Kinetic behaviour of chelating resins with phosphonic functional groups

J. inorg,nucl.Chem.,1971.Vol.33, pp. 3119to 3129. PergamonPress. Printedin Great Britain

KINETIC

BEHAVIOUR PHOSPHONIC

OF CHELATING RESINS FUNCTIONAL GROUPS

WITH

C. H E I T N E R - W I R G U I N and J. K E N D L E R Department of Inorganic and Analytical Chemistry, The Hebrew University, Jerusalem, Israel

(First received 12 March 1970; in revised form 7 September 1970) A b s t r a c t - T h e ion exchange kinetics of various cations in the phosphonic resin Bio-Rex 63 were investigated. The rate-determining step for exchange of univalent and bivalent cations was found to be diffusion through the particle bead, whereas for strong complex-forming tervalent cations the slow step is the chemical reaction at the exchange site. Diffusion coefficients, rate constants, activation energies and equilibrium half-time values are compared for forward and reverse exchange reactions. In buffered solutions the effect of pH did not account for the slow exchange rates obtained with cations which form chelates with the phosphonic group. INTRODUCTION

THE LAST tWO decades have witnessed a growing interest in selective ion exchangers with chelate-forming active groups. The rate of exchange of such resins is much slower and is controlled either by particle diffusion or by a second-order chemical reaction[I], in contrast with exchange in conventional resins, which is more rapid and diffusion controlled. The activation energy for the chelating resins is as high as 15-25 kcal/mole, while with ordinary ion exchangers it is in the range of 2 - 6 kcal/mole. Various opinions have been expressed regarding the kinetic mechanism in chelating resins as to whether the slow rate-determining step is diffusion through the resin bead or chemical reaction at the exchange site[I-8]. Boyd et al.[2] developed equations for exchange kinetics where the slow step is the chemical reaction, while Helferrich [3] pointed out that for resins with chelating groups that form sluggishly-reacting complexes the rate law of a second-order reversible chemical reaction may be obeyed approximately. Heitner-Wirguin and Urbach [9] found that the exchange of the non-chelating cations Ca ~+ and Cu 2÷ on the phosphonic resin Bio-Rex 63 was controlled by particle diffusion, whereas for UO2 z÷ and Th 4+ the chemical reaction is the rate-determining step. In this work an attempt was made to investigate one situation more thoroughly and to determine the feasibility of the chemical reaction at the exchange site being the rate-determining step. Several cations of different valences, both those known to form strong chelates and those which form weak complexes or simple 1. 2. 3. 4. 5. 6. 7. 8. 9.

R. Turse and W. Riemann, I ! i, J. phys. Chem. 65, 1821 ( 1961 ). G. E. goyd, L. L. Myers, Jr. and A. N. Adamson, J. Am. chem. Soc. 69.2836 (1947). F. Helfferich, Ion Exchange, McGraw-Hill, New York (1965). A. Schwartz, Ph.D. Thesis, Israel Institute of Technology. Haifa (1962). C. Heitner-Wirguin and G. Markowitz,J. phys. Chem. 67, 2263 (1963). A. Schwartz, J. A. Marinsky and K. S. Spiegler, J.phys. Chem. 68,918 (1964). A. Varon and W. Riemann, I 1I, J. phys. Chem. 68, 2716 (1964). G. Schmuckler, Talanta 12, 281 (1965). C. Heitner-Wirguin and W. Urbach, J. phys. Chem. 69, 3400 ( i 965 ). 3119

3120

C. H E I T N E R - W I R G U I N

and J. K E N D L E R

salts with the phosphonic or phosphonous acids, were studied under strictlycontrolled conditions. Forward and reverse reactions, and the effect of pH on the rate, have been studied. EXPERIMENTAL Preparation of the resin The resin chosen, Bio-Rex 63 (Bio Rad Laboratories purified Duolite 63), is styrene-based (6% D.V.B.) and its functional group is R-P(~O)(OH)2. The resin was washed with distilled water until the effluent was colorless, and two column volumes (c.v.) of 2N HCI were added. After washing free of HCI, the exchanger was converted to the sodium form with 2c.v. 2N NaOH, with subsequent water washing. This cycle was repeated twice, the last wash in the sodium form involving 2c.v. of 0-01N NaCI and only 1 c.v. of water (to prevent hydrolysis). The resin was then air-dried, sieved (to separate the various mesh sizes) and stored in well-stoppered bottles.

