Prussian Blue Analogues of A2[Fe(CN)6] (A: Cu2+, Co2+, and Ni2+) and Their Composition-Dependent Sorption Performances towards Cs+, Sr2+, and Co2+

Abstract

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 different substituted cations (A₂[Fe(CN)₆] (A: Cu²⁺, Co²⁺, and Ni²⁺)). The synthesized PBAs were characterized and employed for the removal of Cs⁺, Sr²⁺, and Co²⁺ as sorption models, which are commonly found in radioactive waste. Sorption examinations reveal that Cu₂[Fe(CN)₆] has the highest sorption capacity towards Cs⁺, Sr²⁺, and Co²⁺ compared with those of Co₂[Fe(CN)₆] and Ni₂[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 reflection X-ray fluorescence spectroscopy analysis (TXRF). In addition, it was found that Cs⁺ is adsorbed most effectively by PBAs due to the size matching between Cs⁺ ions and the channel windows of PBAs. These findings are important for the design of sorbents with suitable ion-exchange capacity and selectivity toward targeted radioactive wastes.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. 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]. Real-time 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., ¹³⁷Cs (30.2 years) [4–6], ⁹⁰Sr (28.8 years) [7], and ⁶⁰Co (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 ¹³⁷Cs and ⁹⁰Sr can be found in radioactive nuclide-contaminated areas, primarily in the aqueous phase, whereas ⁶⁰Co is found as an impurity in the stainless steel used in nuclear reactors. ⁶⁰Co 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 [11–14] have been extensively developed to remove radioactive nuclides from aqueous solutions. Of great interest is the combination of adsorption- and ion-exchange-based 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) [18–20]. In particular, PB and/or PBAs are constructed via coordination bonds between transition metals (e.g., Fe²⁺, Fe³⁺, Cu²⁺, Co²⁺, and Ni²⁺) and CN^- ligands. 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⁺, Sr²⁺, and Co²⁺(Table 1), there is few research comparing the adsorption capacity of Cs⁺, Sr²⁺, and Co²⁺ions and the correlation between PBA compositions and adsorption activities.

Table 1

Comparison of the adsorption capacity of Cs⁺, Sr²⁺, and Co²⁺ ions on different adsorbent materials.

Adsorbent	pH	Maximum adsorption capacity(mg g−1)	References
Cs+ ion
Copper hexacyanoferrate (CuHCF)	7.0	155.60	This research
Cobalt hexacyanoferrate (CoHCF)	7.0	154.46	This research
Nickle hexacyanoferrate (NiHCF)	7.0	120.31	This research
Copper ferrocyanide functionalized mesoporous silica	7.7	17.1	[37]
Zeolite A	6.0	208.7	[38]
Magnetic PB/GO	7.0	55.6	[39]
Montmorillonite-iron oxide composite	6.5	52.6	[25]
Conjugate adsorbent	7.0	77.7	[40]
Ammonium molybdophosphate-polyacrylonitrile	6.5	81.3	[41]
Cs+-imprinted polymer nanoparticle	9.0	50.0	[42]
Poly(AAc-co-B18C6Am) hydrogels	6.0	74.6	[6]
Prussian blue/Fe3O4	7.0	280.82	[43]
CuHCF-cellulose hydrogel	7.0	309	[44]
CuHCF/MWCNT	7.0	310	[44]
MOF/KNiFC	5.0	153	[45]
Sr2+ ion
Copper hexacyanoferrate(CuHCF)	7.0	59.95	This research
Cobalt hexacyanoferrate(CoHCF)	7.0	32.73	This research
Nickle hexacyanoferrate (NiHCF)	7.0	29.17	This research
Amorphous zirconium phosphates	11.4	134.2	[46]
Zr-MOF	7.0	7.548	[47]
Zirconium phosphate on active carbon	6.0	2.9	[22]
ZrO2-TiO2	9.0	28.01	[48]
Zirconium phosphate	1.0	34	[49]
Fower-like α-ZrP	4.0	293.43	[50]
Titanate nanofibers	7.0	55.2	[51]
PAN-zeolite	7.0	44.43	[52]
Carboxymethylated cellulose	4.0	108.7	[53]
Graphene oxide	6.5	23.83	[54]
ZrP-SO3H	4.0	183.21	[55]
Nb-doped WO3	7.0	54.39	[54]
Co 2+ ion
Copper hexacyanoferrate (CuHCF)	7.0	62.08	This research
Nickle hexacyanoferrate (NiHCF)	7.0	32.34	This research
MWCNT/IO		10.61	[56]
Silica SBA-15		181.67	[57]
SiO2/Nb2O5/ZnO		0.518	[58]
Ordered micro- and mesoporous/SiO2		8.43	[59]
Magnetite-based nanocomposites		43.292	[60]
GO-NH2		116.35	[61]

Herein, we successfully synthesized different PBAs, including A₂ [Fe(CN)₆] (A: Cu²⁺, Co²⁺, and Ni²⁺) and compared theirs adsorption performances with Cs⁺, Sr²⁺, and Co²⁺ ions. It was found that the substitution of the transition metal ions used (Cu²⁺, Co²⁺, and Ni²⁺) 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.

