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International audience; Literature on the design of efficient nonfouling membranes by in-situ modification is poor, which can be explained by the difficulty to control membrane formation mechanisms when a third material is added to the casting solution, or by the lack of stability of matrix polymers with surface-modifiers. We present polyvinylidene fluoride membranes formed by vapor-induced phase separation and modified with a tri-block copolymer of poly(styrene) and poly(ethylene glycol) methacrylate moieties (PEGMA124-b-PS54-b-PEGMA124). After characterizing the copolymer, we move onto membrane formation mechanisms. Membrane formation is well controlled and leads to structure close to bi-continuous. Considering the formulation chosen, PVDF/PEGMA124-b-PS54-b-PEGMA124 solutions are less viscous and more hydrophilic than virgin PVDF solutions. Both effects promote non-solvent transfer, thus decreasing the chances for crystallization. Hydrophilic capability of membranes is increased from about 59 mg/cm3 to 650 mg/cm3, leading to a severe drop of non-specific protein adsorption, up to 85–90%, also depending on its nature. Biofouling at the micro-scale by modified Escherichia coli and Streptococcus mutans is almost totally inhibited. Finally, biofouling is importantly reduced in dynamic conditions, as measured from the water flux recovery ratio of 69.4%, after 3 water-BSA filtration cycles, much higher than with a commercial hydrophilic PVDF membrane (47.3%). These membranes hold promise as novel materials for water-treatment or blood filtration.

Hollow fiber membrane contactors with ionic liquids are promising alternatives to traditional spray towers and amines for carbon dioxide absorption. Ionic liquids have emerged as new alternative solvents because of their zero emission features compared with amines. The aim of this work was to compare fibers based on PVDF and different additives, as well as fibers including two different ionic liquids. On the one hand, 1-ethyl-3-methylimidazolium ethyl sulfate [emim][EtSO4] presents physical absorption, and on the other hand, 1-ethyl-3-methylimidazolium acetate [emim][Ac] presents chemical absorption. To compare the fibers under study, the thickness of the composite fiber was examined using scanning electron microscopy (SEM). The mechanical properties and the bubble point were also evaluated. Permeability tests were conducted, and the gas permeation of the composite hollow fibers was measured using pure CO2. Laboratory-made stainless steel modules were used for the tests. All of the above tests were performed with the fibers in both wet and dry conditions. It was determined that the fibers with the ionic liquid immobilized would be promising for CO2 capture because the CO2 permeance significantly increased. Namely, D+[emim][EtSO4] achieved a 43% increase compared with the fibers without the addition of the ionic liquid, resulting in a CO2 permeance value of 57040NL/(hm2bar), which is higher than the values reported in the literature for PVDF. Moreover, the overall mass transfer coefficient for CO2 capture using the D+[emim][Ac] fibers also presented highly competitive values. This research has been funded by the Spanish Ministry Economy and Competitiveness (Projects ENE2010-14828 and CTQ2013-48280-C3-1-R).

International audience; Previous experimental work has shown that the transfer of organic solutes through ion-exchange membranes depends on the membrane counter-ion and that this dependence is probably linked to the interactions taking place at the nanoscale inside the membrane matrix. In this paper, a computational approach is carried out, combining quantum mechanics and molecular mechanics to determine the interactions occurring at the nanoscale, taking a cation exchange membrane as example. Building blocks are first accurately studied at high level of quantum theory, before being merged in macromolecular models. The computed interactions are then compared to the experimental values of the solute flux in order to point out the nanoscale mechanisms governing the solute transfer. The computed glucose-polymer fragment interactions, related to the sugar solubility inside the membrane, are found to be almost independent from the membrane counter ion. On the contrary, significant variations of the chain-chain interaction, i.e. the interaction energies per trapped water molecule or hydrogen bonding wire connecting the polymer fragments, were observed according to the cation. Moreover, a correlation was pointed out with the experimental sugar fluxes obtained with 3 different sugars. Increasing chain-chain interactions inside the membrane was found to give decreasing sugar flux. Then this work shows that the cohesion energy between the polymer fragments fixes the dependence of the sugar flux versus the membrane counter-ion. The crucial role of the water molecules coordinating the cations is also highlighted.

