When freshly exposed to air, Thallium exhibits a metallic lustre, but soon develops a blueish-grey tinge, resembling lead in appearance. A heavy oxide builds up on Thallium if left in air, and in the presence of water the hydroxide is formed. The metal is very soft and malleable. It can be cut with a knife.
Thallium is used for making low-melting point special glass for highly reflective lenses. Thallium salts are used as reagents in chemical research. Thallium sulphate is still sold in developing countries where it is still permitted as a pesticide, although banned in Western countries. Since its electrical conductivity changes with exposure to infrared light, it is used in photocells. It is used for sink-float separation of minerals. Thallium amalgam is used in thermometers for low temperature, because it freezes at -58 °C (pure Mercury freezes at -38 °C).
Thallium in the environment
Thallium is not a rare element; it is 10 times more abundant than Silver. The element is widely dispersed, mainly in Potassium minerals such as sylvite and pollucite. Thallium minerals are rare, but a few are known, such as crookesirte and lorandite. World production of Thallium compound is around 30 tonnes per year. There has been no assessment of how great the reserves are.
Thallium is partially water-soluble and consequentially it can spread with groundwater when soils contain large amounts of the component. Thallium can also spread by adsorption on sludge. There are indications that Thallium is fairly mobile within soils.
The element and its compounds are toxic and should be handled carefully.
Thallium occurs in the environment naturally
in small amounts. It is not applied very widely by humans, merely
as rat poison and as a substance in electro-technical and chemical
industries. These applications can cause human exposure to
There has been no significant contamination of the environment by Thallium from industry, unlike that caused by its neighbors in the periodic table, Mercury and lead.
Thallium is very toxic to rats and is applied as a rat pesticide because of this. Thallium also has negative effects upon plants, such as colour changes in leaves and growth declines. Mammals, such as rabbits, are just as susceptible to the toxic effects of Thallium as humans.