Colloids and Surfaces, 17 (1986) 123-130 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

123

SODIUM-ZINC EXCHANGE EQUILIBRIA ON MONTMORILLONITE

R.P. SINGH and S.K. SAXENA

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

(Received 8 April 1985; accepted in final form 2 August 1985)

ABSTRACT

An attempt has been made to predict the mechanism of Na’-Zn’+ exchange on mont-

morillonite with the help of thermodynamic parameters. The isotherms and free energy changes show that Zn ‘+ has a lower affinity for montmorillonite than Na’. The findings

were corroborated by enthalpy and entropy changes. Values of activity coefficients of

the adsorbed ions and the excess thermodynamic functions indicate that the equilibrium involving Na+-Zn*+ exchange on montmorillonite behaves differently from a non-ideal system.

INTRODUCTION

Recently, zinc adsorption and reactions in clays and soils have been receiving increasing attention because of frequent zinc deficiencies in plants grown in such soils. Studies on cation and anion interchange of Zn2’ on montmorillonite, as well as its exchange adsorption, have been performed by a number of workers [ 1,2] . In the ion-exchange studies it has been noticed that the two ions may exchange stoichiometrically but they may not, in general, be equally preferred or bound equally strongly on the sur- face. There is a lack of information on this kind of study in alkaline soils rich in Na. Hence, in the present work, the ion-exchange reaction of Zn2+ with Na-montmorillonite has been studied in terms of thermodynamic functions [ 3-71, with the view that such a study will provide a better understanding of the mechanism of the interaction of Na’ and Zn2+ on montmorillonite.

MATERIALS AND METHODS

The montmorillonite was obtained from Ward’s Natural Science Estab- lishment Inc., Rochester, U.S.A. The Na-montmorillonite (< 2pm) was dispersed using the method outlined by Aldrich and Buchanan [8], and the concentration of the suspension was adjusted to 1.2% (w/v). The cation-ex- change capacity (CEC), determined by the ammonium acetate method [9],

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

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was found to be 70 meq per 100 g. For determining the exchange isotherms, 0.03 N ZnS04 solution was added to 10 ml of clay suspension in glass-stop- pered tubes and the mixture was adjusted to constant volume (25 ml) with distilled water. In the first set of experiments the tubes were shaken at 30 f O.l”C for 6 h; and in the second set of experiments at 60 f O.l”C for 6 h in a thermostatic water bath. Supernatants obtained after centrifugation were used for the estimation of sodium by flamephotometry and Zn’+ by atomic absorption spectrophotometry. The corresponding concentrations of Na’ in the clay phases were calculated by obtaining the difference between the CEC and the concentration of the cation in the supernatant liquid, and that of Zn’+ between the Zn*+ added and the Zn*’ in the supernatant liquid.

RESULTS AND DISCUSSION

The exchange reaction for the Na+--Zn*+ system in dilute suspensions can be represented by the equation:

2CN, + CZ, = CZn + 2CN, (1)

where bars represent the equivalent concentrations of the ion concerned in the montmorillonite phase and unbarred quantities the electrolyte con- centration in solution. The equivalent ionic fractions of the Zn*+ and Na’ in montmorillonite and the solution can be expressed as follows:

CZn

Xzn = c 7 xzn czn - CNZi = - ,xNa - CNEI

C c ,XNa - c where c and C are the total electrolyte concentrations in the clay and solution phases, respectively. The values thus obtained both at 30 and 60°C are given in Table 1. The isotherms thus obtained at both temperatures show that Na’ ions have a greater affinity for montmorillonite at all concentra- tions, with greater affinity up to 0.79 Xzn at 60 than at 30°C (Fig. 1). This indicates that the replacement of Na’ by Zn*+ is difficult.

In order to determine the interaction in the solution and montmoril- lonite phase, the selectivity coefficients at 30 and 60°C for different surface composition of Zn*+ were calculated using Eqn (2), assuming the ratio of activity coefficients as unity [lo] in the range of concentrations studied.

