2818
G. E. BOYD,J . SCHUBERT AND A. W. ADAMSON
the elution curves, on continued use. Phenol seemed to meet these requirements and was tested quantitatively. Spore cultures of Aspergillus ‘oryzac and of Aspergillus niger were used for massive spore inoculation of 0.5% citric acid solution adjusted to PH 4.0 with ammonium hydroxide. The following concentrations (g. per 100 ml.) of phenol were added to the citrate solution; 0, 0.01, 0.02, 0.03, 0.05, 0.07, 0.10, 0.20, 0.30, 0.50, 0.70 and 1.0. Growth was evident in the first three in twentyfour hours. After six days, there was some growth in 0.03 and a slight trace in 0.05 but no growth in the higher concentrations; the same situation was maintained after a period of about three weeks. It was decided to use 0.1% of phenol in the eluting agent giving a safety factor of about 2. The effect of 1% phenol on column operation, a concentration ten-fold that employed as a disinfectant, was tested. The control and test columns (16 mm. in diameter and 33 cm. bed length) were charged with Amberlite IR-100 under identical conditions and the complete operation applied in processing 0.5 g. of didymiutn oxide. The elution curves were identical showing that a concentration of phenol as high as 1% did not interfere with the operations. The use of O.l’% phenol in the citrate eluting agent was put into practice in the large-scale production columns. During a period of over five
Vol. 69
months there has been no growth of mold and all columns have been in continuous operation with entirely satisfactory’results. The authors wish to thank V. A. Fassel for the many spectrographic analyses which he performed during the course of this work.
V. Summary Detailed procedures have been given for the large scale separation of the rare earths using Amberlite IR-100 and 0.5% citric acid ammonium citrate solutions as the eluant. The process employed 24 columns 4 inches in diameter and with 8 feet of resin bed length. The pH of the citrate eluant was 3.9; the growth of mold was eliminated by the use of 0.1% phenol. Large samples of rare earths, of high purity were obtained by these procedures, portions of which have been furnished to other members of the Manhattan Project. Examples of the quantity and quality of several fractions on hand include: (1) Neodymium, 800 g. 99.9% Nd203, < 0.1% Pr&; 770 g. 98% Nd203, 2% Pr&; (2) Praseodymium, 35 g. 99% PrGOII, 1% Ndz08; 160 g. 90% Pr6OI1 (impurities Ce, La, and Ca); (3) Samarium, 160 g. > 99.9% Sm203; 600 g. 99% SmzOa,0.5% Eu203, 0.5% Ca. In addition, considerable quantities of slightly impure heavy rare earth salts and yttrium salts have been prepared. AMES, IOWA
[CONTRIBUTION FROM THE CLINTON
RECEIVED JULY 14, 1947
NATIONAL LABORATORY]
The Exchange Adsorption of Ions from Aqueous Solutions by Organic Zeolites. I. Ion-exchange Equilibria’ BY G. E. BOYD,’
J. sCHUBERT3 A N D A.
Introduction The observation that extremely small quantities of dissolved radioactive species could be extensively removed by base-exchanging substances from aqueous solutions of appreciable ionic strength was the starting point for much additional research and development work on the part of many individuals associated with +he Plutonium Project. As a consequence, numerous adsorption techniques have been developed for the extraction, concentration and fractionation of a wide variety of radio-isotopes. Attempts to elucidate the principles governing tkese systems (1) This work was performed under the auspices of the Manhattan District a t the Metallurgical Laboratories of the University of Chicago and at Clinton Laboratories, Oak Ridge, Tennessee, during the period April, 1942, to April, 1946.(2) O n leave from Department of Chemistry, University of Chicago. Present address: Clinton Laboratories, Monsanto Chemical Co., Oak Ridge, Tennessee. (3) Present address: Department of Physiological Chemistry, Medical School, University of Minnesota, Minneapolis 14, Minnesota. (4) Present address: Department of Chemistry. University of Southern California. Loa Angelcs, California.
