Thallium (EHC 182, 1996)

INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY

ENVIRONMENTAL HEALTH CRITERIA 182

THALLIUM

This report contains the collective views of an international group of
experts and does not necessarily represent the decisions or the stated
policy of the United Nations Environment Programme, the International
Labour Organisation, or the World Health Organization.

First draft prepared by Professor G. Schaub, Institute of Zoology and
Parasitology, Ruhr University, Bochum, Germany

Published under the joint sponsorship of the United Nations
Environment Programme, the International Labour Organisation, and the
World Health Organization

World Health Organization
Geneva, 1996

The International Programme on Chemical Safety (IPCS) is a joint
venture of the United Nations Environment Programme, the International
Labour Organisation, and the World Health Organization. The main
objective of the IPCS is to carry out and disseminate evaluations of
the effects of chemicals on human health and the quality of the
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that could produce internationally comparable results, and the
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carried out by the IPCS include the development of know-how for coping
with chemical accidents, coordination of laboratory testing and
epidemiological studies, and promotion of research on the mechanisms
of the biological action of chemicals.

WHO Library Cataloguing in Publication Data

Thallium

(Environmental health criteria ; 182)

1.Thallium - toxicity I.Series

ISBN 92 4 157182 9 (NLM Classification: QV 618)
ISSN 0250-863X

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CONTENTS

ENVIRONMENTAL HEALTH CRITERIA FOR THALLIUM

PREAMBLE

1. SUMMARY

1.1. Identity, physical and chemical properties, and
analytical methods
1.2. Sources of human and environmental exposure
1.3. Environmental transport, distribution and transformation
1.4. Environmental levels and human exposure
1.5. Kinetics and metabolism in laboratory animals and humans
1.6. Effects on laboratory mammals and in vitro test systems
1.7. Effects on humans
1.8. Human dose-response relationship
1.9. Effects on other organisms in the laboratory and field

2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, AND ANALYTICAL
METHODS

2.1. Identity
2.2. Physical and chemical properties
2.3. Conversion factor
2.4. Analytical methods
2.4.1. Sampling and sample preparation
2.4.2. Methods of determination
2.4.2.1 Atomic absorption spectrometry
2.4.2.2 Inductively coupled plasma - mass
spectrometry
2.4.2.3 Other methods
2.4.3. Quality control and quality assurance
2.4.4. Conclusions

3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE

3.1. Natural occurrence
3.2. Anthropogenic sources
3.2.1. Production levels and processes
3.2.2. Uses
3.2.3. Emissions from industrial sources
3.2.3.1 Metal production industries
3.2.3.2 Power-generating plants
3.2.3.3 Brickworks and cement plants
3.2.3.4 Sulfuric acid plants

4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION AND TRANSFORMATION

4.1. Transport and distribution between media
4.1.1. Transport and distribution in air, water
and soil
4.1.2. Soil-vegetation transfer
4.1.2.1 Factors affecting soil-vegetation
transfer
4.1.2.2 Absorption by plants
4.1.2.3 Distribution in plants
4.2. Biotransformation
4.3. Interaction with other physical, chemical, or biological
factors

5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE

5.1. Environmental levels
5.1.1. Air
5.1.2. Water
5.1.2.1 Areas not contaminated by thallium
5.1.2.2 Areas contaminated by thallium from
industrial sources
5.1.3. Rocks, soil and sediment
5.1.3.1 Areas not contaminated by thallium
5.1.3.2 Areas contaminated by thallium from
industrial sources
5.1.4. Plants and animals
5.1.4.1 Plants
5.1.4.2 Animals
5.2. General population exposure
5.3. Occupational exposure during manufacture, formulation
or use

6. KINETICS AND METABOLISM

6.1. Absorption
6.1.1. Animals
6.1.1.1 Aquatic animals
6.1.1.2 Terrestrial animals
6.1.2. Humans
6.2. Distribution
6.2.1. Animals
6.2.1.1 Distribution after administration of
a single dose
6.2.1.2 Distribution after long-term sublethal
administration
6.2.1.3 Transplacental transfer of thallium

6.2.2. Humans
6.2.2.1 Increased concentrations after lethal
poisoning
6.2.2.2 Increased concentrations after
long-term sublethal poisoning
6.2.2.3 Transplacental transfer of thallium
6.3. Metabolic transformation
6.4. Elimination and excretion
6.4.1. Animals
6.4.2. Humans
6.4.3. Methods to estimate daily intake of thallium
6.5. Retention and turnover (Biological half-life)
6.5.1. Animals
6.5.2. Humans
6.6. Kinetics at the cellular level

7. EFFECTS ON LABORATORY MAMMALS AND IN VITRO TEST SYSTEMS

7.1. Single exposure
7.1.1. Toxicity and symptoms
7.1.2. Effects on various organs
7.2. Short-term exposure
7.2.1. Toxicity and symptoms
7.2.2. Effects on various organs
7.3. Long-term exposure: chronic toxicity
7.3.1. Toxicity and symptoms
7.3.2. Effects on various organs
7.4. Skin and eye irritation
7.4.1. Skin and hair
7.4.2. Eye
7.5. Reproductive toxicity, embryotoxicity and teratogenicity
7.5.1. Gonadotoxic effects
7.5.2. Embryotoxicity and teratogenicity
7.5.2.1 Chickens
7.5.2.2 Mammals
7.5.2.3 Delayed effects on development of
offspring
7.6. Mutagenicity and related end-points
7.7. Carcinogenicity
7.8. Neurotoxicity
7.8.1. Central nervous system
7.8.1.1 Histology and ultrastructure
7.8.1.2 Electrophysiological and biochemical
investigations
7.8.1.3 Behavioural toxicology
7.8.2. Peripheral nervous system
7.8.2.1 Histology and ultrastructure
7.8.2.2 Electrophysiological and biochemical
investigations

7.9. In vitro test systems: cell lines
7.10. Factors modifying toxicity
7.10.1. Enhancement of elimination
7.10.2. Selenium
7.11. Mechanisms of toxicity - mode of action

8. EFFECTS ON HUMANS

8.1. General population exposure
8.1.1. Acute toxicity
8.1.2. Effects of long-term exposure: chronic
toxicity
8.2. Occupational exposure
8.3. Subpopulations at special risk
8.4. Target organs in intoxicated humans: pathomorphology
and pathophysiology
8.4.1. Gastrointestinal tract and renal system
8.4.2. Cardiovascular system
8.4.3. Skin and hair
8.4.4. Nervous system
8.4.4.1 Central nervous system
8.4.4.2 Peripheral nervous system
8.4.5. Other organs
8.5. Special effects
8.5.1. Reproduction and developmental effects
8.5.2. Carcinogenicity
8.5.3. Immunotoxicological effects
8.6. Factors modifying toxicity: enhancement of
elimination
8.7. Protective measures against excessive occupational
exposure

9. EFFECTS ON OTHER ORGANISMS IN THE LABORATORY AND FIELD

9.1. Microorganisms
9.2. Aquatic organisms
9.2.1. Plants
9.2.2. Animals
9.3. Terrestrial organisms
9.3.1. Plants
9.3.1.1 Plant photosynthesis
9.3.1.2 Cytotoxic effects
9.3.1.3 Growth of plants
9.3.1.4 Different sensitivities to thallium(I)
and thallium (III)
9.3.1.5 Concentration of trace elements
9.3.1.6 Sensitivity of plants
9.3.2. Wild animals
9.3.3. Household pets and farm animals

10. EVALUATION

10.1. Evaluation of human health risks
10.1.1. Exposure levels
10.1.2. Kinetics
10.1.3. Toxic effects
10.1.4. Dose-response relationship (animals)
10.1.5. Dose-response relationship (humans)
10.2. Evaluation of the effects of thallium on the
environment

11. CONCLUSIONS AND RECOMMENDATIONS

12. FURTHER RESEARCH

REFERENCES

RESUME

RESUMEN

NOTE TO READERS OF THE CRITERIA MONOGRAPHS

Every effort has been made to present information in the criteria
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A detailed data profile and a legal file can be obtained from the
International Register of Potentially Toxic Chemicals, Case postale
356, 1219 Châtelaine, Geneva, Switzerland (Telephone No. 9799111).

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This publication was made possible by grant number 5 U01 ES02617-15
from the National Institute of Environmental Health Sciences, National
Institutes of Health, USA, and by financial support from the European
Commission.

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* Kinetics and metabolism in laboratory animals and humans
* Effects on laboratory mammals and in vitro test systems
* Effects on humans
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WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR THALLIUM

Members

Professor M. Balali-Mood, Poison Control Centre, Imam Reza Hospital,
Mashhad University of Medical Sciences, Mashhad, Islamic Republic
of Iran

Dr P. Doyle, Chemicals Evaluation Division, Environment Canada,
Ottawa, Ontario, Canada

Professor G. Kazantzis, Imperial College of Science, Technology and
Medicine, Centre for Environmental Technology, Royal School of
Mines, London, United Kingdom (Joint Rapporteur)

Dr M. Kiilunen, Department of Industrial Hygiene & Toxicology,
Institute of Occupational Health, Helsinki, Finland

Mr H. Malcolm, Institute of Terrestrial Ecology, Monks Wood
Experimental Station, Huntingdon, Cambridgeshire, United Kingdom

Dr G. Nordberg, Department of Environmental Hygiene, Umea University,
Umea, Sweden (Chairman)

Professor G. Schaub, Department of Zoology, Institute for Zoology and
Parasitology, Ruhr University, Bochum, Germany (Joint Rapporteur)

Dr S. Velazquez, Environmental Criteria and Assessment Office, US
Environmental Protection Agency, Cincinnati, Ohio, USA

Representatives of other organizations

Dr P. Montuschi, Department of Pharmacology, Catholic University of
the Sacred Heart, Rome, Italy (representing the International Union
of Toxicology)

Observers

Dr R. Cornelis, Institute for Nuclear Sciences, State University of
Gent, Gent, Belgium

Secretariat

Dr P.G. Jenkins, International Programme on Chemical Safety, World
Health Organization, Geneva, Switzerland (Secretary)

ENVIRONMENTAL HEALTH CRITERIA FOR THALLIUM

A WHO Task Group on Environmental Health Criteria for Thallium
met in Geneva from 12 to 16 December 1994. Dr P.G. Jenkins, IPCS,
welcomed the participants on behalf of Dr M. Mercier, Director of the
IPCS, and the three IPCS cooperating organizations (UNEP/ILO/WHO).
The Group reviewed and revised the draft and made an evaluation of the
risks for human health and the environment from exposure to thallium.

