Читать книгу Selenium Contamination in Water - Группа авторов - Страница 24

Selenium is a rarer element on earth and ranked as 70th element among naturally occurring elements in order of its natural abundance. It was first discovered by Swedish chemist Jo¨ns Jacob Berzelius in 1818 when preparing sulfuric acid. He named it after the Greek word selene (meaning “moon”). Selenium after its discovery was almost unnoticed for decades (Hamilton 2004), until the identification of its nutritional essentiality in animals in 1957 (Schwarz and Foltz 1957). The interest of researchers in the trace element has grown substantially with the recognition of its nutritional role as antioxidant and toxicity among human beings (Rotruck et al. 1973). It is dispersed widely in low concentration in the Earth’s crust (Berrow and Ure 1989; Russell 2011) and its concentrations in the Earth’s crust and sea water are 0.004 and 0.09 μg/g, respectively. However, the average global concentration of Se in soil is 0.4 μg/g (i.e. ranging from 0.01–2 μg/g) (Dungan et al. 2002) and 3.1 μg/g in soil of Northwest India (Bajaj et al. 2011). However, only bioavailable Se is the key to determine Se in plant tissues ranging from 0.02–4000 μg/g, depending upon low to high seleniferous soil, which enters into food chain. Hamilton (2004) has reported that the trace to moderate amount, i.e. 40–400 μg/day, of Se is necessary for development and normal growth, and to maintain homeostatic functions, respectively. The deficiency of Se in diet (<3 μg/day, reported by Yang et al. (1983), in China) can weaken immune system and can cause Keshan disease and Kashin–Beck disease, (Coppinger and Diamond 2001). On the other hand, a higher concentration (>400 μg/day) of it may cause health hazard to living beings, including gastrointestinal, respiratory, and cardiovascular problems. It may also result in fragile nails, excessive tooth decay, hair loss, garlic‐smelling breath, discoloration and mental problems (Pedrero and Madrid 2009). The intake of Se recommended by World Health Organization (WHO) and different recommended Se intakes followed in different countries are summarized in Table 2.1.

Table 2.1 Recommended daily nutrient requirement of selenium (μg/d) as per WHO and NAS and followed by different countries and their daily Se intakes.

Group	WHOa	NASb	Unique recommended Se intakes (μg/day) followed by countriesc,d
USA	Canada	UK	Europe	Australia	China	India
Men	40	55	55	55	75	55	85	e	e
Women	30	55	55	55	60	55	70	e	e
Daily Se intakes (μg/day)
Men			90a	98–224a	60c	38–48a	e	3–11,a,e	48a
Women			74a		60c		e	1338a,f;
								9–11c,e,
								5000c,f

^a WHO (2005).

^b National Academy of Science (NAS) (2000).

^c Thomson (2003).

^d Thomson (2004).

^e Keshan disease area.

^f Seleniferous area.

g Data not available.

For adults, the recommended dietary allowance (RDA) is 55 μg/day (see Table 2.1) and tolerable upper intake is 400 μg/day, set by the Institute of Medicine of the National Academy of Sciences (NAS 2000) in the US, while more than 5 mg/day can be fatal (Mayer (2010). Amweg et al. (2003) has reported that excessive Se replaces the sulfur from amino acids that subsequently changes the 3D structure of proteins and impairs the enzymatic functions. Levander and Burk (2006) have discussed the biological properties of organoselenium compounds in metabolism and have reported that organoselenium compounds, i.e. di‐/trimethylselenides and selenoamino acids, viz. selenomethionine, selenocystine, and selenohomocystine compounds, are of interest in nutrition and health. Ralston et al. (2009) have reported that dimethylselenide is volatile neutral molecule and readily excreted through breathing and thus provides a natural way to export excess of Se from body. The established “no observed adverse effect level” (NOAEL) in China for human population is 15.5 μg Se/kg body weight as compared to the “recommended daily amount,” i.e. 0.9 μg Se/kg body weight in US (Ralston 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.