Abstract
Geopolymers are a class of alkaline-activated materials that have been considered as promising materials for radioactive waste disposal. Currently, metakaolin-based geopolymers (MK-GPs) are attracting interest for the immobilisation of radionuclides in contaminated water from the Fukushima Daiichi Nuclear Power Station. However, the associated chemical interaction mechanisms and the theoretical prediction of the adsorption behaviour of MK-GP in response to cationic radionuclides have not been thoroughly studied or fully understood. In addition, there is a lack of studies on the adsorption capacity of MK-GP for anionic radionuclides. In this study, two types of metakaolin-based (Metastar501 and Sobueclay) geopolymers were synthesised at a K2O:SiO2:H2O ratio of 1:1:13. The binding capacity and interaction mechanism of MK-GP with Cs+, Sr2+, Co2+, I-, IO3-, SeO32-, and SeO42- were evaluated based on the zeta potential, radionuclide binding, and alkali leaching. The results showed that MK-GP does not have the ability to incorporate anionic radionuclides irrespective of the metakaolin source used, but both types of geopolymers have a high capacity to immobilise cationic radionuclides. The uptake of Cs+ was observed as a one-to-one exchange between Cs+ and K+ whereas both one–two and one–one ion exchanges are possible in the case of Sr2+ and Co2+ with K+. The formation of cobalt blue (CoAl2O4) also contributed to the binding of Co2+. Thermodynamic modelling was conducted according to the ion exchange mechanism which predicts the binding of Cs+ and Sr2+ at low concentrations.
Highlights
The contamination of cooling water with radionuclides and the radioactive waste generated from water is a serious concern in the decommissioning process of the Fukushima Daiichi Nuclear Power Station (FDNPS) (TEPCO, 2013; Tokyo Electric Power Company (TEPCO), n.d.)
The chemical composition of metakaolin determined by X-ray fluorescence (XRF) is listed in Table 1, and the ratios of SiO2:Al2O3 of MS and SC were 1.04 and 1.01 respectively
Geopolymer samples were prepared by mechanically mixing stoichiometric amounts of metakaolin (Metastar/ Sobueclay) with a sufficient quantity of alkaline silicate solution to give Al2O3/K2O = 1
Summary
The contamination of cooling water with radionuclides and the radioactive waste generated from water is a serious concern in the decommissioning process of the Fukushima Daiichi Nuclear Power Station (FDNPS) (TEPCO, 2013; TEPCO, n.d.). The Tokyo Electric Power Company (TEPCO) has been removing radionuclides from cooling water with several types of adsorbents and plans to safely store the generated radioactive waste for a long time. Selection of the most suitable host material, with high solidification and immobilisation capacity for radioactive waste, is necessary for safe disposal. Cement-based materials have been considered to solidify and encapsulate radioactive waste for a long time owing to their low cost and high durability (García-Gutierrez et al, 2018; Koťatkovaet al., 2017; Bar-Nes et al, 2008). Cement-based materials show a good immobilisation capacity for some radionuclides, their incompatibilities with several other radionuclides and high free water content, which may cause problematic hydrogen generation, limit their usage in some applications (Volchek et al, 2011; Arbel Haddad et al, 2017; Cornell, 1993). It is important to develop an environmentally friendly solidification material for safe disposal and environmental remediation of nuclear waste
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