Effect of pH on the ageing of reverse osmosis membranes upon exposure to hypochlorite

Desalination 309 (2013) 97–105

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Effect of pH on the ageing of reverse osmosis membranes upon exposure to hypochlorite Bogdan C. Donose a,⁎, Subash Sukumar a, Marc Pidou a, Yvan Poussade b, c, Jurg Keller a, Wolfgang Gernjak a a b c

The University of Queensland, Advanced Water Management Centre (AWMC), Brisbane, 4072 QLD, Australia Veolia Water Australia, PO Box 10819, Adelaide St Post Office, Brisbane, 4000 QLD, Australia Seqwater, 240 Margaret Street, Brisbane, 4000 QLD, Australia

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

2.5

a r t i c l e

i n f o

Article history: Received 5 July 2012 Received in revised form 23 September 2012 Accepted 24 September 2012 Available online 13 October 2012 Keywords: Polyamide Membrane ageing Ring chlorination Hydrolysis

Membrane B

Membrane A

Membrane C

2.0

Relative permeability (Lp/Lp0)

► The effect of pH on RO membranes aged in Sodium Hypochlorite has been studied. ► Membranes aged at pH 10 have increased water and saline solution permeability. ► Water and salt solution permeability decrease with the reduction of the solution pH. ► Salt rejection is not affected by the pH of the ageing solution. ► FTIR and AFM confirm that significant changes take place at acidic pH.

1.5

1.0

0.5

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ppm·h squares: pH 4; circles: pH 7; triangles: pH 10

a b s t r a c t The declining functionality associated with membranes ageing includes changes in physical parameters, such as, thickness, roughness and density of defects, but also in chemical structure. All these factors impact synergistically on the major performance indicators: permeability and salt rejection. In this study, three types of commercially available RO membranes were statically exposed to hypochlorite solutions and analysed by Fourier transform infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) in conjunction with performance tests. IR data support the chemical structure alteration for samples aged at pHs 7 and 4, where hypochlorous acid (HOCl) is the main oxidant. For two of the membranes, AFM results indicate increasing roughness at pH 4. Performance tests show a reduction of de-ionised (DI) water and brackish water permeability at pH 7 and pH 4, while at pH 10, where hypochlorite ion (ClO −) is abundant, permeability increases. Salt rejection results vary in a narrow interval of 5% and depend on the type of membrane. Based on these results the ppm∙h concept appears to fail to express a simple ageing kinetic over the entire range of pH, owing to the competing mechanisms of ring chlorination and surface hydrolysis of amide groups. © 2012 Elsevier B.V. All rights reserved.

1. Introduction

⁎ Corresponding author at: Advanced Water Management Centre, Level 4, Gehrmann Building (60), The University of Queensland, Saint Lucia, QLD 4072. Tel.: +61 7 3346 3229, +61 413 286 311(Mobile); fax: +61 7 3365 4726. E-mail address: [email protected] (B.C. Donose). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.09.027

The discovery in the early eighties of aromatic thin-film-composite (TFC) membranes [1,2] provided the critical impulse for the development and industrial scale application of nanofiltration (NF) and reverse osmosis (RO). Omnipresent these days in applications such as desalination of brackish, saline and low salt content domestic waters, these

Passive (shaker) LFC1

4; 9 Up to 2000 Kwon et al. 2006[15,16]

15.2

Passive (soaking) Passive (immersion) 4–55 15 Up to 248 8.2 Up to 540 and up to 2000 4; 7; 10 Shemer and Semiat 2011[17] ESPA 2 Kang et al. 2007[21] N/A

SW30HRLE-400 Up to 4000 Ettori et al. 2011[14]

5;6.9;8.0 55–60

5 3.2–11.2 BW30 N/A Do et al. 2011[13] Mitrouli et al. 2010[7]

10–24,000 100–26,000

Passive (soaking)

Permeability (Lp) is increasing with the exposure. Salt rejection (SR) decreases by 10% in active conditions and remains unmodified in passive conditions Lp drops with more than 60%. SR is unaffected. More than 50% drop of the flux in acidic solutions. SR drops by 10.9% at pH 3.2. 26.4% increase of flux in pH 9.2 and SR drop of 4.4. 91.4% increase of flux and 18.1% SR drop in pH 10.2 and 20000 ppm∙h More than 60% drop in permeability at pH 5. Salt rejection dropped with maximum 6% for doses higher than 400 ppm∙h (HOCl). Lp of full SW drops approx 15%. SR drops 2%. Flux drops more than 60% and SR drops with a maximum of 8% on samples aged at pH4. Slight increase of performance at pH 10. At ph 4 the flux dropped by more than 40% and rejection by 2%. At pH 9 flux increases 15 % and SR drops 2%. Active (stirring and applied pressure of 4 bar) and passive (immersion) 6.8; 17.9; 4.8 Passive (mixed on a shaker, no pressure) 6.9; 15.5 Passive (immersion or only top layer contact with the oxidising solution) 12 6 BW30-FR Antony et al. 2010[9]

