Polyvinylpyrrolidone Membranes

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Degradation of Poly(Ether Sulfone)/Polyvinylpyrrolidone Membranes by Sodium Hypochlorite: Insight from Advanced Electrokinetic Characterizations Yamina Hanafi,†,‡ Anthony Szymczyk,*,† Murielle Rabiller-Baudry,† and Kamel Baddari‡ †

Université de Rennes 1, Institut des Sciences Chimiques de Rennes (UMR CNRS 6226), 263 Avenue du Général Leclerc, CS 74205, 35042 Rennes, France ‡ Unité de Recherche Matériaux Procédés et Environnement, Université M’hamed Bougara, Boumerdes, Algeria S Supporting Information *

ABSTRACT: Poly(ether sulfone) (PES)/polyvinylpyrrolidone (PVP) membranes are widely used in various industrial fields such as drinking water production and in the dairy industry. However, the use of oxidants to sanitize the processing equipment is known to impair the integrity and lifespan of polymer membranes. In this work we showed how thorough electrokinetic measurements can provide essential information regarding the mechanism of degradation of PES/ PVP membranes by sodium hypochlorite. Tangential streaming current measurements were performed with ultrafiltration and nanofiltration PES/PVP membranes for various aging times. The electrokinetic characterization of membranes was complemented by FTIR-ATR spectroscopy. Results confirmed that sodium hypochlorite induces the degradation of both PES and PVP. This latter is easily oxidized by sodium hypochlorite, which leads to an increase in the negative charge density of the membrane due to the formation of carboxylic acid groups. The PVP was also found to be partly released from the membrane with aging time. Thanks to the advanced electrokinetic characterization implemented in this work it was possible for the first time to demonstrate that two different mechanisms are involved in the degradation of PES. Phenol groups were first formed as a result of the oxidation of PES aromatic rings by substitution of hydrogen by hydroxyl radicals. For more severe aging conditions, this membrane degradation mechanism was followed by the formation of sulfonic acid functions, thus indicating a second degradation process through scission of PES chains.

1. INTRODUCTION On a number of occasions, commercial conventional separation processes in industry have already been converted to membrane separation processes with significant reductions in cost, energy, and environmental impact.1 In order to restore the membrane performance and inhibit the proliferation of micro-organisms in filtration units used in both the food industry and water treatment, cleaning and disinfection are carried out daily using cleaning chemicals such as alkaline and acidic compounds, surfactants and oxidizing agents. Among these last ones, sodium hypochlorite (NaOCl) is the most widely used disinfecting agent because of its efficiency and relatively low cost. However, exposure of polymer membranes to oxidizing reagents is one of the major causes of process failure in industrial plants and several works have pointed out the role of sodium hypochlorite in the aging process of various polymer membranes (e.g., polysulfone,2−5 polyamide,6−12 poly(ether sulfone)13−22). For instance, the effect of NaOCl on a hollow-fiber membrane made from polysulfone (PSU) and polyvinylpyrrolidone (PVP) was investigated by Rouaix et al.3 who gave evidence for the scission © XXXX American Chemical Society

of PSU chains after prolonged exposure to bleach solutions and the correlation with the modification of the mechanical properties of the membrane. Gaudichet-Maurin and Thominette4 also studied the aging of PSU/PVP hollow-fiber membranes in bleach solutions. From a comparison with additive-free PSU films they showed that chain scission affects PSU and they proposed a mechanism based on radical attack of both sulfone and isopropylidene groups by hydroxyl radicals formed in the bleach solution. The role of hydroxyl radicals in PSU-chain scission was confirmed by Causserand et al.5 who showed that the addition of a radical scavenger such as tertiobutanol in the bleach solution inhibits the chain-scission mechanism in PSU/PVP membranes. Poly(ether sulfone) (PES) is another widely used polymer for the production of membranes used in various industrial fields Received: June 9, 2014 Revised: September 15, 2014 Accepted: November 3, 2014

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dx.doi.org/10.1021/es5027882 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