- Xzn (XNa )” -.-

K, = (riNa)* XZn (2)

The va&es thus obtained are given in Table 1, and the values of log K, versus Xzn have been plotted at both temperatures in Fig. 2. The selectivity coefficients initially decreased and then increased at both temperatures. Such a variation indicates significant interactions between the Zn*+ and the multiple sites of Na-montmorillonite at both temperatures, which underwent

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TABLE 1

Values of equivalent ionic fractions and selectivity coefficients at 30 and 60°C for the Zn’+-Na* exchange on montmorillonite

No. 8,, xzn xNa xNa K, log &

30°C 1

2 3 4 5 6 7 8 9

10 11

60°C 1 2 3 4 5 6 7 8 9

10 11

0.222 0.335 0.778 0.665 0.486 -0.314 0.305 0.556 0.695 0.444 0.224 -0.650 0.440 0.615 0.560 0.385 0.335 -0.475 0.446 0.713 0.554 0.287 0.167 -0.775 0.478 0.759 0.521 0.241 0.135 -0.871 0.458 0.806 0.542 0.194 0.072 -1.143 0.482 0.830 0.518 0.170 0.063 -1.201 0.508 0.846 0.492 0.154 0.057 -1.244 0.607 0.845 0.393 0.155 0.118 -0.952 0.652 0.855 0,348 0.145 0.132 -0.879 0.695 0.861 0.305 0.139 0.165 -0.783

0.160 0.260 0.840 0.740 0.478 -0.321 0.297 0.540 0.703 0.460 0.236 -0.627 0.318 0.664 0.682 0.336 0.116 -0.934 0.380 0.696 0.620 0.304 0.131 -0.882 0.425 0.734 0.575 0.266 0.124 -0.907 0.466 0.799 0.534 0.201 0.082 -1.087 0.472 0.824 0.528 0.176 0.064 -1.197 0.520 0.838 0.480 0.162 0.070 -1.154 0.597 0.839 0.402 0.161 0.114 -0.942 0.611 0.854 0.389 0.146 0.101 -0.996 0.667 0.861 0.332 0.139 0.134 -0.874

gradual saturation one after another. The preference of Zn2’, however, for the solid surface decreased with a rise in its concentration, and with a further rise in concentration it decreased.

For further study the thermodynamic equilibrium constant K was calcu- lated as follows

1

lnK=(ZA-ZB)+ s In Kc tizn (3)

0

where ZA and ZB are the charges on the competing ions. The integral was evaluated from the area under the curves (Fig. 2) using the trapezoidal rule [6] . The values are summarized in Table 2. The values of thermo- dynamic equilibrium constant K were found to be higher at 30°C indicating that the affinity of montmorillonite for Zn2+ decreases with rise in temper- ature, which is in accordance with the deductions drawn from the nature of the isotherms.

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3

Fig. 1. Exchange isotherm for Zn*+ on sodium-montmorillonite at different temper- atures.

TABLE 2

Thermodynamic values for the Zn =+-Na’ exchange on montmorillonite at 30 and SO” C

Thermodynamic 30°C 60°C parameters

K 0.1516 0.1357 A Go (kJ mol-’ ) 4.7690 5.5380 A Ho (kJ mol-’ ) -3.10 A So (kJ mol-’ deg-’ ) -0.026 -0.026

The standard free energies of exchange, AG O , for the interaction [ Eqn (l)]

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, AI?‘, was calculated from the Van ‘t Hoff isochore.

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In (5)

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

AGO = Ap-TAS’ (6)

The values obtained are listed in Table 2. The positive standard free energy, AGO, values for the interaction at both

temperatures indicate that Zn *+ has a lower affinity for montmorillonite than Na’ and the replacement of Na’ by Zn*+ cannot be spontaneous.

The reaction was attended by a negative enthalpy change, which is pre- sumably due to the smaller ionic radii and higher charge of Zn*+ ions com- pared with Na’ ions.

The exchange was also ruled out by the entropy effect. The negative entropy effect for the exchange suggests that the montmorillonite has a greater affinity for the Na’ ions. This is, however, not in accordance with the facts. Entropy is also ruled out by hydration effects. It appears that, - _ in this system, the hydration shell around

-O.*1

the Zn*+ ions remains intact

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7-

Equivalent ionic fraction of Zn*+ in montmorillonite ( Rzn)

Fig. 2. Sodium-zinc selectivity in montmorillonite.

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during its adsorption, contributing to a greater order in the system. Further, with Na’ ions the double layer is more extended and the distribution of cations above the negative montmorillonite surface is more diffuse than with Zn2+ ions.

