Vieillissement chimique de membranes d'osmose inverse

Ce travail porte sur l'étude du vieillissement chimique de membranes d’osmose inverse (OI) en présence de chlore, oxydant couramment utilisé en traitement d'eau. Une analyse multi-échelle d'une couche nanométrique de polyamide aromatique (PA) présente en surface d'une membrane OI commercialisée (SW30HRLE-400 de Dow FilmTecTM) a été réalisée. Des analyses par XPS et FTIR en ATR ont montré que les compositions élémentaire et chimique en surface et en profondeur du PA sont rapidement modifiées par chloration, une réaction de substitution électrophile réalisée en 2 temps, à des doses d'exposition au chlore actif variant de 40 à 4000 ppm.h, auxquelles les membranes OI sont susceptibles d'être exposées en exploitation. L'incorporation du chlore au sein du PA s'accompagne d'une augmentation monotone continue de la fraction de fonctions carbonyle non associées traduisant une mobilité accrue au sein du PA, mécanisme de vieillissement principal identifié dans ce travail. Ces transformations chimiques couplées à des variations de propriétés de transfert en osmose inverse déterminées à l'échelle du laboratoire mettent en évidence trois domaines d'influence de la chloration. Pour une exposition inférieure à 400 ppm.h HOCl, espèce active dans la chloration pour un pH en solution supérieur à 5,0, soit pour un ratio de 1 atome chlore incorporé pour 2 atomes d'azote (élément constitutif de la couche active), la sélectivité eau/soluté de la membrane OI est améliorée. Elle pourrait résulter d'un accroissement de la fraction et/ou de la dimension de volumes libres au sein du PA. L'amélioration des performances en osmose inverse est en revanche réalisée aux dépens de la tolérance de la membrane OI à une exposition consécutive à HOCl, comme mis en évidence par des essais de chloration séquentielle à l'échelle pilote. Au-delà de la dose d'exposition seuil, de 400 ppm.h HOCl, les performances en sélectivité en OI se détériorent et la structure du PA est irréversiblement fragilisée mécaniquement, vraisemblablement par la transition d'un état ductile à un état fragile. Enfin, le vieillissement chimique du PA semble se stabiliser pour une dose supérieure à 2000 ppm.h HOCl, soit pour un ratio de 1 atome de chlore incorporé pour 1 atome d’azote. A ces doses d'exposition, la mobilité des chaines du PA est décrite par des modifications de la perméabilité à l'eau, conditionnées par la pression appliquée et par le flux de transfert initial d'un anion.In this dissertation, a study on the chemical aging of reverse osmosis membranes (RO) with exposure to chlorine, an oxidant commonly used in water treatment, is presented. A multi-scale analysis was performed to investigate the chemical aging of a thin film aromatic polyamide (PA), active layer of a composite RO membrane (SW30HRLE-400, Dow FilmTecTM). XPS and ATR-FTIR analysis show that elemental and chemical composition at the surface and in the bulk of the PA nanolayer are rapidly modified during chlorination, a two-step electrophilic substitution reaction, at free chlorine doses between 40 and 4000 ppm.h, at which RO membranes are likely to be exposed to during operation. A continuous increase in the fraction of non associated carbonyl groups was observed with the incorporation of chlorine within the PA. It is believed to result in an increase in the mobility of the PA layer, main chemical aging mechanism found during this study. Three characteristic chlorine induced PA structure modifications have been identified by combining transport property variations determined during RO experiments at the lab scale and chemical transformations. At a chlorine exposure below 400 ppm.h HOCl, free chlorine active specie at a pH above 5.0, corresponding to a bound chlorine to nitrogen (characteristic element of the PA) 1:2 ratio, water / solute selectivity of the RO membrane is improved. This phenomenon is believed to be caused by an increase in the fraction and/or size of free volumes within the PA. However, enhancement of RO membrane performance is achieved at the expense of its tolerance to subsequent exposure to HOCl, as demonstrated at the pilot scale during a study on the effects of continuous chlorination. At an exposure dose above 400 ppm.h HOCl, RO selectivity capabilities and PA layer mechanical properties are permanently impaired, likely by embrittlement. Finally, PA chemical aging seems to level off for an exposure to 2000 ppm.h HOCl, corresponding to a bound chlorine to nitrogen 1:1 ratio. In these conditions, PA chain mobility can be described by water permeability, as observed from the latter’s variations with the applied pressure and with the initial flux of an anion

Permeability and chemical analysis of aromatic polyamide based membranes exposed to sodium hypochlorite

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

Mass transfer properties of chlorinated aromatic polyamide reverse osmosis membranes

