Separation of Radium and Barium by Ion Exchange Elution W. H. POWER,1
H.
W. KIRBY, W. C. McCLUGGAGE,2 G. D. NELSON,1 and J. H. PAYNE,
Jr.1
Mound Laboratory, Monsanto Chemical Co., Miamisburg, Ohio Radium can be separated from barium-radium mixtures in ratios as high as 4440 to by a single elution from a cation exchange resin. Barium elution characteristics from Dowex 50 resin, position of elution maxima, and maximum barium loadings are correlated with citrate concentration and pH by the use of citrate complex equilibria. Ammonium citrate, 0.32M at pH 5.6, is the most satisfactory eluent of those used from the standpoints of separation factor and freedom from precipitation in the column. Conditions affecting the precipitation of slightly soluble barium citrate during elution with ammonium citrate are discussed. Eluting above 25° C. decreases separation factors and permissible barium levels. 1
of radium from its ore is aided by the addition of a barium salt to act as a carrier. Radium may be concentrated from radiumbarium mixtures by fractional crystallization of halide solutions (6) or by fractional precipitation of radium and barium chromates (8). The final separation of small quantities of radium may be accomplished by elution from a cation exchange resin. The elution method consists of adding a radiumbarium mixture to the upper layer of a cation resin column and preferentially leaching the barium from the column. Subsequent elution strips essentially carrier-free radium from the column. Tompkins (10) separated barium from microgram quantities of radium by ammonium citrate elution from a Dowex 50 column. Modifications of this method were found necessary to permit handling of concentrations suitable for commercial application. The process was demonstrated with the separation of milligram quantities of radium.
The
recovery
EQUIPMENT AND REAGENTS
Beds of 50- to 100-mesh Dowex 50 resin, 13 mm. in diameter by 250 mm. high, were prepared in borosilicate glass Present address, Monsanto Chemical Co., Lindbergh Blvd. and Olive St. Road, St. Louis 24, Mo. 2 Present address, Goodyear Atomic Corp., Portsmouth, Ohio 1
washed with soluble organic material. Conversion of the resin to the ammonium form and elimination of alkaline earths were accomplished by
columns.
The resin
ethyl alcohol to
was
remove
contact with excess 0.6M ammonium citrate, 1.5V in ammonium hydroxide. Radium bromide was purchased from Eldorado Mining and Refining, Ltd. All chemicals used to prepare feed and eluting solutions were of reagent grade. EXPERIMENTAL
Adsorption of Radium and Barium.
Feed solutions contained barium and radium salts dissolved in 0.2V nitric or hydrochloric acid. Flow rate of the feed solutions was limited to 1 ml. per sq. cm. per minute to permit the cations to be adsorbed on the topmost layer of resin. Because anions were not adsorbed by the cation exchange resin, the nature of the feed solutions could be varied for convenience. If the starting form of the barium-radium mixture was a chromate precipitate (result-
ing from a concentration by fractional chromate precipitation), the barium and radium could be fed onto the eluting column and the chromate ion eliminated simultaneously. In this case the barium-radium chromate was dissolved in a small quantity of concentrated nitric acid and diluted to 0.2V acid concentration before feeding to the column. Analysis of Column Effluent. Barium determinations were made by a turbidimetric analysis of barium sulfate precipitate; the lower limit which could be evaluated was 10_4V. Radium determinations were made by alpha counting a sample freed from alphaactive daughters by either aeration (11) or chloride precipitation (1). Elution of Barium and Radium. The elutriant addition rate was fixed at 0.3 ml. per sq. cm. per minute as recommended by Tompkins (10). Most elutions were made at 25° C. Increasing the elution temperature to 50° and 75° C. had an adverse effect on separation factors and increased the tendency for barium citrate to precipitate on the column. The column volume was taken as the volume of water (16.6 ml.) required to fill the interstices of the air-dried ammonium resin form. To check the effectiveness of barium elution by mineral acids and salts, a series of runs was made using 1 to 6V hydrochloric acid and 3.5M ammonium chloride solutions. Of all these elutriants tested, only ammonium chloride
The of ammonium chloride as an elutriant did not provide a good separation of barium and radium, however. Better separations were obtained with citrate solutions at a pH above 5 than with ammonium chloride or citrate solutions of low pH. gave well defined barium peaks. use
ION EXCHANGE EQUILIBRIA
Schubert’s data (9) indicate that barium is more strongly complexed than radium in citrate solution, but that radium ion is more strongly attracted to a cation exchange resin. Consequently, barium can be more easily separated from radium by elution wfith citrate solution than with acid alone. The following expression has been derived (S, 7) relating the column volume
maximum for barium elution, CBa, to the ammonium ion concentration and the ratio of barium ion to total barium in solution (Ba++/Bas): log
CBa
=
log [(Ba++)/(Ba,) ] 2 log (NHt) + log K -
(1)
An approximately constant K, based concentrations instead of activities, may be established by eluting with ammonium chloride solution in which no citrate is present so that the ratio of barium ion to total dissolved barium is unity. The column volume maximum was 13 when barium was eluted with 3.5M ammonium chloride; hence, K is calculated to be 159. Values for the ratio, Ba++/Ba„ were calculated from the dissociation constants of barium citrate complexes as determined from solubility data (5) and the dissociation constants of citric acid (4). The molarities of ammonium ion in fixed concentrations of ammonium citrate solutions required to obtain a given pH were calculated from the equations of Bjerrum and Unmack by Schubert (9). The approximate position of barium maxima as related to pH and citrate concentration can be read from the plot of Equation 1 on
(Figure 1). PRECIPITATE FORMATION
During initial elutions with citrate solutions, precipitates of slightly soluble barium citrate were formed in the columns. Precipitation in the column is VOL. 31, NO. 6, JUNE 1959
·
1077
9r
o o o
CO
=>
_l
o 4
cn
Figure 1. Relation of barium elution maxima to citrate concentration and pH
z>
---Locus of equivalent barium