The first draft was prepared by Professor G. Schaub, Institute
for Zoology and Parasitology, Ruhr University, Bochum, Germany. He
also prepared the second draft, incorporating comments received
following circulation of the first draft to the IPCS contact points
for Environmental Health Criteria monographs.

Dr P.G. Jenkins, IPCS, was responsible for both the overall
scientific content and the technical editing.

The efforts of all who helped in the preparation and finalization
of the monograph are gratefully acknowledged.

ABBREVIATIONS

AAS atomic absorption spectrometry
AES atomic emission spectrometry
AMP amperometric titration
CRMs certified reference materials
DPASV differential pulse anodic stripping voltametry
EDL electrode discharge lamp
EDTA ethylenediaminetetraacetic acid
GABA gamma-aminobutyric acid
GDMS glow discharge mass spectrometry
GFAAS graphite furnace atomic absorption spectrometry
GLP good laboratory practice
ICP inductively coupled plasma
IDMS isotope dilution mass spectrometry
LOEL lowest-observed-effect level
MED minimum effective dose
MIBK methyl isobutyl ketone
MS mass spectrometry
NAA neutron activation analysis
NaDDC sodium diethyldithiocarbamate
NADP nicotinamide adenine dinucleotide phosphate
NOEL no-observed-effect level
PAA photon activation analysis
TLV threshold limit value
tRNA transfer ribonucleic acid

1. SUMMARY

1.1 Identity, physical and chemical properties, and analytical
methods

Elemental thallium is a soft and malleable metal with a
bluish-white colour. When exposed to humid air or water, thallium is
oxidized rapidly on the surface or the hydroxide is formed,
respectively. Thallium has two important oxidation states,
thallium(I) and thallium(III). Monovalent (thallous) compounds behave
like alkali metals, e.g. potassium, whereas the trivalent (thallic)
compounds are less basic, resembling aluminium. In contrast to
inorganic compounds in which the thallium(I) ion is more stable in
aqueous solutions than the thallium(III) ion, the latter is more
stable in organic compounds.

The determination of thallium in environmental samples is
somewhat difficult as concentrations are in the µg/kg range or less.
Generally the limits of determination for minerals, soils and dusts
are about 20 µg/kg, for aqueous solutions about 0.1 µg/litre, and for
biological materials a few µg/kg, when no pre-concentration of
thallium is applied.

Graphite furnace atomic absorption spectrometry (GFAAS) is an
analytical method well suited for applications where high sensitivity
is required from small sample amounts with thallium present at
concentrations of a few µg/kg. Isotope dilution mass spectrometry
(IDMS) and inductively coupled plasma-mass spectrometry (ICP-MS),
possibly combined with isotope dilution, are excellent methods for
determinations offering good precision and accuracy at the µg/kg
level.

1.2 Sources of human and environmental exposure

Thallium is present in the environment as a result of natural
processes and from man-made sources. It is ubiquitous in nature and
occurs especially in sulfide ores of various heavy metals, but
normally in low concentrations. There are only a few areas with a
naturally very high thallium concentration.

Thallium is produced industrially only in small quantities (the
worldwide industrial consumption in 1991 was 10-15 tonnes/year).
Thallium and its compounds have a wide variety of industrial uses.
Its uses as a depilatory agent for humans and as a rodenticide and
insecticide are now severely restricted. The main uses are in the
electrical and electronic industries and in the production of special
glasses. Another important field of application is the use of
radioisotopes in medicine for scintigraphy and the diagnosis of
melanoma and the use of arylthallium(III) compounds in biochemistry.

Losses to the environment mainly occur from mineral smelters
(deposits of waste material and emissions into the atmosphere),
coal-burning power-generating plants, brickworks, and cement plants
(all emissions into the atmosphere). From 2000 to 5000 tonnes/year
are estimated to be mobilized world-wide by industrial processes.
Emissions of thallium from industrial processes vary widely according
to the type of industry.

Emissions from coal-fired power-generating plants can contain a
thallium concentration of 700 µg/m³ exhaust air and those from
cement plants up to 2500 µg/m³. The latter value may be reduced to
< 25 µg/m³ by using other raw materials and changing the production
process. Thallium volatilizes during the burning of coal or raw
material for cement production and recondenses on the surface of ash
particles in cooler parts of the system. These particles contain up to
50 mg thallium/kg fly-ash and are often of small size, so that only
50% of them are held back by filters in cement plants. Also, about
one third of emitted particles from power-generating plants are of the
small particle size which can be deposited in the lower respiratory
tract.

Effluent from mine tailing ponds containing up to 1620 and
36 µg/litre caused elevated levels of 88 and 1 µg/litre, respectively,
in connecting rivers. Rainwater ponds around a cement plant contained
up to 37 µg/litre. In soil maximal concentrations of 60 mg/kg have
been found near waste materials from mines; 2, 0.6 and 27 mg/kg have
been found in the vicinity of base metal smelters, brickworks and
cement plants, respectively.

In contaminated areas the majority of vegetables, fruits and meat
contain less than 1 mg thallium/kg fresh weight. Concentrations are
higher in cabbages (Brassicaceae), with up to 45 mg/kg reported in
green kale. Concentrations of thallium in the tissues of farm animals
correlate with concentrations in the fodder. In the vicinity of some
cement plants, increased concentrations in fodder (e.g., up to
1000 mg/kg in rape) and beef and rabbit meat (up to 1.5 and 5.8 mg/kg,
respectively) have been reported.

1.3 Environmental transport, distribution and transformation

Near point sources such as coal-fired power-generating stations,
some cement plants and metal smelting operations, the major source of
thallium in air is emission of fly ash. The results of one study
indicate that nearly all of the thallium in fly dust from a cement
plant was present as soluble thallium(I) chloride.

The fate of thallium added to soil (in deposited fly ash, for
example) depends largely on soil type. Retention will be greatest in
soils that contain large amounts of clay, organic matter and
iron/manganese oxides. Incorporation into stable complexes causes
enhanced thallium concentrations only in the upper levels of soils.
The uptake of thallium by vegetation increases as soil pH decreases.
In some strongly acid soils significant amounts of thallium can be
leached to local ground and surface water.

Most dissolved thallium in freshwater is expected to be in the
monovalent form. However, in strongly oxidized fresh water and most
seawater trivalent thallium may predominate. Thallium can be removed
from the water column and accumulate in sediment by various exchange,
complexation or precipitation reactions.

Although thallium can bioconcentrate, it is not likely to
biomagnify in aquatic or terrestrial food webs.

1.4 Environmental levels and human exposure

In areas not contaminated by thallium, concentrations in air are
usually < 1 ng/m³, those in water < 1 µg/litre, and those in water
sediments < 1 mg/kg. Mean concentrations in the earth's crust range
from 0.1 to 1.7 mg/kg, but very high concentrations are possible,
e.g., in coal up to 1000 mg/kg, and the rarely found minerals of
thallium consist of up to 60% of the element. Food of plant and
animal origin usually contains < 1 mg/kg dry weight and the human
average dietary intake of thallium appears to be less than 5 µg/day.
Uptake via the respiratory system is estimated to be < 0.005 µg
thallium/day.

There are only limited data about the actual thallium content of
workplace air. The most recent (1980s) concentrations of thallium
observed were < 22 µg thallium/m³ (in the production of a special
thallium alloy and in a thallium smelter). Average urinary
concentrations were determined to be in the range of 0.3-8 µg/litre
for cement workers and 0.3-10.5 µg/litre for foundry workers.

1.5 Kinetics and metabolism in laboratory animals and humans

Thallium is rapidly and well absorbed through the gastro
intestinal and respiratory tracts and is also taken up through the
skin. It is rapidly distributed to all organs and passes the placenta
(as indicated by the rapid fetal uptake) and the blood-brain barrier.
Because of its rapid accumulation in cells, concentrations of thallium
in whole blood do not reflect the levels in tissues. In acute
poisoning of experimental animals or humans, initially high
concentrations of thallium appear in the kidney, low concentrations in
fat tissue and brain, and intermediate concentrations in the other
organs; later the thallium concentration of the brain also increases.

Elimination of thallium may occur through the gastrointestinal
tract (mainly by mechanisms independent of biliary excretion), kidney,
hair, skin, sweat and breast milk. Intestinal reabsorption (mainly
from the colon) may occur with a consequent decrease in total body
clearance. In rats, the main routes of thallium elimination are
gastrointestinal (about two thirds) and renal (about one third), in
rabbits the contribution of the two routes is about equal. Thallium
is also secreted in saliva.

As with many other substances, the excretion of thallium in
humans differs from that in laboratory animals, since the rate of
excretion is generally much lower in humans (rate constant =
0.023-0.069 day-1) than in laboratory animals (average rate constant =
0.18 day-1). Another major difference between humans and animals is
the relative contribution of the different routes of excretion. In
humans, renal excretion seems to be much more important than in
animals, although its relative contribution to the total body
clearance has not been definitively established, due principally to
the lack of sufficient human data. Moreover, exposure levels,
duration of exposure, impairment of excretory organ function,
potassium intake and concomitant treatment of acute poisoning may
considerably influence the results.

In one study renal excretion of thallium was reported to be about
73%, whereas that through the gastrointestinal tract was about 3.7% of
the daily excreted amount. Excretion through hair and skin, and sweat
has been estimated to be 19.5% and 3.7%, respectively.

