Journal of Radioanalytical Chemistry, VoL 4 7 (19 78) 25- 28

CHEMICAL SEPARATION OF MOLYBDENUM FROM URANIUM

AND FISSION PRODUCT NUCLIDES

M. IQBAL, M. EJAZ

Nuclear Chemistry Division, Pakistan Institute of Nuclear Science and Technology, Nilore, Rawalpindi (Pakistan)

(Received March 2, 1978)

A very rapid method for the separation of molybdenum(Vl) from ncutron irradiated uranium and its fission products is described. The procedure is based on the selective extraction of molybdenum(Vl) by a 0.1M solution of 2-hexylpyridine in benzene from 4MHC1 + 0.04M KSCN. Decontamination factors were estimated to be >104 for the radionuelides of niobium, zirconium, ruthenium, lanthanum, cerium, promethium, yttrium, strontium and barium.

Introduct ion

Molybdenum-99 is the parent nuclide of 99mTc, which is widely used in nu-

clear medicine. 99M0 is either obtained by neutron activation of 98M0 in a nu-

clear reactor or as high specific activity fission molybdenum from neutron irradiated

uranium. Several methods of producing 99Mo from irradiated uranium are reported. ~-4

In the present paper, a very easy method of separating 99Mo from uranium and

its fission products is reported. The method is rapid, safe and requires a minimum

of laboratory operations.

Experimental

Reagents, tracers and equipment

2-Hexylpyridine (HPy) was obtained from K and K laboratories. The character-

istics of this compound are given in Ref. s All other reagents used in this study

were of AnalaR grade. 99Mo and other tracers employed in this work were ob-

tained either from Radiochemical Centre, Amersham or were prepared locally by

(n, 7) reaction or by separation of the daughter nuclide from the parent without

a carrier. The equipment used for the radiochemical assay is described in previous articles, s- 6

*Author to whom correspondence should be addressed.

J. Radioanal. Chem. 47 (1978) 25

M. IQBAL, M. EJAZ: CHEMICAL SEPARATION OF MOLYBDENUM

Table 1 Distr ibut ion coefficients of uranium(VI), molybdenum(VI) and some other fission

products between 0.IM 2-hexylpyridine in benzene and

4M hydrochloric acid containing 0.04M }otassium thiocyanate

C o n c e n t r a t i o n in Dis tr ibut ion Metal ion / the initial aqueous

coe f f i c i en t /

t phase, M

: 33 U(VI) I 10 - 3 0.05

99 Mo(VI)

9 s Nb(V)

95 Zr(iV)

~ 6 Ru(IV)

144Ce(ii i )

1 4 0 L a ( i i i )

~ o Ba( l l )

9 o Sr(II)

i a 7Cs

10 -s 15.25

C.F.* 0.01

10 -9 0.01

10 -s 0.0I

10 -7 0.01

C.F.* 0.01

10 -7 0.01

10 -9 0.01

10 -9 0.01

*C.F.: Carrier free.

Procedure

An irradiated uranium sample (50 mg) cooled for about three weeks was dis- solved in 5 rnl of 4M hydrochloric acid. The solution was made 0.04M with respect to potassium thiocyanate and equilibrated once with an equal volume of 0.1M HPy in benzene. The organic phase was separated and scrubbed twice with the barren aqueous phase (4M HC1 + 0.04M KSCN). After scrubbing, molybdenum was back- extracted from the organic phase with 7070 perchloric acid. One ml of concentrated phosphoric acid was added to the back extract which was distilled in an air stream until 1 - 2 ml of HC104 had distilled.