400–10,000

Ageing type Pressure (bar) pH Exposure (ppm∙h) Type Reference

membranes have also great applicability in industrial water treatment [3]. One of the major problems in RO membranes operation for water recycling [4,5] is biofouling, and it is commonly addressed by continuously dosing monochloramine, a mild oxidizing agent, at 1–5 mg/L in RO feed streams. The interaction with the chlorine species' residual (i.e. generally monochloramine, but could be partially other chloramines and sodium hypochlorite) and the membrane has long term damaging results. In combination with cleaning procedures and colloid mediated mechanical interactions, this oxidative damage contributes to the failure of the membranes after years of exposure. Their declining functionality is usually associated with ageing, a term meant to include changes in physical parameters, such as, thickness, roughness and density of defects, but also chemical structure, including, H-bonds density, amide substitution degree and aromatic ring chlorination [6]. Whereas in water reuse monochloramine is dosed continuously, in seawater desalination upstream chlorination is followed by a dechlorination step before the membranes to avoid all the unwanted effects associated to chlorine exposure. Nevertheless, during years of operation, the exposure of the membranes to oxidants is thought to affect their performances. As mentioned previously, in full scale applications, membrane ageing occurs over long periods of time. For practical reason, the impact of these oxidants on RO membranes has been studied in accelerated ageing trials. It is generally assumed that the exposure to oxidants can be quantified by the ppm∙h concept, i.e. the product between the oxidant concentration in ppm (mg/L) and the exposure duration in hours. It is also general industry practice to apply this concept for product warranties. From seven key studies on the effect of hypochlorite on PA membranes currently available in the literature (Table 1), ppm∙h ranging from 10 to 26000 have been investigated corresponding to exposure times between 1 and 54 h and doses between 10 and 4000 ppm. In most of the studies, either one or two fixed chlorine concentrations or exposure times were used with the other parameter being varied. In only one of the studies both parameters had been varied, but this was only done in alkaline conditions (pH 9–11) [7]. To transfer results to full scale application, where membranes are exposed to very low dose for very long periods of time, it is important to evaluate the validity of the ppm∙h concept and also determine the effect of different pH conditions during the test. In the last two decades, significant efforts have been made to study the sensitivity of polyamides (PA) to residual chlorine [7–17] and also to find ways to prevent the degradation [6,12,18–20]. In one of the largely accepted mechanisms, the degradation of PA is considered to start with chlorination of the amide nitrogen being followed by the chlorine migration to the aromatic ring via an intramolecular mechanism, also known as Orton rearrangement [6,21]. In addition, a competing mechanism, based on the hydrolysis of amidic group is believed to contribute to the overall performance, making the polymer's surface hydrophilic, and therefore more permeable for the water [13]. Additionally, it has been claimed that the presence of transition metals in the feed can have catalytic effects on membrane degradation [12,22] in the presence of chloramines. Membrane ageing was generally evaluated by studying the filtration performance (permeability and salt rejection) and monitoring the active layer's chemistry with infrared spectroscopy (ATR-FTIR). Interestingly, for many years, atomic force microscopy (AFM) has been employed in various fields of science to study micro and nano scale surface topographies. RO membranes, in particular, received special attention because of interesting correlations between topographical parameters and performance [23–27]. Surface topography of membranes determined by AFM has been linked in the past with membranes hydrophobicity [23] which in turn, could point to signs of ageing. The aim of the current study was to evaluate the validity of the ppm∙h concept on membrane ageing by hypochlorite under different

Summary of findings

B.C. Donose et al. / Desalination 309 (2013) 97–105

Table 1 Summary of findings for seven key studies of RO membranes ageing in hypochlorite.