2. Materials and Methods

2.1. Materials

Standard solutions (Cs⁺ (1000 mg/L), Sr²⁺ (1000 mg/L), Co²⁺ (1000 mg/L)), CsCl (99.99%, Meck), SrCl₂ (99.99%, Meck), CoCl₂ (99.99%, Meck), K₄[Fe(CN)₆] (99.99%, Meck), CoCl₂·6H₂O (99.99%, Meck), CuCl₂·2H₂O (99.99%, Meck), and NiSO₄·6H₂O (99.99%, Meck) were used as received. pH was adjusted using HNO₃ (0.01–0.1 N) and NaOH (0.01–0.1 N).

2.2. Synthesis of A₂[Fe(CN)₆]

The synthetic protocol for A₂[Fe(CN)₆] ($\text{A}=\text{Co}$, Ni, and Co) was slightly modified from previous Therefore reports [26–29]. For the synthesis of Cu₂[Fe(CN)₆], a 250 mL of 0.05 M K₄[Fe(CN)₆] solution was slowly added to a 750 mL of 0.15 M CuCl₂ 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 Co₂[Fe(CN)₆] and Ni₂[Fe(CN)₆], a CoCl₂ or NiSO₄ solution was, respectively, used in place of CuCl₂ in the aforementioned procedure. The other reaction conditions remained unchanged, unless stated otherwise.

2.3. Adsorption Performance of A₂[Fe(CN)₆] towards Cs⁺, Sr²⁺, and Co²⁺

For the sake of safety, Cs⁺, Sr²⁺, and Co²⁺ used in this study were stable isotopes. A series of reaction flasks containing 50 mL of Cs⁺, Sr²⁺, and Co²⁺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 A₂[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.

The adsorption capacity of A₂[Fe(CN)₆] toward Cs⁺, Sr²⁺, and Co²⁺ is calculated using the following formula: $$\begin{array}{}\text{(1)}& q=\frac{V\times C_i-C_e}{B},\end{array}$$

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⁺, Sr²⁺, and Co²⁺) 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 A₂[Fe(CN)₆]. $$\begin{array}{}\text{(2)}& \text{Langmuir}\text{adsorption}\text{equation}{q}_e=\frac{Q_m\times b\times C_e}{1+b{C}_e},\end{array}$$

where $q_e$ is the amount of Cs⁺, Sr²⁺, and Co²⁺ ions adsorbed by the material (mg/g), $Q_m$ is the maximum adsorption capacity for Cs⁺, Sr²⁺, and Co²⁺ ions, $C_e$ is the initial concentration at a point of adsorption (mg/L), and rate constant $b$ is the adsorption/desorption. $$\begin{array}{}\text{(3)}& \text{Freundlich}\text{adsorption}\text{equation}{q}_e=K\times C_e^{1/n},\end{array}$$

where $q_e$ is the amount of Cs⁺, Sr²⁺, and Co²⁺ ions adsorbed by the material (mg/g), and $K$ and $n$ are the adsorption constant at equilibrium.

2.4. TXRF Analyses of the Samples and Cs⁺, Sr²⁺, and Co²⁺ 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⁺, Sr²⁺, and Co²⁺ before and after adsorption remaining in the solution was also measured by TXRF.

2.5. Characterizations

Crystalline structures of A₂[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 A₂[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 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 X-ray fluorescence (TXRF) S2 Picofox Bruker.

3. Results and Discussion

A₂[Fe(CN)₆] (A: Cu, Co, and Ni) was readily synthesized by precipitating Cu²⁺, Co²⁺, and Ni²⁺ salt with K₄[Fe(CN)₆] aqueous solution at 60°C for 4 h. The chemical reactions for A₂[Fe(CN)₆] are as follows: $$\begin{array}{c}\text{(4)}& 2\text{NiS}{\text{O}}_4+{\text{K}}_4\text{Fe}{\text{CN}}_6\to \text{N}{\text{i}}_2\underline{\text{Fe}}{\text{CN}}_6+2{\text{K}}_2\text{S}{\text{O}}_{4}2\text{CuC}{\text{l}}_2+{\text{K}}_4\text{Fe}{\text{CN}}_6\to \text{C}{\text{u}}_2\text{Fe}{\text{CN}}_6+4\text{KCl}\\ 2\text{CoC}{\text{l}}_2+{\text{K}}_4\text{Fe}{\text{CN}}_6\to \text{C}{\text{o}}_2\text{Fe}{\text{CN}}_6+4\text{KCl}\end{array}$$

