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.