of the organic buffers on the mineral/Fe(II) surface (MOPS > HEPES). These observations suggest an interaction between the organic buffers and the Fe(II)/mineral surface. This interaction showed various effects on the oxidation of Fe(II) by CCl4 (i) with slower reaction rate constants and (ii) higher yield of chloroform with increasing organic sorbate concentrations. The type of interaction between MOPS/HEPES and Fe(II) was further characterized by investigating the competitive electrostatic and steric effects of model compounds (Ca2+ and two sulfonic acids). Neither desorption of Fe(II) nor significant sorption of Ca2+ and the two sulfonic acids were observed. In situ ATR-FTIR measurements of the organic buffers and aqueous Fe(II) excluded the involvement of C-H or C-C bonds in a complex formation and the precipitation of a buffer-Fe(II)- complex could not be observed. Therefore we postulated that free electron pairs of the nitrogen in the heterocyclic ring may form complexes with surface bound Fe(II). Zusammenfassung ix Zusammenfassung Durch die Oxidation von gelöstem zweiwertigem Eisen, das durch Verwitterungsprozesse freigesetzt wird, sind Eisenminerale in Böden und Sedimenten weit verbreitet. Sie sind bekannt dafür, dass sie Elektronenkreisläufe unterstützen und Redoxreaktionen im anoxischen Milieu ermöglichen. Die hohe Affinität der Eisenoxide zu gelöstem zweiwertigem Eisen führt
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