Stability of polyethersulfone membranes to oxidative agents: A review

Accepted Manuscript Stability of polyethersulfone membranes to oxidative agents: A review Misgina Tilahun Tsehaye, Svetlozar Velizarov, Bart Van der Bruggen PII:

S0141-3910(18)30284-2

DOI:

10.1016/j.polymdegradstab.2018.09.004

Reference:

PDST 8629

To appear in:

Polymer Degradation and Stability

Received Date: 22 August 2018 Accepted Date: 11 September 2018

Please cite this article as: Tsehaye MT, Velizarov S, Van der Bruggen B, Stability of polyethersulfone membranes to oxidative agents: A review, Polymer Degradation and Stability (2018), doi: https:// doi.org/10.1016/j.polymdegradstab.2018.09.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Stability of Polyethersulfone Membranes to Oxidative Agents: A Review

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Corresponding author: Tel +32 16 322340, Mobile +32 471 380026. E-mail address: [email protected]

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Misgina Tilahun Tsehayea, Svetlozar Velizarovb, Bart Van der Bruggencd* a LEPMI, Université Grenoble Alpes, Grenoble INP, UMR5279, 38000 Grenoble, France b LAQV-REQUIMTE, Chemistry Dept., FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal. c Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium. d Faculty of Engineering and the Built Environment, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa.

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Abstract

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Polyethersulfone (PES) is one of the most commonly used polymers for preparation of ultrafiltration and nanofiltration membranes. However, oxidative degradation of PES-based membranes, which results from exposing the membranes to oxidative agents, is limiting their operational lifespan and possible areas of application. Despite the high need for a fundamental understanding of the detailed oxidative degradation mechanism(s) of PES membranes in order to improve the effectiveness of cleaning/disinfecting agents and/or develop PES membranes with a higher tolerance to oxidative agents, it still remains an insufficiently understood topic. Therefore, this review aims at analyzing and critically discussing the recent state-of-the-art on the degradation mechanisms of PES membranes, focusing on the effects of chlorine-based oxidants (mainly NaOCl) and H2O2. Strategies that can be useful for minimizing/preventing oxidative PES membranes attack are presented. Finally, further prospective study possibilities to fill in the existing research gaps in this area are highlighted.

Key words: Polyethersulfone membranes, Oxidative degradation mechanism, Sodium

Contents

Introduction .................................................................................................................................... 2 1.1.

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1.

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hypochlorite, Hydrogen peroxide, Degradation prevention

Overview of Polyethersulfone Membranes .............................................................................. 2

1.2 Occurrence and Impact of Oxidative Agents ................................................................................... 4 1.3 2.

Objectives and Scope ............................................................................................................... 6

Oxidative PES Membranes Degradation ........................................................................................... 7 2.1.

PES Membranes Degradation by NaOCl .................................................................................... 8

2.2.

PES Membranes Degradation by H2O2 .................................................................................... 16

2.3. Discussion and Implications ......................................................................................................... 18 3.

Strategies to minimize PES membranes degradation ...................................................................... 21

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4.

3.1.

Use of Free Radical Scavengers .............................................................................................. 22

3.2.

Reinforcing PES membranes with Nanoparticles..................................................................... 22

3.3.

Introducing a Layer of Nanoparticles ...................................................................................... 24

Conclusions and Future Prospects.................................................................................................. 27

1. Introduction 1.1. Overview of Polyethersulfone Membranes

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References ............................................................................................................................................ 29

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Continuous research efforts on developing more sustainable water treatment technologies have been done over the past years in order to reduce scarcity of water and improve its quality. Membrane-based separation techniques have been widely implemented as an efficient and more environmentally friendly alternative water treatment technology. Without any doubt, the use of membranes has revolutionized both drinking water production and wastewater treatment sectors. Nowadays, they are widely used in treating wastewaters and various purifying water applications and they are believed to play an even broader role in the future [1–3].

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Because of the relatively simple pore-forming mechanism and high flexibility, polymers are nowadays the most widely used type of membrane materials [4]. Among the various existing polymeric materials, polyethersulfone (PES), a high-performance thermoplastic, is widely used for preparation of polymeric membranes [5], such as ultrafiltration (UF) [6] and nanofiltration (NF) [7] membranes. This is mainly due to the fact that PES has excellent mechanical properties, a good thermal stability (Fig. 1) and operation over a broad range of temperatures and pH values. Most commercial NF membranes are thin film composite (TFC) polyamide (PA) membranes with a porous polysulfone (PS) support [8,9]. Additionally, PES is also commonly used as the main polymeric material for the preparation of lab-made [10,11] and commercial [12,13] NF membranes.

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Fig. 1. Chemical structure of PES.

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The most common method to prepare PES-based asymmetric membranes is the phase inversion process [14] (Fig. 2a). In this process, the formation of the membrane occurs in a very short time and involves a number of consecutive steps [15]. The PES is first dissolved in an appropriate solvent to form a homogeneous solution, the solution is then cast into a film of the order of 100 - 500 µm thickness, and immersed in a nonsolvent bath (also known as coagulation bath), typically water [16,17]. A number of researchers have studied the effects of various parameters, which determine the resulting membrane morphology, such as polymer material (type, concentration and molecular weight), solvent type, additives (e.g., polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG)), non-solvent type, coagulation bath temperature and casting conditions (relative humidity, temperature and casting speed). Both thermodynamic and kinetic (mutual diffusion among the components) factors control the final morphology of the membrane. The phase equilibrium between the involved components define the thermodynamic part of the system and the thermodynamic states of the solution can be described by a phase diagram [18–20] (Fig. 2b). Titration is used to construct the ternary phase diagram for PES/solvent/non-solvent system [21–24].

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For cleaning and disinfection of (PES) membranes, various chemical agents, typically chlorinated alkaline or oxidizing solutions can be used. These chemicals have been reported to attack the PES polymer matrix [25–27]. We believe that the membrane degradation mechanism in contact with hypochlorite, free bromines, chloroamines and hydrogen peroxide (H2O2) could be similar; however, with a distinct degree of damage under identical testing conditions.

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Fig. 2. (a) Phase inversion steps (solution formation, casting, and immersion in a nonsolvent bath) and (b) Ternary phase diagram during membrane formation. Fig. 2 (b) is taken with permission from [18]. Copyright 2018 Elsevier.

1.2 Occurrence and Impact of Oxidative Agents

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Membrane fouling, especially biofouling, is a commonly observed phenomenon when using PES membranes in practical applications. PES membranes fouling arises from the relatively hydrophobic nature of PES [28] and it limits their performance and lifespan [29,30]. In order to remove the foulants from the membrane surface, thus minimizing the effect of fouling and restoring membrane performance, chemical cleaning/disinfecting and/or physical cleaning (mechanical, hydraulic or electrical) must be performed [31]. Chemical cleaning/disinfection using chlorine-containing solutions has been commonly applied. The purpose of such cleaning is to disinfect the membranes, and increase hydrophilicity of the parent organic foulants by oxidizing the organic foulants to functional groups, which reduces their attraction and deposition on the membrane surface [32,33]. Because of its availability, high cleaning efficiency and reasonable cost, sodium hypochlorite (NaOCl), also called bleach, is often used. However, frequent cleaning using NaOCl has been found to degrade PES membranes [25,34].

