Selenium, found in group 16 of the periodic table, follows a cycle in the environment as it transfers between the biosphere, atmosphere and hydrosphere. It is also an essential element for humans and animals. It can cause both deficiencies and toxic effects if not taken in the correct quantities. This essay outlines the general features and behaviour of Se in the primary environment, its general abundance and its ability to substitute into minerals. Its Eh and pH also are important factors in determining the form which the Se takes, altering its mobility.
This essay also covers the transition of selenium between mediums as it cycles through the environment. Once released through the effects of weathering or erosion, it enters the secondary environment where it interacts with plants animals and humans. Its bioavailability is important to determine the rate at which it can be removed from soils and water. This and its overall abundance in any given area can cause deficiency or toxicity. 1 Primary 1.1 Elemental state and properties Selenium has a proton number of 34 and mass number of 78.96, lying between As and Br on the periodic table and is a non-metal, discovered by Berzelius in 1817. It is a group 16 element, the same group as S. It can vary in colour from grey, purple and reddish and is opaque. It has a good cleavage at 0112o, a hardness of 2, a sub metallic lustre and a red streak.
When selenium forms a crystal, it is often coarse and well formed. When it is disseminated, it is found as small particles in a matrix. In its granular form, it forms anhedral to subhedral grains (Selenium mineral data, no date). Selenium can form six isotopes naturally; Se74, Se76, Se77, Se78, Se80 and the degradable Se79, which has a half-life of 327,000 years (Jörg et al., 2010). In 2016, the uses of Se are metallurgy, 40% of the total used; manufacturing glass, 25%; pigments and chemicals 10%; agriculture, 10%; electronics, 10% and other uses accounted for the remaining 5% (USGS National Minerals Information Center, 2016).
1.2 Abundance Selenium occurs naturally in the environment as selenide (Se2-) which forms in reducing conditions, selenite (SeO32-) forming in mildly oxidating conditions, selenate (SeO42-) forms in oxidating environment. In reducing conditions, Se is immobile and the oxygen content increases, so does the mobility of Se. Selenite is often bound by clays and organic material, whereas selenate is highly soluble and is a primary source of Se to plants (Selinus et al., 2013). Table 2. Abundance of Se isotopes (North Carolina State University, no date). Mass of atom (u) Abundance (%) 74 Se 73.922 0.89 76 Se 75.919 9.37 77 Se 76.92 7.63 78 Se 77.9 23.77 80 Se 79.9 49.61 82 Se 81.9 8.73
The most abundant isotope of Se found is Se-80, accounting for close to half of all Se isotopes. Se-74 has the lowest abundance amongst the stable isotopes (North Carolina State University, no date). The average crustal abundance of Se is 0.05ppm.
Black shales are enriched with selenium and contain up to 700ppm whilst also enriched in coal ash, volcanic emanations and some thermal waters. Also, Chinese stone coal contains 300ppm, phosphatic rocks 300ppm, phosphate rocks 300ppm,* (Selinus et al., 2013). As a result of these concentrations, Se is common in the geospheric cycle and can also, under the correct conditions, can be bioconcentrated. Some plants have the capability to bioaccumulate Se to a concentration above other trace elements. Selenium can then be transferred up the food chain (Kabata-Pendis, 2011).
Selenium mineral Chemical Formula Crookesite (Cu, Tl, Ag)2Se Clausthalite PbSe Berzelianite Cu2Se Tiemannite HgSe Elemental selenium Se Pyrite FeS2 Chalcopyrite CuFeS2 Pyrrhotite FeS Sphalerite ZnS Polymetallic sulphide ores Se-Hg-As-Sb-Ag-Cu-Zn-Cd-Pb Copper-pyrite ores Cu-Ni-Se-Ag-Co Gold-silver selenide deposits Au-Ag-Se From flemming (1980), Neal (1995), and Reimann and Caritat (1998).