Determination of the capacity The weight capacity was determined by contacting 0.5 g resin in the hydrogen form with 50 ml 0.1N N a O H solution in 200 ml stoppered bottles. When equilibrium was attained (no further change in pH) the supernatant liquid was filtered and the free hydroxide was titrated with standard 0.1N HCI solution. The mean weight capacity was 5.1 m-equil./g resin. Titration curves were constructed by the method of Topp and Pepper[10], which was similar to the weight capacity determination procedure. The pH after the attainment of equilibrium (no further change in pH) was plotted against m-equil. N a O H added. The p K values calculated from these curves[3] were PK1 = 3.70 and PK2 = 7.10, which are close to those found by Rumpf and Chavane [11] for aliphatic phosphonic acids. The weight capacity of the resin in the sodium form is very close to the mean value for most of the cations investigated. A low value of 4.1 was found for AIa+ and a high value of 5-7 for Pb ~+, probably because of the presence of carbonate in the lead solution. Kinetic measurements The limited bath technique [ 12] was used. Solutions of the cation (0-002- 0.01N ) in sodium chloride (p. = 1.0) were taken for each measurement and transferred to jacketed reaction vessel. The solutions were stirred vigorously by means of a vertical stirrer so that film diffusion was not rate determining. The amount of cation was rigorously kept equivalent to the amount of exchangeable cation in the resin. The solutions were brought to the desired temperature by constant circulation of thermostaticallyheated water (±0. I'C) through the double wall of the jacketed reaction vessel. When the desired temperature was reached a weighed amount of exchanger was added (generally 0.1 g). At the end of each measured time interval, the solution was rapidly drawn off into a suction flask, the resin being retained on a sintered glass plate; new fractions of resin and solution were taken each time.

Measurement of particle radius The particle radius was determined in two different ways: Microscopically. The diameter of 100 particles, swollen in the appropriate solutions, was measured using a micrometric ocular in a Zeiss microscope. The calculated average value showed a standard deviation of 4.8--8.2 per cent (mean deviation 6 per cent). Density method. Using the method of Kressman and Kitchener[12], 500 swollen particles (in appropriate solutions) were introduced into a pycnometer and their density measured. The effective mean radius is calculated from the formula 4/31rr~.p.N = W, where r---- radius, p = density of the swollen resin and W = weight of N particles.

11. N. E. Topp and K. W. Pepper, J. chem. Soc. 3299 (1949). 12. P. Rumpf and V. Chavane, C.r. hebd. S~anc. Acad. Sci. Paris 224, 919 (1947). 13. T. R. E. Kressman and 3. A. Kitchener, Discuss. Faraday Soc. 4, 90 (1949).

Kinetic behaviour of chelating resins

3121

Analytical methods Li ÷, Na ÷, NH4 ÷, Cs ÷ and Ca 2+ were determined by flame photometry [13] or by atomic absorption spectroscopy [ 14]. Copper, nickel and lead were determined polarographically [15]. Aiuminium was determined colorimetrically with 8-hydroxyquinoline [16] or, when possible, polarographically using the aluminium di-o-hydroxyazo complex with Pontachrome Violet SW in an acetate medium of pH 4.6117]. Indium was determined spectrophotometrically with dithizoine at pH 8.5-9.5118]. Chromium was determined by butanol extraction of the diphenyl complex and its spectrophotometric measurement [ 19], and iron spectrophotometrically with o-phenanthroline [20]. U ranyl ion was determined spectrophotometrically as peroxide [21-23], and thorium spectrophotometrically with Thorin [22].

