Dec., 1957
ADSORPTION OF ZN(II) ON ANION-EXCHANGE RESINS
valent nickel atom, its positive charge is effectively cancelled by the negative charges of electrons involved in the six dative bonds. Accordingly, the distance between the tetracoordinated nickel atom
1651
and the adjacent cyanide group will be shorter than that between the hexacoordinated nickel atom and the cyanide group, as is actually found by Xray analysis.
THE ADSORPTION OF ZINC(I1) ON ANION-EXCHANGE RESINS. I. THE SECONDARY CATION EFFECT BYR. A. HORNE~ Contribution of the Department of Chemistry and Laboratory for Nuclear Science of the Massachusetts Institute of Technology, Cambridge, Mashachusetts Received Jula, 89, 1967
The adsorption of tracer quantities of einc(I1) on the anion-exchange resin Dowex 1-X8chloride from solutions of various chloride concentrations depends on the nature of the cation (secondary cation) of the supporting chloride electrolyte. The adsorption of zinc(I1) from LiCl, CaC12, MgCL, NaC1, LiCl and HCl,. HCl, KCl, CsCl, NH,Cl, and HONHaCl has been studied at 25” and the results interpreted in terms of ionic association of the secondary cation with anionic zinc(I1) chloride complexes (a) in the resin and solution phases and (b) in the resin phase only.
Introduction Kraus, Nelson, Clough and Carlston2 have reported that a number of metals, including zinc(II), adsorb more strongly on Dowex 1 from LiCl solutions from HC1 solutions of comparable concentration. The purpose of the present investigation was to see whether this “lithium chloride effect” is a specific LiCl or HC1 effect or whether the phenomenon in question is more general by extending existing work to a greater number of chloride salts, particularly those of the alkali metals. The anionexchange resin behavior of zinc(I1) from chloride systems has been studied previously by Kraus and Moore,s Miller and H ~ n t e r ,and ~ Jentzsch and Frotscher6 and is discussed briefly in the 1955 Geneva Paper of Kraus and Nelson.6
Experimental The 245 day Zna was prepared by bombarding a copper foil target with deuterons (d,2n) in the M.I.T. cyclotron. The dissolved radioeinc was separated as described by Schindewolf’ from the bulk co per and resultant nickel by an anion-exchange column, t f e Cu and Ni being eluted with 2 M HCI, the Zn(I1) with 2.M “0s. The final concentration of HNOs in the experimental batches was 2 X 10-8 M . These batches contained 0.10 g. of medium porosity 50-100 mesh Dowex 1-X8 chloride, tracer radiozinc and supporting electrolyte of appropriate concentration. The total solution volume was 10 ml. The resin was conditioned by repeated alternate washings with 6 M NaOH, 6 HCl and distilled water and dried over “Anhydrone. The batches were equilibrated for longer than 10 hours with mechanical shaking in a thermostatically controlled bath at 24.8 f 0.2”. The radioactivity of solution aliquots was measured in 5-cc. screw capped vials in a well type yscintillation counter. Experiments with 0.10 g. of glass wool per 10 ml. of solution and no resin showed slight glass adsor tion ( D = 3.8 with no added acid, D = 2.1 in 0.6 M &I); the effect,
,y
-~
(1) Radio Corporation of America, Needham Heights, Mass.
Kraus,F. Nelson, F. B. Clough and R. C. Carlston, J . Am. Cham. SOC.,‘IT, 1391 (1955). (3) K. A. Kraus and G. E. Moore, ibid., T I , 1460 (1953). (4) C. C. Miller and J. A. Hunter, Anolyst, 79, 483 (1954). ( 5 ) D. Jentzsch and I. Frotscher, 2.and. Chem., 144, 17 (1955). (6) K. A. Kraus and F. Nelson, “Collected Papera on the Geneva Conference on the Peaceful Uses of Atomic Energy,” 1955, Paper UN-837. (7) U. Schindewolf, M.I.T. Lab. Nuc. Sci., Teohnical Report No. 6s (1955); Anpew Chsm., in press. (2) K. A.
however, is negligible compared to the resin adsorption. The adsorption coefficient, D, is defined operationally by D = (vol. of soln. in ml.)(radioactivity in resin) (1) (wt. of resin in g.)(radioactivity in soln.)
