Cation Exchange Separation of Thorium from Rare Earths and Other Elements in Methanol-Nitric Acid Medium Containing Trioctylphosphine Oxide Johann Korkisch’ and K. A. Orlandini Chemistry Division, Argonne National Laboratory, Argonne, Ill. The batch distribution coefficients of thorium, rare earths, and many other elements were determined on the strongly acidic cation exchanger Dowex 50 in mixed aqueous-organic solvent systems containing organic phosphorus compounds and nitric acid. These investigations showed that from some of these media, thorium i s not retained on the resin while rare earths and numerous other metal ions are strongly adsorbed making possible their quantitative separation from thorium. The most suitable medium for the complete separation of tracer and milligram amounts of thorium is 0 . 1 ~trioctylphosphine oxide (TOPO) in methanol containing 5 vol % of 12M nitric acid. With this eluent, thorium is eluted ahead of rare earths and numerous other elements with separation factors in the order of > 102-104. Less suitable systems were investigated in which methanol was replaced by acetone, tetrahydrofuran, acetic acid, and methyl glycol or media in which nitric acid was replaced by hydrochloric acid. If in place of TOPO other extractants such as bis(2-ethylhexyl)-orthophosphoric acid (HDEHP) or tri-n-butylphosphate (TBP) are employed, thorium is less readily eluted from Dowex 50. The effectiveness of the organic extractants increased in the order TBP < HDEHP < TOPO.
THORIUM exhibits a n extremely high affinity for strongly acidic cation exchange resins from aqueous hydrochloric, nitric, sulfuric, and perchloric acid media (1-3). Consequently, all methods that have so far been reported for the separation of this element on cation exchangers are based on this strong retention while most other metal ions are much less strongly adsorbed and can be removed by elution with dilute mineral acids (4). Even at relatively high acid concentrations, the adsorption of thorium is remarkably high so that its elution, following the removal of the other metal ions, presents a special problem. The most suitable eluents are 6N sulfuric acid and O.5Moxalic acid or the thorium can be recovered by ashing the resin (4). This separation principle has been employed to separate thorium from most accompanying metal ions which include the rare earths, uranium, alkaline earth elements, alkali metals, and most of the transition elements ( 4 ) . Also from organic solvent-water mixtures containing dilute mineral acids such as hydrochloric or nitric acid, thorium is very strongly retained by cation exchangers--e.g., Dowex 50 t o an extent comparable with its adsorption from pure aqueous 1 Present address, Analytisches Institut der Universitaet 1090 Wien IX, Wahringer-Strasse 38 Austria.
(1) F. W. E. Strelow, ANAL.CHEM.,32, 1185 (1960). (2) F. W. E. Strelow, R. Rethemeyer, and C . J. C. Bothma, ibid.,
37, 106 (1965). (3) F. Nelson, T. Murase, and K. A. Kraus, J. Chromatog., 13, 503 (1964). (4) J. Korkisch, “Modern Methods for the Separation of Rarer Metal Ions,” Chap, 2, Pergamon Press, Headington Hill Hall,
Oxford, England, 1968, in press. 1952
ANALYTICAL CHEMISTRY
solutiotls of the same acid concentrations (5). However, the adsorption of other elements from such mixed aqueous-organic systems is considerably higher than from the pure aqueous solutions of comparable acidity. Therefore, separations of thorium from other elements in these organic solvent systems are of much lower selectivity. Thorium is much more extractable than many other elements from nitric acid media using as extractant trioctylphosphine oxide (TOPO) (6) dissolved in cyclohexane, carbon tetrachloride or other water-immiscible organic solvents. Because TOPO is also readily soluble in water-miscible organic solvents in the presence of a small percentage of aqueous nitric acid, these systems can also be employed as eluents in ion exchange making use of the principle of combined ion exchangesolvent extraction (CIESE) earlier described (7). In the present paper, the suitability of such organic solventTOPO-nitric acid systems for separating thorium on Dowex 50 from the rare earths and many other metal ions was investigated. As a result, a highly effective and selective method for their separation has been developed. A special feature of this technique is that thorium is eluted ahead of the other elements. EXPERIMENTAL