International audience; This work reports on the glow dielectric barrier discharge (GDBD) plasma-induced surface grafting of poly(vinylidene fluoride) (PVDF) membranes with mixed-charge copolymers of [2-(methacryloyloxy)ethyl] trimethylammonium (TMA) and sulfopropyl methacrylate (SA). The aim is to investigate the antifouling properties and the hemocompatibility of this system. We first characterize the physico-chemical properties of the membranes. With SA alone in the coating solution, efficient grafting cannot be achieved as monomer is blown away during grafting. Membranes grafted with a mixture of SA and TMA, or TMA alone do not meet this problem and grafting density ranged between 0.29 and 0.41 mg/cm2. Bovine-serum-albumin and lysozyme adsorption tests (70% reduction) and Escherichia coli attachment test (annihilation of adhesion) unveil that pseudo-zwitterionic PVDF membranes are very efficient to reduce biofouling in static condition. Different fouling resistance behaviors are observed in dynamic conditions. Permeability of virgin membranes progressively decreases over the cycles, arising from a gradual pore blockage and irreversible fouling. All potential adsorption sites of pseudo-zwitterionic membrane and membrane with positive charge-bias are fouled after the first cycle, and flux recovery is maximal in the following cycles. This behavior is ascribed to the lack of homogeneity of the surface grafting. Finally, pseudo-zwitterionic membranes are hemocompatible (resistance to blood cells, low hemolysis activity). Provided a better tuning of surface uniformity, the method and system presented in this work are a promising approach to the new generation of antifouling mixed-charge membranes for water treatment or blood contacting devices.

International audience; This study continues from a previous work on the impact of packing density on the fluid flow distribution in a hollow fiber module [1]. A numerical model was developed to simulate the growth of a particle cake along the surface of a hollow fiber membrane and the subsequent fluid flow during a microfiltration operation. The model accounts for the continuous change in porous domain (cake and porous wall) geometry and permeability as long as filtration occurs. The effect module packing density has upon cake growth is carefully analyzed both for inside/out (I/O) and outside/in (O/I) filtration modes. The results exhibit significant differences in the time variations of cake spatial distribution along the fiber as a function of packing density for both filtration modes. Then a confrontation between forward filtration and backward filtration velocities offers some conclusion on the effect of packing density on the backwash efficiency. This in turn underlines the importance of design parameters in the filtration performance of a hollow fiber module.

The aim of this work is to develop a simulation capability applicable to dead-end filtration of colloidal dispersions in order to investigate the effect of process conditions, such as membrane configuration and operating parameters, on filtration efficiency through the analysis of the appearance of a deposit on the membrane. To reach this goal, a model describing the transport behaviour of a concentrated colloidal dispersion is implemented in a commercial CFD code (ANSYS-CFX). The collective diffusion induced by inter-particle interactions is accounted for from knowledge of the variation of the osmotic pressure with the particle volume fraction. Coupled with a transient, two dimensional hydrodynamic solution, such a model allows description of the mass transport properties both in the dispersed (concentration polarization) and the condensed (deposit) forms of accumulation. Two-dimensional concentration profiles along the membrane are obtained. Simulations are used to understand the role of operating parameters and membrane characteristics on the appearance of a deposit at the membrane surface. This formation is controlled by the hollow fibre configuration, where there are zones working in both cross-flow and dead-end mode due to the particular hydrodynamic conditions.

This paper presents experiments showing the existence of a critical filtered volume (CFV) when operating colloid dead-end filtration. The CFV is here defined as the filtered volume below which there is no irreversible (with respect to a break in the filtration) fouling on the membrane surface: it has thus the same meaning as cross-flow critical flux but applied to a dead-end process. The existence of the CFV is demonstrated when filtering stable latex or clay suspensions in constant-flux filtration experiments with alternating rinses: in contradiction to the current view, an irreversible deposit is not formed as soon as dead-end filtration begins. This critical filtered volume is shown to be dependent on the suspension stability and to be fully linked to the permeate flux: for permeate fluxes of 80 and 110 l h−1 m−2 the CFV is, respectively, 82 and 65 l m−2 for latex particles. Analyses of results are made by depicting the transition between concentration polarisation and deposit formation considering a critical osmotic pressure, which appears to be a characteristic of the fouling potential of a suspension. The results are discussed in the light of how this concept could lead to an interesting way to control and develop a strategy to operate filtration in dead-end mode.

International audience; The aim of this work was to investigate NF as a purification step, i.e. sugar removal, in the production process of lactic acid from sodium lactate fermentation broth. Experiments were carried out with the Desal 5 DK membrane and solutions of increasing complexity, i.e. single-solute solution of sodium lactate and glucose and mixed-solute solutions containing both solutes. Concentrations close to those found in a fermentation broth were chosen. Experimental results were used to get the variations of the intrinsic retention versus the filtration flux in order to achieve comparisons without interference of concentration polarization. Quite distinct retentions were obtained for glucose and sodium lactate in single-solute solutions, so that the purification was expected to be feasible. However, it was pointed out that glucose retention is significantly lower in mixed-solute solutions, i.e. as far as lactate is present. This decrease is such that retentions of both solutes become comparable so that any purification is unachievable. Experiments at different salt concentrations and with a salt of different nature (mineral salt-NaCl) were also performed. Again, it was found that the presence of NaCl tends to decrease glucose retention. Moreover, the phenomenon is shown to be related to salt concentration in both cases, i.e. with sodium lactate and NaCl. Some 1 possible explanations of this effect are provided in this paper. Further investigations are still in progress to improve the knowledge of the involved mechanisms.

