Separation of Uranium by Combined Ion ExchangeSolvent Extraction SIR: Cation exchange procedures employing strongly acid resins for the separation of uranium(V1) have not been widely used because of their lack of selectivity and specificity. Thus in pure aqueous nitric acid solutions of variable concentration in this acid, the uranyl ion cannot be separated from divalent metal ions (13). d separation from elements of higher valency is posiible, however, although the separation factors are in many cases not very favorable for performing simple and iapid separations. Much more suitable conditions prevail if sulfuric acid is used in place of nitric acid with separation factors of uranium (in 1 N wlfuric acid) toward divalent ions varying from about 3 to 4 while for trivalent ions they may be as high as 33 (e.g., for lanthanum) (19, IS). These factors for the divalent ions are, however, low as compared nith those obtained in the medium recommended in the present paper. The most unfavorable conditions for ieparating uranium from other metal ions are found in hydrochloric acid media. In these uranium cannot be ieparated from most divalent and also trivalent metal ions because under these conditions the tendency of the uranyl ion (9) and of theqe metal ions to form anionic chloride complexes effectively competes with their adsorption on the cation exchanger (11 ) . Also in perchloric acid medium the separability of uranium from other metal ions is far from being selective (IO). From this short survey of the literature it is seen that cation exchange in aqueous mineral acid solutions has so far not offered the ideal solution for a selective i5olation of the uranyl ion. When uqing, however, the tetrahydrofurannitric acid medium recommended in this paper a separation and purification of uranyl nitrate can be effected in one -ingle step on Dowex 50. EXPERIMENTAL
Reagents and Solutions. T h e strongly acid cation eschange resin Dowex 50, X8 (100-200 mesh; hydrogen form) was used. For the batch equilibrium experiments, the air-dried form of the resin was employed while for the resin column processes the resin was first soaked in the T H F - H N 0 3 mixture and then transferred t o the ion exchange columns. The THF-HS03 mixture was prepared by mixing 9 parts of tetrahydrofuran (THF) (chemically pure) with 1 part of 6h' aqueous nitric acid. When disregarding the slight volume changes
due to mixing this solution is 90 volume % in THF and 10 volume % in 6 N nitric acid. Standard solutions of uranium and the various other metal ions were prepared by dissolving their reagent grade nitrates in 6N nitric acid. Apparatus. The separation experiments were performed on 1- and 10gram resin columns with diameters of 0.5 and 1 em., respectively. Procedure. Separation of uranium from other elements. Through the resin bed pretreated with 20 ml. of the T H F - H N 0 3 mixture, 5 ml. of the sample solution 90% in THF and 10% in 6'V nitric acid containing uranium and the elements t o be separated from it, is passed a t a flow rate corresponding t o the normal back-pressure of the column. Under these conditions the strongly adsorbed di-, tri-, and tetravalent metal ions are retained on top of the resin bed while uranium, if present in great excess and when the small column is used, is already partly eluted. For the complete elution of uranium the column is then washed with 100 ml. of the THF-HN03 mixture and in the combined effluent and eluate uranium is determined fluorimetrically. If the separation is performed on a 10-gram column the effluent does not contain any uranium (if the amount used does not exceed 10 mg.) and neither does the next 50 ml. of effluent when washing is started with the THFH S 0 3mixture after the 5 ml. of sorption solution have passed. This effluent and washing, however, contains all of the vanadium, molybdenum, gold, and platinum and palladium as well as phosphoric acid. B y continued washing with the same mixture the uranium is completely
