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

Authors: Lan Ha Thi Le; Son An Nguyen; Trung Dinh Nguyen; Van Cam Thi Le; Hai Van Cao; Ngoc Bao Nguyen; Thao Phuong Thi Le
Journal: Journal of Nanomaterials
 (2021)

---

## 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.


---

## Body

## 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-likeα-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] (A=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:
(1)q=V×Ci−CeB,whereq is the adsorption capacity of the adsorbent material (mg/g adsorbent), Ci and Ce 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].
(2)Langmuiradsorptionequationqe=Qm×b×Ce1+bCe,whereqe is the amount of Cs+, Sr2+, and Co2+ ions adsorbed by the material (mg/g), Qm is the maximum adsorption capacity for Cs+, Sr2+, and Co2+ ions, Ce is the initial concentration at a point of adsorption (mg/L), and rate constant b is the adsorption/desorption.
(3)Freundlichadsorptionequationqe=K×Ce1/n,whereqe 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.


## 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] (A=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:
(1)q=V×Ci−CeB,whereq is the adsorption capacity of the adsorbent material (mg/g adsorbent), Ci and Ce 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].
(2)Langmuiradsorptionequationqe=Qm×b×Ce1+bCe,whereqe is the amount of Cs+, Sr2+, and Co2+ ions adsorbed by the material (mg/g), Qm is the maximum adsorption capacity for Cs+, Sr2+, and Co2+ ions, Ce is the initial concentration at a point of adsorption (mg/L), and rate constant b is the adsorption/desorption.
(3)Freundlichadsorptionequationqe=K×Ce1/n,whereqe 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:
(4)2NiSO4+K4FeCN6→Ni2Fe̲CN6+2K2SO42CuCl2+K4FeCN6→Cu2FeCN6+4KCl2CoCl2+K4FeCN6→Co2FeCN6+4KClCrystalline properties of theas-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 1
PXRD of A2[Fe(CN)6] (A: Co, Cu, Ni).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 volumeV (Å3)
Cu2Fe(CN)6
9.92
±
0.1
142.3
1003.9
Co2Fe(CN)6
10.22
±
0.2
309.6
1022.5
Ni2Fe(CN)6
10.26
±
0.2
166.6
1032.5Table 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
59.0432The 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].Figure 2
TEM images and the corresponding particle size distribution of (a) Co2[Fe(CN)6], (b) Cu2[Fe(CN)6], and (c) Ni2[Fe(CN)6].
(a)
(b)
(c)Figure 3
SEM images and the corresponding elemental mapping by EDX of A2[Fe(CN)6].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.Figure 4
FT-IR spectra of(a) Cu2[Fe(CN)6], Ni2[Fe(CN)6], and Co2[Fe(CN)6] exhibit a characteristic peak of C-N around 2000 cm-1. (b) The location of the C-N vibration peak in Cu2[Fe(CN)6], Ni2[Fe(CN)6], and Co2[Fe(CN)6] is at 2099 cm-1, 2097 cm-1, and 2088 cm-1, respectively.
(a)
(b)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 Kα, 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.Figure 5
The Langmuir isotherm model fitted for the adsorption of (a) Cs+, (b) Sr2+, and (c) Co2+ on A2[Fe(CN)6] at pH 7. (d) The maximum adsorption capacity of Cs+, Sr2+, and Co2+ ions of A2[Fe(CN)6] estimated from their corresponding Langmuir model.
(a)
(b)
(c)
(d)Figure 6
The Freundlich isotherm model fitted for the adsorption of (a) Cs+, (b) Sr2+, and (c) Co2+ on A2[Fe(CN)6] at pH 7.
(a)
(b)
(c)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
0.914Figure 7
TXRF spectra analyzing the composition of the waste solution before and after adsorption by A2[Fe(CN)6].
(a)
(b)
(c)
(d)
(e)
(f)


## 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+.



---

*Source: 5533620-2021-03-27.xml*