International audience; Lack of knowledge on their hemocompatibility limit the use of PVDF membranes in biomedical applications. Herein, we investigated the in situ modification of PVDF membranes by a PEGylated copolymer (PS60-b-PEGMA108) using vapor-induced phase separation (VIPS) process. Efforts were first oriented toward the characterization of the effect of copolymer on membrane formation, membranes physical properties and membranes surface chemistry. Then, biofouling was investigated before moving onto the hemocompatibility of membranes. Membranes structure tended to evolve from nodular to bi-continuous with PS60-b-PEGMA108 content, evidencing a change of dominating phenomena during phase inversion (crystallization-gelling vs. non-crystallization gelling), associated to a change ofprevailing crystalline polymorph (β-polymorph vs. α-polymorph). Furthermore, the hydration of membranes was importantly enhanced, affecting nano-biofouling: bovine serum albumin, lysozyme and fibrinogen adsorption were drastically reduced, despite rough surfaces, highlighting the efficiency of the copolymer. Bacterial attachment tests revealed that macro-biofouling was inhibited as well. Results of erythrocytes, leukocytes, and thrombocytes adhesion indicated that membranes prepared from a casting solution containing 5 wt% copolymer are highly hemocompatible, result supported by low hemolysis ratio (1%) and delay of plasma clotting time. Overall, this study unveils that in situ modification coupled to the VIPS method can readily lead to hemocompatible PVDF membranes.

International audience; In drinking water production plants, the use of oxidants such as sodium hypochlorite during in-place cleanings may impair the membrane integrity and radically impact the ultrafiltration process efficiency, leading to potential contamination of the permeate water with pathogens. This study investigates the effects of hypochlorite exposure on the properties of a commercially available UF hollow fiber. Mechanical performances and water permeability appeared to be greatly affected by the contact with hypochlorite. Monitoring the molecular changes by X-ray photoelectron spectroscopy (XPS), attenuated total reflectance infrared spectroscopy (ATR-IR), size exclusion chromatography (SEC) and VITA-mode atomic force microscopy (VITA-AFM) revealed high stability of the main polymer constituting the membrane (i.e. polyethersulfone (PES)) and very high reactivity of the additive (i.e. poly(N-vinyl pyrrolidone) (PVP)) towards immersion in aqueous sodium hypochlorite solution with maximal reaction rate for neutral to slightly basic pH. Correlation of those results unexpectedly leads to the conclusion that the overall membrane properties changes are governed by the additive fate.

International audience; Recent studies have shown that unexpected performances can be obtained with NF membranes, when applied to the treatment of solutions containing significant amount of electrolyte. This study deals with the investigation of the mechanisms governing the mass transfer of neutral species through NF membranes with a focus on the role of ions. More precisely, it consists to determine the apparent molar volume of saccharides, which characterizes the hydration state of solutes in presence of electrolyte, and to assess the relationship between these parameters and those characterizing the mass transfer. The transfer of saccharides of increasing molecular weights (xylose, glucose and sucrose) through a NF membrane is studied in a diffusion regime. Different electrolytes (NaCl, Na₂SO₄, CaCl₂, MgCl₂) are chosen with respect to their hydration level. A specific procedure was developed to dissociate the flux variation due to the modifications of the solute properties and of the membrane material induced by the electrolyte. The results show that the mass transfer modification is mainly due to the influence of the electrolyte on the solute properties. Then, the mass transfer parameters are put in parallel with the apparent molar volume of saccharides measured in solutions of different ionic compositions. For a given electrolyte, Na₂SO₄, a good quantitative relationship is obtained regardless of saccharide nature, concerning the influence of the electrolyte concentration. This result confirms that the saccharide transfer increase can be due to its dehydration in presence of electrolyte. However, from these results, it is not possible to establish a clear relationship regardless of the electrolyte nature.

The cross-flow microfiltration applied to wine filtration has become a legitimate alternative to conventional filtration processes. However, membrane fouling which affects the operating costs and the plant maintenance, limits the widespread application of this technique. The aim of this review is to provide a better understanding of the development of the cross-flow microfiltration in wine industry, as well as the complexity of wine composition and its consequences on membrane fouling. This review covers also the impact of the operating conditions and the membrane characteristics on fouling mechanisms. Strategies to limit fouling as well as the latest innovations and commercial proposal are discussed in this paper.