Results and Discussion The variation of the logarithm of the adsorption coefficient with chloride concentration, maintained by the addition of LiC1,8 CaCL, MgC12, AlC13, NaCI,g LiCl in the presence of 0.5 M HCI, HC1,l0 HCI in the presence of 0.5 M LiC1, KC1, CsCl, NHICl and hydroxylaminehydrochloride,l1 is shown in Fig. 1. The near constancy of D at larger concentrations of the latter is due possibly to the strong hydroxylamine complex forming tendencies of zinc(II).12 The variation of log D with the logarithm of the hydrogen ion concentration in mixed LiC1-HC1 solution of a constant chIoride concentration of 6.0 M is shown in Fig. 2. Point A in Fig. 2 is for a 5.5 M LiC1-0.5 M KCl solution and indicates that the depression of the adsorption is not peculiar to hydrogen ion. Initially let it be said that, although zinc(I1) has appreciable hydrolysis equilibria constants,la the high a;dsorption of zinc(I1) from LiCl solution cannot be attributed to hydrolysis because (a) the LiCl solutions in these experiments were all at least 2 X 10-3 M in H + and thus the concentrations of anionic hydrolysis products were negligible, (b) even if formed such species apparently do not adsorb strongly as will be shown in a subsequent paper in this series, and (c) all of the alkali metal chloride curves should be high and the HC1 curve only low. On the basis of only LiCl and HC1 data Kraus and co-workers2.6 have advanced two possible explanations of the “lithium chloride effect”: (a) differencesin activity coefficients in the resin phase, (8) Independent data of U. Schindewolf, R. H. Holm and M. D. Meyers of this Laboratory and the author. (9) CY. H. Funk and G. Lux, Cham. Tech, (Berlin), 8 , 210 (1956). (10) Independent data of Kraue and co-workers (ref. 3 and 6). U. Schindewolf, R. H. Holm, M. D. Meyers and W. R. Pierson of this Laboratory and the author. (11) Data of M. D. Meyers of this Laboratory. (12) C. J. Nyman. J . Am. Chsm. Soc.. 77, 1371 (1955). (13) J . W. Fulton and D. F. Swinehart, ibid., 76, 864 (1954).
R. A. HORNE
1652
40 Total Chloride Concentration =6.0M 24.8 * 0 2OC.
0
3.5 -
0
'O-
d
-
0
__
-P
where K s = 4 f o r H'and K,=Ofor LI'
25-
20
'
Observed Volues D = 10'+K~X+lr10/X+I/X2 KsX+I*lO/X+I/X*
I
I
I
I
I
I
I
I
I
I
Vol. 61
sists of a resinous matrix penetrated throughout with internal aqueous solution which is essentially the same with respect to the concentrations of its various constituents as the external solution except that (a) the concentration of cationic species in the internal solution may be somewhat reduced by Donnan exclusion, (b) the extent of ionic association may differ in the internal and external solutionsdue to differencesin effective (microscopic) dielectric constant and (c) the internal solution contains the fixed cationic functional groups, R+, of the resin with unassociated anionic species lingering near each t o ensure electrical neutrality. The adsorption process may be represented by
I
1 I
I
I I
Dec., 1957 2M+
'
ADSORPTION OF ZN(II) ON ANION-EXCHANGE RESINS
+ Z n C P = MzZnCln,
1653
4.0
K4 = ( MZZnCL)/( M +)2( ZnClr') with reaction 3 rather than the reversal5of
(5)
2 . O M Total Chloride Conc.
at high chloride concentrations, where M+, extending the suggestion of Kraus and c o - w o r k e r ~ ~ ~ ~ ~ ~ ~ is not only H+ but any secondary cation, then at high chloride concentrations the distribution constant will be
2.0
t
The subscripts.R and S indicate the resin and 1.5 solution phases, respectively. I I I Equations 3 and 6 differ with respect to the I 2 3 4 6 6 7 nature of chloride's bonding in the resin, the former assuming a very loose, the latter a more definite Debye-Huckal Parameter, :, in i. chloride-quaternary amine bond. Terms such Fig. 3.-The dependence of the adsorption on Dowex 1 a t as (MZnC13)s, (MZnCh)R and (RZnC4) have 25" of tracer radiosinc(I1) on the Debye-Huckel parameter been omitted as being comparatively negligible; of the cation of the supporting electrolyte. the (RZnCL-) term will also be dropped, its contribution being assumed to be negligible. As MC1 increases, since (R2ZnC14) decreases and (M2ZnCL)Rand (MzZnC14)sconcurrently decrease, D decreases; the greater K4 (or K 3 )the more abrupt the decrease in D. Turning now t o Fig. 1 at higher supporting electrolyte concentrations the order of increasing desorption effectiveness of the secondary cation and hence of increasing K4 is Li+ < Na+ < H+ < NH4+ < K+ < Cs+ < R+,which is the same, except for hydrogen ion, as their absorption order on a Zeolite cation-exchange resin and the reverse of the order of their Debye-Huckel distance of closest approach parameters, d16 (Fig. 3). Equation 7 becomes, remembering our assumption that (ZnCln2-@)Ris approximated by (ZnCln2-@)sand omitting the (RZnC14-) term as negligible D = I
+ + + +
+
I
+
Kz' f K s ( M + ) 1 ~/KA(CI-) l/KB(C1-)a ... Kp(M+) 1 ~ / K A (Cl-) ~ / K B ( C ~ - ) ~. .