Criteria.['or determination of the kinetic behaviour The kinetic behaviour was investigated in a complex system containing various ions of different valences and mobilities. Moreover, swelling and shrinking during the exchange reactions, as well as changes in pH and making and breaking of chelate bonds, may further complicate the system. Due to this complexity provisions were made and criteria chosen as follows in order to elucidate the kinetics of this system in practice. 1. In order to prevent film diffusion from being rate determining the following provisions were made: (a) the solutions were thoroughly stirred at 800 rev/min: (b) the ionic strength was maintained at 1.0 by adding sodium chloride (except in the experiments in Table 5). Interruption tests [3, 12] have proved that the above conditions led to the expected results. 2. The equations of Kressman and Kitchener[12] for particle diffusion under limited bath conditions are:

F=-O-~=r'Qo_Q~ where F is the extent of exchange, Qt the exchange in time t, Qo the quantity of resin and solute taken in each experiment, Q= the amount of exchange at equilibrium (all the Q values are in milliequivalents), r is the radius of the resin particles in cm. and D is the diffusion coefficient in cm ~ sec -~. This equation predicts that a plot of F vs. X/7-will be a straight line if the exchange is controlled by particle diffusion, and from the slope of this line the diffusion coefficient of the ion examined can be evaluated. This equation, analogous to that of Paterson[24] for the conduction of heat into a sphere from a well stirred bath, was also applied by Barrer [25] to the sorption of gases in zeolites for small values of Qt/Q~. This condition was strictly followed in the present work. 3. For a second-order chemical reaction, the equation of Frost and Pearson[26] was used in the limited bath method: In Z = 2k (Qo - Q~)t/Q~ 13. 14. 15. 16. 17. 18. 19. 20.

2 I. 22. 23. 24. 25. 26.

{2)

J. A. Dean, Flame Photometry. McGraw-Hill, New York (1960). J. N. Robinson, Atomic Absorption Spectroscopy. M. Dekker, New York (1966). 1. M. Kolthoff and J. J. Lingane, Polarography. Vol. 2rid Edn. Interscience, New York (1952). E. Goon, J. E. Pethy, W. H. McMulen and S. E. Wilberley, Analyt. Chem. 25,608 (1953). H. H. Willard andJ. A. Dean, Analyt. Chem. 22, 1264 (1950). 1. May andJ. E. Hoffman,J. Wash.Acad. Sci. 38, 329 (1948). B. E. Saltzman,Analyt. Chem. 24, 1016 (1952). G. Chariot and D. Bezier, Quantitative Inorganic Analysis. Methuen, London (1957). T. R. Scott, Analyst-Chemist75, 100 (1950). M. B. Jacobs, The Analytical Chemistry of lndustrial Poisons, Hazards and Solvents. lnterscience, New York (1949). W. Wagner, C. J. Hull and G. E. Markle, Advanced Analytical Chemistry. Reinhold, New York (1958). A. Paterson, Proc. phys. Soc. 59, 50 (1947). R. M. Barrer, Trans. FaradaySoc. 45,358 (1949). A. A. Frost and R. G. Pearson, Kinetics and Mechanisms p. 173. Wiley, New York (1953).

3122

C. HEITNER-WIRGUIN and J. KENDLER

where

Z = Q,(Qo-2Q=) + QoQ** Qo(Q=-Q,)

(2a)

Here Q is in m-equil/l, and k is the second-order rate constant. If the rate is controlled by a chemical reaction a plot of log Z vs. t should be linear, and the rate constant can be determined from the slope S of this plot.

S

=

2k Qo(Qo- Q®)

(2b)

2-30 Q=

4. The particle size of the resin. According to Equation (I) a diffusion-controlled process is accelerated by a decrease in particle size, while the chemical reaction controlled process is unaffected by this parameter (Equation 2). 5. The solution concentration has little or no effect on a diffusion-controlled process, whereas a process controlled by a chemical reaction should be markedly affected by a change in concentration. 6. The activation energy should be markedly higher for chelate forming cations and much lower for cations which form weak complexes or simple salts. RESULTS A N D D I S C U S S I O N

E x c h a n g e o f univalent and bivalent cations T h e r a t e o f e x c h a n g e o f t h e c a t i o n s Li +, N H 4 +, K ÷, C s +, N i 2+ a n d P b ~+ f o r s o d i u m i o n in t h e e x c h a n g e r is m a r k e d l y d e p e n d e n t o n t h e p a r t i c l e size ( T a b l e 1). A l l t h e e x c h a n g e e x p e r i m e n t s in T a b l e 1 a n d F i g . 1 g a v e l i n e a r p l o t s o f F vs. VT?. F r o m t h e s e p l o t s , t h e diffusion c o e f f i c i e n t s D w e r e e v a l u a t e d ; e a c h v a l u e o f D s h o u l d b e c o n s i d e r e d as an a v e r a g e v a l u e f o r all t h e m e a s u r e m e n t s . T h e e r r o r in t h e s e v a l u e s is c a u s e d b y t h e i n a c c u r a c y in e v a l u a t i n g t h e p a r t i c l e radii. T h e r a t e