Crystalline properties of the as-synthesized A₂[Fe(CN)₆] were examined using PXRD, and the data are shown in Figure 1. Ni₂[Fe(CN)₆], Co₂[Fe(CN)₆], and Cu₂[Fe(CN)₆] exhibit a high degree of crystallinity with a set of diffraction peak characteristic for the PBA family (JCPDS 77-1161) [26, 30, 31]. The respective lattice constant estimated from the PXRD data for Cu₂[Fe(CN)₆], Co₂[Fe(CN)₆], and Ni₂[Fe(CN)₆] is 9.92 ± 0.1 Å, 10.22 ± 0.2 Å, and 10.26 ± 0.2 Å, respectively (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 A₂[Fe(CN)₆] was also characterized using N₂ isotherm adsorption at 77 K, and the results were tabulated in Table 3. The surface area of Co₂Fe (CN)₆ and Ni₂Fe (CN)₆ is around 60 m² g^-1, which are tenfold higher than that of Cu₂Fe (CN)₆. This could be attributed to the slight aggregation of Cu₂Fe(CN)₆ as seen by TEM.

[figure omitted; refer to PDF]

Table 2

Typical parameters estimated from XRD patterns of Cu₂[Fe(CN)₆], Co₂[Fe(CN)₆], and Ni₂[Fe(CN)₆].

Sample	A (Å)	The average crystallite size D (Å)	Unit cell volume $V$ (Å3)
Cu2Fe(CN)6	$$9.92\pm 0.1$$	142.3	1003.9
Co2Fe(CN)6	$$10.22\pm 0.2$$	309.6	1022.5
Ni2Fe(CN)6	$$10.26\pm 0.2$$	166.6	1032.5

Table 3

BJH pore size and BET surface areas of Cu₂[Fe(CN)₆], Co₂[Fe(CN)₆], and Ni₂[Fe(CN)₆].

	Cu2Fe(CN)6	Co2Fe(CN)6	Ni2Fe(CN)6
BJH pore size (nm)	22.105	34.9401	34.0211
BET surface area (m2/g)	5.857	63.9561	59.0432

The particle size and morphological properties of A₂[Fe(CN)₆] were examined using transmission electron microscopy (TEM) (Figure 2). Co₂Fe (CN)₆ shows pseudospherical particles with the size varying between 25 and 55 nm (Figure 2(a)). For Cu₂[Fe(CN)₆], the particles are formed from the aggregation of smaller subparticles, resulting in a wider spectrum of distribution. Among the synthesized PBAs, Ni₂[Fe(CN)₆] shows the smallest size, ranging from 15 nm to 35 nm. Essentially, the elemental composition of A₂[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 Co₂[Fe(CN)₆], Cu₂[Fe(CN)₆], and Ni₂[Fe(CN)₆], respectively. This further confirms the successful synthesis of A₂[Fe(CN)₆].

[figures omitted; refer to PDF]

[figure omitted; refer to PDF]

Infrared spectroscopy (IR) is used to investigate the characteristic bonding information within the structure of A₂[Fe(CN)₆] (Figure 4). In addition to a vibrational band at 590 cm^-1 and 3450 cm^-1 corresponding to Fe-C bond [32, 33] and bending mode of H₂O [34], all of the A₂[Fe(CN)₆ exhibit a characteristic peak assigned to the C-N bonding located at around 2000 cm^-1, which belongs to the CN^- ligand. Principally, the peak position of C-N vibration is indirectly indicative of the bond strength between metal cation and CN^- ligands. It is observed that the location of C-N vibration peak in Cu₂[Fe(CN)₆] is at 2099 cm^-1, which is slightly higher than those of Ni₂[Fe(CN)₆] (2097 cm^-1) and Co₂[Fe(CN)₆] (2088 cm^-1), revealing that the contribution of π-back bonding to the antibonding orbital of CN^- ligand from Cu²⁺ is less significant than those from Ni²⁺ and Co²⁺. In other words, Cu²⁺ within the framework of Cu₂[Fe(CN)₆] binds less strongly to CN ligand than Ni²⁺ and Co²⁺ do in Ni₂[Fe(CN)₆] and Co₂[Fe(CN)₆]. This is an important evidence as it is explicitly correlated with the ion-exchange capacity of A₂[Fe(CN)₆] discussed later.

