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3 Occurrence and Sources of Selenium Contaminationin Soil and Water and its Impacts on Environment
Rashmi Dahake1,2, Amit Bansiwal1, and Asmita Jadhav1
1 CSIR‐National Environmental Engineering Research Institute (CSIR‐NEERI), Nagpur, Maharashtra, India
2 Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India,
3.1 Introduction
Selenium (Se) is considered to be a vital element for living organisms. In 1817 a Swedish chemist, Jo¨ns Jakob Berzelius, discovered this element during sulfuric acid production. The name given was complementary to its sister element tellurium. The meaning of tellurium in Greek is “earth” and selenium is named after the Greek moon goddess, Selene. As Se is an essential micronutrient, its importance has been grown widely in recent years. The global estimates state that approximately 3000 million population of the world suffer from Se deficiency and are prone to the various health impacts. The Se‐deficient countries reported so far include USA, Canada, Japan, China, Finland, Scotland, New Zealand, and Spain (Fordyce 2013). Se requirements in human and animals can be met by consuming food which contains Se. Consumption of crops such as cereals, wheat, and vegetables which contain Se in small concentrations can fulfill the Se requirement of a whole population (Awika 2011). The presence of Se in human populations is majorly dependent upon the level in soil and food that is being produced in such soil. The addition of Se in the food chain is desirable and acceptable when it is in low concentration; however, the increased level of Se leads to an adverse effect on the human population and environment (Rayman 2008).
Dietary Se in human and animals has its own advantages, but overconsumption or exposure can lead to harmful effects. Concentration and speciation of the Se is the deciding factor for the impact of Se on environment. There is a great amount of literature available on Se which majorly focuses on nutrition and its occurrence in food chain. In recent years Se has emerged as a pollutant which may contaminate and cause severe toxicity to the environment. Researchers have started evaluating the occurrence, sources, and harmful effect of Se and are working on different mitigating, removal, and remediation technologies. Brief insights into Se as an emerging contaminant in water and soil are covered in this chapter, along with its natural and anthropogenic sources and uses.
3.1.1 Chemistry of Se
Se is a metalloid with atomic number 34 and molecular mass of 78.971 g/mol, which means it is heavier than Fe, Ni, and Mn, and lighter than Ag, Pb, and Au. In the periodic table it occurs in fourth period and belongs to the XVI (chalcogen) group. Se occurs in different allotropic forms such as amorphous, crystalline, and metallic which varies its physical properties. The two distinct allotropes of amorphous Se are finely divided brick red form and vitreous form. The only difference between these two allotropes is the route of production; otherwise at microscopic level they appear alike. The crystalline allotropes appear in various monoclinic varieties such as red to brown in color and metallic gray or black Se. Both monoclinic and amorphous forms of Se converts spontaneously at 70–120 °C, whereas the hexagonal crystalline allotrope is thermodynamically most stable at temperatures above 110 °C. Se exists in different oxidation states due to its strong metallic properties. It can be reduced to the selenide (Se−2) oxidation state or oxidized to the +4, i.e. selenite (SeO32−) or +6, i.e. selenate (SeO42−) oxidation states (Fernández‐Martínez and Charlet 2009).
It is a trace element with five stable isotopes 74Se, 76Se, 77Se, 78Se, 80Se reported till date, based on half‐life ranging from 20 ms to 295 000 years whereas 24 unstable Se isotopes are present in nature (Aston 1922). The nuclear waste disposal research has drawn attention toward the fission product of 235U which has the isotope of Se as 79Se. 82Se has a very long half‐life of approximately 1020 years, hence it is considered as the stable isotope which yields 80Kr through double beta decay.
Se shows similar chemical behavior to sulfur and similar physiological properties to arsenic. The valance electronic configuration of Se ([Ar] 3d10 4s2 4p4) is similar to that of sulfur and shows that the properties such as size of atom both in ionic and covalent states, electronegativity, ionization potential, and bond energies are similar to that of sulfur, but in biological systems where sulfur tends to reduce, Se tends to oxidize (Wessjohann et al. 2007).
3.1.2 Forms of Se
The occurrence of Se in the environment is mostly in two forms, i.e. organic and inorganic Se. The organic form includes selenoaminoacids, selenopeptides, and selenoproteins whereas different minerals contain it in the form of selenites and selenates, which are inorganic forms of Se. The inorganic forms are more toxic than organoSe compounds and have less bioavailability. The main classes of organoSe compounds include selenols, diselenides, selenoxides, selenones, Se acids, selenides, Se halides, and selenaheterocyclic compounds (Back 2006).
3.1.3 Chemical and Physical Properties
Se is a fairly reactive element. It is 16 Group element with a melting point of 317.3 K, a boiling point of 553.7 K, and density of 1.823 g/cm3. Se is placed in the oxygen group of the periodic table. It usually occurs in combination with metals like mercury, copper, silver, or lead (Guoxiong Hua 2011).
It combines easily with hydrogen, fluorine, chlorine, and bromine. It reacts with nitric and sulfuric acids. It also combines with a number of metals to form compounds called selenides such as magnesium selenide (MgSe). Se dioxide has a characteristic odor of rotten horseradish.
The Pauling scale electronegativity of Se is 2.55, which is slightly lower than oxygen due to the size of atom indicated by van der Waals and ionic radius as 0.14 and 0.198 nm respectively. The energy required to remove the first electron from the valence shell is 940.7 kJ/mol, which is quite lower than that required for the second and third electrons, as removal of one electron leads to stability of the atom (Brown 1914). Se and tellurium are often associated with each other. They tend to occur together in the Earth and have somewhat similar properties.
3.2 Sources and Occurrence of Se in the Environment
Se is widely present in nature. The distribution of Se in soil, water, and air is either by natural routes or by anthropogenic activities. Plant–soil environments undergo different Se functions, speciation, metabolism, and biological transformation which have different consequences in human and animal health (El‐Ramady et al. 2014a). Within the environment, transformation of Se is continuously happening from one chemical form to another. Some of the sources are presented below.
3.2.1 Natural Sources
A variety of elements in the environment are combined with Se due to its complex chemical behavior and these Se compounds are widespread in the rocks, soils, water, and air. It is distributed in the atmosphere by processes such as volcanic activity, hot springs, weathering of soils and rocks, forest wildfires, movement