The solubilities of the Na₂SeO₄-FeSeO₄-H₂O system have been investigated by the physicochemical analysis method. Crystallization of a new double salt Na2SeO4.FeSeO4.2H₂O has been established. TG and DTA measurements indicate that the double salt loses the crystallization water in three steps in the temperature interval from 87 ^oC to 148^oC. The Pitzer ion interaction model has been used for thermodynamic analysis of the experimental solubility data obtained. The simulation has been materialized by i) determination of the Pitzer binary parameters for the binary solutions, ii) determination of the Pitzer mixing parameters, and iii) calculation of the solubility isotherm of the Na₂SeO₄-FeSeO₄-H₂O system. The binary parameters of interionic interaction for Na2SeO4(aq) and FeSeO4(aq) are taken from our previous studies. The ternary parameters θ(Na,Fe) and (Na,Fe,SeO4) and the thermodynamic solubility product (as lnK^o_sp) of the double salt have been calculated on the basis of experimental data on the solubility reported in this study. The experimentally obtained and the calculated solubilities are in very good agreement. The standard molar Gibbs energy of the synthesis reaction D_rG^o_m of the double salt Na2SeO4.FeSeO4.2H₂O from the corresponding simple salts Na2SeO4.10H₂O and FeSeO4.6H₂O and the standard molar Gibbs energy of formation D_fG^o_m of simple salts and double salt have been determined. The experimental data and the model described in this study are of high importance, especially in the development of nuclear waste storage technology, as well as for production and purification of selenate solutions and solid phases.

Acknowledgement: This study was financed by the European Union-NextGenerationEU, project № BG-RRP-2.013-0001