Development of metakaolin-based geopolymer for selenium oxyanions uptake through in-situ ettringite formation

Abstract

Studies have demonstrated the remarkable capacity of metakaolin-based geopolymers for the uptake of cationic radionuclides, such as Cs+ and Sr2+. However, the lack of uptake ability towards anionic radionuclides represents a potential avenue for improvement in utilising this material for nuclear waste disposal applications. Additionally, the prevalence of ettringite as an anion exchange agent formed in alkaline environments, such as cement hydration, is widely acknowledged in the field. However, no previous studies have been conducted to explore the utilisation of the advantageous qualities of both metakaolin-based geopolymer and ettringite in developing a comprehensive incorporation process for both cationic and anionic radionuclides. In this study, compositions and sets of preparation conditions were proposed to modify metakaolin-based geopolymer, resulting in the in-situ formation of ettringite, with the aim of enhancing its uptake capabilities for selenium oxyanions. The uptake behaviour and mechanism of ettringite, and geopolymer with in-situ ettringite, were proposed through co-precipitation experiments, binding and structural analysis. The results reveal that the ettringite can uptake SeO32- and SeO42- through co-precipitation; however, its effectiveness is limited to SeO32- in the uptake process, where the mechanism is contingent on the concentration of Se oxyanions present. On the other hand, modified geopolymer with in-situ ettringite retained its capacity to uptake cationic radionuclides such as Cs+ and Sr2+ while evolving to exhibit an ability to uptake SeO32-. Thermodynamic modelling was carried out according to the ion exchange mechanism, which effectively predicts the uptake of SeO32- at low concentrations. The proposed composition and preparation conditions hold the potential for developing an effective incorporation process for cationic and anionic radionuclides. The outcomes of this research enhance the understanding of the uptake behaviour of Se oxyanions by ettringite and provide insight into the potential of metakaolin-based geopolymers to immobilise anions.