Selenium

• 1. Introduction

  ›

  Selenium sits at atomic number 34 on the periodic table, but it refuses to behave like a single, predictable substance. Depending on how it cools, it can appear as a brick-red powder, a glossy black solid, or a grey metallic-looking material that conducts electricity only when light falls on it. That last property, discovered in 1873, set off a chain of inventions stretching from the photophone to the solar cell to the flat-panel X-ray detector. The element was first identified in 1817 by the Swedish chemist Jons Jacob Berzelius, who thought he had found tellurium before realizing he had stumbled onto something new. He named it after the Moon. How did a byproduct of copper refining end up inside shampoo bottles, glass factories, and hospital imaging systems? And why is something essential to human health toxic enough to cause hair loss and nail discoloration at slightly higher doses? Those questions run through the whole story of selenium.

• 2. A Moon-Named Discovery

  ›

  Jons Jacob Berzelius and Johan Gottlieb Gahn owned a chemistry plant near Gripsholm, Sweden, where they produced sulfuric acid by the lead chamber process. Pyrite samples from the Falun Mine left a red solid precipitate inside the lead chambers, which everyone assumed was an arsenic compound. The smell changed that assumption. When the red precipitate was burned, it gave off an odor like horseradish. That smell was not typical of arsenic, but chemists recognized it from tellurium compounds. Berzelius wrote to the chemist Alexander Marcet in a first letter saying the residue was a tellurium compound. Only after studying the Falun Mine minerals more carefully and finding no tellurium there did Berzelius reanalyze the precipitate. In 1818 he wrote a second letter to Marcet announcing a new element that resembled both sulfur and tellurium. Because tellurium had been named for the Earth (tellus in Latin), Berzelius named the new element for the Moon, from the Greek selene. That pairing of Moon and Earth in the names of two neighboring elements on the periodic table was deliberate and poetic. The red precipitate from a Swedish acid plant had given chemistry a new member of the chalcogen family.

• 3. Light, Electricity, and Early Tech

  ›

  Willoughby Smith noticed in 1873 that the electrical conductivity of grey selenium changed when light struck it. That observation unlocked a decade of rapid invention. Werner Siemens developed the first commercial products to use selenium in the mid-1870s. Then, in 1879, Alexander Graham Bell put a selenium cell at the heart of his photophone, a device that transmitted speech by varying a beam of light. Selenium transmits electric current in proportion to the light falling on its surface, making it a natural candidate for light meters and related sensing devices. The development of selenium rectifiers began in the early 1930s, and they replaced copper oxide rectifiers because they were more efficient. Those selenium rectifiers stayed in commercial service until the 1970s, when silicon devices took over. In photography, selenium toners intensify and extend the tonal range of black-and-white prints while also improving their permanence, and the element was once the workhorse photoconductor inside plain-paper copiers before organic photoconductors began displacing it in the 1980s. One holdout remains: in DC power surge protection, the superior energy capabilities of selenium suppressors still make them preferable to metal-oxide varistors.

• 4. The Many Faces of an Element

  ›

  Black selenium is the form most often sold commercially, shaped into beads by rapid melting. Its structure consists of polymeric rings that can hold up to 1,000 atoms per ring, making it a brittle, lustrous solid that softens at 50 degrees Celsius and converts to grey selenium at 180 degrees Celsius. Grey selenium is the most stable and dense form, built from helical polymeric chains in a chiral hexagonal crystal lattice; unlike every other allotrope, it is insoluble in carbon disulfide and resists attack by nonoxidizing acids. The red forms, labelled alpha, beta, and gamma, are produced by varying the evaporation rate of a solvent, usually carbon disulfide, and each contains puckered eight-membered rings where the average selenium-selenium bond distance is 233.5 picometres. Selenium has seven naturally occurring isotopes, five of them stable. The most abundant is selenium-80, which makes up 49.6% of natural selenium. The isotope selenium-82 is also naturally occurring but radioactive, with a half-life of 8.76 times ten to the nineteenth power years. A synthetic isotope, selenium-75, is used as a gamma source in industrial radiography, with a half-life of 119.78 days.

