A Material Design on Radioactive Cesium Adsorbent: Cs_1-xNa_xMn^II(CN)₃ Prussian Blue Analogue

Abstract

After the accident of Fukushima's nuclear power plant, soil contamination was caused by radioactive cesium. In this study, the new cesium-adsorbent: Cs_1-xNa_xMn^II(CN)₃ Prussian blue analogue was theoretically designed to solve the problem. In Prussian blue (iron cyano-compound), both water defect and iron vacancy are closely related to cesium-adsorption. On the other hand, in Cs_1-xNa_xMn^II(CN)₃, the cesium-adsorption mechanism is completely different. Hybrid Kohn-Sham DFT calculations were performed to investigate the cesium-adsorption mechanism. It was concluded that more stable and efficient cesium-adsorption can be realized by the ion exchange between counter cations (between sodium and cesium ions), under applied voltage.

Figures

Figure 1.

 The cubic crystal structure of CsMn^II(CN)₃ at room temperature. The red, green, blue and yellow dots denote manganese, carbon, nitrogen and cesium, respectively.

Figure 2.

 The cluster models for Cs_1-xNa_xMn^II(CN)₃:(a) CsMn^II₈(CN)₁₂, (b) NaMn^II₈(CN)₁₂ models. Cesium or sodium ion migrates along x axis, following an arrow.

Figure 3.

 The schematic picture on Onishi chemical bonding rule.

Figure 4.

 The cesium-adsorption mechanism.

Figure 5.

 The potential energy curves for (a) CsMn^II₈(CN)₁₂ and (b) NaMn^II₈(CN)₁₂ models, displacing cesium and sodium ions along x axis (x = 0.0 ~ 2.65Å) in CsMn^II₈(CN)₁₂ and NaMn^II₈(CN)₁₂ models, respectively.

Figure 6.

 The molecular orbitals (MOs) related to the outer shell of sodium ion (2s and 2p orbitals) for NaMn^II₈(CN)₁₂ model: (a) the cubic centre (x = 0.0Å), (b) minimum (x = 1.8Å), and (c) bottleneck (x = 2.65Å).

Figure 7.

 The selected molecular orbitals (MOs) related to the outer shell of cesium ion (5s and 5p orbitals) for CsMn^II₈(CN)₁₂ model: (a) the cubic centre (x = 0.0Å), and (b) bottleneck(x = 2.65Å).

Figure 8.

 The schematic picture on the ion exchange between cesium and sodium ions in Cs_1-xNa_xMn^II(CN)₃, under applied voltage.

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