Inorganic Ba–Sn nanocomposite materials for sulfate sequestration from complex aqueous solutions

Abstract

Selective sequestration of sulfate (SO₄²⁻) in the form of barite (BaSO₄) from alkaline solutions of high ionic strength containing carbonate is problematic due to the preferential formation of BaCO₃. Incorporation of sulfate into the insoluble and thermally stable BaSO₄ phase can potentially benefit radioactive waste processing by reducing operational challenges and suppressing volatilization of other waste components such as technetium-99. To enhance selectivity of SO₄²⁻ sequestration, a series of Ba–Sn nanocomposite materials was prepared using simple hydrothermal synthesis from different Sn(II) and Sn(IV) precursors. Structural characterization indicated that all obtained products predominantly contained BaSn(OH)₆ and Ba₂SnO₂(OH)₄·10H₂O nanocrystalline phases which were disrupted upon exposure to SO₄²⁻ due to formation of BaSO₄. Performance of the Ba–Sn materials was tested using complex alkaline solutions simulating radioactive waste containing 0.094 M SO₄²⁻ and 0.5 M CO₃²⁻ among other constituents. About 54–66% of SO₄²⁻ was converted to BaSO₄ when a quantity of Ba–Sn material containing approximately a stoichiometric amount of Ba²⁺ relative to SO₄²⁻ was used. In comparison, previous studies indicate negligible BaSO₄ formation under similar conditions when a simple Ba²⁺ salt is used. This improvement is attributed to the selective replacement of the stannate by SO₄²⁻. Thermal stability of the sulfate-loaded product material up to 1100 °C was demonstrated. The obtained materials promise a convenient and economical option for the selective sequestration or removal of SO₄²⁻ from complex carbonate containing solutions.