MINERALOGICAL MAGAZINE, VOLUME 62A Sorption mechanism of Sr(ll) in natural bentonite V. Garrido M. Martinez J. de Pablo T. E. Eriksen L. Duro J. Bruno Chemical Engineering Department, UPC, 08028 Barcelona, Spain Royal Institute of Technology, Nuclear Chemistry Department, S-100 44 Stockholm 70, Sweden QuantiSci S.L., Parc Tecnol61gic del Vallbs, 08290 Cerdanyola del Vall~s. Spain Bentonite clay has been proposed as a buffer material in the final disposal concepts for spent nuclear fuel and other hazardous materials. As a consequence, a lot of work has been devoted to study the metal sorption capacity, particularly for sybthetic montmorillonites. In general, the sorption capacity has been studied in terms of Kd distribution coefficients, and interpreted as a cation-exchange process between the intterlayer cations metal ions such as Caesium(I) and Strontium(II) [1]. In the case of Strontium, Ka values reported in the literature vary from 0.006 to 2.14 m 3 kg i A more realistic picture of the overall process should include the interaction of the metal with OH-groups of the mineral surface [2], although the first hydrolisis reaction has a low equilibrium constant (log [31 = 0.86). In this work, we have studied the sorption of Strontium (II) on natural bentonite, data modelling has been carried out by considering ion-exchange and surface complexation processes. Materials The natural bentonite used in this work was the American Colloid Co. type MX-80 (Wyoming Na- bentonite). Characterisation of this kind of mineral has been reported by Kruse (3), being montmorrillo- nite 67.2%, kaolinite 7.1%, illite 3.7%, feldspars 8.8%, quartz 10% and carbonate 2%. The whole clay content (2gm particle size) is approximately 85%. All chemicals were of analytical grade. The SSSr isotope was purchased fi'om DuPont Scandinavia as Strontium chloride. Tracer solutions were prepared by adding small aliquots of the active stock solution to the solutions used in the experiments. The Sr2§ concentration was 10 6 mol dm -3. The ionic medium was prepared from sodium chloride stock solution in Millipore deionized, triple distilled water (10, 50, 100, 500 and 1000 mM of NaCl). Procedure The experiments were carried out at ambient temperature. Batch experiments were performed by shaking 110 mg of the solid and 20 cm 3 of the aqueous phase. The pH measurements were made with a Metrohm 632 pH meter and a GK 2331 combined glass/reference electrode. The bentonite was pre-treated at 110~ in order to remove the initial water content. Sr concentration was obtained by measuring the y-activity of the solutions after at least one day shaking, to ensure equilibrium. Results Results are shown in Fig. 1. There are two striking features: 1) the pH dependence of the Sr(II) sorption and 2) the inverse relationship between dissolved Na(I) and Sr(II) concentrations. From the distribution values obtained at pH<4, it is possible to estimate the stoichiometry of the Na/SrX exchange reaction. [Sr]r~tld (mot.din-3) I.E-06 ~ rJ D U ~ 9 o @e 13 04. o G| ~o 1.E~07 _ e #++o o + o o I.E-08 ~+ I I r 1 2 4 6 8 10 pH FIG. 1. Experimental results obtained from Sr sorption onto MX-80 bentonite. 502