Prussian Blue Analogues of A 2 [Fe(CN) 6 ] (A: Cu 2+ , Co 2+ , and Ni 2+ ) and Their Composition-Dependent Sorption Performances towards Cs + , Sr 2+ , and Co 2+

Investigation in radioactive contaminant removal from aqueous solutions has been considered essential upon unexpected nuclear accidents. In this report, we have successfully prepared Prussian blue analogues (PBAs) with di ﬀ erent substituted cations (A 2 [Fe(CN) 6 ] (A: Cu 2+ , Co 2+ , and Ni 2+ )). The synthesized PBAs were characterized and employed for the removal of Cs + , Sr 2+ , and Co 2+ as sorption models, which are commonly found in radioactive waste. Sorption examinations reveal that Cu 2 [Fe(CN) 6 ] has the highest sorption capacity towards Cs + , Sr 2+ , and Co 2+ compared with those of Co 2 [Fe(CN) 6 ] and Ni 2 [Fe(CN) 6 ]. This is mainly attributed to the cation-exchange ability of substituted metal within the framework of PBAs. The sorption mechanism is qualitatively and quantitatively supported by infrared spectroscopy (IR) and total re ﬂ ection X-ray ﬂ uorescence spectroscopy analysis (TXRF). In addition, it was found that Cs + is adsorbed most e ﬀ ectively by PBAs due to the size matching between Cs + ions and the channel windows of PBAs. These ﬁ ndings are important for the design of sorbents with suitable ion-exchange capacity and selectivity toward targeted radioactive wastes.


Introduction
Increasing demand for nuclear power plants (NPP) has caused a large amount of highly radioactive waste, which brings with it the risk of severe impact on humans and the environment in the event of a nuclear accident [1]. Realtime monitoring and removal of such contaminants are considered an important task upon a NPP shutdown [2,3]. Radioactive nuclides with long half-life, e.g., 137 Cs (30.2 years) [4][5][6], 90 Sr (28.8 years) [7], and 60 Co (5.3 years) [8], are the primary species produced in the nuclear reactions and are potentially discharged into the environment during an accident, such as the explosion at the Fukushima Daiichi power plant in 2011 [9]. The isotopes 137 Cs and 90 Sr can be found in radioactive nuclide-contaminated areas, primarily in the aqueous phase, whereas 60 Co is found as an impurity in the stainless steel used in nuclear reactors. 60 Co is also used as a gamma ray source in radiotherapy or used as a disinfectant in the food industry [10]. Thus far, a variety of techniques, e.g., precipitation, extraction, ion-exchange, and adsorption [11][12][13][14] have been extensively developed to remove radioactive nuclides from aqueous solutions. Of great interest is the combination of adsorption-and ion-exchangebased approaches because the combined techniques can considerably enhance removal efficiency and selectivity towards the targeted radioactive waste rather than the coexisting competitors or inhibitors [15]. Therefore, it is highly desirable to develop advanced materials with a high degree of porosity and well-established pore size distribution and controllable ion-exchange capability for improved removal efficiency.
In recent years, there have been several classes of porous inorganic materials that match the aforementioned standards, such as clays [16], zeolites [17], and Prussian blue (PB) and Prussian blue analogues (PBAs) [18][19][20]. In particular, PB and/or PBAs are constructed via coordination bonds between transition metals (e.g., Fe 2+ , Fe 3+ , Cu 2+ , Co 2+ , and Ni 2+ ) and CNligands. In particular, PBAs can be synthesized in a facile and cost-effective manner. Such materials often exhibit high porosity, excellent thermal, and radiation stability [21], which render them highly applicable in many fields, including information/energy storage [22], biomedicine [23], and dye [24] or radioactive waste removal. In addition, PBAs have been regarded as one of the most efficient and selective adsorbents for cesium ions. The selective cesium adsorption is attributable to the size matching between PBAs (3.2 Å) and cesium ions (3.25 Å) [25]. Although a number of publications have demonstrated adsorption performance of PBAs towards individual radioactive nuclides Cs + , Sr 2+ , and Co 2+ (Table 1), there is few research comparing the adsorption capacity of Cs + , Sr 2+ , and Co 2+ ions and the correlation between PBA compositions and adsorption activities.
Herein, we successfully synthesized different PBAs, including A 2 [Fe(CN) 6 ] (A: Cu 2+ , Co 2+ , and Ni 2+ ) and compared theirs adsorption performances with Cs + , Sr 2+ , and Co 2+ ions. It was found that the substitution of the transition metal ions used (Cu 2+ , Co 2+ , and Ni 2+ ) in the framework of PBAs led to improved adsorption capacity and selectivity. Total reflection X-ray fluorescence spectroscopy analysis (TXRF) provides quantitative evidence with respect to the adsorption mechanism of the obtained PBAs.  6 ] (A = Co, Ni, and Co) was slightly modified from previous Therefore reports [26][27][28][29]. For the synthesis of Cu 2 [Fe(CN) 6 ], a 250 mL of 0.05 M K 4 [Fe(CN) 6 ] solution was slowly added to a 750 mL of 0.15 M CuCl 2 solution. The reaction mixture was stirred at 1200 rpm and sonicated, prior to heating to 60°C for 4 h. Upon reaction completion, the product was purified by repeated washing with water and centrifugation and dried at 70°C. For the synthesis of Co 2 [Fe(CN) 6 ] and Ni 2 [Fe(CN) 6 ], a CoCl 2 or NiSO 4 solution was, respectively, used in place of CuCl 2 in the aforementioned procedure. The other reaction conditions remained unchanged, unless stated otherwise. 6 ] towards Cs + , Sr 2+ , and Co 2+ . For the sake of safety, Cs + , Sr 2+ , and Co 2+ used in this study were stable isotopes. A series of reaction flasks containing 50 mL of Cs + , Sr 2+ , and Co 2+ solutions with concentrations of 0.1 mg/L, 1 mg/L, 10 mg/L, 30 mg/L, 50 mg/L, 70 mg/L, 100 mg/L, 150 mg/L, 200 mg/L, 250 mg/L, 300 mg/L, 350 mg/L, 400 mg/L, 450 mg/L, 500 mg/L, 550 mg/L, and 600 mg/L were prepared. To the above solutions, 0.1 g of the as-synthesized A 2 [Fe(CN) 6 ] was added. The pH was adjusted to 7.0, and the mixture was sealed and shaken at 270 times/min for 24 hours at 25°C in order to reach equilibrium. After adsorption completion, the adsorbent was separated by centrifugation (8500 rpm, 10 min), and the remaining solution was filtered through a 220 nm filter for further analysis with TXRF.

