Selenium

Typical wastewaters

• coal power plant flue gas desulfurisation wastewater selenate SeO₄²⁻ with selenite in a high nitrate scrubber water; reduced to elemental Se(0) in anoxic bioreactors BAT limits 70 µg/L daily and 29 µg/L monthly; nitrate is consumed before selenium
• coal gasification wastewater and combustion residual leachate total selenium; gasification wastewater limited to 453 µg/L daily and 227 µg/L monthly a limit, not a measured range
• petroleum refinery effluent dissolved selenium; species not given by the source US EPA source list for the selenium MCL
• mine drainage (coal, phosphate and sulfide mining) dissolved selenium; selenate is favoured where the water is oxic and alkaline, selenite at near neutral and acid pH the US EPA lists mines as a source; no mine water concentration was read
• agricultural drainage from seleniferous soils selenate in high sulfate drainage water, with organic selenium (selenomethionine, dimethylselenide) the case behind the nanofiltration study cited by WHO
• copper refinery anode slime processing selenium in the anode slime processing effluent; species not given by the source
• textile wet processing total selenium, a sample and report only ZDHC parameter; sludge reporting limit 5 mg/kg dry weight

1 · Identity

Symbol, number

Se, 34

Oxidation states in water

+6 selenate, SeO₄²⁻, the soluble, mobile form of alkaline oxidising water; +4 selenite, H₂SeO₃, HSeO₃⁻ and SeO₃²⁻, the form that adsorbs to iron and aluminium oxides; 0 elemental selenium, insoluble, the product of chemical and biological reduction; -2 selenides and hydrogen selenide in strongly reducing sediments; organic selenium (selenomethionine, dimethylselenide) in biota and drainage water.

Note

The element entry covers anode slimes, glass and photovoltaics; this chapter is selenite against selenate.

2 · Occurrence in water

Natural sources

Crustal selenium 50 to 90 µg/kg, higher in some volcanic, sedimentary and carbonate rocks; soils 5 to 1,200,000 µg/kg. Selenites and selenates are soluble, so selenium leaches from well aerated alkaline soils; elemental selenium and selenides are insoluble, so it is retained in wet, poorly aerated soils. Dissolved selenium rises at both high and low pH (WHO background document).

Anthropogenic sources

Flue gas desulfurisation wastewater, gasification wastewater and combustion residual leachate at coal power plants, regulated in 40 CFR 423.13; discharges from petroleum refineries and mines (EPA NPDWR); copper refinery anode slime processing (element entry); agricultural drainage from seleniferous soils, the case behind the nanofiltration study cited by WHO; coal, phosphate and sulfide mining (element entry, MWEI BREF); selenium is a textile sludge and report only wastewater parameter for ZDHC.

3 · Speciation

The common forms in water are selenite, Se(IV), and selenate, Se(VI); conversion of selenite to selenate is slow, both exist together, and neither is oxidised or reduced easily under treatment conditions (WHO citing Sorg and Logsdon 1978). Selenate is more soluble and much harder to remove by coagulation than selenite, so oxidising selenite would be a mistake. Acidic and reducing conditions reduce selenite to elemental selenium; alkaline and oxidising conditions favour selenate. Chlorine oxidises selenite to selenate only under harsh conditions and selenium is unlikely to react with ozone, chlorine dioxide or chloramines.

Solubility

Selenite and selenate salts of the common cations are soluble; elemental selenium and metal selenides are insoluble (WHO). No solubility products are printed in the sources read.

Hydrolysis

Selenous acid and selenic acid dissociate stepwise with pH; the sources read do not print the constants, so none are quoted. In the pH 6 to 9 range selenite is HSeO₃⁻ or SeO₃²⁻ and selenate is the divalent anion.

Complexation

Selenite sorbs on iron and aluminium oxide surfaces and on iron oxide coated sand (capacity about 1 mg/g); selenate sorbs weakly; nitrate interferes with selenium adsorption on soil (WHO). No constants quoted.

Precipitates

Elemental selenium from reduction; selenite carried on ferric hydroxide floc; metal selenides under sulfidic conditions.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Acidify for total selenium; filter 0.45 µm in the field for dissolved selenium. Interconversion of selenite and selenate is slow, so speciation samples are less fragile than arsenic, but hydride methods respond only to Se(IV) and undercount selenate unless the sample is pre-reduced. Hydride generation interferences from transition metals are real in FGD and mine water.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

Essential: about 25 selenoproteins; recommended intakes 26 to 35 µg/day for adults, upper tolerable limit 400 µg/day. Deficiency is Keshan disease and Kaschin-Beck disease; excess is selenosis (brittle hair and nails, skin lesions, nerve changes) at dietary intakes above about 900 µg/day, with symptoms in Chinese villages at about 5 mg/day. Selenite, selenate, selenocysteine and selenomethionine kill laboratory animals at single doses of 1.5 to 6 mg/kg. Not classifiable as a carcinogen; several selenium compounds are anticarcinogenic in animals (WHO).

