3rd-INCC

Physico-chemical investigation of heterogeneous equilibria in the saturated aqueous solutions of uranylsilicates of the alkaline elements

19 Sept 2011, 17:30

1h 30m

Paladini (Città del Mare, Terrasini - Palermo - Sicily - Italy)

poster Radioecology and Geochemistry Poster Section 1

Speaker

Ms Natalya Godovanova (Nizhni Novgorod state University)

Description

Solubilities of uranylsilicates of general formula МIHSiUO6nH2O (MI – Li+, Na+, K+, Rb+, Cs+, NH4+) in aqueous solutions at 25С were investigated. Based on the obtained data constants of equilibrium of the dilution’s reactions of uranylsilicates in acidic and neutral solutions and standard Gibbs’s functions of formation of studied compounds were calculated, the composition of equilibrium solutions was investigated. Physico-chemical description of the equilibrium heterogeneous systems based on thermodynamics laws included set of equilibria established in the systems was proposed. To develop this description it was taken into account that heterogeneous process of dilution in the studied systems was established and homogeneous equilibria where silicon (IV) and uranium (VI) were presented as set of ion and molecular forms, were established, too. Prediction of behavior of given compounds in different conditions was made based upon proposed description of the heterogeneous system state like “solid uranylsilicate – aqueous solution”. Solubility of uranylsilicates of alkaline elements and ammonium quite depends on pH. In acidious solutions their concentration is at the level of 10-3 M and in the neutral ones it decreases to 10-6 M. Type of alkaline element does not have significant influence on solubility of the compounds. Data for compounds in the range pH 3-6 was used to calculate values of solubility products (SP) of the low soluble compounds. At first values of SP decrease in the raw of Li+ - NH4+ - Na+ - K+, then increases again from K+ to Cs+. Modeling of the heterogeneous system state showed that solubility of uranium compounds as function of pH has minimum at pH 11.5, that corresponds to optimal conditions for synthesis of uranylsilicates of alkaline elements.