0"70"8

2/ I /

1~~4'5

0

~e 0.5 O.4 0.2 0.1 I I I I i I I 2I 4.I 16 8r I0i 12 14 16 18 20 22 ;:'I 4 ¢~, SeC

Fig. 1. Effect of particle radius (r) and cation concentration in solution on the exchange rate in Bio-Rex 63-Na+: A--1. 10.16 m-equil. KCI, r = 0.0194 cm, 25°C; 0 - 2 . 1 0 . 1 6 m-equil. KCI, r = 0.0146cm, 25°C; x-3.3.70m-equil. NiC12, r--- 0.0156cm, 21°C; O - 4 . 3.70 m-equil. NiCI~, r = 0.0118cm, 21°C; 0 - 5 . 7 . 4 0 m-equil. NiCI~, r = 0-0118 cm, 21°C.

Kinetic behaviour of chelating resins

3123

Table 1. T h e effect of particle radius and solution concentration on the rate of e x c h a n g e of uni- and bivalent cations (Resin: Bio-Rex 6 3 ) T e m p . 25°C Exchange reaction l.i+/Na +

NH4+/Na 4

K+/Na +

Cs+/Na +

N iZ+/N a +

pb2+/Na +

r x 102 (cm)

Q0 (m-equi..)

S × 102

D × 107 Icm 2 sec -11

E,, (kcal/mole)

1.35 1.86 1.86

10.08 10.08 20.16

9-88 6-64 6.67

1.30 1.00 1.00

6.56

1"42 1 "88 1 "88

10'00 10'00 20"00

11"42 6"64 6"67

1"50 1 '30 1 '30

6"15

1 "46 1 "94 1"94

10' 16 10" 16 20"32

13"98 10"05 1I)'07

1 "80 I '60 1 '60

6.07

1"51 2"04 2"04

10"10 10" 10 20" 211

16"50 10"85 10 "92

2"0 1"70 1"80

5.59

1"18 1 "56 1"56

3"70 3"70 7"40

5"69 3"90 3"90

0"16 0" 13 0" 13

7.94

1"22 1"61 1"61

5-00 5"00 10"00

4"34 3"21 3"24

0" 13 0"10 0'10

11.07

r = particle radius: Qo = n u m b e r of m-equil, in original solution; S = slope of F vs. "~-: D = diffusion coefficient: E~ = activation energy (calculated from experim e n t s at 25°C and 35°C).

tor the univalent cations increases in the order Li+ < NH4 + < K + < Cs +, which is identical with the order of decreasing hydrated ionic radii. The same behaviour was observed for the bivalent cations nickel and lead. The linear plots of F vs. ~ , non-linear plots o f - l o g ( 1 - F) vs. t, strong rate dependence on particle size and the relatively small activation energies (close to those for conventional ion exchangers[27, 28]) suggest that the rate is particle diffusion controlled. The exchange reaction proceeds very quickly, the halftime for equilibrium (t1~2) being 20-50 sec for the univalent cations and a few minutes for the bivalent cations (Table 6). The above cations are assumed to yield weak complexes or simple salts with the phosphonic monomers[29]. This assumption, together with the assumption that Th 4+, UO22+ and Fe 3÷ form relatively strong complexes of the inner chelate type, is based on the determination of the order of selectivity [20-31] of the phosphonic resins, which show the follow27. 28. 29. 30. 31.

G. E. Boyd and B. A. Soldano, J. A m. chem. Soc. 75,6091 ( 1953 ). B. A. Soldano, ,4 nn. N. Y. ,4 cad. Sci. 57, 116 ( 19631. J. K e n n e d y and R. V. Davies, Chem. a n d l n d . 378 11956). J. Kendler, Ph.D. T h e s i s , T h e H e b r e w University of Jerusalem, ( 19691. J. Persoz and R. Rosset, Bull. Soc. chim. Fr. 2197 (1964).

