# Radiochemical Studies on the Separation of Cesium, Cobalt, and Europium from Aqueous Solutions Using Zirconium Selenomolybdate Sorbent

Authors: H. El-Said
Journal: Journal of Chemistry
 (2013)

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## Abstract

A procedure for removal and separation of Cs, Eu and activation product of Co using zirconium selenomolybdate was developed. Interactions of134Cs(I), 152,154Eu(III), and 60Co(II) ions from HNO3 acid solutions with zirconium selenomolybdate matrix, dried at 50°C, have been individually investigated by the batch equilibration method. The sorption behavior of the three ions showed a selectivity sequence in the following order: Cs(I) > Eu(III) > Co(II). The breakthrough capacities of zirconium selenomolybdate for Cs(I), Eu(III) and Co(II) were found to be 0.82, 0.45, and 0.18 mmol/g of the sorbent, respectively. A mixture of the three radionuclides (1×10−2 M each) in 140 mL of 1×10−2 M HNO3 solution was passed through 1 g zirconium selenomolybdate chromatographic column. Thereafter, quantitative elution of the retained Co(II) was achieved with 14 mL of 1×10−1 M HNO3 acid solution leaving Eu(III) and Cs(I) strongly retained onto the column. Quantitative elution of Eu(III) was achieved by passing 22 mL 2.5×10−1 M HNO3. About 89% of the retained Cs(I) was eluted with 32 mL of 2 M NH4Cl solution at a flow rate of 0.5 mL/min.


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## Body

## 1. Introduction

Synthetic inorganic sorbents have many advantages over organic ones. These advantages include high selectivity to ions of some elements of potential benefits, rapid rate of uptake, stability towards high temperature and ionizing radiation doses, and acidic and moderately alkaline media [1, 2]. In addition, they have the ability to be converted into unleachable glass or ceramic form. There are a wide variety of inorganic sorbents utilized successfully for treatment of large volume of nuclear waste effluents to separate and concentrate the radionuclides in small volume before burying and disposal of the treated liquid aquatic system and/or recovery of some valuable radionuclides for reuse in different applications [3–14]. In addition, inorganic ion exchangers can be used for treatment of industrial effluents to remove some toxic heavy metals which are frequently found in these effluents [15–21]. The proper choice is limited to a number of factors such as chemical composition of the medium, reactivity of the radionuclides present and in turn selectivity of the sorbent, solution concentration, pH, and temperature.Heteropolyacids and salts have found versatile radiochemical separation of mixture of radioisotopes from each other and of fission products, parent/daughter isobars separation onto chromatographic columns in the form of radioisotope generators and immobilization of exhausted radiowaste onto installed traps. Heteropolyacid sorbents such as 12-molybdocerate, zirconium-selenomolybdate, and 12-tungstocerate have promising surface adsorption reactions with different metal ions in solution such as Cs, Ba, Co, Eu, Zn, Cd, Pb, Sn, In, and Ag [14, 22, 23]. Their chemical and radiation stability are suitable for their introduction in the field of chromatographic applications.The three radioisotopes,137Cs, 152,154Eu, and 60Co, are important long-lived products in the nuclear waste. 137Cs is a fission product, 60Co is an activation product, and 152,154Eu is fission and neutron activation product. The removal of these radioisotopes decrease the radiation level of the waste. In addition, the recovery of some valuable radionuclide such as 137Cs for reuse in different applications by preparation of 137Cs/137mBa radioisotope generator which is used in medicine and industry in quality control process. 152,154Eu and 60Co are used for producing sealed sources which are used in medicine and industry.The present work aims at (i) preparation of zirconium(IV) selenomolybdate, (ii) determination of batch distribution coefficients of134Cs(I), 152,154Eu and 60Co(II) from HNO3 solutions individually on zirconium(IV) selenomolybdate matrix, (iii) determination of the breakthrough capacities of zirconium(IV) selenomolybdate matrix for these ions, and (iv) separation of these radionuclides from each other by loading their mixture solution in HNO3 onto a chromatographic column of the matrix and eluting them subsequently with HNO3 solutions and/or other solutions of different concentrations.


