Immobilization of sodium-salt wastes containing simulated 137Cs by volcanic ash-based ceramics with different Si/Al molar ratios

In this study, volcanic ash was used as raw material to prepare waste forms with different silicon/aluminum (Si/Al) molar ratios to immobilize sodium-salt waste (SSW) containing simulated 137Cs. Effects of Si/Al molar ratios (3:1 and 2:1) and sodium salts on sintering behavior of waste forms and immobilization mechanism of Cs+ were investigated. Results indicated that the main mineral phase of sintered waste-form matrixes was albite, and the formation of major phases was found to depend on Si/Al molar ratios. Si/Al molar ratio of 2 was favorable for the formation of pollucite, and the formation and crystallization of mineral phases were also decided based on physicochemical characteristics of sodium salts. Furthermore, product consistency test results indicated that the immobilization of Cs+ was related to Si/Al molar ratio, types of sodium salts, and glassy phase. Waste forms with Si/Al molar ratio of 2 exhibited better ability to immobilize Cs+, whereas the influence of sodium salts and glassy phases on the immobilization of SSW showed more complicated relationship. In waste forms with Si/Al molar ratio of 2, Cs+ leaching concentrations of samples containing Na2B4O7·10H2O and NaOH were low. Na2B4O7‧10H2O easily transformed into liquid phase during sintering to consequently achieve low temperature liquid-phase sintering, which is beneficial to avoid the volatilization of Cs+ at high temperature. Results clearly reveal that waste forms with Si/Al molar ratio of 2 and containing Na2B4O7‧10H2O show excellent immobilization of Cs+.