The biogeochemistry of iodine

173 Pages·2016·7.33 MB·English

The biogeochemistry of iodine

A thesis submitted to the University of Manchester for the degree of
Doctor in Philosophy in the Faculty of Engineering and Physical Sciences

2016

Fabiola Guido-Garcia

School of Earth, Atmospheric and Environmental Sciences
Contents
Abstract 9
Declaration 10
Copyright statement 11
Acknowledgements 12
The Author 13

Chapter 1 Project context and thesis structure
1.1 Context 14
1.2 Research Hypothesis and Aims 15
1.3 Thesis structure and contributions 16
References 17

Chapter 2 Radioactive iodine in the environment and iodine
biogeochemistry
2.1 Nuclear power and its environmental challenges 19
2.1.1 Overview 19
2.1.2 Nuclear Power in Britain 20
2.1.3 The nuclear fuel cycle and radioiodine 21
2.1.4 Radioactive waste in the UK 24
2.1.5 Disposal of radioactive waste in the UK 25
2.1.6 Release of radionuclides to the environment 31
2.1.6.1 Accidental releases 31
2.1.6.2 Controlled releases 32
2.1.6.3 Consequences: contaminated land and water 33
2.2 Biogeochemistry 39
2.2.1 Introduction to biogeochemistry 39
2.2.2 Respiration of microorganisms in the subsurface 39
2.2.3 Nitrate reduction 41
2.2.4 Manganese reduction 42
2.2.5 Iron reduction 42
2.2.6 Sulfate reduction and methanogenesis 43
2.3 Environmental chemistry and iodine behaviour 44
2.3.1 Iodine environmental chemistry 44
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2.3.2 Speciation 45
2.3.3 Interaction of iodine with soil and sediment 47
2.3.3.1 Sorption reactions 47
2.3.3.2 Distribution coefficient 48
2.3.3.3 Interaction of iodine with Fe and Mn oxides 49
2.3.3.4 Interaction of iodine with other minerals 51
2.3.4 The biogeochemistry of iodine 52
2.3.4.1 Reduction of iodine species 54
2.3.4.2 Oxidation of iodine species 58
2.3.4.3 Case studies 61
2.4 Summary 63
References 64

Chapter 3 Bioreduction of iodate in sediment microcosms
3.1 Abstract 80
3.2 Introduction 81
3.3 Methods 85
3.4 Results and discussion 87
3.5 Conclusions 90
3.6 Acknowledgements 91
3.7 References 91

Chapter 4 Insights into bio-cycling of iodine in sediment
microcosms
4.1 Abstract 98
4.2 Introduction 99
4.3 Methods 104
4.3.1 Overview 104
4.3.2 Sample collection 105
4.3.3 Manganese and iron enriched bioreduction experiments 105
4.3.4 Abiotic reduction of iodate in reduced sediment 106
microcosms
4.3.5 Release of iodine from natural sediments 107
4.3.6 Reoxidation experiments 107
4.4 Results and Discussion 108
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4.4.1 Manganese and iron poised bioreduction experiments 108
4.4.2 Abiotic reduction 110
4.4.3 Sulfate enriched systems 112
4.4.4 Reoxidation experiments 112
4.5 Conclusions 114
Acknowledgments 115
References 115
Supplementary information 128

Chapter 5 Biological controls on iodine speciation and fate in
Fe-oxyhydroxide mineral systems
5.1 Abstract 130
5.2 Introduction 131
5.3 Experimental 134
5.3.0 Overview 134
5.3.1 Direct bacterial reduction of iodate 136
5.3.2 Amorphous Fe (III) oxide and iodate reduction by 136
Geobacter sulfurreducens
5.3.3 Iodate abiotic reduction in sterile magnetite systems 136
5.3.4 Reoxidation experiments 137
5.4 Results and discussion 138
5.4.1 Direct bacterial reduction of iodate 138
5.4.2 Fe(III) oxide and iodate reduction by Geobacter 138
sulfurreducens
5.4.3 Abiotic reduction of iodate by sterile magnetite 140
5.5 Conclusions 141
Acknowledgements 142
References 142

Chapter 6 Conclusions and recommendations for future work
6.1 Summary and conclusions 151
6.2 Future work 155
References 156

