Experimental Study and Modeling of the Radium Sorption onto Goethite
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Experimental Study and Modeling of the Radium Sorption onto Goethite Sylvain Bassot, Caroline Mallet and Denise Stammose Institut de Protection et de Sûreté Nucléaire, DPRE/SERGD/LESTS, BP-6 92265 Fontenay aux Roses Cedex, France Email: [email protected], [email protected], [email protected] ABSTRACT Iron oxyhydroxides play a significant role in radium behavior in uranium mill tailings. Although they are present at trace level (less than 1% in mass), iron oxyhydroxides are responsible of the retention of significant amounts of radium in these residues. A part of these oxyhydroxides is present as amorphous or ill-crystallized forms, the other part corresponds to crystalline iron oxyhydroxides like goethite. In this paper, we present data concerning radium sorption on a well-characterized goethite (α-FeOOH). Batch experiments have been performed and the influence of different parameters such as contact time, pH and electrolyte concentration of the solution has been examined. The sorption equilibrium was obtained after a contact time of 1 hour. In sodium perchlorate medium, the amount of sorbed radium reaches a maximum for pH higher than 8 and no significant influence of sodium concentration was observed. However, the calcium concentration greatly influences the radium sorption: the greater the calcium concentration, the less radium is sorbed. The kinetics of the radium desorption are slower than those of sorption: more than 30% of radium remains sorbed on goethite after 1 week of contact time. Using the characteristics of the goethite surface (total number of sites, intrinsic acidity constants, intrinsic constants of calcium), modelling of the experimental data with inner sphere complexes gave satisfactory results. Two surface species involving radium can be formed (SORa+ and SORaOH). These sorption constants determined allow prediction of the radium sorption onto goethite in the chemical conditions of uranium mill tailings. INTRODUCTION Large amounts of uranium mill tailings are disposed in surface areas near the mines. These residues still contain radioelements (70% of the initial radioactivity) which may be mobilized and migrate through the pile to reach the geosphere and induce an impact on the environment. These radioelements are involved in numerous processes such as precipitation/dissolution and sorption/desorption onto solid phases. Several minerals originating from the ores or formed by weathering processes can act as radioelements sorbents. Among them, iron oxyhydroxides play a significant role in the radium behavior. Although they are present at trace level (less than 1% in mass), iron oxyhydroxides are responsible of the retention of significant amounts of radium in these residues [1]. A part of these oxyhydroxides is present as amorphous or ill-crystallized forms, the other part corresponds to cristalline iron oxyhydroxides, mainly goethite. In order to quantify the mobility of radium in tailings, we have conducted experiments of radium sorption on a well-characterized goethite (α-FeOOH).
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