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Selenium is the 34th element and is placed in the VIA (or 16th) group of periodic table following the order O, S, Se, Te, Po, and Lv, within the group. Chemical properties of Se are analogous to those of sulfur and it mainly shows following oxidation states selenide (−2), elemental selenium (0), selenite (+4), and selenate (+6). Se occurs in minerals, for example pyrites, where it replaces sulfur partially and also form compounds with other elements (Butterman and Brown 2004).

In nature, elemental selenium (Se⁰) exists in six stable isotopic forms, ⁷⁴Se, ⁷⁶Se, ⁷⁷Se, ⁷⁸Se, ⁸⁰Se, and ⁸²Se, whereas 24 unstable isotopes of Se has also been reported so far, with half‐life 20 ms to 295 000 years (Audi et al. 2003). The details of unstable isotopes, their atomic mass, half‐life, and mode of decay are reported in detail by Perrone et al. (2015). The isotopes ⁷⁸Se and ⁸⁰Se are more common due to their high natural abundance, i.e. 24 and 50%, respectively. Of the radio‐nucleotides, ⁷⁵Se (t_1/2 = 120 days), owing to its reasonably high half‐life value, is suitable for determination of biological traces (Irons et al. 2006), whereas ⁷⁵Se, ^77mSe, and ⁸¹Se may possibly be used for quantitative determination of Se through radiologic diagnostics (Li and Zheng 1990).

Se exists in allotropic forms either in amorphous form or any of the three crystalline forms viz., α‐monoclinic, β‐monoclinic, and hexagonal forms (National Research Council 1983). Amorphous Se is red in color (Jovari et al. 2003) and its viscosity is highly dependent on temperature. At 230 °C it is a free‐flowing liquid and on reducing the temperature up to 80 °C its viscosity increases and it forms polymeric chains. However, on further reducing the temperature there is a decrease in its viscosity and it forms ring‐shaped aggregates. It forms Se₈ rings and has deep red color in its monoclinic crystalline form. The shape of α‐monoclinic Se is flat hexagonal and polygonal crystals while it has needle like shape in β‐monoclinic crystalline form. Hexagonal crystalline form with spiral Se chains is the most stable form of Se and is gray in color. Monoclinic crystalline and amorphous forms are transformed into hexagonal form at temperature >110 and 70–210 °C, respectively. The variation of its physical properties with its allotropic form has been reviewed in detail by Crystal (1972) and Chizhikov and Shchastlivyi (1968).

Since lithosphere, hydrosphere, atmosphere, and biosphere are the integrated components of environment, as a result it is indispensable to understand the cycling of Se in soil, water, and air. During volcanic eruption, at high temperature Se and S vaporize to gaseous form. On cooling, Se condenses and forms a layer over ionized micro‐particulate of atmosphere that eventually precipitates with rainfall and appends to the rocks and/or enters to the water bodies. In several research papers the abundance/concentration of Se is reported in different terms that includes: weight by weight: microgram per gram (μg/g) or (mg/kg) is equivalent to parts per million (ppm); and weight by volume: microgram per liter (μg/l) = 1 ppb Se), here in this chapter μg/g and μg/l units are used throughout. In the earth's crust the average abundance of Se is ~0.09 μg/g (Lakin 1972) and barely found in its native state. Owing to the chemical resemblance of Se to its analogue S, it is widely distributed in environment as a major and minor constituent of most of the sulfide ores (Cooper et al. 1970) or as selenides of nickel (Ni), copper (Cu), silver (Ag), lead (Pb), and Mercury (Hg). Uranium ore contain highest (~600 μg/g) of Se content (Ralston et al. 2009). Rocks contain around 40% of the Se of the total of Earth crust (Wang and Gao 2001), values reported for igneous rocks (0.35 μg/g) (Fordyce 2005), sedimentary rocks (0.0881 μg/g) (Tamari et al. 1990), shales (0.24–277 μg/g) (Lakin and Davison 1967), phosphatic rocks (1.4–178 μg/g) (Robbins and Carter 1970), coal (1–5 μg/g) (Cooper et al. 1970), limestone (0.03–0.08 μg/g) (Fordyce 2005), and sandstone (0–112 μg/g) (Lakin and Davison 1967). Leaching from these Se‐rich sources can elevate the Se concentration in environment up to 1200 μg/g (Paikaray 2016). Rosenfeld and Beath (1964) have compiled the data of Se concentrations in rocks and seleniferous soils. These seleniferous rocks are the major source of Se in soil, ground water, and atmosphere.

