Distribution and management of seleniferous soils

Selenium is one of the few elements absorbed by plants in sufficient amounts that can be toxic to livestock. Volcanic activity, weathering, precipitation of minerals and burning of fossil fuel control the distribution of Se in the environment. In addition, anthropogenic activities such as disposal of fly ash, utilization of underground water for raising of crops and mining operations have contributed substantially to the redistribution and cycling of Se. Changes in topographical features and leaching/erosion processes have played an important role in the development of seleniferous soils in the different parts of the world. Areas of toxic, adequate and deficient Se levels exist side by side. Soils with elevated levels of Se are found in many countries including Australia, China, India, Ireland and USA. Soils containing >0.5 mg Se kg-1 are considered as seleniferous as the forages produced on such soils absorb Se more than the maximum permissible level for animal consumption. Selenium binding onto soils and sediments depends upon the pH, Eh, Se species, competing anions, hydrous oxides of iron and type of clay minerals. Selenium in contaminated soil and water exists mainly as highly mobile toxic inorganic species such as selenate (SeO42-, Se6+) and selenite (SeO32-, Se4+). During the last decade, a number of technologies have been proposed to get rid of excessive Se from the contaminated environments. These are based on either immobilization of Se to biologically unavailable forms or complete removal of Se through phytoextraction or biomethylation. It is the concentration of mobile Se, usually selenate, that determines the need for adopting a specific technology and the extent of change in the mobile concentration is considered as the measure of success. Potential of different remedial technologies leading to the permanent removal of Se oxyanions from the seleniferous soils has also been discussed. © 2003 Elsevier Science B.V. All rights reserved.