After reading this article you will learn about the biochemical properties of selenium, occurrence, uptake by plants, bioaccumulation and its reutilization prospects when generated as waste.
Properties of Selenium:
Selenium was identified as element by Berzelius in 1817. It was noted as a constituent of the slime collected at the bottom of the lead chambers used for the production of sulphuric acid. Selenium is classified in group VI-A of the periodic table of elements. It lies between the group V-A metal arsenic and the group VII-A non-metal bromine. Thus selenium is considered a metalloid having both metallic and non-metallic properties.
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Its atomic properties are summarized as follows:
The electrons of the element are susceptible to excitation by high energy light, being promoted to conduction levels. The intrinsic conduction of selenium is very low, however conduction increases markedly on exposure to light. Elemental Selenium shows allotropy.
It can exist in either an amorphous state or one of three crystalline states. Amorphous Se is hard, brittle glass at temperature below 31°C, is vitreous at 31-230°C, and is a free floating liquid above 230°C. Elemental Se can be reduced to the -2 oxidation state [selenide] or oxidized to the +4 [selenite] or +6 [selenate] oxidation states.
Occurrence of Selenium:
Selenium enters soil primarily as a result of the weathering of Se containing rocks. Much of the Se that is released from rocks weathering under alkaline and well-aerated condition is oxidized to form selenates. Selenates are highly soluble in water and do not form stable adsorption complexes. Therefore they are readily taken up by plants or easily leached into the ground water.
These types of soils are usually found in semi-arid or poorly drained areas, when they are irrigated, Se may be transported into, or leached out of these soils, depending upon the selenate content of the irrigation water. In contrast, Se released from rocks under acid and very moist condition, Se is present in insoluble reduced forms that can form stable adsorption complexes with ferric hydroxide. In this case, Se released is poorly available to plants and is not readily transported by ground water.
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In soils, that are slightly acid to neutral, Se is present in organic compounds formed from the vegetation growing there. Harvesting of crops from such soils, can result in depletion of soil Se by interrupting “the replacement of these organic Se compounds into the soil.
Se enters water as soluble selenites and selenates and as suspended particles of insoluble and organic forms of the element. Therefore the maximum concentrations of Se are found in water systems that drains seleniferous soil. Most of the Se found in air is in aerosol and large particulate forms that result from windblown dust, volcanic action, combustion of fossil fuels, smelting and refining of nonferrous metals and manufacturing of glass and metals. However these forms are not directly available for plants and animals.
Six stable isotopes of Se exist in nature. These are 74Se, 76Se, 77Se, 78Se, 80Se and 82Se. These isotopes have been employed in study of the biological utilization of Se in the foods, in which their quantification has been achieved by neutron activation or by mass spectrometry. Due to emission of gamma radiation, and to its relatively long half-life, Se has been widely employed in biological experimentation and in medical diagnostic work.
Uptake of Selenium by Plants:
The availability of soil Se to plants is determined by the soil characteristics that influence the water solubility of Se. Plant uptake of Se varies with soil pH. Thus the best utilization of soil Se is achieved by plants growing in alkaline soil conditions. Se uptake is generally better from sandy soil than from loamy soils.
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Poorest utilization of soil Se is observed among plants growing in soils that acid to neutral, in which selenite and reduced forms of Se are favoured, and that are loamy, thus fixing the otherwise soluble selenite and making it unavailable for plant uptake. Gissel Nielsen cited that Indian soils tend to be alkaline with low precipitation and Se is present mainly as selenate, which is not fixed in the soil.
In contrast, Scandinavian soils are relatively acidic with greater amount of precipitation and leaching, rendering the remaining Se strongly fixed in the soil. The accumulations of tissue concentration of Se in these regions are ten folds greater in plants in India in comparison to plants in Scandivinia.
Absorption and Translocation of Selenium Compounds:
A two stage process characterize absorption of nutrients and other substances by the plant root. The first stage involves movement of molecule into the apparent free space, an area within the root that is considered equivalent to the region occupied by the cell walls and intercellular spaces. Entry into this free space is normally achieved by diffusion.
The second stage of absorption is the transport of molecules from the free space, across the cell wall and cell membrane and into the cell itself. Transport in to the cell may be either energy requiring or energy independent. Once within the root, substance can travel elsewhere within the plant by the process of translocation through the xylem vessels.
Physiological and Biochemical Events that Underlie Selenium Toxicity:
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Of the different forms of selenium usually present in the soil, only the inorganic anion selenate [SeO4] and selenite [SeO3] are normally toxic to higher plants.
This toxicity can be attribute to a combination of three factors:
1. Both anions are readily absorbed by roots and trans located to other parts of the plant.
2. Both anions are metabolized to organic forms of selenium within the cell.
3. These organic selenium metabolize act as analogues of essential sulfur compounds and thereby interfere with cellular biochemical reactions.
Bioaccumulation and Reutilization of Selenium Generated as Waste:
Anne Simon (1995) has documented the role of plants in toxic metal clean-up. The plant can accumulate the selenium and can remove this from the waste. Toxicity avoidance by accumulation is believed to be related with incorporation of selenium in non-protein amino acids. Selenium toxicity in non-accumulating plants seems to be related to the replacement of Sulfur in Cysteine and Methionine, forming selenocysteine and selenomethionine.
North American plants have been grouped as primary accumulators (several thousand mg per g of tissue), secondary accumulators (up to 1000 mg/g) and non-accumulators or plants that do not contain more than 25 mg/g.
Different plant species show considerable variation in their selenium content. Primary indicators or selenium accumulators mostly the member of genus Astrolagus, are high tolerant of selenium. Selenium tolerant accumulator plants differ in at least two respects from sensitive species.
Large quantities of Se-methyl selenocysteine and selenocys- tathionine, two non-protein selenoaminoacids, rarely detected in non-accumulators, have been isolated from the tissues of selenium accumulators.
In addition, selenium is kept from entering proteins so that the selenium level in proteins of accumulator plants is significantly lower than the levels in selenium-sensitive plants. Exclusion of selenium from the proteins of accumulators is thought to be the basis of selenium tolerance. Discrimination against selenocysteine during protein synthesis seems to prevent incorporation of this selenoaminoacid into proteins of accumulators.
Furthermore, synthisis of Se-methyl selenocysteine and seleno-cystathionine, which results in diversion of selenium away from the synthesis of selenomethionine, will restrict the amount of this compound available for protein synthesis Se-methyl selenocysteine.
The selenium content of fertilizers differs a great deal and the choice of raw materials and manufacturing procedures influence the selenium content of the compound fertilizers, but the contribution to the total selenium content of the plants from the selenium in the fertilizers is negligible, unless highly seleniferous raw materials are used however, fertilizers can be used as carriers for added selenite, though sulfur rich fertilizers as superphosphate may be less effective as such than fertilizer low in sulfur content.