| Cadmium, the name of which derives from Latin cadmia, Greek kadmeia, was discovered in Germany in 1817 by Friedrich Stromeyer. At that time C.H. Roloff, a district doctor, was auditing district drugstores in Magdeburg vicinity when he found a specimen of zinc oxide which looked like it contained arsenic. It had been proved when the reaction between hydrogen sulphide and acid solution of zinc oxide yielded a yellow precipitation. Strohmeyer roasted the zinc oxide specimen; it turned from gray to yellow. In Schonebeck factory it was explained that the color change was caused by iron impurities. Nevertheless Strohmeyer was not satisfied by this interpretation. After thorough analysis she found a new metal, which was easily separated from zinc by hydrogen sulphide. |
Cadmium crustal abundance is 1.6x10-5%, which is close enough to abundance of antimony 2x10-5% and is twice of that of mercury 8x10-6%. Cadmium migrates in underground hot waters with zinc and other elements which are apt to form natural sulphides. It is concentrated in hydrothermal deposits. Volcanic rocks contain 0.2 mg Cadmium per kg. Clays are most cadmium abundant in sedimentary rocks containing up to 0.3 mg/kg of cadmium; limestones and sandstones are less rich - 0.03 mg/kg. Average cadmium soil abundance is 0.06 mg/kg.
Cadmium forms its own minerals: greenockite (Cadmium blend) CdS, otavite CdCO3, monteponite CdO; however, these minerals do not have their own deposits. Zinc ores are the only commercially significance cadmium source which contain 0.01-5% of cadmium. This element is accumulated also in galena (up to 0.02%), chalcopyrite (up to 0.12%), pyrite (up to 0.02%), and stannite (tin pyrite 0.2%). The worldwide Cadmium reserves are estimated to be 20 million tons, among which 600 thousand tons of commercially important.
Plants contain 10-4% (per dry matter) cadmium; some animals such as sponges, coelenterates, worms, echinoderms and tunicates, - 4x10-5-3x10-3. All vertebrate animals collect Cadmium, especially in liver.
Physiological significance of cadmium is not completely clear; however it has been known that cadmium is important for carbohydrate metabolism, hippuric acid synthesis in liver and enzymatic activity. |