The biological half-life of thallium in laboratory animals
generally ranges from 3 to 8 days; in humans it is about 10 days but
values up to 30 days have been reported.

No data on the biotransformation of thallium are available.

1.6 Effects on laboratory mammals and in vitro test systems

There are no striking species-specific differences in the
toxicity of thallium(I) salts. Usually an oral intake of 20 to 60 mg
thallium/kg body weight is lethal within one week. Guinea-pigs are
slightly more sensitive than other experimental animals. The
water-insoluble thallium(III) oxide shows a somewhat lower acute
toxicity by oral or parenteral administration than thallium(I) salts.
Comparison of acute toxicity data indicates a high degree of
bioavailability from all exposure routes. Most organs are affected,
but the signs of poisoning and the sequence in which they occur reveal
some intra- and interspecies variability.

The symptoms of acute intoxication generally follow the following
sequence: firstly anorexia, vomiting and depression, later diarrhoea,
skin changes (inflammation at body orifices, skin furuncles, hair
loss), and then dyspnoea and nervous disorders. Finally, respiratory
failure leads to death.

Symptoms of chronic intoxication are similar to those of acute
intoxication. Loss of hair regularly occurs.

Histological examination reveals necrosis or other cell damage.
Necrotic changes have been observed in the kidneys, liver, intestine,
heart and the nervous system. Swelling of mitochondria and loss of
cristae, dilatations of smooth endoplasmic reticulum, increased
numbers of autophagic vacuoles and lipofuscin granules, and loss of
microvilli have been observed in many cells. The thallium-induced
alterations of functional processes may arise from physical disruption
of the membranes of subcellular organelles. In the heart,
arrhythmogenic effects are restricted to the sinus node.

Thallium intoxication causes selective impairment of certain
behavioural elements, which are correlated with biochemical effects
(which indicate cellular damage) in certain regions of the brain.
Some neurological effects seem to be caused by direct action, e.g.
ataxia and tremor by cerebellar alterations or alterations in
endocrine activity through changes in the hypothalamus. The autonomic
nervous system, mainly the adrenergic, may be activated by thallium.
In peripheral nerves, thallium seems to interfere presynaptically,
with the spontaneous release of transmitter, by antagonizing these
calcium-dependent processes.

The exact mechanism of thallium toxicity is still unknown.
Several, perhaps interconnected, mechanisms have been postulated. An
important aspect of thallium intoxication is the significant increase
in lipid peroxidation and in the activity of the lysosomal enzyme
ß-galactosidase. The resulting deficiency of glutathione leads to the
accumulation of lipid peroxides in the brain and, presumably, finally
to lipofuscin granules. The mode of action of thallium seems to be
mainly due to a disturbance of the function of the mitochondria.

Sexual activity is usually reduced in chronically poisoned
animals, and gonadotoxic effects of thallium are evident in the male
reproductive system. In the testes of rats given 10 mg thallium/litre
in the drinking-water for 16 days, the Sertoli cells were most
sensitive, and desquamation of the spermatogenic epithelium led to
immature sperm cells in the semen. This could explain the decreased
survival rate of embryos or reduced life span of offspring after
sublethal thallium-poisoning of the fathers.

Teratogenic effects, growth inhibition and disturbances in the
development of bones were found to occur in chicken embryos after
injection of thallium into the egg, but such effects in mammals, even
at maternotoxic doses, are controversial. Although transplacental
transfer has been demonstrated, many strains of mice and rats show no
or only slight teratogenic effects.

Two microbiological mutagenicity tests in Salmonella typhimurium
were negative and in vivo tests on sister chromatid exchange were
controversial. However, single studies report chromosomal aberrations
or a significant increase of single-stranded DNA breaks.

Long-term studies on the carcinogenicity of thallium are lacking.

1.7 Effects on humans

Since thallium salts are tasteless, odourless, colourless, highly
toxic, were easily obtainable in the past and still are in some
developing countries, thallium has often been used for suicide,
homicide and attempts at illegal abortion, causing acute thallium
poisoning. Indeed, thallium intoxication is considered one of the
most frequent causes, on a worldwide scale, of purposeful or
accidental human poisoning. Knowledge of chronic thallium
intoxication is limited to occupational exposure, to population groups
in contaminated areas and to cases of homicide involving multiple low
doses.

Symptoms of acute thallium toxicity depend on age, route of
administration and dose. Doses which have proved lethal vary between
6 and 40 mg/kg, being on average 10 to 15 mg/kg. Without therapy this
average dose usually results in death within 10 to 12 days, but death
occurring within 8-10 h has also been reported.

The triad of gastroenteritis, polyneuropathy and alopecia is
regarded as the classic syndrome of thallium poisoning, but in some
cases gastroenteritis and alopecia were not observed. Several other
signs and symptoms also occur, varying in order, extent and intensity.

Symptoms of thallium intoxication are often diffuse and initially
include anorexia, nausea, vomiting, metallic taste, salivation,
retrosternal and abdominal pain and occasionally gastrointestinal
haemorrhage (blood in faeces). Later, constipation is commonly seen
and may be resistant to treatment, thus interfering with antidotal
treatment.

After 2 to 5 days some of the typical thallium disorders slowly
develop, irrespective of the route of exposure. Effects on the
central and peripheral nervous system vary, but a consistent and
characteristic feature of thallium intoxication in humans is the
extreme sensitivity of the legs, followed by the "burning feet
syndrome" and paraesthesia. Involvement of the central nervous system
(CNS) is indicated by symptoms like hallucinations, lethargy,
delirium, convulsions and coma. Common circulatory symptoms are
hypertension, tachycardia and, in severe cases, cardiac failure. Loss
of head hair and sometimes body hair occurs after the second week of
poisoning; dystrophy of the nails is manifested by the appearance of
white lunular stripes (Mee's lines) 3 to 4 weeks after intoxication.
The black regions found in hair papillae are not caused by deposition
of pigments or thallium but are due to small amounts of air entering
the shaft.

In lethal cases the time until death occurs may vary from hours
to several weeks, but most commonly death occurs within 10 to 12 days.
Causes of death are mainly renal, CNS and cardiac failure.

In sublethal poisonings, recovery often requires months.
Sometimes neurological and mental disturbances as well as
electroencephalographic abnormalities and blindness can remain.
Additionally, intellectual functions seem to be adversely affected in
survivors.

In cases of chronic poisoning, symptoms are similar but in
general milder than in cases of acute intoxication. Sometimes
permanent blindness occurs. Complete recovery takes months and can be
interrupted by relapses.

In a well-investigated case of thallium emission around a cement
plant in Lengerich, Germany, thallium concentrations in the hair and
urine of exposed people did not correlate with certain features which
are known to be usually associated with chronic thallium poisoning,
but only with subjective neurological symptoms.

Postmortem examinations or biopsies following thallium poisoning
reveal damage of various organs. For example, after ingestion of
lethal doses, haemorrhages in the mucosa of the intestine, lung,
endocrine glands and heart, fatty infiltrations in liver and heart
tissue, and degenerative changes to glomeruli and renal tubules occur.
In the brain, fatty degeneration of ganglion cells, damage to axons
and disintegration of myelin sheaths can be observed.

Variations in blood pressure may be caused by direct effects of
thallium on the autonomic nervous system. Thallium intoxication
causes symmetric, mixed peripheral neuropathy. Distal nerves are
affected more than proximal nerves, and earlier but lesser degrees of
damage occur in nerves with shorter axons, e.g., cranial nerves.
Axons are swollen and contain vacuoles and distended mitochondria. In
lethal poisoning, severe damage of the vagus nerve, denervation of the
carotid sinus and lesions of the sympathetic ganglia have been
observed. In sublethal poisoning, affected nerves may undergo axonal
degeneration with no or only partial recovery within 2 years.

Retrobulbar neuritis and resulting visual disorders can develop
and persist for months after terminating treatment with thallium-
containing depilatories, and even optic atrophy may occur.

Limited data are available on the effects of thallium on human
reproduction. Menstrual cycle, libido and male potency may be
adversely affected. Effects on sperm are known to occur following
chronic intoxication. As in animal studies, transplacental transfer
occurs; this was seen following a thallium-induced abortion. However,
apart from a relatively low weight and alopecia of newborn babies,
fetal development was not affected in about 20 cases of thallium
intoxication during pregnancy.

No reports of any carcinogenic effects or data on immunological
effects of thallium are available. There is no adequate evidence of
genotoxic effects.

Therapies of thallium intoxication combine forced diuresis, use
of activated charcoal and prevention of re-absorption in the colon by
administration of Prussian blue, potassium ferric hexacyano
ferrate(II).

1.8 Human dose-response relationship

The mean urinary thallium concentration in unexposed populations
is 0.3 to 0.4 µg/litre. As thallium has a short biological half-life,
measured in days, and assuming steady-state conditions, this urinary
concentration can be taken as an indicator of total dose following
inhalation and dietary intake.

The mean urinary thallium concentration in a population sample
living near a thallium atmospheric emission source was 5.2 µg/litre.
A clear dose-response relationship was found between urinary thallium
concentration and the prevalence of tiredness, weakness, sleep
disorders, headache, nervousness, paraesthesia, and muscle and joint
pain. A similar dose-response relationship was also reported when
thallium in hair was used as an indicator of exposure.

The Task Group considered that exposures causing urinary thallium
concentrations below 5 µg/litre are unlikely to cause adverse health
effects. In the range of 5-500 µg/litre the magnitude of risk and
severity of adverse effects are uncertain, while exposure giving
values over 500 µg/litre have been associated with clinical poisoning.

1.9 Effects on other organisms in the laboratory and field

Thallium affects all organisms, but species- and also strain-
specific differences are evident. Different inorganic thallium(I) and
thallium(III) compounds and organothallium compounds can show
different toxicities.

