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CADMIUM

ROBERT L. WOLKE, WASHINGTON POST

T
he word "cadmium" evokes different associations in the minds of different people. I'll consider only two. The proverbial man in the street may think of rechargeable nickel-cadmium batteries. Me? I think of how cadmium got my name into the "Guinness Book of World Records."

Let's start with batteries. Every Ni-Cd battery bears the following words in fine print: "Warning: Do not dispose of in fire." Okay, so we mustn't throw them into the fireplace. But we remain haunted by the thought that our discarded batteries might wind up in an incinerator, causing unknown--but undoubtedly dire--consequences. The hoard of spent Ni-Cd batteries in the possession of people who are afraid to throw them away must be enormous.

Being sealed containers of moist chemicals, all batteries may of course explode when heated. But in addition, cadmium and its compounds are quite toxic, and an incinerated Ni-Cd battery could introduce cadmium into the environment.

The Environmental Protection Agency sets a maximum contaminant level for cadmium in drinking water of 5 ppb. And according to the U.S. Geological Survey's Minerals Information statistics for 2001, about 75% of the U.S. apparent consumption of cadmium, or a total of 2.4 million lb, went into Ni-Cd batteries. No one knows how much of this winds up in our air and water.

As consumers, we must therefore agonize about what to do with our worn-out Ni-Cd batteries, which contain about 4 g of Cd per AA size, other than throwing them in the trash and feeling guilty.

According to EPA's Toxics Release Inventory, industrial sources released 2,292 lb of cadmium into the air in 2002. To that amount, I hereby confess that I may have added 8 g in the form of the two AA batteries that I imprudently discarded. This, in spite of the fact that I live only 40 miles from Ellwood City, Pa., where the sole cadmium recycler in the U.S., the International Metals Reclamation Co., is located. Forgive me, but it was raining, and I just didn't feel like making the drive.

National efforts are under way toward the goal of making it more practical for consumers to recycle their Ni-Cd batteries--even for those who live outside of Pennsylvania.

Now on to the "Guinness Book of World Records." Cadmium consists of eight naturally occurring isotopes, including cadmium-113. Although 113Cd constitutes 12.22% of the cadmium atoms in nature, it is radioactive. Cadmium, therefore, is a radioactive element.

According to the systematics of nuclear stability, no two odd-A isobars (nuclei with the same odd mass number) having adjacent atomic numbers can both be stable; one of the pair must be unstable to -decay. In 1970, there was only one unresolved exception to this rule: 113Cd and 113In, atomic numbers 48 and 49, both of which appeared to be stable.

As soon as their atomic masses had been determined accurately enough to indicate that 113Cd must be the unstable one by an energy excess of some 300 keV, my graduate students and I decided to undertake an all-out search for radioactivity in 113Cd [J. Inorg. Nucl. Chem., 32, 2113 (1970)].

We obtained a small quantity of separated-isotope 113CdO from Oak Ridge National Laboratory, electrodeposited the metal, purified it by distillation, and counted it continuously for over 300 hours in a shielded, super-low-level radiation detector. We were able to observe the disintegration of about two atoms per hour, corresponding to a half-life of (9.3 6 1.9) X 1015 years. (I proudly point out that we nailed down that half-life within an experimental error of less than 2 million billion years!)

With a half-life of 500,000 times the age of the universe, cadmium hardly constitutes a radiation hazard. But our discovery was recorded in the 1979 "Guinness Book of World Records" as the longest lived -emitter at the time.

The record has since been broken by 50V, with a half-life of about 4 X 1017 years. This scientific fact is ignored by today's Guinness people, however, in favor of such Earth-shaking intelligence as the speed record for opening 300 bottles of beer. (One minute, 47 seconds, if you must know, by a team of three Germans.)

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Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh and a syndicated food columnist for the Washington Post. His latest book, "What Einstein Told His Cook: Kitchen Science Explained," was a James Beard Foundation nominee for the best food reference book of 2002.




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CADMIUM AT A GLANCE
Name: From the Latin cadmia, calamine (cadmium carbonite).
Atomic mass: 112.41.
History: Discovered in 1817 by German chemist Fredrich Stromeyer.
Occurrence: Obtained mostly as by-product of smelting of zinc ore, in which a cadmium-containing mineral is a significant impurity.
Appearance: Silvery metal; soft enough to cut with a knife.
Behavior: Not volatile, but toxic in humans. Poisoning rarely occurs because very little can be absorbed. Linked to kidney failure and high blood pressure.
Uses: Electroplating steel, in rechargeable (Ni-Cd) batteries, in alloys, and in some nuclear reactor control rods. Cadmium sulfide is a yellow powder that is used as a pigment. Other cadmium compounds are used in the phosphors of black and white television sets and in the blue and green phosphors in color television sets.

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