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pH conditions, and to assess the repeatability and transfer of the results to different commercially available membranes. We also demonstrate the use of AFM as a supporting tool to assess membrane ageing. 2. Experimental 2.1. Membranes: performance assessment and ageing Three commercially available TFC RO membranes used for brackish water desalination and domestic effluent water reuse were used as model cross-linked, fully aromatic PA barrier layers, coded for the simplicity of presentation as “A”, “B” and “C”. Their codes and manufacturers are as follows: A = Toray TML20, B = Dow Filmtec BW-30 XFR, C = Hydranautics ESPA 2. Membrane B is known to have a proprietary antibiofouling coating. Square samples of 20×20 cm2, cut from virgin spiral-wound modules (A and C) and from a flat sheet coupon (B) have been stored in de-ionised (DI) water at 4 °C as sample stock to prevent microorganisms proliferation. Circular membrane coupons (14.6 cm2) cut from the stocks were statically aged, with only the active side being brought into contact with the surface of the sodium hypochlorite solutions at pH 4, 7 and 10, as opposed to dynamic ageing where chlorination is done under pressure while actively filtering. We chose the static ageing in polypropylene (PP) plastic containers (with good resistance to hypochlorite at room temperature) in order to avoid contamination of the chlorine solution with metals, known to catalyse PA degradation [12,22]. Colorimetric tests, by DPD (N, N-diethyl-p-phenylenediamine) method [28] were done at the end of the ageing stage, consistently showing a reduction of total chlorine of less than 10%. Membranes were exposed to chlorine at 1000, 3000 and 6000 ppm∙h for each of the three pH values. pH of the ageing solutions was adjusted using 0.1 N HCl and 0.1 N NaOH. In order to study the validity of the ppm∙h concept, each ppm∙h value was obtained from 3 combinations of dose and contact time e.g. for 1000 ppm∙h: 50 ppm×20 h, 100 ppm×10 h and 200 ppm×5 h. Before and after chlorination, membranes were subjected to performance tests employing a cylindrical dead-end filtration stirred cell (HP4750 Sterlitech). Salt rejection was determined through conductivity measurements with a Mettler Toledo S30 Conductivity Kit. All the filtration experiments were carried out at an applied pressure of 12 bars and 22 °C, involving a volume of 300 ml of de-ionised (DI) water (Millipore Academic, resistivity larger than 18 MΩ cm) or 1500 ppm NaCl saline solution. All experiments started with a membrane compaction stage required to reach steady performance. 2.2. ATR-FTIR Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectra were recorded on a Nicolet 5700 spectrometer. As an internal reflection element, a flat plate Ge crystal at an incident angle of 45° was used to get a higher contribution of the polyamide skin layer of the membrane to the spectrum. Typical Ge crystal penetration depth was estimated to be 0.5 μm as opposed to diamond which has an estimated penetration depth of approximately 1 μm. A minimum of 128 scans at a resolution of 4.0 cm−1 was signal-averaged. The membranes were placed on the ATR crystal and pressed onto the surface with a cylindrical press. 2.3. Scanning electron microscopy Sample imaging has been done employing a JEOL 7001 thermionic field emission scanning electron microscope. All samples have been Pt coated (Structure Probe Inc Sputter Coater, USA) for 90 s and vacuum oven dehydrated for at least 24 h before imaging. Image acquisition was done at 5 kV accelerating voltage and approximately 10 μm working distance.

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2.4. AFM All AFM imaging was performed in AC mode (intermittent contact) on an Asylum MFP-3D-BIO (Asylum Research, USA) placed on an anti-vibration table (Herzan, USA) within a noise-proof enclosure (TMC, USA). The cantilevers employed for imaging were Etalon (NT-MDT, Russia) with a nominal spring constant of 3 N/m and a nominal contact radius of 10 nm. Samples for AFM were dried prior to analysis for at least 48 hours in a forced-fan (Secador) desiccator. AFM was used to determine roughness parameters by extracting the root-mean-square (RMS) roughness as the best indicator of surface corrugation.

RMS ¼

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !ffi u u 1 2 t

ð1Þ

2

Vnpnts∑Y i

where Vnpnts is the number of height events and Y is the height of the peak. Recent studies [29] have suggested that using RMS alone may be misleading and they also proved that roughness is scale dependent. Consequently, the surface area difference (SAD) which is a measure of the percentile difference between actual surface area and the area of a perfectly flat domain with identical x and y dimensions was also measured. SAD ¼

  Am−Asc ⋅100 Asc

ð2Þ

where Am is the measured surface area and Asc is the imposed scanned area. RMS and SAD were obtained from multiple scans over 10 × 10 μm. 3. Results and discussion 3.1. Hydraulic performance 3.1.1. Baseline data Dead-end filtration experiments were done before ageing in order to establish the membrane coupons performance baselines (Table 2). As it has already been shown [7] coupons cut out from a large membrane sheet may display different performance. It should then be noted that for consistency any membrane coupons with abnormally low performance (under 97% salt rejection) were discarded as this was likely an indication of physical membrane damage or a faulty experimental set-up. This allowed assessing the ageing effects on membranes with very similar initial performance, as confirmed by the low standard deviations. The detailed results for each membrane are provided in supporting information, Fig. S1. However, significant differences could be observed when comparing the three different membranes, with for example clean water permeabilities of 5.0, 3.7 and 6.4 L∙m –2∙h –1∙bar –1 for the membrane A, B and C. Membrane B samples generally displayed lower permeabilities and salt rejections. This may be explained by the fact that coupons were obtained from a single membrane sheet in comparison to complete spiral-wound modules for the other two. Indeed, it can be expected that the

Table 2 Baseline values for the hydraulic performance of the three membranes tested (n = 27). A Clean water permeability (L∙m–2∙h–1∙bar–1) Salt solution permeability (L∙m–2∙h–1∙bar–1) Salt rejection (%)