recovered in the next 600 ml. of eluate in which it is then determined. Interference is caused by the presence of chloride ions which cause iron(II1) to pass into the effluent together with uranium. If phosphoric acid is present system containin the THF-"03 ing the uranium (sorption solution), the solution must be protected from strong sunlight otherwise insoluble uranium(1V)-phosphate precipitates from the solution. h group fractionation of the metal ions which are strongly retained by the resin after uranium has been washed out can be achieved by treating the resin bed with 80% THF-20% 3N hydrochloric acid under which condition a complete elution of iron(III), gallium, indium, lead, copper, zinc, and cadmium is achieved (4) (all these elements form strong anionic chloride complexes). Subsequently cobalt followed by titanium can be eluted in two well defined and separate bands with 90% THF-lO% 6 S hydrochloric acid. The other elements-e.g., nickel, aluminum, and alkaline earth elements, the rare earths, zirconium, thorium, and chromium(II1)-can then be desorbed with strong acid, e.g.. 6N sulfuric acid. The distribution coefficients of uranium and of the other elements were determined using the batch equilibrium method ( 7 ) and the quantitative determinations of the metal ions were performed using suitable volumetric and spectrophotometric procedureq. RESULTS
From Table I , showing the variation of the di5tribution coefficients of uranium and several other elements, with changing percentage of T H F , it is
Table I. Variation of Distribution Coefficients of Uranium and Other Metal Ions with Percentage of THF at the Constant Overall Acidity of 0.6N HNOa ( 5 mg. load per 1 gram of Dowex 50) Percentage of THF Metal ion 0 20 40 60 80 90 UOdII) 40 96 134 243 51 40 36 72 357 >io3 WII) 18 21 38 50 Ca(I1) 102 145 760 >io3 Srf 11) 68 109 228 676 >103 , 1 0 3_ Al(II1) 132 137 258 479 878 >io3 In(111) 173 235 630 >io3 >io3 >io3 Pb(I1) 105 185 463 934 >io3 >io3 Bi(II1) 135 280 630 592 195 47 Zn(I1) 46 58 157 704 >io3 >io4 Cd(I1) 46 49 178 103 104 >io4 Y(II1) >lo? >io3 >io4 >io4 >io4 >io4 La -L Lu(II1) >lo3 >io3 >io4 >io4 >io4 >io4 Ti(1V) 70 127 300 103 >io3 >io3 Zr-Hf( IV) >lo' >lo4 >io4 >lo4 >lo1 >io4 Th(1V) >io4 >lo' >io4 >io4 >lo4 1,420 Cr(II1) 125 230 600 >io3 >lo? >io4 lIn(1I) 40 61 130 405 >io3 >io3 Fe(II1) 171 200 287 550 >io3 >io3 Co(I1) 32 42 119 306 >io3 >io3 Ni(I1) 30 40 115 316 >io3 >io3
-
VOL. 38, NO. 3, MARCH 1966
e
497
seen that in the 90% THF-10% 6N nitric acid medium, uranium has by far the lowest distribution coefficient, while practically all other divalent and higher valent ions (except bismuth) have Kdvalues of the order of > lo3 - lo4 and consequently their separation from uranium is readily achieved by using the working procedure described above. This method was applied t o the separation of amounts of uranium ranging from micrograms to grams from small amounts of divalent and polyvalent cations and the results of some of these experiments are shown in Table I1 from which it is evident that all
separations are quantitative. The separations of 100-mg. amounts of cobalt and iron(II1) from uranium were performed on the 10-gram resin columns. A typical example for the ease with which also very large amounts of uranium can be separated from small amounts of divalent elements is the following: 2 grams of uranyl nitrate hexahydrate plus 2 mg. of cobalt nitrate (cobalt was selected because it has about the same distribution coefficients as uranium in pure aqueous 0.6N nitric acid) (see Table I) are dissolved in 5 ml. of the THF-HNOa mixture and this is passed through a 1-gram resin column as
Table II. Separation of Uranium from Other Elements in HNO, Medium
90%
Amounts, mg. Taken Found
Elements separated Uranium-cobalt UOd 111 Co(I1) UOdII) Co(I1) UOdII) Co(I1) UOd 11) Co(1I) UOZ(I1)
cop)
Uranium-iron UOdII) Fe(II1) UOZ(I1)
THF-lOyo 6N
Error, %
1 1 100 100 1,000 1 10,000 1 10,000 0.1
1.00 1.02 100 99.9 998 1.00 10,000 1.02 10,010 0.098
0 +2 0 -1 -0.2 0 0 $2 +0.01 -2
0.1
0.10 1.03 99.7 0.104 1.00 101.5 1,005 0.10
0 +3 -0.3 +4 0 +1.5 $0.5 0
1.02 0.096 997 0.102
+2 -4 -0.3 $2
0.1 10
0.10 9.98
0 -2
0.1 10
0.097 10.0
-3 0
0.1 10
0.10
10.1
0 +I
1
100 0.1 1 100 1,000
Fe(II1)
UOz(I1)
Fe(II1)
UOP(I1)
0.1
Fe(II1)
Uranium-thorium UOdII) Th(IV) UOdII) ~ h117 () Uranium-gold UOZ(I1) Au(II1) Uranium-vanadium UOu(I1)
1
0.1 1.000 0.1
Y(V)
Uranium-molybdenum UOdII) Mo(V1) Uranium-rthophosphoric acid UOz(I1) &PO4
0.102 9.96
0.1 10
+2 -4
Distribution Coefficients of Uranium in Various Organic Solvent-Water Mixtures Containing 10% 6N "03
Table 111.