The most important effect of thallium on microorganisms seems to
be inhibition of nitrification by soil bacteria. Results of one study
suggest that microbial community structure is disturbed at soil
concentrations in the range of 1-10 mg/kg dry weight, but the form of
thallium used in this experiment was not identified.

Thallium is taken up by all plant parts, but principally by the
roots. After uptake into the cell, it is concentrated unevenly in the
cytosol, probably bound to a peptide. Thallium concentrations found
in plants depend on soil properties (especially pH, clay and organic
matter content), as well as on the developmental stage and on the part
of the plant. Thallium accumulates in chlorophyll-containing
regions, but to a lesser degree in thallium-resistant plants. Oxygen
production is reduced by thallium, presumably by direct action on
electron transfer in photosystem II. Interference with the pigments
is indicated by the occurrence of chlorosis. In addition, impaired
uptake of trace elements seems to be involved in the mechanism of
toxicity. Growth is also affected, roots reacting more sensitively
than leaves or stems. These effects have been reported at
concentrations as low as 1 mg thallium/kg of dry plant tissue, after
exposure to monovalent forms of thallium.

Most studies of effects on aquatic organisms have used soluble
monovalent thallium compounds. The lowest thallium concentration
reported to affect aquatic species is 8 µg/litre, which caused a
reduction in growth of aquatic plants. Invertebrates are often
affected at lower concentration than fish (96-h LC₅₀ values are
2.2 mg thallium/litre for daphnids and 120 mg/litre for a freshwater
fish). The lowest LC₅₀ value, reported after exposure for about 40
days, was 40 µg/litre for fish.

Many cases of thallium intoxication of wildlife have been caused
by its large scale application as a rodenticide. In seed-eating
animals and predators the CNS and/or the gastrointestinal tract are
most severely affected. These effects can also be observed in farm
animals. In addition, thallium causes a loss of dorsal feathers in
ducks, salivation from the nose and mouth of cattle, and reduced
growth in broilers, laying hens, sheep and steers.

2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, AND ANALYTICAL METHODS

Thallium is a soft, malleable, heavy metal with a bluish-white
colour and the chemical symbol Tl. The name thallium derives from the
beautiful green spectral line ( thallos, green shoot), which
identified the element.

An overview on the properties, synonyms and chemical formulae of
pure thallium and some of its compounds is given in Table 1.

2.1 Identity

Thallium is the fifth element in Group IIIB of the Periodic
Table. It occurs naturally as two isotopes thallium-203 and
thallium-205 with abundancies of 29.52 and 70.467%, respectively
(Aderjan et al., in press). The relative atomic mass of thallium is
204.383, the atomic number is 81, and the electron configuration is
(Xe) 4f14 5d10 6s2 6p. Due to its high specific gravity of
11.85 g/cm³, thallium belongs to the heavy metal group, which
comprises all metals with a specific gravity of over 4.5 g/cm³
(Micke et al., 1983).

2.2 Physical and chemical properties

The physical properties of elemental thallium are similar to
those of lead; it is very soft and malleable. Thallium exists in both
the monovalent (thallous) and the trivalent (thallic) form. Because
the 6s electrons possess only a low tendency to be released or bound
covalently, the thallous form is more common and stable and forms
numerous stable salts. Thallium(III) is easily reduced to thallium(I)
by reducing agents at high temperatures (Tl⁺/Tl³⁺ = +1.12v) (Micke
et al., 1983; Schoer, 1984; Stokinger, 1987).

Below 234°C the metal crystallizes in a hexagonal close-packed
form (alpha-thallium), while at 234°C it converts to the ß-form, a
cubic body-centred lattice. Thallium begins to volatilize at 174°C.
It has a melting point of 303°C, a boiling point of 1457°C and a
normal potential of Tl/Tl⁺ -0.335v (Micke et al., 1983). Thallium
is a very reactive metal. When exposed to air and moisture, it is
superficially oxidized, forming a coating of thallium(I) oxide
(Tl₂O). At higher temperatures it reacts with a lovely green flame
to form thallium(III) oxide (Tl₂O₃). Thallium carbonate
(Tl₂CO₃) is the only heavy metal carbonate that is very soluble in
water (Micke et al., 1983; Stokinger, 1987).

Table 1. Physical and chemical properties of thallium and some selected thallium compounds^a

Name Chemical CAS registry Relative Specific Melting Boiling Colour Solubility
formula number atomic/ gravity point point in water
molecular mass (g/cm³) (°C) (°C) (g/litre)

Thallium Tl 7440-28-0 204.38 11.85 303.5 1457 bluish-white, insoluble
metallic

Thallium(I) TlC₂H₃O₂ 563-68-8 263.43 3.765 131 - silky white very
acetate soluble

Thallium TlAl(SO₄)₂*₁₂H₂O 52238-56-9 639.66 2.306 91 - colourless 117.8
aluminium
sulfate

Thallium(I) TlBr 7789-40-0 284.29 7.557 480 815 pale yellow 0.5
bromide (17.3°C) (25°C)

Thallium(I) Tl₂CO₃ 29809-42-5 468.78 7.110 273 - white 40.3
carbonate (15.5°C)

Thallium(I) TlCl 7791-12-0 239.84 7.004 430 720 white 2.9
chloride (30°C) (15.5°C)

Table 1 (contd).

Name Chemical CAS registry Relative Specific Melting Boiling Colour Solubility
formula number atomic/ gravity point point in water
molecular mass (g/cm³) (°C) (°C) (g/litre)

Thallium(III) TlCl₃ 13453-32-2 310.74 - 25 decomposes colourless, very
trichloride hygroscopic soluble
TlCl₃*₄H₂O 13453-33-3 382.80 - 37 100 (-H₂O) colourless 862

Thallium TlOC₂H₅ 20398-06-5 249.44 3.493 -3 130 colourless -
ethylate (20°C) (decomposes)

Thallium(I) TlF 7789-27-7 223.38 8.23 327 655 colourless 786
fluoride (4°C) (15°C)

Thallium(III) TlF₃ 7783-57-5 261.38 8.36 550 - olive decomposes
trifluoride (25°C) (decomposes) green to TlOH

Thallium TlOH 1310-83-4 221.39 - 139 - pale 259
hydroxide (decomposes) yellow

Thallium(I) TlI 7790-30-9 331.29 7.29 440 (ß) 823 (ß) yellow 0.006
iodide (alpha) (20°C)

Table 1 (contd).

Name Chemical CAS registry Relative Specific Melting Boiling Colour Solubility
formula number atomic/ gravity point point in water
molecular mass (g/cm³) (°C) (°C) (g/litre)

Thallium(I) TlNO₃ 10102-45-1 266.39 - 206 430 white 95.5
nitrate (alpha) (20°C)

Thallium (III) Tl(NO₃)₃*₃H₂O 13453-38-8 444.44 - 105-107 decomposes colourless decomposes
nitrate
trihydrate

Thallium(I) Tl₂O 1314-12-1 424.77 9.52 300 1080 (-O) black decomposes
oxide (16°C) to TlOH

Thallium(III) Tl₂O₃ 1314-32-5 456.76 10.19 717 ± 5 875 (-O₂) black insoluble
oxide (22°C)

Thallium(I) Tl₂SO₄ 7446-18-6 504.82 6.77 632 decomposes white 48.7
sulfate (20°C)

Thallium(I) Tl₂S 1314-97-2 440.85 8.46 448.5 0.2
sulfide (20°C)

^a From: Stokinger (1987); Budavari (1989); Lide (1990)
In contact with water, thallium(I) hydroxide is formed from the
metal. Thallium is very soluble in HNO₃ and H₂SO₄, but only
slow dissolution takes place in HCl, because of the low solubility of
the halides. It is insoluble in alkali bases. Thallium combines with
fluorine, chlorine and bromine at room temperature, and reacts with
iodine, sulfur, phosphorus, selenium and tellurium after heating. The
metal does not react with molecular hydrogen, nitrogen or carbon. It
forms alloys with other metals and readily amalgamates with mercury
(Micke et al., 1983).

The ionic radii and the electronegativity constant of monovalent
thallium are very similar to those of other alkali metals.
Thallium(I) hydroxide, carbonate and sulfate, like the corresponding
potassium compounds are very soluble in water. With respect to their
physical and chemical properties, e.g., poor water solubility,
thallium(I) oxide, sulfide and halides show similarities to the
corresponding compounds of silver, mercury and lead
(Trotman-Dickenson, 1973). In contrast to inorganic thallium
compounds, covalent organothallium compounds are only stable in the
trivalent form (McKillop & Taylor, 1973). Thallium(I) is not strongly
complexed by humic acids, whereas thallium(III) forms stable complexes
of the [TlX₄]- or [TlX₆]³- type (Schoer, 1984).

2.3 Conversion factor

1 g thallium = 0.0049 mol
1 mol thallium = 204.38 g

2.4 Analytical methods

Classical analytical methods, the introduction of new techniques
and a combination of both with enrichment or separation processes
provide suitable methods for the quantitative detection of thallium in
various media. Because thallium concen trations in environmental
samples are very low, determination directly from the sample or from
the digestion solution usually lacks sufficient accuracy. Therefore,
preconcentration procedures are necessary (Schoer, 1984; Sager & Tölg,
1984).

2.4.1 Sampling and sample preparation

Thallium losses during sampling, sample preparation and
determination are a major source of analytical error. Contamination
hazards need to be anticipated, as thallium is present in laboratory
ware and is leached out by solutions (Kosta, 1982). Glass contains
about 1-10 µg thallium/kg. Leaching of polythene containers with 6M
HCl for 1 week brought 1-10 ng thallium/cm2 into solution. In

addition, thallium(I) in 0.1M HNO₃ solution adsorbs onto container
walls made of polyethene, polypropene, glassware or rubber. This
effect depends on the chemical properties of the surface of the
container walls and on the concentration of matrix ions. At a
thallium concentration of 1 mg/litre, no losses to borosilicate
surfaces at pH < 4 were reported, but extensive adsorption occurred
at pH > 10 (Sager & Tölg, 1984).