[figures omitted; refer to PDF]

The adsorption isotherms of A₂[Fe(CN)₆] towards Cs⁺, Sr²⁺, and Co²⁺ were examined at 25°C and pH 7 (Figures 5 and 6). The parameters of the isothermal adsorption of Cs⁺, Sr²⁺, and Co²⁺ ions on A₂[Fe(CN)₆] estimated from Langmuir and Freudlich models are shown in Table 4. It is interesting to note that Cu₂[Fe(CN)₆] shows much higher maximum adsorption capacity (Qm) towards Cs⁺ (155.60 mg g-1), Sr²⁺ (59.95 mg g-1), and Co²⁺ (62.08 mg g-1) than those of Ni₂[Fe(CN)₆] (120.31, 29.17, and 32.34 for Cs⁺, Sr²⁺, and Co²⁺, respectively) and Co₂[Fe(CN)₆] (154.46 and 59.95 for Cs⁺ and Sr²⁺, respectively). Considering crystallographic similarity among the structures of Cu₂[Fe(CN)₆], Ni₂[Fe(CN)₆], and Co₂[Fe(CN)₆], the difference in adsorption capacity can be associated with the ion-exchange capability of the metal nodes in the framework of A₂[Fe(CN)₆]. More specifically, the metal nodes that bond less strongly to the CN^- ligand are more likely to participate in ion-exchange with adsorbate (i.e., Cs⁺, Sr²⁺, and Co²⁺). 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), Sr²⁺ (14.2 keV), and Co²⁺ (6.93 keV) in the solution before and after adding Cu₂[Fe(CN)₆], Ni₂[Fe(CN)₆], and Co₂[Fe(CN)₆] into the solution. As seen, after the sorption reaches equilibrium, the peak intensity corresponding to Cs⁺, Sr²⁺, and Co²⁺ decreases, revealing the sorption process of those cations by A₂[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 any peaks corresponding to those of Ni²⁺ or Co²⁺. These data imply that only Cu²⁺ cations within the framework of Cu₂[Fe(CN)₆] meaningfully participate in the sorption via ion exchanging with the adsorbates. This is in concert with the IR data in which Cu²⁺ binds less strongly to CN^- ligands, thus readily subjected to readily ion exchange with Cs⁺, Sr²⁺, and Co²⁺. 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 Sr²⁺ (4.12 Å) and Co²⁺ (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.

[figures omitted; refer to PDF]

[figures omitted; refer to PDF]

Table 4

Adsorption isothermal parameters of Cs⁺, Sr²⁺_, andCo²⁺ by A₂[Fe(CN)₆] extract from Langmuir and Freundlich models.

		Langmuir			Freundlich
Ion	Adsorbent material	$Q_m$ (mg/g)	$K_L$ (L/mg)	$R^2$	$K_F$ (mg/g)	$n$	$R^2$
Cs+	Cu2[Fe(CN)6]	155.60	0.996	0.927	62.69	6.797	0.907
	Co2[Fe(CN)6]	154.46	0.010	0.954	9.44	2.269	0.890
	Ni2[Fe(CN)6]	120.31	0.272	0.973	53.49	6.570	0.708
Sr2+	Cu2[Fe(CN)6]	59.95	0.068	0.980	13.65	3.804	0.871
	Co2[Fe(CN)6]	32.73	0.008	0.961	1.79	2.233	0.953
	Ni2[Fe(CN)6]	29.17	0.009	0.989	1.58	2.204	0.965
Co2+	Cu2[Fe(CN)6]	62.08	0.078	0.961	18.03	4.585	0.827
	Ni2[Fe(CN)6]	32.34	0.028	0.935	5.81	3.523	0.914

[figures omitted; refer to PDF]

4. Conclusions

Prussian blue analogues (PBAs) with different substituted cations (A₂[Fe(CN)₆] (A: Cu²⁺, Co²⁺, and Ni²⁺)) were successfully synthesized and applied for the removal of Cs⁺, Sr²⁺, and Co²⁺, which are commonly found in radioactive waste. It was found that Cu₂[Fe(CN)₆] exhibits the highest sorption capacity towards Cs⁺, Sr²⁺, and Co²⁺ compared with those of Co₂[Fe(CN)₆] and Ni₂[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 Sr²⁺ and Co²⁺.

References

[1] J. Jewell, "Ready for nuclear energy?: An assessment of capacities and motivations for launching new national nuclear power programs," Energy Policy, vol. 39 no. 3, pp. 1041-1055, DOI: 10.1016/j.enpol.2010.10.041, 2011.

[2] Anonymus, Radiological Characterization of Shut Down Nuclear Reactors for Decommissioning Purposes, 1998.

[3] M. Laraia, "Introduction to nuclear decommissioning: definitions and history," Nuclear Decommissioning,DOI: 10.1533/9780857095336, 2012.

[4] M. Xing, S. Zhuang, J. Wang, "Efficient removal of Cs(I) from aqueous solution using graphene oxide," Progress in Nuclear Energy, vol. 119, article 103167,DOI: 10.1016/j.pnucene.2019.103167, 2020.

[5] H.-M. Yang, K. S. Hwang, C. W. Park, K.-W. Lee, "Sodium-copper hexacyanoferrate-functionalized magnetic nanoclusters for the highly efficient magnetic removal of radioactive caesium from seawater," Water Research, vol. 125, pp. 81-90, DOI: 10.1016/j.watres.2017.08.037, 2017.