B

C

5.0 ± 0.3

3.7 ± 0.3

6.4 ± 0.2

4.0 ± 0.2

3.2 ± 0.3

5.2 ± 0.1

99.1 ± 0.2

98.1 ± 0.3

99.3 ± 0.1

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such an extent that increasing the dose would not affect them further. It can then be expected that for the same ppm∙h with lower doses and longer exposure times differences might appear. Interestingly, at pH 7, with proportionally lower HOCl concentrations, greater differences could be observed between the different combination of exposure time and dose. One of the most critical examples is membrane B at 3000 ppm∙h. Indeed, at an exposure time of 20 h a permeability reduction of about 7% was measured; however, when the exposure time was decreased to 5 h and the dose proportionally increased the permeability dropped by 60%. This is a clear example that depending on the experimental conditions very different outcomes and conclusions could be obtained. As described above, a neutral to acidic pH dose was found to have a greater effect on membrane ageing than exposure time. Considering that in full scale applications membranes are exposed to very low concentrations for years, these results suggest that accelerated ageing trials will significantly overestimate the impact of the chlorine damage on membranes. Interestingly, in alkaline conditions (pH 10) when hypochlorite ions (OCl −) are the dominant species, the opposite trend was observed. The change in performance due to exposure to the solutions, characterised by an increase of the permeability (Lp / Lp0 > 1), was generally found to be greater when the exposure time was increased. For example, the relative permeability for membrane A at 3000 ppm∙h decreased from 1.29 to 1.18 and 0.95 when the exposure time was decreased from 20 h to 10 h and then 5 h. Surprisingly, this could suggest that only very small concentrations would affect the membranes but the phenomenon is slow and reaction time becomes the limiting factor. It is more probable that these results are due to a combination of reactions. As shown previously, the impact of HOCl was found to cause a significant decrease of the permeability. Here, the effect is opposite (note that these mechanisms will be discussed in details in the following sections). At pH 10, the sodium hypochlorite will dissociate at 99.7% into OCl − and the remaining 0.3% will be HOCl. When considering the tests at 3000 ppm∙h, this then means that for the exposure times of 20, 10 and 5 h HOCl concentrations of 0.5, 1.0 and 2.0 ppm, respectively, were present in the ageing solutions. Assuming that HOCl has a significant impact on the membranes even at a very low dose, the attack mechanism must be different in order to explain the trend of increasing permeability.

membrane sheet was handled and stored differently than full-scale modules from the time of production to its use. In addition, the proprietary coating of membrane B could have also an impact on its overall performance. Further studies would be required to determine the specific coating effect on performance. In any case, this generally shows that different performance can be expected from different TFC membranes and consequently these membranes can also be expected to behave differently in the ageing trials. In order to best assess the ageing of the hypochlorite solution at various pH, tests were carried out to evaluate the impact of pH alone on the membranes (Figs. S2 and S3 in the Supplementary content). The membranes were then exposed to DI water with pH adjusted to 4 and 10, at three different exposure durations, 20, 10 and 5 h. At pH 10, an increase of the clean water permeability of up to 9% was observed. In contrast, at pH 4, a reduction of about 4% of the permeability was measured. In both cases no change was observed on the salt rejection. These results will be taken in consideration when discussing the results from the ageing trials. 3.1.2. Evaluation of the ppm∙h concept For each ppm∙h studied, three combinations of exposure time and dose were tested on all three membranes and for the different pH conditions (4, 7 and 10). Validation of the ppm∙h concept implies that for one set ppm∙h, the exact same impact should be observed on a membrane, regardless of the combination of exposure time and dose. However, as it can be seen in Fig. 1, in most cases, variations were observed. In neutral and acidic pH, when hypochlorous acid (HOCl) is the dominant species, the observed impact characterised by a decrease of the permeability (Lp / Lp0 b 1), was found some cases to be more severe when the dose was increased and consequently the exposure time decreased. Nevertheless, sample size, the limited number of sampling locations on the sheet and experimental conditions inconsistencies may be responsible for the observed scattered trends, especially at pH 7 and 10. The smallest variations were recorded for all the tests at pH 4. This could be explained by the fact that at this pH a much greater impact was generally observed with permeability reductions from at least 50% up to 90%, even for the lowest dose and/or exposure time tested. This suggests that the changes in the membrane structure reached 1000 ppm.h

3000 ppm.h

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2

Membrane C

0

Membrane B

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1

1

0 4

7

10

4

7

10

4

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pH circles: 20h; squares: 10h; triangles: 5h Fig. 1. Relative water permeability dependence on pH, total dose and exposure times for all three types of membranes.

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Based on these observations, it is clear that performance variations can be expected when varying either the exposure time and dose and care should be taken when comparing data from different studies or transferring results to full scale operation. Generally, these results suggest that accelerated ageing trials should be carried out with exposure time as long as possible to limit the stronger effect of the dose observed in this study. 3.1.3. Hypochlorite ion dominated ageing The results presented above have highlighted the need to work with low doses and high exposure times in accelerated ageing trials for a better transferability of the findings. Considering this, the evaluation of the impact of the pH on ageing for the different membranes was then based on the results obtained with the longer exposure time tested here of 20 h (Fig. 2). For all three types of samples, at pH 10, where the predominant chlorine species is the hypochlorite anion [30], permeability is increasing in comparison to the initial condition (Lp > Lp0) (Fig. 2). The increase observed is many times larger than the one presented in the case of blank tests, where samples have been exposed to solutions of pH 10 in the absence of sodium hypochlorite (Figs. S2 and S3). This effect is irreversible for blanks as it persists in the subsequent permeability measurements carried out at neutral pH in the absence of chlorine. It is also apparent that higher ppm∙h exposure is responsible for higher permeabilities, owing to a more hydrophilic surface. Such a behavior suggests that the chlorine attack in alkaline environments is responsible for what has been hypothesised to be hydrolysis of the amide groups at the interface [13]. An overall increase in the number of surface negative charges, mainly as result of the dissociation of already existing and newly formed carboxylic acid groups of the PA, would allow an easier passage of water and hydrated ions molecules. Salt rejection is fluctuating close to the values before ageing, in a window of less than 5% and without any distinctive trends (Fig. S3, Supplementary information), which is in good agreement with previous studies (see Table 1). In addition, trends of relative salt passage (Fig. 3), defined as: RSP ¼ SP final =SP initial