3124

C. H E I T N E R - W I R G U I N and J. K E N D L E R

ing order of decreasing affinities[30]: Th 4+ > UO~2+ > Fe a+ > Cr a+ > Ins+ > AI3+ > Pb 2+ > H + > Cu 2+ > Ni 2+ > Ca 2+ ~> Li+ > Na + > NH4 + > K + > Cs +. This order is the reverse of that found with conventional sulphonic cation exchangers, and the same as that found on the chelating exchanger Dowex A-1 (when the chelation is through oxygen and not through amino nitrogen [32]). This order is also the same as the usual sequence of metal chelate when oxygen is the only ligand donor[33]. From the high affinity of the resin for polyvalent cations and the order of selectivity, Kennedy e t a/.[34] concluded that stable four-membered chelate rings are formed with the resin functional groups. As one equivalent of acid is liberated for each equivalent of ferric or uranyl ion sorbed[34] it may be assumed that two four-membered rings are formed with uranyl ion and three such rings with the tervalent ions (thus bringing the total coordination number to six in both cases). It is expected that steric hindrance (i.e. the need for three phosphonic groups to be suitably located in the polymeric matrix) will render the tervalent chelates less stable than the uranyl chelate. Stability constant data for the monomeric phosphate chelates show that the uranyl and ferric species have about the same stability [35]. Forward and reverse exchange for the bivalent cations calcium, nickel and copper, summarized in Table 2, is different for the sodium and the hydrogen forms of the phosphonic resin. When the resin is initially in the bivalent form the rate of exchange for sodium ion is always smaller than in the reverse reaction. Table 2. Forward and reverse exchange of bivalent cations (Resin: BioRex 63; Temp. 25°C)

Qo

Exchange reaction

Initial

pH Final

(m-equil.)

r

102 (cm)

D X 10a (cm2. sec -1)

Ca~+/Na +* Na+/Ca 2+ Ca2+/H + H+/Ca 2+

4.60 9.85 4.60 1.70

5.05 5.15 2.35 5.00

3-70 3-70 3.70 3-70

1.50 1.68 1.70 1.02

1.46 0.25 0.69 1-68

Ni2+/Na + Na÷/Ni 2+ Ni2+/H + H+/Ni 2+

4.85 9.90 4.80 1.70

5.40 5.30 2.30 5-18

3.70 3.70 5.00 5.00

1.56 1.71 1.77 1.08

1.32 0.18 0.63 1.47

Cu2+/Na+ Na+]Cu 2+ Cu2÷/H+ H+/Cu 2+

5.00 9.90 4.90 1.70

5.45 5.50 2"32 5.30

5.00 5.00 5.00 5.00

1.58 1.72 1-80 1.10

1.28 0-16 0.57 1.42

=

* The cations on the right side are o n the resin initially. 32. R. Rosset, Bull. d'lnformations Scientifiques et Techniques du Commissariat d l'Energie Atomique No. 85, (1964), 33. A. E. Martell and M. Calvin, Chemistry of the Metal Chelate Compounds. Prentice-Hall, N e w Jersey (1952). 34. J. Kennedy, R. V. Davies, H. Small andB. K. Robinson,J. appL Chem. 9, 32 (1959). 35. Stability Constants, Special Publications of the Chemical Society, London, No. 17 (1964).

Kinetic behaviour of chelating resins

3125

This behaviour may be explained by the higher mobility of the univalent sodium ion compared with the bivalent cations, since the rate is higher when the more mobile ion is initially on the resin. The kinetic behaviour of the hydrogen form of the resin is quite different. When the resin is in the hydrogen form the rate of exchange of the bivalent cations is always smaller than when the resin is in the sodium form. This is due to the fact that the phosphonic group acts as a weak acid and therefore has a strong affinity for the proton. The low pH and minimal swelling of the hydrogen form of the resin are added factors which contribute to the lower rate. The rate of the reverse exchange will consequently be higher.

Exchange of tervalent cations The influence of particle radius and solution concentration on the rate of exchange of the tervalent cations aluminium, indium, chromium and iron are presented in Table 3 and Fig. 2. The rate is independent of particle size, and an increase in the cation concentration increases the rate appreciably. In these systems it must be assumed that some of the ions in the solutions are hydrated and others complexed by chloride. As may be seen further, complex formation in solution affects the rate of exchange but not the mechanism (Table 5). Linear plots of log Z vs. t indicate that the rate of exchange is controlled by a secondorder chemical reaction at the exchange site (plots of F vs. VTwere non-linear in all these cases). Table 3 also gives the second-order rate constants evaluated from the experimental data. These constants are consistently slightly higher when Table 3. Effect of particle radius and solution concentration on the rate of exchange of tervalent cations (Resin: Bio-Rex 63; Temp. 25°C) Exchange reaction

r x 102 (cm)

Qo (m-equil./1.)