In this study, a cross-linked aromatic polyamide based reverse osmosis membrane was exposed to variable sodium hypochlorite ageing conditions (free chlorine concentration, solution pH) and the resulting evolutions of membrane surface chemical and structural properties were monitored. Elemental and surface chemical analysis performed using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR), showed that chlorine is essentially incorporated on the polyamide layer of a commercially available composite RO membrane, when soaked in chlorine baths, presumably through a two step electrophilic substitution reaction governed by the concentration of hypochlorous acid (HOCl), at pH values above 5. Deconvolution of the FTIR vibrational amide I band experimentally confirmed previous assumptions stated in the literature regarding the weakening of polyamide intermolecular hydrogen bond interactions with the incorporation of chlorine. An increase in the fraction of non associated Cdouble bond; length as m-dashO groups (1680 cm−1) and a decrease of hydrogen bonded Cdouble bond; length as m-dashO groups (1660 cm−1) were observed with an increase in the concentration of the free chlorine active specie. The relative evolution of pure water permeability was assessed during lab-scale filtration of MilliQ water of a membrane before and after exposure to chlorine. Filtration results indicate polyamide conformational order changes, associated with the weakening of polyamide intermolecular H bonds, as observed with the increase in the packing propensity of the membrane, dominant for HOCl doses above 400 ppm h. In addition, water–sodium chloride selectivity capabilities permanently decreased above this HOCl concentration threshold, further suggesting polyamide chain mobility. However, under controlled exposure conditions, i.e., HOCl concentration, operating conditions (applied pressure or permeation flux) can be improved while maintaining similar RO membrane separation performance.

The study shows the respective roles of skin and support of an ultrafiltration membrane in the retention mechanisms of bacteria (Escherichia coli). For this, pinholes defects of 5–200 μm in diameter were performed through ultrafiltration polymeric membranes and their impact was assessed on bacterial retention in a stirred cell when the transmembrane pressure is set at 0.5 bars. Various techniques have been used to make the defects such as a microhardness tester or femtosecond lasers. As long as the selective skin is not altered through its whole thickness, the membrane keeps a retention efficiency equivalent to the one of an uncompromised membrane. The retention by the macroporous support is also investigated. In case of membrane with defects of cylindrical geometry, experimental results are compared to calculated data obtained with a pore flow model, and the validity of this model is discussed.

Using the extension of the concept of universal calibration parameter, yielding a relation between the hydrodynamic volume of molecules and the elution volume in size exclusion chromatography (SEC), to retention coefficients in ultrafiltration (UF), we propose a direct calibration of UF membranes against chromatography columns. Plotting the retention coefficient by one given UF membrane of a series of probe molecules versus their elution volume in SEC chromatography provides a calibration curve for this membrane. For a wide range of retentions, such calibration can be directly used to predict the retention of any molecule: one only needs to measure its exclusion volume by the SEC column, and read the retention by the calibrated membrane on the calibration curves. The method has been tested with dextran and PEG for the calibration, and milk proteins as test molecules, for three different membranes. The predicted values of the retention are in rather good agreement with those experimentally measured in a UF cell.

This paper discusses the consequences of possible distribution of critical flux (DCF) in cross-flow filtration. These distributions are described here by a normal function with a mean critical flux and its standard deviation. The DCF model allows the description, through an analytical relationship, of the variation in steady state permeate flux with trans-membrane pressure. Both strong and weak forms of critical flux, which can be observed on a membrane operating in cross-flow mode, are depicted. A simple graphical method to determine the mean critical flux and its standard deviation from experimental results is derived from the theoretical model. The theoretical trends are compared to experimental data and show good agreement for cross-flow filtration of latex and BSA suspensions. The distribution function parameters obtained by fitting the DCF model to experiments are compared to critical flux measured via a pressure step method. We thus propose a tool to analyse filtration results and to determine new global parameters for critical conditions (mean value and its standard deviation), which appears to be a good way to account for fouling complexity.

International audience; In electrophoretic membrane contactors, a porous membrane is used to put into contact two flowing liquids between which an electrically driven mass transfer takes place. These processes offer the possibility to scale up electrophoresis as well as to extend the field of application of electrodialysis. This paper deals with the study of the mass transfer mechanisms involved in such electroseparation processes for the separation of binary mixtures. This study is carried out by associating an experimental work and a theoretical approach. The parameters involved in the model are calculated according from the experimental variations of the solvent and solute transfer. From the dependence of these parameters with the operating conditions, the importance of electrostatic interactions with respect to the separation performances is pointed out. Finally, the process performances are studied considering the separation of alpha-lactalbumin and bovine haemoglobin as a case study.