101

where SP = 1-salt rejection, show that at pH 10 and high dose, RSP decreases as the ppm∙h increases. These data seem to support the idea that an increasing negatively charged membrane acts as a stronger barrier for the positively charged salt ions. At 6000 ppm∙h and highest dose (1200 ppm, triangles) the relative salt passage decreases linearly with the pH increase for all membranes suggesting an electrostatic rejection mechanism. Overall, the relative salt passage appears to depend on the type of membrane, for A and B fluctuating around unity at 1000 ppm∙h and 3000 ppm∙h and for type C being always higher than unity, therefore suggesting a stronger ageing effect. 3.1.4. Hypochlorous acid driven ageing At pH 7 and 20 hours of exposure (Fig. 2), where the molar ratio between hypochlorous acid (pKa = 7.53) and hypochlorite ion is 3.3 [30], permeability drops under the pre-ageing values (Lp b Lp0) for membranes A and C, with membrane B being slightly more resistant to ageing than the others, possibly because of its proprietary coating. For the non-coated membranes, A and C, in contrast to ageing performed at pH 10, permeability decreases linearly with the ppm∙h increase. Such behaviour comes to support the theory according to which ageing follows a mechanism based on combined surface group hydrolysis and ring chlorination, with the later being more pronounced due to the increasing amount of hypochlorous acid. Exposure to hypochlorous acid at pH 4 causes the most dramatic changes in performance as depicted in Fig. 2. At this pH, the reduction of permeability is reaching the highest percentage. Since the H bonds scaffold of the polymeric structure is collapsed because of ring chlorination, a significant compaction of the membrane is to be expected. The three membranes have distinctive trends in terms of salt rejection and such behaviour could be linked to the hydrophobicity of the polymer at the fluid interface. Insufficient data of the proprietary coating and synthesis procedure limits our data interpretation in terms of salt rejection. The recent results of Do et al. [13] obtained at pH 5, suggest that in the case of BW30 membrane aged with 1000 ppm∙h hypochlorite, salt permeability is similarly and significantly dropping, owing to the mixed mechanism of ring chlorination and hydrolysis. In addition, the authors suggest that in severe chlorination conditions, the coating layer could be partially removed along with changes in hydrophilicity. Similar cross-flow studies of Ettori

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Membrane B

Membrane A

Membrane C

Relative permeability (Lp/Lp0)

2.0

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1.0

0.5

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1000

3000

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squares: pH 4; circles: pH 7; triangles: pH 10 Fig. 2. Relative water permeability dependence on pH and ppm∙h for all membranes at the longest exposure time of 20 h.

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1000 ppm.h

8

3000 ppm.h

6000 ppm.h Membrane C

6

Relative salt passage (Spf/Spi)

4 2 0

Membrane B

6 4 2 0

Membrane A

6 4 2 0 4

7

10

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7

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7

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circles: 20h; squares: 10h; triangles: 5h Fig. 3. Relative salt rejection (1500 ppm NaCl saline solution) results from the ageing trials at an exposure time of 20 h and different ppm∙h parameters (1000, 3000 and 6000) for the three different membranes.

et al. [14] report more than 60% drop in water permeability at pH 5 and points to the changes in packing propensity of the membranes as results of the H-bonding weakening. In their study, salt rejection has been reported to decrease with 2-4% at doses above 400 ppm∙h also in agreement with the results presented in our current paper. 3.2. Advanced analytical tools 3.2.1. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy In order to obtain a better understanding of the complex phenomena taking place upon accelerated ageing, we employed FTIR spectroscopy as

a tool for the assessment of chemical changes. Fig. S5 shows characteristic IR absorbance peaks of all three membranes before ageing as well as those of the polyether sulphone (PES) reinforcing substrate. It can be observed that the PES peaks can be found in the membrane IR signature since the light probe has a penetration depth between 0.5 and 2.0 μm, while the active polyamide layer has an estimated thickness of only few hundreds of nanometres. As previously reported [9,13,14,21] FTIR can offer information regarding the types of functional groups present in the chemical structure of membranes. The most significant changes, after ageing, were recorded in the zones corresponding to N–H bending motion of amide, at 1541 cm−1, and aromatic amide and carbon double bond, at 1609 cm−1 [9,13,14,21]. An

Pristine membrane pH10 3000 ppm·h pH7 3000 ppm·h

Absorbance

pH4 3000 ppm·h

1,800

1,700

1,600

1,500

1,400

1,300

1,200

Wavenumbers (cm-1) Fig. 4. Typical FTIR spectra of membranes exposed to sodium hypochlorite at different pH (membrane A exposed at 3000 ppm∙h).