(5-mg. load per 1 gram of Dowex 50)
Organic solvent, yo Solvent Methanol Ethanol n-ProDanol Isopropanol n-Butanol Isobutanol
Acetone THF Methyl glycol Ethyl glycol Acetic acid ~
498
0 40 40 40 40 40 40 40 40 40 40 40
20 49 52 85 67 51 95 85 96 54 45 76
~
ANALYTICAL CHEMISTRY
40 85 119 168 159 77 30 127 134 110 141 159
60 150 224 207 364 26 18 364 243 207 330 350
80 350 535 500 890 198 283 207 51 434 350 480
90 750 750 890 >io3
455 395 141 40 314 390 >io3
described in the working procedure. During the washing step (see procedure) the pink zone due to the adsorbed cobalt does not move down the column bed to any appreciable extent and even if two liters of the THF-HN03 mixture is passed, the cobalt zone moves to a very small extent only while more than 90% of the uranium is already eluted with the first 50 ml. of the THF-"03 mixture. This shows that cobalt could easily be separated from quantities of uranium by far exceeding two grams. Similar experiments carried out also using very large amounts of uranium showed that such separations can be achieved with all the other divalent and also tri- and tetravalent metal ions (see Table 11). Consequently this technique can also be used to purify uranyl nitrate and to enrich these strongly adsorbed elements before their determination. The method, therefore, appears to be highly suited for the analysis, purification, and preparation of highpurity uranium. Analyses by means of this method would especially then be of importance if the content of the impurities is low so that very large samples must be analyzed. Furthermore, this technique offers the possibility of rapidly removing fission products from uranyl nitrate solutions and the subsequent group separation (qee working procedure) of the adsorbed radionuclides. Table I1 includes also the results of experiments which involve the separation of uranium from phosphoric acid, gold(III), molybdenum(VI), and vanadium(V)-Le., from elements which under these conditions are not retained by the resin and were found to have distribution coefficients in the order of 1 to 2. Platinum and palladium are also not retained under the.e conditions. Because all theqe elements including phosphoric acid are cleanly separated from uranium and, consequently, also from the strongly adsorbed elements, the method may also be suitable for the analysis of phosphates, vanadates, and molybdates for uranium and/or the strongly adsorbed elements. h reduction of vanadium and molybdenum to lower valency states does not take place because these elements form rather stable complexes with the T H F under these conditions. Investigations with respect to the influence of nitric acid concentration on the adqorption of uraniuni from 90% T H F mixtures showed that the adsorption decreases rather strongly with increasing concentration of nitric acid. Thus, when 10% of 0.6N nitric acid is present, the Kd of uranium is greater than lo3 while in the medium u-ed for the separation experiments, it is only 40 (see Table I). \Vhile in the concentration range from 100 fig. to 10 mg. thr Kd of uranium is
practically independent of the uranium concentration, it was found to decrease when greater amounts of uranium were present. DISCUSSION
Measurements of the adsorption behavior of uranium in mixtures with varying amounts of organic solvents (0 to 90%) and all containing 10% of 6N nitric acid and water up to 100% gave the results shown in Table 111. From these data it is seen that in the T H F and acetone media, adsorption maxima occur at 60% of these organic solvents (and a small maximum a t 80% methyl glycol) whereafter the distribution coefficients decrease with an increase of the percentage of the nonaqueous components of the mixtures. In all the other media there is a steady rise of the distribution coefficients from 0 to 90% except of the minima in nbutanol and isobutanol media which are caused by the appearance of two liquid phases. This estraordinary behavior shown by uranium especially in the tetrahydrofuran-nitric acid media (less so in the acetone system) of very high concentration of organic solvent can