Colloids and Surfaces, 20 (1986) 239-246 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

239

Calcium-Zinc Exchange Equilibria on Montmorillonite

R.P. SINGH and KUSUM KUMARI

Section of Plant Pathology and Nematology, Department of Botany, Aligarh Muslim University, Aligarh-202 001 (India)

(Received 9 December 1985; accepted in final form 5 May 1986)

ABSTRACT

An attempt is made to predict the mechanism of Ca * + -Zn* + exchange on montmorillonite with the help of thermodynamic parameters. From the exchange isotherms at 30 and 60 ’ C the changes in free energy, enthalpy and entropy of the reaction are calculated and some predictions made for the exchange processes taking place in the system. The activity coefficients are also calculated which give the excess free energies, enthalpies and entropies of the mixing.

INTRODUCTION

Zinc adsorption and reactions in clays and soils are receiving increasing attention. Calcium is an important constituent of calcareous soils. Zinc defi- ciencies are common in plants growing on such soils. There is an important relationship between ion exchange and plant nutrition. Cation and anion interchange of Zn2+ on montmorillonite, as well as its exchange adsorption have been studied by several workers [ l-31. In ion exchange studies it has been noticed that although two ions may exchange stoichiometrically, they may not, in general, be equally preferred or bound equally strongly on the sur- face. In this study, the ion-exchange reaction of Zn2+ with Ca-montmorillonite is studied in terms of thermodynamic functions, using thermodynamic models developed previously [ 3-91. It was felt that such a study would be of consid- erable importance in a better understanding of the mechanism of the interac- tion of Ca2+-Zn2+ on montmorillonite.

MATERIALS AND METHODS

Montmorillonite, obtained from Ward’s Natural Science Establishment Inc., Rochester, U.S.A., was dispersed in distilled water and centrifuged. To obtain pm < 2 pm Na-montmorillonite it was equilibrated with 2 N NaCl and a small

0166-6622/86/$03.50 0 1986 Elsevier Science Publishers B.V.

240

quantity of 0.1 N HCl for 30 min and the supernatant salt solution decanted [lO].Thistr t ea ment was repeated three times. The Na-montmorillonite sus- pension was washed free of excess salts with distilled water and alcohol until the clay dispersed and the conductivity of the aqueous suspension was of the same order as that of distilled water. Calcium saturated montmorillonite was prepared from the sodium clay suspension by saturating it three times with a normal solution of CaCl, and then washing it as above to remove the excess salt. The suspension was then quickly used for cation-exchange experiments to avoid any ageing effects. The concentration of the suspension was 0.7%.

To determine the exchange isotherm we took 10.0 ml of Ca-montmorillonite suspension in glass-stoppered tubes. Various volumes of 0.03 NZnSO, solution were immediately added and the mixtures adjusted to constant volume (25 ml) with distilled water. The tubes were shaken in the first and second set of experiments at 30 t 1 and 60 ‘_’ 1 ‘C, respectively, for 6 h in a thermostatic water bath. The mixtures were then centrifuged immediately and the concentrations of Ca2+ and Zn2+ in the supernatant liquids were determined by atomic absorption spectrophotometry. The cation-exchange capacity (CEC) value of montmorillonite was determined by the ammonium acetate method [ 111 and was found to be 70 meq per 100 g. The corresponding concentrations for Ca2+ in the montmorillonite phases were obtained by the difference between the CEC and the concentration of Ca2+ in the supernatant liquid, and that of Zn2+ between Zn2+ added and Zn2+ in the supernatant liquid.