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Appendices
Appendix A Research methods 159
A1 Batch experiments and microcosms 159
A1.1 Sediment microcosms 159
A1.2 Mineral enriched microcosms 160
A1.3 Microbiological experiments 161
A1.4 Sorption experiments 163
A2 Aqueous analyses 163
A2.1 Solution pH and Eh 163
A2.2 ICP-MS 164
A2.3 HPLC-ICP-MS 164
A2.4 Ion chromatography 165
A2.5 Nitrite in solution 165
A2.6 Manganese in solution 166
A2.7 Iron in solution 166
A3 Solid analysis 167
A3.1 Iron speciation in sediments 167
A3.2 Iodine in sediments 167
A3.3 Powder X-ray diffraction 168
A3.4 X-ray fluorescence spectrometry 169
A4 Mineral synthesis 169
A4.1 Ferrihydrite synthesis 169
A4.2 Manganese oxide (δ-MnO2) synthesis 170
References 171
Appendix B Other experience 173

List of Figures
Figure 2.1 Nuclear fuel cycle. 22
Figure 2.2 Geological disposal concept model. 28
Figure 2.3 Engineered barriers concept. 28
Figure 2.4 Relative activity and decay of relevant isotopes in high 29
level radioactive waste.
Figure 2.5 Liquid discharged of 129I from spent nuclear fuel 33
reprocessing plants.
Figure 2.6 Map of 129I contaminated groundwater plumes in 34
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Savannah River Site.
Figure 2.7 Identified surface aquifer plumes of different 36
radionuclides in the Hanford Site.
Figure 2.8 129I in Sellafield groundwater 2014. 38
Figure 2.9 Theoretical biogeochemical zones scheme according to 41
depth.
Figure 2.10 Pourbaix diagram of iodine speciation in water under 46
standard conditions.
Figure 2.11 Biogeochemical cycling of iodine. 53
Figure 2.12 Expected oxidation states of actinides as a function of 55
Eh at different stages of the bioreduction cascade.
Figure 2.13 Transformations of iodine promoted by bacteria. 61
Figure 3.1. Sediment microcosm incubation time-series data. 96
Figure 3.2. Iodate reduction time-series data. 97
Figure 4.1 Manganese enriched sediment microcosm incubation 123
time-series.
Figure 4.2 Abiotic reduction time series data from Mn(IV)- 124
reduction; Fe(III)- reduction and SO 2- -reduction.
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Figure 4.3 Time series data from the experiments exploring release 125
of iodine from natural sediments during Fe(III)- and SO 2- -
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reduction.
Figure 4.4 Sediment microcosm undergoing re-oxidation with air. 126
Figure 4.5. Sediment microcosm undergoing re-oxidation with 127
nitrate.
Figure 5.1. Direct iodate reduction by Geobacter sulfurreducens 147
incubation time-series.
Figure 5.2. Fe(III) oxide and iodate reduction by Geobacter 148
sulfurreducens incubation time-series.
Figure 5.3. Reoxidation of biogenic magnetite systems incubation 149
time-series.
Figure 5.4. Abiotic reduction of iodate by sterile magnetite 149
incubation time-series.
Figure 5.5. Reoxidation of sterile magnetite systems time-series. 150

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List of Equations

Equation 2.1 Reaction of reduction of nitrite to nitrous oxide using 42
acetate as electron donor.
Equation 2.2 Distribution coefficient. 49

List of Tables

Table 1.1 Author contributions to Chapter 3. 16
Table 1.2 Author contributions to Chapter 4. 16
Table 1.3 Author contributions to Chapter 5. 17
Table 2.1 Fission yield of iodine. 24
Table 2.2 Radioactive waste categories, definitions and 25
composition.
Table 2.3 Standard Gibbs free energy (∆G ̊) per mole for the major 40
respiratory pathways, with acetate as electron donor and at
sediment concentrations.
Table 2.4 Sorption of iodine species onto Fe oxides. 51
Table 2.5 Sorption of iodine species onto minerals. 51
Table 4.1 Summary of sediment microcosm experiments. 104
Table 4.2 Groundwater composition. 106
Table 4.3 Sediment microcosm characteristics during abiotic 111
reduction under Mn(IV), Fe(III), and sulfate reduction with
corresponding results of speciation of iodine in solution.
Table A.1 Summary of sediment microcosm experiments. 160