The activity coefficients of Zn’+ and Na’ ions were calculated from the following expressions [ 111 :

XZn

ln fin = (,“iz,-l)ln&- J ln & mzn (7)

0

and

Xzn

ln fNa =rTzn In&-- J In& mzn (8)

0

The values obtained are given in Table 3. At both temperatures the activity coefficients of Na’ were less than unity while those of Zn2+ were higher than unity. Such variations indicate a much greater heterogeneity in the distribution of ions at the clay surface with significant interaction between the Na’ and Zn2+ ions at the neighbouring sites of the surface. The results thus support the work of Howery and Thomas [ 111, Robinson and Stokes [12] and Diest and Talibuddeen [13], who also reported high values for activity coefficients in solid phases.

To examine further the deviation of these heterogeneous systems from ideality, the excess thermodynamic functions of these systems were cal-

TABLE 3

Values of the activity coefficients for the Zn *+-Na+ exchange on montmorillonite at 30

and 60°C

No. 30°C 60°C

Xzn fin fNa Xzn fzn fNa

1 0.222 1.760 0.855 0.160 1.868 0.892 2 0.305 3.078 0.689 0.297 3.200 0.755 3 0.440 2.415 0.809 0.318 5.189 0.603 4 0.446 3.721 0.622 0.380 4.717 0.619 5 0.478 4.010 0.540 0.425 4.542 0.563 6 0.458 5.905 0.422 0.466 6.404 0.524 7 0.482 6.240 0.394 0.472 7.606 0.484 8 0.508 6.640 0.379 0.520 6.867 0.486 9 0.607 5.077 0.567 0.597 5.966 0.685

10 0.652 4.827 0.636 0.611 5.760 0.574 11 0.695 4.545 0.750 0.667 5.333 0.714

culated from the following expressions [ 14,151:

AG$, = RT (zZn In fin + XNa In fNa)

and

AS% = AH&-Task

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(9)

(10)

(11)

where AG&, AHg and Ash are the excess free energy, enthalpy and entro- py of mixing, respectively. The values are given in Table 4. During the ex- change, free energy has positive values and shows a gradual rise with increase in the concentration of Zn2+ at both temperatures. It thus appears that the heterogeneous mixture of Na’ and Zn2’ ions is less stable than the pure forms and the stability further decreases with a rise in concentration of Zn2’. Thus, the departure from ideality occurs in the form of a more or less stable mixture on the clay surface depending upon the nature of the ex-

TABLE 4

Values of the excess free energies, enthalpies and entropies of mixing for the Zn’+-Na+ exchange on montmorillonite at 30 and 60” C

No. 8,, A Gz(kJ mol-’ ) A Hk(kJ mol-’ ) A S&(kJ mol-’ deg-’ )

30°C 1 0.222 0.010 -1.061 -0.004 2 0.305 0.211 -1.930 -0.007 3 0.440 0.679 -4.396 -0.017 4 0.446 0.815 -2.658 -0.011 5 0.478 0.866 -1.820 -0.009 6 0.458 0.872 -2.359 -0.011 7 0.482 1.008 -3.822 -0.016 8 0.508 1.221 -1.781 -0.010 9 0.607 1.926 -3.674 -0.018

10 0.652 2.193 -8.576 -0.036 11 0.695 2.443 -2.551 -0.016

60°C 1 0.160 0.010 -1.239 -0.004 2 0.297 0.422 -2.339 -0.008 3 0.318 0.508 -1.403 -0.006 4 0.380 0.811 -2.723 -0.011 5 0.425 0.868 -1.893 -0.008 6 0.466 1.443 -2.844 -0.013 7 0.472 1.592 -4.587 -0.019 8 0.520 1.818 -2.124 -0.012 9 0.597 2.535 -4.420 -0.021

10 0.611 2.367 -2.084 -0.013 11 0.667 2.786 -2.920 -0.017

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changeable ions and the equivalent ionic fraction of Zn’+. The AGX, is also ruled out by enthalpy and entropy effects. The AH& and AS& values were negative at both temperatures. The AS& could be a measure of the nature of the distribution of Na’ and Zn2+ ions on the clay surface compared with the pure forms. Its negative values for our mixed system points to a more ordered distribution of the mixture of Zn*+ and Na’ ions on the clay surface compared with the pure forms at all concentrations under study.

ACKNOWLEDGEMENT

These studies are financed by University Grants Commission, New Delhi, India.

REFERENCES

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