Apparatus. For the ion exchange separations, ion exchange columns of 0.5-cm diameter and 25-cm length have been employed. Reagents and Solutions. ION EXCHANGE RESIN. Dowex A G 50W-X8 (100-200 mesh; hydrogen form) was used for the column separations and for the batch experiments. Before use, the resin was purified by washing it in succession with 6M nitric acid, distilled water, and methanol; then it was dried in air and stored in an amber bottle. For 1 kg of resin, 2 liters of 6M nitric acid, 5 liters of distilled water, and 2 liters of methanol have been used. ORGANIC SOLVENTS.The following reagent grade solvents were employed : methanol (MeOH), acetone (Ac), tetrahydrofuran (THF), methyl glycol (ethylene glycol monomethyl ether) (MG), and glacial acetic acid (HAC). ORGANIC PHOSPHORUS COMPOUNDS. Trioctylphosphine oxide (TOPO) (Eastman Organic Chemicals, Rochester, N. Y.),bis(2-ethy1hexyl)orthophosphoric acid (HDEHP) (Laboratory stock), and tri-n-butylphosphate (TBP) (Laboratory stock; unpurified). ELUENTSOLUTION. 0.1M TOPO in 95% methanol-5x 12M HN03 (v/v). This solution is prepared by mixing 19 parts of methanol with 1 part of 12M nitric acid in the presence of the required amount of TOPO--e.g., 3.86 grams of (5) J. Korkisch, “Ion Exchange in Mixed and Non-aqueous
Media,” Progress in Nuclear Energy Series IX, Analytical Chemistry, Vol. 6, Pergarnon Press, Oxford-London-New YorkParis, 1966, p 1. (6) T. Ishimori and E. Nakamura, Japan At. Energy Res. Inst. Rep?., 1047,1963. (7) J. Korkisch, Separation Science, 1, 159 (1966).
System MeOH-”03 MeOH-HC1 Ac-HNOs Ac-HC1 THF-HNOa THF-HC1 MG-”03 MG-HC1 HAC-HNOs HAC-HC1
Table I. Distribution Coefficients of Thy Sc, Yb, Ce, and U on Dowex 50 in 0.1M TOPO-Organic Solvent Media Containing 5 Vol % 12M H N 0 3 or 12M HCl Distribution coefficients 30Th(IV) ~~SC(III) 1 6 9b(111) ~ l 4 4Ce(III) 0.4 346 420 16,350 105 5,000 17,960 >io4 8.3 204 >lo’ 3,438 182 8.3 3,220 >lo4 2.6 445 5,220 166,000 460 >1 io4 29,000 136 251 11,045 >lo4
System MeOH-”03 MeOH-HC1 Ac-H NO 3 Ac-HCI THF-H NOs THF-HC1 MG-HNOa MG-HCl HAc-HNO~ HAC-HC1
Table 11. Distribution Coefficients of Thy Sc, Yb, Ce, and U on Dowex 50 in 0.1M HDEHP-Organic Solvent Media Containing 5 Vol 12M H N 0 3 or 12M HCI Distribution coefficients ~~SC(III) ~~QY~(III) 1 4 4ce(111) * 30Th(IV) >lo5 >io5 180 6,W 1,043 83,000 >loa 59 >lo5 >1oj 41 9 >io3 5,374 13 180 >io5 141 >lo5 17 2,253 44,450 107 3.7 >lo5 >io5 400 18 100,000 >lo& 200 180 >1oj 3,000 >105 >io3 9,080 >io3 >io3 265
TOPO are dissolved in 50 ml of methanol, 5 ml of 12M nitric acid is added, and the mixture is diluted with methanol to 100 ml) (molecular weight of TOPO = 386.65). TRACERS.Nitric acid and hydrochloric acid solutions of many radioactive tracers were used. These included: 23Th (ionium), 16gYb, 144Ce,46Sc, 233U,l8lHf, g6Zr, 241Am,j4Mn, 59Fe, W o , 22Na, I3jCs, 4 F a ,85Sr,44Ti,and 66Zn. The radioactive measurements were performed by using standard counting techniques. Determination of Distribution Coefficients. The batch distribution coefficients (Kd values) of thorium, rare earths, and of the other investigated elements were determined by using the same technique as has been described earlier (8). Working Procedure (Separation of thorium from rare earths and other elements.). PRErREATMENr OF RESINBED. One gram of the resin is soaked for about 5 min in a few milliliters of the eluent solution and then the slurry is transferred to the ion exchange column. The resulting resin bed is supported by a pad of glass or quartz wool and is subsequently washed with 5-10 ml of the eluent solution. SORPTION AND ELUTION. 0.5 ml of 12M nitric acid containing up to 100 mg of thorium and rare earths and other elements is mixed with 9.5 ml of methanol containing 386 mg of TOPO and after 5-10 min this solution is passed through the pretreated resin bed at a flow rate corresponding to the back-pressure of the resin column (about 0.35 ml/min). Under these conditions the rare earths, alkaline earth metals, alkali metals, and many other elements are strongly retained on the resin while more than 90% of the thorium passes into the effluent. The complete elution of thorium (residual < l o % ) is effected by subsequent passage (at the same flow rate as above) of 20-30 ml of the eluent solution and in the combined fractions thorium is determined radiometrically.
