Characterization of the intermediate in formation of selenate-substituted ettringite

Precipitation of SeO₄^{2 −}-substituted ettringite is an efficient method to immobilize SeO₄^{2 −} under alkaline conditions. The effect of SO₄^{2 −} on SeO₄^{2 −} immobilization was investigated, because SO₄^{2 −} 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 SeO₄^{2 −}-substituted ettringite with and without SO₄^{2 −}. When SO₄^{2 −} and SeO₄^{2 −} coexisted, both oxoanions were immediately coprecipitated with ettringite after adding a Ca source. Although SO₄^{2 −} was partially substituted by SeO₄^{2 −} in ettringite, no other phases were formed during the process. Without SO₄^{2 −}, SeO₄^{2 −}-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 SeO₄^{2 −} in ettringite is dependent on coexisting SO₄^{2 −} in aqueous environments.