Maghemite nanoparticles (γ-Fe 2 O 3 NP), and maghemite/silica nanocomposite microspheres (γ-Fe 2 O 3 /SiO 2 MS), have been evaluated as magnetic heterogeneous Fenton catalysts. The catalysts were fully characterized by electronic microscopies, magnetometry, XRD, UV-Vis-NIR spectroscopy, and sorption volumetry. It was found that the two materials differ in size, morphology, porosity and microstructure, although the maghemite nanoparticles are not modified by their encapsulation into the silica. Both catalysts have a strong magnetic susceptibility, but only the MS catalyst can be easily recovered by magnetic settlement. The mineralization and decolorization of aqueous solutions containing a model pollutant in presence of the catalysts were comparatively studied. Three model pollutants differing in their structure and their electrostatic charge were tested. The obtained reaction rates depend on the nature of the pollutant and catalyst. The results indicate the existence of a correlation between the amount of adsorbed pollutant and the decolorization rate. The free NP are usually more active than the MS catalyst, but larger velocity can also be obtained with the MS catalyst when the pollutant is strongly adsorbed on this material. Moderate mineralization rates were observed for both catalysts illustrating the larger stability toward oxidation of the uncoloured organic intermediates resulting from the primary degradation of the model pollutants. Moreover the efficiency and stability of the MS catalyst were established since this material showed an activity for a pollutant during five consecutive tests. This was also confirmed by characterization of the catalyst after these tests.

In this paper, we present a new simple two-dimensional forced Rayleigh-scattering (FRS) experimental setup for determination of the nanoparticle-diffusion coefficient (D M) and the Soret coefficient (S T) in colloids. For this purpose, we give a two-timescale model for the evolutions of temperature and colloid concentration (similar to that given for a former one-dimensional FRS method) and a complete description of the signals diffracted by a squared-lattice grating. Both transport properties in ferrofluids (magnetic colloids) determined with this new setup are in good agreement with those found with samples already studied using the one-dimensional technique. This work is completed by studying new samples. Experimental results we obtained confirm and make clearer the following: (i) the strong Soret effect in ferrofluids has a nanoparticle origin and (ii) furthermore, this origin lies in the immediate surroundings of the nanoparticles (ionic or surfacted coating and dispersion liquid).

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