## 2. Experimental

All chemicals were of A. R. grade. Distilled water was used for different solution preparations and washing. Radiometric identification and measurements were made by using a multichannel analyzer (MCA) of “Inspector 2000” model, Canberra Series, made in USA, coupled with a high-purity germanium coaxial detector (HPGe) of “GX2518” model.


### 2.1. Radiotracers of134Cs, 152,154Eu, and 60Co

Radionuclides of134Cs and 60Co were produced by thermal neutron irradiation of CsCl and CoCl2 target materials in the water-cooled ETRR-2 Research Reactor (Egypt) for 4 h at a thermal neutron flux of 1 × 1014 n cm−2 s−1. Radionuclides of 152,154Eu were produced by thermal neutron irradiation of Eu2O3 target materials in the water-cooled ETRR-1 Research Reactor (Egypt) for 48 h at a thermal neutron flux of 1 × 1013 n cm−2 s−1.


### 2.2. Preparation of Zirconium Selenomolybdate

Zirconium selenomolybdate was prepared by mixing amounts of zirconium oxychloride, selenous acid, and ammonium molybdate with molar ratio 2 : 2 : 1 with constant stirring. The pH was adjusted to be 3 by adding ammonia solution. After standing for 24 h, the precipitate was separated by suction and washed with water. The separated precipitate was dried at 50°C, packed in chromatographic column and converted into the H+-form by passing 10−1M HNO3 acid solution. The obtained exchanger was washed again with water and redried at 50°C.


### 2.3. Batch Distribution Studies

The distribution coefficients for134Cs(I), 152,154Eu, and 60Co(II) ions in aqueous HNO3 acid solutions on zirconium selenomolybdate matrix were individually determined by the static batch equilibration technique using the following equation:
(1)Kd=Ai-AfAf×Vm(mL/g),
where Ai and Af are the counting rates of the aqueous phase before and after equilibration with the gel matrix, respectively, V is the volume of the aqueous phase (10 mL), and m is the weight of the gel matrix (0.1 g).


### 2.4. Capacity Studies

Chromatographic column breakthrough investigations were conducted by passing HNO3 solutions (10−2 M) of appropriate volumes 10−2 M of each of 134Cs(I), 152,154Eu, and 60Co(II) individually through glass columns (0.6 cm i.d.) packed with 1 g of zirconium selenomolybdate matrix at a flow rate of 0.5 mL/min. The breakthrough capacities (Q’s) of these ions were calculated from the following equation:
(2)Q=C0V50%w(mmol/g),
where C0 is the initial metal ion concentration of the corresponding nuclide (M) in its feeding solution, V50% is the effluent volume (mL) at C/C0=0.5 (as indicated by measuring the counting rates of the initial solution and different effluent fractions), and w is the weight of the column matrix.


### 2.5. Chromatographic Separation

A mixture solution of134Cs(I), 152,154Eu, and 60Co(II) ions in HNO3 acid (10−2M and pH 2) was passed through a chromatographic column (0.6 cm i.d.) containing 1 g of zirconium selenomolybdate matrix at a flow 0.5 mL/min. The radionuclides were separated from each other by eluting the loaded column with HNO3 acid and NH4Cl solutions of different volumes and concentration.


## 2.1. Radiotracers of134Cs, 152,154Eu, and 60Co

Radionuclides of134Cs and 60Co were produced by thermal neutron irradiation of CsCl and CoCl2 target materials in the water-cooled ETRR-2 Research Reactor (Egypt) for 4 h at a thermal neutron flux of 1 × 1014 n cm−2 s−1. Radionuclides of 152,154Eu were produced by thermal neutron irradiation of Eu2O3 target materials in the water-cooled ETRR-1 Research Reactor (Egypt) for 48 h at a thermal neutron flux of 1 × 1013 n cm−2 s−1.


