Effects of Chlorine Exposure Conditions on Physiochemical Properties and Performance of a Polyamide Membrane—Mechanisms and Implications (original) (raw)
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Degradation of Polyamide Nanofiltration and Reverse Osmosis Membranes by Hypochlorite
Environmental Science & Technology, 2012
The degradation of polyamide (PA) nanofiltration and reverse osmosis membranes by chlorine needs to be understood in order to develop chlorine-resistant membranes. Coated and uncoated fully aromatic (FA) and piperazine (PIP) semi-aromatic PA membranes were treated with hypochlorite solution and analyzed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). XPS results showed that in chlorine treated FA PA membranes the ratio of bound chlorine to surface nitrogen was 1:1 whereas it was only 1:6 in the case of PIP PA membranes. Surface oxygen of uncoated FA and PIP membranes increased with increasing hypochlorite concentration whereas it decreased for coated FA membranes. High resolution XPS data support that chlorination increased the number of carboxylic groups on the PA surface, which appear to form by hydrolysis of the amide bonds (C(O)−N). FTIR data indicated the disappearance of the amide II band (1541 cm −1 ) and aromatic amide peak (1609 cm −1 ) in both coated and uncoated chlorinated FA membranes, consistent with the N-chlorination suggested by the XPS results. Furthermore, the surface charge of chlorinated membranes at low pH (<6) became negative, consistent with amide-nitrogen chlorination. Chlorination appeared to both increase and decrease membrane hydrophobicity depending on chlorination exposure conditions, which implied that Nchlorination and hydrolysis may be competing processes. The effects of property changes on the membrane performance were also observed for NF90, BW30, and NF270 membranes.
Desalination, 2009
The physiochemical properties of reserve osmosis (RO) and nanofiltration (NF) polyamide (PA) membranes are largely determined by their PA chemistry and coatings, if any. Knowledge on such inherent relationship is critically needed in advancing membrane technology. This paper presents a consistent and in-depth characterization on diagnosing the chemistry of polyamide and the presence of any coating or modifying agent. Fourier-transform infrared (FTIR) and x-ray photoelectron spectra (XPS) of 17 commonly used commercial thin film composite polyamide RO and NF membranes are presented. The FTIR spectra for fully aromatic trimesoyl chloride and 1,3benzenediamine based membranes had an amide II band (1541 cm -1 ) and an aromatic amide band (1609 cm -1 ) that were absent for the semi-aromatic membranes. Consistent with that, the XPS binding energy shift for carbon atoms in fully aromatic amide groups was higher than that for semi-aromatic ones likely due to the more electron withdrawing environment. An additional intermediate peak with a binding energy shift of 1.1-1.6 eV was present in the XPS spectra of C(1s) for some commercial RO and NF membranes. The additional peak, coupled with FITR analysis over the high wave number region and XPS elemental analysis, provided consistent evidence that these membranes were either coated with an additional coating layer or had a modified PA chemistry.
Desalination, 2009
The physiochemical properties of reserve osmosis (RO) and nanofiltration (NF) polyamide (PA) membranes are largely determined by their PA chemistry and coatings, if any. Knowledge on such inherent relationship is critically needed in advancing membrane technology. This paper presents a consistent and in-depth characterization on diagnosing the chemistry of polyamide and the presence of any coating or modifying agent. Fourier-transform infrared (FTIR) and x-ray photoelectron spectra (XPS) of 17 commonly used commercial thin film composite polyamide RO and NF membranes are presented. The FTIR spectra for fully aromatic trimesoyl chloride and 1,3benzenediamine based membranes had an amide II band (1541 cm-1) and an aromatic amide band (1609 cm-1) that were absent for the semi-aromatic membranes. Consistent with that, the XPS binding energy shift for carbon atoms in fully aromatic amide groups was higher than that for semi-aromatic ones likely due to the more electron withdrawing environment. An additional intermediate peak with a binding energy shift of 1.1-1.6 eV was present in the XPS spectra of C(1s) for some commercial RO and NF membranes. The additional peak, coupled with FITR analysis over the high wave number region and XPS elemental analysis, provided consistent evidence that these membranes were either coated with an additional coating layer or had a modified PA chemistry.
Journal of Membrane Science, 2013
In this study, chlorine-induced structural changes of fully-aromatic and semi-aromatic polyamide (PA) active layers used in reverse osmosis (RO) and nanofiltration (NF) membranes, respectively, were investigated by a combination of mechanical property measurements and performance studies. Our results indicated that chlorination causes quite different changes in mechanical properties of the active layer depending on the chemical nature of the PA, as identified by increased brittleness for a fullyaromatic PA and improved ductility for a semi-aromatic PA. Moreover, the results revealed that the mechanical responses of the PA active layers after chlorine exposure correlate to the overall membrane performance. A significant increase in water flux and a large decrease in salt rejection were observed for the RO membrane after chlorination, which can be ascribed to the increased fragility and resultant defects of the oxidized fully-aromatic PA network. In sharp contrast, the chlorination of the NF membrane resulted in a slightly reduced water flux accompanied with improved salt rejection, suggestive of structural compaction and densification of the semi-aromatic PA network induced by enhanced chain flexibility. We contend that our thin film measurement methodology provides key mechanical property measurements of the PA active layer and begins to bridge the gap between compositional chemical analyses and membrane performance measurements.
