1 Analysis of breakthrough curves of Np(V) in clayey sand packed column in terms of mass transfer kinetics. Christine ANDRE*, Michel SARDIN**, Pierre VITORGE*, Marie-Hélène FAURE* *CEA DCC/DESD/SESD Saclay, 91191 Gif-sur-Yvette cedex, France.CONTACT.pierre.vitorge(at)cea.fr. **LSGC-CNRS, ENSIC(INPL), 1 rue Grandville, BP 451, 54001 NANCY Cedex, FRANCE Abstract The transport properties of Np(V) were studied in a column packed with a mixture of silica sand and natural clay minerals (8% w/w), essentially montmorillonite and kaolinite with goethite (6.5% of clays fraction). The clayey sand packing is 1.6 cm in diameter and 7 cm in length; the pore velocity is 3.6 m/day. Np(V) was injected as a concentration pulse of 8.0 10 -6 mole/l in a solution containing sodium perchlorate and sodium carbonate at a given pH. Np(V) was detected at the outlet and the distribution coefficient, K D ,was measured from the first moment of peaks. The curve log(K D ) vs pH displays a characteristic shape : Log(K D ) firstly decreases from 1.5 at pH 8.2 to 0.5 at pH 9.8, value for which a minimum is observed. Then, when pH increases from 10.0 to 11.8, log(K D ) value increases to 1.3. The theoritical interpretation of equilibrium properties as a function of pH takes into account Na + /H + /NpO 2 + cation exchange on a specific site of clay minerals. Analysis and modelling of peak shapes show that the stronger the retention, the higher the reduced variance of the Np(V) peaks. This behaviour is interpreted in the framework of linear chromatography theory, which leads to attribute the evolution of reduced variance of peaks vs pH to external and/or internal mass transfer limitations. Introducing the characteristic times of a first order kinetics allows one to determine the nature of mass transfer kinetics and the characteristic length of clayey sand aggregates. 1. Introduction The determination of radionuclide transport properties from column experiments is interesting when it addresses the questions : (1) to display coupling between various retention mechanisms such as precipitation, ion exchange, surface complexation ; (2) to measure adsorption equilibrium properties in a chemically perfectly controlled medium (control of the pH, ionic composition, etc.); (3) and/or to determine the mass transfer kinetics in the presence of flow between the mobile phase and the stationary phase containing the solid active for solutes retention. In the first case, the aim is to interpret the consequences of a compositional disturbance at the system inlet on the composition transition number and position at the system outlet and to deduce the nature of the interactions which are responsible for them (Schweich et al., 1993). In the second case, one is interested in the location of a concentration front or peak of one single species at the column outlet in a chemically perfectly controlled system and, if possible, in linearly interacting conditions (Sardin et al., 1991). Numerous works illustrate these two first chromatographic methods (Lefèvre et al., 1993 and 1995, Kohler et al., 1996), but few of them are devoted to the interpretation of the breakthrough curve or peak shape in terms of mass transfer kinetics. Generally, the gaps observed between the theory of local equilibrium transport and experiments are ignored. At most, either one assigns a hydrodynamic dispersion coefficient which is different for the water tracer and the interactive solute, or one uses a first order (linear driving force principle) model which represents the peak or front shape observed at best by means of a parametrical fitting.