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Similarly, other cleaning agents, such as H2O2, a powerful oxidizing agent, can be used. However, in addition to decomposing the organic foulants, H2O2 has been found to attack the chains of the polymer . . membrane matrix [35]. Moreover, H2O2 can be converted into free radicals (OH and OOH ) [36], with a

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very high reactivity (Table 1), during Fenton’s reaction that can further degrade the membrane in presence of metal ions, such as Fe2+ and Cu2+. Furthermore, as a chemical product, H2O2 is commonly used in many industries, such as in textile, paper, food, pharmaceuticals, bio-refineries and semiconductor processing (and/or producing) industries [37–40]. Lignocellulose (composed of mainly lignin, cellulose and hemicellulose), which is the most abundant renewable biomass on earth, employs hydroxyl radical derived from H2O2 for the degradation of the lignin component. The dissolution of most hemicelluloses and almost 50% of lignin has been achieved using a solution of 2% H2O2 at 30 oC [41,42]. In fact, H2O2 (up to 1.5 v/v%) is commonly used as an oxidative pretreatment to decrystallize cellulose in the biorefinery process [40]. In food-processing industries, H2O2 is used for sterilization (35% H2O2), for the removal of unwanted substances (such as residual chlorine) and product purification. Similarly, H2O2 has also been used in pharmaceutical applications for disinfection of instruments. It is mainly used for hair care products preparations. For these applications, 3 to 12% H2O2 in dilute solutions is common [43,44]. As a consequence, many industrial wastewaters contain H2O2 as a contaminant [37,45]. Therefore, when employing PES-based membranes for treating such wastewaters, the H2O2 or intermediate radicals could attack the PES polymer. In our recent work, H2O2 immersed pristine and commercial PES NF membranes were observed to decrease their separation performance [35]. The occurrence and effect of the commonly used oxidative agents are summarized in Fig. 3. Table 1. Oxidation potential of various oxidizing species [25,46] Oxidation potential (Volts)

Fluorine (F2) . Hydroxyl radical (OH )

Ozone (O3)

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Hydrogen peroxide (H2O2)

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Oxidant

3.0 2.8 2.1 1.8 1.7

Chlorine dioxide (ClO2)

1.5

Hypochlorous acid (HClO)

1.49

Chlorine (Cl2)

1.4

Hypochlorite ions OCl-)

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Potassium permanganate (KMnO4)

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Fig. 3. Occurrence, and impacts of oxidative agents on PES membranes.

1.3 Objectives and Scope

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The relevant literature clearly demonstrates that oxidative degradation of PES membranes is an inevitable side effect of employing disinfecting/cleaning agents [25,26,34]. Therefore, there is a growing concern for its minimization [35]. This calls for developing novel PES membranes with a higher tolerance to oxidative agents [35]. Furthermore, to extend the application of PES membranes for treating industrial wastewaters containing oxidative agents, there is a need for a comprehensive understanding of the associated degradation mechanism(s).

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So far, most studies reported in the literature have been mainly dedicated to chemical cleaning/disinfection. Review papers by Regular et al. [47] on chemical cleaning/disinfection and ageing of different polymeric UF membranes and by Shi et al. [31] on fouling and cleaning of UF (including PES-based) membranes can be mentioned. Compared to other oxidative agents, the effect of bleach solution on PES memranes is a better explored topic. However, generally speaking, the mechanism(s) controlling oxidative ageing of PES membranes remains a less understood topic. The purpose of this paper is to review recent studies on the degradation of PES membranes due to commonly used oxidative agents, focusing on NaOCl and H2O2. Degradation mechanisms of PES membranes are elucidated in detail. Finally, prospective strategies for protection of PES membranes and minimization of their oxidative ageing are discussed.

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2. Oxidative PES Membranes Degradation

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Polymer degradation may be defined as any undesirable change in the physical or chemical properties of the polymer as a result of externally applied stimuli [48]. Membrane degradation is commonly grouped into chemical/electrochemical, thermal and mechanical degradation categories [49,50]. Of these three classifications, (PES) membrane degradation due to chemicals (also known as oxidative degradation) is the main subject of this review paper.

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The tolerance of (PES) membranes to oxidative/cleaning agents is usually studied using immersion techniques [51,52]. The membranes are immersed in a solution, containing the oxidizing agent, for a certain period of time with or without other components present, such as metal ions and radical scavengers. On the other hand, if the oxidizing agent is found to be part of wastewater as a contaminant, degradation/stability testing by passing the solution through the membrane (in a throughput operation mode) can provide more relevant data regarding its degradation/stability compared to the membrane immersion mode. To conclude on the stability of the membranes to an oxidative agent, the surface morphology and separation performance before and after exposing to the working solution are usually compared. An increased pure water flux (and/or decline in target solute(s) rejection performance) and alterations in membrane integrity and morphology, such as cracking or pitting are typical indications of chemical degradation.

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Various tools can be used to characterize oxidative degradation of membranes. These assessment techniques are categorized into five major groups (Fig. 4): change in filtration performance, change in surface characteristics, change in chemical characteristics, change in morphological characteristics and change in mechanical and thermal characteristics [53].

Fig. 4. Membrane degradation assessment tools. Adapted from [53]. Different PES membranes degradation mechanisms: chain-scission and free radical oxidation (attack) have been reported in the literature. The chain-scission degradation mechanism would result in PES polymer terminated by different end groups, such as sulfonic acid or phenyl chloride [25,26]. The studies on oxidative degradation of PES membranes by chlorine-based agents (typically, NaOCl), H2O2 and other 7

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oxidative chemicals are reviewed. The first part is devoted to degradation provoked by NaOCl, whereas degradation by H2O2 is discussed in section 2.2. Other oxidative agents are discussed in section 2.3.

2.1. PES Membranes Degradation by NaOCl

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Unlike the degradation of PES membranes by other oxidative agents, the effect of exposing various polymeric membranes, such as PVDF [54,55], PAN [56], polyamide [57], PS [58,59], PES/PVP [60,61] and PES [25–27,62,63] membranes to NaOCl has been better explored. In this section, the effect of treating pure PES and PES/PVP membranes with NaOCl is discussed. Additionally, the stability testing conditions, degradation monitoring tools and main remarking results of the significant studies available in the literature are discussed. The chlorine stability of PES membranes are discussed at the end of this section by comparing their lifespan with commonly used polymeric (typically, PA for reverse osmosis) membranes.

