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Mineral Formation and Sorption Mechanisms in Marine Ferromanganese-Rich Sediments

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Mineral Formation and Sorption Mechanisms in Marine Ferromanganese-Rich Sediments Mineral Formation and Sorption Mechanisms In Marine Ferromanganese-rich Sediments Amy Leanne Atkins Submitted in accordance with the requirements for the degree of Doctor of Philosophy The University of Leeds School of Earth and Environment October 2014 The candidate confirms that the work submitted is her own, except where work, which has formed part of jointly authored publications has been included. The contributions of the candidate and the other authors to this work have been indicated overleaf. The candidate confirms that appropriate credit has been given where reference has been made to the work of others. This copy has been supplied on the understanding that it is copyright material and that no quotation from this thesis may be published without proper acknowledgement. ii Declaration Chapter 4 is a reproduction of a peer-reviewed publication in the journal Geochimica et Cosmochimica Acta as: Atkins A. L., Shaw S., Peacock C. L. (2014) Nucleation and growth of todorokite: Implications for trace-metal cycling in marine sediments. Geochim. Cosmochim. Acta. 144, 109-125. TEM images presented in chapter 4 were collected by Michael Ward at the Leeds Electron Microscopy and Spectroscopy centre with the guidance of the candidate. All other experimental work has been conducted by the candidate. The interpretation of the experimental data, and preparation of the manuscript has been undertaken by the candidate with the help and guidance of her supervisors: Caroline L. Peacock and Sam Shaw. Chapter 5 is a reproduction of a manuscript currently in the final stages of preparation for submission to the journal Geochimica et Cosmochimica Acta as: Atkins A. L., Shaw S., Peacock C. L. (2014) Transformation of Ni-rich birnessite: Implications for the fate of bioessential Ni. TEM-EDS data presented in chapter 5 was collected by Michael Ward at the Leeds Electron Microscopy and Spectroscopy centre with the guidance of the candidate. ICP-OES analysis described in the methods section (chapter 3) was conducted by Bob Knight at the Department of Chemistry, University of Hull, UK. Micro thin-sections were prepared by Bob Jones and John Ford at the National Oceanography Centre, Southampton. Caroline Peacock was extensively involved in the collection and processing of the XAS and -XAS data presented in chapter 5. All other experimental work has been conducted by the candidate. The interpretation of the experimental data was undertaken by, and the manuscript was prepared by the candidate with the help and guidance of her supervisor: Caroline L. Peacock. Chapter 6: All mineral synthesis procedures and Ni sorption experiments described in chapter 6 were conducted by the candidate. All of the Ni isotope analysis procedures outlined in the methods chapter (chapter 3) were conducted by Louise Gall at the Department of Earth Sciences, University of Oxford UK, and at the Institute for Research in Environmental Science, University of Colorado, USA. Interpretation of the iii data presented in chapter 6 was undertaken by the candidate with guidance from Louise Gall and her supervisor, Caroline L. Peacock. iv Acknowledgements Thanks are due to my supervisors Caroline Peacock and Sam Shaw for their invaluable support and guidance throughout the project. Liane Benning for being part of my research support group and for the useful discussions on crystallization. The Natural Environmental Research Council and the School of Earth and Environment for funding my PhD and various trips to national and international conferences. Sam Allshorn for his assistance during lab work. Michael Ward for collecting countless TEM images, Lesley Neve for training me on the XRD and Bob Knight at the University of Hull for ICP-OES analysis. Everyone from the Cohen Geochemistry group, especially the people in office 8.154. Finally, I would like to thank my friends and family for their continued support. v Abstract The phyllomanganate birnessite is the dominant Mn-bearing phase in oxic marine sediments and through coupled sorption and redox reactions exerts a strong control on the oceanic concentrations of micronutrient trace metals. However, during oxic diagenesis and under mild hydrothermal conditions, birnessite undergoes transformation to the tectomanganate todorokite. The mechanistic details of the transformation are important for the speciation and mobility of metals sequestered by birnessite, and are necessary in order to quantify the role of marine sediments in global trace element cycles. This study provides new insight into the crystallization pathway and mechanism of todorokite formation from birnessite under conditions analogous to those found in marine diagenetic and hydrothermal settings. Using a combined approach employing X-ray diffraction, electron microscopy, infrared spectroscopy, X- ray absorption spectroscopy and wet chemical methods, I propose a new four-stage process for the transformation of birnessite to todorokite, beginning with todorokite nucleation, then crystal growth from solution to form todorokite primary particles, followed by their self-assembly and oriented growth via oriented attachment to form crystalline todorokite laths, culminating in traditional crystal ripening. Furthermore, the results of this study indicate that contrary to current understanding, the bioessential trace metal Ni impedes the transformation of birnessite to todorokite, and is eventually released into sediment porewaters. This mineralogical transformation may therefore provide a benthic flux of Ni and possibly other micronutrient trace metals to seawater. Finally, I find that the uptake of Ni to the phyllomanganate birnessite under varying physiochemical conditions is accompanied by Ni stable isotope fractionation. During fractionation, the light Ni isotope is preferentially sorbed to birnessite, leaving the remaining solution heavy with respect to its Ni isotopic composition. These findings raise important questions about the mechanisms and processes responsible for the heavy δ60Ni isotopic compositions recently measured in marine ferromanganese-rich sediments. vi Contents Declaration ...................................................................................................... ii Acknowledgements ....................................................................................... iv Abstract ........................................................................................................... v Figures ............................................................................................................ ix Tables ............................................................................................................ xiv Chapter 1 Introduction .................................................................................... 1 1.1 Background information ........................................................................ 1 1.2 Research aims and objectives ................................................................. 3 1.3 Thesis outline .......................................................................................... 4 Chapter 2 Literature review ............................................................................ 5 2.1 The role of manganese minerals in trace-metal cycling ........................... 5 2.2 Manganese oxides in the marine environment ........................................ 6 2.3 Manganese oxide precipitates in the marine environment ....................... 6 2.3.1 Hydrogenetic ferromanganese crusts ......................................................... 6 2.3.2 Diagenetic ferromanganese nodules .......................................................... 7 2.3.3 Hydrothermal ferromanganese-rich deposits .............................................. 8 2.4 Birnessite and todorokite mineralogy ....................................................... 8 2.4.1 Birnessite formation .................................................................................. 13 2.4.2 Todorokite formation ................................................................................. 14 2.5 Sorption processes at the mineral-water interface ................................. 17 2.5.2 Inner-sphere complexation ....................................................................... 19 2.5.3 Structural incorporation ............................................................................. 19 2.6 Oceanic biogeochemistry of Ni .............................................................. 24 Chapter 3 Methods ........................................................................................ 29 3.1 Laboratory based methods. ................................................................... 30 3.1.1 Synthesis of c-disordered birnessite ......................................................... 30 3.1.2 Transformation of c-disordered birnessite to todorokite ............................ 30 3.1.3 Co-precipitation of c-disordered birnessite in the presence of Ni. ............. 32 3.1.4 Transformation of Ni incorporated c-disordered birnessite to todorokite ... 32 3.1.5 Synthesis of reference Manganese Oxide phases .................................... 33 3.1.6 Determination of average Mn oxidation state via potentiometric titration ... 35 3.1.7 Ni Isotope fractionation during sorption to hexagonal birnessite ............... 38 3.2 Spectroscopic and analytical tools. ........................................................ 41 vii 3.2.1 Powder X-ray diffraction (XRD) ...............................................................
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