be explained by assuming that under these conditions extraction and ion exchange mechanisms are operative simultaneously. This is corroborated by the fact that uranyl nitrate is readily extractable into ethers or ketones from dilute nitric acid solutions and is also very soluble in such systems. In these media therefore two processes are operative namely extraction (viz., liquid anion exchange, see below) and cation exchange with the former having a much greater effect on uranium adsorption at high T H F concentrations while a t lower percentages of this solvent (Le., up to 6070), the cation exchanger is oompeting much more succesafully with the extraction mechaniam so that in this region of T H F percentages a steady increase of the Kd value of uranium is observed. At higher concentrations the extraction effect shows a much stronger influence SO that the adsorption of uranium on the cation exchanger decreases considerably. Because under these conditions both ion exchange and the principle of solvent extraction are effective simultaneouqly, separations carried out in such media may be termed as such performed under conditions of "combined ion exchange-solvent extraction" (CIESE for short) ( 3 ) . In analogy to the extraction of uranyl nitrate with diethyl ether or other solvents (e.g., a long chain amine from nitric acid solution), it is proposed that in the liquid phase (e.g., in the 90% THF-lO% 6N nitric acid medium) the following reactions 1 and 2 occur (schematically presented).
CHz-CH2/ T H F (ether) acting as Lewis base 2(THFH)+NO3-
Liquid anion exchanger in the nitrate form (THFH) +NOs-for short (oxonium salt)
+ [VO2(NO,),]-'
At high concentrations of T H F the liquid anion exchanger formed by the reaction shown by Equation 1 is formed to a much higher extent and, hence, it can much more effectively compete with the solid cation exchanger Dowex 50 for the uranyl ion (present as anionic nitrate complex) which means that compound A formation (see Equation 2) will be enhanced. I n other words the anion exchange reaction shown by Equation 2 can effectively compete with the cation exchange reaction shown by Equation 3. 2RaS03-H+ f uozz+ (&S03-)2uoz R, = resin matrix
+ 2H+
(3)
That this compound 4 formation does not occur in the other organic solvent mixtures (80-9070) [e.g.. methanol, ethanol, acetic acid, etc. (see Table III)] is very likely due to the fact that these solvents are much weaker Lewis bases than T H F or acetone so that the reactions shown by Equations 1 and 2 do not occur to a n appreciable extent so that the distribution coefficient rises steadily with increasing concentration of organic solvent-Le., with decreasing dielectric constant of the mixtures when the percentage of an organic solvent in a certain mixture (e.g., methanol-nitric acid) is increased from 0 to 90%. With respect to analytical separations the 90% THF-lOyo 6.47 nitric acid medium thus combines both the advantages of ion exchange and solvent estraction because this compound A formation is evidently not shown by most of the elements investigated (see Table I)-Le., these metal ions behave as uranium in the alcohols and other solvents (see Table 111). That even in these solvents the CIESE principle is active to some extent, however, is evidenced by the fact that the distribution coefficient of uranium in the 90% organic solvent mixtures, for example, is in most cases lower than that of divalent transition elements (e.g., cobalt and nickel) which show about the same degree of adsorption as uranium in the pure aqueous nitric acid medium (compare adsorption values of Table 111 with those of Table I). -4behavior similar to that of uranium
(1)
+