## 2.2. Preparation of Zirconium Selenomolybdate

Zirconium selenomolybdate was prepared by mixing amounts of zirconium oxychloride, selenous acid, and ammonium molybdate with molar ratio 2 : 2 : 1 with constant stirring. The pH was adjusted to be 3 by adding ammonia solution. After standing for 24 h, the precipitate was separated by suction and washed with water. The separated precipitate was dried at 50°C, packed in chromatographic column and converted into the H+-form by passing 10−1M HNO3 acid solution. The obtained exchanger was washed again with water and redried at 50°C.


## 2.3. Batch Distribution Studies

The distribution coefficients for134Cs(I), 152,154Eu, and 60Co(II) ions in aqueous HNO3 acid solutions on zirconium selenomolybdate matrix were individually determined by the static batch equilibration technique using the following equation:
(1)Kd=Ai-AfAf×Vm(mL/g),
where Ai and Af are the counting rates of the aqueous phase before and after equilibration with the gel matrix, respectively, V is the volume of the aqueous phase (10 mL), and m is the weight of the gel matrix (0.1 g).


## 2.4. Capacity Studies

Chromatographic column breakthrough investigations were conducted by passing HNO3 solutions (10−2 M) of appropriate volumes 10−2 M of each of 134Cs(I), 152,154Eu, and 60Co(II) individually through glass columns (0.6 cm i.d.) packed with 1 g of zirconium selenomolybdate matrix at a flow rate of 0.5 mL/min. The breakthrough capacities (Q’s) of these ions were calculated from the following equation:
(2)Q=C0V50%w(mmol/g),
where C0 is the initial metal ion concentration of the corresponding nuclide (M) in its feeding solution, V50% is the effluent volume (mL) at C/C0=0.5 (as indicated by measuring the counting rates of the initial solution and different effluent fractions), and w is the weight of the column matrix.


## 2.5. Chromatographic Separation

A mixture solution of134Cs(I), 152,154Eu, and 60Co(II) ions in HNO3 acid (10−2M and pH 2) was passed through a chromatographic column (0.6 cm i.d.) containing 1 g of zirconium selenomolybdate matrix at a flow 0.5 mL/min. The radionuclides were separated from each other by eluting the loaded column with HNO3 acid and NH4Cl solutions of different volumes and concentration.


## 3. Results and Discussion

### 3.1.  Distribution Coefficients of Cs(I), Eu(III), and Co(II) on Zirconium Selenomolybdate Matrix

Individual interactions of134Cs(I), 152,154Eu(III) and 60Co(II) ions (~1 × 10-4M for each) in HNO3 acid solutions with zirconium selenomolybdate matrix, dried at 50°C, were investigated by the batch equilibration method under comparable experimental conditions to make possible evaluation of the obtained Kd values for the retention and separation of these ions. Figure 1 displays the variation of the Kd values of 134Cs(I), 152,154Eu(III), and 60Co(II) radionuclides individually in HNO3 acid solutions on zirconium selenomolybdate as a function of the acid concentration in the range from 1 × 10−3 to 1 M HNO3. It is observed that the Kd values 134Cs(I), 152,154Eu(III), and 60Co(II) radionuclides decrease with increasing the acid concentration in the equilibrating solutions. The obtained results may be attributed to the greater competition between the respective cations in solution to exchange with H+-ions on the surface of zirconium selenomolybdate. The retention behaviour of 134Cs(I) and 152,154Eu(III) on zirconium selenomolybdate is characterized by plateaus (of almost Kd values of 1160, 280 mL/g, resp.) in dilute acid solutions of concentration up to 8 × 10−2 and 3 × 10−2 M, respectively. Thereafter, the corresponding Kd values linearly decrease with increasing the acid concentration. It is also observed that the Kd values of the three nuclides increase in the following order:
(3)Cs(I)>Eu(III)>Co(II).
This selectivity sequence may be attributed to the fact that Cs(I) ions are more able to diffuse through the sorbent and reach a higher number of the exchange sites than Eu(III) and Co(II). This may be attributed to that Cs(I) ions have larger effective ionic radii than Eu(III) and Co(II) ions and can easily be dehydrated [2]. In addition, The higher Kd values for Cs(I) compared with Eu(III) and Co(II) on zirconium selenomolybdate may be attributed to the high selectivity of the heteropolyacids for large monovalent cations (LMC’s) such as Cs(I), Ag(I), and Tl(I), which form insoluble heteropolyacid salts [2, 23]. Table 1 compiles the calculated separation factors(α) of Cs(I), Eu(III) and Co(II) from each other.Table 1
The calculated separation factors (α) of Cs(I), Eu(III) and Co(II).
Acid concentration, M
Separation factors,α
K
dCs(I)
/
K
dEu
(
III
)
K
dCs(I)
/
K
dco
(
II
)
K
dEu
(
III
)
/
K
dco
(
II
)
3
×
10
-
3
4.2
38.7
9.4
1
×
10
-
2
4
118.5
29.5
4
×
10
-
2
4.1
395.7
96.7
8
×
10
-
2
8.4
666.7
80
3
×
10
-
1
50
1250
25
1
100
700
7Figure 1
Distribution coefficients of134Cs(I), 152,154Eu(III), and 60Co(II) in HNO3 acid solutions on zirconium selenomolybdate matrix as a function of the acid concentration.


