Polythermal Investigation Of Equilibrium
POLYTHERMAL INVESTIGATION OF EQUILIBRIUM
AND KINETICS FOR ION SORPTION
BY COMPLEXING SILICAS
Y. V. Kholin, S. A. Myerniy, I. V. Khristenko
V.N. Karazin Kharkov National University
Svoboda Square 4, Kharkov, 61077, Ukraine e-mail kholin@univer.kharkov.ua
The authors discuss the chemisorption of Hydrogen ion accompanied with the transfer of counter-ions into the surface layer of silicas with grafted aminogroups.
The object under study was silochrome (specific area 120 m2×g-1) chemically modified with secondary amine (0.46 mmol×g-1). To investigate the protolytic properties the slurries of the sorbent in aqueous solutions were pH-metrically titrated with hydrochloric acid at (20–50)oC. The pH values of aqueous phase were measured (error of determination ±0.05) sixfold per second till the sorptional system attained the equilibrium state. Ionic strengths of solutions were maintained in the range 0.1–1 mol×l-1 by NaCl and NaNO3.
The kinetics of chemisorption is discussed in the framework of “a semi-infinite diffusion model”1 which can be represented in the form
c(t) / c(0) » a - 2×b×t1/2, (1)
where t is the time, s, c(0) is the initial sorbtive concentration in solution, c(t) is the current concentration, a and b are the fitting parameters. The values of diffusion coefficient D are easily received from b. The typical example of the c(t) / c(0) vs. t1/2 dependence is presented in the figure.
The calculated D values varied from ~10-7 m2×s-1 to ~10-5 m2×s-1 dependently on the temperature, the ionic strength of solution and the protonization degree of grafted species.
The protolytic properties of grafted reagents were described with the reaction
, (2)
where Q is the grafted amine, A- is the counter-ion, the line above the formulas marks grafted species.
The plot of c(t) / c(0) vs. t1/2.
For all ionic strengths the mixed constants KH were calculated and then the thermodynamic constants were found on the base of the Pitzer’s method2. The values of log varied from 8.2 to 9.2 dependently on the temperature and the kind of the counter-ion. Equation was applied to estimate
(3)
thermodynamic characteristics of reaction (2). It was found that both DHo and DSo are positive and dependent highly on the kind of the counter-ion. At the first glance, the endothermic character of grafted amine protonization seems to be surprising. To explain this phenomenon one has to take into account that the transfer of hydrogen ions from the bulk solution into the surface layer is accompanied with the transfer of counter-ions and their strong fixation near the charged surface centers. It is impossible without the partial loss of the hydrate water molecules by counter-ions. Hence, the total process is of the endothermic nature and proceed under the entropy growth.
References
1. B.V. Zhmud, A.B. Pecheniy and A.A. Golub, Functional Materials, 2(1995) 44.
2. Yu.V. Kholin, A.A. Bugaevskii, D.S. Konyaev and Yu.V. Shabaeva, Ukr. Chem. J., 65 (1999) 110.
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