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Wienk et al. [60] investigated the effect of NaOCl (3000 ppm) treatment on PES/PVP membranes at three different pH values (3.3, 6.9 and 11.5). A decrease in the molecular weight of PVP (due to the selective removal of PVP from the membrane matrix) was confirmed using gel permeation chromatography (GPC). The decrease in molecular weight of PVP was the highest at pH 11.5. A chain scission due to a radical mechanism was presumed. In this study, no PES-chain scission was observed using steric exclusion chromatography (SEC), especially at pH 11.5. This could be due to the fact that in highly alkaline solutions, OCl- with oxidation potential of only 0.94 V is the dominant oxidant. It seems that at this high pH value, OCl- is able to remove PVP but not PES from the PES/PVP membrane. However, contrasting results were reported from studies done by Qin et al. [61] and Rabiller-Baudry et al.[64]. Qin et.al . [61] found out that a PES/PVP membrane-treated with NaOCl (3000 ppm) showed a five times increment in post-treatment water flux and a narrower pore size distribution. Moreover, the rate of degradation of the membranes when exposed to NaOCl was found to be accelerated by the presence of PVP in the work of Rabiller-Baudry et al. [64]. However, it is difficult to compare these three studies because the experiments were not performed under identical conditions. For instance, the pH of NaOCl solution, a critical factor, was not consistent, if not even unmentioned, as e.g., in [61].

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The degradation mechanism of PES membranes by NaOCl was first proposed by Thominette et al. [65], who demonstrated the formation of sodium sulfonate- unstable compound. Deterioration of mechanical strength of the NaOCl-treated membranes as a result of chain scission at the sulfone groups of the PES was reported. A similar degradation mechanism was proposed, later on, by Arkhangelsky et al. [25], who studied the impact of cleaning of PES UF membranes using a commercially available bleach solution at pH 7.2. They assumed the formation of phenyl sulfonate, which was confirmed by using X-ray photoelectron spectroscopy (XPS) and Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, as a result of chain scission of the PES polymer (Fig. 5a). This resulted in a mechanical strength decrease of the tested membranes. It seems that HOCl, which is a strong non-radical oxidant, attacks the C-S bond of PES and converts the SO2 groups into SO3 groups as explained by Lewin and Sello [66], in a similar way to the degradation of PES membranes by H2O2 [35]. Many researchers reported that PES membranes treated with NaOCl showed a decline in mechanical strength and an increase in water flux. Begoin et al. [27] concluded that long term treatment of NaOCl (7600 ppm, pH 11.5) on spiral UF PES membranes caused 8

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breakage of the C-S bond of the polymer, confirmed by FTIR-ATR (Fig. 5b). As a result, cracking of the PES layer were observed (Fig. 5d). The membranes were reported to be stable in acid and alkaline solutions. Fig. 5 shows degradation mechanism proposed by [25] (a), and the ATR-FTIR spectra (b and c) and photograph (d and e) of pristine and NaOCl-treated PES membranes [27]. The relative absorbance (A) was calculated using:

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Relative absorbance = Hx/H1240, where Hx is band height at a given wavenumber (x cm-1) and H1240 is band height at 1240 cm-1 (PES membrane, pure band).

Fig. 5. Proposed PES degradation by NaOCl (a), Raw ATR-FTIR spectra of virgin and aged in PES membranes at pH = 11.5 (b) and 9.0 (c) and Photograph of the PES membranes after long-term treatment in bleach at pH = 11.5 (d) and pH = 9 (e). Fig . 5a taken with permission from [25], Copyright 2018 Elsevier. Fig. 5b-e taken with permission from [27], Copyright 2018 Elsevier. Susanto and Ulbricht [67] investigated the change in surface chemistry, hydrophobicity and water flux of PES UF (pristine and PES/additives) membranes after immersing in a NaOCl solution (400 ppm) for up to 10 days. In addition to a significant increase in membranes water flux, a slight decrease in water contact angles of pristine PES membranes was also reported. The authors speculated that an increase in porosity of the membranes as a result of the PES degradation by NaOCl should have 9

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occurred. No direct comparison can be made with other related works as the pH during degradation was not mentioned. By lowering the concentration of NaOCl to 200 ppm, Nasrul Arahman et al.[68] studied the stability of pristine and blended PES membranes soaked in NaOCl solution for 10 days; no change in permeability of the pristine membranes was detected. Additionally, the hydrophilicity of the membranes was not changed. On the other hand, the modified PES membranes, such as PES/PVP, showed a change in permeability and hydrophilicity after soaking in the NaOCl solution. The pristine PES membranes were stable probably because of the low NaOCl concentration used. At industrial scale, 200 ppm of NaOCl solution (pH 11.5) is commonly used for disinfection purposes [27]. Moreover, no information was provided about the pH of the NaOCl solution.

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The next significant study on this topic was done by Yadav et al.[26], who investigated the effect of 700 ppm NaOCl solutions (at 55 0C and at pH 9 and 12) for two exposure times (10, 000 ppm-day and 25,000 ppm-day) on PES UF membranes. A new reaction mechanism, which involves chain scission of the PES backbone into sulfonic acid groups and phenyl chloride groups was proposed (Fig. 6a). The sulfonic acid was formed as a result of hydrolysis of sodium sulfonate, a sodium salt, in water. The effect of NaOCl as a cleaning agent was prominent in altering the surface morphology, whereas the tensile strength of the membranes did not show much alteration. Surface pitting was visible on the surface of the membranes exposed to NaOCl at pH 9 and not at pH 12 which indicates the strong pH dependence of NaOCl activity and dissociation. Moreover, as shown in Fig. 6c, the decrease in absorbance of aged PES UF membranes was prominent at pH 9 (compared to pH 12), which could be due to depletion of PES from the membrane surface. Fig. 6 presents the proposed reaction mechanism and the SEM and FTIR-ATR results reported in this study.

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Fig. 6. (a) Proposed PES degradation by NaOCl, (b) SEM image of hypochlorite-treated PES (Koch) membrane pH 9, 25,000 ppm-day treated membrane at 1000x and (c) FTIR-ATR spectra of new and Koch PES membranes aged in different NaOCl solutions. Adapted with permission from [26]. Copyright 2018 Elsevier.

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Pellegrin et al. [69] confirmed the degradation of PES/PVP membranes at pH 8, as a result of a radical induced PVP degradation. Similarly, appearance of chlorine was not detected in the work of Prulho et al. [63], who provided evidence for the formation of phenol groups instead, as a result of the radical oxidation of the aromatic rings of PES (Fig. 7a), when PES/PVP membranes were exposed to NaOCl (4000 ppm of chlorine at pH of 8 and 12). As shown in Fig. 7b, the new band formed at 1030 cm-1 in the ATR-FTIR spectra belongs to the phenol vibration. Formation of radicals from bleach solutions can be also concluded from this study, which was suggested in [70].

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Fig. 7. (a) Phenol formation through oxidation of PES in the PES/PVP blends and (b) Evolution of the IR spectra after immersing a 50/50 PES/PVP blend film in a hypochlorite solution for 48 h. Taken with permission from [63]. Copyright 2018 Elsevier.