Adsorption Performance of A 2 [Fe(CN)
The adsorption capacity of A 2 [Fe(CN) 6 ] toward Cs + , Sr 2+ , and Co 2+ is calculated using the following formula: where q is the adsorption capacity of the adsorbent material (mg/g adsorbent), C i and C e are the concentrations of adsorbate (i.e., Cs + , Sr 2+ , and Co 2+ ) before and after adsorption, respectively, V is the volume of the solution, and B is the mass of the adsorbent used.
Langmuir and Freundlich models were used to assess the adsorption performance of A 2 [Fe(CN) 6 ].

Langmuir adsorption equation
where q e is the amount of Cs + , Sr 2+ , and Co 2+ ions adsorbed by the material (mg/g), Q m is the maximum adsorption capacity for Cs + , Sr 2+ , and Co 2+ ions, C e is the initial concentration at a point of adsorption (mg/L), and rate constant b is the adsorption/desorption.

Freundlich adsorption equation
where q e is the amount of Cs + , Sr 2+ , and Co 2+ ions adsorbed by the material (mg/g), and K and n are the adsorption constant at equilibrium.

TXRF Analyses of the Samples and Cs + , Sr 2+ , and Co 2+
Solution prior to and after Adsorption. After adsorption completion, the adsorbents were washed several times with distilled water and dried at 60°C. The sample elemental contents were analyzed by total reflection X-ray fluorescence (TXRF) to monitor the change in the composition of the material before and after the reaction. The content of Cs + , Sr 2+ , and Co 2+ before and after adsorption remaining in the solution was also measured by TXRF.

Characterizations. Crystalline structures of A 2 [Fe(CN) 6 ]
were investigated by powder X-ray diffraction (PXRD) performed with a Bruker D8 Advance diffractometer using Cu Kα radiation (wavelength 1.541 Å) in focused beam and in the range 10-80°. The morphologies and elemental composition of A 2 [Fe(CN) 6 ] were characterized using field emission transmission electron microscopy (FE-TEM; JEM 2100-Jeol, Japan) and energy dispersive X-ray spectroscopy (EDS; JEM 2100-Jeol, Japan). Gas adsorption isotherms at 2 Journal of Nanomaterials 77 K are obtained using TriStar II-Micromeritics, America. The IR spectra of the samples were recorded in the 399-4000 cm -1 range using KBr pellets on a Nicolet iS10 (Thermo Scientific, America). The composition of the material before and after the reaction was analyzed using total reflection Xray fluorescence (TXRF) S2 Picofox Bruker.  6 ] are as follows: 3 Journal of Nanomaterials