List of abbreviations
CoRWM Committee on Radioactive Waste Management
BERR Department for Business, Enterprise and Regulatory Reform
BIOS Bacteriogenic iron oxides
DECC Department of Energy and Climate Change
DEFRA Department for Environment, Food and Rural Affairs
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DTI Department of Trade and Industry
EPA United States Environmental Protection Agency
GDF Geological Disposal Facilities
HAW Higher Activity Wastes
HLW High Level Wastes
HOI Hypoiodous acid
HPLC-ICP-M S High Performance Liquid Chromatography ICP-MS
IAEA International Atomic Energy Agency

IC Ion Chromatography

ICP-MS Inducted Coupled Plasma Mass Spectrometry

ILW Intermediate Level Wastes

LLW Low Level Wastes

I- Iodide
I Elemental iodine
2
IO - Iodate
3
K Distribution coefficient
d
MRWS Managing Radioactive Waste Safely White Paper

NDA Nuclear Decommissioning Authority

NIREX Nuclear Industry Radioactive Waste Executive

OD Optical Density at 600 nm
600
TBP/OK Tri-n-butyl phosphate in odourless kerosene

TEA Terminal Electron Acceptors

TEPC Tokyo Electric Power Company

TMAH Tetramethylammonium hydroxide
United Nations Scientific Committee on the Effects of Atomic
UNSCEAR
Radiation
VLLW Very Low Level Wastes

WHO World Health Organization

WNA World Nuclear Association

XRD X-Ray Diffraction
XRF X-Ray Fluorescence

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Abstract
The University of Manchester
Fabiola Guido-Garcia
Doctor of Philosophy
The Biogeochemistry of Iodine
March 2016
Iodine-129 is a high-yield fission product of 235U and 239Pu; is produced in nuclear
power plants and is therefore present in substantial quantities in radioactive wastes.
In the environment, iodine exists as a range of species: iodate (IO -), iodide (I-),
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elemental iodine (I ), HOI and organic species are the most common. The behaviour
2
of iodine in the environment is linked to its speciation which can be affected by
different factors such as pH, redox potential and enzymatic reduction. Previous
research has shown that iodine speciation can determine its fate in the environment;
however the mechanisms of redox cycling amongst the different species are not yet
fully understood. This research project has focused on improving the understanding
of the changes on speciation of iodine in sediment and mineral systems undergoing
redox cycling reactions.
The fate and changes in iodine speciation were studied under reducing and oxidising
conditions, with all experiments conducted under circumneutral pH conditions.
Overall the results showed that when microbial activity is promoted in a sediment
system, iodate is reduced to iodide with the reduction occurring during manganese
reduction. Further, when nitrate is present at high concentrations the reduction of
iodate is retarded. A net release of native iodine from sediments was observed in all
experiments conducted with sediments; confirming previous observations that
sediment bound iodine is released from sediments under reducing conditions. Modest
abiotic reduction of iodate was observed under manganese and iron reduction; and
iodate reduction happened faster in a mixed system with iron and pure culture
bacteria than solely by the pure culture alone or via abiotic reduction with Fe(II).
When reduced experiments were exposed to air, concentrations of iodide decreased
with no iodate ingrowth or losses in total iodine in solution. This suggested that
iodide had been oxidised to intermediate species (I , HOI) that were not detected,
2
although this reaction has been described in past research. Finally, when no
microbial activity is promoted, iodine remains as iodate which showed modest
sorption onto sediment systems and Fe(III) oxide.
Overall, these results highlight the important role that bacteria play in the reduction
of iodate. This research also confirms that iodine speciation impacts on the fate of 129I
throughout the environment, where it may be less mobile in an oxic environment
than under reducing conditions. Moreover, some techniques of bioremediation (e.g.
promoting metal reducing conditions) may cause the release of radioactive iodine to
solution.
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Declaration
No portion of the work referred to in the thesis has been submitted in support of an
application for another degree or qualification of this or any other university or
institute of learning.

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