Magnetite Sorption Capacity for Strontium as a Function of pH
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Magnetite Sorption Capacity for Strontium as a Function of pH Joan de Pablo 1,2, Miquel Rovira 1,2, Javier Giménez 1,2, Ignasi Casas1 and Frederic Clarens2 Dept. Chemical Engineering, Universitat Politècnica de Catalunya (UPC), Avda. Diagonal 647, 08028 Barcelona, Spain 2 CTM Centre Tecnològic, Avda. Bases Manresa 1, 08240 Manresa, Spain 1
ABSTRACT The ubiquity of iron oxide minerals and their ability to retain metals on their surface can represent an important retardation factor to the mobility of radionuclides. In a deep repository for the spent nuclear fuel, the intrusion of the groundwater might produce the anoxic corrosion of the iron, with magnetite as one of the end-products. In this study, as expected considering the strontium speciation in solution, strontium is sorbed onto magnetite at alkaline pH values while at acidic pH the sorption is negligible. Magnetite is able to sorb more than the 50% of the strontium from a 8·10-6 mol·dm-3 solution at the pH range representative of most groundwater (79). A surface complexation model has been applied to the experimental data, allowing to explain the results using the Diffuse Layer Model (DLM) and considering the formation of the innersphere complex >FeOHSr2+ (with a calculated logK=2.7±0.3). Considering these data, the magnetite capacity to retain strontium and other radionuclides is discussed. INTRODUCTION In the high-level radioactive waste repository concept, spent nuclear fuel is designed to be encapsulated in steel canisters which represent the first physical barrier to radionuclide migration. Besides constituting a physical barrier for an eventual groundwater intrusion to the repository, the steel canister provides an effective chemical trap for the release of several radionuclides present in the fuel. Magnetite (Fe3O4) has been identified as a corrosion product of carbon steel canisters intended to be used as containers in a HLW final deep repository [1]. For this reason, the study of radionuclide retention role of magnetite is an issue of crucial importance for the performance assessment of spent nuclear fuel iron-containing canisters. In previous works [2-4] we have studied the magnetite sorption capacity for different radionuclides while in this work we present the results obtained with the sorption of strontium. In this work, the study of the interaction between Sr(II) and commercial magnetite is reported. In the literature, several works have been focused on the study of Sr(II) interaction with hydrous ferric oxides. Todorovic et al. [5] studied the sorption of Sr(II) at trace level concentrations onto magnetite and hematite. For magnetite, sorption extent increased with pH, being almost negligible at pH 4, and for hematite, sorption was found to be very small at the pH values studied (4 and 9.4). Ebner et al. [6] determined sorption isotherms for Sr onto magnetite as well as in magnetite (80%)-silica composite at the pH range 5-9, and they found that solid saturation increased with pH and with the presence of silica. Sorption processes are incorporat
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