Bioaccumulation

Selenium is taken up into proteins as selenocysteine and selenomethionine and moves through food webs; selenate, selenite and the seleno amino acids are teratogenic in birds and fish, the basis of the aquatic bird deformities in central California cited by WHO and of the fish tissue form of the US criterion.

Ecotoxicity

US EPA 2016 freshwater criterion: egg or ovary 15.1 mg/kg dry weight, whole body 8.5, muscle 11.3, water column 1.5 µg/L in lentic and 3.1 µg/L in lotic systems (30 day averages), with an intermittent exposure element; fish tissue governs except when selenium inputs are increasing. Saltwater criteria 290 µg/L acute and 71 µg/L chronic (1999).

Flags

• The WHO natural water ranges are US compilations from 1937 to 1968 and vary by region; no recent survey was read.
• The seawater figure (0.2 µg/L) is a single abundance figure from Jefferson Lab via PubChem.
• Dissociation constants of selenous and selenic acid are not printed in the sources read; none are quoted.
• The selenate and selenite reduction half reactions are electron balances written here; the EPA and WHO describe the reductions in words.
• The US FGD limits carry compliance dates and were changed again by the 2024 supplemental rule, which was not read in full.
• Abu Dhabi values cover two media (marine 0.02 mg/L, sewer 10 mg/L); other GCC states not read.
• EPA 200.8 detection limits for selenium are 1994 quadrupole figures without a collision cell.
• The WHO selenium document names a branded activated alumina in the 98 percent figure; the brand is omitted here.

Gaps

• No measured selenium concentrations in FGD wastewater, refinery effluent, mine drainage or municipal wastewater were read; only the regulated levels.
• No removal percentages for full scale biological FGD treatment beyond the limits it was designed to meet.
• Selenium speciation methods (HPLC-ICP-MS, hydride with and without pre-reduction) were not read as numbered standards.
• No solubility products, adsorption constants or acid dissociation constants are printed in the sources read.
• The 2024 steam electric supplemental rule and other GCC discharge standards were not read.
• Selenium in the EU EQS directive and in the AMR or pharma sector lists was not checked.
• The zero valent iron and ferrous hydroxide reductions are written as electron balances from the reagents and pH values WHO gives; neither source prints a stoichiometry, and no carbon source stoichiometry was read for the biological selenate reduction step.

Sources

Identity

Name and symbol

Selenium, Se

Atomic number

34 protons

Position

group 16 · period 4 · p-block · polyatomic nonmetal

CAS number

7782-49-2

Atomic structure

Atomic mass

78.971 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴
[Ar] 4s²³d¹⁰⁴p⁴

Electrons per shell

2, 8, 18, 6

Valence electrons

6 outer shell

Physical properties

State at room temperature

Solid

Melting point

493.65 K (220.5 °C)

Boiling point

958 K (684.85 °C)

Density

4.809 g/cm3

Appearance

grey metallic-looking, red, and vitreous black (not pictured) allotropes

Thermal conductivity

amorphous: 0.519 W/(m·K)

Electrical resistivity

not in sources

Electrical conductivity

not in sources

Crystal structure

hexagonal

Molar heat capacity

25.363 J/(mol·K)

Chemical properties

Oxidation states

+6, +4, -2

Electronegativity

2.55 (Pauling Scale)

Ionisation energy

9.752 eV
1st 941, 2nd 2,045, 3rd 2,973.7 kJ/mol

Electron affinity

2.021 eV

Atomic radius

empirical 120, covalent 120, van der Waals 190 pm

Ionic radius

Se²⁻ 198; Se⁴⁺ 50; Se⁶⁺ 42 pm

Reactivity

A group 16 non-metal below sulfur with several allotropes; grey selenium resists air and non-oxidising acids at room temperature but burns in oxygen and is oxidised by nitric acid, showing the -2, +4 and +6 states.

with water

Does not react with water.

with oxygen, air

Stable in air at room temperature; burns in oxygen with a blue flame to selenium dioxide: $\ce{Se + O2 -> SeO2}$.

with acids

Resists non-oxidising acids; nitric acid oxidises it to selenous acid: $\ce{3 Se + 4 HNO3 + H2O -> 3 H2SeO3 + 4 NO}$.

with halogens

Burns in fluorine to selenium hexafluoride: $\ce{Se + 3 F2 -> SeF6}$; with chlorine it gives SeCl4 and Se2Cl2.