1.3 Substitution Table 1. Content of Se in Au, Ag, Hg and Sb deposits (Geological Survey Bulletin, 11 12-A, 2010). Maximum Se content (%) Silver and gold deposits: Naumannite 30.2 Aguilarite 14.8 Argentite 0.001 Berzelianite 40 Eueairite 32.5 Umangite 45.1 Clausthalite 31.4 Klockmannite 55.4 Guanajuatite 34.33 Paraguanajuatite 36.2 Mercury deposits: Tiemannite 29.2 Metacinnabar 8.4 Cinnabar 0.0001 Antimony deposits Stibnite 0.0009
Table * shows the quantity of Se that can substitute into certain metal deposits. It is also possible for Se to replace carbon in calcite (Aurelio et al., 2008). Further to the minerals shown in the table above, sulphur containing minerals are a common candidate for Se substitution. Selenium and S are both group 16 elements yet they do vary in characteristics. Isomorphism is unrestricted between the two, allowing Se substitution to occur in 45 different minerals, many of them found in ore-forming minerals. Pure selenides are rarely found in the environment, requiring extreme physical and chemical conditions with little S present. Galena and bismuthine are two elements that are often found to contain Se. In these and other sulphide elements, Se does not exceed 20 wt. %. Goldfieldite contains up to 12 wt.% Se, skippenite contains 19-20 wt.% and kavasulite contains 12-15 wt.% Se (Nekrasov, 1996).
When conditions are endogenic, bivalent negative Se ions react with S to form sulphoselenides or selenides of metals such as Fe, Cu, Ni, Cd, Pb, Hg and others. Under hypergene conditions, unstable selenides will oxidise to poorly soluble selenites. As a result of this, Cu and Ag deposits are enriched in Se compounds up to a concentration where it is commercially important. Tellurium is another group 16 element that often substitutes into minerals in the place of S. It is then interchangeable with Se. Selenotellurium can form up to 30 wt. % of a mineral (Nekrasov, 1996).
1.4 Radioactivity The most common isotopes of Se are stable. Currently 24 unstable Se isotopes have been discovered (Audi et al., 2003). The half lives of these isotopes vary from 20ms to 295000 years as shown in table ****. They are primarily used for the production of industrial and medical bromine radioisotopes. Isotopes with a longer half life are used in the study of the long term effects of geological disposal of radioactive waste (Preedy, 2015).
Transition and release
Table 4. Influx of Se to the environment (****************************). Cycle Selenium flux (tonnes per year) Anthropogenic 76,000-88,000 Marine 38,250 Terrestrial 15,380 Atmospheric 15,300 Modified from Haygarth (1994) Essentials of medical geology A complex dispersion of Se occurs, resulting in an uneven distribution of Se across the globe. The largest source of Se is in the Earth’s crust. Selenium, with in an average crustal abundance of 0.05-0.09mg/kg is classed as a trace element.
Magmatic rocks are not likely to be found exceeding this concentration. However, volcanoes are a major source, contributing 0.1g for each cm2 on the earth’s surface. Selenium is found in large quantities in volcanic ash and gas, reaching 6-15mg/kg in volcanic soils on Hawaii Phosphate based fertilizers, sewage sludge and manure all are major sources of Se. To counteract the potential harm caused by implementing a 25mg/kg of Se in sewage sludge in the UK (Selinus et al., 2013).
Secondary Abundance in soils / water and distribution
As table ** shows, carbon shale China, Chinese stone coal and mudstone have the potential to hold the largest source of Se in the geosphere. In general, igneous rocks are low in selenium; volcanic igneous rocks are somewhat higher in Se content than other igneous rocks. Sedimentary rocks generally are higher in Se content than igenous rocks. Table *** shows an uneven balance of Se across the globe. Countries such as China, India and Sri Lanka have areas of Se deficiency. Selenium deficiency can also be caused by the conditions present rather than the total Se content.