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More importantly, Hanafi et al.[71] used an advanced electrokinetic characterization tools to understand the degradation of commercial PES/PVP UF (HFK-131) and NF (NP030) membranes in 400 ppm NaOCl at pH 8. Streaming current measurements (using a SurPASS electro-kinetic analayzer) were supported by ATR-FTIR spectroscopy analysis. By combining both techniques, the formation of first phenol groups and later on sulfonic acid groups on the surface of the membranes were demonstrated, which suggest two different possible degradation mechanisms. The phenol groups, which are very weak acids, were first formed as a result of radical oxidation of PES by OH. radicals, according to the same reaction mechanism as proposed by Prulho et al. [63] (Fig. 7a). The formation of these weak acid groups is represented in Fig. 8a, which shows a net charge density increment for pH values around 10. After a prolonged exposure to NaOCl, sulfonic acid groups were formed as a result of PES chain-scission. This is in line with the findings reported in the work of Yadav et al. [26]. The formation of these groups is shown in a new band of the FTIR data presented in Fig. 8b.

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Fig. 8. (a) pH dependence of the zeta potential of HFK-131 membranes for various exposure times to 400 ppm of NaOCl solutions at pH 8 and (b) 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. Taken with permission from [71]. Copyright 2014 American Chemical Society.

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Moreover, recently, Hanafi et al. [72] studied the effect of ageing of commercial HFK-31 UF and NTR 7459 NF PES/PVP membranes with 200 ppm NaOCl solutions at 20 + 2 oC. Interestingly, they investigated the effect of HClO and OCl- components by adjusting the pH of the NaOCl solution to 6.0 (HClO predominance), 8.0 (coexistence of HClO and OCl-) and 11.5 (OCl- predominance). The readers should refer to Fig. 10 for more information regarding the ratio of HClO and OCl as a function of pH. Under these testing conditions, PES chain-scission occurred at pH 6.0 and 8.0; however, no PES chain-scission was detected at pH 11.5, which implies that OCl was not responsible for PES chain-scission. However, PVP was degraded at all solution pH studied. From both works of . Hanafi et al., it can be concluded that HOCl and OH are the responsible species for PES chainscission. Furthermore, HOCl was identified to have a greater impact than that of the free radicals. Table 2 presents the main observations of published studies on degradation of PES membranes by NaOCl.

[NaOCl]

pH

Stability/degrada tion test and monitoring tools 13 C-NMR, IR and GPC

PES and PES/PVP

3000 ppm

3.6, 6.9 and 11.5

PES/PVP

4000 ppm

-

PES

0.4 wt%

PES

-

8

Spectroscopic methods (IR, RMN), SEC and tensile tests

PES

150 ppm

7.2

XPS, ATR-FTIR, and AFM

PES

700 ppm

9 and 12

FTIR-ATR,

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Membrane materials

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Table 2: Summary of published works on degradation of PES and PES/PVP membranes by NaOCl

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SEM and flux change

9 and 11.5

SEM-EDX and FTIR-ATR

Main remarks

Ref.

-decrease in molecular weight of PVP -PES resistant to NaOCl treatment -removal of PVP -five times membrane flux increment -narrower pore size distribution -breakage of C-S bond, Cl-S bond formation -PES degradation -degradation of PES in the membrane -chain scissions localised at the sulfone group

[60]

-Chain scission of the PES polymer (formation of phenyl sulfonate). -decrease in mechanical strengths of the membranes and loss in membrane integrity -Surface pitting and cracking was

[25]

[61]

[27]

[65]

[26] 14

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-

contact angle, zeta potential and FTIR spectroscopy

PES, PES/Plu, PES/PVP, and PES/Tel

200 ppm

-

Filtration performance, hydrophilic property and AFM

PES

700 ± 50 ppm

9, 10, 11 water flux, whey and 12 proteins rejection and SEC

PES/PVP

350 ppm

8

PES/PVP

4000 ppm

8 and 12

PES/PVP

400 ppm

8

PES/PVP

200 ppm

6,8 and 11.5

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XPS, ATR-IR, SEC and VITAAFM UV visible spectra, FTIR and SEC

Streaming current measurements and ATR-FTIR Filtration performance, Streaming current measurements, XPS and ATR – FTIR spectroscopy

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400 ppm

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PES, PES/PEG, PES/PVP, PES/Plu

observed -chain scission of the PES polymeric backbone into two parts (terminated by a sulfonic acid group and a phenyl chloride group) -significant increase in PES membranes water flux -a slight decrease in contact angle of pristine PES membranes -UF performance of PES membrane does not changed -Modified membranes showed change in hydrophilic property and UF performance - Scission and pitting of PES polymer -increased water flux and reduced protein rejection -degradation of PES/PVP membrane as a result of a radical induced PVP degradation -formation of phenol groups instead, as a result of the radical oxidation of the aromatic rings of PES - formation of first phenol groups and later on sulfonic acid groups on the surface of the membranes -Degradation of PVP at all chosen pH values. - HOCl and OH. found to be responsible for PES chainscission

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FESEM, TGA, DMA and X-ray spectroscopy

[67]

[68]

[62]

[69]

[63]

[71]

[72]

The above studies are focused on the use of NaOCl for cleaning PES membranes. However, in practice, NaOCl is not only used to clean membranes but also it is used for water disinfection. Chlorine is obviously the most valuable agent for disinfecting/sanitizing potable water by protecting bacterial contamination. However, similar to cleaning with oxidative agents, chlorine has been reported to attack different polymeric membranes, such as PES and PA. In fact, a serious drawback of PA-based membranes, a common type of reverse osmosis membranes, is its instability to chlorine [73]. The formation of N-chloro products, as a result of chlorine reaction with amide nitrogen [74] and ring chlorination are considered to take place.

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As a result, many PA reverse osmosis membranes manufacturers recommend to use a free chlorine concentration of less than 0.1 ppm [74]. However, inorganic chloramines concentration between 1.5 and 1.8 ppm is required to kill all the pathogenic bacterial species and reduce total amount of bacterial to an acceptable range [74]. Moreover, in practice, PA based membranes are reported to lose their performance after about 1000 ppp.h (concentration *exposure time) of chlorine treatment [74]. In this study, the PATFC membranes were found to degrade after about 200 – 1000 h of continuous 1 ppm free chlorine exposure.

2.2. PES Membranes Degradation by H2O2

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As an alternative, chlorine tolerant, modified membranes have been studied [75]. Polymers with oxygen and sulfur functional groups, such as PES membranes are a good choice [73]. PES can generally tolerate up to 100 ppm [74]. It is showed that cleaning PES UF membranes with a cleaning intensity between 0.5 and 1 ppm/h sufficiently improves the membranes functionality [74]. Pristine PES membranes are believed to be tolerant to above 250,000 ppm.h of chlorine exposure [25]. It seems that PES indeed has a far better tolerance to chlorine compared to PA.