### 3.2. Breakthrough Capacities of Cs(I), Eu(III), and Co(II) on Zirconium Selenomolybdate Matrix

Figure2 illustrates the breakthrough behaviour of 134Cs(I), 152,154Eu(III), and 60Co(II) from 1 g zirconium selenomolybdate column (0.6 cm i.d.) matrix fed with 140 mL 1 × 10−2M 134Cs(I), 152,154Eu(III), and 60Co(II), in 1 × 10−2M HNO3 acid at a flow rate of 0.5 mL/min. Figure 2 indicates (i) an immediate breakthrough of 60Co(II) and 100% breakthrough at 60 mL effluent volume and (ii) quantitative adsorption of 134Cs and 152,154Eu up to ~10 and 50 mL effluent volume, respectively, after which concentration in the effluent is gradually increases. Substituting the values of the effluent volume corresponding to 50% 134Cs(I), 152,154Eu(III), and 60Co(II) breakthrough, the initial concentration (C0 = 1 × 10−2M) and amount of the bed matrix (m=1 g) (2), a value of 0.82, 0.45 and 0.18 mmol/g of zirconium selenomolybdate for 134Cs(I), 152,154Eu(III), and 60Co(II), respectively.Figure 2
Breakthrough curves of Cs(I), Eu(III) and Co(II) from 1 g zirconium selenomolybdate column (0.6 cm i.d.) matrix using individual feeding solution of 1 × 10−2M in 1 × 10-3M HNO3 acid at a flow rate 0.5 mL/min.


### 3.3. Separation of Cs(I), Eu(III), and Co(II) Using a Zirconium Selenomolybdate Chromatographic Column

A mixture of Cs(I), Eu(III), and Co(II) (1 × 10−2M each) in 140 mL of 1 × 10−2M HNO3 acid solution was passed through a 1 g zirconium selenomolybdate column (0.6 cm i.d.) at a flow rate of 0.5 mL/min. Figure 3 shows the elution profiles of Cs(I), Eu(III), and zirconium selenomolybdate column (0.6 cm i.d.) at a flow rate of 0.5 mL/min. It is observed that Co(II) of the lower affinity for the bed matrix immediately passed along the column bed matrix leaving Eu(III) and Cs(I) of relatively higher affinity build up onto the bed matrix. The retained Co(II) was eluted from the column bed by passing 14 mL 1 × 10−1M HNO3 acid solution at a flow rate 0.5 mL/min. Thereafter, quantitative elution of Eu(II) was achieved by passing 22 mL 2.5 × 10−1M HNO3 acid solution at a flow rate of 0.5 m/min. Cs(I) was strongly adsorbed onto the matrix because of the formation of insoluble heteropolyacid salts which is often the case with large monovalent cations and heteropolyacids. Consequently, it typically requires another monovalent cations of very similar bar ionic radius to exchange Cs(I). NH4Cl was used for the elution of Cs. About 89% of Cs(I) was eluted with 32 mL 2 M NH4Cl solution at a flow rate of 0.5 mL/min. Complete recovery of Cs(I) could be achieved after decomposing the adsorbent.Figure 3
Elution profiles of Cs(I), Eu(III), and Co(II) from 1 g zirconium selenomolybdate column (0.6 cm i.d.) with HNO3 and NH4Cl solutions, at a flow rate 0.5 mL/min.As a heteropolyacid salt zirconium selenomolybdate is very selective for cesium. But comparing zirconium selenomolybdate with 12-tungstocerate we found that Cs was recovered completely with 2 M HNO3 acid solution but in our study about 89% of the retained Cs(I) was eluted with 2 M NH4Cl solution which means it is adsorbed strongly enable us in a following study to separate parent(137Cs)/daughter(137mBa) isobars onto chromatographic columns in the form of radioisotope generators.


