 Occurrence
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| Selenium occurs naturally in a number of inorganic forms, including selenide, selenate and selenite. In soils, selenium most
often occurs in soluble forms like selenate (analogous to sulfate), which are leached into rivers very easily by runoff.
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| Selenium has a biological role, and is found in organic compounds such as dimethyl selenide, selenomethionine and selenocysteine.
In these compounds selenium plays an analogous role to sulfur.
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| Selenium is most commonly produced from selenide in many sulfide ores, such as those of copper, silver, or lead. It is obtained as a byproduct of the processing of these ores, from the anode mud of copper refineries and the mud from the lead chambers of sulfuric acid plants. These muds can be processed by a number of means to obtain free selenium.
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| Natural sources of selenium include certain selenium-rich soils, and selenium that has been bioconcentrated by certain toxic
plants such as locoweed. Anthropogenic sources of selenium include coal burning and the mining and smelting of sulfide ores
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