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The studies on degradation of membranes used for polymer electrolyte membrane fuel cells (PEMFC) by H2O2 showed that the degree of H2O2 and related radicals attack depends on various parameters, such as pH, temperature, exposure time, concentration, and presence/absence of metal ions and radical scavengers, etc [52,76]. On the other hand, in the literature, there are only few papers regarding the stability of PES membranes to H2O2.

H2O2 + Fe2+ → Fe3+ + HO. + HO-, HO. + RH → H2O + R., HO. + H2O2 → HOO. + H2O,

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According to a Filmtec technical manual on the use of H2O2 as a disinfecting agent, temperature and pH were found to greatly influence the rate of attack on a specific TFC membrane consisting of a polyester support, PS interlayer and PA selective layer [77]. A maximum temperature of 25 oC for treatment with a H2O2 disinfecting solution was recommended. Furthermore, the presence of Fe and other transition metals in the H2O2 solution were found to cause a faster membrane degradation [78]. This is because these metals can catalyze membrane degradation, as shown in the reactions below [79] (Fe2+ is chosen as an example). The polymer is represented by R.

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. where, HO and HO are the hydroxyl radical and ion, respectively. As shown in the Fenton reaction, the major step is the oxidation of iron (II) to iron (III) by the peroxide [79]. Subsequently, the radical (HO.) attacks the membrane by H-abstraction [63,71]. Linden et al. [80] discussed an alternative mechanismthe reaction steps- that can occur in the presence of peroxide radicals (shown below). The first step of these reactions is the hydrogen abstraction from the polymer, which can be followed by numerous propagation steps:

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RH + HO. → R. + H2O, R. + O2 → RO2., RO2. + RH → ROOH + R., RH + HO2. → R. + H2O2, RH + HO2. → H. + HO2. + R.. 16

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As shown in the above two possible reaction mechanisms, degradation of PES membranes by H2O2 is . . attributed to attacks of free radicals, HO and hydroperoxide ( OOH) [63]. In addition to the possible conversion of H2O2 into strong free radicals, it is also important to keep in mind that H2O2 itself is a powerful oxidizer (1.8 V) (Table 1).

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Recently, the stability of PES NF membranes to H2O2 was studied [35]. As shown in Fig. 9a, the H2O2 attack is believed to result in loss of the S=O links, i.e., conversation of the -SO2 groups to charged – SO3 groups. The pristine membranes exposed to a 5 wt.% of H2O2 solution showed significant cracks on their surface, as confirmed by SEM (Fig. 9b), due to the H2O2 and/or related radicals attack on the PES [85]. Similar cracks or pitting on PES membranes surfaces were reported when PES membranes were exposed to NaOCl [26,27]. Moreover, as shown in Fig. 9c, self-made (pristine) and commercial (PES10 and NP030) NF membranes immersed in H2O2 solutions (1 and 5 wt.%) for a specified number of days showed prominent increase in their pure water permeability, which implies an attack by H2O2 and/or related radicals on the PES membranes. The increase in water permeability was combined with a decrease in solute rejection. It was concluded that an increase in H2O2 concentration, from 1 to 5 wt.%, led to a more severe degradation of PES membranes. Fig. 9c shows the change in normalized water permeability (with time) of the PES NF (self-made and commercial) membranes immersed in these solutions.

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Fig. 9. (a) Proposed degradation of PES membranes by H2O2, (b) SEM images of H2O2 treated pristine PES membrane and (c) Change in pristine and commercial membranes normalized permeability in the exposure of 1 wt.% (left) 5 wt.% (right) aqueous solution of H2O2. Adapted with permission from [35]. Copyright 2018 Elsevier.

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2.3. Discussion and Implications With regard to the stability/degradation of PES membranes, contrasting results have often been reported in the literature. This could be because of the different testing conditions and stability testing/characterizing techniques used. This made it difficult to perform direct comparisons between the findings reported over the years. However, basically, two oxidative degradation mechanisms of PES membranes can be distinguished. These are PES chain-scission, as a result of the C-S bond breakage, and radical oxidation of PES by free radicals. It seems that in addition to the chemical agent employed, as one might expect, the intermediate components formed are also playing roles in attacking the membranes. Therefore, in addition to the very chemical stability nature of the polymer, the concentration, exposure time, pH and temperature of the testing solution, containing an oxidative agent(s) are crucial in determining the stability/degradation of (PES) membranes. The effect of pH, for the case of NaOCl as a typical example, on PES membranes degradation will be discussed first. 18

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NaOCl + H2O ↔ HClO + Na+ + HO-, HClO + OH- ↔ OCl- + H2O,

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As presented in the above studies (typically, section 2.1), the pH of the NaOCl solution indeed greatly affects the degree of PES membranes attack. Thus, it is interesting to revise the dissociation chemistry of NaOCl in an aqueous solution. First, when NaOCl is introduced into water, it undergoes a dissociation reaction to form HClO, as shown in reaction I. Subsequently, HClO can react with OHand produce another non-radical oxidizing agent, OCl-. The HClO oxidation chemistry is very complex and mostly dependent on, but not limited to, pH and to a lower extent on temperature [86] (Fig. 10). As shown in Fig. 10, the pH affects the ratio of HClO to OCl- in a given solution. Even though both forms are oxidants, HClO (1.49 V) is a more powerful oxidizing agent than OCl- (0.94 V) (Table 1). Yadav et.al [62] concluded that water flux increment of NaOCl-exposed PES membranes was more prominent in a lower pH NaOCl solution and at longer exposure times. This could be due to the fact that HOCl (a more powerful oxidizing agent than OCl-) is dominantly available at lower pH. The effect of HClO and OCl- on PES membranes has been investigated in [72].

Fig. 10. Concentration of HOCl and OCl- as function of pH [87].

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Therefore, since HOCl is a strong oxidizing agent, it could directly attack PES membranes [66], as shown in scheme 1. Second, it can indirectly participate in degradation of PES membranes through formation of OCl-. Additionally, as mentioned in the work of Caussarand et al. [88], HOCl can also form radicals through different ways (possible mechanisms are discussed below). The first route, which was proposed by Wienk et al. [60], is the formation of radicals by reacting with OCl-, as shown in mechanismI below. They proposed a reaction mechanism of OCl- with PVP, which resulted in pyrrolidone-ring opening. The second mechanism is the direct dissociation of HClO to a hydroxyl radical and a chloride radical (mechanism-II) [89]. On a related topic, the effect of NaOCl on PS membranes in the presence of metallic ions, such as Fe2+ and Cu2+ was investigated by [59,88] (mechanism-III). As expected, they reported that their presence further facilitate the deterioration of PES membranes.

