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

A procedure for removal and separation of Cs, Eu and activation product of Co using zirconium selenomolybdate was developed. Interactions of Cs(I), Eu(III), and Co(II) ions from HNO3 acid solutions with zirconium selenomolybdate matrix, dried at 50C, have been individually investigated by the batch equilibration method. e sorption behavior of the three ions showed a selectivity sequence in the following order: Cs(I) > Eu(III) > Co(II). e breakthrough capacities of zirconium selenomolybdate for Cs(I), Eu(III) and Co(II) were found to be 0.82, 0.45, and 0.18mmol/g of the sorbent, respectively. A mixture of the three radionuclides (1 × 10M each) in 140mL of 1 × 10M HNO3 solution was passed through 1 g zirconium selenomolybdate chromatographic column.ereaer, quantitative elution of the retained Co(II) was achieved with 14mL of 1×10MHNO3 acid solution leaving Eu(III) and Cs(I) strongly retained onto the column. Quantitative elution of Eu(III) was achieved by passing 22mL 2.5 × 10MHNO3. About 89% of the retained Cs(I) was eluted with 32mL of 2M NH4Cl solution at a �ow rate of 0.5mL/min.


Introduction
Synthetic inorganic sorbents have many advantages over organic ones.ese advantages include high selectivity to ions of some elements of potential bene�ts, 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.ere 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][4][5][6][7][8][9][10][11][12][13][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][16][17][18][19][20][21].e 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 �ssion 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 12tungstocerate 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].eir chemical and radiation stability are suitable for their introduction in the �eld of chromatographic applications.
e three radioisotopes, 137 Cs, 152,154 Eu, and 60 Co, are important long-lived products in the nuclear waste. 137Cs is a �ssion product, 60 Co is an activation product, and 152,154 Eu is �ssion and neutron activation product.e removal of these radioisotopes decrease the radiation level of the waste.In addition, the recovery of some valuable radionuclide such as 137 Cs for reuse in different applications by preparation of 137 Cs/ 137m Ba radioisotope generator which is used in medicine and industry in quality control process. 152,154Eu and 60 Co are used for producing sealed sources which are used in medicine and industry.
e present work aims at (i) preparation of zirconium(IV) selenomolybdate, (ii) determination of batch distribution coefficients of 134 Cs(I), 152,154 Eu and 60 Co(II) from HNO 3 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 HNO 3 onto a chromatographic column of the matrix and eluting them subsequently with HNO 3 solutions and/or other solutions of different concentrations.

Experimental
All chemicals were of A. R. grade.Distilled water was used for different solution preparations and washing.Radiometric identi�cation 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. 134Cs, 152,154 Eu, and 60 Co.Radionuclides of 134 Cs and 60 Co were produced by thermal neutron irradiation of CsCl and CoCl 2 target materials in the watercooled ETRR-2 Research Reactor (Egypt) for 4 h at a thermal neutron �ux of 1 × 10 14 n cm −2 s −1 .Radionuclides of 152,154 Eu were produced by thermal neutron irradiation of Eu 2 O 3 target materials in the water-cooled ETRR-1 Research Reactor (Egypt) for 48 h at a thermal neutron �ux of 1 × 10 13 n cm −2 s −1 .

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.e pH was adjusted to be 3 by adding ammonia solution.Aer standing for 24 h, the precipitate was separated by suction and washed with water.e separated precipitate was dried at 50 ∘ C, packed in chromatographic column and converted into the H + -form by passing 10 −1 M HNO 3 acid solution.e obtained exchanger was washed again with water and redried at 50 ∘ C.

Batch Distribution Studies. e distribution coefficients
for 134 Cs(I), 152,154 Eu, and 60 Co(II) ions in aqueous HNO 3 acid solutions on zirconium selenomolybdate matrix were individually determined by the static batch equilibration technique using the following equation: where   and   are the counting rates of the aqueous phase before and aer equilibration with the gel matrix, respectively,  is the volume of the aqueous phase (10 mL), and  is the weight of the gel matrix (0.1 g).

Capacity Studies.
Chromatographic column breakthrough investigations were conducted by passing HNO 3 solutions (10 −2 M) of appropriate volumes 10 −2 M of each of 134 Cs(I), 152,154 Eu, and 60 Co(II) individually through glass columns (0.6 cm i.d.) packed with 1 g of zirconium selenomolybdate matrix at a �ow rate of 0.5 mL/min.e breakthrough capacities ('s) of these ions were calculated from the following equation: where  0 is the initial metal ion concentration of the corresponding nuclide (M) in its feeding solution,  50% is the effluent volume (mL) at / 0 = 0.5 (as indicated by measuring the counting rates of the initial solution and different effluent fractions), and  is the weight of the column matrix.

