Adsorption Properties of Sr(II) on Zeolite Type Adsorbents and Their Irradiation Stabilities

Radioactive waste treatment is planned in LWTF (Low-level radioactive Waste Treatment Facility, JAEA) for LLW generated from the Tokai-reprocessing facility. The target LLW consists of highly concentrated sodium nitrate (5 M NaNO3) containing low-level 90Sr. In this study, selective adsorption properties of Sr2+ for highly functional A type zeolites (A51-JHP, A51-J (Union Showa) and A-4, X type zeolite (F-9) and Titanic acid-PAN (polyacrylamide) were clarified by batch and column adsorption methods. The irradiation stabilities of these adsorbents were also evaluated. The distribution properties of Sr2+ on different adsorbents were compared in simulated waste solution (5 M NaNO3, 0.1 ppm Sr2+, 85Sr as tracer). The order of distribution coefficients (Kd,Sr) was Titanic acid-PAN > A51-JHP > A51-J > A-4 > F-9. The largest value of Kd,Sr for titanic acid-PAN was estimated to be 218 cm3/g, while the saturated capacity (Qmax) was very small. Titanic acid-PAN had also the largest uptake rate of Sr2+ ions and the uptake attained equilibrium within 8 h. On the other hand, A51-JHP had a relatively large Kd,Sr value above 100 cm3/g and a Qmax value of 0.65 mmol/g. The breakthrough properties of Sr2+ were examined by varying cations present (single and mixed solutions) and flow rate (0.08 and 0.17 cm3/min). The components for the single solution were 400 g/L NaNO3, 100 ppm Sr2+, 85Sr as tracer, and the mixed solution contains 200 ppm Cs+, 100 ppm Ca2+, 50 ppm Mg2+, 50 ppm RuNO3+ in addition to the single solution components. The breakthrough curve for Titanic acid-PAN column using single solution had an S-shaped profile, while the “concentration phenomenon” exceeding C/C0 (breakthrough ratio) = 1 was observed in the case of mixed solution. As for the A51-JHP column, the breakthrough curve for single solution was similar to that for mixed solution and the 5% breakpoint was enhanced by decreasing the flow rate. The A51-JHP was stable under 60Co-irradiation up to 2.54 MGy; Kd,Sr and Qmax values were almost constant. In contrast, Titanic acid-PAN was affected above 0.28 MGy, due to the radiolysis of PAN matrix, and this surface alteration led to the release of active component of titanic acid. The novel A type zeolite (A51-JHP) is thus expected for the selective removal of Sr2+ in LWTF. The optimization of particle size and flow rate should be examined before practical use.