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Scheme 1: Various mechanisms in which HOCl can participate in PES membranes attack

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In a nutshell, it can be concluded that the degradation of PES membranes occurs via chain scission and/or radical oxidation mechanisms. The PES chain scission degradation mechanism depends on various factors, such as the type of agent used and the pH of testing solutions. This mechanism should be handled as a function of, at least, pH. For instance, when a pH value below neutral (e.g., 6) of NaOCl is used, a PES chain scission is reported to occur. On the other hand, keeping other parameters such as concentration and temperature constant, when a high pH, (e.g., 11) is used, a lower degree of PES degradation is expected. This is due to the fact that OCl- is now the main degradation agent. However, OCl- has been found to remove PVP from PES/PVP membranes. Therefore, when dealing with NaOCl as oxidizing agent, it is important to always keep the dissociation chemistry of HOCl as function of pH in mind, and the oxidative potentials of HOCl and OCl-.

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The radical oxidation of PES membranes can occur via identical mechanisms, even if different radicals are used and involved. However, the degree of attack could be different depending on the oxidative potentials of the chemical agents. Depending on the site of attack, radical oxidation of PES membranes results in phenol groups formation or conversation of the SO2 groups into SO3 groups (scheme 2).

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Scheme 2: Graphical representation of PES attack by radicals. Adapted with permission from [26,35,63]. Copyright 2018 Elsevier.

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It is important to develop a generalized degradation mechanism of PES membranes. An interesting question to raise can be how would the degradation mechanism of PES membranes by other oxidizing compounds, such as ozone and permanganate be. We believe that a similar degradation mechanisms to the degradation of PES by NaOCl and H2O2 should be expected. For instance, ozone (O3) has been used as an effective product for alleviating PES membranes fouling in the drinking water industry [90]; however, extended exposure of PES membranes to O3 has been reported to cause their degradation. This was claimed to be as a result of the presence of C=C bonds in the PES polymer main chain in aromatic rings since polymers with single C-C or Si-C bonds, such as PP and PDMS showed a certain resistance to O3 oxidation [91]. Cleavage of ether bridges was reported for O3 exposed PS membranes [92].

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Furthermore, KMnO4 treated PES/PVP membranes were found to decrease in PVP content and disappearance of the SO3 group was confirmed by FTIR. A new peak representing sulfonic groups denoting an oxidation degradation reaction of PES was observed in these KMnO4 exposed PES membranes [93]. Moreover, the resistance of PES membranes has been reported to decrease by 30% after exposure to ultraviolet (UV) light for 24 h [94]. This is due to the fact that PES contains sulfur atoms, which are highly susceptible to a UV radiation [95]. The formation of oxidative compounds during the photocatalytic reaction were proposed to be responsible for the degradation of the PES membranes tested [94]. It seems that degradation through radical oxidation has been the main mechanism behind the PES membrane attack.

3. Strategies to minimize PES membranes degradation As discussed in the previous sections, a number of factors affect the rate of degradation of PES membranes. Thus, when fabricating PES membranes with stability to NaOCl, H2O2 or other oxidative agents, the various parameters that influence the rate of (free radical) attacks, such as temperature, pH, exposure time, concentration, presence/absence of radical scavengers and metal ions should be taken into consideration. In this section, three strategies that can minimize oxidative degradation of PES membranes are discussed. These techniques are: (i) incorporation of free radical scavengers, (ii) reinforcing PES membranes with nanoparticles and (iii) the formation of a H2O2 decomposition catalyst layer on the membranes surface. 21

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3.1. Use of Free Radical Scavengers

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One of the approaches that has been used as a strategy to minimize oxidative degradation of polymeric membranes (used in PEMFC), mainly perfluorosulfonic acid membranes, is the use of radical scavengers to quench free radicals [96], such as OH. radicals, which are known to be the most reactive oxygen species [97]. Scavengers are primary antioxidants, which are concerned with free radical termination [98]. They are widely used in treating oxidative stress-related human diseases [99].

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In PEMFC, the residence time of reactive oxygen species in the membrane were found to determine the degradation rate of the membranes [100]. The basic assumption here is that the polymeric membranes chain degradation is done mainly by the free radicals. This assumption contradicts with the finding of Hanafi et al. [72], who recently concluded that HClO has a more severe impact than HO. radicals on PESchain scission. In this study, tertiobutanol (tBuOH) was used as a free radical scavenger to compare the impact of the radicals and HClO.

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Ce3+ + HO. + H+ → Ce4+ + H2O Ce4+ + H2O2 → Ce3+ + HOO. + H+ Ce4+ + HOO. → Ce3+ + O2 + H+

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Various materials have been used to mitigate radical attacks in the field of PEMFC. Pearman et al. [76] studied the degradation mitigation effect of synthesized and commercial cerium oxide nanoparticles in perfluorosulfonic acid (2.0 V oxidizing potential) PEMs. The ceria particles were found to scavenge the hydroxyl and hydroperoxyl radicals. The scavenging reaction mechanisms of CeO2 nanoparticles is believed to follow the reaction mechanism given below [76,96]. As shown in the reactions, the key factor for a nanoparticle to be used as a free radical scavenger is its ability to undergo a reversible redox reaction between Ce3+ and Ce4+ oxidation states [101]. In another related work, Lorenz et al. [82] did a simulation on mitigating the radical attack in the polymer electrolyte fuel cell by the incorporation of cerium and manganese as radical scavengers. As a result, they found out a decreased radical attack on the polymer. The HO. scavenging activity of ceria nanoparticles has been proved with simple photometric measurement [102]. Similarly, multi-walled carbon nanotubes were found to retard oxidation of various polymers (polystyrene, polyethylene, polypropylene and poly vinylidene fluoride) [103].

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Other researchers used different radical scavengers, other than nanoparticle-based radical scavengers. Ling et al. [51] added methanol into the feed water solution to act as a OH. quencher. Methanol was found to be protective by minimizing the free radical attack on PA TFC membranes only in the first few days, after which the methanol were depleted due to a physical process. Similarly, Causserand et al. [88] reported a slower PS chain oxidation exposed to chlorine solutions by adding tBuOH to bleach solution. The use of free radical scavengers to mitigate PES membranes degradation has not been discussed in the literature. However, incorporation of radical scavengers, such as cerium nanoparticles and methanol, could be anticipated to have similar mitigation effects during exposure of PES-based membranes to oxidative agents.