Chromatographic Separation.
A mixture solution of 134 Cs(I), 152,154 Eu, and 60 Co(II) ions in HNO 3 acid (10 −2 M and pH 2) was passed through a chromatographic column (0.6 cm i.d.) containing 1 g of zirconium selenomolybdate matrix at a �ow 0.5 mL/min.e radionuclides were separated from each other by eluting the loaded column with HNO 3 acid and NH 4 Cl solutions of different volumes and concentration.the   values of the three nuclides increase in the following order:

Results and Discussion
is 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).is 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, e higher   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].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 HNO 3 acid solution but in our study about 89% of the retained Cs(I) was eluted with 2 M NH 4 Cl solution which means it is adsorbed strongly enable us in a following study to separate parent( 137 Cs)/daughter( 137 Ba) isobars onto chromatographic columns in the form of radioisotope generators.

Conclusion
Chromatographic column of zirconium selenomolybdate was used successfully for separation of cesium, europium, and cobalt from their aqueous mixture solution by elution with HNO 3 and NH 4 Cl.
values for the retention and separation of these ions.Figure1displays the variation of the   values of 134 Cs(I), 152,154 Eu(III), and 60 Co(II) radionuclides individually in HNO 3 acid solutions on zirconium selenomolybdate as a function of the acid concentration in the range from 1 × 10 −3 to 1 M HNO 3 .Distribution coefficients of 134 Cs(I), 152,154 Eu(III), and 60 Co(II) in HNO 3 acid solutions on zirconium selenomolybdate matrix as a function of the acid concentration.
3.1.DistributionCoefficients of Cs(I), Eu(III), and Co(II) onZirconium Selenomolybdate Matrix.Individual interactions of 134 Cs(I),152,154Eu(III) and 60 Co(II) ions (∼1 × 10 −4 M for each) in HNO 3 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 134 Cs(I) and 152,154 Eu(III) on zirconium selenomolybdate is characterized by plateaus (of almost   values of 1160, 280 mL/g, resp.) in dilute acid solutions of concentration up to 8 × 10 −2 and 3 × 10 −2 M, respectively.ereaer, the corresponding   values linearly decrease with increasing the acid concentration.It is also observed that

Table 1
Figure2illustrates the breakthrough behaviour of 134 Cs(I),152,154Eu(III), and 60 Co(II) from 1 g zirconium selenomolybdate column (0.6 cm i.d.) matrix fed with 140 mL 1 × 10 −2 M 134 Cs(I),152,154Eu(III), and 60 Co(II), in 1 × 10 −2 M HNO 3 acid at a �ow rate of 0.5 mL/min.Figure2indicates (i) an immediate breakthrough of 60 Co(II) and 100% breakthrough at 60 mL effluent volume and (ii) quantitative adsorption of 134 Cs and 152,154 Eu up to ∼10 and 50 mL effluent volume, respectively, aer which concentration in the effluent is gradually increases.Substituting the values of the effluent volume corresponding to 50% 134 Cs(I),152,154Eu(III), and 60 Co(II) breakthrough, the initial concentration ( 0 = 1 × 10 −2 M) and amount of the bed matrix (  1 g) (2), a value of 0.82, 0.45 and 0.18 mmol/g of zirconium selenomolybdate for 134 Cs(I), 152,154 Eu(III), and 60 Co(II), respectively.× 10 −2 M in 1 × 10 −3 M HNO 3 acid at a �ow rate 0.5 mL/min./ dEu(III)  dCs(I) / dco(II)  dEu(III) / dco(II) × 10 −1 M HNO 3 acid solution at a �ow rate of 0.5 m/min.Cs(I) was strongly adsorbed onto the matrix because of the formation of insoluble heteropolyacid salts which is oen 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).NH 4 Cl was used for the elution of Cs.About 89% of Cs(I) was eluted with 32 mL 2 M NH 4 Cl solution at a �ow rate of 0.5 mL/min.Complete recovery of Cs(I) could be achieved aer decomposing the adsorbent.
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.e retained Co(II) was eluted from the column bed by passing 14 mL 1 × 10 −1 M HNO 3 acid solution at a �ow rate 0.5 mL/min.ereaer, quantitative elution of Eu(II) was achieved by