Results and discussion

The distribution data of molybdenum(V/) and some other fission products and uranium(V/) in 4M hydrochloric acid solution containing 0.04M KSCN, for 0.1M HPy/benzene, are shown in Table 1. It is to be seen that molybdenum(VI) is quan- titatively extracted under these conditions, while the extraction of other fission products and uranium is poor. The selectivity of molybdenum(VI) extraction with this reagent has been utilized to work out the reported procedure. In the first series of experiments we studied the extraction behaviour of uranium(VI) and

26 J. Radioanal. Chem. 47 (1978)

M. IQBAL, M, EJAZ: CHEMICAL SEPARATION OF MOLYBDENUM

molybdenum(VI) from aqueous hydrochloric acid media without thiocyanate ions. Interestingly, it has been found that the anionic chlo~o complexes of uranium(VI), which exist in this acidity range (1-10M), are very poorly extracted by this pyri- dine amine. This behaviour is different from that of all commonly used aliphatic amines and is probably due to the low basicity of HPy, 7 which results in the formation of an unstable pyridinium cation, (HI~H) § so that the extraction of anionic complexes through ion-pair formation becomes difficult. Even the addition of 0.04M potassium thiocyanate does not improve extraction either because anionic chloro complexes still predominate in aqueous solutions or mixed anionic chloro- thiocyanato complexes are formed, which are inextractable. The extraction of mo- lybdenum(VI) from hydrochloric acid was not quantitative although oxyanions of chromium(VI) are extracted fairly well from hydrochloric acid solutions by this pyridine. ~ The low extraction of molybdenum(VI) from hydrochloric acid could be due to the relatively stronger hydration of molybdate ions or to the absence of condensed species of molybdenum at such low (<10-SM) concentrations, since macroamounts are easily polymerized to polymolybdates, which are generally more extractable than monomeric species. The presence of thiocyanate ions was found to improve extraction in the hydrochloric acid concentration range of 2-6M with a maximum at 4M. In this region of hydrochloric acid concentration, molybdenum(VI) exists chiefly in the form of cationic species of the type MoO~ § with an equilibrium shifting in favour of MoO~-or MoO2CI~ n+2 as the aqueous acid concentration de- creases or increases. The extraction of molybdenum in this region of acid concentra- tion in the presence of thiocyanate ions could be due to MoO2(SCN)2. The extrac- tion of thiocyanate complexes of certain metal ions, through solvation, by this pyridine has been reported. 9 Since the extraction of molybdenum is fairly good, it can easily be separated from uranium and its fission products in a single extrac- tion, and one or two scrubbing stages would not affect the recovery of molyb- denum very much. Ruthenium, technetium and iodine were found to contaminate the organic extract and were eliminated through volatilization, after complexing molybdenum with phosphoric acid. However, iodine, which may contaminate the off gas system, can also be removed by prior extraction with carbon tetrachloride. The absence of actinides in the organic phase was checked by alpha spectroscopy. Both the ~-spectra and decay studies showed that the separated fraction was more than 99% pure. The decontamination factors estimated were greater than 104 for several radionuclides although from the distribution coefficients reported, their cal- culated separation factors are about 5 .104 . This is probably due to scrubbing of the organic extract.

J. Raarfoanal. Chem. 47 (1978) 27

M. 1QBAL, M. EJAZ: CHEMICAL SEPARATION OI: MOLYBDENUM

References

1.W.D. TUCKER, M. W. GREENE, A. P. MUFFENHOFF, Atompraxis, 8 (1962) 163. 2. Radioisotope Production and Quality Control, Techn. Reports Series No. 128, IAEA, Vienna,

1971 p. 709. 3. R. E. LEWIS, Int. J. Appl. Radiation Isotopes, 22 (1971) 603. 4. M. EJAZ, Radiochim. Acta, 22 (1975) 51. 5. M. IQBAL, M. EJAZ, S. A. CHAUDHARY, R. AHMED, Separation Science, 11 (1976) 255. 6. M. EJAZ, Separation Science, 10 (1975) 425. 7. S. A. CHAUDHARY, R. AHMED, M. EJAZ, J. Radioanal. Chem., 35 (1976) 235. 8, M. IQBAL, M. EJAZ, J. Radioanal. Chem., 42 (19781 28. 9. M. IQBAL, M. A. QURESHI, M. EJAZ, Talanta, (in press).

28 J. Radioanal. Chem. 47 (1,978)