3.2. Reinforcing PES membranes with Nanoparticles The role of nanoparticles as a H2O2 decomposition catalyst, which then would minimize radical oxidative attack and degradation of membranes, has been proposed and tested in PEMFC [51,52,76,104]. A PESbased NF membrane with a promising stability to H2O2 was fabricated and characterized by our group 22

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recently [35]. It was found that the membrane integrity of conventional pristine and commercial PES membranes is problematic, while newly developed membranes reinforced with TiO2 nanoparticles had a remarkably better stability performance. The sample membranes were immersed for 45 days in 1 wt.% and 30 days in 5 wt.% H2O2 solutions. The stability/degradation of the membranes was studied by investigating the change in water permeability and surface morphology before and after immersion in the H2O2 solutions. The synthesized composite NF membrane was found to have a better tolerance to H2O2 than the pristine and commercial PES NF membranes, which suggests that the fabricated PES(TiO2) membranes were not easily attacked by H2O2 and related free radicals. This was confirmed by the good process performance (Fig. 11 a and b) and morphological stability (Fig. 12b) of the composite membranes after prolonged exposure to H2O2 containing solutions. For instance, as shown in Fig. 11 (b), after being immersed in 5 wt.% H2O2 solution for a month, the PES/TiO2 increased by only 1.8 times than the original permeability, which is a far better tolerance than that of the reference membranes. The PES/TiO2 membrane’s direct red 23 dye rejection decreased from 94.9% to 90.0% after immersed in 1 wt.% H2O2 solution for 45 days and to 66.0% after immersed in 5 wt.% H2O2 solution for a month.

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Fig. 11. Comparison of blended (PES/TiO2), pristine (PES) and commercial references (PES-10 and NP030) PES membranes separation performance. Change in membranes normalized permeability in the exposure of 1 wt.% (a) 5 wt.% (b) aqueous solution of H2O2. Adapted with permission from [35]. Copyright 2018 Elsevier.

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Furthermore, the separation performance of the membranes was accompanied by a membrane morphology study by SEM (shown in Fig. 12a and b). As shown in Fig. 12a, cracks were clearly visible on the surface of pristine PES membranes that were exposed to 5 wt.% H2O2 solution. In contrast, the PES/TiO2 membranes showed no cracks at all on its surface after being treated for a month.

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Fig. 12. SEM images of H2O2 treated (a) pristine PES and (b) PES (TiO2) membranes. Adapted with permission from [35]. Copyright 2018 Elsevier.

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Both the permeability and morphology studies showed that the TiO2 indeed improved the stability of the PES membrane to H2O2. Three possible reasons were proposed as an explanation for this improvement in stability of the membranes: (i) the nanoparticles might have decomposed the H2O2 into water and oxygen, (ii) the bond formed between the TiO2 and the PES (via self-assembly) might have avoided the interaction between the H2O2 and the polymer and (iii) since there is a significant amount of TiO2 nanoparticles on the surface of the PES membranes, the H2O2 might have preferentially contacted the nanoparticles, which have better have a better stability to H2O2 than the PES polymer. However, the exact reason why the composite membranes showed a better tolerance to H2O2 than the reference (pristine and commercial) PES membranes remains still unclear. It could be due to the combined effect of some or all the three proposed mechanisms. Further research is also required to optimize the size, type and concentration of the nanoparticles.

3.3. Introducing a Layer of Nanoparticles

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Another promising technique to deal with H2O2 attack is to introduce a thick layer of nanoparticles that can be used as a peroxide decomposition catalyst on the top surface of the (PES) polymeric membranes [49,105]. The main role of the decomposition catalyst is to minimize the concentration of H2O2 [106], by decomposing the H2O2 into water and oxygen. The deposition of TiO2 or activated carbon, in a similar way, was proposed in the review paper of Yang et al. [105]. Therefore, the deposition of stable, homogenously dispersed, nanoparticles on the membrane surface, in acidic environment [107], results in decomposition of H2O2 into H2O and O2 [49,105], which is a thermodynamically favorable reaction [108]. In this section the possible use of TiO2 in anatase form as a H2O2 decomposition catalyst is discussed. TiO2 nanoparticles have been used for various applications, such as for water treatment [109,110], antibacterial [111,112], flux enhancement and fouling mitigation [113,114], photocatalysts [115] and for photo-degradation of aqueous environmental pollutants and disinfections [115,116]. Its low cost, nontoxicity and high chemical stability, combined with its chemical and biological inertness and resistance to chemical and photo-corrosion, made TiO2 the most commonly used nanomaterial for the development of photocatalytic membranes [117,118]. In the literature, four different ways of incorporating TiO2 nanoparticles into (PES) membranes have been mentioned: (1) via self-assembly, i.e., dipping the already prepared membranes in a solution containing nanoparticles [119,120]; (2) using binding agents such as 24

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dopamine between the nanoparticles and the membrane surface; (3) via phase inversion, i.e., dispersing the nanoparticles in the polymer/solvent solution directly and casting the solution to form mixed matrix membranes [4]; (4) preparing a separate nanoparticle solution, mixing it with the casting solution and preparing the mixed matrix membrane via phase inversion. The self-assembly of TiO2 on the PES membrane surface occurs in two ways [120]. One is via TiO2 binding with two oxygen atoms of carboxylate group by a bidentate coordination to Ti4+ cation. The other way is to form hydrogen bonds between a carbonyl group and the surface hydroxyl group of TiO2 [121].

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Mussel-inspired deposition of nanoparticles is a method widely used to prepare multifunctional coatings. Polydopamine (PDA) (also known as “bioglue”) is a polymer derived from dopamine, a chemical agent that contains amine and catechol groups. It has been shown to attach to all types of surfaces (organic and inorganic surfaces). It is able to attach even to adhesion-resistant materials, such as poly (tetrafluoroethylene), PTFE [122]. In recent years, many researchers have used this method to achieve various membrane modifications. Yang et al. [123] prepared uniformly dispersed silver nanoparticles inside PDA coating layer on a TFC RO, polyamide membrane. Their results implicate that PDA has the ability to be used for membrane modification with various surface chemistries and morphological features. In another work [124], researchers were able to (in situ) immobilize Cu nanoparticles on PDA coated graphene oxide and use it for determination of H2O2 as an alternative technique to the other analytical methods employed for the determination of hydrogen peroxide (chemiluminescence, spectrophotometry, titrimetry and electrochemistry) [125].

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Another related study, which is worth mentioning is the work done on the preparation of a self-protected TiO2/PDA/PS composite UF membrane with a self-cleaning property performed, by Feng et al. [126]. Similarly, a nanohybrid UF membrane made of PS capable of self-cleaning and self-protection was prepared by doping TiO2-PDA into the casting solution (via blending), as discussed by Wu et al. [127]. In this work, PDA spheres were used as an adhesive substrate to hold the photocatalytic TiO2. One of the issues with the use of PDA coatings was the long time it requires (10 h – few days) [128]. To overcome this drawback, Zhang et al. [129] studied a strategy to greatly accelerate the deposition of PDA coatings using a small amount of CuSO4/H2O2 as a trigger. The Cu2+ and H2O2 produced reactive oxygen species in an alkaline medium. Additionally, the PDA coatings were found to exhibit a high uniformity. Furthermore, a rapid PDA deposition triggered by a simple and green FeCl3/H2O2 system under acidic conditions has been reported recently [130]. This technique was found to shorten the deposition time and improve the stability of PDA coatings onto various membranes.

