Reversibility of Strontium Sorption on Fracture Fillings
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REVERSIBILITY OF STRONTIUM SORPTION ON FRACTURE FILLINGS DAQING CUI AND TRYGVE E. ERIKSEN
Department of Chemistry, Nuclear Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden. ABSTRACT
We have carried out a comparative study of sorption and desorption of strontium in groundwater on separated size and magnetic fractions of fracture fillings from deep granite. Complete reversibility of the sorption process was demonstrated by identical Freundlich isotherms, isotopic exchangeability and pH dependence of the distribution coefficient Rd. INTRODUCTION
Granite has been chosen by several countries as a major candidate for deep geologic disposal of radioactive waste [1]. The most probable escape pathway for radionuclides from a repository is the water carrying fractures in the surrounding rock. This transport will be retarded by sorption on surfaces of minerals in the fractures. Transport models usually assume that the sorption processes are reversible and that local equilibria prevail along the flow path, but available evidences for the sorption kinetics and reversibility are very few. The radionuclide sorption on a heterogeneous solid surface generally follows the Freundlich isotherm q=KCN, where q and C are the equilibrium concentrations in sorbent and aqueous phase, respectively, and K and N are empirical constants [2 to 8]. If the sorption is completely reversible, the same isotherm should be valid for the sorption and desorption processes under the same experimental conditions. Hence, the Nd/N. and Kd/K, ratios [5, 6], where the subscripts s and d denote sorption and desorption respectively, as well as the isotopic exchangeability 2[7, 8], have been used to assess the reversibility of cation sorption. Sorption of Sr ÷on granite [9 to 12] and other rocks [2, 3 and 13] have been widely studied. The sorption isotherm has often been observed to be linear or near linear and the reversibility has often been invoked in spite of the fact that the desorption process has been given relatively little attention. In the present work we have studied the sorption of strontium on fracture fillings with respect to kinetics, sorption and desorption isotherms, isotopic exchangeability and pH dependence, and present evidences for complete reversibility of the sorption process.
EXPERIMENTAL
The experiments were carried out at ambient temperature (22±2oC). All chemicals were of analytical grade and used as received. 85Sr was purchased from Amersham as strontium chloride in aqueous solution. Synthetic groundwater with the composition given in Table I was prepared from Millipore deionized, triple distilled water. The pH measurements were made with a Metrohm 622 pH meter and a GK 2331 combined glass/reference electrode. the electrode was previously calibrated 85using standard buffer solutions (pH 4.0, 7.0 and 10.0). Solution samples were analyzed for Sr in a 2"x2" well type Nal detector connected to a 256 channel analyzer. The amounts of solid, aqueous solution and samples of the aqueous phase were measured gravimetrical
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