+
+
is,
if (3) is the principal adsorption and (4) the principal desorption process or D = &' f K4(Mf)2 f 1 + 1/KA(c1-) l/Kb(Cl-)a . . . Kd(M+)'
+
+ 1 + 1/KA(cI-) + l/KB(Cl-)' +
+
(9)
if (3) is the principal adsorption and ( 5 ) the principal desorption process where Kz' is K2(R+)2and K A and K B are given by
Fig. 4.-The adsorption on Dowex 1 a t 25' of tracer radiozinc(I1) from various supporting electrolyte solutions.
ZnClt-
constants of the sinc(I1) chloro complexes. Estimating K2' to be about lo4 one obtains the curves shown in Fig. 4 for functions 8 and 9 for the values of Ka and K4 indicated. Both (M+) and (Cl-) are represented by X in Figs. 2 and 4. Observed curves are shown for comparison. The curve in Fig. 2 is drawn on the basis of function 8 and the assumption that Ka for H+ is 4 but that KS for Li+ is very nearly equal to zero. In view of the failure t o consider activity coefficient effects and the formation of RZnCL and MZnCL and the assumntion
+ C1- = ZnCL-, KA = (ZnC14-)/(ZnCI,-)(Cl-) ZnClz + 2C1- = ZnCL-,
=
0.1 (10)
K B = (ZnCL-)/(ZnCl,)(Cl-)* = 1 (11)
These values of K A and KB are calculated from Marcus' estimates16 of the successive formation (14) Xraus and co-worker8 do not commit themselves t o any specific resin model. (16) G. E. Boyd, J. Schubert and A. W. Adamson, J . Am. Chham.
Soc., 69, 2818 (1947). (16) Y. Marcur, private communication, 1866.
1654
R. A. HORNE
concerning the constancy of (R+) these functions correspond quite closely with the observed data. Unfortunately, although the acids and alkali metal salts of halo-metallic complexes solvent extract,17.1athere is no justification for supposing that in aqueous solution they are other than strong electrolytes. Nor can association in aqueous solution be attributed to ion-pair formation in the present case without assuming unreasonably small values of the Bjerrum distances of charge center separ& tion. Yet the ambiguities concerning this parameter are so great, our knowledge of its dependence on ionic structure and solvation so imperfect,’9S20 that this difficulty cannot be considered an unequivocal disqualification of the foregoing hypothesis. Furthermore, this difficulty ceases t o be serious if the dielectric constant of the external solution is sufficiently lowered by means of mixed solvents. Under these circumstances the above type of development becomes applicable, as will be shown by data in a subsequent paper in this series. The other explanation of the “lithium chloride effect” ventured by Kraus and co-workers2.6 attributed the effect to differences in activity coefficients in the resin phase. One manner in which such apparent differences could arise is through ionic association predominantly in the resin phase rather than comparably in both internal and external solution phases as discussed above, Diamond,21recognizing the many similarities between the anion-exchange resin adsorption and solvent extraction of metals, treats the resin phase after the fashion of an organic solvent, assuming that the effective dielectric constant in the resin phase is small enough t o encourage ion-pair formation and that invasion of the resin phase by non-exchange electrolyte is extensive at high solution concentrations. The principal adsorption process is now equation 6 and the principal desorption process its reversal. The reaction RC1
+ XnClr- = RZnCla + C1-
(12)
may also be an important adsorption process But, in addition, process 5 and M + 3- ZnCls- = MZnC18 (13) occurring only in the low dielectric constant medium of the resin phase give rise to apparent adsorption. The order of formation of M2ZnC14 (or MZnC13) is Li+ > Na+ > Hf > K+ > CS+, the order of ionic sizes being reversed, tending to approach that of the crystal radii, due to ion dehydration in concentrated s ~ l u t i o nthus , ~ ~making ~~~ lithium ion the smallest and most strongly ionpairing. There is a correlation between log D for the adsorption of zinc(I1) and the crystal radii of the secondary cation (cf. Fig. 3), log D decreasing (17) H. Irving. F. J. C. Rossotti and R. J. P. Williams, J. Chem. Soc., 1906 (1955). (18) R. Irving and F. J. C. Rossoti. {bid., 1927 (1956). (19) H. S. Harned and B. B. Owen, “The PhyBioal Chemistry of Electrolytic Solutions,” Reinhold Publ. Corp.. New York, N. Y., 1943, pp. 202-206. (20) C. A. Kraus, TRIEJOURNAL, 60, 129 (1956). (21) R. M. Diamond, private communication, 1956. (22) R. M. Diamond, J. Am. Cham. Soc., 77, 2978 (1956). (23) I. Nelidow and R. M. Diamond, THIEJOURNAL,119, 710 (1956).
