Study of the radium sorption/desorption on goethite

The oxi-hydroxides, present at trace level in uranium mill tailings, are responsible of about 70% of the 226 radium sorption, half being fixed on crystallized forms. This radionuclide (half time = 1622y), present at high level (50 to 100kBq.kg), can be released in groundwater, involving a possible contamination of the food chain (actual concentration limit = 0.37Bq.l). So, it is very important to point out the mechanisms of the radium sorption/desorption on crystallized oxi-hydroxides as a function of chemical conditions of the system. The radium sorption on synthetic goethite α-FeOOH has been studied as a function of contact time, initial radium activity, pH, sodium and calcium concentrations. The results show that, after one hour of contact time (necessary to reach equilibrium), the radium sorption increases widely in a pH range 6-7. The increase of Na concentration is without influence on the radium sorption, indicating the low interactions between sodium and surface sites. At the opposite, the presence of calcium in solution decreases widely the radium sorption, that indicates a competition between calcium and radium for the same kind of sorption sites of the oxi-hydroxide surface. The percentage of radium desorbed increases widely with time, from 1 to 120h and becomes constant at a time higher than 120h. This long equilibrium time for desorption in comparison with sorption one can be explain by a local evolution of the sorption sites of the solid, which become less accessible for the solution in contact. INTRODUCTION The main minerals contained in uranium mill tailings from acid treatment are clastic minerals corresponding to the nature of uranium rocks and to its paragenesis: quartz, sodic and potassic feldspars, muscovite, biotite, smectites, calcite. After deposit of the acid uranium mill tailings, different geochimical phenomena occur, linked to diagenetical evolution of the tailings. This evolution involve different mechanisms like mineral dissolution (pyrite dissolution during system oxidation), development of ferrous smectite or formation of authigenous minerals, which appear as cement or coating : gypsum, barytine, jarosite, Fe and Mn oxi-hydroxides, hexavalent uranium minerals (uranium phosphate, uranium vanadate, lead and uranium silicates) [1]. Sequential extractions realized on Ecarpiere tailings in Vendée [2] (acid treatment) show that Fe anf Mn oxihydroxides are responsible of about 70% of 226 radium sorption, half being fixed on crystallized forms. The rest of the 226 radium is fixed on gypsum, barytine and silicates phases (clays, feldspars, micas, quartz). In uranium mill tailings from acid treatment, the ferric oxi-hydroxides, although present at trace level, predominate in tailings in comparison with others metallic oxi-hydroxides because of ferric salts addition during acid treatment of the uranium ores. Objectives of these studies are to determine mechanisms of 226 radium sorption/desorption on goethite and the model to use for simulate this sorption and to evaluate the 226 radium behaviour in uranium mill tailings. Numerous studies have been realized on amorphous and cristallized ferric oxi-hydroxides, especially on uranium VI sorption [3, 4, 5, 6] and on heavy metal sorption [7, 8]. These studies describe the surface complexation site-binding model used to simulate uranium sorption on oxi-hydroxide and show the high influence of uranium complexation in solution on sorption capacity of oxi-hydroxides. Some studies have been realized on 226 radium sorption on manganese oxides [9, 10, 11], on different hydroxides (Fe, Ti, Zr, Mn) [12] and on different sediments [13]. But these data don’t allow to modelling radium behaviour in presence of oxides. In this paper, the radium sorption/desorption on synthetic goethite has been studied as a function of different parameters (contact time, equilibrium radium activities, pH, sodium and calcium concentrations).