FORMS OF RADIONUCLIDES (U AND Th) MIGRATION IN NATURAL WATERS UNDER DIFFERENT GEOCHEMICAL CONDITIONS BASED ON COMPUTATIONAL AND EXPERIMENTAL DATA

Abstract

The relevance of the study is caused by the necessity to take into account the diversity of the migration forms of radionuclides under geochemical conditions changes, as wellas prognosis of the radioecological status of hydroecosystems. On the territory of the Semipalatinsk Test Site, the migration of natural radionuclides in water systems is poorly investigated, the calculation of the migration forms of uranium and thorium with a change in geochemical conditions will be informative for understanding the migration of a wide range of transuranium radionuclides. The aim of the research is to determine the forms of migration of uranium and thorium in natural waters depending on the particle size and, taking into account a wide range of components, to calculate the forms of their migration in waterstreams and lakes. Objects: water of streams of the Degelen mountain massif, water of lakes of experimental sites of the Semipalatinsk Test Site, model hydrogeochemical systems. Methods. Speciation of natural radionuclides were investigated in situ using the method of cascade fractionation. The pH and Eh of the waters were determined by potentiometry, total dissolved solids was determined by conductometry. The main water ions and some elements were determined by titrimetric (HCO3–, CO32–, Cl–, Ca2+, Mg2+), optical emission spectrometry (Na, K, Ca, Mg, Fe, Si), and ion chromatography (Cl–, SO42–). The content of dissolved organic substances was determined by the permanganate and dichromate oxidation, the method of catalytic combustion, and the intensity of the UV signal. The content of U and Th was determined by mass spectrometry. Results. The features of U and Th transfer in different water bodies of Semipalatinsk Test Site are determined and the processes that control them are indicated. It was found that the sulfate ion in waterstreams is an indicator of water exchange intensity and the rate of oxidation of sulfide minerals, which determine the release of uranium significant concentrations into the solution. Uranium is associated with organic and organomineral complexes and is subject to transformation of migration forms in the event of changes in the geochemical conditions of the environment. Thorium is more characterized by colloidal transport than uranium. Suspended components of natural waters are the dominant migratory form of these elements. The Th/U ratio in dissolved form is rather stable compared to colloidal and suspended forms. For waters with various hydrodynamic regimes, there is a difference in the severity of the influence of the terrigenous component.

Keywords

• uranium
• thorium
• colloids
• modeling
• geochemical conditions
• migration
• calcium uranyl carbonate