Coupling of SEM-EDX and FTIR-ATR to (quantitatively) investigate organic fouling on porous organic composite membranes M Rabiller-Baudry *,1 , F. Gouttefangeas 2 , J. Le Lannic 2 and P. Rabiller 3 1 Université Rennes 1, UMR 6226 « Institut des Sciences Chimiques de Rennes » CNRS, 263 avenue du Général Leclerc, CS 74205, case 1011, 35042 Rennes cedex, France * corresponding author : murielle.rabiller-baudry@univ-rennes1.fr 2 Université Rennes 1, CMEBA, 263 avenue du Général Leclerc, CS 74205, batiment 10A, 35042 Rennes cedex, France 3 Université Rennes 1, UMR 6251 « Institut de Physique de Rennes» CNRS, 263 avenue du Général Leclerc, CS 74205, 35042 Rennes cedex, France Membrane processes are widely used at industrial scale for separation/concentration purpose. Among filtered fluids are tape water and food fluids, both containing biological components. For this later case the most used membranes are composite organic materials made of three layers that are consecutively crossed by the filtered fluid. The main drawback of membrane processes is the built-up , in the course of the filtration, of a more or less thick fouling layer that can be located both in/on the membrane. As both fouling components and membrane are organic compounds, a specific approach is proposed in this chapter to be able to distinguish carbon, nitrogen and oxygen issued from the membrane and the fouling layer, respectively, by using a post-treatment of raw SEM-EDX results. The methodology is detailed in the case of ultrafiltration membrane made of PES fouled with skim milk , a complex media leading to membrane fouling mainly made of proteins. A complementary analysis is achieved using Fourier Transformed infrared in the total attenuated reflection mode (FTIR-ATR) and the use of a similar kind of raw signal post-treatment than that of SEM-EDX one is also explained. Keywords SEM-EDX, FTIR-ATR, industrial organic membrane, organic fouling 1. About membrane processes Membrane based separation techniques which are efficient at molecular level are widely used at industrial scale for concentration or selective extraction of target component(s) from a liquid medium. The main applications concern the drinking water production and effluent treatment as well as treatment of food fluid, and particularly dairy fluids. The separation of the fluid components is achieved trough a porous medium, called membrane, thanks to a driving force obtained by a pressure gradient applied across the membrane material. The initial feed is then separated in two fractions: a retained one, called retentate, and a transport fraction through the porous medium, called permeate (Figure 1a). Several complex mechanisms are required to explain the solute transfer toward the permeate side; among them are molecular sieving and electrostatic interactions between the membrane and the charged solute to be filtered. According to the rejected solute size different techniques are commonly distinguished: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) [1, 2]. MF retains suspension of the order of a micron as well as bacteria. UF is applied for retention of viruses and soluble polymers as proteins and polysaccharides. NF is able to retain small organic solutes within the range of molecular weight from 200 g.mol -1 to 300 g.mol -1 . RO is a well- known desalination technique able to retain small inorganic ions as Na + , Cl - and so on. The most commonly used membranes are composite organic materials generally made of at least three layers which are consecutively crossed during filtration by the filtered fluid transmitted toward the permeate side (Figure 1). A sub- micron layer is directly in contact with the feed fluid to be filtered and is commonly called active or selective layer. It can be made of several polymer types (Figure 2) such as polyethersulfone (PES) with or without polyvinyl pyrolidone (PVP) added in order to increase the hydrophilic character, polysulfone (PSU), polyvinylidenedifluoride (PVDF), aromatic polyamides (PA), cellulose based polymers and so on. The intermediate layer is often made of polysulfone (PSU) or sometimes PES. Finally a macro-porous support often made of unwoven polyester gives its mechanical resistance to the whole membrane. Because of the inherent hydrophobic character of the membrane polymers, it easily induces attractive interactions with all types of organic molecules contained in the filtered fluid, the first consequence of it being logically the build up of an organic adsorbed layer on/in the membrane. Thanks to the applied pressure, the formation of the adsorbed layer is generally quickly followed by the build up of a thicker fouling layer, the cohesion of which is due to interactions developed between solutes. Moreover, inorganic components having perhaps less affinity toward the membrane material can be entrapped in the organic layer and thus participate to the global fouling. Systematic fouling is observed when filtering biological and food fluids with polymeric membranes. This is the main drawback of membrane processes and it is well admitted that understanding of membrane fouling by proteins remains a challenge [3]. Current Microscopy Contributions to Advances in Science and Technology (A. 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