(THFH)2+[COz(NOs)a]-' 2N03- (2) liquid anion exchanger loaded with the uranyl nitrate complex (compound A) (Ion association complex) in the 90% THF-lOyo 6 N nitric acid medium is shown by bismuth and thorium which from this medium are also much less adsorbed on Dowex 50 than from media of lower concentration of T H F (see Table I) so that also in these two cases the reactions shown by Equations 1 and 2 occur to a more (bismuth) or less (thorium) extent. [That thorium, although it forms a much stronger anionic nitrate complex (I), is much stronger adsorbed on Dowex 50 than uranium or bismuth is due to the fact that because of its higher valency it has a much greater affinity for the resin than these elements.] Under similar conditions (90% THF10% 5N nitric acid) uranium, thorium, and bismuth are also not strongly adsorbed on the anion exchanger Dowex 1 (7) although especially thorium and bismuth are known to form stable anionic nitrate complexes which are strongly adsorbed from both pure aqueous nitric acid systems ( 1 ) and from organic solvent-nitric acid mixtures containing high percentages of aliphatic alcohols ( 8 ) , for example, isopropanol (2) and acetic acid (6). This is strong evidence that the CIESE system THF-nitric acid can also effectively compete with a strongly basic anion exchange resin for these metal ions. Another indirect proof that the CIESE principle is also operative with thorium when acetone-nitric acid is employed as the adsorption medium and Dowex 1 as the adsorbent is evidenced by the fact that the distribution coefficient of thorium increases much less with increasing acetone concentration than that of lanthanum so that in 90% acetone-10% 6N nitric acid the Kd values of thorium and lanthamum are 530 and 1740, respectively (5). At acetone concentrations below 80% the reverse is trueLe., the Kd value of thorium is much higher which is in accordance to the observations made in pure aqueous nitric acid media (1) and in other organic solvent-nitric acid mixturgs (8). Thus in a mixture consisting of 60% acetoae-30% water-lO% 6.47 nitric acid the distribution coefficients of thorium and lanthanum are 85.2 and 7.2, respectively. A similar explanation as in the case given above for the relatively weak VOL. 38, NO. 3, MARCH 1966
499
adsorbability of uranium on Dowex 50 from the THF-nitric acid medium can be given (3, 4) with respect to the nonadsorbability of iron(III), gold(III), gallium, and other elements on Dowex 50 and also Dowex 1 from T H F and acetone media of similar composition but containing hydrochloric acid in place of the nitric acid used here. The greatest advantage of this CIESE principle over the conventional ion exchange and solvent extraction procedures is that immediately after removal of the “extractable” species as in our case uranium, a further fractionation of the adsorbed metal ions can be achieved on the resin while with, for example, the conventional extraction of uranium with ether from a nitric acid solution a further separation of nonextractable elements is not readily achieved and conditions are also complicated by the
presence of the relatively large amounts of salting-out agents that have to be added to the aqueous phase before the uranium can be extracted with the ether. The advantages of the CIESE method over the conventional cation exchange procedures used for the separation of uranium have already been pointed out in the introduction to this paper and are amply demonstrated by the results presented in Table I. LITERATURE CITED
(1) Faris, J. P., Buchanan, R. F., USAEC, Rept. ANL-6811, July 1964. ( 2 ) Fritz, J. S., Greene, R. G., ANAL. CHEM.36, 1095 (1964).
(3) Korkisch, J . , 2.Anal. Chem., in press. (4) Korkisch, J., Ahluwalia, S. S., Anal. Chim. Acta, in’press. ( 5 ) Korkisch, J., Ahluwalia, S. S., J. Inorg. Nucl. Chem., in press, (6) Korkisch, J., Arrhenius, G., ANAL.
CHEM.36, 850 (1964).
G., Talanta 9,957 ( 1 (9) Kraus, K. A., Moore. F. L.. Nelson. F . , J . A m . Chem. SOC.7 8 , 2692 (1956). (10) Nelson, F., Murase, T., Kraus. K. A.. J . Chromatoo. 13., 503 1196 -._ .-.-4). (ll)’Strelow, F. O W . - E., ANAL. CHEM. 32, 1185 (1960). (12) Strelow, F. W . E., J. S. African Chem. Znst. 16, 38 (1963). (13) Strelow, F. W. E., Rethemeyer, R., Bothma, C. J. C., ANAL. CHEM.37. 106 (1965). JOHANN KORKISCH
s. s. AHLUWALIA
Analytical Institute University of Vienna, IX Wahringerstrasse 38 Vienna, Austria RESEARCH sponsored by the International Atomic Energy Agency and the U. S. Atomic Energy Commission under Contract 67/US (AT(30-1)-2623).
