UDC 661.183.12

STUDY OF EXCHANGE SORPTION AND EXCHANGE KINETICS OF ALKALI METAL IONS ON KU-2 CATION EXCHANGER IN AQUEOUS–ALCOHOL SOLUTIONS

G. S. TIKHONOV, E. R. GOFMAN

(Presented by the scientific seminar of the Faculty of Physics and Technology)

The sorption and exchange kinetics of alkali metal ions in aqueous–alcohol solutions of various concentrations on the KU-2 cation exchanger are considered. It is shown that the exchange constants of lithium, sodium, potassium, and magnesium ions increase with increasing concentration of propyl alcohol in the solution, while the diffusion coefficients decrease in comparison with aqueous solutions.
Tables 3, references 12.

Ion-exchange processes occurring in mixed aqueous–organic solutions differ in many respects from ion-exchange processes in aqueous salt solutions. This is associated with changes in various physicochemical properties of the ion exchanger–solution system [1–5]. Additions of organic solvents to water cause large differences in the solubility, dissociation of salts, and solvation of ions, as well as in the behavior of the ion exchanger itself. Interest in the study of ion-exchange processes in aqueous–organic solutions is due to the ever-increasing use of ion-exchange materials for the sorption and separation of substances. In a number of cases, aqueous–organic solutions make it possible to obtain results that cannot be achieved using aqueous solutions.

In the present work we studied the processes of sorption and kinetics on the KU-2 × 8 cation exchanger in aqueous–organic solutions. As organic additives we used a homologous series of saturated alcohols with more than two carbon atoms. These alcohols are characterized by low solubility in water (except for propyl and isopropyl alcohols) and fairly high values of dielectric permittivity.

Sorption of ions from aqueous–alcohol solutions on KU-2

To study the exchange sorption of ions, KU-2 × 8 cation exchanger was used, prepared according to the generally accepted procedure [6]. Experiments on the study of ion-exchange equilibrium were carried out by the method described in [7]. Into a series of flasks with ground-glass stoppers, with a volume of 150–200 ml, 1.0 g of ion exchanger of known moisture content was placed; into one of them was added 0.5 mg-eq of an aqueous–alcohol solution of the chloride of one of the alkali metals (lithium, sodium, magnesium); into another, 1.0 mg-eq; into a third, 3 mg-eq, etc. The experiments were conducted at a temperature of 20°C until equilibrium was reached. In most cases the equilibrium solution

was quantitatively transferred to a volumetric flask. In one aliquot of the solution the metal concentration was found by determining the chloride bound to it by Mohr’s method; in another aliquot the concentration of hydrogen ions was determined titrimetrically. The described procedure eliminates the unavoidable error in the analysis of the equilibrium solution associated with absorption of water by the ion exchanger. As a result of the experiments it was shown that additions of alcohols that are only slightly soluble in water (from 9.5% for isobutyl alcohol to 0.4% for heptyl alcohol) do not affect, within experimental error, the sorption of ions by the KU-2 cation exchanger. The results of the study of the sorption of Li\(^+\), Na\(^+\), K\(^+\), and Mg\(^{2+}\) ions from aqueous propyl alcohol solutions of different concentration are presented in Tables 1 and 2.

Table 1

Table 2

Sorption of sodium, potassium, and magnesium ions from aqueous propyl alcohol solutions of different concentration by the KU-2 cation exchanger

It is seen from Tables 1 and 2 that the sorption of lithium, sodium, potassium, and magnesium ions from aqueous solutions of propyl alcohol increases with increasing alcohol concentration in the solution.

