The fractionation of the nitrogen isotopes 14N and 15N in the denitrification process has been studied in laboratory experiments for the step NO2− → N2O. This study has been carried out on natural soils under anaerobic conditions (helium atmosphere). Reduction of N2O is blocked by introduction of a small amount of acetylene in the incubator atmosphere. Variation of experimental conditions of incubation (temperatures, direct or after air-drying incubation of the soil, addition of glucose), greatly modify the reaction rate. 15N is enriched in the substrate during denitrification. The isotopic enrichment factor changes with experimental conditions from about −33 to −11‰. Greatest enrichment is obtained for the lowest reduction rate (low temperature, undried soil). For high rates of denitrification (higher temperature, addition of glucose), the isotopic fractionation decreases. An exponential relation is found between isotopic enrichment, εp/s, and the reaction rate.

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We investigate water infiltration in porous matrices made of bitumen and salts with different solubilities. Dispersion, inside bitumen, of either SrSO4 or MgSO4 at 40% mass fraction was achieved, as a way to model materials used in the context of nuclear waste conditioning. Pulsed field gradient nuclear magnetic resonance (PFG-NMR) and environmental scanning electron microscopy (ESEM) measurements allow us to characterize the evolution of the porous structure as a consequence of water infiltration (due to a leaching phenomenon) for durations up to 1.5 year. PFG-NMR enables performing 1D-imaging of water at different times to monitor its slow seeping inside the material. Profiles observed at the maximum time interval (1.5 year) demonstrate that different leaching behaviors exist depending on whether the salts dispersed inside bitumen are of the soluble or insoluble type. NMR relaxation and diffusion measurements were also taken. Remarkably, when performed in combination with 1D-imaging, such measurements yield information on the surface-to-volume ratio of the water-filled porous network, at different times, as a function of depth. In the case of the matrix-containing insoluble salts, relaxation measurements lead to discrimination between two different water populations differing by their T 2 or T 1 values. These values are described in the framework of surface-driven relaxation. A two-step model of leaching is proposed that globally accounts for the different observations (NMR 1D-imaging, relaxation/diffusion, as well as ESEM). The present work gives insight into the leaching behavior of porous bitumen-salt matrices and could be used as an input for modeling their evolution on longer timescales.