EURO-ECO 2010 Hanover 2 - 3 Dezember 2010	Environmental, Engineering - Economic and Legal Aspects for Sustainable Living
	European Academy of Natural Sciences, Hanover European Scientific Society, Hanover University of Bremen, Bremen

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Abstracts EURO-ECO 2010

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Natural radionucides are the source radioactive contamination of natural waters as the result both natural migration and anthropogenic dissipation by working of enterprises of nuclear fuel cycle, metallurgical, chemical and mining plants (that is not usually related to radiation-dangerous works). As the rule methods of determination of the isotopic composition of elements in natural substances differ from methods of determination of the total quantities of elements because of their high complexity and laboriousness. Exacting accuracy and sensitivity demands are made to them because as a rule isotopic ratio variations are slight and contents of determined elements are little. When methods of isotopic content determination are realized we have to exercise processes of chemical separation and/or concentration; that is connected with low content of uranium and thorium and necessity of purification for preparing of high quality source for α-spectrometry.

Methods of sorption concentration using inorganic sorbents based on titanium hydroxide and also coprecipitation with iron and zirconium hydroxides were researched for concentration of uranium and thorium from different natural waters. Every suggested method has its advantages and disadvantages. The peak value of distibution coefficient is reached in coprecipitation because the extraction of radionuclides occurs simultaneously with formation of hydroxide which has maximal sorption activity. The disadvantage of coprecipitation is the necessity of treatment of large volumes of water that causes considerable difficulties with quantitative coprecipitation and filtration of the sediment. The proposed method of uranium and thorium coprecipitation with iron (III) and zirconium hydroxides and with following extraction of the sediment on the cellulose under dynamic condition allows to treat of large volumes of water and to save of advantages of coprecipitation connected with full implementation of sorption capacity of freshly made hydroxide.

The dependences of efficiencies of sorption of U and Th by sorbents based on titanium hydroxide, efficiency of colloids formation (using the ultrafiltration method) and efficiencies of coprecipitation with iron and zirconium hydroxides on pH were studied for determination of the best conditions of concentration. The dependence of efficiency of coprecipitation on pH is the bulb function for uranium and the classic sigmoid curve for thorium; this dependences correlate with percentages of colloids of uranium and thorium obtained by ultrafiltration method. The dependences of distribution coefficients of U and Th in sorption on pH are explicit bulb functions. The peak values of distribution coefficients under pH = 6 – 7.5 for uranium and under pH = 3 - 6 for thorium were observed. Decrease of efficiency of uranium sorption under increase of pH is connected with the formation of carbonate complexes of uranyl; in case of thorium it is probably connected formation of pseudocolloids.

Distribution coefficients of U and Th were 10³ – 10⁵ mL/g for sorption by inorganic sorbent based on titanium hydroxide (T-5) and up to 10⁶ mL/g for coprecipitation with iron and zirconium hydroxides.

Subsequent extraction of U and Th was realized under conditions of frontal chromatography and recirculation. The alkaline-treated cellulose was used in coloumns for extraction of hydroxides on which uranium and thorium were concentrated after coprecipitation. Distribution coefficients as of U as of Th at the rate of n • 10⁶ mL/g were realized when coprecipitation with hydroxides and subsequent extraction by alkaline-treated cellulose was combined. The combination of advantages of coprecipitation and dynamic process was success due to this method.

Thus, the following optimal conditions of concentration stage of determination of uranium and thorium isotopes in natural water samples were determined: pH of the solution, ratio of solution volume and sorbent weight, flow rate, number of filter cycles (for recirculation), concentration of iron or zirconium for coprecipitation. Chemical yield of uranium and thorium was not less than 90% under these optimal conditions.