Rapid and Precise Determination of Carbon Dioxide from Carbonate-Containing Samples Using Modified Dynamic Sorption Apparatus SIR: Apparatus developed by Nelsen and Eggertsen (IS) for determining the surface area of solids by a continuous flow method has proved useful, with slight modifications, in studies involving thermal decomposition of carbonates and oxide sintering ( I T ) , chemisorption on solid catalysts ( I @ , and pore-size distribution (IO). With further modifications the apparatus can be transformed readily into a sensitive analytical tool for determining the quantity of gas evolved from certain types of samples through thermal or chemical decomposition or by chemical reaction. This paper is concerned with the use of such apparatus in the quantitative determination of carbon dioxide evolved from carbonate-containing samples after acidification. Commercial apparatus, such as the Perkin-Elmer Sorptometer, should pose no problems for the modifications to be described. The extensive literature on carbonate analysis offers a number of methods for quantitatively determining carbon dioxide evolved from a sample. The method generally regarded as standard is that in which the liberated carbon dioxide is absorbed and weighed (11, 18). However, for optimum results this method usually requires 45 minutes to an hour per determination. Examples of other methods that have been used with varying degrees of accuracy, precision, speed, and adaptability include manometric (8, 18), titrimetric (4,7 , 15), gas volumetric ( I ) , dilatometric (S), controlled loss-on-ignition 500
ANALYTICAL CHEMISTRY
(o), infrared ( 1 4 , and gas chromatography ($1. In studies on the thermal decomposition of carbonates referred to above, precursory data were reported in the determination of the carbon dioxide evolved, but the method did not prove as convenient nor as precise as the method outlined in this paper. Here, in brief, carbon dioxide is released within the apparatus after the sample is acidified. The gas is carried by helium through an absorption train to remove water and undesirable acidic gases and then through a thermal conductivity cell. A determination can be made in about 15 minutes. The method is applicable to various sample types ranging from those containing the highest carbonate content to those containing well below 0.1%. Although samples with a carbonate content lower than O . l ~ ,have not been investigated in detail here, consideration of instrumental stability and of the signal generated by small quantities of carbon dioxide under test conditions leaves little doubt that the method can be further extended, even into the parts per million range, if desired. EXPERIMENTAL
Apparatus. A schematic diagram of the apparatus is shown in Figure 1. A thermal conductivity cell assembly (Gow-Mac Model 9193-TE-11) is contained in an oil bath a t 27.0’ =t0.1’ C. A coil of copper tubing, l/h X 18 inches, is attached to each half (reference and
measuring) of the cell assembly and is also immersed in the oil bath to help provide temperature equilibrium for the incoming gas. The reference and measuring detectors form part of wellknown bridge circuitry (Gow-Mac Bulletin TCTH-6-62-311) in which a total bridge current of 140 ma. is supplied by a 12-volt storage battery. An attempt was made to use the more convenient Gow-Mac Power Supply Control Unit (Model 405-C : l), but it did not provide satisfactory current stability for this application. A Sargent Model MR (multirange) recorder equipped with a Disc integrator (Disc Instruments, Inc., Santa Ana, Calif.) is used for recording bridge signals and integrating peak areas. Helium, used as a carrier gas, is delivered a t 10 p.s.i.g. Pressure is maintained by a Cash-Acme (Decatur, Ill.) type A-360 regulator. A needle valve controls helium flow, which is determined and monitored by a soapfilm flowmeter and rotameter, respectively. The U-shaped sample tube, illustrated in Figure 2, is constructed from two 12/5 standard ball joints sealed to a 5-inch section of 12-mm. i.d. glass tubing. A short side arm that accommodates a rubber septum is sealed into the larger portion of the sample tube. The over-all length (between joints) of the sample tube is about 12 inches. Three U-shaped tubes immediately follow the sample tube in the gas flow scheme. The first contains anhydrous magnesium perchlorate for removal of water from the gas stream, the second contains anhydrous copper sulfate for removal of undesirable acidic gases