For determinations with spectrophotometric, mass spectrometric,
voltametric and other methods, digested samples are needed. With
respect to the high volatility of the metal and the low boiling points
of some of its compounds, only closed systems are recommended for the
digestion of organic matrices to prevent thallium losses. Fusion, dry
ashing and fuming with HF and H₂SO₄ or HClO₄ may lead to severe
losses (up to 40%) of the thallium present (Matthews & Riley, 1969).
High-pressure digestion in closed quartz vessels with concentrated
acids, e.g., HNO₃ or HNO₃ and HF, at temperatures up to 300°C is
the most suitable procedure for nearly all matrices (Knapp, 1985). HF
interferes with analysis by GFAAS or ICP-AES and needs to be removed
by heating to dryness with H₃BO₃ (Han et al., 1982).

The volatility of thallium and its oxide or chloride makes it
possible to separate these with a gas stream of O₂, H₂ or HCl from
other elements that do not form volatile components under the same
conditions and subsequently capture them in a cool trap. This
procedure can be used as a preconcentration step when large quantities
of sample are available (Geilmann & Neeb, 1959; Han et al., 1982;
Sager, 1984).

Other methods of preconcentration are coprecipitation, anodic
electrolysis, ion exchange and liquid-liquid extraction.
Coprecipitation is not selective, but it leads to a high concentration
factor and results in a definite matrix, which might be useful in some
methods (Griepink et al., 1988). For example, coprecipitation with
Fe(OH)₃ leads to separation from a salt matrix (K⁺, NH⁴⁺).
Electrolytic deposition or cementation with zinc powder yields an
excellent separation, although this procedure is time-consuming. Ion
exchange, which gives a specific separation in certain cases, is also
time-consuming. Liquid-liquid extraction with chelating agents is
virtually nonspecific, but it is a fast and easy method. A
disadvantage is the relatively low concentration factor (Sager & Tölg,
1984).

Isotope dilution methods have been applied to avoid ionization
matrix effects. Thallium is measured as thallium-205; the thallium-203
isotope can be used as a spike for isotope dilution (Sager, 1986).

2.4.2 Methods of determination

Thallium is almost always determined as total metal, rather than
as specific thallium compounds. Among the analytical techniques that
can be used are spectrophotometry, mass spectrophotometry (MS), atomic
absorption spectrometry (AAS), voltametry, neutron activation analysis
(NAA), X-ray fluorimetry, and inductively coupled plasma (ICP)
techniques (Sharma et al., 1986). A selection of analytical methods
is summarized in Tables 2 and 3.

2.4.2.1 Atomic absorption spectrometry

The most widely used method of thallium determination is atomic
absorption analysis, using measurement at 276.8 nm with a thallium
hollow cathode lamp. The sensitivity can be improved by the use of an
electrode discharge lamp (EDL), owing to its higher intensity.
Graphite furnace atomic absorption spectrometry (GFAAS) is a
well-established technique for the monitoring of trace elements in
nearly all kinds of matrices. The technique has sufficiently low
detection limits and is well-suited to applications where high
sensitivity is required for small sample amounts. In Table 3 some
methods for GFAAS are summarized.

The platform furnace concept in the temperature-stabilized mode,
together with Zeeman effect background correction, allows almost
interference-free determinations of many elements. Sample
pretreatment is not necessary, which greatly reduces the risk of
substance losses or contamination of the sample prior to analysis
(Minoia et al., 1990).

Matrix modifiers permit higher pyrolysis temperatures, so that
the desired element can be isolated from matrix elements and compounds
in an ideal case. Letourneau et al. (1987) found that additions of
H₂SO₄ as a matrix modifier were inadequate and that interferences
could not be corrected by Zeeman background compensation. Modifying
the matrix with palladium and magnesium nitrate has been suggested to
be generally applicable, but this is not as effective for thallium as
it is for other elements (Welz et al., 1988a). A combination of 6 mg
palladium with 100 mg ammonium nitrate allows the direct determination
of thallium in ten-fold diluted blood against matrix-free standards
(Yang & Smeyers-Verbeke, 1991).

Paschal & Bailey (1986) determined thallium concentrations in
urine. The samples were diluted 1:1 with a matrix modifier consisting
of magnesium nitrate, HNO₃, Triton X-100 and water. The detection
limit was calculated to be 0.5 µg/litre.

Table 2. Instrumental methods for the determination of thallium

Method^a Matrix Oxidation Sample Parameters Interferences Detection Reference
state pretreatment of method limit

PAA metals - ²⁰³Tl (gamma,n) ²⁰²Tl gamma440 keV Segebade &
30 MeV bremsstrahlung Schmitt (1987)
post-irradiation
separation of Tl
from the matrix

NAA biological post- drying, 2-M 10¹³ n/cm².sec 3-7 other isotopes 1 µg absolute Itawi &
material irradiation days ²⁰³Tl (n,gamma) than 204Tl Turel (1987)
extraction ²⁰⁴Tl 0.77 MeV ß^-
measurement

AMP water Tl (I) Na₂CO₃, NaHCO₃, -0.47 vs sat. calomel Mn(VII), Co(II), - Agrawal &
thiomalic acid electrode Sn(II), Tl(III) Khatkar (1988)

DPASV urine, - Na acetate, HClO₄, -1.0 vs sat. calomel Cd, Pb 0.2 µg/litre Vandenbalck &
saliva EDTA electrode Patriarche
(1987)

ICP-MS rocks - HNO₃, HF, H₂O₂ - polyatomic 70 ng/litre Date et al.
interferences (1988)

Table 2 (contd).

Method^a Matrix Oxidation Sample Parameters Interferences Detection Reference
state pretreatment of method limit

GDMS indium - - at pressure 3.10^-4 mbar - 30 µg/kg Guidoboni &
discharge voltage 1 kV Leipziger
discharge current 3 mA (1988)
accelerating voltage
8 kV resolution 4000

ICP-AES air - HNO₃/HClO₄ (4:1) 190.9 nm F^- 17 µg/litre NIOSH (1984)
particulates leachate

ICP-AES biological - Parr bomb F^- 0.05-0.1 Que Hee &
material mg/litre Boyle (1988)

ICP-MS water - HNO₃ ²⁰⁵Tl - 0.1 µg/litre Henshaw et al.
(1989)

ICP-MS tissues - HNO₃ - - 18 µg/kg Templeton
et al. (1989)

Spectrometry environmental Tl (I) dithizone, CHCl₃, - Ag, Hg 1 µg/litre Sager (1986)
EDTA, citrate,
cyanide

^a AMP = amperometric titration; DPASV = differential pulse anodic stripping voltametry; GDMS = glow discharge mass spectrometry;
ICP-AES =inductively coupled plasma - atomic emission spectrometry; ICP-MS = inductively coupled plasma - mass spectrometry;
NAA = neutron activation analysis; PAA = photon activation analysis; PPS = proton-induced prompt low energy photon high resolution
spectrometry

Table 3. Methods for determining thallium (Tl) with graphite furnace atomic absorption spectrometry (GFAAS)

Sample Separation Injected solution Detection limit Interferences Reference

Fly ash, soil digestion and preconcentration diluted H₂SO₄, HNO₃ 3.3 ng/litre HBr De Ruck et
by extraction of Tl(III) with including the 400 × al. (1989)
diisopropylether evaporation preconcentration step)

Urine complex with tri-n-octylamine, organic layer diluted 0.3 µg/litre max. charring Flanjak &
extraction with ethanol into 5 mg with ethanol and H₂SO₄ temp. 400°C Hodda (1988)
metallic n-butyl acetate gallium

Gallium - gallium 200 µg/kg - Hiltenkamp &
Jackwerth (1988)

Urine - spiked urine, diluted 2 µg/litre NaCl Berndt &
Sopczak (1987)

Urine chelation with NaDDC, extraction MIBK extract 0.05 µg/litre - Apostoli et
with MIBK al. (1988)

Mineralized - HNO₃, H₂SO₄, ascorbic 5 µg/litre NaCl Leloux et al.
faeces and acid, Triton X-100 (1987a)
tissues

Table 3 (contd).

Sample Separation Injected solution Detection limit Interferences Reference

Blood, serum - HNO₃ 10 µg/litre Leloux et al.
(1987a)

Erythrocytes - HNO₃ 12 µg/litre Leloux et al.
(1987a)

Soil, extraction with 20 µg/kg Cu, Zn, Pb Ebarvia et
sediments tri-octyl-methylammonium, MIBK al. (1988)

Coal fly ash - HNO₃ - - Bettinelli et
al. (1988)

MIBK = methyl isobutyl ketone; NaDDC = sodium diethyl dithiocarbamate

Chemical interferences due to chloride ions are important. These
interferences are caused by volatilization of thallium chloride in the
pyrolysis stage and, in part, by formation of TlCl(g) during the
atomization stage. Even matrix modification gives unsatisfactory
results. Welz et al. (1988b) showed that addition of palladium
nitrate as a modifier and application of argon with 5% H₂ as a purge
gas leads to interference-free determination with, for instance, NaCl
loads of up to 100 mg. A special pre-pyrolysis step is necessary to
reduce palladium to the metal state, thus enabling adsorbed H₂ to
react with the chloride compounds to form volatile HCl. Similar
results were obtained by Manning & Slavin (1988).

De Ruck et al. (1987) reported an oxidation technique for natural
waters with cerium(IV) sulfate and a subsequent preconcentration step
on an anion-exchange column. A preconcentration factor of 400 was
achieved, and the resultant detection limit was 3.3 ng/litre using
GFAAS. Flame atomic absorption is a reliable method for measurement
of thallium concentrations at the level of mg/litre or more. The
determination is easy and free from interference (Welz, 1983; Griepink
et al., 1988).