[6] H.-R. Yu, J.-Q. Hu, Z. Liu, X.-J. Ju, R. Xie, W. Wang, L.-Y. Chu, "Ion-recognizable hydrogels for efficient removal of cesium ions from aqueous environment," Journal of Hazardous Materials, vol. 323 no. Part B, pp. 632-640, DOI: 10.1016/j.jhazmat.2016.10.024, 2017.

[7] A. M. Emara, F. H. El-Sweify, S. F. Abo-Zahra, A. I. Hashim, T. E. Siyam, "Removal of Cs-137 and Sr-90 from reactor actual liquid waste samples using a new synthesized bionanocomposite-based carboxymethylcellulose," Radiochimica Acta, vol. 107 no. 8, pp. 695-711, DOI: 10.1515/ract-2018-3005, 2019.

[8] G. L. Brownell, C. J. Maletskos, "Half-Life ofFe55andCo60," Physical Review, vol. 80 no. 6, pp. 1102-1103, DOI: 10.1103/PhysRev.80.1102.2, 1950.

[9] M. Manolopoulou, E. Vagena, S. Stoulos, A. Ioannidou, C. Papastefanou, "Radioiodine and radiocesium in Thessaloniki, northern Greece due to the Fukushima nuclear accident," Journal of Environmental Radioactivity, vol. 102 no. 8, pp. 796-797, DOI: 10.1016/j.jenvrad.2011.04.010, 2011.

[10] D. Kim, A. Jo, H.-M. Yang, B.-K. Seo, K.-W. Lee, T. S. Lee, "Colorimetric detection and removal of radioactive co ions using sodium alginate-based composite beads," Journal of Hazardous Materials, vol. 326, pp. 69-76, DOI: 10.1016/j.jhazmat.2016.12.007, 2017.

[11] C. A. Kozlowski, J. Kozlowska, W. Pellowski, W. Walkowiak, "Separation of cobalt-60, strontium-90, and cesium-137 radioisotopes by competitive transport across polymer inclusion membranes with organophosphorous acids," Desalination, vol. 198 no. 1-3, pp. 141-148, DOI: 10.1016/j.desal.2006.02.005, 2006.

[12] C. A. Kozlowski, W. Walkowiak, W. Pellowski, "Sorption and transport of Cs-137, Sr-90 and co-60 radionuclides by polymer inclusion membranes," Desalination, vol. 242 no. 1-3, pp. 29-37, DOI: 10.1016/j.desal.2008.03.029, 2009.

[13] B. Ma, S. Oh, W. S. Shin, S.-J. Choi, "Removal of Co 2+ , Sr 2+ and Cs + from aqueous solution by phosphate-modified montmorillonite (PMM)," Desalination, vol. 276 no. 1-3, pp. 336-346, DOI: 10.1016/j.desal.2011.03.072, 2011.

[14] P. Sylvester, A. Clearfield, "The removal of strontium and cesium from simulated Hanford groundwater using inorganic ion exchange materials," Solvent Extraction and Ion Exchange, vol. 16 no. 6, pp. 1527-1539, DOI: 10.1080/07366299808934593, 1998.

[15] A. Nilchi, H. Atashi, A. H. Javid, R. Saberi, "Preparations of PAN-based adsorbers for separation of cesium and cobalt from radioactive wastes," Applied Radiation and Isotopes, vol. 65 no. 5, pp. 482-487, DOI: 10.1016/j.apradiso.2006.12.003, 2007.

[16] A. F. Seliman, Y. F. Lasheen, M. A. E. Youssief, M. M. Abo-Aly, F. A. Shehata, "Removal of some radionuclides from contaminated solution using natural clay: bentonite," Journal of Radioanalytical and Nuclear Chemistry, vol. 300 no. 3, pp. 969-979, DOI: 10.1007/s10967-014-3027-z, 2014.

[17] T. Kubota, S. Fukutani, T. Ohta, Y. Mahara, "Removal of radioactive cesium, strontium, and iodine from natural waters using bentonite, zeolite, and activated carbon," Journal of Radioanalytical and Nuclear Chemistry, vol. 296 no. 2, pp. 981-984, DOI: 10.1007/s10967-012-2068-4, 2013.

[18] N. R. de Tacconi, K. Rajeshwar, R. O. Lezna, "Metal hexacyanoferrates: electrosynthesis, in situ characterization, and applications," Chemistry of Materials, vol. 15 no. 16, pp. 3046-3062, DOI: 10.1021/cm0341540, 2003.

[19] T. Vincent, C. Vincent, Y. Barré, Y. Guari, G. le Saout, E. Guibal, "Immobilization of metal hexacyanoferrates in chitin beads for cesium sorption: synthesis and characterization," Journal of Materials Chemistry A., vol. 2 no. 26, pp. 10007-10021, DOI: 10.1039/c4ta01128g, 2014.