## 3.1.  Distribution Coefficients of Cs(I), Eu(III), and Co(II) on Zirconium Selenomolybdate Matrix

Individual interactions of134Cs(I), 152,154Eu(III) and 60Co(II) ions (~1 × 10-4M for each) in HNO3 acid solutions with zirconium selenomolybdate matrix, dried at 50°C, were investigated by the batch equilibration method under comparable experimental conditions to make possible evaluation of the obtained Kd values for the retention and separation of these ions. Figure 1 displays the variation of the Kd values of 134Cs(I), 152,154Eu(III), and 60Co(II) radionuclides individually in HNO3 acid solutions on zirconium selenomolybdate as a function of the acid concentration in the range from 1 × 10−3 to 1 M HNO3. It is observed that the Kd values 134Cs(I), 152,154Eu(III), and 60Co(II) radionuclides decrease with increasing the acid concentration in the equilibrating solutions. The obtained results may be attributed to the greater competition between the respective cations in solution to exchange with H+-ions on the surface of zirconium selenomolybdate. The retention behaviour of 134Cs(I) and 152,154Eu(III) on zirconium selenomolybdate is characterized by plateaus (of almost Kd values of 1160, 280 mL/g, resp.) in dilute acid solutions of concentration up to 8 × 10−2 and 3 × 10−2 M, respectively. Thereafter, the corresponding Kd values linearly decrease with increasing the acid concentration. It is also observed that the Kd values of the three nuclides increase in the following order:
(3)Cs(I)>Eu(III)>Co(II).
This selectivity sequence may be attributed to the fact that Cs(I) ions are more able to diffuse through the sorbent and reach a higher number of the exchange sites than Eu(III) and Co(II). This may be attributed to that Cs(I) ions have larger effective ionic radii than Eu(III) and Co(II) ions and can easily be dehydrated [2]. In addition, The higher Kd values for Cs(I) compared with Eu(III) and Co(II) on zirconium selenomolybdate may be attributed to the high selectivity of the heteropolyacids for large monovalent cations (LMC’s) such as Cs(I), Ag(I), and Tl(I), which form insoluble heteropolyacid salts [2, 23]. Table 1 compiles the calculated separation factors(α) of Cs(I), Eu(III) and Co(II) from each other.Table 1
The calculated separation factors (α) of Cs(I), Eu(III) and Co(II).
Acid concentration, M
Separation factors,α
K
dCs(I)
/
K
dEu
(
III
)
K
dCs(I)
/
K
dco
(
II
)
K
dEu
(
III
)
/
K
dco
(
II
)
3
×
10
-
3
4.2
38.7
9.4
1
×
10
-
2
4
118.5
29.5
4
×
10
-
2
4.1
395.7
96.7
8
×
10
-
2
8.4
666.7
80
3
×
10
-
1
50
1250
25
1
100
700
7Figure 1
Distribution coefficients of134Cs(I), 152,154Eu(III), and 60Co(II) in HNO3 acid solutions on zirconium selenomolybdate matrix as a function of the acid concentration.