(8) K . A . Orlandini and J. Korkisch, USAEC Rep2 ANL-7415,
January 1968.
233u(v1)
0.6 6
2 12 0.1 io3
~~(III) >io4
Distribution coefficients 109~b(111)
144ce(111)
>105 >105
> 105 100,000
28 539
17,700 >lo5
>io5
>io5
>io3
360
20,000
>io4
>io3
>lo6
269 >io3
540
74 > 105 8,450
>io3
>io5
>io3
651
>lo6
>io5
4,200 > 106 450,000 > 106 11,880
>io4 >io5
>lo5
>105 >lo'
233u(v1)
200 4 14 0.8 3 0.05
26 0.3 46 1.2
Table IV. Distribution Coefficients of Th,Sc, Yb, Ce, and U on Dowex 50 in 95 Vol % Methanol-5 Vol 1 2 M "03 Media Containing Varying Concentrations of TOPO Molarity of TOPO 0.5 0.1 0.05 0.01 0.001 0.000
2aTh(IV) 0.02 0.4 5.2 >io3
>lo3 >io3
~~SC(III)
Distribution coefficients '6QYb(III)
14
4ce(111)
3.4 346 1,500 40,000
4 420 1,550 48,000
2,700 16,350
>io4
7,000
34,500
>io4
>io4
>io4
>io4 >io4
*33u(v1)
0.3 0.6 4.5 44 75 77
Table V. Distribution Coefficients of Th, Sc, Yb, Ce, and U on Dowex 50 in 0.1M TOPO-Methanol Systems Containing 5 Vol % Nitric Acid of Varied Molarity Molarity of nitric acid 12 6 3
1
230Th(IV) 0.4 11 109 >io3
~~SC(III)
346 2,840 10,700 12,300
From the results shown in Tables I1 and 111, it is seen that in the HDEHP and TBP systems the distribution coefficients of thorium are much higher than in the TOPO systems of comparable composition (see Table I). I n most cases, the distribution coefficients of thorium and also of the other listed elements are smaller in the H D E H P than in the TBP systems. Consequently, the effectiveness of these organic extractants as eluents for thorium and other metal ions increases in the order TBP < HDEHP < TOPO. However, this general trend does not always apply t o the behavior of uranium in all systems. Because the system MeOH-HNO,-TOPO was the most suitable for the effective elution of thorium, all further investigations were directed toward its close study. Thus, it was found that a decrease of the methanol concentration from 9 5 z t o 90% did not cause a change in the adsorption characteristics of thorium and the other elements from that recorded in Table I. If, however, the percentage of methanol is further decreasede.g., t o 80%-TOPO is precipitated so that no column separations can be performed under these conditions. Consequently, all further studies were carried out at a methanol concentration of 9 5 z . These studies involved measurements of distribution coefficients of thorium, scandium, ytterbium, cerium, and uranium at varying concentrations of TOPO and nitric acid. The results are presented in Tables IV and V. From the data shown in Table IV, it is seen that a decrease of the TOPO concentration invariably increases the distribution coefficients of thorium and of all other elements. This is 1954
ANALYTICAL CHEMISTRY
Distribution coefficients 16$Yb(III)
14
4ce(111)
420 1,900 6,300
16,350
>io4
>io4
>io4
>io4
*33u(v1)
0.6 1.2 3.3 21
Table VI. Distribution Coefficients of Several Elements on Dowex 50 in 0.1M TOPO-Methanol4 Vol 12MNitric Acid Distribution Distribution Metal ion coefficient Metal ion coefficient 330 30Th(IV) 0.4 4Ti(IV) 233u(v1) 0.6 54~n(11) 7,300 2,630 23QPU(IV) 0.4 6 QFe( I I I) 181Hf(IV)
