Characterization of the intermediate in formation of selenate-substituted ettringite

Abstract

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.