2.4.2.2 Inductively coupled plasma - mass spectrometry

ICP-MS is a promising method for concentrations in the µg/kg
range or less, and has good precision and accuracy. It is a multi-
element technique with sub-ppb detection limits for many elements.
Additional advantages of mass discrimination include its suitability
for isotope ratio analysis and stable isotope tracer analysis, and the
extended range of elements that can be studied. Some ICP-MS methods
are summarized in Table 2.

The application of ICP-MS to the analysis of thallium in
iron-rich ores was described by Date et al. (1988). No polyatomic
interferences for iron were detected in acid solutions. The addition
of 500 mg iron/litre to a solution of 1 mg thallium/litre in 1% HNO₃
resulted in a 0.1% increase in the thallium peak. The detection limit
was found to be 0.07 µg/litre.

Templeton et al. (1989) examined thallium concentrations in rat
liver and blood plasma samples which were submitted to acid digestion
and reported a detection limit of 0.09 µmol/kg (18 µg/kg).

More than 250 water samples from lakes were analysed for thallium
(thallium-205) by ICP-MS after acidification with HNO₃. The
detection limit was found to be 0.1 µg/litre; the recovery of spiked
analytes amounted to 112 ± 4% (Henshaw et al., 1989).

2.4.2.3 Other methods

Methods other than AAS and ICP-MS are summarized in Table 2.
Spectrophotometric determination with rhodamin B after liquid/liquid
extraction is a quick and easy method, but it is less sensitive and
has a high incidence of interference. The method is suitable for a
quick visual test, when a massive intoxication with thallium compounds
is suspected. Determinations down to 10 µg/litre are possible in
environmental matrices (Griepink et al., 1988).

Inductively coupled plasma - atomic emission spectrometry
(ICP-AES) is a rapid multi-element technique, but it does not provide
the detection limits required to measure thallium concentration in
uncontaminated samples. The NIOSH method for determining thallium in
air particulates has a detection limit of 17 µg/litre of leaching
solution (NIOSH, 1984).

Differential pulse anodic stripping voltametry (DPASV) is a
sensitive method for the quantitative determination of thallium in
water samples or urine. Voltametric methods also offer the advantage
of simultaneous determination of several metals from one solution.
The lower limit of detection for thallium(I) is 10-100 ng/litre
(Klahre et al., 1978; Vandenbalck & Patriarche, 1987; Griepink et al.,
1988).

Neutron activation analysis (NAA) is applicable for the
determination of thallium in various environmental samples, but it is
relatively slow and impractical for the routine analysis of large
numbers of samples. The detection limit is determined by the
irradiation time, neutron flux, the choice of a radiochemical
separation of the radio-isotope to remove interfering matrix
radio-isotopes and the measurement time. Levels down to the absolute
amount of ng of thallium can be determined (Schoer, 1984). This
method can therefore be used for the determination of low thallium
concentrations in biological samples. In bovine liver a detection
limit of 1.5 µg/kg was found after digestion, separation and
concentration procedures (Henke, 1991).

Thallium(I)-sensitive electrodes are not sensitive enough for
trace determinations, and high concentrations of alkali ions reduce
the selectivity. Sensitivity problems must also be considered for the
usual X-ray fluorimetry techniques. Other methods, like excitation
with charged particles and photon activation radiochemical isotope
dilution, are seldom used.

2.4.3 Quality control and quality assurance

Sample collection, analysis and data presentation should be
carried out according to a protocol which ensures adequate validation
of biological monitoring procedures (Vesterberg et al., 1993).

There is an urgent need for strict quality control and quality
assurance of the analytical data on thallium in clinical and
environmental samples. It is only when proof is given for the accuracy
of the published data that they become unequivocally useful to
establish critical concentrations and dose-response relationships in a
given population or ecosystem. General considerations of quality
control and quality assurance have been recommended by WHO (WHO, 1986;
Aitio, 1988).

To date, very few of the many studies on thallium have provided
the necessary evidence concerning the quality of the data throughout
the analytical procedure. The recognized way to control and ensure
this involves good laboratory practice (GLP), including intra- and
inter-laboratory analysis of materials with certified concentrations
of thallium. Such Certified Reference Materials (CRMs) should have
the same (or a similar) matrix as the sample to be analysed and be
certified for thallium concentrations (similar to those in the sample)
by an internationally recognized body. This implies suitable levels
for thallium in serum, whole blood, urine, faeces, animal tissues and
plants, as well as levels typical for exposed individuals, animal
studies or eco-systems (Cornelis, 1988).

Available reference materials with clinical and environmental
interest are listed in Table 4. This immediately reveals the very
poor picture for CRMs certified for thallium. Whole blood and serum
samples are totally lacking, while urine of exposed individuals is
handled by the BI CUM 2 and 3 products with assigned values for
thallium only. The BCR milk powders and the NBS liver samples carry a
reference value. Thallium has also been reported in some
environmental samples (fly ash, etc.) without being certified.

There appears to have been only one inter-laboratory survey on
thallium in two spiked urine samples (Geldmacher-von Malinckrodt et
al., 1984). The 35 participating laboratories used one of the three
routine methods, AAS, DPASV or photometry, after thallium extraction.
The samples were also analysed by IDMS (isotope dilution mass
spectrometry) and attributed reference values of 66.3 and 483 µg
thallium/litre, respectively. The evaluation of this inter-laboratory
survey revealed that about 70% of the laboratories met the goal.

2.4.4 Conclusions

There are several methods available for the determination of
thallium in biological and environmental samples. As routine methods
these are GFAAS (the most widely used), DPASV, ICP-MS and photometry.
They all require a very careful sample pretreatment and, in the case
of DPASV and photometry, perfect mineralization of the sample without
losses due to volatilization or adsorption onto the container walls.

The same remarks apply to the methods including a preconcentration
step. In the case of GFAAS and ICP-MS, direct analysis of the diluted
sample is feasible. It is strongly recommended that all analyses be
accompanied by a quality assurance programme. At present, it is
possible to determine thallium concentrations of about 0.1 µg/litre or
0.1 µg/kg.

Table 4. Reference materials for thallium determinations in biological and
environmental materials^a

Matrix Origin^b Code Thallium Remarks
concentration

Liver NBS SRM 1577 50 µg/kg lyophilized bovine liver
SRM 1577A 3 µg/kg lyophilized bovine liver

Milk BCR CRM 63 1.3 µg/kg natural skim milk powder
powder CRM 150 1.0 µg/kg spiked milk powder
CRM 151 0.8 µg/kg spiked milk powder

Urine BI CUM 2 93 ± 13 µg/litre^c lyophilized synthetic urine
CUM 3 603 ± 78 µg/litre^c lyophilized synthetic urine

City BCR BCR-CRM- 2850 µg/kg certified; error 6.7%
waste 176

Coal IRANT IRANT-ECO 14 000 µg/kg not certified
fly ash

Coal NIST NIST-SRM- 5700 µg/kg certified; error 3.5%
fly ash 1633a

Gas coal BCR BCR-CRM- 2200 µg/kg not certified
180

Steel IRANT IRANT-OK < 3000 µg/kg not certified
plant
flue dust

^a According to Muramatsu & Parr (1985) and Cortes Toro et al. (1990)
^b BCR: Measurement and Testing Programme, DG XII, BCR, Commission of the
European Union, Wetstraat 200, B-1049 Brussels, Belgium BI: Behring
Institute, PO box 140, D-3350 Marburg 1, Germany IRANT: Institute of
Radioecology and Applied Nuclear Techniques (CSSR) NBS (new name NIST):
Room B 311, Chemistry Building, National Institute for Standardization
and Testing, Gaithersburg, MD 20899, USA NIST: National Bureau of Standards
(USA)
^c assigned values for a particular lot only

3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE

3.1 Natural occurrence

Thallium is ubiquitous in nature, but occurs at low
concentrations (< 2 mg/kg) (section 5.1), especially in sulfide ores
of various heavy metals (zinc, copper, iron and lead) and in minerals
of potassium, caesium and rubidium (Micke et al., 1983; Kemper &
Bertram, 1984; Ohnesorge, 1985; Stokinger, 1987; Manzo & Sabbioni,
1988). Although the concentration of thallium is low, about 700 000
tonnes of thallium are contained in worldwide identified resources of
coal and 19 000 tonnes in zinc resources (US BM, 1989). There are
only a few areas with a naturally very high thallium concentration,
e.g., the Alsar in the Former Yugoslav Republic of Macedonia (Zyka,
1972). Minerals of thallium, e.g., lorandite (TlAsS₂) and
crookesite ((Cu,Ag,Tl)₂Se), with thallium concentrations of up to
60%, are rarely found and usually not used for production of thallium
(Micke et al., 1983; Kemper & Bertram, 1984; Briese et al., 1985;
Kazantzis, 1986).

3.2 Anthropogenic sources

3.2.1 Production levels and processes

Since thallium is used only in small amounts by industry,
worldwide production of pure thallium is low. In 1975 about 8 tonnes
were produced in Germany and 2 to 3 tonnes in the USA (Zitko, 1975a),
while in 1987 and 1988 worldwide production was about 17 tonnes (US
BM, 1992). In 1981 the production of thallium in the USA was
discontinued. Sources for the production of thallium are zinc, lead
and sometimes copper or iron smelters and sulfuric acid plants. Flue
dust in particular is used as a thallium source (Zitko, 1975a; Smith &
Carson, 1977; Micke et al., 1983; Briese et al., 1985). Procedures
for the separation of thallium from other metals depend on the
proportions of the different minerals and, therefore, vary
considerably between the different smelters (Sanderson, 1952; Smith &
Carson, 1977; Micke et al., 1983; Kemper & Bertram, 1984; Briese et
al., 1985).

3.2.2 Uses

Thallium(I) sulfate was once used in medicine to reduce sweating
and to cure various infections, e.g., venereal diseases, ringworm of
the scalp, typhus, tuberculosis and malaria, and as a depilatory
agent, which caused many intoxications (Munch, 1934b; Smith & Carson,
1977; Emsley, 1978; Briese & Nessler, 1985a). However, therapeutic
uses of thallium have been discontinued because of its toxicity.

