There is no evidence that cadmium (Cd) is biologically essential or beneﬁcial; on the contrary, it has been implicated as the cause of numerous human deaths and various deleterious effects in ﬁsh and wildlife. In sufﬁcient concentration, it is toxic to all forms of life, including microorganisms, higher plants, animals, and man. It is a relatively raremetal, usually present in small amounts in zinc ores, and is commercially obtained as an industrial by-product of the production of zinc, copper, and lead. Major uses of cadmium are in electroplating, in pigment production, and in the manufacture of plastic stabilizers and batteries. Anthropogenic sources of cadmium include smelter fumes and dusts, the products of incineration of cadmium-bearingmaterials and fossil fuels, fertilizers, and municipal wastewater and sludge discharges; concentrations are most likely highest in the localized regions of smelters or in urban industrialized areas (Hammons et al. 1978; U.S. Environmental Protection Agency [USEPA] 1980; Nriagu 1980, 1981; Hutton 1983b; Eisler 1985; Scheuhammer 1987; U.S. Public Health Service [USPHS] 1993; Cooke and Johnson 1996).Industrial consumption of cadmium in the United States, estimated at 6000 metric tons in 1968, is increasing; projected use in the year 2000 is about 14,000 tons, primarily for electroplating of motor parts and in the manufacture of batteries. The cadmium load in soils and terrestrial biota in other industrialized countries also appears to be increasing and is of great concern in Scandinavia(Tjell et al. 1983), Germany (Markard 1983), and the United Kingdom (Hutton 1983a).
Cadmium, as cadmium oxide, is obtained mainly as a by-product during the processing of zincbearing ores and also from the reﬁning of lead and copper from sulﬁde ores (USPHS 1993). In 1989, the United States produced 1.4 million kg of cadmium (usually 0.6 to 1.8 million kg) andimported an additional 2.7 million kg (usually 1.8 to 3.2 million kg). Cadmium is used mainly for the production of nickel–cadmium batteries (35%), in metal plating (30%), and for the manufacture of pigments (15%), plastics and synthetics (10%), and alloys and miscellaneous uses (10%) (USPHS 1993). Cadmium is a silver-white, blue-tinged, lustrous metal that melts at 321˚C and boils at 767˚C. Thisdivalent element has an atomic weight of 112.4, an atomic number of 48, and a density of 8.642 g/cm3. It is insoluble in water, although its chloride and sulfate salts are freely soluble (Windholz et al. 1976; USPHS 1993). The availability of cadmium to living organisms from their immediate physical and chemical environs depends on numerous factors, including adsorption and desorption rates of cadmiumfrom terrigenous materials, pH, Eh, chemical speciation, and many
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other modiﬁers. The few selected examples that follow demonstrate the complex behavior of cadmium in aquatic systems. Microbial extracellular polymeric substances (EPS) — ubiquitous features in aquatic environments — actively participate in binding dissolved overlying and pore-water metals insediments. Organic sediment coatings in the form of bacterial EPS equivalent to about 0.5% organic matter can adsorb cadmium under estuarine conditions (Schlekat et al. 1998). EPS aggregates rapidly sorbed up to 90% of cadmium from solution. Changes in pH affected cadmium sorption, with the proportion of freed Cd to sorbed Cd changing from 90% at pH 5 to 5% at pH 9; desorption was enhanced with increasingsalinity (Schlekat et al. 1998). Adsorption and desorption processes are likely to be major factors in controlling the concentration of cadmium in natural waters and tend to counteract changes in the concentration of cadmium ions in solution (Gardiner 1974). Adsorption and desorption rates of cadmium are rapid on mud solids and particles of clay, silica, humic material, and other naturally...