chapter 9 arsenic species determination

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Durham E-Theses
Arsenic in the environment
Jones, Iwan E. LI.
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ARSENIC IN THE ENVIRONMENT
by
Iwan E. LI. Jones
A dissertation submitted to the University of Durham in fulfilment of the
requirement for the degree of MSc by Research
The copyright of this thesis rests
with the author. No quotation
from it should be published
without the written consent of the
author and information derived
from it should be acknowledged.
September 1997
m
5 MAR
ACKNOWLEDGEMENTS
In the writing of this work, I must first thank Dr Stephen Thomas for the initial
opportunity to study for a PhD. Since then, he has been a continued help in the search
for funding in various shapes and forms, and although this work is not in his field of
expertise, he has maintained a continued interest in its progress, and occasionally
provided me with an opportunity to work back in the 'real world'. I must also thank my
'secondary' supervisor, Dr John Wilson for help with the many things bureaucratic that
universities seem to love so much, his patience and humour with such things causes
nothing much short of complete admiration.
Thanks must also go to a number of researchers outside Durham University, who
kindly provided me with reports and reprints of their research papers. These include
Prof. Bill Davison and Dr Hao Zhang of Lancaster University; Dr Andrew Hursthouse
of the University of Paisley and Michael Riley of S. S. Papadopulos & Associates, Inc.
of Boulder, Colorado.
I must sincerely thank my parents for their continued support over the course of
the past three years, the good humour of my sisters, Angharad and Catrin and their
families, and also the patience exhibited by my somewhat long suffering girlfriend,
Rachel.
My friends at Durham Amateur Rowing Club have provided me with an excuse
not to go completely mad, although perhaps they might not be so sure. My thanks also
go to a number of friends who have helped me with comments of pure cynicism when I
was in a mood to throttle someone! Such dubious characters are too numerous to name
individually, but include Andrew Adams, Tony Leopold, Fran Brearton, Lynne Beaton,
Emily Williams, John McGroary, Ros Martin, Paula Russell, Richard Mortimer, GwiUym
and Pamela Roberts, y teulu Bodlew, Adonis Giolas, Doug Langton, Jeremy Lloyd, Pete
and Aila Bursnall, Dave-the-Leg, Aids Stevens; Dave Barham, Mark Briggs, and of
course Rod and Anna White.
ABSTRACT
Arsenic, long synonymous in people's mind with poison exhibits a varied, fascinating
and dynamic biogeochemistry. Chemically and biologically reactive, its chemical form, or
speciation, changes with slight variations in chemical or biological conditions. Depending upon
the extent to which any arsenic containing system is dominated by physical/chemical or
biological process, the forms of arsenic may change between the various inorganic and
methylated species, and may alter rapidly with varying conditions.
Early research revolved around the formulation of pigments, and later in the
development of effective medicines. Later stiU, thanks due to its long history as a poison, arsenic
was included in numerous agricultural practices, mainly as a herbicide or pesticide. It has also
seen service in the rather more specialised field of chemical warfare, and still poses threats as a
result of improper disposal.
Much of the recent research has focused on the identification of previously unknown
organoarsenic species foimd in estuarine and marine waters. This work is building up an
understanding of the biological pathways involved in the biochemical cycling of arsenic. Littie
work has been carried out with respect to the cycling of arsenic in freshwaters in comparison to
that in marine and estuarine waters. Similarly, there has been less work performed on the
speciation of arsenic in freshwater sediment interstitial waters, than there has on marine
sediments, or intertidal sediments.
The characterisation of arsenic in dynamic porewater poses a set of unusual and difficult
problems, not the least being the procurement of representative, discrete samples. A number of
potential sampling methods are reviewed, and variations on the thin film gel sampling technique
are thought to provide perhaps the best option, although this will depend upon the type of
intertidal sediment being investigated, and the information sought.
It may be impossible to propose a general model of arsenic cycling either at a local scale
or at a global level. This is of course due to the great diversity in ecosystems, each having
different controls over arsenic speciation, and containing different biological commimities. Once
a given system has been described, the patterns of arsenic speciation (both spatially and
temporally) are explainable, and potential impacts can be identified,
but they carmot be transferred to another system. The continuing accumulation of information
regarding arsenic speciation in different systems is helping in the unravelling of the larger global
arsenic cycle. Such an understanding can only be a benefit in the development of safe and
efficient remediation schemes for contaminated soil and aquatic systems.
