Immobilization mechanism of Se oxyanions in geopolymer: Effects of alkaline activators and calcined hydrotalcite additive

Geopolymers have been widely adopted to stabilize the cationic pollutants. However, few studies have focused on the immobilization of anionic species. In this study, the immobilization of SeO₃^2– and SeO₄^2– was explored for the first time using geopolymer activated by different alkaline solutions (NaOH and Na₂SiO₃) with and without calcined hydrotalcite (CHT), characterized by TCLP, XRD, FTIR, TG, NMR, XAFS, and N₂ adsorption-desorption isotherm. Na₂SiO₃-activated geopolymers without CHT additive showed lower leaching percentages of SeO₃^2– and SeO₄^2– (approximately 10 % and 18 %) than NaOH-activated geopolymers (approximately 58 % and 74 %). It has been proven that electrostatic interaction is the main association mode of SeO₃^2– and SeO₄^2– in both NaOH- and Na₂SiO₃-activated geopolymers. Hence, compactness plays a vital role in the Se leaching from geopolymer. The addition of CHT reduced the compactnesses of both NaOH- and Na₂SiO₃-geopolymers. Due to the formation of hydrotalcite, the CHT additive contributed to immobilize SeO₃^2– and SeO₄^2– in NaOH-activated geopolymers. However, this phenomenon was not observed in Na₂SiO₃-activated geopolymers. Thus, the leaching amount of Se greatly increased from Na₂SiO₃-activated geopolymers with CHT additive. This study provides new insights on the application of geopolymer to immobilize anionic pollutants.