Selective Adsorption Properties and Stable Solidification of Cs by Insoluble Ferrocyanide Loaded Zeolites

In Fukushima NPP-1, large amounts of HALW （high-activity-level water） accumulated in the reactor, turbine building and the trench in the facility is treated by circulating injection cooling system. The development of highly functional adsorbents and stable solidification method contributes to the advancement of the decontamination system and environmental remediation. The present study deals with： （1） preparation of insoluble ferrocyanide loaded zeolites; （2） selective uptake of Cs~ in seawater; and （3） estimation of Cs immobilization ratio and stable solidification. Various kinds of Cs-selective composites loaded with insoluble ferrocyanides （CoFC, NiFC） into the zeolites （zeolite A （A51, A-51J）, zeolite X （LSX）, chabazite （modified chabazite） and natural mordenite （SA-5）） matrices have been prepared to use successive impregnation/precipitation methods by Tohoku University. As for Cs~ adsorption, these composites had relatively large uptake （%） over 95%, distribution coefficients （Kd） above 103 cm3/g and excellent adsorption kinetics even in seawater. The immobilization ratio （%） of Cs for the CoFC saturated with Cs＋was estimated at different calcination temperatures up to 1,200 ℃ in advance. The immobilization ratio was less than 0.1% above 1,000℃, indicating that the adsorbed Cs~ ions are completely volatilized and insoluble ferrocyanides had no immobilization ability for Cs. In contrast, the insoluble ferrocyanide-loaded zeolites had excellent Cs immobilization ability; in the case of insoluble ferrocyanide-loaded natural zeolites （NiFC-SA-5, CoFC-modified chabazite）, the immobilization ratio was above 99% and 96% even after calcination at 1,000 ℃ and 1,100 ℃, respectively, indicating that nearly all Cs ions are immobilized in the sintered solid form. On the other hand, the immobilization ratio for the insoluble ferrocyanide-loaded A and X zeolites （NiFC-A （A51, A51J）, NiFC-X） tended to decrease with calcining temperature; for example, the immobilization ratio for NiFC-X at 1,000 ℃and 1,100 ℃ was estimated to be 74.9% and 55.4%, respectively, and many spots concentrating Cs were observed on the surface. The difference in immobilization behavior between natural zeolites and synthetic ones is probably due to the phase transformation and surface morphology at higher temperature above 1,000 ℃. The stable solidification of insoluble ferrocyanides was thus accomplished by using the excellent Cs immobilization abilities of zeolite matrices （Cs trapping and self-sintering abilities）.

Selective adsorption and stable solidification of radioactive cesium ions by porous silica gels loaded with insoluble ferrocyanides

Development of highly functional cesium selective adsorbents for the decontamination of high-activity-level water(HALW) from the Fukushima NPP-1 accident is very urgent. In order to selectively adsorb the radioactive cesium, three kinds of novel porous silica gels loaded with insoluble ferrocyanides(SLFC) were prepared using a successive impregnation/precipitation method. Based on the results of previous research, the SLFC composites have relatively large uptake ratio above 95%, distribution coefficients(Kd) above 103 cm3/g, and excellent adsorption kinetics even in seawater. The solidification results also indicate that zeolites have an excellent Cs immobilization characteristic, gas-trapping and self-sintering abilities, and low leachability. We chose three kinds of SLFC composites to achieve the optimization of solidification by mixing with nine kinds of additives at high temperatures(up to 1200 °C). The Cs contents in the three composites were estimated to be below 30% of the initial contents and decreased with the three stages at calcination temperatures ranging from 25 to 1200 °C. By contrast, the Cs immobilization ratio was markedly lowered by mixing with additives: of those, allophane had the best immobilization result. By increasing the additive ratio to 50 wt%, the Cs immobilization ratio became almost 100% and no volatilization of Cs was detected even after calcination at 1200 °C. This result indicates that calcination of the mixture of SLFC composites after adsorbing Cs+ ions and specific additives under appropriate ratio is effective for stable solidification.