Addition of Al 2 O 3 nanoparticles to bentonite: effects on surface charge and Cd sorption properties
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Addition of Al2O3 nanoparticles to bentonite: effects on surface charge and Cd sorption properties Natalia Mayordomo, Ursula Alonso, Tiziana Missana, Ana Benedicto, Miguel García-Gutiérrez CIEMAT, Department of Environment, Avenida Complutense 40 28040 Madrid (SPAIN) ABSTRACT Compacted bentonite barrier in radioactive waste repositories is expected to prevent radionuclide migration, due to its high sorption capability for many radionuclides. This study analyses whether the addition of Al2O3 nanoparticles (NPs) enhances the sorption properties of bentonite. The study was carried out with 109Cd, highly pollutant heavy metal and divalent fission product. Sorption experiments were conducted in NaClO4 at different ionic strengths (5·10-4 to 10-1 M) and pH (2 to 10), using mixtures of sodium homoionised bentonite and Al2O3 in different proportions. It has been probed that addition of Al2O3 NPs to bentonite enhances Cd sorption at pH higher than 6. The effect of Al2O3 NPs addition on the surface properties of bentonite colloids was also analyzed by measuring particle size and surface charge in all studied systems. INTRODUCTION Bentonite clay, mainly composed by smectite, is a buffer and backfill material considered in nuclear waste repositories, owing to its swelling properties and high sorption capability for many radionuclides. When hydrated, smectite particles take on a permanent negative surface charge that promotes retention of positively charged ions, while radionuclides whose dominant aqueous species were anionic can be mobile. The charge on the edge of bentonite particles is positive or negative, depending on pH conditions. The aim of this study is to evaluate whether the addition of Al2O3 nanoparticles (NPs) enhances the sorption properties of bentonite. The addition of nanoparticles (diameters < 50 nm) positively charged may favour the sorption of negatively charged species or modify sorption of cations. The enhancement of bentonite sorption properties has been tested with magnetic Fe nanoparticles, promoting contaminant reduction[1, 2], but no previous study with Al2O3 was reported. FEBEX clay, a Spanish Ca-Mg bentonite, was selected for the study [3]. The FEBEX clay was homoionised in Na (Na-bentonite) and mixed at different weight proportions with Al2O3 NPs (nominal size < 50 nm), in NaClO4 at different ionic strengths. The surface properties (size and surface charge) of the independent Na-bentonite clay and Al2O3 NPs suspensions and of the bentonite /Al2O3 mixtures were analysed. In the binary systems interaction of particles with very different surface charge properties may promote changes in charge or particle coagulation that affects contaminant retention, as previously observed for hematite, magnetite and TiO2 /clay mixtures [4, 5]. The sorption properties of the Na-bentonite /Al2O3 mixtures were evaluated with 109 Cd(II). Cadmium is a heavy metal of great environmental concern that has a low fission product yield, whose dominant aqueous species are positive. Previous studies on Cd sorption
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