et al. 2009). It has also been reported by the group that for humans there is about ~20‐fold factor between nutrient dietary intake and the threshold intake that result in a toxic level of Se. On considering its narrow tolerance range of nutritional deficiency (<40 μg/day) and toxic level (>400 μg/day) (Levander and Burk 2006) for humans, Se is being termed as “essential toxin” (Stolz et al. 2002) or “double‐edged sword” (Fernández‐Martínez and Charlet 2009) element.
Apart from essential dietary value of Se, it has several industrial uses including ceramic, pigment, pharmaceuticals, organic catalyst, as photocopying material, in glass manufacturing industries as dopants, photocell devices, electronic equipment, batteries, antidandruff products, etc. (Mayland et al. 1989; Russell 2011). For commercial applications it is produced worldwide from mud deposited on the anode during electrolytic refining of copper (Russell 2011). Literature reports state that the annual production of Se is around 1500 tones and recycled amount from industrial waste is approximately 150 tones. Therefore, in addition to natural sources, emission of Se‐using industries mentioned above and combustion of fossil fuel including coal burns are other main sources of Se in environment that results Se as an emerging pollutant. Sharma and Kumar (2006) have reported that in Baurani industrial area of Bihar, India, groundwater area up to 30 km away has polluted by the effluents from oil refineries, chemical fertilizers, and thermal power plants. A study conducted by the National Irrigation Water Quality Program (NIWQP), in various irrigated regions, in different states of western US between 1986 and 1993 (Seiler 1995, 1998). The study demonstrated that when irrigated aquifers are underlain by marine shales (as mentioned in Eq. 2.1), the agricultural drainage waters reaches to toxic Se level. Bioavailability of Se in many areas of the countries like USA, Australia, Ireland, China, and India is high enough to cause toxicity in soil and water (Presser and Piper 1998). In low land areas and ponds the deposition of solid wastes is the major source of elevated Se concentration in ground water. Except certain seleniferous areas concentration of Se in drinking water is usually less than 10 μg/l. Vegetables and crops are responsible for a Se‐rich diet; the reason may be the adsorption of particulate Se onto them. Moreover, non‐vegetarian food is more Se‐rich than vegetarian food (Paikaray 2016). Se intake through inhalation is much lesser (<0.1%) in comparison to drinking water and food. Even in highly seleniferous soils, the contribution of Se by drinking water is relatively low in comparison to food stuffs. In the polluted areas water and soil are highly enriched with Se and are a concern for human and animal health. This attracts the attention of scientific community worldwide, in recent years. Therefore it is essential to know the chemistry of Se including its oxidation states, isotopes and allotropic forms, chemical, and physical properties of Se. Such properties are helpful to understand the nature of Se compounds and their structure. Additionally it would be easy to illustrate a relationship between environmental conditions responsible for distribution of different chemical compounds of Se in biogeological samples at different sites.
2.2 Environmental Distribution and Forms
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 (t1/2 = 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
