Cadmium (atomic number 48; relative atomic mass 112.40 g/mol) is a metal that belongs, together with zinc and mercury, to group IIB in the Periodic Table. It is a relatively rare element and is not found in the pure state in nature. Cadmium is mainly associated with the sulfide ores of zinc, lead, and copper, although purification first took place in 1817 from zinc carbonate. Commercial production only became significant at the beginning of this century. Cadmium is often considered as a metal of the 20th century; indeed, over 65% of the cumulative world production has taken place in the last two decades.
Cadmium has a relatively high vapour pressure. Its vapour is oxidized rapidly in air to produce cadmium oxide. When reactive gases or vapour, such as carbon dioxide, water vapour, sulfur dioxide, sulfur trioxide or hydrogen chloride are present, cadmium vapour reacts to produce cadmium carbonate, hydroxide, sulfite, sulfate or chloride, respectively. These compounds may be formed in stacks and emitted to the environment. Some cadmium compounds, such as cadmium sulfide, carbonate, and oxide, are practically insoluble in water.
There is, however, a lack of data on the solubility of these compounds in biological fluids, e.g., in the gastrointestinal tract and lung. These water-insoluble compounds can be changed to water-soluble salts in nature under the influence of oxygen and acids; cadmium sulfate, nitrate, and halides are water-soluble. Most of the cadmium found in mammals, birds, and fish is probably bound to protein molecules.
The speciation of cadmium in soil, plants, animal tissues, and foodstuffs may be of importance for the evaluation of the health hazards associated with areas of cadmium contamination or high cadmium intake. Very few data on the occurrence and speciation of cadmium compounds in nature are available. Cadmium is commonly regarded as a pollutant of worldwide concern. The metal has been reviewed by the International Register of Potentially Toxic Chemicals of the United Nations Environment Programme. As a result, it has been included on the list of chemical substances and processes considered to be potentially dangerous at the global level.
Using Area of Cadmium
Cadmium has a limited number of applications but within this range the metal is used in a large variety of consumer and industrial materials. The principal applications of cadmium fall into five categories: protective plating on steel; stabilizers for poly-vinyl chloride (PVC); pigments in plastics and glasses; electrode material in nickel-cadmium batteries; and as a component of various alloys.
Detailed consumption statistics are only available for a limited number of countries but from these it is apparent that the pattern of use can vary considerably from country to country. Examination of the reported trends in cadmium consumption over the last 25 years reveals considerable changes in the relative importance of the major applications. The use of cadmium for electroplating represents the most striking decrease in 1960 this sector accounted for over half the cadmium consumed worldwide, but in 1985 its share was less than 25%. In contrast, the use of cadmium in batteries has shown considerable growth in recent years from only 8% of the total market in 1970 to 37% by 1985.
The use of cadmium in batteries is particularly important in Japan and represented over 75% of the total consumption in l985 . Of the remaining applications of cadmium, pigments and stabilizers are the most important, accounting for 22% and 12%, respectively, of the world total in 1985. The share of the market by cadmium pigments remained relatively stable between 1970 and l985 but the use of the metal in stabilizers during this period showed a considerable decline, largely as a result of economic factors. The use of cadmium as a constituent of alloys is relatively small and has also declined in importance in recent years, accounting for about 4% of total cadmium use in l985.
Sources of Cadmium
Cadmium is a relatively rare element and current analytical procedures indicate much lower concentrations of the metal in environmental media than did previous measurements. At present, it is not possible to determine whether human activities have caused a historic increase in cadmium levels in the polar ice caps. Cd is widely distributed in the earth’s crust at an average concentration of about 0.1 mg/kg. However, higher levels may accumulate in sedimentary rocks, and marine phosphates often contain about 15 mg Cd /kg.
Weathering also results in the riverine transport of large quantities of Cd to the world’s oceans and this represents a major flux of the global Cd cycle; an annual gross input of 15 000 tonnes has recently been estimated. Volcanic activity is a major natural source of cadmium release to the atmosphere. Emissions of Cd take place both during episodic eruptions and continuous low-level activity. Difficulties exist in quantifying the global flux from this source but an estimate of 100-500 tonnes has been made.
Deep sea volcanism is also a source of environmental Cd release, but the role of this process in the global Cd cycle remains to be quantified. Ice and snow deposits from the polar regions represent a unique historical record of pollutants in atmospheric precipitation. However, the problems of contamination are great and no reliable data are at present available from historic samples; this prevents an insight into temporal changes in the cycling of Cd . Nevertheless, current ice samples have been analysed; those from the Arctic contain on average 5 pg/g, while corresponding values from the Antarctic (0.3 pg/g) are much lower.