Results and Discussion
Crystalline properties of the as-synthesized A 2 [Fe(CN) 6 ] were examined using PXRD, and the data are shown in Figure 1.  (Table 2). Although the estimated lattice constants show a slight deviation, presumably due to the size difference among the metal ions, these results are highly consistent with the lattice constant of the face-centered cubic (Pm3m) of PBAs previously reported [26]. The specific surface area of A 2 [Fe(CN) 6 ] was also characterized using N 2 isotherm adsorption at 77 K, and the results were tabulated in Table 3. The surface area of Co 2 Fe (CN) 6 and Ni 2 Fe (CN) 6 is around 60 m 2 g -1 , which are tenfold higher than that of Cu 2 Fe (CN) 6 . This could be attributed to the slight aggregation of Cu 2 Fe(CN) 6 as seen by TEM.
The particle size and morphological properties of A 2 [Fe(CN) 6 ] were examined using transmission electron microscopy (TEM) (Figure 2). Co 2 Fe (CN) 6 shows pseudospherical particles with the size varying between 25 and 55 nm (Figure 2(a)). For Cu 2 [Fe(CN) 6 ], the particles are formed from the aggregation of smaller subparticles, resulting in a wider spectrum of distribution. Among the synthesized PBAs, Ni 2 [Fe(CN) 6 ] shows the smallest size, ranging from 15 nm to 35 nm. Essentially, the elemental composition of A 2 [Fe(CN) 6 ] was confirmed using X-ray energy dispersion spectroscopy (EDX) (Figure 3). The data show that the elements Co, Cu, and Ni were uniformity distributed throughout the examined area in Co 2 [Fe(CN) 6 (Figure 4). In addition to a vibrational band at 10 6 6 ]. More specifically, the metal nodes that bond less strongly to the CNligand are more likely to participate in ion-exchange with adsorbate (i.e., Cs + , Sr 2+ , and Co 2+ ). In order to further understand the sorption mechanism, TXRF was used to investigate the solution composition before and after sorption (Figure 7). Figures 7(a)-7(c), respectively, demonstrate the change in the peak intensity of Cs + (4.3 keV), Sr 2+ (14.2 keV), and Co 2+ (6.93 keV) in the solution before and after adding Cu 2 [Fe(CN) 6 ], Ni 2 [Fe(CN) 6 ], and Co 2 [Fe(CN) 6 ] into the solution. As seen, after the sorption reaches equilibrium, the peak intensity corresponding to Cs + , Sr 2+ , and Co 2+ decreases, revealing the sorption process of those cations by A 2 [Fe(CN) 6 ]. Interestingly, the peak located at 8.05 keV, which is assigned to Cu K α , is clearly observed after the sorption process in all solutions (Figures 7(d)-7(f)); however, we could not observe   6 ] meaningfully participate in the sorption via ion exchanging with the adsorbates. This is in concert with the IR data in which Cu 2+ binds less strongly to CNligands, thus readily subjected to readily ion exchange with Cs + , Sr 2+ , and Co 2+ . Ion-exchange-based sorption for removal of radioactive waste was also previously reported for PBAs [35]. In addition, among the tested cations, Cs + was found to be the most effectively adsorbed PBAs. This is mainly attributed to the size similarity between Cs + cation (3.25 Å) [25] and the channel window of PBAs (3.2 Å), while the size of Sr 2+ (4.12 Å) and Co 2+ (4.23 Å) [36] is comparably larger than the window size. These are important points as these findings can allow for the potential design of adsorbent with designed ion-exchange capacity, so that we could further control the sorption process as well as enhance the selectivity.

Conclusions
Prussian blue analogues (PBAs) with different substituted cations (A 2 [Fe(CN) 6 ] (A: Cu 2+ , Co 2+ , and Ni 2+ )) were successfully synthesized and applied for the removal of Cs + , Sr 2+ , and Co 2+ , which are commonly found in radioactive waste. It was found that Cu 2 [Fe(CN) 6 ] exhibits the highest sorption capacity towards Cs + , Sr 2+ , and Co 2+ compared with those of Co 2 [Fe(CN) 6 ] and Ni 2 [Fe(CN) 6 ]. IR and TXRF data reveal that the cation-exchange ability of substituted metal within the framework of PBAs has a significant impact on the sorption performance of PBAs. In addition, the similarity between the Cs + size and the channel window size of PBAs leads to a preferential sorption of Cs + over Sr 2+ and Co 2+ .

Data Availability
The data has been provided by the DaLat University.

Conflicts of Interest
The authors declare that they have no conflicts of interest.  10 Journal of Nanomaterials