After reading this article you will learn about:- 1. Uses of Cadmium 2. Emissions of Cadmium 3. Biochemical Effects.
Uses of Cadmium:
Cadmium (Cd) is a divalent metal, chemically similar to Zinc and Mercury. Although it is widely distributed in the lithosphere, Cadmium is usually found at quite low concentrations in crystal rocks, the mean concentration being about 200-300 ppb.
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Sometime Cd concentrations as high as 100 ppm are found in phosphatic rocks (due to presence of fossilized fish teeth).
Cadmium may also be leached from sulfide ores of zinc, copper and lead. The world production of Cadmium is over 20,000 metric tons per year (2000), about 50% greater than the present rate of mercury production. The major producing countries are the US, Russia, Japan, Canada, Belgium, and France.
Cadmium is used in various industrial purposes viz.
(a) For electroplating of steel, iron, copper, brass and other alloys to prevent corrosion.
(b) For colouring pigment production which are used in fabrics, textiles and paints, etc.
(c) For uses as plastic stabilisers.
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(d) For manufacture of alloys,
(e) For manufacture of Ni-Cd batteries,
(f) For production of certain pesticides, luminescent dials, X-ray screens, etc.
Emissions of Cadmium to the Environment:
The estimates of Cd emissions to the environment in different countries vary widely. There are a number of sources of Cadmium emission.
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An estimate as measured in US during 1968 is presented in Table 25.6:
i. Toxicity:
The Cadmium ion (Cd2+) is believed to exert no toxic effect. In human system the efficiency of cadmium absorption by the intestines is only about 5-6%, but once ingested, Cadmium is transported to all parts of the body by the blood stream. Although almost all organs probably absorb some Cd, the highest concentrations are invariably found in the liver and kidneys.
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The toxic effects of cadmium are presumably associated with the metal’s affinity for organic ligands containing sulphur, nitrogen or other electronegative functional groups. The metal appears to be particularly attracted to enzyme containing Zinc.
The most serious effects of Cd poisoning usually involve damage to the kidney, particularly the renal tubes. These symptoms are associated with proteinurea, glucosuna and high alkaline phosphatase in the blood. Cd poisoning is also associated with bone softening, fatal lung damage, high rise of blood pressure and other heart diseases.
As such as per the US drinking water standard, Cd levels should not exceed 10 ppb. Chronic toxicity of Cd is evident at 30-60 ppb in fresh water fish, 5 to 50 ppb at marine fishes. In general, Cd toxicity can be partially reduced by use of Zn.
In 1955 two Japanese physicians reported the occurrence of a mysterious disease in the Jintsu basin of Japan near the city of Toyama.
The disease was characterised by severe pain in the back, joints and lower abdomen, development of a waddling or duck like gait, kidney lesions, proteinurea, glycosuria and loss of calcium from the bones leading in some cases to multiple bone fractures. They designated the disease as Initi-itai’ disease.
Initial efforts to identify the cause of the disease were unsuccessful, but they tried to link the same disease with some nutritional toxicity. However, in 1970s, it was possible to identify that the said disease was due to chronic Cd toxicity (EPA, 1972, 1976).
ii. Emission Control and Other Measures:
The technology for removing Cd from industrial wastewater or from flue dust is well-established. In wastewater, dissolved Cd can be precipitated with sodium sulphide, cemented by the addition of Zinc or separated out by ion exchange.
If the Cd is incorporated into particulates, the Cd can be dissolved by the addition of acid and then separated by one of the above techniques, or the solids can be settled out and the Cd removed with the sludge.
Cadmium released to the atmosphere via smoke stacks is primarily in the form of fine particulate materials, which can be separated from the stack gases by wet scrubbers, fabric filters or electrostatic precipitators (OECD, 1975).
Removal of Cd by any of the above techniques may lead to a solids disposal problem if Cd is not to be recovered or recycled. Dumping of Cd or Cd-containing compounds at sea or in fresh water is prohibited. Land disposal is the obvious solution but only in areas where the soil is neutral or basic so that the cadmium is not mobilised by percolating groundwater.
In many cases the use of Cd is substituted by other metallic compounds, viz.:
(a) Zinc could be substituted for Cadmium in many electroplating applications;
(b) The Edison Cell (iron/KOH/Nickel oxide) is the chief alternative to the Ni-Cd battery;
(c) Yellow and red pigments made from iron oxides could be substituted for Cadmium for some processes.
Biochemical Effects of Cadmium:
Cd occurs in nature in association with zinc minerals. Growing plants require Zn and they also take up and concentrate Cd with the same biochemical apparatus. The outbreak of Cd poisoning occurred in Japan in the form of itai itai “Ouch ouch” disease. Many people suffered from this disease in which their bones became fragile. At high levels, Cd causes kidney problems, anemia and bone marrow disorders.
The major portion of Cd ingested into our body is trapped in the kidneys and eliminated. A small fraction is bound most effectively by the body proteins, metallothionein, present in the kidneys, while the rest is stored in the body and gradually accumulates with age. When excessive amounts of Cd2+ are ingested, it replaces Zn2+ at key enzymatic sites, causing metabolic disorders.