Sorption of Caesium on Commercial Magnetite with low Silica Content: Experimental and Modelling
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Sorption of Caesium on Commercial Magnetite with low Silica Content: Experimental and Modelling Miquel Rovira1,2, Joan de Pablo1,2, Ignasi Casas2, Javier Giménez2 and Frederic Clarens2 1
Centre Tecnològic de Manresa, Technical University of Catalonia, Av. Bases Manresa 1, 08240 Manresa, Spain 2 Chemical Engineering Department, Technical University of Catalonia, 08028 Barcelona, Spain ABSTRACT Interaction of Cs on commercial magnetite has been studied under different experimental conditions at high solid/liquid ratios. At pH 8.2 the Freundlich isotherm was able to describe an important range of the experimental data and at high Cs concentration surface precipitation was found likely to occur. Special attention has been given to silica, present in the system as an impurity of magnetite, since it plays an important role on the sorption mechanism in which a ternary complex magnetite-Si-Cs is formed. INTRODUCTION In the context of high level radioactive wastes (HLNW) repository, magnetite is expected to form as a corrosion product of carbon steel, which is one of the candidate material for overpacking the radioactive material [1]. Cs sorption on magnetite, in spite of the simple solution chemistry of this element since has no hydrolysis and complex formation, is remarkably complex and has been studied by several authors, in the frame of radionuclide migration from radioactive wastes [2-7]. From the experiments reported in the literature, the main conclusion obtained is that although Cs does not bind on pure magnetite, a non negligible sorption is observed when working with natural magnetite containing silica as impurity. Studies of Cs-magnetite interaction in the presence of silica added to the system, allowed to justify this behaviour on the basis of a mechanism in which silicates, dissolved from silica, act as a bridge between magnetite and Cs forming ternary complexes [7]. Despite of the role of silicates on Cs sorption has been unambiguously demonstrated, the proposed surface complexation model is still matter of controversy [8, 9]. In Table I, studies found in the literature focused on Cs sorption on magnetite are summarised. The presence of Si in magnetite is important to be studied because this element is usually present in the composition of stainless steel. Table I. Summary of Cs-magnetite sorption works published in the literature Reference
Type of magnetite synthetic natural natural
Silica (% wt) ? 8.4 * 2.4
Surface area 2 -1 (m .g ) ? 33.6 18.3
Ionic strength [Cs]o -1 -3 (mol.kg ) (mol.dm ) 2 0.1 0.1 -9 3 10 →0 -5 4 → 0, 0.01 and 2⋅10 to 8⋅10 5 0.1 -4 -5 1.8 0.01 5 synthetic 4⋅10 4.3⋅10 6 synthetic ? 2 0.01 variable -5 7 (a) as ref. 5 ** 0.01 4⋅10 -5 7 (b) as ref. 5 *** 0.01 4⋅10 * Value calculated assuming that all the Si content in magnetite is in the form of silica -4 -3 ** Aqueous silicates (1⋅12 10 mol.dm ) were added to the system *** Silica (10 % wt with respect to magnetite) was added to the system
1
mass/volume -3 (kg.dm ) 0.01 0.02 -3 2⋅10 8.6⋅10
-4
to 0.012 0.01 -3 6.0⋅10 -3 6.6⋅10
Takin
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