Since 1920, thallium(I) sulfate has been used as a rodenticide, in
Europe chiefly against rats and in the USA chiefly against ground
squirrels (Howe, 1971; Smith & Carson, 1977). Formerly it was used as
an insecticide (against ants and cockroaches). However, thallium is
no longer on sale as a rodenticide in most industrial countries
(Bruère et al., 1990), but is still used in developing countries
because of its cheapness.

Other areas in which thallium is used (Howe, 1971; Smith &
Carson, 1977; Micke et al., 1983; Briese et al., 1985; Sharma et al.,
1986; Kazantzis, 1986; Manzo & Sabbioni, 1988; ATSDR, 1992) are as
follows:

a) low temperature thermometers (down to -59°C) made from a
mixture of mercury and thallium;

b) special glasses with a high resistance and a low melting
point, containing thallium and selenium;

c) mixed crystals for infrared instruments, composed of arsenic
or thallium(I) salts and halogenides (TlI-TlBr), and
Tl₃VS₄, Tl₃NbS₄, and Tl₃PSe₄ for acusto-optic
and laser equipment;

d) electronic devices, e.g. thallium(I) sulfide for
semiconductors and scintillation counters;

e) mercury lamps (addition of thallium(I) halogenids increases
the yield of light and changes its spectrum);

f) alloys with lead, zinc, silver and antimony enhance
resistance to corrosion;

g) catalysing organic reactions, e.g., oxidations of
hydrocarbons and olefins (thallium compounds are being
increasingly used for organic synthesis); patents summarized
by Smith & Carson (1977);

h) radioactive isotopes, used in physics for measurement of
exact time periods (thallium-205), in industry for measuring
the thickness of material (thallium-204), and in medicine
for scintigraphy of heart, liver, thyroid and testes, and
for the diagnosis of melanoma (thallium-201) (Rao et al.,
1983; Müller-Brand et al., 1984; Urbain et al., 1986);

i) other uses, e.g., in the production of imitation jewellery,
fireworks, pigments and dyes, the impregnation of wood and
leather against bacteria and fungi, and in mineralogical
analysis;

j) minor amounts of thallium are used in biochemistry, e.g.,
arylthallium(III) compounds for modification of proteins and
tRNA (Douglas et al., 1990).
Worldwide industrial consumption in 1991 was estimated to be 10
to 15 tonnes. Between 1940 and 1980 consumption in the USA varied
considerably between 0.5 and 11 tonnes/year (Schoer, 1984), and
between 1984 and 1988 it was 1.1-1.5 tonnes/year (US BM, 1985, 1989).
In the USA it is used mainly in the electrical and electronic
industries and the 650 kg used in 1983 in the German Democratic
Republic was mainly for making special glass (Smith & Carson, 1977;
Micke et al., 1983; Briese et al., 1985; Kazantzis, 1986; Kemper &
Bertram, 1991).

3.2.3 Emissions from industrial sources

There is an enormous difference between the amount of thallium
mobilized (released into air, water or disposed of on land) and the
thallium consumption of 12 tonnes/year (section 3.2.2). Worldwide a
total of 2000-5000 tonnes of thallium is estimated to be mobilized per
year, especially through the combustion of fossil fuels, refinement of
oil fractions, the smelting of ferrous and non-ferrous ores, and also
by some other industrial processes such as cement production (Gorbauch
et al., 1984; Ewers, 1988; Nriagu & Pacyna, 1988). Smith & Carson
(1977) estimated that about 15% (240 tonnes) of total mobilized
thallium is transferred annually to the atmosphere. However, only a
small fraction is released into the atmosphere or wastewater during
production processes or from waste materials (Table 5). Summarizing
estimations for the USA by Smith & Carson (1977), Schoer (1984)
emphasized that in the USA each year nearly 1000 tonnes of thallium
are released into the environment, of which 350 tonnes are emitted in
vapours and dusts, 60 tonnes bound to non-ferrous metals, and more
than 500 tonnes contained in fluid and solid wastes.

3.2.3.1 Metal production industries

It has been estimated that worldwide over 600 tonnes of thallium
are processed per year during the smelting of lead, copper and zinc
ores (Micke et al., 1983). Thallium emissions from smelters can vary
greatly from plant to plant, depending upon the thallium content of
the raw materials and the technology used. For this reason, and
because of the lack of recent emission data, global releases can be
only roughly quantified. On the basis of the data in Table 5, a total
of about 90 tonnes of thallium may be released each year into the
atmosphere from non-ferrous metal production operations in the USA,
Canada and Germany. Dust in one zinc smelter was reported to contain
380-3700 mg thallium/kg before and 60-9700 mg/kg after starting the
production of thallium (Briese et al., 1985). Although it is not
possible to estimate the losses of thallium from mineral waste
materials, releases from these materials are generally expected to be
small.

Table 5. Estimated emissions of thallium (tonnes/year) into the environment

Emission source USA Canada Germany Europe World

Coal combustion
into air 180^a 7.5^b 7^c 54^d
140^e 4^f 80^e 600^e
6^g
into soil/water 170^a
into total environment 240^c

Coal combustion (into air)
from electric utilities 155-620^h
from industry and domestic 495-990^h

Ferroalloy production
using manganese ores
into air 140^a
into soil/water 220^a

Raw iron production and
related coal combustion
into air 6^a 35^g 30^d
into total environment 205^a

Production of nonferrous
metals
into air 38^a 44ⁱ 11^g
total emission 496^a

Potash-derived fertilizers
into total environment 5^a

Cement plants
into air 25^g 2670-5340^h

Brick works 28^b

Table 5. (cont'd).

Emission source USA Canada Germany Europe World

Oil fuel combustion, mining
and processing of oil shales
into soil/water 8^a
total emission 8^a

Waste combustion < 1^g

^a Smith & Carson (1977) ^f Brumsack et al. (1984)
^b Brumsack (1977) ^g Davids et al. (1980)
^c Sabbioni et al. (1984b) ^h Nriagu & Pacyna (1988)
^d Bowen (1979) ⁱ Kogan (1970)
^e Schoer (1984)

Data from the USA (Smith & Carson, 1977) indicate that relatively
large amounts of thallium are present in waste materials from
non-ferrous metal (mainly copper) and iron and steel production
(Table 5). Although no precise data were available on thallium levels
in waste from ferroalloy production using manganese ores, Smith &
Carson (1977) suggested that emissions from this source could be
significant. Atmospheric releases resulting from the production of
iron and steel in the USA were estimated to be relatively small (about
5 tonnes from steelmaking and 1 tonne in iron blastfurnace gases). In
the main area of iron and steel production in Germany, annual thallium
emissions into air have been estimated to be about 0.8 tonnes (Ewers,
1988).

3.2.3.2 Power-generating plants

Power-generating plants represent a major source of thallium
emissions, especially those using some brown coal or coal of the
Jurassic period. Most coals contain only about 0.5 to 3 mg/kg, mainly
incorporated in sulfide inclusions. Some of these impurities can be
removed by washing and mechanical cleaning. It has been estimated
that about half of the thallium content of coal is emitted into the
atmosphere and represents the biggest anthropogenic source (Smith &
Carson, 1977) (Table 5). In such estimations, losses from collected
fly ash are not taken into consideration, because its use may vary.
Only a minor amount is used in cement making. If it is used as a soil
stabilizer, contami nation of the environment is much higher (Smith &
Carson, 1977).

Natusch et al. (1974) found that coal-fired power-generating
plants emitted about 700 µg thallium/m³ flue gases, resulting in a
local level of air emission of about 700 ng/m³. This would result
in an estimated daily absorbed amount of 4.9 µg airborne thallium per
person (US EPA, 1980). In the European Union, coal-fired
power-generating plants were estimated to have caused a total
mobilization of 240 tonnes of thallium during 1990, about one third of
this being concentrated in the smallest particles, and atmospheric
emissions of 7 tonnes (Sabbioni et al., 1984b).

In coal burners, thallium volatilizes and recondenses onto the
surface of ash particles in cooler parts of the system. As a result,
2 to 10 times higher concentrations of thallium may occur in the
fly-ash than was present in the coal (Galba, 1982). Fly-ash thallium
content is negatively correlated with particle size (Manzo & Sabbioni,
1988). Thus, thallium and other toxic trace elements are concentrated
in the smallest particles, which pass through conventional
power-generating plant filters, remain suspended in the atmosphere for
long periods and are respirable. For instance, particles with a
diameter of 1.1-2.1 µm contain 76 mg thallium/kg fly-ash, those with a
diameter of 2.1-7.3 µm contain 62-67 mg/kg and those with a diameter
of 7.3-11.3 and > 11.3 µm contain 40 and 29 mg/kg, respectively.
Particles with a diameter of less than 74 µm contain only 7 mg
thallium/kg (Natusch et al., 1974). These particles are highly toxic,
since thallium and other heavy metals are preferentially concentrated
on the particle surfaces and therefore are relatively bioavailable
(Linton et al., 1976; Natusch, 1982).

3.2.3.3 Brickworks and cement plants

Total thallium emissions from brickworks in Germany have been
estimated to be 28 tonnes/year. This compares with emissions of
7.5 tonnes/year from the burning of coal (Brumsack, 1977).

The emission potential of cement plants was not recognized until
1979. The first effects on vegetation around a cement plant in
Lengerich, Germany were observed in 1977 (Pielow, 1979; LIS, 1980),
but only the gradual hair-loss in a rabbit led to the suspicion that
thallium was the cause of the toxic effects (LIS, 1980; Brockhaus et
al., 1981b; Dolgner et al., 1983). The source of thallium was found
to be residues of pyrite roasting added as a ferric oxide additive to
powdered limestone in order to produce special qualities of cement and
the addition of the filter fly-dust (LIS, 1980). Studies at other
plants showed much lower emission levels, so that the emission at
Lengerich was caused by the exceptional circumstances. Production
alterations in Lengerich caused a reduction in the emissions of more
than 99% (Pielow, 1979; Prinz et al., 1979; LIS, 1980).

