Characterization of the intermediate in formation of selenate-substituted ettringite

Precipitation of SeO₄²⁻-substituted ettringite is an efficient method to immobilize SeO₄²⁻ under alkaline conditions. The effect of SO₄²⁻ on SeO₄²⁻ immobilization was investigated, because SO₄²⁻ is one of the major ions in aqueous and soil environments and it is easily incorporated in ettringite. We aimed to systemically elucidate the formation mechanisms of SeO₄²⁻-substituted ettringite with and without SO₄²⁻. When SO₄²⁻ and SeO₄²⁻ coexisted, both oxoanions were immediately coprecipitated with ettringite after adding a Ca source. Although SO₄²⁻ was partially substituted by SeO₄²⁻ in ettringite, no other phases were formed during the process. Without SO₄²⁻, SeO₄²⁻-substituted AFm phase was formed as an intermediate, confirmed by X-ray diffraction peak at d 2.878, as well as by scanning electron microscopy with energy dispersive X-ray spectroscopy and zeta potential measurements. It is clear that the incorporation mechanism of SeO₄²⁻ in ettringite is dependent on coexisting SO₄²⁻ in aqueous environments. - Highlights: • SO₄²⁻ is preferentially incorporated in ettringite over SeO₄²⁻. • Coexisting SO₄²⁻ influences immobilization of SeO₄²⁻. • SO₄²⁻–hydrocalumite is proved as an intermediate. • The zeta potential and SEM-EDX support the XRD results.