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integral suppression of the 1609 cm−1 peak accompanied by a significant reduction of the 1541 cm−1 peak has been always observed in the case of samples aged at pH 4, compare with no change characteristic for samples aged at pH 10 (Fig. 4). The case presented in Fig. 4, membrane A at 3000 ppm∙h, is representative for all three membranes regardless of the ppm∙h parameter. Such behaviour has been associated with ring chlorination by the Orton rearrangement mechanism [10] and linked with an increase of the packing density of the membrane, by increasing the mobility of the structure as described elsewhere [14,15]. The fact that peak depletion is absent in all of the ageing experiments at pH 10, regardless of concentration, supports the intrinsic relation between structural alteration and the presence of hypochlorous acid in solution. At this pH, zeta potential is reaching values close to −50 mV and in some cases −60 mV, always more negative for chlorinated membranes compared to virgin ones [13]. FTIR and zeta potential results lead to the conclusion that hydrolysis of the surface groups is the major mechanism governing ageing in the alkaline domain, owing to the effect of the minute amount of hypochlorous acid. In addition, since most of the carboxylic acid groups on the PA structure are fully dissociated at pH 10, the dominant ionic group in the ageing solution, the hypochlorite ion, would be electrostatically repelled. Chung et al. [31] advanced the idea that the PA chemical changes in the presence of hypochlorous acid happen within the first few hours of the exposure and that the mechanical changes, such as polymer “embrittlement”, continues after the oxidant attack for many hours. Such hypothesis built on the mechanical properties tests done at nanoscale and supported by X-ray photoelectron microscopy and FTIR are relevant for membrane behaviour at pH 4. A similar conclusion, that chlorination attack at acidic pH is very quick, was also reached by Kwon and Leckie [16]. In a recent publication of Cran et al. [11] it has been shown by employing FTIR, that ageing with chloramines is affecting only the PA barrier layer. Calculations of the absorbance ratio between 1549 cm −1 (amide II) and 1487 cm −1 (CH2 stretch in polyether sulphone [32]) peak intensities showed that, in time, the chemical structure is modified. Our attempt to estimate the absorbance ratio in the same manner failed. We observed that in the case of ageing with sodium hypochlorite, the intensity of the peaks corresponding to PES, 1487 cm −1 and 1240 cm −1 were affected by what has been previously reported to be a radical oxidation attack highlighted through relative elongation at break experiments involving membranes soaked in sodium hypochlorite at different pH [33–36].

3.2.2. Atomic force microscopy and scanning electron microscopy To test the effects of ageing on the topography of membranes, we employed AFM and SEM. Typical AFM and SEM images of untreated membranes are presented in Fig. 5. Membrane structure with random areas of possibly denser PA (lighter colour–higher topography) can be observed for samples A and B in both, AFM and SEM micrographs. For sample C, a more uniform structure is highlighted by the AFM and supported by the high resolution SEM scans. It can be concluded that the topography of the three membranes, even though they are made of a similar polyamide, is quite different, and in combination with proprietary coatings will have a significant impact in the operation, especially when fouling is involved. One can infer that membrane A, having a lower roughness could have better antifouling properties. Such a conclusion would have to be based though on additional information regarding the surface charging. Aged samples of the three membranes have been imaged (Fig. 6) after ageing in extreme exposure conditions (6000 ppm∙h, pH 4 and pH 10). The magnitude of roughness parameters obtained by averaging values from nine different 10× 10 μm locations is shown in Table 3. For membranes A and B, at pH 4, roughness is increasing from 42 nm to 98 nm (A) and from 89 nm to 105 nm (B). Membrane C has a

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small reduction in roughness, from 117 nm to 109 nm. SAD parameters are also scattered for the three membranes. AFM measurements confirm that at pH 4 two of the membranes underwent major structural transformations as a result of the hypochlorous acid action. These results indicate as well an increase in the two membranes' hydrophobicity as result of the hypochlorous acid attack. A minimal effect is observed in the case of membrane C. Such a variation between sample types could be explained by the particular nature of each membrane but also by the structural nonuniformities along the entire membrane sheet. It is therefore necessary to further the studies of membrane surface topography as a result of ageing and also to carefully consider the membranes’ manufacturing process, the coatings used to improve performance and the relation between surface parameters and the scanned area size in the microscopic measurements. 4. Conclusions Results from this study can be summarized as follows: - Membranes aged with NaOCl at pH 10 have increased water and saline solution permeability within the range of operational conditions tested. For all three membranes permeability is gradually increasing as exposure parameters increase. - As soon as hypochlorous acid concentration is increasing in the system, the permeability starts to decline, having the most significant effect at pH 4. All three membranes have decreasing permeability with increasing exposure parameters and as the medium become more acidic. - The ppm∙h concept fails to be valid in describing the performance evolution upon accelerated ageing in sodium hypochlorite, since the oxidant concentration is an important factor. It is worth pointing out though that accelerated ageing at high oxidant doses is a conservative measure of the actual ppm∙h that a membrane would sustain before damage occurs at lower oxidant concentrations typical of operational errors, where residual free active chlorine is put in contact with the membrane. - Contrary to chlorine exposure experiments in previous studies performed under filtration, in static conditions without transmembrane flux and within the range of our experimental range (up to 6000 ppm∙h chlorine exposure), salt rejection is only slightly affected by chlorination, each membrane behaviour depending on its own manufacturing proprietary recipe. - The mechanism responsible for the recorded performance trends is based on the hydrolysis of surface groups and bulk polyamide ring chlorination. - FTIR confirms that chemical changes reach a maximum in accelerated ageing under acidic conditions. - AFM studies show roughness increase at pH 4 only for two of the membranes pointing to the necessity of careful consideration in data interpretation when different types of samples are investigated. These results prove that static ageing experimentation has significant importance from a fundamental point of view because it allows eliminating the interferences resulted from the combination between pressure, flow, chlorine species and metal catalysts which may additionally accelerate the ageing process. Acknowledgements The authors wish to acknowledge financial support and provision of membranes to Veolia Water Australia and Seqwater. We also acknowledge Dow (Chemical) Australia for the provision of membrane samples. This work was performed in part at the Queensland node of the Australian National Fabrication Facility (ANFF-Q), a company established under the National Collaborative Research Infrastructure Strategy to provide nano and microfabrication facilities for Australia's