w.ADAMSON‘
led to the undertaking of quantitative studies of the equilibrium and rate processes governing ionexchange adsorption. Further, these efforts led to the development of exact methods for describing adsorption processes in dynamic systems as, for example, when traces of solute are extracted from a solution flowing through a deep bed of adsorbent. The more important results from this program of basic research will be presented in a series of papers, of which the current article is the first. Here, the problem of the accurate formulation of the relations governing the equilibrium behavior of exchange adsorption systems, the experimental testing of these equations, and the generalizations concerning the adsorbability of cations arrived a t will be treated. Incidentally, the researches to be described will illustrate many of the advantages in the employment of radio-isotope techniques in dealing with physical chemical problems. By virtue of the convenience, sensitivity and accuracy with which changes in the amounts of radioactive substances can be estimated, the phenomenon of the exchange
Nov., 1947
ION-EXCHANGE
EQUILIBRIA ON
adsorption of ions present in extreme dilution could be studied in solutions of widely variable ionic strength. Formulation of Heterogeneous Ion-exchange Processes According to Langmuir’s Adsorption Mechanism.-The relationship which has been found to hold between the amount of base exchange and the equilibrium concentration cmf added electrolyte appears a t first sight to be similar to an ordinary adsorption isotherm (Fig. 1). For a given amount of ion-exchanger and a fixed total volume of solution the curve has the same form. The similarity, however, seems to disappear on consideration of the equilibrium taking a constant amount of exchange adsorbent:, and an increasing solution volume. The experi.mental results in this case reveal that the amount of uptake is independeiit of the concentration of the adsorbed electrolyte. T o explain the foregoing observations, consider the dynamic interchange of ions between the zeolite and the surrounding aqueous solution a t equi.librium. Upon :any unit area there will be a corn.petition between the cations in solution for the available anionic groups structurally bound to the adsorbent. Plt any time, under constant conditions, then, the surface will be covered by ions both from the solution and initially in the ex:changer. Consider now the most simple case of the simultaneous competitive adsorption of two singly charged cations, A+ and B+. By formal analogy with the Langmuir equation for adsorption from a binary gaseous m i x t ~ r ethe , ~ equation for the adsorption of one of the ions, A+, from a dilute electrolyte solution is then
where ( x / m ) A + is the amount of A+ ion adsorbed per unit weight of adsorbent; CA+and CB+ are the respective equilibrium concentrations (activities) of ions A+ and B+ in solution; and k, b 1 and bz are constants. The number oF unsaturated anionic exchanging groups ii. e., :tmount of bare surface in the Langmuir picture) must always be very small, for otherwise a n appreciable frce negative surface charge would be created leading to particles of adsorbent bearing a high negative charge, which is not the case. Accordingly, to a good approximation the quantity unity i n the denominator of Equation (1) can be neglected relative to the quantity, (blCA+ bzCB+)
+
For convenient application. to experimental data, Equation ( 2 ) may be written in a linear form (3) (5) I . Langmuir, THIS J O U R N A L ,38, 2221 (1916); see H. S. Taylor, “Treatise on Physical Chemistry,” D. Van Nostrand Co., New York,
N.Y..1980, Vol. 11. pp. 1073 6 .
ION
EXCHANGE RESINS
2819
SO that a ( C A + / C B + ) / ( x / m ) A + , C A + / ~ B plot + should give a straight line if the treatment is valid. When very small initial concentrations of the ion A+ are present in solutions relatively concentrated in ion B+, even the complete adsorption of A + will not desorb sufficient quantities of B + to cause a detectable change in CB+. As a consequence, a plot of CA+/(x/m)A+against CA+ would be expected to be linear. An example of this is afforded by Fig. 2 which was constructed from the data of Fig. 1 obtained in preliminary experiments.
1.40 4
9
eSI 1;20 a
p:
e ?
1.00
-9
% 0.80 M
k
3 0.60
eSI
2
2 0.40 E
.e
E!
2 0.20 ,
i
-
L
0.02 0.04 0.06 0.08 Equilibrium concn. moles per liter. Fig. 1.-Adsorption of cations a t 25’ by organic ion exchanger.
Inspection of Equation (2) reveals that the amount of base exchange, ( x / ~ ) A + ,must depend upon the ratio of the equilibrium concentrations of the exchanging ions, and hence t h a t the extent of adsorption will be independent of the dilution. For CB+ >> CA+the adsorption of A+ becomes directly proportional t o CA+to a good approximation; for CA+>> CB+the adsorption of A+ becomes constant and independent of concentration. Measurement of the adsorption a t CA+ = CB+makes possible the evaluation of the adsorption affinity, given by (b&). I n the more general case, the exchange adsorption of two cations of unequal charge, A’+ and By+,will be governed by the equation
Finally, the expression for the adsorption of a
G. E. BOYD, J.
SCHUHEK.1 A N I )
r\. b‘. ADAMSON
Vol. ti9
sitions are relatively widely spaced in the adsorbent. Further, it was taken that none of.the anions initially present in solution was adsorbed. If a physical adsorption of anions should occur, this would be accompanied by an adsorption of cations in excess of t h a t from base-exchange, Finally, it has been supposed that no more than one layer of cations was adsorbed, or, when the anionic groups of the zeolite are saturated no further uptake of cations occurred. When looked a t in this sense, ion-exchange adsorption can be regarded as a species of chemisorption. Since equations of the same form as the above may be derived from the law of mass action when applied to the exchange equilibrium of equally charged ions, the agreement of experimental data with Equation (2) cannot be taken as evidence that base-exchange is an “adsorption” phenomenon in any special sense. Formulation of Heterogeneous Ion-exchange Processes According to the Law of Mass Action. -An alternative to the foregoing “adsorption” treatment is t o assume t h a t the ion-exchange is analogous to an ordinary metathesis reaction and that, a t equilibrium, the Mass Law applies t o the heterogeneous system : ion-exchanger aqueous solution. The exchange reaction involving two monovalent cations, A + and B+, then may be written as
+
0.4 0.8 0.12 Equil. eoncn. of adsorbed ion, CY’+ moles per liter. Fig. 2.-Plot of the data of Fig. 1 according to equation (3).
