The Switchable Uptake and Elution for Cesium Ions Using Electroactive Membrane of Copper Hexacyanoferrate Nanoparticles

The switchable uptake and elution for cesium ions using electroactive membrane of copper hexacyanoferrate nanoparticles

Rongzhi Chen^a, Hisashi Tanaka^a,*, Tohru Kawamoto^a, Miyuki Asai^a, Chikako Fukushima^a,

Masato Kurihara^a,b, Masayuki Watanabe^c, Makoto Arisaka^c, Takuya Nankawa^c

^aNanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, 305-8565, Japan.

^b Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, Yamagata 990-8560, Japan

^c Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, Tokai-mura, 319-1195, Japan

1. Introduction

Nuclear power is one of the reliable and abundant power sources with an attractive advantage of zero carbon dioxide emission. However, the safety of spent nuclear fuel is of concern to the general public. The earthquake and subsequent tsunami happened on 11 March 2011, caused huge damage to the Fukuashima nuclear plant, Japan. This accident had wide spread radiation health effects including those caused by ¹³⁷Cs and ¹³⁴Cs. Several techniques such as chemical precipitation, ion exchange, and evaporation can be used for the removal cesium from radioactive waste solutions. Among these processes, ion exchange appears to be the most effective to control radioactive wastes due to its simplicity, selectivity, and efficiency ^[1].

Transition metal hexacyanoferrates (MHCFs) are preferred to be competitive ion exchangers over other materials due to their selectivity and high capacity ^[2]. Cu^IIHCF^III has been much used as precipitants for alkali metal cations, especially cesium, to remove them from aqueous radioactive wastes ^[3]. Instated of the conventional adsorption by using copper hexacyanoferrate powder, we developed and coated the Cu^IIHCF^IIInanoparticles on the electrode substance, and then performed Cs removal in an Electrochemical Adsorption System (EAS). The objective of this study was to develop a nanoparticle film of copper (II) hexacyanoferrate (III) for Cs removal, and to determine the effect of solution pH. Attentions were also paid to explore the possible interaction mechanism, and regeneration performance.

2. Experimental

The CuHCF precipitates were synthesized following the equation ^[4]:

3Cu(NO₃)₂•2H₂O +2K₃[Fe(CN)₆] → Cu₃[Fe(CN)₆]₂ +  6KNO₃

Then, the precipitation CuHCF core was mixed with Na₄[Fe(CN)₆] •10H₂O to improve its hydrophilicity for water soluble. Finally 5% CuHCF ink was obtained for membrane coating.

The Cs adsorption was performed in a three-electrode cell in which an Hg/Hg₂Cl₂/KCl (saturated solution) as reference electrode, a platinum electrode (as counter electrode), and the CuHCF membrane coated adsorbent electrode were used as the working electrodes. The redox reaction for ion exchange process using applied potentials of 1.3V~0V (vs. Hg/Hg₂Cl₂) is proposed as follows:

Cu₃[Fe^III(CN)₆]₂ (sorbent electrode) + 2e⁻ + 2Cs⁺↔ Cs₂Cu₃[Fe^II(CN)₆]₂

The reaction was performed for 30min, the Cs loaded electrode were transfer into a 50 mL NaNO₃solution for Cs elution for another 30 min

3. Results and Conclusion

The field emission scanning electron microscopy (FE-SEM) images of the Cu^IIHCF^III NPs film were shown in Fig. 1a, heterogeneous NPs were uniformly dispersed throughout the ferrocyanate network, which predicted that the uptake kinetic might be very fast. The enlarged image (Fig. 1b) indicated that the NPs size ranged from 20 to 40 nm. X-ray diffraction (XRD) pattern was showed in Fig. 1c, the broadened peaks of the Cu[Fe(CN)₆]_0.667 suggested its nanocrystallite appearance. According to Sherrer’s formula, D = K*λ/β* cos θ, the crystallite size of CuHCF NPs can be evaluated from the full width at half maximum β of the diffraction peak with the Bragg angle of θ. Since the particles are cubic, then Kis equal to 0.943 and the average crystallite size is about 20 nm. The rough surface of CuHCF NPs film was also observed by Atomic force microscopy (AFM) in Fig. 1d. Due to the estimation of the NPs agglomeration, the size ranged from 20-80 nm, which was a little larger than results from SEM and XRD.  

The Cs removal efficiency (R%) showed unremarkable change in the pH range from 0.3 to 9.2, and then decreased dramatically with increasing pH to 11.5. At low pH competition with hydrogen ions for exchange sites or dissolution of a portion of sorbent will result in a decrease of ions sorption. However, the produced charges made contributions to electrostatic attraction and ion exchange, thus the sorption capacities almost showed no change under acidic conditions. At pH greater than 9.2, the cesium hydroxides began to form and some of them was negatively charged Cs(OH)₂^-which resulted in the decrease of Cs adsorption due to electrostatic repulsion.

The correlation coefficients of PFO (0.982) and PSO (0.990) were decent, indicating that both can well describe the adsorption kinetic with a little prefer to the PSO. Interestingly, we noticed that kinetic rate could be also described as two stages (red dashed). The first stage (from initial to 60 min) followed the PFO (the R² was revised as 0.9978), since it is initial stage of adsorption, more charge were produced. The Cs concentration was relatively high; the rate may only depend on one reactant, Cs ions. At this stage, the physical electrostatic attraction may be the predominant sorption mechanism. With the time increase (from 60 min to equilibrium), the surface of Cu^IIHCF^III film was electro-actived, producing more adsorption sites, then the rate was determined by both Cs ions and Cu^IIHCF^III. This chemical process preferred to follow the PSO (revised R² is 0.9955).