Tracer studies of adsorption of Sr ions

Tracer Studies of Adsorption of Sr Ions Radiotracer techniques can be used to study the separation of ions in the solution by adsorption on solid surfaces. This is a more direct, sensitive and rapid means than other methods in understanding the mechanism of the different adsorption processes (1-7), especially at low concentrations. However, information on the adsorption of Sr ions on metallic nickel is not available in the literature and it was considered interesting to investigate the adsorption of Sr ions on a metallic nickel surface as a function of time, concentration of strontium nitrate solution, and temperature, using the radio-tracer technique. Circular coupons were punched from pure nickel sheets of electrolytic quality and these were mechanically and chemicaaly polished following the procedure of Siejka and Campbell (8). Sr(NO3h solution labeled with Sr a° was taken in a polythene bottle and two nickel coupons clamped vertically in a formica stand were immersed in the solution in the bottle. After each period of adsorption, the coupons were removed and washed. After drying, activity of each of the two faces of the coupon was measured in turn with a G.M. counter and scaling arrangement under conditions of constant geometry and expressed in counts per minute (CPM). A plot of activity of Sr ions adsorbed on the coupons against time of immersion (Fig. 1) shows that there is an increase of activity with time and a saturation value is reached only after about 40 hours of immersion, if the adsorption is not continuous. If

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FIG. 1. Plot of activity of nickel surface (diameter 2.0 cm) as a function of time (short-time intervals) at 25°C.

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FIG. 2. Plot of activity of nickel surface (diameter 2.23 cm) as a function of time (long-time intervals) at 25°C with various concentrations of Sr(NOz)z solution. the adsorption is carried out continuously, after five hours, the saturation value is reached (Fig. 2). The saturation value of true activity of the nickel coupons at various concentrations of Sr(NOa)2 solution were obtained from Fig. 2 and these values, when plotted as a function of concentration on a log-log plot, give a straight line within limits of experimental error (Fig. 3). No induction period in the adsorption was observed and this is in agreement with adsorption of cations of Co 8°, Sr a°, and Ce '44 on aluminum, lead, zinc, and stainless steel surfaces (8). It is seen in our case (Fig. 2) that the adsorption passes through a maximum with time and then falls off quite rapidly at first and then more slowly. Hackerman and Stephens (9) have also found such behavior in the adsorption of SO4 ions on an iron surface. The decrease in the activity might be due to the desorption of Sr ions. Chemical polishing of the nickel coupons would have made it free from any oxide on its surface. However, if the adsorption is not carried out continuously (Fig. 1), it would require the exposure of the nickel coupon to air for a considerable length of time for drying and measurement of activity, thereby causing formation of nickel oxide on its surface. A possible mechanism for Sr ion adsorption with a nickel oxide substrate is as follows:

578

0021-9797/78/0653-0578502.00/0 Copyright © 1978 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 65, No. 3, July 1978

579

LETTERS TO THE EDITORS H O

H

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This mechanism would require the adsorption of Sr ions to be pH dependent. The mechanism described above can also account for the decrease in adsorbed amount at long exposure times. If the nickel coupon is corroding during exposure to the nitrate solution, it is possible that the pH of the solution will gradually decrease. Hence, the amount of adsorbed Sr ions will decrease. Pradhananga (10) has studied the adsorption of Sr ions on copper surface using a similar experimental set up and has observed that the fall in activity with time is not to the same extent as in the case of nickel

surfaces. This could be due to the difference in adsorption potential in the two cases. The adsorption potential has a lower value in copper, thereby giving a higher binding energy to Sr ions than that in the case of nickel and hence the amount of desorption or fall in activity will be greater in nickel than in copper.

REFERENCES 1. Arnikar, H. J., and Mehta, O. P., Indian J. Chem. 7, 616 (1969). 2. Arnikar, H. J., Daniels, E. A., and Kulkarni, S. V., J. Indian Chem. Soc. 48, 555 (1971). 3. Arnikar, H. J., Daniels, E. A., and Kulkarni, S. V., J. Indian Chem. Soc. 52, 711 (1975). 4. Arnikar, H. J., Daniels, E. A., and Kulkarni, S. V., J. Indian Chem. Soc. 49, 649 (1972). 5. Haissinsky, M., and Laflamme, Y.,J. Chim. Phys. 55, 510 (1958).

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FIG. 3. Plot of logarithm of saturation value of activity as a function of logarithm of concentration of Sr(NO3)2 solution.

FxG. 4. Plot of activity per square centimeter of nickel surface as a function of time at 25 and 30°C.

Journal of Colloid and Interface Science, V o l . 65, N o . 3, J u l y 1978

580

LETTERS TO THE EDITORS

6. Camarcat, M., Bouissieres, G., and Haissinsky, M., J. Chim. Phys. 46, 153 (1949). 7. Palacious, J., and Baptista, A., Nature 170, 665 (1952). 8. Siejka, J., and Campbell, I. G., Nukleonika (Special issue), 119 (1958). 9. Hackerman, N., and Stephens, S. J., J. Phys. Chem. 58, 904 (1954). 10. Pradhananga, T. M., Degree of Science in Chemistry Thesis. Tribhuwan University, Kathmandu, Nepal, September 1976, unpublished.

Journal of Colloidand Interface Science, Vol.65, No. 3, July 1978

NARENDRA N. ROY1 Indian Cooperation Mission, Kathmanda, Nepal M. P. BARAL K. K. SRESHTHA

Department of Chemistry Tribhuwan University Kathmandu, Nepal Received March 17, 1977 1 Present address: Department of Chemistry, Regional Institute of Technology (P.O.), Jamshedpur, Bihar, India.