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

Mechano-chemical ageing of PES/PVP ultrafiltration membranes used in drinking water production

In water treatment by microfiltration and ultrafiltration, a major concern is the integrity loss or failure of membrane induced by onsite operations, potentially leading to permeate water contamination. This study aims to provide a better understanding of the phenomena responsible for membrane damage by analyzing its causes and effects. The role of sodium hypochlorite exposure conditions and the impact of mechanical stress on membrane characteristics were investigated. Monitoring of hydraulic response, mechanical properties and the evolution of the chemical structure showed, on multiple scales, strong indications of membrane chemical degradation, involving radical mechanisms, accelerated by tensile stress application

Kinetics of chlorine-induced polyethylene degradation in water pipes

The presence of chlorine in drinking water supplies in many countries creates the undesirable side effect of causing a relatively under investigated degree of polymer degradation in the polyethylene pipes used for transport. In order to predict pipe lifetimes and ensure safe water supplies, a kinetic model using data for the degradation rates of polyethylenes immersed in chlorine solutions, was developed. In order to replicate phenomena that normally occur very slowly at low concentrations of chlorine, accelerated ageing studies were necessary. These were carried out at high chlorine concentrations under well-defined experimental conditions (70, 400 and 4000 ppm). Results showed that, for the chlorine concentrations studied, a chain scission process associated with carbonyl formation is occurring. It was also shown that the rate of this degradation does not depend on the presence of stabilizer. A kinetic model, taking into account the chlorine concentration, is proposed in order to simulate the molar mass changes occurring. This will facilitate the prediction of the degree of polyethylene embrittlement and ultimately the lifetime

Multi-scale analysis of hypochlorite induced PES/PVP ultrafiltration membranes degradation

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 them olecular changes by X-rayphoto electron spectroscopy (XPS), attenuated total reflectance infraredspectroscopy (ATR-IR), size exclusion chromatography (SEC) and VITA-mode atomic forcemicroscopy (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 over all membrane properties changes are governed by the additive fate

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

Caractérisation et vieillissement d'une membrane d'ultrafiltration d'eau

During disinfections, water ultrafiltration membranes are mechanically and chemically stressed. These stresses lead to an early membrane ageing implying a drop of the quality of ultrafiltrated water. The aim of this study is to examine the long term behaviour of an ultrafiltration polysulfone hollow fibre in contact with bleach. After the initial characterisation of the fibre and its main component (polysulfone), we broached watermembrane physical interaction. The sorption behaviour of this asymmetric membrane could be interpreted within a dual mechanism constituted of two simultaneous phenomena: pore filling and water absorptiondiffusion in the pore walls. As chemical ageing could induce large modifications on membranes interaction and transport properties, we studied these phenomena via structure-properties relationships. The bleach-membrane chemical interaction shows a degradation of the polysulfone, in spite its well-known chemical stability. The degradation, which generates an embrittlement of the fibre, occurs by chains scission (determined by Size Exclusion Chromatography) with the hydroxyl radical - OH formed in the bleach solution. These chain scissions take place on the isopropylidene and sulfone groups to form final vinylidene and sulfonate groups respectively. Analytic investigations (IR and proton NMR) show that the degradation of additives happens separately without interfering on the ageing of the PSU but alters the behaviour of the fibre. The lifetime of the fibre depends on the total chlorine concentration of the ageing solution but also on its pH which drives the formation of hypochlorous acid and hypochlorite ion in great proportion, essential condition for the formation of hydroxyl radicals.Sur site, les membranes d'ultrafiltration d'eau sont soumises à des sollicitations mécaniques et chimiques lors de leur désinfection. Ces sollicitations conduisent à un vieillissement précoce de la membrane pouvant aller jusqu'à la rupture mécanique et générer une baisse de la qualité de l'eau ultrafiltrée. Le but de cette étude est d'examiner le comportement à long terme d'une fibre creuse d'ultrafiltration au contact d'une solution d'eau de Javel. Après une caractérisation initiale de la fibre et de son constituant majoritaire, le polysulfone, nous avons étudié les interactions physiques de cette membrane avec l'eau. Le comportement en sorption de cette fibre asymétrique doit être considéré comme un mécanisme dual constitué de deux phénomènes simultanés : remplissage des pores et absorption-diffusion de l'eau dans les parois des pores. Le vieillissement chimique, engendré par la désinfection des fibres, est susceptible de modifier les propriétés d'interaction et de transport de l'eau dans les parois et ainsi affecter les propriétés d'utilisation des membranes. Il nous a donc paru nécessaire de bien comprendre ces phénomènes, et ce par le biais des relations structure-propriétés. L'étude de l'interaction chimique membrane-eau de Javel, a permis, quant à elle, de mettre en évidence une dégradation du PSU en immersion, réfutant sa stabilité chimique reconnue. Cette dégradation, à l'origine de la fragilisation de la membrane, se produit par coupure de la chaîne principale du PSU (mise en évidence par chromatographie d'exclusion stérique) sous l'action du radical - OH formé au sein de la solution de Javel. Ces ruptures se produisent au niveau des groupements isopropylidènes et sulfones et forment respectivement des groupements vinylidènes et sulfonates terminaux. Les investigations analytiques (IR et RMN 1H) montrent que la dégradation des additifs contenus dans la fibre (agents hydrophile et porogène) se produit indépendamment sans interférer avec le vieillissement du PSU, mais altère néanmoins le comportement de la fibre. La durée de vie de la fibre dépend non seulement de la concentration en chlore total de la solution de lavage mais également de son pH qui conditionne la formation en proportion significative des espèces HClO (acide hypochloreux) et ClO- (ion hypochlorite), indispensables pour la formation des radicaux hydroxyles

