**4. Sources of selenium in the environment and its location**

#### **4.1 In soils**

Selenium occurs in soils via the erosion of rocks that contains selenides and selenites associated with sulfide minerals with mass fractions less than 1 mg/kg. Selenium is mostly found in soils either in its organic form or elemental selenium like selenite salts and ferric selenite. The common forms of selenium in soils are the anionic forms like Selenite (SeO3 <sup>2</sup><sup>−</sup>) and selenate (SeO4 <sup>2</sup><sup>−</sup>) and they are soluble and potentially toxic. Organic forms of selenium in soils are mostly from plants decomposition [24, 25].

The selenium content in soil varies depending on the organic matter, soil texture along with the level of rainfall. The rate of assimilation of selenium by plant is further influenced by the physicochemical factors of the soil, such as microbial activity, pH and redox status. The concentration of selenium in the soil varies from 0.1 to 0.7 mg kg<sup>−</sup><sup>1</sup> . Tropical soils have a concentration between 2 and 4.5 mg kg<sup>−</sup><sup>1</sup> , while clay soils are between 0.8 and 2 mg kg<sup>−</sup><sup>1</sup> . Granites and volcanic soils are usually low in selenium while the soils around mountains are rich in selenium. Normally, selenium is more concentrated in soils of the driest regions of the world. The toxic effect of selenium on animals occurs on these soils [26, 27]. The rate of soil acidity determines the amount of selenium in crops and plants; more selenium is released in alkaline soil than in acidic ones. In alkaline soils, selenite undergoes oxidation into a more soluble form that is easily assimilated by plants (selenite). However, in acidic soils, selenite combines with iron hydroxide making it to be permanently fixed by the soil [28].

#### **4.2 Plant sources**

The concentration of selenium in plants is dependent on the level of selenium levels in the soils. The physiological conditions and species of the plant also determine how the selenium are taken in and distributed by the plant root. The aerial silks normally contain about 80% selenite and 65% selenate [29]. Forages contain selenium from about 0.2 to 0.6 ppm and livestock are at risk of selenium poisoning [30]. In arid regions of the United States and China, some plants contain very high selenium content, as high as 10,000 ppm [4]. Some species of *Astragalus* usually accumulates very high level of selenium making them toxic to animals [31]. Wheat plants normally store selenium in their seeds in form of selenomethionine with varying levels depending on the environment [27, 32]. Plants assimilate selenite more than selenite. Selenate and selenite share similar chemical features with sulfur hence they both undergo metabolism via the same route (in the choloroplast). The first reaction is when selenite is activated by ATP sulfurylase-adenosine 5′-phosphoselenate which is followed by its reduction to selenite by adenosine

5′-phosphosulfate reductase and finally the selenite is further reduced by the action of glutathione into selenide. Depending on the rate of selenium accumulation by plants, two mechanism of metabolism exist.

## **4.3 Selenium in water**

Selenium in water is initiated from ambience deposits or drainage in the soil, its concentration varies but does not exceed 9 mg L<sup>−</sup><sup>1</sup> . The World Health Organization recommends that the content of selenium in water for consumption should not exceed 10 μg L<sup>−</sup><sup>1</sup> [24]. When farm lands are supplemented with fertilizers, selenium content increases. Selenate sodium and selenide are mostly found in surface waters, while freshwater contains majorly selenite.

### **4.4 Food and feed sources of selenium**

The content of selenium in vegetables and grains is largely dependent on selenium found in the soil. Vegetables like beans, peas and carrots can contain up to 6 mg g<sup>−</sup><sup>1</sup> of selenium, while onions contain more. However fruits generally contain a very low level of selenium, but nuts with high protein levels are also known for their high selenium concentration [33–35].
