Selenium is a chemical element with symbol Se and atomic number 34. It is a nonmetal with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium. It rarely occurs in its elemental state or as pure ore compounds in the Earth’s crust. Selenium (Greek σελήνη selene meaning “Moon”) was discovered in 1817 by Jöns Jacob Berzelius, who noted the similarity of the new element to the previously discovered tellurium (named for the Earth).
Selenium is found in metal sulfide ores, where it partially replaces the sulfur. Most selenium in the environment comes from burning coal. Minerals that are pure selenide or selenate compounds are known but rare. lthough small amounts of selenium are used in products as diverse as high strength alloys and vulcanized rubber, the chief commercial uses for selenium today are glassmaking and pigments. Selenium is a semiconductor and is used in photocells. Applications in electronics, once important, have been mostly supplanted by silicon semiconductor devices. Selenium is still used in a few types of DC power surge protectors and one type of fluorescent quantum dot.
While selenium is an important trace nutrient, high concentrations of selenium salts are toxic.
Selenate is the fully oxidized oxo anion of selenium. It is a divalent anion and is well removed by strong base anion resins. However, throughput capacity is often limited by high concentrations of sulfate also common in wastewaters that contain selenate.
The only current commercial use of selenides is as metal selenide in quantum dots. Selenide is not commonly found in wastewaters unless that water is significantly reducing (low redox potential).
Selenite is the most common form of selenium found in wastewaters and is generally considered the easiest form to remove.