Caractérisation et vieillissement d'une membrane d'ultrafiltration d'eau

Sur site, les membranes d'ultrafiltration d'eau sont soumises à des sollicitations mécaniques et chimiques lors de leur désinfection. Ces sollicitations conduisent à un vieillissement précoce de la membrane pouvant aller jusqu'à la rupture mécanique et générer une baisse de la qualité de l'eau ultrafiltrée. Le but de cette étude est d'examiner le comportement à long terme d'une fibre creuse d'ultrafiltration au contact d'une solution d'eau de Javel. Après une caractérisation initiale de la fibre et de son constituant majoritaire, le polysulfone, nous avons étudié les interactions physiques de cette membrane avec l'eau. Le comportement en sorption de cette fibre asymétrique doit être considéré comme un mécanisme dual constitué de deux phénomènes simultanés : remplissage des pores et absorption-diffusion de l'eau dans les parois des pores. Le vieillissement chimique, engendré par la désinfection des fibres, est susceptible de modifier les propriétés d'interaction et de transport de l'eau dans les parois et ainsi affecter les propriétés d'utilisation des membranes. Il nous a donc paru nécessaire de bien comprendre ces phénomènes, et ce par le biais des relations structure-propriétés. L'étude de l'interaction chimique membrane-eau de Javel, a permis, quant à elle, de mettre en évidence une dégradation du PSU en immersion, réfutant sa stabilité chimique reconnue. Cette dégradation, à l'origine de la fragilisation de la membrane, se produit par coupure de la chaîne principale du PSU (mise en évidence par chromatographie d'exclusion stérique) sous l'action du radical OH formé au sein de la solution de Javel. Ces ruptures se produisent au niveau des groupements isopropylidènes et sulfones et forment respectivement des groupements vinylidènes et sulfonates terminaux. Les investigations analytiques (IR et RMN 1H) montrent que la dégradation des additifs contenus dans la fibre (agents hydrophile et porogène) se produit indépendamment sans interférer avec le vieillissement du PSU, mais altère néanmoins le comportement de la fibre. La durée de vie de la fibre dépend non seulement de la concentration en chlore total de la solution de lavage mais également de son pH qui conditionne la formation en proportion significative des espèces HClO (acide hypochloreux) et ClO- (ion hypochlorite), indispensables pour la formation des radicaux hydroxyles.During disinfections, water ultrafiltration membranes are mechanically and chemically stressed. These stresses lead to an early membrane ageing implying a drop of the quality of ultrafiltrated water. The aim of this study is to examine the long term behaviour of an ultrafiltration polysulfone hollow fibre in contact with bleach. After the initial characterisation of the fibre and its main component (polysulfone), we broached watermembrane physical interaction. The sorption behaviour of this asymmetric membrane could be interpreted within a dual mechanism constituted of two simultaneous phenomena: pore filling and water absorptiondiffusion in the pore walls. As chemical ageing could induce large modifications on membranes interaction and transport properties, we studied these phenomena via structure-properties relationships. The bleach-membrane chemical interaction shows a degradation of the polysulfone, in spite its well-known chemical stability. The degradation, which generates an embrittlement of the fibre, occurs by chains scission (determined by Size Exclusion Chromatography) with the hydroxyl radical OH formed in the bleach solution. These chain scissions take place on the isopropylidene and sulfone groups to form final vinylidene and sulfonate groups respectively. Analytic investigations (IR and proton NMR) show that the degradation of additives happens separately without interfering on the ageing of the PSU but alters the behaviour of the fibre. The lifetime of the fibre depends on the total chlorine concentration of the ageing solution but also on its pH which drives the formation of hypochlorous acid and hypochlorite ion in great proportion, essential condition for the formation of hydroxyl radicals.PARIS-Arts et Métiers (751132303) / SudocSudocFranceF