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PES component. In contrast to what was suggested by Thominette et al.14 and Arkhangelsky et al.,16,17 Prulho et al.22 proposed a mechanism based on the oxidation of aromatic rings of PES by hydroxyl radicals involved in the degradation mechanism of PVP, leading to the formation of phenol groups on the PES backbone. According to these authors, the formation of a new band located at 1030 cm−1 on the FTIR-ATR spectra of aged PES/PVP blends was to be attributed to the formation of phenols on the PES and not to the formation of sulfonic acid groups. The aging process of PES/PVP membranes by sodium hypochlorite therefore results from a series of complex mechanisms that are not yet fully understood. In the present work, we show how an advanced electrokinetic characterization of aged membranes can be beneficial to gain insight in the molecular mechanisms involved in the degradation of PES/PVP membranes exposed to bleach solutions. Notably we demonstrate for the first time the presence of both phenols and sulfonic acid groups on the surface of aged ultrafiltration and nanofiltration PES/PVP membranes, thus giving evidence for the degradation of PES by two different mechanisms upon exposure to sodium hypochlorite.

(water treatment, dairy industry···). The inclusion of hydrophilic polymers such as PVP has become a standard method to obtain hydrophilized PES membranes23 with lower propensity for fouling and water permeation resistance than pure PES membranes.20 PES is considered as a polymer highly resistant to oxidants and tolerant to a wide range of pH (between 2 and 12).13,16,20 Its stability in hypochlorite solutions is related to the absence of labile hydrogen atoms, thus limiting the initiation and the propagation of radical oxidation.22 However, several works have demonstrated the modification of surface morphology and mechanical properties15−18 as well as of the performance of PES/ PVP membranes13,19−21 after exposure to bleach solutions. By comparing pure PES films and PES/PVP blends, Delaunay24 and Rabiller-Baudry et al.23 showed that the degradation of PES by sodium hypochlorite is accelerated in the presence of PVP. Wienk et al.13 proposed a mechanism for the reaction of PVP with hypochlorite (ClO−) in the basic pH range leading to pyrrolidone-ring opening with the formation of carboxylic acid groups. By means of potentiometric titrations these authors showed that only 1% ring opening occurred after exposure of PES/PVP membranes to 3000 ppm of NaOCl solutions for 48 h. By means of steric exclusion chromatography Wienk et al. also demonstrated that PVP-chain scission occurs and explained this phenomenon by a radical-oxidation mechanism.13 The reaction between PVP and sodium hypochlorite solutions at pH 8 was studied very recently by Prulho et al.22 who gave evidence for the formation of succinimide groups after radical oxidation of PVP by hydroxyl radicals. These authors also proposed a mechanism in which the formation of succinimide groups is followed by their hydrolysis and a ring opening leading to the formation of carboxylic acid groups. Nonetheless, Pellegrin et al.21 showed that the formation of succinimide groups occurs only if the concentration of the free chorine is high enough and so they concluded that the degradation mechanism of PES/PVP membranes is not ruled only by the overall hypochlorite dose (i.e., hypochlorite concentration x exposure time) the membranes have been exposed to. The impact of bleach solutions on PES is even more controversial. Wienk et al.13 used steric exclusion chromatography and did not observe any chain scission with PES after exposure to sodium hypochlorite. On the other hand, Thominette et al.14 concluded that PES-chain scission occurs and proposed a chain-scission mechanism based on PES cleavage at the ether-sulfone linkage with the formation of sulfonate (SO3−) groups. The same conclusion was drawn by Arkhangelsky et al.16 who attributed the more negative streaming potentials they measured with PES membranes aged in bleach solutions to the formation of SO3− groups (to our knowledge, ref 16 is the only reported work that made use of electrokinetic measurements to investigate aging of PES membranes). Yadav et al.18 proposed another mechanism for PES-chain scission in which the PES polymeric backbone breaks in two parts with one end terminated by a phenyl chloride group (the presence of chlorine on the membrane surface was revealed by electron dispersion spectroscopy) and the other part terminated by a sulfonic acid group (whose formation was justified by the appearance of a new band located at 1030 cm−1 in the ATR-FTIR spectra of aged membranes). However, the formation of sulfonic acid groups resulting from a PES-chain scission mechanism after exposure to sodium hypochlorite was recently challenged by Prulho et al.22 An important finding made by these authors is that the degradation of PES/PVP blends in sodium hypochlorite involves radical oxidation of the PVP component which further promotes oxidation of the

