Soil Cd Contamination

The release of Cd into the environment constitutes a significant pollution problem. The release of Cd from anthropogenic activities is estimated to be about 4,000 to 13,000 tons per year, with major contributions from mining activities, and burning of fossil fuels (ATSDR 1999). The Cd-yellow oil colours used by landscape painters, including Claude Monet (Fig. 21.1a) is just one of the many valuable uses of Cd, contrasting with the pernicious effects that Cd can cause in plants (Fig. 21.1b). Other important applications of Cd are in metallurgical industry and in the manufacture of nickel-cadmium batteries,

Pollution Claude Monet

Fig. 21.1 Cadmium, abeautiful toxic colour. (a) Poplars in the sun by Claude Monet, 1891 (www.monet-on-canvas.com/prod197.htm) , showing the powerful use of

Cd yellow pigments and (b) a lettuce leaf reflecting the toxic effects of Cd exposure (arrow: Cd-induced necrosis)

Fig. 21.1 Cadmium, abeautiful toxic colour. (a) Poplars in the sun by Claude Monet, 1891 (www.monet-on-canvas.com/prod197.htm) , showing the powerful use of

Cd yellow pigments and (b) a lettuce leaf reflecting the toxic effects of Cd exposure (arrow: Cd-induced necrosis)

pigments, plastic stabilizers and anti-corrosive products, phosphors for television sets, scintillation counters and X-ray screens, semiconductors and ceramic glazes (Robards and Worsfold 1991). As a consequence of this widespread and diverse usage, large quantities of Cd end up in sewage.

Treated sewage sludge ("biosolids") and phosphate fertilizers (He and Singh 1994a, b; McLaughlin et al. 2006; Singh and Agrawal 2007; Speir et al. 2003) are important sources of Cd contamination in agricultural soils. The usage of Cd in developed countries has, however, begun to decline because of its toxicity. For instance, Cd is one of the six substances banned by the European Union's Restriction on Hazardous Substances directive, which bans carcinogens in computers (2002/95/EC 2002).

Wagner (1993) estimated that non-polluted soil solutions contain Cd concentrations ranging from 0.04 to 0.32 mM. Cadmium concentrations in non-polluted soils are, however, highly variable, depending on sources of minerals and organic material. For instance, Eisler (1985) reported Cd concentrations of 0.01-1.00 mg/kg in soils of nonvolcanic origin and up to 4.50 mg/ kg in soils of volcanic origin. Soil solutions that have a Cd concentration varying from 0.32 to about 1 mM are considered as moderately polluted (Sanita di Toppi and Gabbrielli 1999). Topsoil concentrations are often more than twice as high as subsoil levels as the result of atmospheric fallout and contamination (Pierce et al. 1982) . Cadmium levels up to 800 mg/kg have been reported for soils in polluted areas (IARC 1993). Jung and Thornton (1996) have found Cd concentrations up to 40 mg/kg in surface soils taken from a mining area in Korea; and more recently, Cd contaminated river water (65-240 mg/l, 0.58-2.13 mM) downstream from a mining area in Bolivia has increased the soil concentration of Cd to 20 mg/kg and the concentration of Cd in soil solutions to 27 mg/l (0.24 mM) (Oporto et al. 2007).

Contamination of topsoil is likely the most important route for human exposure to Cd, mediated through uptake of soil Cd into edible plants (IARC 1993). Cadmium concentrations of 0.5 mg/kg or more have been found in rice grown in Cd-polluted areas of Japan (Nogawa et al. 1989) and China (Cai et al. 1990). Furthermore, in a recent field study in Europe performed by Peris et al. (2007) the Cd content in edible parts of vegetables such as lettuce was found to be above the maximum levels established by the Commission Regulation no. 466/2001 for horticultural crops (466/2001/EC 2001).

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