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Selenium Mobility in Some Egyptian Soils
Abdou A. Soaud1, Rushdi M. El-Kilani1, Nader R. Habashy2 and Eman M. Abd El-Razik2
1Department Soil Science, Faculty of Agriculture, Cairo University, Giza, Egypt.
2Soils, Water and Environment Research Institute, Agricultural Research Center, Giza, Egypt.
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Abstract
Selenium is an essential element for humans, animals and some species of microorganisms. The Egyptian soils are low in selenium content but information about its distribution and mobility in Egyptian soils are limited.Selenium application could lead to soil, plant and water pollution.Forms of selenium found in soils influence its mobility, uptake, and metabolism by plants. The major forms in alkaline, oxidizing environments which are available for plant uptake are selenium-VI (as selenate, SeO42-) and selenium-IV (as selenite, SO32-). The major influences on uptake are soil pH and salinity. High salinity and pH favor selenium anion adsorption onto clays and metal oxides. Selenite is adsorbed much more strongly than selenate, leaving selenate as the major form available for plant uptake. Some soil anions, such as phosphate, increase plant selenium uptake because increased soil-solution anion concentrations compete with selenium anions for adsorption sites. Other anions, such as chloride or sulfate, actually enhance or inhibit uptake by affecting plant metabolism. The objectives of this study was to study selenium mobility in some soils of Egypt using breakthrough curves(BTCs) technique and determined the values of some parameters related to Se transport in soils such as retardation factor, Peclet numbers and hydrodynamic dispersion coefficient.Two soils differ in texture were collect from two locations in Egypt. First one was clay loamfrom Mansura and the second one sandy loam from Toshka.Six columns (7 cm in diameter and 40 cm length) for each soil type were filled with homogenous soil sample. Half of the columns wereunsterilized and other half weresterilized using the chlorophorm solution.To obtain the breakthrough curve,the columnswere saturated with CaCl2 from the bottom of the column for five days and then washed with distilled water till the excess calcium was removed. The columns were leached continually with 1 mg L-1 Na2SeO3solution at a constant hydraulic head (10 cm). The leaching solution were collected atdifferent times, filtered and analyzed for selenateuntil upper breakthrough points for selenate were obtained.The values of the retardation factors of the two different soils,either unsterilized or sterilized, were less than one which means that the velocity of solute was higher than the water velocity due to negative expulsion. The peclet numbers of the studied soils were much greater than one due to convection of selenium being higher than diffusion. The values of the hydrodynamic dispersion coefficient were ranged from 9.04×10-5 to 1.98 ×10-3 m2s-1 for clay loam soil and 4.54 ×10-3 to 2.75 ×10-2 m2s-1 for sandy loam soil.The value of relative maximum concentration (Cmax) of selenium was 0.871 and 0.880 for sterilized clay loam and sandy loam soil, respectively.While it was 0.827 and 0.889 for unsterilized clay loam and sandy loam soils, respectively. The large difference between Cmax values of sterilized and unsterilized clay loam soil was probably due to the microbial action in methylation of selenium and its volatilization. The low hydraulic conductivity of the clayey soils allows enough time for the microbial action to have more time to volatilize selenium and loss from soil. In conclusion, selenium could be easily leached from studied soils
Keywords
Selenium; breakthrough curves; retardation factors; peclet numbers; hydrodynamic dispersion coefficient.