2. THEORETICAL BACKGROUND The zeta potential of membranes can be inferred from electrokinetic methods such as streaming potential25−28 and streaming current.29−32 The experimental simplicity of through-pore (or transversal) streaming-potential measurements (with respect to streaming current) has led this technique to become a standard characterization method of porous membranes.33−36 However, a meaningful interpretation of data obtained from the throughpore measurements is cumbersome when applied to composite membranes,37,38 especially if the skin layers of membranes have ion-rejection properties.39,40 To overcome these difficulties streaming potential measurements in the tangential mode (i.e., the measurement is no longer performed through the membrane pores but across the surfaces of two identical membrane samples facing each other) have been paid more and more attention in the past decade. However, several works have pointed out the difficulties associated with the interpretation of tangential streaming potential because of the contribution of the porous body of membranes to the measured electric conductance.41,42 This additional contribution was first suggested by Yaroshchuk and Ribitsch41 and was further proved experimentally by Fievet et al.42 and Sbai ̈ et al.43 with ceramic and polymer membranes, respectively. The recent marketing of electrokinetic analyzers allowing accurate measurements of tangential streaming current is clearly beneficial since the streaming current phenomenon is affected neither by the surface conductance nor by the electrical conduction through the porous sublayers of membranes.44 If a laminar flow occurs only between two rectangular membrane samples separated by a distance hch (referred to as channel height) much larger than the Debye length in the background solution, the zeta potential (ζ) can be inferred from streaming current (Is) measurement as follows (considering conventionally that the streaming current and the zeta potential have the same sign): Is =

Whchε0εr ΔP ζ ηL

(1)

where L and W are the length and the width of the membrane samples, respectively, ΔP is the pressure difference applied B



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fixed using double-sided adhesive tape. The solution flow was created by a pair of syringe pumps and streaming current was measured with a pair of reversible Ag/AgCl electrodes (surface area: 10 cm2). The streaming current was measured and recorded for increasing pressure differences up to 300 mbar, the flow direction being changed periodically (every 30 s). Using electrodes with a large surface area and alternating the direction of solution flow limits electrode polarization during streaming current measurements.46 All experiments were performed with a 10−3 mol L−1 KCl background solution at room temperature (20 ± 2 °C) under a controlled atmosphere (nitrogen gas) according to the experimental protocol described elsewhere.47 Such an experimental protocol allows more reliable and accurate measurements in the basic pH range. For instance, it has been shown recently that the dissolution of carbon dioxide in the background solution could hide the electrokinetic response of very weak acid functions.47 3.5. ATR-FTIR Spectroscopy. Streaming current measurements were complemented by ATR-FTIR spectroscopy performed with a Spectrum 100 Fourier transform infrared spectrometer (PerkinElmer) equipped with a diamond crystal ATR element (single reflection; angle: 45°). Each spectrum was averaged from 20 scans collected from 650 to 4000 cm−1 at 2 cm−1 resolution. Membrane samples were carefully dried for 2 days under dynamic vacuum before performing ATR-FTIR spectroscopy experiments.

between the channel ends, ε0 is the vacuum permittivity, εr is the dielectric constant of the background solution and η its dynamic viscosity. For channel heights (hch) much larger than the Debye length of the background solution, the net (or effective) charge density σnet (defined as the opposite of the electrokinetic charge density σek so that σnet + σek = 0) of the membrane samples can be determined from ζ by means of the following relation derived from the Gouy−Chapman theory for flat interfaces:45 ⎛ ⎛ z Fζ ⎞ ⎞ σnet = −σek = sgn(ζ ) 2ε0εr RT ∑ ci⎜exp⎜ − i ⎟ − 1⎟ ⎝ ⎝ RT ⎠ ⎠ i (2)

where sgn(ζ) denotes the sign of the zeta potential, ci and zi are the concentration and the charge number of ion i, respectively, F is the Faraday constant, R is the ideal gas constant and T is the temperature.

