Benoît Madé

Surface reactivity of α-Al2O3 and mechanisms of phosphate sorption: In situ ATR-FTIR spectroscopy and ζ potential studies

We have investigated the effect of solution parameters on the adsorption of phosphate ions and on charges and structures, i.e., on the nature of species, at the alpha-Al(2)O(3) colloid/solution interface by using the batch method, zeta potential measurements, and in situ ATR-FTIR spectroscopy. The uptake of phosphate decreases with the extent of surface deprotonation (i.e., pH), imparts negative charges to the colloid surface, and induces IEP shifts showing chemical sorption. Use of complementary techniques provides evidence that phosphate is sorbed at low pH (3.3) by a combination of surface reactions of complexation and precipitation, whose relative contributions depend on phosphate loading. Surface complexation includes fast reactions of ligand exchange with single coordinated hydroxyls, and electrostatic attraction of H(2)PO(4)(-) ions at positively charged surface sites. This is supported by experiments at low coverage showing sharp and linear decrease of zeta potential (i.e., surface charge) with amount of phosphate sorbed. At high coverage, zeta potential values are low and independent of phosphate loading. Formation of surface precipitates of Al-phosphate is inferred from the assignment of the ATR-FTIR absorption band at 1137cm(-1), whose intensity increases with phosphate solution content and reaction time, to the P-O-stretching vibration mode for phosphate sorbed at high concentrations on alpha-Al(2)O(3). In situ ATR-FTIR spectroscopy reveals also structural reorganizations of surface hydroxyls with time, due to surface hydration and to surface precipitation continuing over extended periods along alumina dissolution.

Uranium transport around the reactor zone at Bangombé and Okélobondo (Oklo): examples of hydrogeological and geochemical model integration and data evaluation

The sites at Bangombé and Okélobondo (Oklo) in Gabon provide a unique opportunity to study the behaviour of products from natural nuclear reactions in the vicinity of reactor zones which were active around two billion years ago. The Commission of the European Communities initiated the Oklo Natural Analogue Programme. One of the principal aims was to study indications of present time migration of elements from the reactor zones under ambient conditions. The hydrogeological and hydrochemical data from the Oklo sites were modelled in order to better understand the geochemical behaviour of radionuclides in the natural system, by using independent models and by comparing the modelling outcome. Two modelling approaches were used: M3 code (hydrochemical mixing and mass balance model), developed by the Swedish Nuclear Fuel and Waste Management Company (SKB) and HYTEC (reactive transport model) developed by Ecole des Mines de Paris. Two different reactor zones were studied: Bangombé, a shallow site, the reactor being at 11 m depth, and OK84 at Okélobondo, situated at about 450 m depth, more comparable with a real repository location. This allowed the validation of modelling tools in two different sedimentary environments: one shallow, with a more homogeneous layering situated in an area of meteoric alteration, and the other offering the opportunity to study radionuclide migration from the reaction zone over a distance of 450 m through very heterogeneous sedimentary layers. The modeling results indicate that the chemical reactions retarding radionuclide transport are very different at the two sites. At Bangombé, the decomposition of organic material consumes oxygen and at Okélobondo the oxygen is consumed by inorganic reactions resulting, in both cases, in uranium retardation. Both modelling approaches (statistic with M3 code and deterministic with HYTEC code) could describe this situation. The goal of this exercise is to test codes which can help to describe and understand the processes taking place at the sites, validate the models with in situ data, and thus build confidence in the tools used for future site characterization. Ultimately, this allows identifying and selecting processes and parameters that can be used as input into repository performance assessment calculations and modelling exercises.

Mechanisms of uranyl and phosphate (co)sorption: Complexation and precipitation at α-Al2O3 surfaces

This study presents new in situ electrophoretic and ATR-FTIR data on the surface species controlling the cosorption of uranyl and phosphate ions in alpha-Al(2)O(3) suspensions at acidic pH (3.3). It was shown that the uranyl sorption (i) was promoted in the presence of phosphate, (ii) induced significant changes in zeta potential of P-loaded alumina, and (iii) was governed by two mechanisms, surface complexation and surface precipitation, with the predominant species being mainly dependent on phosphate surface coverage. Formation of surface precipitates of uranyl phosphate at high phosphate surface coverage was inferred from the high negative charges imparted to the surface by uranyl and phosphate (co)sorption, and from assignments of IR bands at 1107, 1024, and 971 cm(-1) to P-O-stretching vibrations for phosphate coordinated to uranyl, at the alumina surface. The ATR-FTIR study showed that the precipitates of uranyl phosphate formed at the surface of alpha-Al(2)O(3) for aqueous concentrations of uranyl at trace levels. It also evidenced that formation of surface precipitates of U(VI)-phosphate was occurring along with the transformation of alumina into secondary surface precipitates of Al-phosphate, at very high phosphate concentrations. These findings are relevant to the mechanisms of adsorption of trace uranyl on naturally occurring oxide surfaces, in soils with low pH where cosorption of phosphate and uranyl ions is known to play a crucial role in the long-term retention of U.