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Thallium [Tl, 81]
49.1 INTRODUCTION
Thallium (Tl), a soft gray metal of group 13 in the periodic table of elements, is
discolored when exposed to air. Its average content of the Earth’s crust has been estimated to be 0.7 mg/kg, within the range of 0.9–1.8 mg/kg. Its contents increase with
increasing acidity of igneous rocks and with increasing clay contents of sedimentary
rocks. Its amounts in coal average at about 1 mg/kg, but may be concentrated up to
about 50 mg/kg. It is also associated with K-based ores.
The oxidation states of Tl are +1, +2, and +3, the most important being +1. It has
lithophilic properties and is associated with several minerals, but in higher amounts
it is present mainly in lorandite (TlAsS 2 ) and avicennite (Tl 2 O 3 ). Other minerals,
such as crookesite [(Cu,Tl,Ag) 2 Se] and hutchinsonite [(Pb,Tl) 2 As 2 S 9 ], do not have
any mining potential. It is usually obtained as a by-product of the production of sulfuric
acid, or as a by-product of refining Zn and Pb ores.
The world production (excluding the United States) of Tl in 2010 was 10,000 kg
(USGS 2011). Thallium is used mainly in high-temperature superconductors. There
are no uses for metallic Tl, because it quickly combines with O 2 and H 2 O vapor
from the atmosphere, forming a black, powdery substance. Thallium metal and its
compounds are highly toxic and are strictly controlled to prevent a threat to humans
and the environment.
Thallium, used in conjunction with S, Se, and As, forms low melting glass. Its
sulfate (Tl 2 SO 4 ) is an odorless and tasteless compound, and has been used as a
rat and ant poison in the United States, since 1974, but it has been banned from
household use. Other thallium compounds, sulfide (Tl 2 S), iodide (TlI), and bromide (TlBr), are all used in devices to detect infrared radiation. Thallium previously used in pesticides and various poisons, which have been strictly forbidden
(ATSDR 2002b).
Radioisotope 201 Tl is used in medicine for scintigraphy and diagnosis of some
diseases, including myocardial disorders and various cancer detections.
49.2 SOILS
The worldwide range of Tl in soils is 0.0014–2.8  mg/kg, at an average value of
0.5  mg/kg. Its higher levels are in cambisols and histosols, which reveals its
association with both clay fraction and soluble organic matter (SOM). Its occurrences in uncontaminated soils of various countries are reported as follows
(in mg/kg): Canada, 0.17–0.22; China, 0.21–2.4; France, 1.54–55; Japan, 0.39–0.50;
Poland, 0.01–0.41; Sweden, <0.04–0.52; and the United States, 0.02–2.8 (KabataPendias and Mukherjee 2007). Background Tl levels of soils in China are given as
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