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.