RESULTS AND DISCUSSION

The exchange reaction for the Ca2+-Zn2+ system in dilute suspensions can be represented by the equation:

G*+ Cz, - - Gn + Cc* (1)

where bars represent the equivalent concentrations of the ion concerned in the montmorillonite phase and unbarred quantities as the electrolyte concentra- tion in solution. The equivalent ionic fractions of the Zn2 + and Ca2 + in mont- morillonite and solutions can be represented by the following expressions:

where ?_? and C are the total electrolyte concentrations in the clay and solution phases, respectively. The values obtained both at 30 and 60°C are given in Table 1. The data yielded the exchange isotherm show in Fig. 1. The isotherms were sigmoid at both temperatures and showed selectively reversal. At in low values of X,,, Ca2+ was preferred and at higher concentrations Zn2+ was pre- ferred, the preference of Ca 2 + at 60” C in the low concentration range and of Zn2+ in the higher concentration, being somewhat greater than at 30 o C. It was

241

TABLE 1

Values of equivalent ionic fractions and selectivity coefficients at 30 and 60” C for the Zn2+ -Ca’+ exchange on montmorillonite

rr,, XZ, rr,, &a KC 1% Kc

30°C 0.127 0.149 0.873 0.851 0.831 - 0.081 0.233 0.246 0.767 0.754 0.931 -0.031 0.262 0.395 0.738 0.605 0.594 - 0.265 0.387 0.379 0.613 0.621 1.034 0.015 0.509 0.449 0.491 0.551 1.272 0.105 0.591 0.505 0.409 0.495 1.416 0.151 0.652 0.535 0.348 0.465 1.628 0.212 0.713 0.550 0.287 0.450 2.033 0308 0.752 0.659 0.248 0.341 1.569 0.196 0.790 0.754 0.210 0.246 1.227 0.089 0.852 0.811 0.148 0.189 1.342 0.128

60°C 0.092 0.209 0.248 0.366 0.507 0.614 0.695 0.738 0.814 0.842 0.862

0.196 0.908 0.804 0.416 - 0.381 0.277 0.791 0.723 0.690 -0.161 0.393 0.752 0.607 0.509 - 0.293 0.405 0.634 0.595 0.848 - 0.072 0.456 0.493 0.544 1.227 0.089 0.478 0.686 0.522 1.737 0.240 0.500 0.305 0.500 2.279 0.358 0.560 0.262 0.440 2.213 0.345 0.636 0.186 0.364 2.505 0.399 0.710 0.158 0.290 2.177 0.338 0.755 0.138 0.245 2.027 0.307

clear that the repIaceability of Ca2+ by Zn2+ was much more difficult in the low concentration range.

To examine the interaction in the solution and montmorillonite phase, the selectivity coefficients (K, ) at 30 and 60’ C for different surface compositions of Zn2+ were calculated from the expression

assuming the ratio of activity coefficients as unity [ 141 in the concentration range studied. The values are listed in Table 1, and the plots of log Kc against &, at 30 and 60°C are given in Fig. 2. The values of& initially increased with temperature, and then decreased at highe~values of X,,. At both temperatures, Kc increased with xx,, followed by a small fall. Such a variation was in general indicative of the fact that at both the temperatures there were significant inter-

242

z 4J B

0.2

.z g w

0.0 0.0 0.2 0.4 06 0.8 1.0

Equivalent Ionic Fraction 01 Zn2+ in solution (Xz,)

Fig. 1. Exchange isotherm for Zn2+ on Ca-montmorillonite at different temperatures.

44 0 3ok

0 6O+Z

a3

0.2 -

u” 0.1 -

g ; rc 0.0 -

.Y .I!

t 8 -O.l-

b 0

:z -0.2 - /

s -0.3

?

0

-0.4

I I I I I I

0.8 1.0 0.0 0.2 0.4 0.6 Equivalent Ionic Fraction of Zn2+ in Montmorillonite ( Kz,)

Fig. 2. Calcium-zinc selectivity in montmorillonite.

243

TABLE 2

Thermodynamic values for the Zn 2+ -Ca2 + exchange on montmorillonite at 30 and 60’ C

Thermodynamic parameters

30°C 60°C

K 1.216 1.252 AC” (Cal mol-‘) -22.4 -28.1 AH” (cal mol-‘) 34.8 AS” (cd mol-’ deg-‘1 0.19 0.19

actions between the Zn2+ and the multiple sites of the Ca-montmorillonites which underwent gradual saturation one after another. The preference of Zn2 + for the solid surface is parallel to this.