Like power-generating plants, cement plants emit thallium mainly
bound to particles with a diameter of 0.2-0.8 µm (LIS, 1980).
Thallium concentrations in fly-dust emitted by the cement plant in
Lengerich were about 2.5 mg/m³ air, of which nearly all was
water-soluble thallium(I) chloride. Whereas the filter efficiency was
99% with respect to cement dust, it was only 50% with respect to the
thallium-containing particles. As a result, about 140 to 200 g
thallium/hour was emitted (Pielow, 1979; Prinz et al., 1979;
Weisweiler et al., 1985). Changing the production process reduced the
thallium content to less than 25 µg/m³ (< 200 mg/kg dust). In
other cement plants the concentrations in the filter dust were reduced
from 3066 mg/kg to about 100 mg/kg, and after this reduction only 13%
of the thallium was soluble in water (LIS, 1980).

3.2.3.4 Sulfuric acid plants

The sulfuric acid plant which had been the source of the roasted
pyrite used in the cement plant in Lengerich used pyrite containing
about 400 mg thallium/kg. However, in the roasted pyrite about 7% of
the thallium was water-soluble. During production of sulfuric acid, a
100-fold enrichment of thallium was found (LIS, 1980). As a
consequence, increased levels of thallium were found in Duisburg,
Germany around the sulfuric acid plant but never such high
concentrations as around the cement plant (Gubernator et al., 1979).

4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION AND TRANSFORMATION

4.1 Transport and distribution between media

4.1.1 Transport and distribution in air, water and soil

Near point sources such as coal-fired power-generating stations,
cement plants using pyrite and some metal smelting operations, the
major source of thallium in air is emission of fly-ash (section
5.1.1). Although data on the forms of thallium in these emissions are
limited, results of one study indicate that nearly all of the thallium
in fly dust from a cement plant (in Lengerich, Germany) was present as
soluble thallium(I) chloride (LIS, 1980). No data on the amounts or
forms of thallium transported from soil into air during the dry season
were identified.

Assuming that 4 × 10¹² kg of crustal rocks weather each year,
Bowen (1979) calculated that 2.4 × 10⁶ kg thallium/year become
available to plants. However, as Smith & Carson (1977) have noted,
thallium tends to be retained during rock weathering, and
concentrations in soils tend to be somewhat enriched in soils compared
to the original bedrock.

The fate of thallium added to soil in deposited fly-ash probably
depends greatly on soil type (Crössmann, 1984). Data from Smith &
Carson (1977) suggest that retention should be greatest in soils that
contain large amounts of clay, organic matter and iron/manganese
oxides. According to McCool (1933) significant amounts of thallium
can be removed from solution in soils by ion exchange. Thallium can
also be incorporated into stable humus complexes (Crössmann, 1984),
which are resistant to rapid "wash-out" (Schoer, 1984).

Results of studies in several areas indicate that thallium
deposited from the atmosphere tends to accumulate in the surface
layers of soils (Smith & Carson, 1977; Heinrichs & Mayer, 1977; LIS,
1980). For example, after prolonged emissions from a cement plant in
Germany (LIS, 1980), thallium was found to remain in the upper levels
of soil (Schoer, 1984); material from depths 0-10, 40-50 and 60-70 cm
contained 4.2, 1.3 and 0.1 mg thallium/kg, respectively (Schoer,
1984). Retention processes will, however, be less effective in acidic
soil. For example, results of studies by Heinrichs & Mayer (1977)
indicate that about 40% of the thallium deposited from the atmosphere
onto relatively uncontaminated acidic (pH = 3.9-4.3) forest soil in
Germany was leached from the top 50 cm to lower soil horizons.
Elevated concentrations of thallium in groundwater (up to 40 µg/litre)
and in an irrigation canal (up to 96 µg/litre) in China, near a site
where waste materials from the mining of mercuric ore and coal
containing 25 to 106 mg thallium/kg were deposited (Zhou & Liu, 1985),
indicate that under some circumstances significant amounts of thallium
can move from soil into local water.

Although there is little information on the forms of thallium in
natural water, most dissolved thallium in fresh water is expected to
be present as the monovalent Tl⁺ ion (Smith & Carson, 1977). In
strongly oxidizing fresh water and in most seawater (Sager & Tölg,
1984), however, trivalent thallium is probably the predominant
dissolved form. Both forms of thallium can be removed from solution
by exchange and complexing reactions with suspended solid phases.
Trivalent thallium is also susceptible to reduction and precipitation
processes. According to Cotton & Wilkinson (1988), trivalent thallium
is extensively hydrolysed to form the colloidal oxide over the pH
range of natural water. Depending upon the relative kinetics of
reduction and hydrolysis, precipitation of thallium(III) hydroxide may
be an effective mechanism for removing thallium from solution. When
thallium(III) (precipitated as the oxide or hydroxide) settles into
organic-rich anaerobic sediment, it will be reduced to the monovalent
form, which can in turn be fixed in the sediment by reaction with
sulfide to form insoluble Tl₂S (US EPA, 1978). Thallium is thus
relatively depleted in seawater where thallium(III) predominates and
can be enriched in sediments where organic matter accumulates under
undisturbed, anaerobic conditions (Smith & Carson, 1977).

The partitioning of thallium among the water, sediment and biotic
compartments of aquatic systems has rarely been investigated. In one
study, however, in which thallium (100 µg/litre as thallium(I)
nitrate) was added to a 7-litre glass aquarium containing washed sea
sand, goldfish and submergent aquatic angiosperms, thallium was
distributed among all of the compartments. Concentrations in water
decreased gradually, while those in the fish and vegetation increased,
throughout the 9-day duration of the experiment, indicating that
thallium was being exchanged among these media (Wallwork-Barber et
al., 1985). Concentrations in the sand increased rapidly to a
relatively low value (0.05 mg thallium/kg), and remained relatively
stable thereafter, suggesting that there was little exchange between
the sediment and the other compartments. The limited accumulation of
thallium in the sediment was attributed in part to the short duration
of the study and to the absence of organic matter and clay in the
sand.

4.1.2 Soil-vegetation transfer

4.1.2.1 Factors affecting soil-vegetation transfer

In general, the solubility of thallium compounds governs the
availability of the metal to vegetation (discussed in detail by
Cataldo & Wildung, 1978). Crössmann (1984) mentioned that so far no
method had been developed to quantify the amount of thallium in soil
that is easily available for plants. However, Schoer & Nagel (1980)
emphasized the good correlation between soil-vegetation transfer and
the concentration determined following ammonium acetate extraction
from soil. Other authors favour an EDTA/ammonium acetate extraction
(Scholl & Metzger, 1982).

Transfer is influenced by various factors, e.g., pH (section
5.1.3.2) and the type of the contaminated soil. Green rape, bush
beans and rye grass were found to take up less thallium from weakly
acidic soil (pH 6.2) than from more acidic soil (pH 5.6), and thallium
supplied by cement factory dust was more available to plants than
thallium in soil (Makridis & Amberger, 1989a). Rape plants grown on
two samples of soil from a contaminated area, one sample (A)
containing a 3-fold higher concentration of thallium than the other,
showed identical concentrations of thallium, while other vegetables
grown on sample A even showed a lower thallium content. It was
concluded, that plant availability cannot be correlated to total soil
thallium content as determined after extraction with concentrated
nitric acid (Hoffmann et al., 1982). Only 4.4% (± 2.7%) of the
thallium content of soil from a lead-zinc mining waste material area
was available to vegetation, compared to 17.5% (± 10.7%) in soil from
a cement plant area (Schoer & Nagel, 1980). In a similar study with
soil from a cement plant and with stream sediments from a mining
district (Wiesloch, Germany), rape plants took up about 20% of soil
thallium from the cement plant sample but only 1.4 to 5.1% from the
stream sediments, although the latter contained 2- to 3-fold higher
thallium concentrations; 8- to 80-fold higher concentrations of
plant-available thallium were calculated for the soil from the cement
plant (Scholl & Metzger, 1982). Comparing the uptake of thallium by
rape seedlings from soil contaminated by emissions from a cement plant
(mainly with thallium(I) chloride or iodide) with that from
uncontaminated soil (traces of thallium(I) sulfide), a 7.5-fold higher
uptake from the contaminated soil was found (Lehn & Bopp, 1987).

At lower thallium concentrations, some plant species took up a
higher percentage of the available thallium than at higher
concentrations, perhaps in part because of the stronger toxic effects
at higher concentrations. However, the total amount of thallium found
in the plants and the thallium content of the artificial soil
solutions were correlated, reaching up to 1000 mg/kg dry weight in
green kale following one week's exposure to a concentration of
10 mg/litre (Schweiger & Hoffmann, 1983).

The transfer from soil to plant also depends on a number of
factors relating to the plant, e.g., root system, kinetics of membrane
transport, metabolism of thallium (Cataldo & Wildung, 1978), so that
the total amount of thallium taken up is species-specific (section
5.1.4.2). This is shown by the bioconcentration factor (concentration
of thallium in the plant (fresh or dry weight) in relation to its
concentration in dry soil) found for different plants grown in soil
contaminated by mining waste materials or collected from sites with
naturally high concentrations (Table 6) (Schoer & Nagel, 1980; Lehn &
Bopp, 1987). Calculations based on the concentrations in plant ash

and dry soil show that the concentration factor is usually less than
20 (Smith & Carson, 1977). The concentrations of thallium in
vegetables reported by these authors are one to two orders of
magnitude higher than those found by Geilmann et al. (1960) in
vegetation grown on uncontaminated soil (Schoer & Nagel, 1980)
(sections 5.1.4.1 and 5.1.4.2). Trees can be a long-term reservoir of
thallium. As a result of emission by cement plants, the bark and