Chloride-sulphate exchange on anion resins — kinetic investigation, IX. Direct, isotopic and reverse exchange

Desalination, 46 (1983) 55-66 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

CHLORIDE-SULPHATE EXCHANGE ON ANION RESINS - KINETIC INVESTIGATION, IX. DIRECT, ISOTOPIC AND REVERSE EXCHANGE

L. LIBERTI,* Istituto

R. PASSINO AND D. PETRUZZELLI

di Ricerca Sulle Acque,

C.N.R.,

5 Via De Blnsio, 70123 Bari (Italy)

(Received November 16, 1982)

SUMMARY

Experimental kinetics of direct, isotopic and reverse Cl-/SO, exchange on different anion resins are presented. At very low concentration (C = 6 X 10T3N) extremely high resin selectivity toward sulphates occurs (separation factors ranging from 52 to 589). Consequently chemical interaction of interdiffusing species within the resin seems to influence the rate determining step, exception made for isotopic exchange. Application of model equations from Nernst-Planck and SN? rate theories is discussed.

SYMBOLS

c

c D_4i(B) D A(B)

DVB fdc pdc ISV k, kf

ki

-total solution concentration (meq/cm3) - resin exchange capacity (meq/cmz) - diffusion coefficient in solution of ion A(B) (cm2 /s) - diffusion coefficient in resin of ion A(B) (cm2 /s) - divinylbenzene - film diffusion control - particle diffusion control - infinite solution volume - rate parameter according to pseudo-SN, mechanism (s-’ ) (see Eq. (16)) - rate parameter according to fdc-ISV mechanism (s-l ) (see Eq. (4)) - rate parameter according to fdc-ISV mechanism for isotopic exchange (s-l ) (see Eq. (4) with CL,,, = 1) - rate parameter according to pdc-ISV mechanism (s-l ) (see Eq. (6))

*To whom correspondence OOll-9164/83/$03.00

should be addressed. o 1983 Elsevier Science Publishers B.V.

L. LIBERTI, R. PASSINO AND D. PETRUZZELLI

56

r SN2

t to.5

toA(B) u

‘A(B) %/so,

! X Y x A(B) 0

- mean radius of the resin bead (wet sieved) (cm) Bnd-order nucleophilic substitution mechanism - time (s) half time of exchange reaction (s) - transference number of ion A(B) - fractional attainment of equilibrium - valence of ion A(B) separation factor, defined by Eq. (1) - retardation factor - film thickness (cm) - equivalent fraction of SO, in solution at equilibrium - equivalent fraction of SO, in resin at equilibrium - ionic mobility of ion A(B) in solution (cm2/ohm) - refers to infinite dilution

INTRODUCTION

A systematic investigation of the Cl-/SOT exchange on anion resins has shown that the exchange rate for the direct (i.e., Cl-SO4) conversion of the resins is greatly influenced by resin selectivity [l, 21. At very low solution concentration (C = O.O06N), in particular, when resin selectivity towards the divalent ion is enhanced by the electroselectivity effect, kinetics are unfavorably dependent on selectivity, i.e., the preferred species is taken most slowly by the most selective resin. This result is just the opposite of what one would expect from familiar diffusional rate mechanisms of ion exchange based on the Nernst-Planck theory [ 31. In this paper the investigation has been extended to include also isotopic (Sod-SO4) and reverse (SO43Cl) exchange of anion resins and the experimental kinetics are discussed in the light of the existing rate theories.

EXPERIMENTAL

Three strictly comparable anion resins (see characteristics in Table I) were used, namely Amberlite IRA 458 and IRA 67 from Rohm and Haas, and Kastel A 102 N from Montedison, representing a strong base, a medium base and a weak base anion resin, respectively. After conversion in Cl or SO4 form by 0.006N NaCl or Na2 SO, solution at pH 3, the resins were liquid sieved and the mesh opening was assumed as the wet resin bead diameter. Equilibrium determinations were performed batchwise by standard procedure. Weighed amounts of Cl-resin were added to known volumes of 0.006 Na2 S04/H2S04 solution at pH 3 in a stoppered Erlehmayer flask and intermit-

CHLORIDE-SULPHATE TABLE

EXCHANGE

ON ANION

RESINS

57

I

MAIN PHYSICO-CHEMICAL

Resin

PROPERTIES

Matrix

Amberlite IRA 458

gel

OF THE ANION

RESINS INVESTIGATED

Amino type functional

Exchange capacity

Cross-linking degree (% DVB)

group

C (meslcmf)

