Vol.9 No.3 2017

Research paper : Radioactive cesium decontamination technology for ash (T. Kawamoto et al.)−142−Synthesiology - English edition Vol.9 No.3 (2017) 2.2.1 Development of PB nanoparticles for use as radioactive cesium adsorbentsThe benefits of the PB nanoparticles, the material of our technology for use with radioactive cesium, is its high selectivity and efficient adsorption. Although PB tends to form fine particles, the cesium adsorption performance can be increased through optimization of the chemical composition as well as its particle size.[30][32] The particle size and composition of the PB nanoparticles is shown in Table 1 in comparison with “konjo” (905; Dainichiseika Color & Chemicals Manufacturing Co., Ltd.), a commercially available form of Prussian blue.[30] The points are the optimization of the composition and the control of the particle size. In general, the Prussian blue composition is expressed as AyFe[Fe(CN)6]1−x, implying the vacancies of [Fe(CN)6]’s or alkali cations insertion in its structure. The x and y in the chemical composition respectively represent the amount of the [Fe(CN)6] vacancies, and the amount of cation introduced. Prussian blue is obtainable by mixing the Feα+ ion water solution and [Fe(CN)6]β− ion solution. It is possible to control the composition by appropriate choice of the mixing ratio, types of counter ion, and valence numbers of the ions. Results show that the adsorption capacity changed greatly according to the composition.[45]To increase the adsorption performance, we reduced the material to nanoparticles. The size of our nanoparticles was about 8–20 nm (Fig. 5(a)). The primary particle size of nanoparticles is dependent on the synthesis method. For example, it is possible to reduce the particle size by rapid mixing. In the case of copper hexacyanoferrate, a Prussian blue analog, we achieved improvement of 7.7 times in adsorption speed compared with the case of conventional synthesis by the reduction of the particle size by micro-mixer synthesis.[33] The particle size can also be controlled to a certain degree by controlling the ionic valence number or temperature during mixing.Powder size, the secondary particle size, also affects the adsorption performance. The diffusion coefficient of radioactive cesium in a powder is low compared to that in liquid, implying that a higher adsorption rate is possible by reducing the secondary particle diameter. For example, for powder synthesis by drying of the PB nanoparticle suspension, an appropriate choice of the drying method is necessary to achieve the small secondary particle size.Figure 5(b) presents the dependence of the cesium adsorption rate on the liquid–solid ratio when various adsorbent powders were added to water for washing the incinerated ash. When our PB nanoparticles were used, almost 100 % adsorption was observed with a liquid–solid ratio of 5,000 or 200 ppm contents. However, the adsorption rate of commercial PB was approximately 80 %, and for zeolite, it was used frequently as a cesium adsorbent: it was 20 % or less. The reasons for the difference of the adsorption rate against the commercial PB are its different particle size and different chemical composition. The poor adsorption of zeolite derives from its 40 nm5 µmLiquid/Solid (mL/g)Adsorption ratio (%)ZeoliteCommercial PBNPs (60 µm)NPs (11 µm)1006000400020000806040200(a)(b)9 µm60 µm11 µmSecondary particle diameter36 nm8.8 nmPrimary particle diameter(NH4)0.64Fe[Fe(CN)6]0.91Fe [Fe(CN)6]0.75CompositionCommercially available PBNanoparticle (60 µm)Nanoparticle (11 µm)NameTable 1. Outline of Prussian blue nanoparticle[30]Fig. 5 (a) Electron microscope image of PB nanoparticle powder. (b) Liquid–solid ratio dependency of adsorption performance when various cesium adsorbents are added to the water after washing the incinerated ash[3][30]

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