Vol. 61
as the ionic radius decreases with, again, the exception of Na+ and H+. However, the curves are not linear, the situation probably being complicated by varying degrees of completeness of ion dehydration under the experimental conditions. The distribution coefficient now takes the form D = C
+
+ +
++
(RZZnCla) (RZnC4) (MRZnC14) (MsZnclc)R f (MZnCldrt . . . (ZnC4”)s (ZnCls-)~ (ZnCl&+ . . . .
+
(14)
where C is a constant which can be made equal to unity by proper choice of resin phase concentration units. A number of functions have been derived from expression 14, none of which fit the observed data as well as equation 8; in particular, they fail to account for the observed differences in log D at 2.0 M MCI. However, the derivation of these functions is more difficult, involving more assumptions, than the derivations of functions 8 and 9, hence this comparative failure does not necessarily mean that equation 7 is to be preferred to 14. Furthermore, equation 14 readily accounts, in a qualitative manner, for the observed adsorption of tracer bromide ion from LiCl and HC1 solutions6 whereas (7) apparently does not. Furthermore, a low dielectric constant and hence enhanced ionpair formation in the resin phase might account for the weak anion-exchange resin adsorption of metals from aqueous media in which they presumably do not form anionic complexes such as lead(I1) and possibly bismuth(II1) from “ 0 3 and NH4N03.24 Two more significant difficulties deserve mention. Ion dehydration and subsequent reversal of selectivity order does not become important until the concentrations of strongly hydrophilic species become equal to about 5 yet metallic species adsorb more strongly from LiCI than from HCl even at external electrolyte concentrations considerably below this value. Gold(III), for example, adsorbs more strongly from 1.0 M LiCl than from 1.0 M HCl.25 The concentration of the resin functional group, R+,and its counterion is, of course, large in the resin phase. Yet anions and quaternary amine cations do not exhibit strong hydration tendencies26and thus should not compete seriously with the secondary cations for water of solvation. Then, too, Boyd, Schubert and Adamsonl6 have concluded that for cation exchange the important quantity is the hydrated radius, even although the concentrations of the macroscopic component went as high as 10 M in their experiments; and Wheaton and Bauman27 have found that the order of selectivity for halides in anion exchange depends on the order of decreasing hydrated radii. Secondly, if there are differences in the extent of hydration in the resin phase or if the dielectric constant of the resin phase is significantly less than that for the external solution resulting in extensive ion-pair formation, these differences should be strongly reflected in differences in activity co(24) F.
(1954).
Nelson and K. A. Kraus, J. Am. Chem. Soc., 76, 5916
(25) U. Schindewolf, private communication, 1956. (26) B. A. Soldano and G. E. Boyd, J . Am. Chem. Soc., 78, 6099 (1953). (27) R. M. Wheaton and W. C. Bauman, 2nd. Eno. Chem., 48, 1088 (1961).
I
f
I
,
Dec., 1957
RATEOF ADSORPTION OF ZN(II) ON ANION-EXCHANGE RESINS
efficients in the resin and solution phases. However, Kraus and Nelsons have found little difference between the activity coefficients of LiCl and HCl in the resin phase, nor, except for certain irregularities at low concentrations wgch they attribute to resin impurities, did they find that the resin and solution concentrations varied widely for a given species although the activity coefficients for the resin phase are somewhat less than for the external aqueous sol~tion.~8.~9 One might argue that the effective dielectric constant of the resin phase is less than the Bjerrum limiting value for ion-pairs such as M2ZnC14, MZnC4- and MZnCls but greater than that for MC1. But, while such an argument might seem reasonable for the doubly charged ZnCIr, it cannot account for the “lithium chloride effect” in the cases of ions such as AuC4without, again, and with the same reservations noted previously assuming unrealistic values of the Bjerrum distances of charge center separation. Despite the difficulties just discussed the two (28) K. A. Kraus and G . E. Moore, J . A m . Chem. Soc., 7 6 , 1457 (1953). (29) J. 5. Mackie and P. Meares, PTOC.Roy. Soc. (London), 2S2, 485 (1955).
