LOW TEMPERATURE FORMATION OF FERRIC ARSENATE ROBERT ALEC LEWIS A Thesis submitted for the degree of Doctor of Philosophy of Imperial College London & the Diploma of Imperial College (D.I.C.) Imperial College London, Department of Earth Science & Engineering, Exhibition Road, London, SW7 2AZ, U.K. 2 Abstract The safe disposal of arsenic (As) is important for the metal extraction industry worldwide. Previous work has shown that crystalline ferric arsenate (FeAsO 4·2H 2O), which is the synthetic version of the natural mineral scorodite, fulfils most of the criteria for safe disposal. Presently, this requires the use of expensive pressurised equipment in industrial applications. There is increasing evidence that microorganisms play an important part in the geochemical cycle of As. In environmental samples, bacteria are often closely associated with mineral precipitates, as well as in bioleaching samples. Strains of acidophilic bacteria ( Acidithiobacillus spp., Leptospirillum spp., Sulfobacillus spp. and Thiomonas spp.) have been enriched and isolated in liquid and on solid media from rock samples from former mine sites in the Czech Republic and the UK, and experiments carried out to determine whether the bacteria can catalyse the formation of ferric arsenate. Crystalline ferric arsenate has been identified in material precipitated by an environmental strain of Acidithiobacillus spp. in liquid media within 150 h -1 at 35°C and pH 3.0. Acidithiobacillus spp. also precipitated poorly crystalline ferric arsenate at pH 2.4 25°C. An environmental strain of Leptospirillum spp. also showed slightly crystalline scorodite at pH 1.5 25° and at pH 3.0 35°C. There is no evidence that the selected bacteria can precipitate ferric arsenates on solid media, however, despite varying concentrations of As added as As 3+ or As 5+ . Material formed has been x-ray amorphous due to rapid formation, and FTIR analyses have shown no increase in crystallinity with age over the months in which the precipitates have been studied. 3 Acknowledgements I would like to thank Professor John Monhemius and Professor Jane Plant for their supervision during this research. Dr. Bill Dudeney and Dr. Brenda Chan are thanked for their discussions in the lab. I would like to thank Dr. Michal Filippi and Petr Drahota for their assistance in the collection of samples in the Czech Republic, and Dr. Richard Scrivenor (BGS) and Bruce Grant in the UK. Thanks to Dr Kevin Hallberg and the Bangor Acidophile Research Team (BART) at Bangor University for donating pure strains of bacteria. Anton Kearsley and the team in the Electron Microprobe and Micro-Analysis (EMMA) labs at the Natural History Museum, London for SEM work and Martin Gill for XRD analyses. I would also like to thank the Institute of Minerals, Materials and Mining (IOM 3) for their financial support in the form of the Stanley Elmore Fellowship Fund, and thanks to Anglo American Plc. and Chris Oates for their partial support of this research. Finally I would like to thank Verity and my family for all their continued support and patience. 4 Contents Abstract .........................................................................................................................3 Acknowledgements.......................................................................................................4 Contents ........................................................................................................................5 List of Figures ...............................................................................................................7 List of Tables ..............................................................................................................10 Chapter 1 Introduction ..........................................................................................11 1.1 General Overview ..............................................................................................11 1.2 Aims & Objectives of Research.........................................................................14 1.3 Structure of Thesis .............................................................................................16 Chapter 2 Literature Review ................................................................................17 2.1 Chemistry of Arsenic .........................................................................................17 2.2 Arsenic Toxicity & Health.................................................................................22 2.3 Mineralogy.........................................................................................................25 2.4 Safe Disposal .....................................................................................................31 2.5 Bacteria Possibilities..........................................................................................38 2.6 Bacterial Species................................................................................................52 Chapter 3 Materials & Methods ...........................................................................57 3.1 Fieldwork ...........................................................................................................57 3.2 Culturing Bacteria..............................................................................................63 3.3 Trace Elements...................................................................................................69 3.4 Analytical Techniques .......................................................................................70 Chapter 4 Characterisation of Natural Scorodite ...............................................74 4.1 X-Ray Diffraction ..............................................................................................74 4.2 Scanning Electron Microscopy..........................................................................79 4.3 Fourier Transform Infra-Red Spectroscopy.......................................................89 4.4 Bacterial Enrichment .........................................................................................93 5 Contents 4.5 Trace Elements...................................................................................................94 4.6 Discussion..........................................................................................................97 Chapter 5 Liquid Media Precipitates .................................................................107 5.1 Liquid Media Nucleation .................................................................................107 5.2 Acidithiobacillus spp........................................................................................108 5.3 Leptospirillum spp. ..........................................................................................116 5.4 Thiomonas spp. ................................................................................................127 5.5 Discussion........................................................................................................130 Chapter 6 Solid Media Precipitates ....................................................................138 6.1 Solid Media Nucleation ...................................................................................138 6.2 Acidithiobacillus spp........................................................................................139 6.3 Sulfobacillus spp. .............................................................................................147 6.4 Thiomonas spp. ................................................................................................151 6.5 Discussion........................................................................................................157 Chapter 7 Conclusions & Further Work ...........................................................163 7.1 Conclusions......................................................................................................163 7.2 Further Work....................................................................................................166 References ................................................................................................................168 Appendices Appendix A – Natural Scorodite Occurrences.......................................................179 Appendix B – Bacterial Culturing Media ..............................................................182 Appendix C – Sample Decomposition...................................................................191 6 List of Figures Chapter 2 Figure 2.1 Cycle of arsenic inter-conversions in nature...........................................18 Figure 2.2 Predicted solubility of arsenopyrite as a function of Eh and pH at 25°C and 1 atm pressure. .................................................................................20 Figure 2.3 Eh-pH diagram for the system As-Fe-S-H2O including the stability field of scorodite..............................................................................................21 Figure 2.4 Activity versus pH diagram for As and Fe .............................................22 Figure 2.5 Arsenic mineral succession.....................................................................29 Figure 2.6 SEM image of scorodite replacing arsenopyrite.....................................30 Figure 2.7 Backscattered electron image of an arsenopyrite mineral grain with scorodite rim and core surrounded by silicate minerals..........................30 Figure 2.8 Effect of