Selenium

Selenium is a non-metallic chemical element with the symbol Se and atomic number 34, belonging to Group 16 (the chalcogen group) of the periodic table, alongside oxygen, sulphur, and tellurium. It displays both metallic and non-metallic properties and occurs in several allotropic forms, including red, grey, and black varieties. Selenium is notable for its photoconductivity, semiconducting behaviour, and biological significance, which make it valuable in a wide range of everyday, industrial, and economic applications — from electronics and glassmaking to nutrition and environmental technologies.

Discovery and General Characteristics

Selenium was discovered in 1817 by the Swedish chemist Jöns Jacob Berzelius, who identified it as a by-product during the production of sulphuric acid from pyrite ores. The element was named after Selene, the Greek goddess of the Moon, due to its close resemblance to tellurium (named after Tellus, the Earth).
Key physical and chemical properties include:

• Atomic number: 34
• Atomic mass: 78.96 u
• Density: 4.82 g/cm³ (grey form)
• Melting point: 217°C
• Boiling point: 685°C
• Allotropes: Amorphous red, crystalline grey (most stable), and vitreous black
• Electrical behaviour: Semiconductor; electrical conductivity increases with light intensity (photoconductive)
• Oxidation states: −2, +4, +6 (common in compounds such as hydrogen selenide, selenium dioxide, and selenic acid)

Selenium is a trace element essential to life, required in small amounts for various biological functions but toxic at higher concentrations.

Occurrence and Extraction

Selenium is not found in elemental form in nature. It occurs mainly as an impurity in sulphide ores of metals such as copper, nickel, and lead, particularly in copper anode slimes formed during electrolytic refining.
The principal sources of selenium are:

• Copper refining residues (primary source)
• Dusts from lead and nickel smelting
• Flue dust from coal combustion (minor source)

Major producing countries include Japan, Canada, Germany, Russia, the United States, and China. Selenium is typically recovered as a by-product, making its production economically tied to the copper mining industry rather than to direct mining of selenium-bearing minerals.

Everyday Applications

Selenium’s unique properties make it useful in many products and processes encountered indirectly in daily life:

• Glass and ceramics: Selenium compounds are used to decolourise glass by neutralising the green tint caused by iron impurities and to produce red, pink, and amber tints in glass and enamel.
• Photocopiers and scanners: Amorphous selenium was widely used as a photoconductive layer in early xerographic copying and laser printers, where light-induced conductivity helped form printed images.
• Electronics: Selenium rectifiers, once common in electrical devices, converted alternating current (AC) to direct current (DC) before being largely replaced by silicon semiconductors.
• Health supplements: In trace amounts, selenium is an essential micronutrient required for the synthesis of antioxidant enzymes such as glutathione peroxidase. Selenium-enriched supplements and foods (e.g. nuts, cereals, and fish) contribute to dietary intake.
• Pigments: Selenium is used in plastic colourants, paints, and inks, producing stable red and maroon shades.

These applications illustrate selenium’s role in improving the functionality, aesthetics, and performance of products used in everyday environments.

Industrial Applications

Selenium’s versatility extends to numerous industrial sectors, particularly where its semiconducting, catalytic, and colouring properties are valuable:

• Glass manufacturing: One of selenium’s largest industrial uses. It is added to glass batches to counteract colour tints and provide consistent clarity in bottles, windows, and fibreglass.
• Metallurgy: Selenium improves the machinability of stainless steel and copper alloys by reducing brittleness and acting as a grain refiner.
• Solar and electronic devices: Selenium is used in photovoltaic cells and thin-film solar panels, especially in the compound copper indium gallium selenide (CIGS), which converts sunlight efficiently into electricity.
• Chemical catalysts: Selenium compounds act as catalysts in oxidation–reduction reactions in the manufacture of plastics, lubricants, and pharmaceuticals.
• Lubrication: Selenium disulphide (SeS₂) is used in anti-dandruff shampoos and industrial lubricants, offering both antimicrobial and friction-reducing properties.
• Battery technologies: Research is exploring selenium-based cathode materials as high-capacity alternatives to traditional lithium-ion battery components.

Medical and Biological Significance

Biologically, selenium is vital for both humans and animals. It is a cofactor in antioxidant enzymes, supports thyroid function, and contributes to immune regulation.
Key roles and applications include:

• Nutrition: Selenium deficiency can lead to disorders such as Keshan disease (a form of cardiomyopathy) and weakened immunity. Conversely, excessive intake may cause selenosis, characterised by fatigue and hair loss.
• Pharmaceuticals: Selenium compounds are studied for their anticancer properties, owing to their ability to protect cells from oxidative damage.
• Personal care products: Selenium disulphide is widely used in anti-dandruff shampoos and skin treatments for its antifungal properties.
• Medical diagnostics: Radioactive isotopes such as selenium-75 are used in diagnostic imaging of the pancreas and liver, as well as in metabolic studies.

Environmental and Energy Applications

Selenium plays a role in environmental monitoring and clean energy innovation:

• Solar energy: CIGS thin-film technology has positioned selenium as a key component in renewable energy production, contributing to sustainable electricity generation.
• Pollution control: Selenium-based compounds are used to treat industrial wastewater containing mercury and arsenic by facilitating precipitation and removal.
• Environmental monitoring: Selenium isotopes are used as tracers in soil and water studies to track pollutant movement.

However, selenium can become an environmental contaminant when present in excess, especially near mining or agricultural runoff sites, where it can bioaccumulate in wildlife. Careful management is therefore essential.

Economic Importance

Selenium’s global economic significance is rooted in its wide range of technological and industrial uses:

• Market demand: The largest consumers of selenium are the glass, metallurgical, and electronics industries.
• Price and supply: Selenium prices fluctuate with copper production, as its supply is directly linked to by-product recovery rather than dedicated mining.
• Trade and production: Annual global production averages 2,000–3,000 tonnes, with demand influenced by solar panel development, electronics manufacturing, and dietary supplement trends.
• Strategic material: Selenium’s role in clean energy and high-technology sectors makes it a strategically important element, particularly for nations investing in renewable technologies and electronics manufacturing.

Environmental and Safety Considerations

Selenium’s dual nature — essential at trace levels but toxic in excess — necessitates careful management in industrial and environmental contexts:

• Health risks: Inhalation or ingestion of large amounts of selenium or its compounds can cause poisoning, with symptoms such as nausea, fatigue, and nerve damage.
• Industrial handling: Workers in selenium-processing facilities require protection from dust and vapour exposure.
• Waste management: Regulations ensure that selenium-containing waste is properly treated to prevent leaching into soil or water.
• Environmental impact: Selenium pollution from mining or fossil fuel combustion can harm aquatic ecosystems by bioaccumulation, affecting fish and birds.

Thus, its use is subject to strict occupational and environmental standards to safeguard both human health and ecosystems.

Scientific and Technological Research

Current research continues to expand selenium’s technological potential:

• Nanotechnology: Selenium nanoparticles are being developed for drug delivery, antimicrobial coatings, and cancer therapies.
• Advanced semiconductors: Selenium-based materials are used in optoelectronic devices, including sensors and laser systems.
• Battery materials: Experimental selenium-sulphur composites are under study for high-efficiency energy storage.
• Agricultural studies: Selenium-enriched fertilisers are used to combat micronutrient deficiency in soils and crops, improving food quality.

These developments underscore selenium’s ongoing relevance in healthcare, materials science, and sustainable technologies.