[20] J. Wang, S. Zhuang, Y. Liu, "Metal hexacyanoferrates-based adsorbents for cesium removal," Coordination Chemistry Reviews, vol. 374, pp. 430-438, DOI: 10.1016/j.ccr.2018.07.014, 2018.

[21] M. S. Denton, M. J. Manos, M. G. Kanatzidi, "Highly selective removal of cesium and strontium utilizing a new class of inorganic ion specific media–92670," Northwestern University, Department of Chemistry, vol. 2145, pp. 60208-63113, 2009.

[22] B. Aguila, D. Banerjee, Z. Nie, Y. Shin, S. Ma, P. K. Thallapally, "Selective removal of cesium and strontium using porous frameworks from high level nuclear waste," Chemical Communications, vol. 52 no. 35, pp. 5940-5942, DOI: 10.1039/C6CC00843G, 2016.

[23] L. Catala, T. Mallah, "Nanoparticles of Prussian blue analogs and related coordination polymers: from information storage to biomedical applications," Coordination Chemistry Reviews, vol. 346, pp. 32-61, DOI: 10.1016/j.ccr.2017.04.005, 2017.

[24] A. Nowak-Król, F. Würthner, "Progress in the synthesis of perylene bisimide dyes," Organic Chemistry Frontiers, vol. 6 no. 8, pp. 1272-1318, DOI: 10.1039/C8QO01368C, 2019.

[25] X. Liu, G.-R. Chen, D.-J. Lee, T. Kawamoto, H. Tanaka, M.-L. Chen, Y.-K. Luo, "Adsorption removal of cesium from drinking waters: a mini review on use of biosorbents and other adsorbents," Bioresource Technology, vol. 160, pp. 142-149, DOI: 10.1016/j.biortech.2014.01.012, 2014.

[26] S. Adak, L. L. Daemen, M. Hartl, D. Williams, J. Summerhill, H. Nakotte, "Thermal expansion in 3 d -metal Prussian Blue Analogs-a survey study," Journal of Solid State Chemistry, vol. 184 no. 11, pp. 2854-2861, DOI: 10.1016/j.jssc.2011.08.030, 2011.

[27] S. Ayrault, B. Jimenez, E. Garnier, M. Fedoroff, D. J. Jones, C. Loos-Neskovic, "Sorption Mechanisms of cesium on Cu II 2 Fe II (CN) 6 and Cu II 3 [Fe III (CN) 6 ] 2 hexacyanoferrates and their relation to the crystalline structure," Journal of solid state Chemistry, vol. 141 no. 2, pp. 475-485, DOI: 10.1006/jssc.1998.7997, 1998.

[28] S. N. Ghosh, "Infrared spectra of the Prussian blue analogs," Journal of Inorganic and Nuclear Chemistry, vol. 36 no. 11, pp. 2465-2466, DOI: 10.1016/0022-1902(74)80454-9, 1974.

[29] K. Itaya, I. Uchida, V. D. Neff, "Electrochemistry of polynuclear transition metal cyanides: Prussian blue and its analogues," Accounts of Chemical Research, vol. 19 no. 6, pp. 162-168, DOI: 10.1021/ar00126a001, 1986.

[30] L. Hu, P. Zhang, Q. W. Chen, J. Y. Mei, N. Yan, "Room-temperature synthesis of Prussian blue analogue Co 3 [Co(CN) 6 ] 2 porous nanostructures and their CO 2 storage properties," RSC Advances, vol. 1, pp. 1574-1578, DOI: 10.1039/C1RA00624J, 2011.

[31] S. Xu, X. Qian, G. Li, "Size and morphology-controlled Ni 2 [Fe(CN) 6 ] ∙ x H 2 O Prussian Blue analogue fabricated via a hydrothermal route," Materials Research Bulletin, vol. 43 no. 1, pp. 135-140, DOI: 10.1016/j.materresbull.2007.02.009, 2008.

[32] F. Karadas, H. el-Faki, E. Deniz, C. T. Yavuz, S. Aparicio, M. Atilhan, "CO 2 adsorption studies on Prussian blue analogues," Microporous and Mesoporous Materials, vol. 162, pp. 91-97, DOI: 10.1016/j.micromeso.2012.06.019, 2012.

[33] J. Lejeune, J.-B. Brubach, P. Roy, A. Bleuzen, "Application of the infrared spectroscopy to the structural study of Prussian blue analogues," Comptes Rendus Chimie, vol. 17 no. 6, pp. 534-540, DOI: 10.1016/j.crci.2014.01.017, 2014.

[34] X. Li, J. Liu, A. I. Rykov, H. Han, C. Jin, X. Liu, J. Wang, "Excellent photo-Fenton catalysts of Fe-Co Prussian blue analogues and their reaction mechanism study," Applied Catalysis B: Environmental, vol. 179, pp. 196-205, DOI: 10.1016/j.apcatb.2015.05.033, 2015.