## 3.2. Breakthrough Capacities of Cs(I), Eu(III), and Co(II) on Zirconium Selenomolybdate Matrix

Figure2 illustrates the breakthrough behaviour of 134Cs(I), 152,154Eu(III), and 60Co(II) from 1 g zirconium selenomolybdate column (0.6 cm i.d.) matrix fed with 140 mL 1 × 10−2M 134Cs(I), 152,154Eu(III), and 60Co(II), in 1 × 10−2M HNO3 acid at a flow rate of 0.5 mL/min. Figure 2 indicates (i) an immediate breakthrough of 60Co(II) and 100% breakthrough at 60 mL effluent volume and (ii) quantitative adsorption of 134Cs and 152,154Eu up to ~10 and 50 mL effluent volume, respectively, after which concentration in the effluent is gradually increases. Substituting the values of the effluent volume corresponding to 50% 134Cs(I), 152,154Eu(III), and 60Co(II) breakthrough, the initial concentration (C0 = 1 × 10−2M) and amount of the bed matrix (m=1 g) (2), a value of 0.82, 0.45 and 0.18 mmol/g of zirconium selenomolybdate for 134Cs(I), 152,154Eu(III), and 60Co(II), respectively.Figure 2
Breakthrough curves of Cs(I), Eu(III) and Co(II) from 1 g zirconium selenomolybdate column (0.6 cm i.d.) matrix using individual feeding solution of 1 × 10−2M in 1 × 10-3M HNO3 acid at a flow rate 0.5 mL/min.


## 3.3. Separation of Cs(I), Eu(III), and Co(II) Using a Zirconium Selenomolybdate Chromatographic Column

A mixture of Cs(I), Eu(III), and Co(II) (1 × 10−2M each) in 140 mL of 1 × 10−2M HNO3 acid solution was passed through a 1 g zirconium selenomolybdate column (0.6 cm i.d.) at a flow rate of 0.5 mL/min. Figure 3 shows the elution profiles of Cs(I), Eu(III), and zirconium selenomolybdate column (0.6 cm i.d.) at a flow rate of 0.5 mL/min. It is observed that Co(II) of the lower affinity for the bed matrix immediately passed along the column bed matrix leaving Eu(III) and Cs(I) of relatively higher affinity build up onto the bed matrix. The retained Co(II) was eluted from the column bed by passing 14 mL 1 × 10−1M HNO3 acid solution at a flow rate 0.5 mL/min. Thereafter, quantitative elution of Eu(II) was achieved by passing 22 mL 2.5 × 10−1M HNO3 acid solution at a flow rate of 0.5 m/min. Cs(I) was strongly adsorbed onto the matrix because of the formation of insoluble heteropolyacid salts which is often the case with large monovalent cations and heteropolyacids. Consequently, it typically requires another monovalent cations of very similar bar ionic radius to exchange Cs(I). NH4Cl was used for the elution of Cs. About 89% of Cs(I) was eluted with 32 mL 2 M NH4Cl solution at a flow rate of 0.5 mL/min. Complete recovery of Cs(I) could be achieved after decomposing the adsorbent.Figure 3
Elution profiles of Cs(I), Eu(III), and Co(II) from 1 g zirconium selenomolybdate column (0.6 cm i.d.) with HNO3 and NH4Cl solutions, at a flow rate 0.5 mL/min.As a heteropolyacid salt zirconium selenomolybdate is very selective for cesium. But comparing zirconium selenomolybdate with 12-tungstocerate we found that Cs was recovered completely with 2 M HNO3 acid solution but in our study about 89% of the retained Cs(I) was eluted with 2 M NH4Cl solution which means it is adsorbed strongly enable us in a following study to separate parent(137Cs)/daughter(137mBa) isobars onto chromatographic columns in the form of radioisotope generators.


## 4. Conclusion

Chromatographic column of zirconium selenomolybdate was used successfully for separation of cesium, europium, and cobalt from their aqueous mixture solution by elution with HNO3 and NH4Cl.



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