The distribution of seleniferous rocks and deposits in geologic and topographic map indicate the areas of Se‐rich soil and water. Soil and water at distant places can become seleniferous when soluble and/or suspended Se species are imported to such areas through surface water. In the environment, concentration of Se varies from place to place; this uneven distribution of Se is mainly governed by the processes including weathering, interaction of rocks and water, and microbial activities. These processes control the transportation of Se from rocks to soil, water, and air. In soil the amount of Se is primarily influenced by parent material and possible leaching from rocks during soil formation. The confined environmental surroundings such as properties of soil, aeration, organic matter, pH, and microbial activity are also the responsible factors for Se distribution in soil. The average range and mean global concentration of Se in soil is 0.01–2 and 0.4 μg/g, respectively (Dungan et al. 2002). Further, the reactivity and bioavailability of Se, in addition to total concentration, also depends on different chemical forms available in soil and water. Here bioavailability means the part of substance that becomes soluble and accessible for adsorption by means of membrane (Reeder et al. 2006; Ruby et al. 1996) of living organisms.

The toxic, tolerable, and deficient areas of Se level exist alongside and for these different Se levels local environmental conditions are responsible. Depending upon the sampling done for the available Se concentration in vegetation and plants grown in the soil, seleniferous soil has been categorized as toxic, moderate, and low level of Se. Soil that provides an adequate amount of Se to make toxic plants is referred to as toxic seleniferous soil. Contrary to this, the soil may have high Se level as exhibited by toxic Se soil but provide less Se to the plants, known as non‐toxic seleniferous soil. From deficient to most‐seleniferous soil, the concentration of Se reported is 0.01 and 1200 μg/g (Fleming 1980; Jacobs 1989; Neal 1995). Many countries have elevated level of Se including USA (Presser 1994), India (Dhillon and Dhillon 2003), Ireland (Seby et al. 1997), and China (Wang and Gao 2001). Central region and Great Plains of North USA and Prairie region of Canada (Ihnat 1989) is formed from Cretaceous shale (2 μg/g) and exhibit relatively high concentration. In Australia, Ireland, and various other countries with toxic Se level, shales are the parent material (Johnson 1975). Florida, South Carolina, and Tennessee are ranging lower (0.8–9 μg/g) in Se due to phosphatic rocks of that region (Rader and Hill 1935). A low level of Se has been documented in Finland and New Zealand. Se content in Hawaiian and Japanese volcanic sulfur ranged from 1026 to 2000 and 67–206 μg/g, respectively (Lakin and Davison 1967). Many parts of Africa were recognized with low Se; however, in Asia both high and low Se concentrations have been reported (National Research Council 1983). Most of the parts of the world are characterized as moderate to low bioavailability as compared to high Se soil content. Among most studied Se‐contaminated water bodies, Kesterson Reservoir of San Joaquin Valley, California USA is one of them. The main Se source is Se‐rich marine sedimentary rocks (mean values = 8.9 μg/g) of the coastal range, which raise the Se content to the reservoir by weathering and other beneath mechanisms (Milne 1998; Presser and Piper 1998). Human and industrial activities are also responsible for the discharge of Se in rivers and lakes. Dhillon and Dhillon (2003) have compiled a comprehensive review on seleniferous soils.