m
CONTENTS
Page Number
Section
Title 1
Acknowledgements ii
Abstract iii
Contents iv
List of tables X
xiii
List of figures
Chapter 1 Introduction
Chapter 2 Occurrence of Arsenic
6
2.1 Introduction
7
2.2 Rocks
10
2.2.1 Rock arsenic speciation
10
2.3 Soils
12
2.3.1 Soil arsenic speciation
14
2.4 Natural waters
14
2.4.1 Groundwater and geothermal waters
16
2.4.2 Seawater
16
2.4.2.1 Photic zone
19
2.4.2.2 Deeper waters
20
2.4.3 Estuarine waters and sediments
22
2.4.4 River and lake waters and sediments
24
2.5 Air masses and precipitation
2.5.1 Atmospheric arsenic speciation 26
28
2.6 Living things
30
2.6.1 Accumulation
31
2.6.2 Marine organisms
32
2.6.3 Freshwater organisms
33
2.6.4 Marine plants
34
2.6.5 Freshwater plants
35
2.6.6 Terrestrial organisms
38
2.6.7 Terrestrial plants
41
2.7 Anthropogenic inputs
41
2.7.1 Mining, smelting and power generation
43
2.7.2 Soil additions
45
2.7.3 Pollution and disposal
45
2.7.3.1 Out-of-sight-out-of-mind
2.7.3.2 Political reasoning 46
48
2.7.3.3 Industrial incidents
48
2.7.3.4 Non-point source
IV
Chapter 3 Uses, Production and Global Mass Balance
50
3.1 Introduction
50
3.2 Uses of Arsenic
50
3.2.1 Alloys
50
3.2.2 Medicines
51
3.2.3 Pigments and poisonings
54
3.2.4 Glass manufacture
54
3.2.5 Agriculture
56
3.2.6 Wood preservatives
56
3.2.7 Electronics
57
3.2.8 Warfare
57
3.2.9 Embalming
58
3.2.10 Mineral prospecting
58
3.3 Arsenic production
61
3.3.1 Arsenic substitutes
62
3.3.2 Resources
62
3.4 Global arsenic mass balance
Chapter 4 Redox Reactions and Solubility
70
4.1 Introduction
71
4.2 pH
72
4.3 Eh
75
4.4 Eh and pH in natural systems
76
4.4.1 pH-Eh or pH-pE diagrams
78
4.4.2 Drawbacks with pH-Eh or pH-pE diagrams
79
4.5 Interactions with solids (oxidation/reduction processes)
81
4.6 Arsenic as a redox indicator
82
4.7 Arsenic redox rates
84
4.8 Removal from solution/solid phase and transport
85
4.8.1 Inorganic arsenic adsorption
88
4.8.2 Organoarsenic adsorption
89
4.8.3 Adsorption isotherms
4.8.4 Sorption (coprecipitation and adsorption) 91
4.8.4.1 Iron 93
4.8.4.2 Manganese 95
4.8.4.3 Aluminium and clay 96
4.8.4.4 Sulphide 97
4.8.4.5 Organic matter 99
100
4.9 Arsenic solubility
4.10 Arsenic mobility in the environment 101
4.10.1 Atmospheric arsenic redox reactions 102
4.10.2 Sand columns, muds and sediments 103
106
4.10.3 Mine spoil
108
4.10.4 Stratified lakes
110
4.10.4.1 Seasonality
4.10.4.2 Porewaters 111
111
4.10.5 Sediment-water interface
Chapter 5 Biochemistry and Ecotoxicology
115
5.1 Introduction
120
5.2 Methylation
122
5.3 Formation of methylarsenicals
123
5.3.1 Methyl transfer from SAM
125
5.3.2 Vitamin B^ dependent methyl transfer
128
5.4 ExoceUular or abiotic methylation
129
5.5 Demethylation
130
5.6 Biologically mediated redox rates
132
5.7 Seasonality
133
5.8 Complex organoarsenicals
134
5.9 Ecotoxicology
136
5.9.1 Essentiality
137
5.9.2 Arsenic toxicity
139
5.9.3 Routes of exposure
141
5.9.4 Rate of excretion
142
5.9.5 Method of action
144
5.9.6 Effects of arsenic intoxication
147
5.9.7 Phytotoxicity
148
5.9.8 Carginogenicity?
150
5.9.9 Arsenic health risk assessment
Chapter 6 Sample Collection
154
6.1 Introduction
155
6.2 Porewater sampling methods
156
6.2.1 Centrifugation
157
6.2.2 Squeezing
160
6.2.3 Vacuum filtration
161
6.2.4 Dialysis
6.2.5 Thin film gels 164
164
6.2.5.1 Diffusive equihbrium
166
6.2.5.2 Diffusive gradient
6.2.6 Proposed porewater sampling device 168
6.2.6.1 Sampler materials 178
179
6.3 SoU and water sampling
179
6.3.1 Soil sampling
179
6.3.2 Surface water sampling
180
