Spatial and temporal in-situ evolution of concentration profile probed by SAXS during ultrafiltration of casein micelles

International audienceUltrafiltration is a wide spread pressure driven membrane filtration process. In spite of its intensive industrial use, mechanisms involved in the reduction of permeation flux during operations are not completely interpreted. Numerous theories and numerical simulations have been developed to explain and quantify flux decrease accurately. A lot of them give satisfactory results but are not validated experimentally. Some precedent works [1] have tried to characterize the polarization layer growth and the structural organization of deposit during membrane filtration. In order to better understand mechanisms involved in ultrafiltration process, we suggest considering the phenomena of colloid's accumulation itself with an in-situ technique. Characteristics such as concentration, size, structure and organization of particles can then be analyzed during their deposition. With the aim to combine sub-micron and macroscopic measurements, specific ultrafiltration cells have been developed at the “Laboratoire de Rhéologie” [2, 3]. This permitted to follow the polarization concentration layer growth in-situ with a 70 µm accuracy. Several colloids concentration profiles between 280 µm and 1 mm from membrane surface were obtained over the five first hours of casein micelles' frontal ultrafiltration (Fig. 1). Casein micelles suspensions with initial concentration C0 (corresponding to casein micelles content of skimmed milk) have been filtered. The use of small angles x-ray scattering (SAXS) [4] allowed accessing to structural information at length scales going from 1 to 420 nm inside of accumulated matter. The innovating use of SAXS has been correlated with permeation flux measurements. For the first time, in frontal mode filtration of casein micelles suspensions, time evolution of concentration profile has been observed (Fig. 2). This evolution correlated with the corresponding structural modifications and with permeation flux, gives precious information on mechanisms responsible for output losses. The high reduction of permeation flux in the early stage of filtration is associated to an exponential increase of the concentration during time followed by a slowing down of this rise above a critical concentration of about Cc = 157 g of casein per liter. The corresponding scattering intensities exhibit an increase of the interactions between the casein micelles and/or a modification of their mean size. Fig 2. (i) Evolution of the polarization concentration profile during frontal filtration of casein micelles suspensions at initial concentration C0 = 29 g/L and pH = 7.2. (ii) Time dependence of concentration at different distance from membrane surface. Under critical conditions of cross flow filtration [5], scattering measurements were made. It demonstrates that the polarization layer involved during tangential ultrafiltration of casein micelles suspensions is at least smaller than 280 µm even when permeation flux is highly reduced. Keywords: Casein micelles suspensions; Ultrafiltration; Small angles x-ray scattering; Concentration polarization; Deposit, Structure. References [1] J.C. Chen, Q. Li, M. Elimelech, Adv. Colloid Interf. Sci., 107, 83-108 (2004). [2] F. Pignon, A. Alemdar, A. Magnin, T. Narayanan, Langmuir, 19, 8638-8645 (2003). [3] F. Pignon, G. Belina, T. Narayanan, X. Paubel, A. Magnin, G. Gésan-Guiziou, J. Chem. Phys., 121(16), 8138-8146 (2004). [4] T. Narayanan, O. Diat, P. Bösecke, (2001), Nucl. Instrum. Phys. Res. Meth. A, 467-468, 1005-1009 (2001). [5] G. Gésan-Guiziou, E. Boyaval, G. Daufin, J. Membrane Sci. 158, 211-222 (1999)