Sorption of multivalent cations on titanosilicate obtained from natural raw materials. The mechanism and thermodynamics of sorption

A selective adsorbent of multivalent cations – titanosilicate with a sitinakite structure – was prepared via hydrothermal treatment available natural raw materials: products of processing leucoxene concentrate of the Yaregskoe deposit (Komi Republic, Russia) and loparite concentrate of the Lovozero deposit (Kola Peninsula, Russia). The synthesized materials were tested as adsorbents of stable cations of strontium (II), barium (II), cesium (I) in aqueous solutions. Titanosilicate synthesized from a loparite concentrate enrichment product at ambient conditions (20 °C, atmospheric pressure) has the highest sorption capacity: 145 mg/g for Sr2+; 193 mg/g for Ba2+; 297 mg/g for Cs+. The titanosilicate synthesized from leucoxene concentrate has a capacity lower by 42–45% for the indicated cations. The use of leucoxene concentrates as a precursor allows one to reduce the synthesis time to 12 h while lowering the temperature to 210 °C leads to the formation of titanosilicate with an extended specific surface without changing the phase composition.The thermodynamic characteristics – equilibrium constants, average values of enthalpy and entropy – have been calculated on the proposed model of the sorption process. It has been established that the sorption of cations on titanosilicate is a to a two-stage mechanism proceeding in the form of adsorption and/or absorption. For samples with an extended surface the absorption of cations takes place in the pores of the sorbent, while for sorbents with a low specific surface area the adsorption on the surface of dispersed phase particles is predominant.It has been shown on data of differential thermal analysis and X-ray phase analysis that the synthesized material is a promising material for producing mineral-like Synroc-type ceramics used for the long-term burial of radionuclides.