Novel composite nanofiltration membranes containing aromatic-cycloaliphatic polyamide skin layer on a reinforced polyethersulfone ultrafiltration membrane were prepared by in situ interfacial polymerization of 1,3-cyclohexanebis(methylamine) (CHMA) in water with trimesoyl chloride in hexane under different conditions. The composite membranes were exposed to 1, 3 or 5 ppm NaOCl solution containing 2000 ppm NaCl for a period of 1-10 days. The membranes were characterized by infrared spectroscopy (FT-ATIR), scanning electron microscopy (SEM), contact angle measurements, and separation performance by permeation tests using 2000 ppm NaCl solution. The IR spectra of virgin TFC membrane has shown strong amide-I (C O stretch) and amide-II (C N H) bands at 1644 and 1543 cm −1 , respectively. The IR spectra of chlorine exposed membranes have shown a split in both the bands with peaks around 1655 and 1640 cm −1 for amide-I band, and around 1580, 1560, 1545, and 1512 cm −1 for amide-II band. SEM analysis has sown a change in the surface morphology from smooth granular structure of the virgin membrane to rough particle structure in chlorinated membranes. The contact angles of the membrane surface have been increased from 57 • for the virgin membrane to 60-65 • for the membranes treated with NaOCl solution within 24 h. NaCl rejection of the membranes has been decreased from the initial 78 to 63-65%, and water flux was decreased from the initial 73 to 32-38 l m −2 h −1 , depending upon the chlorine concentration, within 24 h of exposure. The higher the chlorine concentration the higher was the reduction in flux and rejection. Further decrease in the performance has occurred at a considerably slower rate. The faster initial decrease in both salt rejection and water flux may be attributed to the faster conversion of the hydrogen bonding amide N H group to the non-hydrogen bonding N Cl group and thus resulting a change in the polyamide skin layer chemistry and morphology. The results clearly indicate that N-chlorination itself adversely affects the membrane performance in the absence of Orton rearrangement and cleavage of the polyamide chain.
Stability of NF membranes under extreme acidic conditions
Journal of Membrane Science, 2004
Two commercial nanofiltration (NF) membranes (FilmTec NF-45 and Desal-5 DK) and two new NF-1 membranes made by BPT (Bio Pure Technology) for the purpose of a European Union funded research project (RENOMEM) were tested under extreme acidic conditions. The polyethersulphone (PES) ultrafiltration (UF) supports used for casting the BPT-NF-1 membranes were also tested under similar conditions. The 006 and 015 UF supports were found to be stable in 5% nitric acid at 20 and 80 • C for 4 and 3 months, respectively. Both supports (006 and 015) showed a significant reduction in flux after immersion in sulphuric acid at both temperatures. The BPT-NF-1 membranes showed excellent resistance to 20% sulphuric acid for up to 4 months at 20 • C but were attacked by the nitric acid solution. The resistance of the two commercial membranes in 20% sulphuric acid at 20 • C was generally lower than that of the BPT-NF-1 membranes. The NF-45 membrane was slightly more stable in 5% nitric acid at 20 • C. Degradation of the membrane occurred only after 2 months while both the Desal-5 DK and BPT-NF-1 membranes degraded during the first month. At the higher temperature of 80 • C in 5% nitric acid all membranes degraded in the first month.
Chlorine-resistance of reverse osmosis (RO) polyamide membranes
Polyamide (PA) reverse osmosis (RO) membranes suffer performance decay when exposed to oxidizing species, limiting their lifetime and increasing operation costs. This article aims at reviewing the effect of chlorine species on the performance and characteristics of PA-membranes. Experimental evidence supporting different competing mechanisms for chlorine-polymer interaction will be presented, together with the influence of operational parameters. Additionally, an overview of different modification methods that exist to render PA-membranes more chlorine-resistant is given.
Journal of Membrane Science, 2011
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 C O groups (1680 cm −1 ) and a decrease of hydrogen bonded C O 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.
Modification to the polyamide TFC RO membranes for improvement of chlorine-resistance
Journal of Membrane Science, 2011
Most current high-performance composite membranes comprise aliphatic or aromatic amines condensed with acyl chlorides or other reactive groups that usually contain substituted amide linkages. Aromatic rings bonded to the N-H group of amide linkages are sensitive to attack by chlorine radicals because of their high electron density. Consequently, the N-H group is converted to an N-Cl group by N-halogenation. This causes the failure of the polyamide reverse osmosis (RO) membrane resulting in decreased salt rejection and increased water flux after exposure to chlorine. In this study, the performance of a silane-coated RO membrane was investigated and its surface analyzed using field emission-scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The stability of the coated membranes in the presence of chorine was also studied in order to ascertain the effect of incorporating different silane compounds with alkyl, aryl and vinyl substituents. In the uncoated membrane, degradation of the polyamide network of the RO membrane by free chlorine in the feed solution resulted in decreased salt rejection and increased water flux after 15,000 ppm h. However, the silane-coated membrane maintained a salt rejection of above 99.0% even after 25,000 ppm h.
Degradation by Free Chlorine of Aromatic Polyamide Active Layers of Thin-Film Composite Membranes
Polyamide-based thin-film composite membranes are the current technology of choice to meet the growing demand for drinking water desalination applications. One significant drawback to the use of this class of membranes is the high sensitivity of their polyamide active layer to oxidation by free chlorine. The current understanding of the mechanisms of chlorine uptake and eventual polymer degradation that lead to membrane failure has been gained mostly through the quantitative study of the effects of chlorination on molecular model polyamide compounds and membrane surfaces (top~5nm) with X-ray photoelectron spectroscopy (XPS), as well as through qualitative analyses of chlorinated membrane samples with attenuated total reflectance-Fourier transformed infrared (ATR-FTIR)spectroscopy. The physico-chemical changes induced by chlorination within the bulk of the active layer, however, have not been characterized by means other than ATR-FTIR, and therefore it has not been confirmed in the l...