Study of the Kinetics of Ion Exchange on KU-2×8 Cation Exchanger in Aqueous–Alcohol Solutions

The study of the exchange kinetics was carried out on KU-2×8 cation exchanger in the \(H^\pm\)-form. KU-2×8 cation exchanger was placed in an ion-exchange column, through which a \(1N\) aqueous–alcohol salt solution was passed at a rate of \(5\ \mathrm{ml/min}\). At specified time intervals the solution was sampled, and the amount of exchanging ions was determined titrimetrically in the sample. The diffusion coefficients were calculated from the kinetic data. To calculate the diffusion coefficients, the equation for diffusion in a sphere [8] was used

\[ \frac{x}{x_\infty}=1-\frac{6}{\pi^2}\sum_1^n \frac{1}{n^2} e^{-Dn^2\pi^2 t/r^2}, \tag{1} \]

where \(x\) is the amount of exchanged ions by time \(t\), mg-eq/g;
\(x_\infty\) is the total exchange capacity of the resin sample, mg-eq/g;
\(D\) is the diffusion coefficient in the cation-exchanger bead, \(\mathrm{cm^2/sec}\);
\(r\) is the radius of the swollen resin beads, cm.

On the basis of the obtained values of \(x/x_\infty\), using tables [9], the values \(Bt\) were found, where \(B=D\pi^2/r^2\). For the initial time intervals, plots of the dependence \(Bt=f(t)\) were constructed. From them the values of \(B\) were found, and the diffusion coefficients were calculated by the formula \(D=Br^2/\pi^2\).

The sizes of the resin particles needed for calculating the diffusion coefficients were calculated from the values of the degree of swelling, determined by the centrifugation method [10]. The mean radius of the swollen beads was calculated by equation [11]

\[ \overline{r}_{\mathrm{sw}}=r_c \sqrt[3]{\frac{W_{\mathrm{sw}}}{W_c}}, \tag{2} \]

where \(r_c\) is the radius of a dry ionite bead, cm, \(r_c=3\cdot10^{-2}\ \mathrm{cm}\);
\(W_{\mathrm{sw}}\) is the total weight of the swollen beads, g;
\(W_c\) is the total weight of the beads in the dry state, g.

The values of the diffusion coefficients of lithium, sodium, potassium, and magnesium ions and the values of the radii of swollen beads in aqueous–alcohol solutions are presented in Table 3.

Table 3

It is seen from Table 3 that, with an increase in the concentration of alcohol in the solution, the diffusion coefficients decrease.

Discussion of the Results

In studying ion-exchange sorption from aqueous–alcohol solutions of various concentrations, it was found that with an increase in concentration

the alcohol in the solution, the sorption of ions increases. Of great importance for this is the decrease in dielectric permittivity with increasing alcohol concentration in the solution and the change in the solvation state of the ions. Thus, in [12] it was shown that, when the alcohol concentration is increased (0–90%), the solvation shell of the lithium ion decreases by 4–5 molecules, which leads to an increase in sorption in alcoholic solutions as compared with sorption in aqueous solutions. The increase in exchange with increasing alcohol concentration cannot be explained solely by dehydration of the ions; apparently, other factors also play a role here (changes in the medium, changes in the tendency to form ion pairs, etc.).

From the data presented in Table 3 it is evident that the diffusion rate of alkali-metal cations on KU-2 increases as the mobility of the ions increases and the radius of the hydrated ion decreases. The decrease in the diffusion coefficients of ions in aqueous-alcoholic solutions occurs as a result of the reduced swelling of the ion exchanger, as well as because of the increased viscosity of the solution and the decreased dielectric permittivity. A certain decrease in the radius of the solvated ion, as compared with the radius of the ion in water, does not compensate for the decrease in the diffusion coefficients in aqueous-alcoholic solutions.

Conclusions

1. The sorption of ions by the KU-2×8 cation exchanger from aqueous-alcoholic solutions has been studied. Sorption from solutions containing less than 20 wt.% alcohol differs little from sorption from aqueous solutions. With an increase in the concentration of alcohols in the solution, the exchange of the ions studied increases.

2. The kinetics of ion exchange in mixtures of alcohols with water has been studied; it has been found that the diffusion coefficients in aqueous-alcoholic solutions decrease in comparison with water.

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