Experimental study of the crack depth ratio threshold to analyze the slow crack growth by creep of high density polyethylene pipes

International audienceTo assess the durability of drinking water connection pipes subjected to oxidation and slow crack growth, a comprehensive database was constructed on a novel specimen geometry: the pre-cracked NOL ring.135 tests were carried out consisting of initial crack depth ratio ranging from 0.08 to 0.6; single or double longitudinal cracks: tensile with steady strain rate and creep loading. A threshold value of the crack depth ratio of 0.2, induced by the oxidation was determined by analyzing several mechanical parameters. This threshold value was shown to be independent on the strain rate effects, single or double crack configuration and the kind of loading: tensile or creep. Creep test results with crack depth ratio larger than 0.2 were then utilized to establish a failure assessment diagram. A methodology allowing the prediction of residual lifetime of in-service pipes was proposed, using this diagram

Interactions chimiques des tubes en polyéthylène avec les désinfectants en eau potable

Distribuer une eau potable de qualité constante et conforme aux impératifs sanitaires constitue une des préoccupations majeures de VEOLIA EAU. Le respect de cette exigence implique à la fois une maîtrise des facteurs qui influencent la qualité de la ressource et une connaissance approfondie de l’état structurel des réseaux. Dans le cadre du consortium VEOLIA/Suez/Saur/ENSAM, il est clairement démontré depuis 2006 que le dioxyde de chlore présent dans l’eau distribuée consomme les antioxydants contenus dans le polyéthylène. Cette altération est ciblée en paroi interne sur le premier millimètre de tuyau en contact avec l’eau. Elle s’accompagne d’une oxydation du polyéthylène qui induit une fragilisation structurelle du branchement et explique les ruptures prématurées observées sur sites depuis 2003. Ce phénomène est accentué par la température, la pression et leurs variations dans le réseau. Cet article porte sur l’interaction chimique du polyéthylène avec le dioxyde de chlore et le chlore, et leurs conséquences respectives sur la tenue mécanique résiduelle du branchement. À partir d’essais à l’échelle pilote en conditions de vieillissement accéléré au dioxyde de chlore (70 mg.L-1, 40 °C) et de prélèvements «terrain» sur des sites exploités en conditions moyennes d’utilisation (~0,5 mg.L-1 ClO2, 20 °C, 3 bar), un mécanisme de rupture mécanique d’un branchement PEHD en service a été établi et sert de base pour les futures expertises en routine. L’oxydation du polyéthylène par le chlore a été démontrée à l’échelle du laboratoire en conditions extrêmement accélérées (400 et 4000 mg.L-1 de chlore). L’examen de prélèvements «terrain» exploité en conditions particulières (0,8–1 mg.L-1 de chlore résiduel, 8 bar, Teau = 25−27 °C) met en évidence des altérations semblables (couche oxydée et réseau de microfissures en paroi interne) à celles observées en présence de dioxyde de chlore en conditions moyennes d’exploitation. Le tube multicouche Excel Plus®, élaboré en partenariat avec ALIAXIS R&D, ARKEMA et VEOLIA Environnement, offre une réponse concrète à l’action de ces désinfectants et se place comme un candidat pertinent pour le remplacement des branchements PEHD