Typical compounds

• SeO₂ selenium dioxide polymeric solid, dissolves to selenous acid
• H₂Se hydrogen selenide extremely toxic gas, intolerable at 1.5 ppm
• H₂SeO₃ selenous acid from the dioxide in water; selenite salts
• SeF₆ selenium hexafluoride toxic gas, more reactive than SF6
• ZnSe zinc selenide metal selenide semiconductor
• SeS₂ selenium disulfide anti-dandruff shampoo ingredient

Occurrence, production and use

Crustal abundance

5×10-2 milligrams per kilogram

Oceanic abundance

2×10-4 milligrams per liter

Occurrence and sources

• selenium in copper sulphide ores recovered from the anode slimes of electrolytic copper refineries; reserves 92,000 tonnes counted as the selenium content of copper reserves, largest in Russia, Peru, the United States, Canada and China
• crooksite, clausthalite and other rare selenium minerals minor; selenium was historically taken from copper smelter flue dust
• selenium in coal, and in lead, nickel and zinc ores coal generally contains significant selenium but recovery from fly ash is not economic; selenium is discharged from base metal, precious metal, industrial mineral, coal and uranium extraction

Extraction, production

Recovery from copper anode slimes

Anode slimes are roasted with soda ash or sulphuric acid, or smelted with soda and nitre, to release the selenium, which is then refined to 99.5 percent and higher; two United States refineries produced crude selenium and slimes in 2024 but refined none. The sources name the reagents but no reaction products, so no equation is written. World refinery production about 3,700 tonnes in 2024 (estimate), price about 24 dollars a kilogram in the United States and Europe.

Uses

Selenium's resistance to the flow of electricity is greatly affected by the amount of light shining on it. The brighter the light, the better selenium conducts electricity. This property has made selenium useful in devices that respond to the intensity of light, such as electric eyes, photo cells, light meters for cameras and copiers. Selenium can also produce electricity directly from sunlight and is used in solar cells. Selenium is also a semiconductor and is used in some types of solid-state electronics as well as in rectifiers, devices which convert alternating current electricity into direct current electricity. In addition to its use in electrical devices, selenium is also used to make a ruby-red color in glasses and enamels, as a photographic toner and as an additive to stainless steel.

Selenium forms few inorganic compounds, none of which are commercially important. They include selenious acid (H2SeO3), selenium dichloride (SeCl2) and selenium oxychloride (SeOCl2).

Selenium is used in Xerography for reproducing and copying documents, letters, etc. It is used by the glass industry to decolorize glass and to make ruby-colored glasses and enamels. It is also used as a photographic toner, and as an additive to stainless steel.

• Metallurgy: selenium dioxide additive to raise yields in electrolytic manganese metal production; free machining copper, lead and steel alloys; gun bluing and plating solutions metallurgy including electrolytic manganese 40 percent of global consumption in 2024 (USGS, world figure)
• Agriculture and food: dietary supplement for livestock and humans as an essential micronutrient; fertiliser additive to raise plant tolerance of stress agriculture and animal health 20 percent of global consumption in 2024 (USGS)
• Glass: decolourising the green iron tint of container and soda lime glass; red and bronze tinted architectural glass glass 20 percent of global consumption in 2024 (USGS)
• Electronics and photovoltaics: copper indium gallium diselenide thin film solar cells, photocells and rectifiers electronics and photovoltaics 10 percent of global consumption in 2024 (USGS)
• Chemicals: cadmium sulphoselenide red pigments for ceramics, paint and plastics; catalysts for selective oxidation, rubber vulcanising agents, antidandruff shampoo actives, blasting cap delays chemicals and pigments 5 percent of global consumption in 2024 (USGS)
• Mining: selenium is listed among substances discharged from base metal, precious metal, industrial mineral, coal and uranium extraction (MWEI BREF)

Safety, toxicity

Hydrogen selenide at a concentration of 1.5 ppm is intolerable to man. Selenium occurs in some solid in amounts sufficient to produce serious effects on animals feeding on plants, such as locoweed, grown in such soils. Exposure to selenium compounds (as Se) in air should not exceed 0.2 mg/m3 (8-hour time-weighted average - 40-hour week).

GHS classification, signal word Danger

• H301 Toxic if swallowed Acute toxicity, oral
• H331 Toxic if inhaled Acute toxicity, inhalation
• H413 May cause long lasting harmful effects to aquatic life to the aquatic environment, long-term hazard
• H373 May causes damage to organs through prolonged or repeated exposure Specific target organ toxicity, repeated exposure
• H370 Causes damage to organs Specific target organ toxicity, single exposure
• H372 Causes damage to organs through prolonged or repeated exposure Specific target organ toxicity, repeated exposure
• H361 Suspected of damaging fertility or the unborn child Reproductive toxicity

Discovery and name

Discovered by

Jöns Jakob Berzelius and Johann Gottlieb Gahn

Discovered

1817

First isolated

not in sources

Named by

not in sources

Origin of the name

after Selene, Greek goddess of the moon

Selenium exists in several allotropic forms, although three are generally recognized. Selenium can be prepared with either an amorphous or a crystalline structure. The color of amorphous selenium is either red (in powder form) or black (in vitreous form). Crystalline monoclinic selenium is a deep red; crystalline hexagonal selenium, the most stable variety, is a metallic gray.

Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point, selenium is a p-type semiconductor and has many uses in electronic and solid-state applications.

Elemental selenium has been said to be practically nontoxic and is considered to be an essential trace element; however, hydrogen selenide and other selenium compounds are extremely toxic, and resemble arsenic in their physiological reactions.