Environmental Sources of Cadmium

Most cadmium is produced as a by-product of zinc smelting, since the two metals usually occur together. Some environmental contamination by cadmium often occurs in the areas surrounding zinc, lead, and copper smelters. As is the case for the other heavy metals, burning coal introduces cadmium into the environment. The disposal by incineration of waste materials that contain cadmium is also an important source of the metal to the environment.

A major use of cadmium is as an electrode in rechargeable nicad (nickel-cadmium) batteries used in calculators and similar devices. When current is drawn from the battery, the solid elemental metal cadmium electrode partially disintegrates to form insoluble cadmium hydroxide, Cd(OH)2, by incorporating hydroxide ions from the medium into which it dips. When the battery is being recharged, the solid hydroxide, which was deposited on the metal electrode, is converted back to cadmium metal:

Each nicad battery contains about 5 g of cadmium, much of which is volatilized and released into the environment if the spent batteries are incinerated in garbage. The metallic cadmium preferentially condenses on the smallest particles in the incinerator smoke stream, which are precisely the ones that are difficult to capture by pollution-control devices inserted in the gas stack. In order to avoid releasing airborne cadmium into the environment upon combustion, some municipalities require nicad batteries to be separated from other garbage. The recycling of metals from such batteries has also begun in some areas. However, the European Union has banned the use of nicad batteries, except in cordless power tools and systems used for safety and medical purposes. Some U.S. states have banned the disposal of nicad batteries. Battery manufacturers hope to replace nicad batteries soon with ones that do not contain cadmium.

In ionic form, the main use of cadmium is as a pigment. Because the color of cadmium sulfide, CdS, depends on the size of the particles, cadmium pigments of many hues can be prepared. Both CdS and CdSe have been used extensively to color plastics. For several centuries, painters have used cadmium sulfide pigments in paints to produce brilliant yellow colors and thus oppose any ban on them, since at present there are no suitable replacements.

Van Gogh could not have painted his famous Sunflowers canvas without cadmium yellows, although it is speculated that cadmium poisoning may have contributed to the painter's anguished mental state.

Cadmium is released into the environment during the incineration of plastics and other materials that contain it as a pigment or as a stabilizer. It is also released into the atmosphere when cadmium-plated steel is recycled, since the element is fairly volatile when heated (its boiling point is 765°C).

Continue reading here: Human Intake of Cadmium

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