Selenium is a naturally occurring trace element found throughout the environment in various forms, both inorganic and organic. While essential in small amounts for many organisms, higher concentrations can be toxic. Its global distribution is not uniform, influenced by geological and environmental processes.
Selenium in Earth’s Crust and Minerals
Selenium is widely distributed within the Earth’s crust, though typically in minute quantities. Its average abundance is estimated to be around 0.09 parts per million (ppm). While rare in its pure elemental form, selenium is most commonly found as an impurity. Due to its chemical similarity to sulfur, selenium often replaces it in various metal sulfide ores, such as pyrite, chalcopyrite, and bornite.
Igneous rocks, particularly those containing sulfide minerals, tend to have higher concentrations of selenium. Sedimentary rocks, including shales, sandstones, and limestones, also contain selenium, with shales often exhibiting the highest concentrations, sometimes reaching up to 277 ppm in black shales. Specific minerals where selenium is a major component, such as selenides like clausthalite or eucairite, are generally uncommon.
Selenium in Soil and Water
Selenium is released into soil and water through natural processes like the weathering of rocks and minerals. Its concentration in soil can vary significantly, generally ranging from 0.01 to 2.0 milligrams per kilogram (mg/kg), though seleniferous soils can contain over 5 mg/kg. This variability is influenced by local geology, parent material, and climatic factors, with arid regions often having higher content due to less leaching.
Soil pH is a significant factor in determining selenium’s mobility and availability. In alkaline soils, selenium forms soluble selenates, making it more available for uptake and prone to leaching into water bodies. In acidic soils, selenium forms selenites or elemental selenium, which are less available and less mobile. Organic matter content and redox potential also play a role in selenium’s speciation and mobility. Selenium can enter water bodies through runoff and leaching from soils, with concentrations varying widely, from 0.06 to 400 micrograms per liter in some natural waters.
Selenium in Living Organisms
Selenium is naturally incorporated into biological systems. Plants absorb selenium from the soil through their roots, primarily as selenate or selenite. The uptake and translocation of selenium in plants depend on the plant species, the form and concentration of selenium in the soil, and soil properties like pH. Some plant species, known as “selenium accumulators” or “hyperaccumulators,” can concentrate exceptionally high levels of the element in their tissues, sometimes thousands of times more than typical plants, without experiencing toxicity.
Selenium then moves up the food chain as animals consume these plants or other animals that have accumulated the element. In living organisms, selenium is often found in organic forms, such as the amino acids selenomethionine and selenocysteine, which are incorporated into proteins. Microorganisms also play a part in the cycling of selenium compounds, transforming them between different forms in the environment.