3. EXPERIMENTAL SECTION 3.1. Chemicals. Hypochlorite solutions used for aging experiments were prepared from dilution of a commercial bleach solution (La Croix, Fr−96,000 ppm of NaOCl). Electrolyte solutions used in electrokinetic measurements were prepared from KCl (Fisher Scientific, analytical grade) and deionized water (resistivity: 18 MΩcm). Decimolar solutions of hydrochloric acid and potassium hydroxide (Fischer Scientific, analytical grade) were used to adjust the pH of the above-mentioned solutions. Ethanol (>99%; Fischer Scientific) was used to prepare water/ alcohol mixtures used to remove membrane conservatives. 3.2. Membranes. HFK-131 ultrafiltration (Koch Membrane Systems) and NP030 nanofiltration (Nadir) membranes were used in this work. Those membranes are composite materials with a skin layer made of PES/PVP23 (see Figures S1 and S2 in the Supporting Information (SI)) on top of a non woven support in polyester. Prior to characterization, the membrane samples used for both ATR-FTIR and streaming current experiments were immersed in a water−ethanol mixture and sonicated for 5 min in order to remove conservatives. The samples were further rinsed twice with deionized water (resistivity: 18 MΩ cm) and sonicated (2 × 2 min). 3.3. Aging Protocols. To perform accelerated aging experiments, membrane samples were soaked in sodium hypochlorite solutions with 400 ppm of total free chlorine at pH 8. Exposure times ranged from 1 h to 60 days. Sodium hypochlorite solutions were renewed every day. Complementary experiments were conducted with a more concentrated bleach solution (total free chlorine concentration: 96 000 ppm) at pH 8. All aging experiments were performed at room temperature (20 ± 2 °C). 3.4. Streaming Current Measurements. Streaming current measurements were performed with a SurPASS electrokinetic analyzer (Anton Paar GmbH, Austria). All measurements were conducted with an adjustable-gap cell which makes it possible to vary the distance between the two membrane samples with micrometric screws without dismounting the cell (a detailed description of the cell is available in references29 and46). The distance between the membrane samples was set to 100 ± 2 μm. Each membrane sample was cut and adjusted to the dimensions of the sample holders (i.e., L = 2 cm and W = 1 cm) and

4. RESULTS AND DISCUSSION Figures 1 and 2 show the variation of zeta potential (determined from eq 1) of HFK-131 and NP030 membranes for pH ranging from ∼2.7 to ∼11.5 and different aging times. It must be stressed that zeta potential of composite membranes deduced from streaming current measurements performed for a single height channel (hch) represents only an apparent zeta potential since the streaming current is likely to flow through the underlying porous structure of the membrane.29,31 In other words, eq 1 is not strictly applicable (even if the condition hch ≫ κ−1 is fulfilled, where κ−1 stands for the Debye length) when a fraction of the streaming current can flow through the porous sublayer(s) of the membranes since this equation considers the geometry of the channel formed by the two membrane surfaces facing each other instead of the actual (but unknown) geometry through which the streaming current flows. After exposure to sodium hypochlorite, both membranes become more negatively charged as revealed by the increase (in absolute value) of the zeta potential (Figures 1 and 2). Since the trend of the different curves remains similar to a pseudo plateau for pH higher than ∼7, these results can be attributed to an increase of the amount of weak acid groups for aged membranes, which is in good agreement with the mechanism of PVP oxidation proposed by Wienk et al.13 leading to the formation of carboxylic acid groups through a ring opening mechanism (Scheme 1). As shown in Figures 1 and 2, weak acid groups are already present on the surface of both pristine membranes, which suggests that those membranes are bleach pretreated by manufacturers prior the commercialization. Indeed, it has been shown by Wienk et al.13 that exposure of PES/PVP membranes to bleach solutions allows increasing significantly membrane permeabilities. These results are supported by those obtained from ATRFTIR spectroscopy, which reveal an obvious change in the C



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Figure 1. pH dependence of the zeta potential of HFK-131 membranes for various exposure times to 400 ppm of NaOCl solutions at pH 8.

Figure 2. pH dependence of the zeta potential of NP030 membranes for various exposure times to 400 ppm of NaOCl solutions at pH 8.