1655
theories presented above show promise of providing a satisfactory interpretation of the dependence of the anion-exchange resin adsorption of metal species on the nature of the cation of the supporting electrolyte. Both expressions 7 and 14 are simplifications of the definition D-
total concn. of Zn(I1) species in resin phase total concn. of Zn(I1) species in soln. phase ( 1 5 )
differing only in the terms that each assumes to be significant. When the dielectric constant of the external solution is comparable with the effective dielectric constant in the resin phase expression 7 will be applicable, when the dielectric constant of the external solution is significantly higher than that of the resin phase equation 14 will be applicable. Acknowledgment.-The author wishes to express his gratitude for the valuable assistance of Professors C. D. Coryell and J. W. Irvine, Jr., of M.I.T. and R. M. Diamond of Cornel1 University and for facilities and funds supplied by the United States Atomic Energy Commission through the Division of Sponsored Research of the Massachusetts Institute of Technology.
THE ADSORPTION OF ZINC(I1) ON ANION-EXCHANGE RESINS. 11. STOICHIOMETRY, THERMODYNAMICS, LOADING STUDIES, DOWEX-2 ABSORPTION AND FACTORS INFLUENCING THE RATE OF THE ADSORPTION PROCESS BY R. A. HORNE,’R. H. HOLM AND M. D. MEYERS Contribution of the Department of Chemistry and Laboratory for Nuclear Science of the Massachusetts Institute of Technology, Cambridge, Massachusetts Received J u l y 89,1967
The adsorption of macroscopic amounts of ainc(I1) on Dowex 1-X8chloride has been studied a t room temperature; the chloride content of the resin doubles with the equivalent adsorption of zinc(I1) species. The adsorption of tracer sinc(I1) on Dowex 1-X8 chloride from LiCl and HC1 solutions a t 0,25 and 49’ has been studied, and the results indicate the presence of a secondary process resulting in apparent resin adsorption at high LiCl concentrations. Loading studies indicate that the equivalent capacity of Dowex 1 for ainc(I1) chloro-complex ions is the same as for chloride ion, thus rendering un!ikely the supposition that the adsorption process is other than ion exchange with the resin functional groups. The adsorption of tracer ainc(I1) on Dowex 2 from HC1 and LiCl solutions, while somewhat less, is substantially the same as on Dowex 1. The influence of resin mesh and amount, the concentration of various constituents in the solution phase, and solvent composition on the rate of the adsorption and desorption processes has been studied together with the rate of exchange of radioainc(I1) between resin and solution phases.
Introduction the determination of resin phase activity coef. ~ purpose of this and the other paWhile cation-exchange resin adsorption and, to a f i c i e n t ~ . ~The lesser extent, the adsorption of simple anions on pers in the present series is to describe certain preanion-exchange resins have been studied exten- liminary investigations into the nature of the adsively12the details of the adsorption of complex an- sorption process, particularly inasmuch as it has ions on anion-exchange resins have as yet received been felt that the anion-exchange resin adsorption very little attention. Kraus and Nelson3 have of complex ions differs significantly from that of surveyed the applications of anion-exchange resin simple ions. adsorption of metal complexes t o separation proExperimental cedures and have done initial work in exploring the The preparation of the tracer 245 day Zn” and the deterbasic physical chemistry of these systems, especially mination of the distribution coefficient, D (1) Radio Corporation of America, Needham Heights, Mass. (2) R. Kunin, F. X. McGarvey and A. Barren, Anal. Chem., Z8, 729 (1 956).
(3) K. A. Kraus and F. Nelson, Collected Papers of the Geneva Conference on t h e Peaceful Uses of Atomic Energy, 1955, paper UN837.
of soln. in ml.)(radioactivity in resin) D = (vol. (wt. of resin in g.)(radioactivity in soln.) (1) from batch runs at 24.8
0.2’ together with the method for
(4) K.A. Kraus and F. Nelson, J . Am. Chsm. Soc., 76,984 (1954).