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A

B

C

5 µm

5 µm

1 µm Fig. 5. AFM micrographs of the three membranes (top) obtained by scanning an area of 30 × 30 μm (height scale bar is 800 nm, identical for all three images) and SEM micrographs of the same samples scanned at 5000× magnification (middle) and 15,000× magnification (bottom).

Fig. 6. 30 × 30 μm AFM micrographs of the aged membranes (A, B and C) at pH 4, pH 10 (6000 ppm∙h). Height scale bar is 800 nm.

B.C. Donose et al. / Desalination 309 (2013) 97–105 Table 3 Roughness parameters, RMS and SAD, as a function of the ageing conditions (error intervals correspond to standard deviations of nine measurements). Membrane type and conditioning A

B

C

Untreated pH 4 pH 10 Untreated pH 4 pH 10 Untreated pH 4 pH10

RMS (nm)

SAD (%)

42 ± 11 98 ± 25 48 ± 12 89 ± 8 105 ± 1 87 ± 10 117 ± 4 109 ± 13 81 ± 1

2±1 30 ± 1 11 ± 1 28 ± 1 28 ± 1 22 ± 1 50 ± 1 39 ± 1 36 ± 1

researchers. SEM imaging has been done at the Centre for Microscopy and Microanalysis. We would also like to thank to Dr Lauren Butler for the FTIR measurements and Dr Elena Taran for the AFM imaging and analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.desal.2012.09.027. References [1] J.E. Cadotte, R.S. King, R.J. Majerle, R.J. Petersen, Interfacial synthesis in the preparation of reverse-osmosis membranes, J. Macromol. Sci. Chem. A15 (1981) 727–755. [2] J.E. Cadotte, R.J. Petersen, R.E. Larson, E.E. Erickson, New thin-film composite seawater reverse-osmosis membrane, Desalination 32 (1980) 25–31. [3] L.F. Greenlee, D.F. Lawler, B.D. Freeman, B. Marrot, P. Moulin, Reverse osmosis desalination: water sources, technology, and today's challenges, Water Res. 43 (2009) 2317–2348. [4] J.S. Baker, L.Y. Dudley, Biofouling in membrane systems — a review, Desalination 118 (1998) 81–89. [5] C. Bellona, J.E. Drewes, P. Xu, G. Amy, Factors affecting the rejection of organic solutes during NF/RO treatment—a literature review, Water Res. 38 (2004) 2795–2809. [6] J. Glater, S.-k. Hong, M. Elimelech, The search for a chlorine-resistant reverse osmosis membrane, Desalination 95 (1994) 325–345. [7] S.T. Mitrouli, A.J. Karabelas, N.P. Isaias, Polyamide active layers of low pressure RO membranes: data on spatial performance non-uniformity and degradation by hypochlorite solutions, Desalination 260 (2010) 91–100. [8] W.R. Adams, The effects of chlorine dioxide on reverse osmosis membranes, Desalination 78 (1990) 439–453. [9] A. Antony, R. Fudianto, S. Cox, G. Leslie, Assessing the oxidative degradation of polyamide reverse osmosis membrane—accelerated ageing with hypochlorite exposure, J. Membr. Sci. 347 (2010) 159–164. [10] S. Avlonitis, W.T. Hanbury, T. Hodgkiess, Chlorine degradation of aromatic polyamides, Desalination 85 (1992) 321–334. [11] M.J. Cran, S.W. Bigger, S.R. Gray, Degradation of polyamide reverse osmosis membranes in the presence of chloramine, Desalination 283 (2011) 58–63. [12] M.K. da Silva, I.C. Tessaro, K. Wada, Investigation of oxidative degradation of polyamide reverse osmosis membranes by monochloramine solutions, J. Membr. Sci. 282 (2006) 375–382. [13] V.T. Do, C.Y. Tang, M. Reinhard, J.O. Leckie, Degradation of polyamide nanofiltration and reverse osmosis membranes by hypochlorite, Environ. Sci. Technol. 46 (2012) 852–859.