• 5. From Copper Refineries to Glass

  ›

  Selenium rarely turns up as a pure ore. It spends most of its geological life substituting for sulfur inside metal sulfide deposits, and it reaches industrial quantities only as a byproduct. The anode mud left over from electrolytic copper refining is the richest commercial source. A standard industrial route begins by oxidizing that residue with sodium carbonate to produce selenium dioxide, then acidifying with water to form selenous acid, and finally bubbling sulfur dioxide through the acid to precipitate elemental selenium. About 2,000 tonnes were produced worldwide in 2011, with Germany contributing 650 tonnes and Japan 630 tonnes. In that same year the price reached 65 US dollars per pound, up from a relatively stable 30 dollars per pound during 2004-2010. The largest single use, accounting for roughly half of all consumption, is glassmaking. Selenium compounds give glass a red color that cancels out the green or yellow tints introduced by iron impurities, and various selenite and selenate salts are added for that purpose. Metallurgy and agriculture each claimed significant shares of 2010 consumption, and China was already the dominant consumer at 1,500-2,000 tonnes per year.

• 6. Solar Cells and X-Ray Detectors

  ›

  William Grylls Adams and his student Richard Evans Day demonstrated the first solid-state solar cell in 1876, using selenium as the photoabsorbing layer. Only a few years later, Charles Fritts fabricated the first thin-film solar cell, also using selenium. Silicon's rise in the 1950s pushed selenium solar research aside, and the efficiency record of 5.0% set by Tokio Nakada and Akio Kunioka in 1985 stood for more than 30 years without a challenger. In 2017, researchers from IBM achieved a new record of 6.5% by redesigning the device structure. Then in 2024, the first selenium-based tandem solar cell was demonstrated, pairing a selenium top cell with a silicon bottom cell in a single integrated device. On a separate track, amorphous selenium thin films became the sensing material inside flat-panel X-ray detectors. A typical device is about 0.2 millimetres thick, and roughly 98% of the electrons and holes produced by incoming X-rays are collected at the electrodes without being trapped. Films up to 1 millimetre thick can be deposited at a rate of 1-5 micrometres per minute, and their lack of grain boundaries improves image quality. Selenium was chosen partly because it is less toxic than compound semiconductors containing arsenic or heavy metals such as mercury or lead.

• 7. Essential Yet Toxic

  ›

  Jane Pinsent, a biochemist, found the first hints of specific biological functions of selenium in microorganisms in 1954. Three years later it was shown to be essential for mammalian life. By the 1970s it had been identified in two independent sets of enzymes, and the discovery of selenocysteine in proteins followed. During the 1980s, selenocysteine was shown to be encoded by the codon UGA, and the recoding mechanism was worked out first in bacteria and then in mammals. In humans, selenium is estimated to total 13-20 milligrams in the body, where it acts as a cofactor for antioxidant enzymes including the glutathione peroxidase family, which removes reactive oxygen species such as hydrogen peroxide. The thyroid gland depends on selenium as a cofactor for three of the four known types of thyroid hormone deiodinase. The US Recommended Dietary Allowance for teenagers and adults is 55 micrograms per day, and Brazil nuts are the richest dietary source, though their selenium content depends on the soil where the trees grew. The gap between beneficial and harmful doses is narrow. Elemental selenium and selenium salts are toxic even in small doses, and selenosis produces diarrhea, fatigue, hair loss, joint pain, nail brittleness or discoloration, nausea, headache, tingling, vomiting, and fever, roughly in that order of frequency. In June 2015, the US Food and Drug Administration published a final rule establishing minimum and maximum selenium levels in infant formula.

• 8. Pollution and Aquatic Harm

  ›

  At Belews Lake in North Carolina, wastewater discharged from a Duke Energy coal-fired power plant between 1974 and 1986 carried selenium concentrations of 150-200 micrograms per liter and eliminated 19 fish species from the lake. At the Kesterson National Wildlife Refuge in California, thousands of fish and waterbirds were poisoned by selenium that drained from agricultural irrigation. Selenium enters waterways through coal burning, mining, smelting, crude oil processing, and farming runoff that leaches natural selenate compounds through dry soils into rivers. It bioaccumulates: organoselenium compounds can be concentrated more than 200,000 times by zooplankton when water concentrations sit in the 0.5-0.8 microgram per liter range. Phytoplankton concentrates inorganic selenium by a factor of 3,000. Reproductive harm sets in at low thresholds; mallard duck reproduction is impaired at dietary concentrations of 7 micrograms per liter, and regulators consider 2 micrograms per liter in water highly hazardous to sensitive fish and aquatic birds. Fish affected by selenium may develop swollen gill lamellae, liver degeneration, heart swelling, damaged egg follicles, cataracts, and malformed offspring. Adult fish can appear healthy while being entirely unable to produce viable young, a detail that makes selenium poisoning particularly difficult to detect before a population has already collapsed.