In-Situ characterization by small angle x-ray scattering (SAXS) of accumulated layers during ultrafiltration of casein micelles dispersions
Résumé
The main obstacle limiting the performance of ultra and microfiltration processes is the accumulation of matter at the membrane surface (concentration polarization, gelation and/or deposit build-up). Improvement of these processes and possible prediction of the performance depends then on the understanding of mechanisms involved in the formation of deposits in the vicinity of the membranes. In the past, experiments, modeling and numerical simulations have been attempted to accurately explain and quantify the decrease of flux due to the development of concentrated layers at the membrane surface. Some of them give satisfactory results concerning the macroscopic behavior but have limited validity at the microscopic scale. Moreover the lack of pertinent experimental data prevents direct validation of these models.
The focus of this paper is to characterize, in-situ, at nanometer length scale, the structures and concentration profiles of colloids accumulated in the vicinity of the membrane. Casein micelles, which represent 80% of the proteins in cow milk, are a good candidate for this study since ultra- and microfiltration are widely used for the concentration of milk proteins. Small-angle X-ray scattering (SAXS) has been applied on filtration cells to scan the interior of the concentrated layers during its growth and relaxation. Experiments were performed using a frontal filtration cell, equipped with a polyethersulfone membrane (100 kDa) mounted on a motorized sample stage that enabled to translate the cell across the incident beam vertically and horizontally. The spatial and temporal resolution of the measurements enabled access of structure and colloid concentration profiles between 100 μm and 2 mm from the membrane surface. The results obtained with SAXS have been compared to characterization of casein micelles done in dense regime [1].
This study shows that in dead-end filtration mode, the high reduction of permeation flux in the early stage of filtration was associated with an exponential increase of the concentration at the membrane surface. The analysis of concentration profile highlights three main regimes:
In the first concentration regime (C<115g/L) the changes in SAXS intensities are restricted to low-q region (q: scattering vector) where the shoulder of the curve I(q) that corresponds to distances between micelles, shifts toward shorter distances. In this regime, the dispersions are liquid and turbid and casein micelles interact as repelling spheres with free volume between them. The internal structure of casein micelle is not modified and the repulsive interactions between micelles increase resulting in an increase in diffusion. In this regime, concentrated layers are totaly reversible. In a second concentration regime (>115g/L), the micelles strongly interact and progressively get into close contact. At a concentration > 150 g/L the micelles are in close packing and then tend to deform and deswell as concentration increases. In this last regime, casin micelles form an gel, that does not disappear after pressure relaxation and rinse of the filtration cell.