S = 103

hn (sec)

k x 10~ (I. mole -1 s e c - b

Ea (kcal/mole)

Ala+/Na +

1.75 1.56 1.56

5.00 5.00 10.00

4.21 4.26 6.70

433 425 364

9.4 9.4 11.7

10.74

1-79 1'58 1"58

5.00 5.00 10.00

3 "06 3'14 4.82

506 492 400

8"0 8"1 10-1

10.80

1.84 1.29 1-59 1'59

5"10 5"10 5' 10 10.20

1-39 1.39 1.39 1'74

776 776 776 683

2-7 2"7 2-7 3 '5

1.85 1-31 1-61 1-85

5.10 5'10 5.10 10'20

1'27 1'27 1"27 2.00

845 845 849 591

2-5 2'5 2.6 4"0

Tna+/Na +

Cr3+/Na +

Fe3+/Na +

14-53

15.37

S = slope of log Z vs. t: k --- second order rate constant: E. = activation energy (calculated from experiments performed at 25°C and 35°C).

3126

C. HE1TNER-WlRGUIN and J. KENDLER

0.028

,3,,f /

/

/@~

0.02'4 0.020 I,NI 0.016

/.///" i i / J

oo12

0.008/,//'b

/

0.004 ] 2

I 4

I 6

I 8

I I0 rain

/,

L 12

I 14

I 16

I 18

Fig. 2. Effect of particle radius (r) and concentration of iron in solution on the exchange rate in Bio-Rex 63 --Na÷: © - 1.5.10 m-equil./l. FeC13, r = 0.0131 cm, 25°C; ×-2.5.10 m-equil./l. FeCI3, r = 0.0185 cm, 25°C; • -3.10.20 m-equil./l. FeCI~, r = 0.0185 cm, 25°C. the concentration of the solution is doubled. This increase in the values of the constants must be taken as an inherent error due to the fact that the system is far from ideal. (A twofold increase in concentration of tervalent cations will cause appreciable changes in activity coefficients.) T h e exchange reactions proceed very slowly, the half-time for equilibrium changing from 7 to 14 min for the tervalent cations and reaching 4 0 - 1 0 5 min for uranyl and thorium ions. Relatively high values for the activation energies also imply that the rate is controlled by a second-order chemical reaction at the exchange site. E v e n higher activation energies were found for uranyl and thorium ions [20 and 23 kcal/mole respectively [9ll. Table 4 summarizes the forward and reverse reactions of tervalent chromium and iron as compared with uranyl and thorium ions. It may be seen from this Table that the rate of exchange of the tervalent cations is higher when the resin is in the sodium form than when it is in the hydrogen form; the proton competes strongly with the exchanging cations in the exchange process, whereas sodium is easily replaced in the resin. During the reverse reaction, i.e. the resin being initially loaded with tervalent cation, the exchange was slower in all the cases investigated and it may be assumed that the chelating cations are preferred by the resin.

Effect of pH T h e phosphonic group of the resin is a weak acid, with a high affinity for the proton. Since most of the cations forming strong chelates with the resin are acidic, it was suspected that at low pH the proton activity (i.e. the small extent of swelling) may account for the slow exchange rates obtained with the acid cations and may also affect the reaction mechanism. T o elucidate the effect of pH, the experiments in Table 5 were carried out. T h e s e experiments were similar to the others, except

Kinetic behaviour of chelating resins

3127

Table 4. F o r w a r d and reverse e x c h a n g e of s o m e strongly chelating cations on BioRex 63 (Temp. 25°C) Exchange reaction

Initial

pH Final

Cr3+/Na +* CrZ~+/H÷ H +/Cr a+

3.40 3.35 1.78

4.25 2.08 3.05

Fe3~/Na + Fe3+/H * H + / F e 3~

2.60 2.60 1.75

3.45 1.75 2.30

Qo

r × 102

(m-equil./l.)