6.3.3 Groundwater sampling
Chapter 7 Sample Preservation
182
7.1 Introduction
183
7.2 Filtration and filter membrane characteristics
187
7.3 Sediments and soils
188
7.4 Fresh waters
189
7.5 Sea and estuarine waters
191
7.6 Groundwater
191
7.7 Sediment and soil porewaters
193
7.8 Sample contamination and other interferences
VI
7.8.1 Sample container materials 194
7.8.2 Labware cleaning procedures 197
7.8.3 Preparation of standards 199
7.8.3.1 Arsenite [As(in)] 199
7.8.3.2 Arsenate [As(V)] 200
7.8.3.3 Monomethylarsonic (MMAA)
and dimethylarsinic (DMAA) acids 200
7.8.3.4 Preservation of standards 201
7.8.4 Quality assurance and quality control 202
7.9 Summary 203
Chapter 8 Arsenic Species Separation
8.1 Introduction 220055
8.2 Total arsenic 205
8.2.1 Wet ashing 206
8.2.2 Dry ashing 207
8.2.3 Fusion 207
8.2.4 Oxygen combustion 207
8.3 Arsenic speciation 208
8.4 Separation systems 209
8.4.1 Fractionation based on selective sizmg 211
8.4.1.1 Sieving 211
8.4.1.2 Centrifugation 211
8.4.1.3 Ultrafiltration 211
8.4.1.4 Dialysis 212
8.4.1.5 Gel permeation chromatography 212
8.4.1.6 Field flow fractionation 213
8.4.2 Differentiation based on charge and size effects 214
8.4.2.1 Electrophoresis 214
8.4.2.2 Ion exchange columns 215
8.4.2.3 Chelating resins 220
8.4.2.4 Adsorption columns 220
8.4.2.5 Liquid-liquid extraction 221
8.4.3 Chromatographic methods of separation 224
8.4.3.1 Open column chromatography 224
8.4.3.2 High performance
liquid chromatography (HPLC) 224
8.4.3.3 Ion chromatography 227
8.4.3.4 Gas chromatography (GC) 228
8.4.3.5 Supercritical fluid chromatography 230
8.4.3.6 Planar chromatography 231
8.4.4 Selective chemical extraction 231
8.4.5 Hydride generation techniques 236
8.4.5.1 pH dependency and reduction 237
8.4.5.2 Other arseno-hydrides 238
8.4.5.3 Interferences 238
8.4.5.4 Carrier gas 240
8.4.5.5 Sodium borohydride 241
8.4.5.6 Acid concentrations 241
vu
8.4.5.7 Molecular rearrangement 243
8.4.5.8 Generation methods 243
8.4.5.9 Arsine collection methods 244
Chapter 9 Arsenic Species Determination
247
9.1 Introduction
249
9.2 Atomic spectrometry
249
9.2.1 Atomic emission spectrometry (AES)
252
9.2.2 Atomic absorption spectrometry (AAS)
255
9.3 Electrothermal atomic absorption spectrometry (ETAAS)
259
9.4 Colorimetry or molecular absorption spectophotometry
259
9.4.1 Silver-diethyldithiocarbamate (SDDC)
260
9.4.2 Ammonium molybdate
261
9.4.3 Other complexes
261
9.5 Electron capture and flame ionisation detection
261
9.5.1 Electron capture detection
262
9.5.2 Hame ionisation detection
262
9.6 Electro-analytical speciation techniques
263
9.6.1 Ion-selective electrodes
264
9.6.2 Polarography
266
9.6.3 Stripping voltammetry studies
9.6.3.1 Anodic stripping
voltammetry (ASV) 266
9.6.3.2 Potentiometric stripping
voltammetry 268
9.6.3.3 Cathodic stripping
voltammetry 268
9.6.4 Amperometric titrations and
268
electro-chemical detectors
269
9.7 Microwave-emission spectrometry
9.8 X-ray fluorescence and atomic
270
fluorescence spectrometry (XRF)
271
9.9 Mass spectrometry
271
9.9.1 Multiple ion detection (MID)
9.9.2 Inductively coupled-plasma
272
mass spectrometry (ICP-MS)
274
9.9.3 Molecular mass spectrometry (MMS)
275
9.10 Neutron activation analysis
276
9.11 Proton-induced X-ray emission
276
9.12 DC discharge spectral emission
277
9.13 Kinetic methods
279
Chapter 10 Discussion
289
References
vm
APPENDICES
Appendix A Properties of arsenic
Appendix B Arsenic compounds
Appendix C UK and EC legislation
Appendix D Arsenic Eh/pH and pH/pE diagrams
IX