and biological redox reactions beneath the soil, and mineral formations. The occurrence of Se in the soil or in water sources depends mainly on the climate, topography, and parental material (El‐Ramady et al. 2014b). Minerals such as sulfide minerals are associated with selenite‐ and selenide‐containing rocks. The erosion of these rocks is responsible for the occurrence of Se in soils in the form of elemental Se, for instance selenite salts, ferric selenite, or in its organic form (Malisa 2001). The anionic forms of Se, i.e. selenite and selenate, are highly soluble, bio‐available, mobile, and toxic above certain limits. Decomposition of plant which accumulate Se are the sources of organic forms of Se.
The available level of Se in soil is mainly dependent on the texture of soil, rainfall, the kind of soil, and the concentration of organic matter. The lowest concentration of Se is obtained in sandy soils while the highest is in organic and calcareous soil. The major Se‐controlling factors in the soil are Eh and pH whereas other parameters like organic ligands, clay, and hydroxides also play a significant role. The worldwide occurrence of Se is in very broad range from 0.005 to 3.5 mg/kg. The concentration of Se in plants depends on the surrounding soil’s Se levels (Sakizadeh et al. 2016). A greater amount of selenate is taken up by plants than selenite and it is metabolized in chloroplasts by pathways similar to sulfur.
Se originates in water from the soil leaching and atmospheric deposits. The Se can form a stable complex with particulate or colloidal matter or sediments or can be dissolved in water. The movement of Se from the top layer is governed by the transport phenomenon and hence Se can occur in water as suspended or dissolved fraction (El‐Ramady et al. 2014b).
The concentration of Se in sea water varies between 0.1 to 0.35 μg/l (Gaillardet et al. 2003). Natural waters have a concentration less than 1 μg/l (Conde and Sanz Alaejos 1997). Global Se average concentration in river waters is given in the range of 0.02–0.5 μg/l while in groundwater and surface water it ranges from 0.06 to 400 μg/l (World Health Organization 2011). The concentration of Se in water is dependent upon pH. The compounds which are soluble in water are converted at high and low pH and this results in an increase in concentration.
Se can be transported to the air by natural processes such as volcanic eruptions, soil erosion, forest fires, and evaporation from ocean and sea. Mosses and peats in marine regions with increased Se levels when volatilized release Se into the air in the form of hydrogen selenide and elemental Se, selenites, and selenates in particulate form. Hydrogen selenide and Se dioxide are unstable in the air and are converted by oxidation into Se and H2O and into selenicious acid in moist conditions (Belcher et al. 1980). The ambient air Se concentration is generally very low and varies from 0.1 to 10 ng/m3 (Lee and Duffield 1979; Gilbert and Fornes 1980).
The presence of Se in grains and vegetables is mainly accountable for the Se contents of soil in which they grow. The natural concentration of Se is very low, i.e. approximately 6 mg/g in vegetables like carrot, tomatoes, potatoes, cucumber, etc., even if they are grown in seleniferous soils. Some vegetables effectively accumulate Se from seleniferous soil, for instance onions and asparagus accumulate up to 17 μg/g. Fruits accumulate very low Se which is less than 10 μg/kg (Whanger 2004).
3.2.2 Anthropogenic Sources of Contamination in Environment
Some industrial waste and processes may contribute for the addition of high levels of Se in the environment. The major source of Se contamination in air is the combustion of fossil fuels (Shah et al. 2007). The elemental Se burns to form Se dioxide in the air. The main atmospheric Se emission includes oil refining factories, mining, and milling end product manufacturing. In 1978 Harr has reported that the fossil fuel burning, emissions from industries, and municipal wastes release almost 1500, 2700, and 360 tons of Se annually, respectively (Fishbein 1983). In Canada nonferrous smelters emitted 3.02 tons of Se in 1993 as reported by Skeaff and Dubreuil. Different kind of paper also contains Se (Skeaff and Dubreuil 1997).
Incineration of papers, municipal waste, and rubber tyres are additional sources of Se in the atmosphere. Few of the sources of Se in the atmosphere have been quantified yet. Fugitive dust emissions from the fly ash settling ponds and hazardous waste sites where Se compounds were disposed of are the potential sources of Se in air (Santhanam et al. 1979). In addition to the burning of fossil fuels, smelting, mining, the disposal of industrial waste, and the processing of nuclear fuel waste also release high levels of Se into the environment. Fossil fuels and coal contain significant amounts of Se which will be added to the atmosphere during its processing in industries. The Se is present in the crude oil in the form of seleniferous marine shale which contains a fairly good amount of Se which is sent into the atmosphere through waste water generated during refining process. Levels of release of Se through different refining processes are different. The desalting process releases less Se, and atmospheric distillation causes elemental Se to be formed and partitioned with suspended matter, and processed stream water discharges high levels of Se from hydro‐treatment and catalytic cracking units. In the end step, the processed oil still contained increased levels of Se, which was examined during sour water treatment (Willig 2014).
Many electronics industrial waste also contains Se as Se is an essential component in the electronics accessories such as capacitors, printers/toners, and photocopiers etc. It is also used in semiconductors, photoelectric cells, glass, and pharmaceuticals production, the waste of which goes into the landfills and incineration and is added to environment. Leachates obtained from landfilling units of such industries wastes show increased levels of Se.
The radioactive isotope of Se, i.e. 79Se34 may also be a component of nuclear waste. The release of this isotope contributes Se content in soil, plants, and water when accidentally released from nuclear waste repositories or power plants. Smelting of ores such as nickel, zinc, and copper also releases Se in atmosphere through volatization. The Se concentration in the environment is increasing day by day and may lead to environmental pollution which is very harmful for human beings (Duce 1987).
3.3 Drinking Water Standards and Criteria
The permissible limit of Se in the drinking water was fixed by different countries based on the dietary intake through food and water. The permissible limit for drinking water is 10 times more than that for surface water as aquatic animals require less Se than mammals (Barron et al. 2009). World Health Organization (WHO) has set the standard for drinking water at 40 μg/l (WHO 2012). The permissible value set by the United States Environment Protection Agency (USEPA) is 50 μg/l (EPA 2020). Different countries have set their standards based on the availability of different factors; for example, the Bureau of Indian Standard has fixed the permissible limit to 10 μg/l (BIS 2012). The water quality standards for drinking water of different countries are tabulated in Table 3.1.
3.4 Effect of Se in Human, Terrestrial, and Aquatic Life
Se is a vital necessity to human, aquatic, and terrestrial organisms. The presence of Se in the diet has nutritional benefits to the organisms on the Earth while its deficiency or excess consumption can lead to an adverse effect in humas as well as in aquatic and terrestrial life.
3.4.1 Human Population
Se is an essential micronutrient required for the human body. It is an important part of selenoproteins which will be required for various biological functions such as antioxidants defense, thyroid hormone formation, DNA synthesis, fertility, and reproduction. In organisms Se is converted into different metabolites. Se plays an important role in slowing down the aging process and improvement of endurance of muscles besides vitamin E. Methylselenol also plays a part in cancer prevention (Papp et al. 2007).
Table 3.1 Drinking water and surface water limits of different countries (EPA 2020; WHO 2012).
| Country | Limit (μg/l) |
|---|---|
|
Drinking water
|