(cm)

k × 102 (1. mole -1 sec. -~)

5.0 5.0 5-0

1.56 1.65 1.18

1.46 0.75 0.69

5.0 5.0 5.0

1.61 1.75 1.10

2.55 1.29 1.18 k×10 a (I. mole -1 sec. -1

UO22+/Na + UO22+/H + H+/UOz 2+

3.55 3.55 1.70

4.10 2.20 3.04

3.70 3.70 3.70

1.60 1.66 1.28

2.94 1.31 0.68

Th4+/Na + Th4+/H + H +/Th 4+

3.70 3.70 1.70

4.32 2.10 3.10

5.0 5-0 5-0

1.62 1.74 1.34

0.52 0.24 0.11

* T h e cations on the right side are in the resin initially.

that the resin was previously saturated with the appropriate buffer solution. The results show that a change in pH does not influence the kinetic mechanism. Linear plots of F vs. X/Twere found for the bivalent cations and linear plots of log Z vs. t for iron and uranyl ions. D for nickel in citric acid at pH 2.4 is lower than in 1M NaCI at pH 4.85; this may be due to the stronger affinity of the proton for the resin at the lower pH. Nevertheless, the D values in citrate and ammonium buffers at pH 7.9 and 9.2 respectively are also lower than in chloride media. The effect of these buffers may be explained by the need to break the chelating bonds of the cation with the buffer. Similar results were obtained with copper at pH 6.0 and 9.5. The somewhat higher rate of exchange for copper at pH 9.5 compared with pH 6.0 may be assumed to be due to hydrolysis of the citrate complex and the formation of complex species of the type CuOH 1÷ which may be absorbed by the resin in the complex form. The same conclusions may be drawn from the experiments with the chelating iron and uranyl cations, the rate being lower when additional chelating bonds have to be broken in the buffer solution. It may therefore be concluded that the affinity of the resin for the proton cannot account for the slow exchange at relatively low pH values with chelate-forming cations.

Concluding remarks The above finding that the slow step in the exchange of the tervalent cations with the phosphonic group is the chemical reaction may seem surprising but is not at all unusual. All these cations form chelates with the phosphonic group. The

3128

C. HEITNER-WIRGUIN and J. KENDLER Table 5. Effect of pH on the rate of exchange (Resin: Bio-Rex 63; Temp. 25°C)

Exchange reaction

Electrolyte or buffer

pH Initial Final

r × 102 (cm)

Qo

D × 10s

(m-equil.)

(cm* sec-0

Ni2+/Na+

1 M HaCit 1 M NaCl l M NaaCit(II) l M NHa-NH4CI

2.40 4.85 7.90 9-20

2.62 5"40 7.95 9.20

1.53 1.56 1.61 1.62

3.70 3.70 3-70 3.70

1.23 1.23 1.28 1.16

CuZ+/Na+

1M NaCI 1 M Na3Cit(l) 1 M Na3Cit(II)

5-05 6-00 9.50

5.35 8-06 9.50

1.58 1.61 1.66

3"70 3.70 3.70

1.26 1'02 1.21 k× 102 (1. mole-x sec -1)

m/l. Fea+/Na+

I M HaCit 1 M NaCI 1 M Na3Cit(ll)

1.40 2.60 7.95

1.55 3.15 7.98

1.17 1.61 1"42

5.0 5.0 5.0

UO2Z+/Na+

I M NaCI 1 M Na3Ver(lIl) 1 M Na3Cit(II)

3.55 8.08 9.50

4.10 8.08 9.50

1.60 1.66 1.69

3.70 3.70 3.70

0.86 2.55 2"23 k×lOo (I. mole-l sec. -l) 2.94 2.32 2.15

Sodium citrate + 10 ml of pyridine. The pH was adjusted to 6.0 with conc. perchloric acid. II -Sodium citrate + 10 ml ammonia. The pH was adjusted to the desired pH (8-0 or 9.5) with perchloric acid. I I I - Michaeli's buffer (sodium veronal + hydrochloric acid). I -