Scheme 1. Oxidation Mechanism of PVP by Hypochlorite in Alkaline Conditions.13

according to the mechanism shown in Scheme 2.22 A similar trend was observed with NP030 membranes even though a longer exposure time was required to observe the formation of succinimide groups (see Figure S3 in the SI). Indeed, 40 days of exposure to sodium hypochlorite were required to make out the PVP oxidation products in the case of NP030 whereas succinimide bands were already present in HFK-131 spectra of membranes aged for 20 days. Finally it can be noted that PVP degradation can also be seen through the progressive decrease in

spectra of pristine and aged membranes in the region around 1670 cm−1 (vibration of the amide bond of PVP). As can be seen from Figure 3 the spectra of aged HFK-131 membranes show a decrease and broadening of the amide band of PVP, typically ascribed to the ring opening of the PVP component accompanied by the formation of carboxylic acid groups (Scheme 1). After sufficiently long exposure times, the formation of two new bands at 1700 and 1770 cm−1 is observed and ascribed to the formation of succinimide groups (PVP-oxidation products) D



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Figure 3. Evolution of ATR-FTIR spectra of HFK-131 membranes in the region 1800−1400 cm−1 for various exposure times to 400 ppm of NaOCl solutions at pH 8.

Scheme 2. Degradation Mechanism of PVP Resulting from Succinimide Formation.22

the intensity of the weak band at 1440 cm−1 (see Figure 3 and SI Figure S3) which is attributed to the C−H vibration from the CH2CO group of PVP.48,49 The results obtained from ATR-FTIR spectroscopy therefore give evidence for the degradation of PVP by hypochlorite but they might also indicate a partial loss of PVP as a result of the aging process. Electrokinetic results reported in Figures 1 and 2 show a non monotonous variation of the zeta potential of PES/ PVP membranes as a function of aging time, with maximum negative zeta potential values observed for 5 days and 40 days of immersion in hypochlorite for HFK-131 and NP030, respectively. This finding can be attributed to the partial removal of oxidized PVP (carrying carboxylic acid groups) and corroborates ATR-FTIR results shown in Figure 3 and SI Figure S3. Chain scission of PVP could explain its partial release from the membrane.13 Besides, the modification of the chemical structure of PVP after exposure to sodium hypochlorite might diminish the interaction with PES thus promoting the removal of PVP from the membrane. Also, it is worth mentioning that the increase in the membrane porosity resulting from the release of PVP could have consequences on the evolution of the membrane mechanical properties. Indeed, Thominette et al. reported a

faster embrittlement rate of porous PES membranes compared with dense films.14 At high and low pH, the addition of KOH and HCl solutions leads to a substantial increase in the electric conductivity of the background solution, which impacts electrokinetic measurements47 by decreasing the zeta potential (in absolute value), or by limiting its increase, due to the compression of electrical double layers.50 That is likely to hide some important information and to lead to a misleading interpretation of electrokinetic data. This issue can be overcome somehow by determining the net charge density (σnet) of the membrane from the zeta potential and eq 2 since σnet accounts for the variation of the electric conductivity of the background solution by considering the actual concentrations of all ionic species in solution at a given pH. Figures 4 and 5 show the variation of the net charge densities (determined from eq 2 and the zeta potentials shown in Figures 1 and 2) of HFK-131 and NP030 membranes as a function of pH for various aging times. For pH ranging from ∼4 to ∼9, the variation of the net charge density with pH is qualitatively similar to that of the zeta potential (see Figures 1 and 2) and indicates the presence of weak acid E



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Figure 4. pH dependence of the net charge density of HFK-131 membranes for various exposure times to 400 ppm of NaOCl solutions at pH 8.

Figure 5. pH dependence of the net charge density of NP030 membranes for various exposure times to 400 ppm of NaOCl solutions at pH 8.

Scheme 3. Mechanism of Phenol Formation through Radical Oxidation of PES in PES/PVP Blends.22

functions as discussed above. On the other hand, for pH higher than ∼10, analyzing electrokinetic data in terms of net charge densities highlights a sharp increase in the net charge density of aged membranes whereas zeta potential was found to level off or even to decrease slightly (see Figures 1 and 2) due to the increasing solution conductivity as discussed above. Such an increase in the net charge density for pH higher than ∼10 is the signature of the deprotonation of surface groups with very weak acid properties. These findings are in very good agreement with the recent work by Prulho et al.22 who gave evidence for the formation of phenol groups in PES/PVP blends exposed to sodium hypochlorite. These authors proposed a mechanism based on the radical oxidation of PES aromatic rings leading to the formation of phenol groups (Scheme 3). It can be noted that phenols are very weak acids,51 which agrees well with electrokinetic results shown in Figures 4 and 5.