Water (A) and solute (B) permeability of aromatic polyamide (PA) reverse osmosis membranes (RO) were monitored under varying applied pressure, solute nature and concentration to assess their evolution after exposure of the membrane to free chlorine. Above a threshold value of 400 ppm h HOCl water permeability was influenced by permeation conditions during both filtration of ultrapure water (UP water) and reverse osmosis of salts performed sequentially. Water permeability decreased during the filtration of UP water performed at a constant applied pressure of 60 bar. During the reverse osmosis of an electrolyte solution, performed at a constant permeation flux of 31 L h¯¹ m¯², A was observed to increase continuously with time, e.g. up to a factor of 3 after exposure to 3120 ppm h HOCl, most severe dose used. Differences in the charge density of mono- and divalent cations did not influence the rate of increase of A with time, which was however shown to depend on salt flux and ascribed to a diffusion limited relaxation process presumed to occur within the dense hydrated PA network. The relative and opposite impact of applied pressure and of salt permeation highlighted the importance in distinguishing conditions under which the water permeability (A) of a chlorinated membrane is measured, whether during the filtration of UP water or of a salt

Pilot scale study of chlorination-induced transport property changes of a seawater reverse osmosis membrane

A pilot-scale study was performed to assess variations of reverse osmosis (RO) membrane water permeance (A) and salt retention (Robs) induced by chlorination and to compare them with those observed at the lab-scale. A chlorination protocol was adapted to expose only the surface active layer (an aromatic polyamide)of a composite RO membrane to consecutive free chlorine doses ranging from 40 to 4000 ppm h, at pH 6.9. Along the long-term filtration of seawater, performed with a 4" spiral wound RO module, we monitored the variations of A, the decrease of Robs and the rate of increase of A with time, and found themquantitatively similar to those reported in previous studies performed at the lab-scale under accelerated exposure conditions. The elemental analysis of the feed and permeate streams revealed that the rejection of divalent ions remained constant (ca. 100%), irrespective of the free chlorine dose reached, whereas the rejection of monovalent ions of the seawater (mainly sodium, chloride and bromide ions) decreased as the exposure dose increased. Overall, transposing the characterization procedure to the pilot-scale further supports that chlorination of PA, under pH conditions usually found in desalination plants (6.9 to 8.0), is controlled by the concentration of HOCl, as observed from elemental analysis of the surface by XPS

The Future Landscape of High-Redshift Galaxy Cluster Science

The Decadal Survey on Astronomy and Astrophysics (Astro2020) of the US National Academies of ScienceWe describe the opportunities for galaxy cluster science in the high- redshift regime where massive, virialized halos first formed and where star formation and AGN activity peaked. New observing facilities from radio to X-ray wavelengths, combining high spatial/spectral resolution with large collecting areas, are poised to uncover this population

The Future Landscape of High-Redshift Galaxy Cluster Science

The Decadal Survey on Astronomy and Astrophysics (Astro2020) of the US National Academies of ScienceWe describe the opportunities for galaxy cluster science in the high- redshift regime where massive, virialized halos first formed and where star formation and AGN activity peaked. New observing facilities from radio to X-ray wavelengths, combining high spatial/spectral resolution with large collecting areas, are poised to uncover this population

The Hot and Energetic Universe: A White Paper presenting the science theme motivating the Athena+ mission

This White Paper, submitted to the recent ESA call for science themes to define its future large missions, advocates the need for a transformational leap in our understanding of two key questions in astrophysics: 1) How does ordinary matter assemble into the large scale structures that we see today? 2) How do black holes grow and shape the Universe? Hot gas in clusters, groups and the intergalactic medium dominates the baryonic content of the local Universe. To understand the astrophysical processes responsible for the formation and assembly of these large structures, it is necessary to measure their physical properties and evolution. This requires spatially resolved X-ray spectroscopy with a factor 10 increase in both telescope throughput and spatial resolving power compared to currently planned facilities. Feedback from supermassive black holes is an essential ingredient in this process and in most galaxy evolution models, but it is not well understood. X-ray observations can uniquely reveal the mechanisms launching winds close to black holes and determine the coupling of the energy and matter flows on larger scales. Due to the effects of feedback, a complete understanding of galaxy evolution requires knowledge of the obscured growth of supermassive black holes through cosmic time, out to the redshifts where the first galaxies form. X-ray emission is the most reliable way to reveal accreting black holes, but deep survey speed must improve by a factor ~100 over current facilities to perform a full census into the early Universe. The Advanced Telescope for High Energy Astrophysics (Athena+) mission provides the necessary performance (e.g. angular resolution, spectral resolution, survey grasp) to address these questions and revolutionize our understanding of the Hot and Energetic Universe. These capabilities will also provide a powerful observatory to be used in all areas of astrophysics