Natural Environmental Sources
Selenium, a metalloid with chemical similarities to sulfur, is widely distributed throughout the planet's lithosphere, hydrosphere, and atmosphere. However, it rarely occurs in its pure elemental form. The journey of selenium begins deep within the Earth's crust and is influenced by geology and environmental factors.
Earth's Crust and Mineral Deposits
Most of the world's selenium is found combined with metal sulfides in various ores, where it substitutes for sulfur. These deposits are the ultimate origin of the element, though they are not economical to mine directly for selenium alone. Commercially, selenium is recovered as a byproduct during the refining process of these metal ores, particularly copper. Key minerals containing selenium include berzelianite (copper selenide), clausthalite (lead selenide), and tiemannite (mercury selenide). Major deposits are found in areas with past volcanic activity and certain sedimentary rock formations.
Soil Distribution
Soil is one of the most critical reservoirs of selenium, directly influencing its concentration in plants and, subsequently, in the entire food chain. The level of selenium in soil is highly variable and depends on several factors:
- Parent material: The geological composition of the underlying rock plays a decisive role in determining the soil's initial selenium content. Soils derived from volcanic materials tend to be poor in selenium, while those from shale can be rich.
- Geographic location: Some regions are naturally high in selenium (seleniferous) and include parts of the U.S. (like the Great Plains), Canada, and China. Conversely, areas like New Zealand and much of Europe are known to have selenium-deficient soils.
- pH and redox potential: The availability of selenium to plants is heavily influenced by soil pH. In alkaline soils, selenium exists as highly soluble selenates ($SeO_4^{2-}$), making it more accessible for plant uptake. In acidic soils, it often exists as less available selenites ($SeO_3^{2-}$) that bind more tightly to iron hydroxides.
- Organic matter: Soil organic matter can absorb and fix selenium, affecting its movement and availability.
Water and Atmospheric Sources
Selenium is also found in both fresh and saltwater. In water, it primarily occurs as selenates and selenites. The concentration in surface water can be low, but groundwater in seleniferous areas can have higher levels. Human activities, such as agricultural runoff from irrigated seleniferous fields and industrial waste, can increase selenium levels in local water systems. In the atmosphere, natural processes like volcanic activity and anthropogenic sources such as the burning of fossil fuels release selenium into the air.
Dietary Sources: The Food Chain's Role
The amount of selenium in food is a direct result of its presence in the soil and water where the food was grown or raised.
Plant-Based Foods
Some plants are known to be significant accumulators of selenium, while others are not.
- Brazil Nuts: This is famously the richest dietary source of selenium, with a single nut potentially providing more than the recommended daily intake. However, the exact concentration can vary depending on the soil where the tree was grown.
- Grains and Cereals: For many people in countries with high-selenium soils, grains, cereals, and fortified breads and pastas are the main dietary contributors.
- Vegetables: The selenium content in vegetables varies widely based on soil levels. Garlic and members of the cabbage family can accumulate higher concentrations than other produce.
Animal-Based Foods
Animals that eat selenium-rich plants will accumulate the mineral in their tissues.
- Seafood: Fish and shellfish, particularly tuna, crab, and shrimp, are excellent sources of selenium.
- Meat and Poultry: Beef, pork, and chicken also contribute significantly to dietary selenium intake.
- Dairy and Eggs: Eggs, especially the yolks, and dairy products contain measurable amounts of selenium.
Industrial and Commercial Sources
Beyond its natural distribution, selenium is also procured and used in several industrial applications.
Production from Copper Refining
As mentioned, commercial selenium is primarily a byproduct of the electrolytic refining of copper. This process extracts the element from the anode sludge left behind.
Industrial Uses
- Glass Manufacturing: Selenium is widely used in the glass industry as a decolorizer to remove the green tint caused by iron impurities and to create red-colored glass.
- Electronics: It is utilized in photocells, rectifiers, and thin-film solar cells, including CIGS cells (containing copper, indium, gallium, and selenium).
- Pigments and Alloys: Selenium is used to produce pigments for paints and plastics, and as an additive in metal alloys to improve machinability.
Commercial Products
- Dietary Supplements: Selenium is sold as a standalone supplement and is a common ingredient in multivitamins. It often comes in inorganic forms like sodium selenite or organic forms like selenomethionine.
- Anti-dandruff Shampoos: Selenium sulfide is an active ingredient in some shampoos to treat seborrheic dermatitis.
Comparison of Selenium Sources
| Feature | Dietary Sources (Brazil Nuts) | Soil Conditions (Alkaline vs. Acidic) |
|---|---|---|
| Availability | Highly concentrated but geographically variable. | Available to plants, but depends heavily on soil chemistry. |
| Chemical Form | Organic form, primarily selenomethionine. | Inorganic forms, selenate (alkaline) and selenite (acidic). |
| Bioavailability | High bioavailability for human absorption. | Selenate is more bioavailable for plant uptake than selenite. |
| Health Impact | Can easily lead to toxicity if overconsumed. | Determines the health of entire food chains in a region. |
| Variability | High variability from nut to nut based on soil content. | Varies with pH, organic matter, and rainfall, leading to localized differences. |
Conclusion
Selenium's distribution is a complex interplay between geology, environmental chemistry, and biology. The answer to where is selenium usually found? is multi-faceted, encompassing its geological origins in the Earth's crust, its presence in soil and water, and its journey through the food chain to plants and animals. While humans can obtain selenium from a variety of food sources, the mineral's concentration in these foods is fundamentally tied to the soil's composition in the producing region. Moreover, industrial byproducts from metal refining and specific commercial products also represent significant sources. Ultimately, understanding its diverse origins is crucial for appreciating its role as both an essential nutrient and a potential environmental toxin. For more information on the environmental impact of selenium, see the U.S. Geological Survey.