Commercial Cd production started at the beginning of this century. The pattern of Cd consumption has changed in recent years with significant decreases in electroplating and increases in batteries and specialized electronic uses. Most of the major uses of cadmium employ cadmium in the form of compounds that are present at low concentration; these features constrain the recycling of cadmium. Restrictions on certain uses of cadmium imposed by a few countries may have widespread impact on these applications. Cd is released to the air, land, and water by human activities.
In general, the two major sources of contamination are the production and consumption of Cd and other non-ferrous metals and the disposal of wastes containing Cd . Areas in the vicinity of non-ferrous mines and smelters often show pronounced cadmium contamination. Increases in soil Cd content result in an increase in the uptake of cadmium by plants; the pathway of human exposure from agricultural crops is thus susceptible to increases in soil cadmium. The uptake by plants from soil is greater at low soil pH. Processes that acidify soil (e.g., acid rain) may therefore increase the average cadmium concentrations in foodstuffs.
The application of phosphate fertilizers and atmospheric deposition are significant sources of Cd input to arable soils in some parts of the world; sewage sludge can also be an important source at the local level. These sources may, in the future, cause enhanced soil and hence crop Cd levels, which in turn may lead to increases in dietary Cd exposure. In certain areas, there is evidence of increasing Cd content in food. Edible free-living food organisms such as shellfish, crustaceans, and fungi are natural accumulators of Cd .
There are increased levels of Cd in the liver and kidney of horses and some feral terrestrial animals, as in the case of humans. Regular consumption of these items can result in increased exposure. Certain marine vertebrates contain markedly elevated renal Cd concentrations, which, although considered to be of natural origin, have been linked to signs of kidney damage in the organisms concerned.
Effects on Humans
Exposure to Cd produces a wide variety of effects involving many organs and systems. From the point of view of preventive medicine, the detection of early effects on the kidneys is of particular importance in order to prevent more serious renal effects and those on the lungs or bones. Recent studies have been indicated that chronic exposure to Cd may give rise to cancer.
Acute Cd poisoning and, in some cases, death have been reported among workers shortly after exposure to fumes when cadmium metal or Cd -containing materials have been heated to high temperatures. Food contamination arose when acid foods and drinks were prepared and stored in contact with Cd -plated surfaces. Rapid onset with severe nausea, vomiting, and abdominal pain were characteristic symptoms.
Effects also occurred following the consumption of drinks with a Cd concentration of approximately 16 mg/litre from an automatic vending machine in which drinking-water was cooled in a tank constructed with Cd -containing solder. Lower Cd concentrations with longer periods of exposure than those described above will cause chronic cadmium poisoning. Fully developed poisoning among industrial workers shows two main effects: renal dysfunction and emphysema. The kidney is most frequently the critical organ, but under certain conditions (short-term peak exposures) it may be the lung. For people in the general environment, exposure is usually by the oral route and the kidney is the critical organ.
The available data show that Cd can affect calcium, phosphorous, and bone metabolism in both industrial workers and people exposed in the general environment. These effects may be secondary to the Cd effects on the kidneys but there have been few studies of calcium metabolism in people with excess exposure to Cd . The increased prevalence of renal stones reported from certain industries is probably one manifestation of the cadmium-induced kidney effects. It is not known if factors other than Cd play a role.
The International Agency for Research on Cancer and the US National Toxicology Program has both concluded that there is adequate evidence that Cd is a human carcinogen. This designation as a human carcinogen was prompted primarily by repeated findings of an association between occupational Cd exposure and lung cancer, as well as very strong rodent data, which included the pulmonary system as a target site.
Thus, the lung is the most definitively established site of human carcinogenesis from Cd exposure. In some studies, occupational or environmental cadmium exposure has also been associated with development of cancers of the prostate, kidney, liver, hematopoietic system and stomach. Clearly, further epidemiological and experimental work is necessary to determine the target sites and nature of the carcinogenic risk from Cd exposure to humans.
Removal of excesses of Cd ions from wastewaters or plasma is essential due to their extreme toxicity towards aquatic life and humans. Heavy metal ions from wastewaters are commonly removed by chemical precipitation, ion-exchange, and reverse osmosis processes. These methods have several disadvantages such as unpredictable metal ions removal, high reagent requirements, and the generation of toxic sludges which are often difficult to dewater and also require extreme caution in their disposal. These disadvantages can become pronounced at low metal concentrations in contaminated groundwaters, mine tailings effluent and other industrial wastewaters.
There is a need for innovative treatment technologies for the removal of heavy metal ions from wastewater. Besides these methods, affinity chromatography is another method which is used for removal of Cd . This method can be adjusted high selectivity to Cd ions and also it can be more effective in low concentrations compared to the other methods. In our study, we used affinity chromatography medthod and chose specific matrix for Cd2+ ions because of advanatages of affinity chromatography mentioned above.
"Read more article about Science"