[35] J. Liu, X. Li, A. I. Rykov, Q. Fan, W. Xu, W. Cong, C. Jin, H. Tang, K. Zhu, A. S. Ganeshraja, R. Ge, X. Wang, J. Wang, "Zinc-modulated Fe–Co Prussian blue analogues with well-controlled morphologies for the efficient sorption of cesium," Journal of Materials Chemistry A, vol. 5 no. 7, pp. 3284-3292, DOI: 10.1039/C6TA10016C, 2017.

[36] E. R. Nightingale, "Phenomenological theory of ion solvation. Effective radii of hydrated ions," The Journal of Physical Chemistry, vol. 63 no. 9, pp. 1381-1387, DOI: 10.1021/j150579a011, 1959.

[37] T. Sangvanich, V. Sukwarotwat, R. J. Wiacek, R. M. Grudzien, G. E. Fryxell, R. S. Addleman, C. Timchalk, W. Yantasee, "Selective capture of cesium and thallium from natural waters and simulated wastes with copper ferrocyanide functionalized mesoporous silica," Journal of Hazardous Materials, vol. 182 no. 1-3, pp. 225-231, DOI: 10.1016/j.jhazmat.2010.06.019, 2010.

[38] A. M. El-Kamash, "Evaluation of zeolite A for the sorptive removal of Cs + and Sr 2+ ions from aqueous solutions using batch and fixed bed column operations," Journal of Hazardous Materials, vol. 151 no. 2-3, pp. 432-445, DOI: 10.1016/j.jhazmat.2007.06.009, 2008.

[39] H. Yang, L. Sun, J. Zhai, H. Li, Y. Zhao, H. Yu, "In situ controllable synthesis of magnetic Prussian blue/graphene oxide nanocomposites for removal of radioactive cesium in water," Journal of Materials Chemistry A, vol. 2 no. 2, pp. 326-332, DOI: 10.1039/C3TA13548A, 2014.

[40] M. R. Awual, T. Yaita, T. Taguchi, H. Shiwaku, S. Suzuki, Y. Okamoto, "Selective cesium removal from radioactive liquid waste by crown ether immobilized new class conjugate adsorbent," Journal of Hazardous Materials, vol. 278, pp. 227-235, DOI: 10.1016/j.jhazmat.2014.06.011, 2014.

[41] N. L. Torad, M. Hu, M. Imura, M. Naito, Y. Yamauchi, "Large Cs adsorption capability of nanostructured Prussian blue particles with high accessible surface areas," Journal of Materials Chemistry, vol. 22 no. 35, pp. 18261-18267, DOI: 10.1039/c2jm32805d, 2012.

[42] M. Shamsipur, H. R. Rajabi, "Flame photometric determination of cesium ion after its preconcentration with nanoparticles imprinted with the cesium-dibenzo-24-crown-8 complex," Microchimica Acta, vol. 180 no. 3-4, pp. 243-252, DOI: 10.1007/s00604-012-0927-x, 2013.

[43] J. Jang, D. S. Lee, "Magnetic Prussian blue nanocomposites for effective cesium removal from aqueous solution," Industrial & Engineering Chemistry Research, vol. 55 no. 13, pp. 3852-3860, DOI: 10.1021/acs.iecr.6b00112, 2016.

[44] Y. Kim, Y. K. Kim, S. Kim, D. Harbottle, J. W. Lee, "Nanostructured potassium copper hexacyanoferrate-cellulose hydrogel for selective and rapid cesium adsorption," Chemical Engineering Journal, vol. 313, pp. 1042-1050, DOI: 10.1016/j.cej.2016.10.136, 2017.

[45] S. Naeimi, H. Faghihian, "Performance of novel adsorbent prepared by magnetic metal-organic framework (MOF) modified by potassium nickel hexacyanoferrate for removal of Cs + from aqueous solution," Separation and Purification Technology, vol. 175, pp. 255-265, DOI: 10.1016/j.seppur.2016.11.028, 2017.

[46] Y. JIAN, W.-J. MU, N. LIU, S.-M. PENG, "Removal of Sr 2+ ions by ta-doped hexagonal WO 3 : zeta potential measurements and adsorption mechanism determination," Acta Physico-Chimica Sinica, vol. 32 no. 8, pp. 2052-2058, DOI: 10.3866/PKU.WHXB201604213, 2016.

[47] Y. Zhu, T. Shimizu, T. Kitajima, K. Morisato, N. Moitra, N. Brun, T. Kiyomura, K. Kanamori, K. Takeda, H. Kurata, M. Tafu, K. Nakanishi, "Synthesis of robust hierarchically porous zirconium phosphate monolith for efficient ion adsorption," New Journal of Chemistry, vol. 39 no. 4, pp. 2444-2450, DOI: 10.1039/C4NJ01749H, 2015.