To study the equilibrium further, the thermodynamic equilibrium constant K was calculated from the relationship

1

lnK= (2,-Z,)+ I

lnK,dXs, (3) 0

where ZA and ZB are the charges on the competing ions. The integral was eval- uated from the area under the curves (Fig. 2) using the trapezoidal rule [ 7,101. The values are summarized in Table 2.The value of thermodynamic equilib- rium constant K was found to be higher at 60’ C indicating that the affinity of montmorillonite for Zn2+ increased with rise in temperature, which is in accordance with the deductions drawn from the nature of isotherms.

The standard free energies of exchange, AGO, for the interaction, Eqn (1) , were calculated from the expression:

AGO=-RTlnK (4)

where R is the universal gas constant and T the temperature in degrees Kelvin. The standard enthalpy change, AHO, was calculated from the Van’t Hoff

isochore

(5)

and the standard entropy change, AS’, from the equation

AG’=AHO--TASO (6)

The values are listed in Table 2. The negative AGO values indicate.a higher preference for Zn 2+ . This is, however, not conclusive because the exchange was also accompanied by an increase in enthalpy, pointing to a stronger binding of Ca2+. The values given in Table 2 indicate that the interaction was affected

244

TABLE 3

Values of the activity coefficients for the Zn’+-Ca2+ exchanges on montmorillonite at 30 and 60°C

30°C 60°C

rr,” f Zn f Ca rr,” f Zn f Ca

0.127 1.176 0.978 0.092 2.220 0.922 0.233 1.070 0.996 0.209 1.437 0.991 0.262 1.606 0.873 0.248 1.827 0.930 0.387 1.040 1.075 0.366 1.284 1.090 0.509 0.930 1.183 0.507 1.043 1.280 0.591 0.885 1.253 0.614 0.898 1.560 0.652 0.840 1.369 0.695 0.813 1.851 0.713 0.785 1.595 0.738 0.821 1.817 0.752 0.842 1.321 0.814 0.801 2.008 0.790 0.840 1.031 0.842 0.820 1.785 0.825 0.877 1.176 0.862 0.829 1.679

both by enthalpy and entropy effects. The positive value of enthalpy change is due to the temperature dependence. The increase of K with temperature means that the preference of the surface for Zn2+ increases with temperature. Entropy gain accompanying Zn 2+ adsorption further justified this assumption. It indi- cated more diffuse and disordered arrangement of Zn2+ ions in the Goiiy layer with Ca2+ forming a more ordered arrangement in the Stern layer. Valence, electrostatic considerations, and the smaller size of Ca2+ ions as compared with the Zn2+ ions justified the contilusion.

The activity coefficients of Zn2+ and Ca2+ ions were calculated from the following expressions [ 131

xzn

lnfz,= (Xz,-1) In Kc- s

In Kc az, 0

(7)

and -

lnfcc,=&, In Kc-T In Kc fiz, (8) 0

The values obtained are given in Table 3. The lack of similarity of the activity coefficients fee and fin against ionic fraction of zinc indicates that both zinc and calcium behaved differently on the montmorillonite surface. The deviation of the activity coefficient from unity is caused by all the usual energy interac- tions at the clay surfaces that prevent ideal behaviour. Thus, it seems that the

245

TABLE 4

Values of the excess free energies, enthalpies and entropies of mixing for the Zn2+ -Ca’+ exchange on montmorillonite at 30 and 60 “C

AG:, AH; (Cal mol-‘) (cal mol- ’ )

ASfn (cal mol-’ deg-‘)

30°C 0.127 0.233 0.262 0.387 0.509 0.591 0.652 0.713 0.752 0.790 0.825

60°C 0.092 0.209 0.248 0.366 0.507 0.614 0.695 0.738 0.814 0.842 0.862

0.00 - 187.212 -0.617 7.638 -393.753 - 1.324

14.373 - 492.305 - 1.672 35.896 - 547.680 - 1.926 27.703 - 587.989 - 2.032 12.151 - 598.355 - 2.015

