IT'S ELEMENTAL! behavior of thallium make for a fertile field for research but provide a minefield in any attempts to adequately assess its risks in the environment. Thallium elicits some of the most comJEROME 0. NRIAGU, UNIVERSITY OF MICHIGAN, ANN ARBOR plex and serious toxicities in living things, involving a wide range of organs and tisO MANY PEOPLE, THALLIUM IS mental chemistry and toxicity of thallium. sues. A recent study with planktonic comsynonymous with rat poison. It is Thallium is an enigmatic element to munities showed thatTl3+ ions, a common more toxic to mammals than study because of its highly divergent propform in aquatic environments, mercury, cadmium, and lead and erties predicated upon its oxidaare about 34,000 times more has been responsible for many tion status. Its paradoxical nature toxic than cadmium ions [Envideliberate, accidental, occupational, and became obvious after its discovron. Sci. Technol., 37, 2720 therapeutic poisonings of people since its ery as attempts were made to (2003)}. This is serious, considdiscovery in 1861. For the past 30 years or place the new element in the peering that organisms (especially so, I have been fascinated by the risks asriodic classification of the elemammals) at the top ofthe food sociated with the ongoing release of this ments. The physical properties chain are the most susceptible highly toxic and unpredictable metal into of elemental thallium, including to thallium toxicosis. Recently, our environment. specific gravity, hardness, apCELEBRATING we have found elevated levels of pearance, melting point, and elecC&EN'S Public angst and concern were first 80TH thallium in Great Lakes fish trical conductivity, were found to drawn in the late 1960s to reports ofwideANNIVERSARY [Bull. Environ. Contam. Toxicol, be similar to those of lead. The spread contamination of the Great Lakes' 67,921 (2001)}, which has raised chemical properties of manyTl(I) ecosystems with toxic metals. At the time, the issue of bioaccumulation and biomagI was a graduate student at the University (thallous) salts resemble those of lead, but nification of this element in aquatic food the valency and other features of thallium of Toronto. Of the various groundbreakwebs and the potential risks to fishing studies on the hazards of heavy < eating members of the food chain. I metals in the Great Lakes basin, I was =; keep wondering whether the dismissal captivated by reports that alluded to I of the early reports of thallium poithe fact that symptoms typical of thal£ soningofbald eagles and other wildlife lium poisoning were being observed | should, in fact, be regarded as a major in many wildlife populations in the 1 scientific oversight of our time. Great Lakes basin, and especially one I Comparedwithmanyheavymetals, report that claimed that nine out of thallium has a short history and what the 34 bald eagles found sick or dying appears to be a rosy future. Its tradiin 1971-72 in parts of the basin in the tional uses (in rodenticides and insecU.S. were poisoned by thallium. For ticides, pigments, wood preservatives, some inexplicable reasons, these earand ore separation; in mercury lamps to ly reports on poisoning of wildlife by increase the intensity and spectrum of thallium were ignored by the scientifthe light; as catalysts in chemical synic community and no efforts were theses; and so forth) are being phased made to conduct necessary follow-up CHECKUP Scan of a normal human heart out in deference to its toxicity. At the studies. Nevertheless, they aroused my after injection of radioactive thallium-201. The same time, there is increasing demand curiosity and served as my initiation test can reveal defects in blood supply. for thallium in the high-technology and into the literature on the environfuture-technology fields. were found to be too divergent from the
THALLIUM
T
History: Discovered in 1861 by Sir William Crookes. Occurrence: Thallium can be obtained as a by-product of producing sulfuric acid or refining zinc or lead. Appearance: Silvery white metal. Behavior: Thallium compounds are extremely toxic. Uses: Tl is used in fiber and low-melting glass. Many traditional uses have been phased out because of its toxicity.
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lead family of elements. On the other hand,Tl(I) resembles the alkali metals in flame spectra; solubility of the hydroxide, sulfate, and carbonate in water; ready oxidation of the metal in air; existence of thallium alums; and isomorphism of some of its salts with those of potassium, cesium, and rubidium. But the absence of isomorphism and divergent properties of many of its common salts precluded thallium from the family of alkali metals. In his periodic classification of the elements, Dmitry Mendeleyev offered the clinching argument in favor of placing thallium among the aluminum group of elements based on the chemical properties of thallic salts. Variations in chemical personality and nonconforming
Among the growing uses for thallium are in the semiconductor and laser industry, in fiber (optical) glass, in scintillographic imaging, in superconductivity, and as a molecular probe to emulate the biological function of alkali-metal ions. Recycling of thallium in commercial products is not yet a serious business, and one has to be concerned about the long-term environmental effects of the growing technological applications of one of the most toxic and eccentric metals known to humankind. Jerome 0. Nriagu is aprofessor in the School of Public Health at the University of Michigan, Ann Arbor. He is the editor of "Thallium in the Environment" as well as many other volumes on heavy metals in the environment. C&EN / SEPTEMBER 8, 2003
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