Another important result shown in Figures 4 and 5 is the shift of the isoelectric point (i.e.p.) of aged membranes. The i.e.p. of the pristine HFK-131 membrane is found to be ∼3.2 (Figure 4). It shifts progressively toward lower values with aging time and F



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eventually vanishes after sufficiently long exposure times. The same qualitative behavior is obtained with the NP030 membrane although a bit less pronounced (Figure 5). The disappearance of i.e.p. is the signature of the progressive formation of strong acid groups as a result of aging. This result can be explained by the formation of sulfonic acid groups52 as a result of PES-chain scission (Scheme 4) as suggested by

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ASSOCIATED CONTENT

* Supporting Information S

Chemical structures of PES and PVP, ATR-FTIR spectra of PES/ PVP film, pristine and aged HFK-131 and NP030 membranes, net charge density of HFK-131 and NP030 membranes aged for 2 days in a 96 000 ppm of NaOCl solution at pH 8. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Scheme 4. Formation of Sulfonic Acid Groups As a Result of PES Chain-Scission.18

AUTHOR INFORMATION

Corresponding Author

*Phone: +33 2 23236528; fax: +33 2 23235765; e-mail: anthony. [email protected]. Notes

The authors declare no competing financial interest.

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Gaudichet-Maurin and Thominette,4 Delaunay24 and Yadav et al.18 The formation of sulfonic acid groups could have a negative impact on the mechanical strength of membranes (as suggested by Arkhangelsky et al.17) as it is accompanied by a scission of PES chains. In the literature the formation of sulfonic acid functions has been suggested on the basis of the appearance of a new band around 1030 cm−1 (attributed to sulfonic acid groups) in ATRFTIR spectra of membranes after exposure to NaOCl.4,18,24 Our own ATR-FTIR experiments confirmed the formation of this new band the intensity of which was found to increase with aging time (see for instance Figure S4 in the SI for the HFK-131 membrane). However, Prulho et al.22 have recently shown that this weak band around 1030 cm−1 could be explained by the formation of phenol groups, which is confirmed by our electrokinetic measurements at high pH. Moreover, Prulho et al. concluded in the absence of sulfonic acid groups on the surface of aged lab-made PES/PVP blends based on the basis of (i) the absence of the band corresponding to hydrogen-bonded −SO3H vibration (3275 cm−1), and (ii) the decrease in the intensity of the band at 1030 cm−1 band after a 72-h treatment of aged PES/PVP blends with SF4. In order to confirm electrokinetic results shown in Figures 4 and 5 we carried out additional streaming current experiments with membranes aged for 2 days in a more concentrated bleach solution (free chlorine concentration: 96 000 ppm; pH 8). Results obtained for both HFK-131 and NP030 membranes are shown in Figure S5 in the SI. As can be seen from SI Figure S5 the disappearance of the i.e.p. is obvious for both aged membranes, which confirms the formation of strong acid functions. It can be noted that the net charge density of aged membranes remains approximately constant up to pH ∼10 beyond which the deprotonation of phenol groups is observed. These results indicate that most carboxylic acid groups resulting from PVP degradation have disappeared under these severe aging conditions, which suggests a significant loss of (degraded) PVP from the membranes. This argument is confirmed by ATR-FTIR spectroscopy that shows the disappearance of the characteristic bands of PVP (1440 and 1680 cm−1) together with the formation of succinimide groups (1700 and 1770 cm−1 bands) (Figure S6 in the SI). The reason for the apparent contradiction between results reported by Prulho et al.22 and those obtained in the present study remains unclear and further work is needed to identify what are the aging conditions and the PES/ PVP blend properties required for generating sulfonic acid groups.

ACKNOWLEDGMENTS Financial support from the French National Research Agency is gratefully acknowledged (project n° ANR-09-BLAN-0055-01).

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REFERENCES

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