Selenite and selenate uptaken in ettringite: Immobilization mechanisms, coordination chemistry, and insights from structure

Although ettringite is a crucial material in terms of Se immobilization, the immobilization mechanisms, atomic configuration, and intercolumn structure of Se sorbed in ettringite are unclear. The immobilization mechanism of Se oxoanions was evaluated through structural insight into ettringite. It is contrasting between SeO₃ ^{2 −} and SeO₄ ^{2 −} in chemical property of the solid residues after immobilization. The oxoanion exchange with structural SO₄ ^{2 −} is the main mechanism for immobilization of SeO₄ ^{2 −}. In contrast, SeO₃ ^{2 −} is easily immobilized to form inner-sphere complexes in ettringite. In addition, it is necessary to reveal the SeO₃ ^{2 −} complexation sites for understanding the mechanisms in immobilization of SeO₃ ^{2 −}. Based on the characterization results with the bond valence theory, the location sites of sorbed SeO₃ ^{2 −} in ettringite structure were proposed. The results obtained in this work are relevant to the understanding of Se and its isotopes immobilized in cement or alkaline environments, especially for nuclear waste management.