literature indicates that many reactions involving formation and breaking of c h e l a t e b o n d s a r e s l o w e v e n in a q u e o u s s o l u t i o n ; t i t r a t i o n s o f A I ( I I I ) , C r ( I I I ) , F e ( I I I ) a n d M n ( I I I ) w i t h E D T A m u s t b e c a r d e d o u t at h i g h e r t e m p e r a t u r e s to a c c e l e r a t e t h e r e a c t i o n [36]. F u r t h e r m o r e , r e a c t i o n s t h a t a r e r a p i d in s o l u t i o n m a y b e v e r y s l o w w h e n o n e o f t h e r e a g e n t s is i n c o r p o r a t e d in a p o l y m e r [ 3 7 ] . T h e s t a b i l i t y c o n s t a n t s o f t h e r e s i n f o r c o v a l e n t l y - b o u n d c a t i o n s s u c h as UO22+ a n d F e z+ a r e g r e a t e r b y an o r d e r o f 1> 10~ t h a n t h o s e o f t h e c o r r e s p o n d i n g m o n o m e r s [29]. T h i s i n c r e a s e in t h e v a l u e o f s t a b i l i t y c o n s t a n t s will r e s u l t in l o w e r m o b i l i t y d u e to t h e s p e c i f i c c h e m i c a l i n t e r a c t i o n , as e x p l a i n e d b y S p i e g l e r a n d W y l i e [38]. It has already been mentioned that for resins with chelating groups that form sluggishly-reacting complexes the rate law of a second-order reversible chemical r e a c t i o n m a y b e o b e y e d [ 3 ] . T h e s e l f - e x c h a n g e o f C r a+ o n D o w e x A-1 is a l s o s u p p o s e d to b e c o n t r o l l e d b y a c h e m i c a l r e a c t i o n [ 6 ] . I n t h e p r e s e n t w o r k , t h e s i z e o f t h e r e s i n b e a d s a n d t h e effect o f p H o n t h e e x c h a n g e r a t e h a v e s h o w n t h a t 36. F. J. Welcher, The Analytical Uses of Ethylenediaminetetraacetic Acid. Van Nostrand, New York (1958). 37. L. Luttinger and H. G. Cassidy,J. polymer Sci. 20,417 (1956). 38. K. S. Spiegler and M. R. J. Wyllie, Physical Techniques in Biological Research Voi. 2. p. 301. Academic Press, New York (I 956).

Kinetic behaviour of chelating resins

3129

these factors cannot account for the slow exchange rates obtained with chelate forming cations (Table 6). The kinetic mechanism proposed by Helfferich[39] for reactions which consume the counter-ion originating from the solution may perhaps be suitable for the exchange of univalent and bivalent cations on the phosphonic resin. This kinetic model based on interdiffusion through a converted "outer shell' cannot Table 6. Half-time of equilibrium (h;2) for various cations (Resin: Bio-Rex in N a ÷ form) Cation

tit2 (sec)

Cation

t~/2(sec)

Cs + K+ Li + Ca 2+ Ni 2+ Cu 2+ Pb 2÷

20 23 51 122 180 181 241

AI a+ In a+ Cr 3+ Fe 3+ UOz z+ Th 4+

421 483 780 841 2825 6242

be accepted for cations which yield strong complexes with the phosphonic functional group. The main objections and drawbacks of this kinetic model are as follows. The complexity of the actual system includes continuous changes in pH, and swelling and shrinking during the exchange. This leads to changes in particle radius, in contrast with the basic assumption of the Helfferich kinetic model of a constant particle radius during the exchange. Furthermore, swelling progresses from the "outer shell"; this will permit faster diffusion of counter-ions and therefore cannot be the slow step influencing the rate. Another basic assumption of the above kinetic model is that the chemical reaction at the exchange site is very fast, the interdiffusion of the counter-ions being the slow step controlling the rate. It has been shown in this work and elsewhere that the making and breaking of the chelate bonds markedly slow down the rate. For diffusion-controlled reactions the exchange is faster when the mobile ion is initially in the resin. For either the hydrogen or sodium forms of the resin these ions are much more mobile than the chelate-forming cations (UO22÷, Fe 3÷) in solution, and therefore the reactions are very slow. It may finally be concluded that, insofar as the complexity of the system has permitted, and according to the criteria chosen, the slow step controlling the rate of exchange of chelate-forming cations on the phosphonic resin is the chemical reaction at the exchange site. 39. F. Helfferich in Ion Exchange (Edited by J. Marinsky), pp. 65-97. Marcel Dekker, N e w York (1966).