1v-y

1.19

11

IIIarY

1.34

11

IIarY

1.76

11

polyacrylic Amberlite

IRA 67

gel polyacrylic

Kastel A 102 N

gel polyacrylic

tently shaken until equilibrium was reached (usually 1 month). Equilibrium composition of both phases was obtained either by analysis of supernatant solution or by regeneration of resin with warm NaOH solution. Mean value of the separation factor, ucI,so 4 calculated as

a Cl/SO‘q

=

y/u - Y) x/u -Xl

(1)

was obtained by the least square method applied to log-log plots of Eq. (l), as shown in Fig. 1. Kinetic determinations were performed by the Kressman-Kitchener technique, already described in detail elsewhere [ 11. About 10 mg of dry resin, previously converted in Cl- or S*OF form with the corresponding solution, were loaded in the stirrer-reactor; this was then immersed, already rotating (3,000 rpm), into 0.8 1 of Na,S04 or NaCl solution (0.006 N), in direct or reverse exchange respectively, thermostated at 25 rt O.l”C. Cl- release from the resin during direct exchange was followed potentiometrically by Cl-selective electrodes. S*OT release from the resin during isotopic or reverse exchange was monitored by liquid scintillation on a Tri-Carb 2425 Spectrometer, using Insta-Gel as scintillation cocktail, both from Packard Instrument Co., Illinois. (S*O, represents sulphate containing S35 , a beta emitter with max energy 0.167 MeV, half life 87.2 d, specific activity 79.8 mCi/mmol, supplied by the Radio-chemical Centre, Ltd, Amersham, UK). Both potentiometric and radioactivity data were automatically collected and processed on line by a Hewlett-Packard 2100s computer to obtain U (i.e., conversion degree of the resin) vs. time plots. During direct exchange, the infinite solution volume (ISV) condition was

Fig. 1. Equilibrium isotherms of Cl-/SOT exchange of anion resins with different basicity (C = 6 X 10W3 N, pH = 3, 25’C, 20/30 mesh). A - experimental data and exchange isotherms based on a constant separation factor. B - determination of separation factors by log-log plotting of Eq. (1).

0

0.25

0.50

Y

0.75

1.00

CHLORIDE-SULPHATE

EXCHANGE

ON ANION

RESINS

59

ensured by a ratio w z 0.008 meq in resin/meq in solution, indicating a negligible concentration in solution of the ion released throughout the exchange. Due to high selectivity of all the anion resins towards sulphates at this concentration, even such a low value of w was not sufficient to allow complete conversion of the resin from SO4 to Cl form during the reverse exchange. The experimental procedure was accordingly modified as follows: a stainless steel wire basket, containing about 5 g of Kastel A 102N resin in Cl-form, was introduced into the reaction vessel during the reverse exchange to remove any sulphate from solution. The basket was replaced with a fresh one at prefixed times, so that a negligible build-up of sulphates in solution was certainly attained throughout the exchange, as checked radioanalytically. By separate regeneration with 2N NaOH of the resin contained in each basket, the amount of sulphates progressively released during the reverse exchange was easily calculated. Other experimental details are given in previous articles of this series.

RESULTS

AND DISCUSSION

Fig. 1 shows the equilibrium isotherms for SO, uptake by Cl-form resins investigated. As can be seen, extremely high sulphate selectivities occur, with a regular increase of ac,,so, value in the order: selectivity of IVq

< IIIW < IIW amino-type resins

(2)

as expected for this system [l]. (It should be noted from the lower part of Fig. 1 that the mean value of acilS04, quite constant, obtained for each resin by linear regression analysis of experimental points, is highly representative of the whole isotherm). As frequently remarked during this investigation, such a high selectivity is responsible for a peculiar kinetic performance of resins in the system under study. Experimental kinetic data have been analyzed theoretically according to the following treatment based on the Nernst-Planck theory for ion exchange kinetics between resin in A form and ion B. i) Film diffusion control (fdc) The limiting step of the exchange rate is assumed to be ion interdiffusion through the stagnant liquid film that surrounds the particle of resin. In ISV condition, with ions of equal mobility (X”,, = 76.3, Xg, = 79.8 cm’/ohm at 25°C) and different valences z, this mechanism is described by the equation [4,51, 1

In l--U

-

U=kft,

(3)

60

L. LIBERTI, R. PASSINO

AND D. PETRUZZELLI

where k

3D*C

=

f

(4)

rGCa,,,

Thus, fdc-ISV rate control may be confirmed by a linear plot of the left side of Eq. (3) vs. time. ii) Particle diffusion control (pdc) The limiting step of exchange rate is now ion interdiffusion in the resin bead. In ISV condition, for uni-divalent exchange, this corresponds to the linear plot [6,7], In