[48] L. Nagy, G. Török, N. Vajda, I. Gerlei, Preparation of Zirconium-Phosphate on Support Material and its Application for the Sorption of some Radioactive Ions, 1979.

[49] H. Tel, Y. Altaş, M. Eral, Ş. Sert, B. Çetinkaya, S. İnan, "Preparation of ZrO 2 and ZrO 2 -TiO 2 microspheres by the sol- gel method and an experimental design approach to their strontium adsorption behaviours," Chemical Engineering Journal, vol. 161 no. 1-2, pp. 151-160, DOI: 10.1016/j.cej.2010.04.053, 2010.

[50] W. Mu, Q. Yu, R. Zhang, X. Li, R. Hu, Y. He, H. Wei, Y. Jian, Y. Yang, "Controlled fabrication of flower-like α -zirconium phosphate for the efficient removal of radioactive strontium from acidic nuclear wastewater," Journal of Materials Chemistry A, vol. 5 no. 46, pp. 24388-24395, DOI: 10.1039/C7TA07803J, 2017.

[51] T. M. Nenoff, J. E. Miller, S. G. Thoma, D. E. Trudell, "Highly selective inorganic crystalline ion exchange material for Sr 2+ in acidic solutions," Environmental Science & Technology, vol. 30 no. 12, pp. 3630-3633, DOI: 10.1021/es9605331, 1996.

[52] S. Sungworawongpana, S. Pengprecha, "Calcination effect of diatomite to chromate adsorption," Procedia Engineering, vol. 8, pp. 53-57, DOI: 10.1016/j.proeng.2011.03.010, 2011.

[53] H. Faghihian, M. Iravani, M. Moayed, M. Ghannadi-Maragheh, "Preparation of a novel PAN-zeolite nanocomposite for removal of Cs + and Sr 2+ from aqueous solutions: Kinetic, equilibrium, and thermodynamic studies," Chemical Engineering Journal, vol. 222, pp. 41-48, DOI: 10.1016/j.cej.2013.02.035, 2013.

[54] B. Liu, W. Mu, X. Xie, X. Li, H. Wei, Z. Tan, Y. Jian, S. Luo, "Enhancing the adsorption capacity of Sr 2+ and Cs + onto hexagonal tungsten oxide by doped niobium," RSC Advances, vol. 5 no. 20, pp. 15603-15611, DOI: 10.1039/C4RA13161D, 2015.

[55] W. Mu, S. du, Q. Yu, X. Li, H. Wei, Y. Yang, S. Peng, "Highly efficient removal of radioactive 90 Sr based on sulfonic acid-functionalized α -zirconium phosphate nanosheets," Chemical Engineering Journal, vol. 361, pp. 538-546, DOI: 10.1016/j.cej.2018.12.110, 2019.

[56] M. U. Ibezim-Ezeani, F. A. Okoye, O. Akaranta, "Studies on the ion exchange properties of modified and unmodified orange mesocarp extract in aqueous solution," International Archive of Applied Science & Technolgy, vol. 1, pp. 33-40, 2010.

[57] W. Guo, R. Chen, Y. Liu, M. Meng, X. Meng, Z. Hu, Z. Song, "Preparation of ion-imprinted mesoporous silica SBA-15 functionalized with triglycine for selective adsorption of Co(II)," Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 436, pp. 693-703, DOI: 10.1016/j.colsurfa.2013.08.011, 2013.

[58] A. Rámila, B. Muñoz, J. Pérez-Pariente, M. Vallet-Regí, "Mesoporous MCM-41 as drug host system," Journal of Sol-Gel Science and Technology, vol. 26 no. 1/3, pp. 1199-1202, DOI: 10.1023/A:1020764319963, 2003.

[59] J. Andersson, J. Rosenholm, S. Areva, M. Lindén, "Influences of material characteristics on ibuprofen drug loading and release profiles from ordered micro- and mesoporous silica matrices," Chemistry of Materials, vol. 16 no. 21, pp. 4160-4167, DOI: 10.1021/cm0401490, 2004.

[60] S. Tizro, H. Baseri, "Removal of cobalt ions from contaminated water using magnetite based nanocomposites: effects of various parameters on the removal efficiency," Journal of Water and Environmental Nanotechnology, vol. 2 no. 3, pp. 174-185, 2017.

[61] F. Fang, L. Kong, J. Huang, S. Wu, K. Zhang, X. Wang, B. Sun, Z. Jin, J. Wang, X.-J. Huang, J. Liu, "Removal of cobalt ions from aqueous solution by an amination graphene oxide nanocomposite," Journal of Hazardous Materials, vol. 270,DOI: 10.1016/j.jhazmat.2014.01.031, 2014.