- 2.499 - 505.946 - 1.662 - 23.260 -421.114 - 1.313 -36.452 - 779.727 - 2.453 - 79.361 - 587.819 - 1.678 - 53.815 - 29.011 0.079

0.00 - 95.307 - 0.286 45.326 - 422.537 - 1.405 63.105 - 587.840 - 1.954 96.756 - 630.848 -2.185 94.696 - 709.626 -2.415 70.049 -688.201 - 2.277 28.844 - 504.961 - 1.603 7.098 - 492.947 - 1.501

-33.117 - 616.961 - 1.753 - 50.057 - 490.812 - 1.323 -59.977 - 4.611 0.166

distribution and the freedom of movement of the two ions in the Goiiy and Stern layers varies according to the concentration of zinc added to Ca-montmorillonite. The results are supported by the work of Diest and Tal- ibudeen [ 141 on ion exchange in soils.

To examine further the deviation of these heterogeneous systems from ide- ality, the excess thermodynamic functions for these systems were calculated from the following expressions [ 15,161

AGF,,=RT(%, lnfi,,+%, InA)

and

246

AG;=AH&-TAS; (11)

where AC”,, AH”, and AS: are the excess free energy, enthalpy and entropies of mixing, respectively. The values are given in Table 4. During the exchange, the excess free energy has initial positive values and thereafter negative with increase in the concentration of Zn2+ at both temperatures. It thus appears that the heterogeneous mixture of Ca2+ and Zn2+ ions on the clay surface is initially less stable than the pure homoionic forms and thereafter more stable depending upon the concentration of Zn’ + . Thus, deviation from ideality occurs in the form of a more or less stable mixture depending upon the equivalent ionic fraction of Zn’+.

The negative values of enthalpies and entropies of mixing (Table 4) indicate that the ions were strongly bound with each other at the clay surface and that the distribution of the mixture of Ca2+ and Zn2+ ions was more ordered on the heteroionic exchanger compared with the pure homoionic forms.

ACKNOWLEDGEMENTS

These studies were financed by the University Grants Commission (India) under a coordinated project.

REFERENCES

1 M.M. Elgably and H. Jenny, J. Phys. Chem., 47 (1943) 399. 2 J.J. Jurinak and N. Bauer, Soil Sci. Sot. Am. Proc., 20 (1950) 466. 3 R.P. Singh and S.K. Saxena, Colloids Surfaces, 17 (1986) 123. 4 G.L. Gains and H.L. Thomas, J. Chem. Phys., 21 (1953) 714. 5 M.M. El-Sayed, R.G. Burau and K.L. Babcock, Soil Sci. Sot. Am. Proc., 34 (1970) 397. 6 J.P. Singhal, R.P. Singh, J. Soil Sci., 24 (1973) 271. 7 J.P. Singhal, R.P. Singh and D. Kumar, Colloid Polym. Sci., 233 (1975) 139. 8 K.G. Varshney, R.P. Singh and S. Rani, Acta Chim. Acad. Hung., 115 (1984) 403. 9 K.G. Varshney, R.P. Singh and S. Rani, Proc. Indian Nat. Sci. Acad. Part A, 50 (1984)75.

10 D.G. Aldrich and J.R. Bauchanan, Soil Sci. Sot. Am. Proc., 22 (1958) 281. 11 M.L. Jackson, Soil Chemical Analysis, Printice HalI, NJ, 1958, p. 60. 12 R.A. Robinson and R.H. Stokes, Electrolyte Solutions, Butterworths, London, 1959, p. 481. 13 D.G. Howery and H.C. Thomas, J. Phys. Chem., 69 (1965) 531. 14 J. Diest and 0. Talibudeen, J. Soil Sci., 18 (1967) 125. 15 E.F. Vasant and J.B. Uytterhoeven, Clays Clay Miner., 20 (1972) 47. 16 R.G. Gast and W.D. Klobe, Clays Clay Miner., 19 (1971) 311.