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

Abstract

A procedure for removal and separation of Cs, Eu and activation product of Co using zirconium selenomolybdate was developed. Interactions of ¹³⁴Cs(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.

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 of ¹³⁴Cs(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 of ¹³⁴Cs, ^152,154Eu, and ⁶⁰Co

Radionuclides of ¹³⁴Cs 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 for ¹³⁴Cs(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: 𝐾_𝑑=𝐴^𝑖−𝐴_𝑓𝐴_𝑓×𝑉𝑚(mL/g),(1) where 𝐴_𝑖 and 𝐴_𝑓 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: 𝑄=𝐶⁰𝑉_50%𝑤(mmol/g),(2) where 𝐶₀ is the initial metal ion concentration of the corresponding nuclide (M) in its feeding solution, 𝑉_50% 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 of ¹³⁴Cs(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 of ¹³⁴Cs(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 𝐾_𝑑 values for the retention and separation of these ions. Figure 1 displays the variation of the 𝐾_𝑑 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 𝐾_𝑑 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 𝐾_𝑑 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 𝐾_𝑑 values linearly decrease with increasing the acid concentration. It is also observed that the 𝐾_𝑑 values of the three nuclides increase in the following order: Cs(I)>Eu(III)>Co(II).(3) 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 𝐾_𝑑 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).

Figure 1 

Distribution coefficients of 134Cs(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

Figure 2 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 × 10−2 M in 1 × 10-3 M 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.

Acknowledgment

The authors would like to express their appreciation for support of colleagues in Radioactive Isotopes and Generators Department, Hot Labs. Center, Egyptian Atomic Energy Authority.