-

1

1 - u2

=kpt,

(5)

where k, = 7~’ fl

Di r2

fl is a function of the ratio DA /& , which assumes the expression fl =

1 0.64 + 0.36 (-IsA/&)o.668

= 0.89

(7)

= 1.19

(8)

or fl

=

1

0.44 + 0.56 (DA /.&)“.777

for direct (zq/zg =-l/2, DA/DB = o,,pso 4 ) or reverse exchange (_zA/z~ = 2, & 10, = D,, /Del), respectively. (D,, = 7.7 X lo-’ cm2/s and Dso, = 5 X lo-’ cm2/s, as %etermined elsewhere [2]). iii) Chem-control The ion exchange reaction in the fixed group in the resin now exhibits the slowest rate. As in a Bnd-order nucleophilic substitution re_action (SN2 ), the following rate equation has been derived for the Cl-/SO, exchange in ISV condition [8] . 1 In 1-u

=k,

t

In isotopic, and, more generally, non-selective exchanges (aA ,e = l), Eqs. (9) and (3) become similar. By analogy to Eq. (4), hence, the following expression has been assumed for k,

CHLORIDE-SULF’HATE

P3D,

k,=Wf=rgca

C

EXCHANGE

ON ANION

RESINS

61

(10)

A/B

where the retardation factor /3accounts for chemical interactions within each resin, as discussed in a previous paper [9]. Although only fdc or them-control were reasonably expected in this very diluted and very selective system, pdc has also been checked for all resins investigated by use of Eq. (3) to (10) for direct, isotopic and reverse exchange. A) Direct exchange As shown in Fig. 2, linear plots of experimental kinetics for all the resins investigated, at the various mesh sizes, cannot be obtained with Eq. (3) or (5), but only when Eq. (9) is used. (D,, in Eq. (10) has been obtained by the anion transference number, t$ and aqueous diffusion coefficient for NaCl, DONaCPat infinite dilution [lo], according to DE, = t& Do,.,, = 0.604 X 1.611 X 10m5 = 0.973 X loss cm2/s). Quite surprisingly, thus, either film or particle diffusion control should be excluded in favor of them-control in this system. According to this conclusion, by use of Eq. (10) the values of the retardation factor fl reported in Table II were obtained for the three resins investigated. As can be seen, /3increases with selectivity of resins towards the entering sulphate ion, so that the maximum retardation occurs with the most selective resin. B) Iso topic exchange From the experimental data in Fig. 3, a pure fdc-ISV mechanism can clearly be assumed for the S04-+S04 isotopic conversion of these resins in the conditions investigated. Use of Eq. (3), modified to account for uAIB = 1, allowed a value of 6 = 0.81 X 10e3 cm to be obtained for the film thickness in this thoroughly stirred solution (see constant slope in Fig. 3B). No retardation effects are encountered during isotopic exchange, provided that no preference may be obviously shown by the resin between the entering and the leaving counterions. C) Reverse exchange A third situation occurs during the reverse exchange, where the preferred sulphate ions are now leaving the resin, exchanged by Cl ions. Accordingly, neither the previous hypothesis of film diffusion or of SN? them-control seems to apply to this case, as shown by the data in Fig. 4. (Dz,, = tg04 X = 0.632 X 1.229 X 10d5 = 0.78 X lo-’ cm*/s [lo] ). The existence G&O4 of such a strong selectivity of the resin towards the ion to be released permits to exclude a purely diffusional control (either through the film or through the particle) in favor of them-control, as in the case of direct ex-

62

L. LIBERTI, R. PASSINO AND D. PETRUZZELLI

Fig. 2. Experimental kinetics and model equations for direct (Cl----+SOT) exchange. A - Eq. (3) for fdc, ISV mechanism, B - Eq. (5) for pdc, ISV mechanism, C - Eq. (9) for pseudo-SNz them-control, ISV mechanism. (Amberlite IRA 458 : 0 20/30 mesh, @ 18/20 mesh, l 16/18 mesh; Amberlite IRA 67 : LJ20/30 mesh, * 18/20 mesh, A16/18 mesh; Kastel A 102 N : 0 20/30 mesh, q 18/20 mesh, n 16/18 mesh; experimental data interpolated by dotted lines; C = 6 x 10e3 N, pH = 3, 25OC).

119

589

Amberlite IRA 67

Kastel A 102 N

(s)

(s-1 )

10.4 6.2 2.8

68 103 253

236

77

59

too5

k, x lo3

2.9

9.2

12.1

(s-l )

k, x lo3

18/20 mesh

to.5 (s)

16/18 mesh

t Calculated according to Eq. (1). tt k, calculated according to Eq. (10). $ kf calculated according to Eq. (4) with an/B = 1. Z$ Calculated from the slope of line in Fig. 2C.