Water applied for irrigation purpose to soil can mobilize the soluble Se species from upper soil surface and create a hydraulic gradient, and as a result Se is discharged to surface water. Apart from natural Se sources, this agriculture drain water and other anthropogenic activities can also discharge Se to water bodies and is available to aquatic life. The aquatic bodies have also played an important role in the transportation of Se from one place to other. The estimated value for the fluxes of Se is ~14 000 tons per year that are relocated from continents to oceans along aquatic pathway (Nriagu 1989, 1991). The mode of Se transfer is either via selenium‐bearing sediments (~85%) or through soluble species (Cutter and Bruland 1984). Leaching from soil to groundwater is another segment of hydrological transport of selenium species. However, contribution through leaching processes is not very clear because of the heterogeneity of the interfaces between water‐rocks, ‐soils and ‐sediment (Dhillon et al. 2008). Ihnat (1989) has reported that the average residence period of selenium in the deep ocean is ~1100 years, almost of the order of residence time of water. The soluble selenium species from industrial waste move downward through infiltration and elevate the Se level in ground water during monsoon season (Kumar and Riyazuddin 2011). Se concentration in water is ranged between 1 and 10 μg/l; due to the emission of geogenic and/or anthropogenic Se, this may exceed 100 μg/l occasionally in outstanding cases. In industrial effluents Se concentration is in 10–100 μg/l range, under rare conditions it can exceeds up to 1000 μg/l. Typical Se concentrations in ambient water are <1 μg/l in the dearth of direct Se sources. The existing Se concentration of marine waters is 0.02–0.04 μg/l. However, for fresh water the background Se concentration is considered to be comparable to marine water. Ghosh et al. (2008) reported that the elevated concentration of Se in soil (1100 μg/l) and water (3 μg/g) in localities of Mumbai and Punjab of India is due to the alkaline pH (pH 8–9.2) of soil in industrially polluted areas. The concentration of Se is high in the saturated water aquifers which have fluctuations in their water table. Se concentration in aquifers also depends on mixing ratio of discharge stream into receiving water. In addition, the probable interaction of discharge to Se removing sediments is relatively more important than its dispersion in aqueous phase. The water quality criterion for Se in water is primarily determined by ecotoxicological considerations for aquatic living organism, e.g. predatory fish and waterfowl, which are at the top of aquatic food chains. Countries like the US and Canada provide regulatory guidelines that include 5 and 1 μg/l (with varying guidelines for individual regions up to 100 μg/l), respectively (US Environmental Protection Agency 2008). The relationship between Se concentration in water and environmental toxicology is not fully understood because of the existing complexity of the biogeochemical Se cycle. However, several efforts are in progress to establish water quality measures on considering different ecosystems that are subjected to emission of Se in particular areas specific for sites.

In addition to this, adsorption and desorption of elements, precipitation of minerals, and incineration of municipal wastes (Plant et al. 2004) have also contributed to the insertion of Se into atmosphere. Organometallic compounds of Se are introduced partly to the atmosphere by chemical or microbial redox reactions and to soil and water by metabolic uptake and release by animals and plants (McNeal and Balistrieri 1989). Consequently, it enters into the food chain through crops, plant, and aquatic lives (Paikaray 2016). Frost (1967) has reported that sea water, earth crust, animals, and plants contain 0.004, 0.09, 1–20, and 0.02–4000 μg/g, respectively, which indicates that plants and animals have ability to concentrate Se from earth crust. It again enters into environment through the decomposition of these species and has excreted from human body (~50–80% through urine) to environment. For this in‐and‐out pathway of Se into environment, several cycles have been proposed including geological cycling of Se where animals and plants had a role, proposed by Moxon et al. (1939) and Lakin and Davidson (1967). Shrift (1964) and Frost (1973) have proposed a biological cycle of Se in which involvement of reduction–oxidation (redox) reactions of Se by plants, bacteria, and fungi have been incorporated. Allaway et al. (1967) and Olson (1967) have reviewed the cycling of low and high levels, respectively, of Se in soils, plants, and animals.

Se forms more than 170 solid compounds, whereas very limited compounds of Se are known that are in liquid (SeF₄, Se₂Cl₂, and CSe₂) and gaseous (H₂Se and SeF₄) state (Butterman and Brown 2004). Owing to its varied oxidation states it forms different type of compounds with sulfur (selenium sulfide (Se₂S₂) and polysulfides), oxygen (selenate, SeO₄⁻⁻ and selenite SeO₃⁻⁻), hydrogen selenide (H₂Se), and some organometalic compounds with methyl such as dimethylselenide ((CH₃)₂Se), dimethyl selenone ((CH₃)₂SeO₂), and dimethyl diselenide ((CH₃)₂Se₂) that are widely distributed in biogeologic forms. On substitution of sulfur it forms selenocysteine, selenocystine, and selenomethionine with amino acids (Chabroullet (2007); Losi and Frankenberger (1997); Haygarth (1994); Fernández‐Martínez and Charlet 2009). Therefore, it is clear that a range of inorganic and organic molecular forms of Se are present in environmental material such as rocks, soils, the aquatic system, and in air.