singly charged cation, Mi, from a mixture of singly charged cations in solution may be written :
A+
+ BR = AR + B +
(7)
where R has been taken to designate the insoluble structurally bound ariionic part of the organic zeolite. The thermodynamic equilibrium constant, Kat for this reaction is defined by
K.
= ag+ a A d a A +
am
(8)
where aA+, a B + are the respective activities in aqueous solution of the ions A+ and B +, and where a m and a B R are the activities in the solid state. Evidently, then, the greater ionic strength of It may be said that the Mass Law is obeyed if, for the mixed electrolyte the less the exchange ad- a variety of compositions of the system containing the exchanging ions plus adsorbent, the approsorption of any single cationic species held a t con- priate activity values when substituted into stant concentration. The constants bl and b2 in the aboce equations Equation (8) lead to a constant value, K,,indeare related to the energy of adsorption of the ions, pendent of the composition. Further considerations must be made before EA+ and EB+,by the relation Equation (8) is applied to experimental basebdbt = (&+/PA+) exp - [(EB+ - E A + ) / R ~(6) ~ exchange data. First, there is the problem of the These energies, actually free energies as will be evaluation of the activities of the ions present in shown later, are determined largely by electro- the aqueous phase. As is well appreciated, these static factors connected with the charge and with may be related to the product of their concenthe size of the ion in solution, The constant, k, trations in solution by their respective ionic acmay be identified with the total exchange capac- tivity coefficients. These ion activity coefficients in turn may be related to the mean activity coity, E , of the adsorbent. The applicability of the foregoing treatment is efficients of the electrolyte, which usually have dependent upon a number of assumptions. been determined fbr pure aqueous solutions. The Several of these seem worthy of mention: Firstly, appropriate activity coefficient to be employed, i t was assumed only one type of adsorbing surface however, is t h a t for the ion in the mixed electro(i. e., exchanging anion) was present in the ex- lyte in equilibrium with ion-exchanger. The problem of the evaluation of the activities changer. It also has been assumed that there was no interaction between neighboring adsorbed ions, for the solid phase calls for a special treatment. which appears reasonable if the exchange po- If the reaction given by Equation (7) were analo-
Nov., 1947
ION-EXCHANGE EQUILIBRIA ON IONEXCHANGE RESINS
gous to a chemical reaction where AK and BR. occur as independent solid phases, i t would be possible to take their activities as constant and equal to unity. Kerr6 reported, however, that such an assumption was not valid with the cation exchange reactions studied by him. This has been confirmed by Vanselow7 and by the results from the present work. Actually, since the ions freely substitute for one another in the exchanger, it is better, as Vanselow pointed out, to suppose that. the components of the solid phases form completely miscible solid solutions with one another Justification for treating the’organic zeolites employed in this work as solid solutions has been supplied by X-ray diffraction measurements8 which have revealed a random distribution of the metal cations throughout the adsorbent. Quite generally, the activities of the components of the binary solid solution must be taken as complex functions of their respective mole fractions. If, however, the ions in the solid are singly charged and if they are not too widely different in size, their solid solutions may be taken as ideal. In this case (EAR and a B R may tie set equal to the mole fractions X A R and XBRwith Equation (8) becoming K. =
( U B + ) ~ A R / ( ~ A + ) ~ B R (~B+)(~AR>/(~A+)(~BR)
(9)
282 1
(10) and (ll),if desired. From the value of log Ka obtained i t becomes possible to calculate the standard free energy of exchange adsorption, A P . The foregoing has been based. upon the exchange of two monovalent cations; a brief further discussion of the somewhat more general case of the exchange of two ions of unequal positive charge, AY+ and B”+ where Y; > v+ will now be given. Writing the exchange reaction A?+
+ (v+/v;)BR
= AR
+ (v+/v;)B*:
(14)
The expression for the mass law equilibrium constant assuming ideal solid solutions is then
where symbols have the same meaning as in Equations (8) and (9). In the special case when ?zAR U+T' >> Ra++ >> Cs+ > R b + > K + > Na+ > H' > Li+. 3 . Adsorption affinities were shown to be determined chiefly by the magnitude of the charge and the hydrated radius of the inns i n solution. P. 0. Box W, OAKRIDGE, TENN. RECEIVED J U L Y 7 , 1917