52

Mean value of separation factor aCllSo t (20/20 mesh:

Amberlite IRA 458

Resin

Direct exchange ++

197

65

42

(s)

to.5

690

3.5

370

193

300 11.3

(s)

to.s

149

0”

2.81

3.69

4.16

(s-l )

k; x lo3

20/30 mesh

175

17.9

(s-l )

k, x lo3

20/30 mesh

Isotopic exchanget

THERMODYNAMIC AND KINETIC DATA FOR THE Cl-/SO: EXCHANGE ON THE ANION RESINS INVESTIGATED (C = 6 x 1O-3 N, pH = 3, 25%)

TABLE II

600

25,400

2,500

(s)

to.5

20130 mesh

Reverse exchange

64

L. LIBERTI, R. PASSINO AND D. PETRUZZELLI

0

XV

60G

10011

tts?

1400

Fig. 3. Experimental kinetics for isotopic (S*O=4-SOs) exchange (C = 6 X 10e3 N, pH = 3, 25OC, 20/30 mesh). A - experimental kinetics for resins Amberlite IRA 458 (1 : A, 0); Amberlite IRA 67 (2 : 0, 0) and Kastel A 102 N (3 : D, 0). B - Application of model Eq. (3) for fdc, ISV mechanism modified for uA ,B = 1,

change. However, the SN2 mechanism, and the corresponding use of a retardation factor /3, seem not to offer an adequate picture of the process involved.

CONCLUSIONS

With the only exclusion of isotopic exchange (S0,wS04), where the influence of resin selectivity toward the interdiffusion species can be neglected,

CHLORIDE-SULPHATE

0

4

EXCHANGE

0

ON ANION RESINS

12

16

.

20

65

24

t x 103 (s) Fig. 4. Experimental kinetics and model equations for reverse (SOT-Cl-) exchange: A - Eq. (3) for fdc, ISV mechanism; B - Eq. (5) for pdc, ISV mechanism, C - Eq. (9) 0 Amberlite IRA 458, A Amberlite IRA 67 for pseudo-SNz them-control mechanism. and 0 Kastel A 102 N, experimental data interpolated by dotted lines; (C = 6 x 10e3 N, pH = 3, 25’C, 20/30 mesh).

66

L. LIBERTI, R. PASSINO AND D. PETRUZZELLI

both direct (Cl+SO,) and reverse (SO4 -Cl) ion exchanges for the Cl-/ SOT system on anion resins cannot be described by familiar diffusional mechanisms based on Nernst-Planck theory. Due to low solution concentration (C = 6 X lop3 N), very high selectivities are shown by the anion resins investigated towards the sulphate ion (uc,,so4 ranging from 52 to 589). This, in turn, requires that chemical interactions among the interdiffusing species and the fixed groups inside the resin be properly accounted for. In the direct exchange, that is when the preferred SO, ion enters the resin, the exchange rate is controlled by the chemical reaction, according to a mechanism similar to that of the nucleophilic substitution mechanism (SN2 ). During the reverse exchange, however, when the preferred ion leaves the resin, a different situation exists, which has not been explained yet in terms of a reaction mechanism. Further research is required to elucidate the resin exchange kinetics of strongly selective systems, like Cl-/SO, exchange. With this aim, use of autoradiography, which allows to visually follow the exchange, may prove useful to overcome uncertainties of mathematical models.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

L. Liberti, G. Boari and R. Passino, Env. Prot. Eng., 4 (2) (1978) 101. L. Liberti, G. Boari and R. Passino, Desalination, 25 (1978) 123. F. Helfferich, Ion Exchange, McGraw-Hill, New York, NY, 1962, Chap. 6. R. Schlogl, F. Helfferich, J. Chem. Phys. 26 (1957) 5. G. Dickel and A. Meyer, Z. Elektrochem, 57 (1953) 901. F. Helfferich and M.S. Plesset, J. Chem. Phys., 28 (1958) 418. M.S. Plesset, F. Helfferich and J.N. Franklin, J. Chem. Phys., 29 (1958) 1064. L. Liberti and G. Schmuckler, Desalination, 27 (1978) 253. L. Liberti, R. Passino and D. Petruzzelli, Desalination, 41 (1982) 199--208. H.S. Harned and B.B. Owen, The Physical Chemistry of Electrolite Solutions, Reinhold Publishing Co., New York, NY, 1958, p. 699.