In-situ structural characterization by SAXS and flow properties of colloidal suspensions during cross-flow ultrafiltration
Résumé
Understanding the mechanisms that control the filtration of complex colloidal suspensions is a major challenge in the development of membrane-based processes in industry. One of the main limiting factors for the development of these processes lies in the formation of a polarization layer and/or deposit near the separating membrane. Elucidating the structure of the first layer of deposited colloids during the first steps of filtration, constitutes a considerable technological jump in the understanding of the physical mechanisms implied in the formation of these deposits. It is also important to identify the effects of the physicochemical properties, the rheological behavior and the hydrodynamic fields on the organisation of the colloids during filtration. The focus of this work is the in-situ characterization of the induced structures and flow properties of the polarization layers of colloidal suspensions when simultaneously subjected to a transmembrane pressure and tangential flow over the membrane. To fulfill this challenge a new tangential ultrafiltration SAXS cell has been developed at the "Laboratoire de Rhéologie" Grenoble, France. This cell allowed combining small angle x-ray scattering (SAXS) at the European Synchrotron Radiation Facility (ID02 beamline) with membrane separation processes. Systems studied are anisotropic colloidal aqueous dispersions composed of sepiolite fibers 1 micrometer long and 0.01 micrometers in diameter, a natural non-swelling fibrous clay. The initial concentration of the filtered suspensions was in the semi-dilute domain for which the suspensions exhibit a shear thinning rheological behavior. During filtration, a transmembrane pressure was applied and successive tangential flows were imposed. The permeation flux was continuously measured and simultaneously the x-ray beam (40 x 250 micrometer) crossed the lateral dimension of the cell to probe the structure of the deposit with time at different heights above the membrane. As already described in previous studies concerning frontal filtration mode [1-2], a calibration curve relating the absolute scattering intensity to the particles concentration has allowed to deduce the concentration profiles in the deposits at distances to the membrane above 300 micrometers. During these experiments the high level of concentration reached in the deposit up to 50 times the initial concentration, and the highly anisotropic structure formed has been identified as one of the main mechanisms controlling the filtration flux decrease. The results prove the possibility to obtain pertinent structural information in the vicinity of membrane surfaces during ultrafiltration. It offers essential experimental data necessary for improvement in theoretical and numerical modeling of the filtration process. [1] F. Pignon, A. Alemdar, A. Magnin, and T. Narayanan, Langmuir, 19, 8638 (2003). [2] F. Pignon, G. Belina, T. Narayanan, X. Paubel, A. Magnin and G. Gésan-Guiziou, The Journal of Chemical Physics, 121(16), 8138 (2004).