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Inspired by the recent literature on using PDA for membrane deposition of nanoparticles for various applications, PES/PDA-TiO2 was explored for membrane synthesis. In this work, PDA was used as a binding agent. The TiO2 nanoparticles was then deposited on the top surface of the PDA layer (Fig. 13a). For this membrane (PES/PDA-TiO2), PES membranes (18 wt.%) were first prepared using DMSO (relatively non-toxic) [131] as a solvent using non-solvent phase inversion method. The membrane samples were washed with deionized (DI) water and dried using an air dryer prior to the surface modification. The PDA deposition was carried out as described in the work of Wang et al. [132]. Precisely, dopamine was dissolved in Tris solution (pH 8.5, 10 mM) to prepare 30 mL (dopamine) solution of 2 mg/mL concentration. Then, CuSO4 (39.9 mg) and H2O2 (0.1 mg) as the trigger of the rapid deposition was added into the solution and mixed for two minutes using magnetic stirrer. The freshly prepared solution was gently poured into the active areas of the ring, which held the prepared membranes (Fig. 13b). The membrane samples were shaken continuously, in order to avoid the aggregation of 25

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dopamine, for 30 min in air. The modified membranes (PES/PDA membranes) were then rinsed with DI for 3 min to remove any loosely adhered PDA.

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The second stage of the modification process was the deposition of TiO2 nanoparticles to the surface of the prepared PES/PDA membranes. This was done as described in the work of Zhang et al. [133], which is a similar procedure to the PDA deposition. The TiO2 powders (0.5 w/v%) were added to the Tris solution (30 mL) and stirred first at 1000 rpm for 3 h and then further treated with a low speed ultrasonic vibration in order to avoid aggregation of particles. The prepared TiO2 solution was then poured to the membrane surface. The membrane samples were shaken continuously for 1 h. The membranes modified this way are denoted as PES/PDA-TiO2.

Fig. 13. (a) Preparation of PES/PDA-TiO2 membrane and (b) Peroxide decomposition catalyst on the surface of membrane, adapted from [49,105]. For such membranes, the stability of the TiO2 nanoparticles is one of the factors that determine the longterm applicability of the membrane. Hence, a high filtration velocity rinsing experiment (1 h, room temperature), using a lab-made cross-flow NF filtration setup, was used to challenge the bound nanoparticles. The change in surface morphology and water contact angle before and after the rinsing experiment was compared as a method to estimate the stability of the TiO2 nanoparticles on the surface of the PES/PDA membrane. The nanoparticles which were loosely adhered were removed by washing with DI water. The water contact angle of PES/PDA-TiO2 membrane before and after rinsing has been tested, and the results are shown in Table 3. As shown in Table 3, the contact angle was found to be almost 26

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constant before and after rinsing, which shows a good binding between the PES/PDA and the TiO2 nanoparticles. This has been further confirmed by SEM images (Fig. 14). Fig. 14 shows SEM micrographs of PES/PDA-TiO2 membrane before (Fig. 14a) and after rinsing (Fig. 14b). Table 3. Water contact angle of PES/PDA-TiO2 membrane before and after rinsing After rinsing (O) 49.8

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Before rinsing (O) 48.0

Membrane PES/PDA-TiO2

Fig.14. SEM micrographs of the surface of the PES/PDA-TiO2 membrane (a) before and (b) after high velocity cross flow rinsing experiment.

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A significant amount of TiO2 nanoparticles is still visible on the surface of the membrane after a high velocity rinsing (Fig. 14b). Therefore, both discussed methods confirm that the nanoparticles were firmly bound to the PES/PDA.

4. Conclusions and Future Prospects

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The use of oxidative chemicals to clean fouled PES membranes, use of chlorine for disinfection purposes and presence of oxidative agents as contaminants in wastewaters has been shown to have negative impacts on the surface morphology and separation performance of the membranes. Therefore, the development of PES membranes with a higher tolerance to these chemicals is of utmost importance. Optimized levels of oxidizing agents and conditions for their application that avoid/minimize PES membrane attack can also be implemented. To achieve these objectives, clear understanding of the mechanisms involved in the oxidation degradation PES membranes is first required. However, as presented in this review paper, insufficient research has been so far devoted to this subject. Moreover, contradictory oxidative degradation mechanisms of PES membranes, mainly in the case of NaOCl, have been reported in the literature. Most of the differences arise from the fact that the chemistry of NaOCl solution is rather complex and highly dependent on the working conditions, especially on the pH. Therefore, to fully understand the effect of NaOCl, H2O2 and other common oxidizing agents on morphology, separation performance and overall integrity of PES membranes, detailed analyses using advanced characterization tools in a broad range of pH, temperature, concentration and exposure time are required.

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PES chain scission and radical oxidation degradation mechanisms are believed to occur when PES membranes are exposed to strong oxidizing agents. This has been discussed by providing different supporting studies of NaOCl interaction with PES membranes available in the literature. A similar degradation mechanism is anticipated to take place up on exposure of PES membranes to other oxidizing agents, especially those involving formation of free radicals.

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Compared to the number of studies on degradation of PES membranes by NaOCl, the stability/tolerance of PES membranes to H2O2 is less explored and reported in the literature. Considering the possible appearance of H2O2 in wastewaters as a contaminant, in addition to the use of H2O2 as a disinfecting agent, more attention should be paid to this topic. Therefore, more research is required to fully understand the role of H2O2 (and related radicals formed) in the degradation of PES membranes. The effect of the presence of scavengers and metal ions in a wide range of pH and temperature values using advanced characterizing tools should be explored.

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Despite the vast occurrence of PES membranes exposure to oxidizing agents, very few studies on strategies to minimize oxidative degradation of PES membranes are available in the literature. In this review work, three of these strategies have been discussed. Further research on the long term applicability and compatibility with the current cleaning techniques of these proposed strategies is required. TiO2 nanoparticles have shown to improve the oxidative (H2O2) chemical stability of self-made PES membranes. However, optimization of the concentration of the nanoparticles is yet to be carried out. A better result could be anticipated by optimizing the concentration (and size) of the TiO2 nanoparticles. A similar result is expected by using other nanoparticles as well.

Funding

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Forming layer of nanoparticles on polymeric membranes surface can be considered a novel way to at least minimize oxidative agents attack on membranes. Further research on other methods to deposit nanoparticles on the surface of the prepared membranes is required to form a (TiO2) nanoparticles layer that is able to stand harsh environments.

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Conflict of interest

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This research was financed by the scholarship of the European Commission- Education, Audiovisual and Culture Executive Agency (EACEA), under the program: Erasmus Mundus Master in Membrane Engineering - EM3E (FPA No 2011-0168, Edition I, http://em3e-4sw.eu).

The authors declare that there is no conflict of interest.

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The recent state-of-the-art on oxidative degradation of PES membranes is presented. Main degradation mechanisms have been identified and critically discussed. Possible strategies for minimizing oxidative degradation of PES membranes have been outlined. Remaining research gaps and challenges have been identified and required actions proposed.

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