Studies of the Liquid-Solid Ratio Effect on the Distribution Ratio R d , in Sorption
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STUDIES OF THE LIQUID-SOLID RATIO EFFECT ON THE DISTRIBUTION RATIO Rd IN SORPTION J. BORS*, H. N. ERTEN** and S. ASSEMI** * **
Center of Radiation Protection and Radioecology, University of Hannover, Herrenhduser StraBe 2, 30419 Hannover, Germany Department of Chemistry, Bilkent University, 06533 Bilkent, Ankara, Turkey
ABSTRACT
of2+aqueous to solid mass The variation of the distribution ratio Rd . as a function S • 2+ volume 2+ ratio (V/M) for several sorption systems involving the cations Zn , Co , Ba , the anion I and different solid matrices has been extensively examined. Empirical relationships for the concentrations of the sorbed species in the aqueous and solid phases as well as for the distribution ratio Rd are proposed. Generally Rd is found to increase with increasing V/M for tracer concentrations. A gradual increase in the initial concentration of the sorbed species eventually leads to a decrease in Rd with V/M.
INTRODUCTION
In experimental sorption studies of radionuclides on various solid matrices, the batch method is generally used. In the batch technique the distribution of a radionuclide between the solid and the aqueous phase is measured. During such measurements several different volume of solution (V) to mass of solid (M)ratios are used by various experimenters. These ratios (VIM) are generally much higher than those found in geological environments. Since the distribution ratio, Rd, is a function of concentrations of the sorbed ion in the solid and liquid phase, it should not be affected by the variations in the V/M ratio. In the same manner, the initial concentration of the sorbed species should not affect the Rd. It has been reported however that the distribution ratio, Rd, varied with V/M [1-4]. The systematic study of the variation of Rd as a function of V/M ratio is thus important with respect to migration modelling calculations. In this work an extensive study has been carried out on the effect of both the V/M ratio and the initial concentration of the sorbed species, on Rd. The sorbed species studied were the matrices were a variety of soil and clay cations Zn2+ , Co2+, Ba2+, and the anion I- and the and mineral samples from various regions of Turkey solidGermany.
EXPERIMENTAL
The solid matrices, the ionic species and the tracer radioisotopes used in the studies are summarised in Table I. Detailed structural and physical characteristics of the solid matrices and the compositions of the aqueous phases used in the batch experiments are given in refs. [5-9]. The aqueous phases consisted of synthetic groundwaters with compositions similar to natural groundwaters where the solid samples were recovered. Duplicate soil or clay samples were
Mat. Res. Soc. Symp. Proc. Vol. 353 @1995 Materials Research Society
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suspended in aqueous solutions containing the ion of interest, its radiotracer and shaken for 6-8 days at room temperature. The two phases were then separated by centrifugation (6000 rpm). The distribution ratios, Rd, were calculated from the measured activities in the liquid
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