A number of techniques are availble to determine the bioavailability of Se in soils. The most commone is the water-soluble concentration. (Fordyce et al., 2000; Jacobs, 1989; Tan 1989). Typically, between 0.3% and 7% of the Se present is in a dissolved form and therefore bioavailble (Lollar, 2005). According to WHO guidleines, the Se concentration in drinking water should not exceed 10µg/L. This standard varies however as the US EPA have set a standard of 50µg/L (Lollar, 2005). In most natural waters, the concentration is often less than 1µg/L and it is rare that it exceeds that standards above (Fordyce et al., 2000a; Vinceti et al., 2000).
However, in China, groundwaters have been shown to contain 1000µg/L. In lakes found in USA, China, Pakistan and Venezuela concentrations of 2000µg/L have been reported. Where waters contain 10-25µg/L, often it will emanate a garlic odour. Waters containing 100-200µg/L will have an unpleasant taste. Groundwaters will generally contain a higher concentration of Se than surface waters through the interaction with bedrock (Frankenberger and Benson, 1994; Jacobs, 1989). In precipitation, Se derives from volcanic sources, fossil fuel combustion, earth surface volatilization and incineration of waste.
The concentration of Se in water is typically between 0.04 µg/L and 1.4 µg/L (Hasimoto and Winchester, 1967)***. Selenate is weakly adsorbed by oxides and clays at a pH of 7. When selenite oxideses, it enhances its mobility in natural waters. Selenate is found in high concentrations of agricultural drainage waters in arid areas. Seleniferous soils can contain several hundred µg/L when formed above black shales. Toxicity can also be found in river water, particularly in semi-arid areas. The Soan-Sakesar Valley of Punjab, Pakistan, the average concentration of Se is 302 µg/L in springs and streams and 297-2100 µg/L in lake water. The highest concentrations can be found in low lying, salinized areas (Lollar, 2005).
Table 5. Species of plant that Se accumulates within. Type Examples (genus, family or species) Primary accumulator G. Astragalus (eg., milk vetch G. Machaeranthera (woody aster) G. Haplopappus (North and South American goldenweed) G. Stanleya (Prince’s Plume) G. Morinda (rubiaceous trees and shrubs, Asia/Australia) F. Lecythidaceae (South American trees) Sp. Neptunia (Legume Asia/Australia)
Secondary accumulator G. Aster G. Astragalus G. Atriplex (saltbrush) G. Castilleja (North and South American perennials) G. Grindelia (gummy herbs of western North and Central America) G. Gutierrezia (perennial herbs of western North and South America) G. Machaeranthera G. Mentzelia (bristly herbs of western America) Sp. Brassica (mustard, cabbage, broccoli, cauliflower)
Non-accumulator Sp. Pascopyrum (wheat grass) Sp. Poasecunda (blue grass) Sp. Xylorhiza (woody aster) Sp. Trifolium (clover) Sp. Buchloe (buffalo grass) Sp. Bouteloua (North and South American tuft grass) Sp. Beta (sugar beet) Sp. Horedeum (barley) Sp. Triticum (wheat) Sp. Avena (oats) From rosenfield and beath (1964, Jacobs (1989) and Neal (1995).
An increase in the concentration of PO43- results in a dilution of Se in plants by increasing plant growth. (Selenus, 2013) Jacobs, 1989; mayland 1994; Neal 1995. Total Se content in soils is not always indicative of Se toxicity. In the United States, soils above Cretaceous bedrock in the mid-West contained 1-10mg/kg Se with a 60% bioavailability in an alkaline and semi-arid environment. This combination caused signs of Se toxicity. In contrast, soils in Hawaii contain up to 20mg/kg Se yet a lower bioavailabilty prevented any signs of toxicity.
Selenium is not an essential element in plants yet Se atoms become integral to plant structure. Plants classified as Se accumulator plants are well adapted to surviving in Se rich environments, capable of absorbing over 1000mg/kg. Secondary Se absorbers concentrate 50-100mg/kg and the third group commonly accumulates less than 50mg/kg. The total range is 0.005mg/kg in crops deficient in Se and 5,500mg/kg in exceptional cases although most cases less than 10mg/kg of Se is present.