105

[14] A. Ettori, E. Gaudichet-Maurin, J.-C. Schrotter, P. Aimar, C. Causserand, Permeability and chemical analysis of aromatic polyamide based membranes exposed to sodium hypochlorite, J. Membr. Sci. 375 (2011) 220–230. [15] Y.-N. Kwon, J.O. Leckie, Hypochlorite degradation of crosslinked polyamide membranes: I. Changes in chemical/morphological properties, J. Membr. Sci. 283 (2006) 21–26. [16] Y.-N. Kwon, J.O. Leckie, Hypochlorite degradation of crosslinked polyamide membranes: II. Changes in hydrogen bonding behavior and performance, J. Membr. Sci. 282 (2006) 456–464. [17] H. Shemer, R. Semiat, Impact of halogen based disinfectants in seawater on polyamide RO membranes, Desalination 273 (2011) 179–183. [18] N.H. Lin, M.M. Kim, G.T. Lewis, Y. Cohen, Polymer surface nano-structuring of reverse osmosis membranes for fouling resistance and improved flux performance, J. Mater. Chem. 20 (2010) 4642–4652. [19] T. Kawaguchi, H. Tamura, Chlorine-resistant membrane for reverse-osmosis .1. Correlation between chemical structures and chlorine resistance of polyamides, J. Appl. Polym. Sci. 29 (1984) 3359–3367. [20] T. Kawaguchi, H. Tamura, Chlorine-resistant membrane for reverse-osmosis .2. Preparation of chlorine-resistant polyamide composite membranes, J. Appl. Polym. Sci. 29 (1984) 3369–3379. [21] G.-D. Kang, C.-J. Gao, W.-D. Chen, X.-M. Jie, Y.-M. Cao, Q. Yuan, Study on hypochlorite degradation of aromatic polyamide reverse osmosis membrane, J. Membr. Sci. 300 (2007) 165–171. [22] C.J. Gabelich, J.C. Frankin, F.W. Gerringer, K.P. Ishida, I.H. Suffet, Enhanced oxidation of polyamide membranes using monochloramine and ferrous iron, J. Membr. Sci. 258 (2005) 64–70. [23] K. Boussu, B. Van der Bruggen, A. Volodin, J. Snauwaert, C. Van Haesendonck, C. Vandecasteele, Roughness and hydrophobicity studies of nanofiltration membranes using different modes of AFM, J. Colloid Interface Sci. 286 (2005) 632–638. [24] V. Freger, Nanoscale heterogeneity of polyamide membranes formed by interfacial polymerization, Langmuir 19 (2003) 4791–4797. [25] V. Freger, J. Gilron, S. Belfer, TFC polyamide membranes modified by grafting of hydrophilic polymers: an FT-IR/AFM/TEM study, J. Membr. Sci. 209 (2002) 283– 292. [26] I. Koyuncu, J. Brant, A. Luttge, M.R. Wiesner, A comparison of vertical scanning interferometry (VSI) and atomic force microscopy (AFM) for characterizing membrane surface topography, J. Membr. Sci. 278 (2006) 410–417. [27] G.J. Vancso, H. Hillborg, H. Schonherr, Chemical composition of polymer surfaces imaged by atomic force microscopy and complementary approaches, Adv. Polym. Sci. 182 (2005) 55–129. [28] A.T. Palin, Determination of free chlorine and combined chlorine in water by the use of diethyl-p-penylene diamine, J. Am. Water Works Ass. 49 (1957) 873. [29] P.C.Y. Wong, Y.-N. Kwon, C.S. Criddle, Use of atomic force microscopy and fractal geometry to characterize the roughness of nano-, micro-, and ultrafiltration membranes, J. Membr. Sci. 340 (2009) 117–132. [30] M. Deborde, U.v. Gunten, Reactions of chlorine with inorganic and organic compounds during water treatment — kinetics and mechanisms: a critical review, Water Res. 42 (2008) 13–51. [31] J.Y. Chung, J.-H. Lee, K.L. Beers, C.M. Stafford, Stiffness, strength, and ductility of nanoscale thin films and membranes: a combined wrinkling and cracking methodology, Nano Lett. 11 (2011) 3361–3365. [32] M.R. Pereira, J. Yarwood, ATR-FTIR spectroscopic studies of the structure and permeability of sulfonated poly(ether sulfone) membranes .1. Interfacial waterpolymer interactions, J Chem Soc Faraday Trans 92 (1996) 2731–2735. [33] C. Causserand, S. Rouaix, J.P. Lafaille, P. Aimar, Degradation of polysulfone membranes due to contact with bleaching solution, Desalination 199 (2006) 70–72. [34] C. Causserand, S. Rouaix, J.P. Lafaille, P. Aimar, Ageing of polysulfone membranes in contact with bleach solution: role of radical oxidation and of some dissolved metal ions, Chem Eng Process 47 (2008) 48–56. [35] E. Arkhangelsky, D. Kuzmenko, V. Gitis, Impact of chemical cleaning on properties and functioning of polyethersulfone membranes, J. Membr. Sci. 305 (2007) 176–184. [36] E. Arkhangelsky, D. Kuzmenko, N.V. Gitis, M. Vinogradov, S. Kuiry, V. Gitis, Hypochlorite cleaning causes degradation of polymer membranes, Tribol Lett 28 (2007) 109–116.