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Unveiling the Susceptibility of Functional Groups of Polyethersulfone/Polyvinylpyrrolidone Membranes to NaOCl: A Two-Dimensional Correlation Spectroscopic Study Zhongbo Zhou, Guocheng Huang, Yi Xiong, Minghao Zhou, Shaoqing Zhang, Chuyang Y. Tang, and Fangang Meng Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03952 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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Environmental Science & Technology

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Unveiling the Susceptibility of Functional Groups of

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Polyethersulfone/Polyvinylpyrrolidone Membranes to NaOCl:

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A Two-Dimensional Correlation Spectroscopic Study

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Zhongbo Zhou a,b, Guocheng Huang a,b, Yi Xionga,b, Minghao Zhoua,b, Shaoqing Zhang a,b, Chuyang Y. Tang c, Fangang Menga,b*

a

School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510006, PR China b Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Sun Yat-sen University, Guangzhou 510275, China c Department of Civil Engineering, The University of Hong Kong, Pokfulam, Hong Kong

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*

Corresponding author.

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Fangang MENG, Ph. D., Email: [email protected]

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ACS Paragon Plus Environment


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TOC:

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ABSTRACT: A clear understanding of membrane ageing process is essential for the

30

optimization of chemical cleaning in membrane-based facilities. In this study,

31

two-dimensional

32

spectroscopy (COS) analysis was firstly used to decipher the sequential order of

33

functional group changes of NaOCl-aged polyethersulfone/polyvinylpyrrolidone

34

(PES/PVP) membranes. The synchronous maps showed twelve major autopeaks in

35

total. Based on the asynchronous maps, a similar ageing sequence of membrane

36

groups was clearly identified at three pHs (i.e., 6, 8 and 10): 1463, 1440, and 1410

37

(cyclic C-H structures) > 1662 (amide groups) > 1700 (succinimide groups) > 1320,

38

1292 (S=O asymmetric) > 1486, 1580 (aromatic structures) > 1241 (aromatic ether

39

bands) > 1105, 1150 cm-1 (O=S=O symmetric). Among them, membrane chlorination

40

occurred at 1241, 1410 and 1440 cm-1. Moreover, the initial degradation of PVP and

41

the subsequent transformation of PES could be highly responsible for the increased

42

water permeability and the enlargement of membrane pores, respectively, both

43

leading to serious fouling with humic acid filtration. In summary, the 2D-FTIR-CoS

44

analysis is a powerful approach to reveal the interaction mechanisms of

45

NaOCl-membrane and could be also useful to probe the process of membrane

46

fouling and chemical cleaning.

(2D)

Fourier

transformation

infrared

47 48 49 50 51 52 3


(FTIR)

correlation


53

INTRODUCTION

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Over the past few years, membrane technologies have been increasingly used in

55

diverse fields (e.g., wastewater treatment and reuse, drinking water purification, pure

56

water production and desalination) because of their good capability to reject a wide

57

spectrum of contaminants1, 2. Despite of these advantages, membrane fouling

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remains a critical issue and often requires periodical chemical cleaning3, 4. The harsh

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environment during chemical cleaning can result in accelerated membrane ageing,

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with major changes in pore size, surface properties, and chemical bonds5. These

61

changes, in turn, impacts the interaction between membranes and foulants, and thus

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the redevelopment of fouling6. Therefore, a comprehensive understanding on the

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membrane ageing process is of great importance for the optimization of chemical

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cleaning in membrane-based facilities.

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Sodium hypochlorite (NaOCl) is one of the most popular cleaning reagents3.

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Nevertheless, NaOCl is known to cause chemical degradation of common membrane

67

materials such as polysulfone (PSf) and polyethersulfone (PES)5. For example,

68

exposure of PES to NaOCl can result in the chain scission of C-S bonds7-9 and the

69

breakage of PES chains10, which often leads to increased pore sizes and decreased

70

tensile strength of membranes10,

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complicated by the presence of macromolecular additives12-14 such as polyethylene

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glycol (PEG)15 or polyvinylpyrrolidone (PVP)16-18. Despite the vast literature on

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membrane ageing, the dynamic ageing process is not fully understood. Traditional

74

one-dimensional Fourier transform infrared (FTIR) spectra, a most widely used tool

11

. In fact, membrane ageing can be further

4


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for studying the changes in chemical bonding information, often suffer in

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overlapping problems among different bands19, resulting in poor resolution of the

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molecular-level details. In particular, traditional FTIR spectra fail to clarify the

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sequential order of different functional group changes during the membrane aging

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process. More recently, electrokinetic characterizations were proposed to provide

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important clues on the sequential changes of some ionizable groups of NaOCl-aged

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membranes20. Nonetheless, the changes in considerable functional groups of

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membranes cannot be well identified. Hence, more direct molecular-level evidences

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on the sequential interactions between different functional groups of membranes and

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NaOCl need to be clarified. Moreover, the corresponding changes in membrane

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performance as a function of membrane group degradation need to be systematically

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studied.

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Two-dimensional correlation spectroscopy (2D-CoS), based on a set of FTIR

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spectra in response to some external perturbations (e.g., pH, temperature, and

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oxidant concentration), can potentially reveal the sequential changes of different

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functional groups21, 22. In recent years, 2D-FTIR CoS analysis has been successfully

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applied to study the interaction mechanism of metals23 and nanomaterials24, 25 with

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various substrates such as macromolecules26 and bacteria27. The method was also

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capable to resolve the dynamic diffusion process of water and CaCl2 in a

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polyacrilonitrile membrane28. These inspiring studies prompted us to apply

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2D-FTIR-CoS to characterize the dynamic ageing process of membranes exposed to

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NaOCl. It can be expected that the use of 2D-FTIR-CoS can provide a clearer picture 5



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on the degradation mechanisms of membranes during the exposure to NaOCl.

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In this study, we employed the FTIR spectra integrated with 2D-CoS analysis to

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systematically examine chemical structure variations of a PVP modified PES

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membrane under various NaOCl exposure conditions. Our study can provide direct

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molecular-level insights on the ageing sequence of functional groups of NaOCl-aged

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membranes and the consequent impact on the membrane properties (e.g., pure water

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permeability, surface morphology and fouling behavior).

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MATERIALS AND METHODS

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Membranes. A commercial flat-sheet PES membrane with a molecular weight

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cut-off of 150 kDa (UP150, Microdyn-Nadir, Germany) was used. Based on FTIR

107

and X-ray photoelectron spectroscopy (XPS) analysis, the active layer of the PES

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membraneis blended with PVP (i.e., a PEV/PVP membrane). The support and

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drainage layer of the membrane is composed of non-woven polyester fabric.

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According to the manufacturer, the maximum chlorine resistance of the membrane is

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up to 500,000 ppm·h, and the membrane can be operated at a wide range of pH (1-14)

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and at high temperature (max. 95 °C). Prior to ageing assays, disks of the virgin

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membrane (diameter=85 mm) were rinsed with and soaked in ultrapure water (18.2

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MΩ cm, RSJ Corp., China) for approximately 24 h to remove impurities.

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Chemicals. All the solutions were prepared with the ultrapure water. NaOCl (reagent

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grade, available chlorine 4.00-4.99%) was purchased from Sigma-Aldrich. The free

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chlorine

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(N,N-diethyl-p-phenylenediamine) method29. Solution pH was adjusted using 1 M

concentration

was

determined

6


by

the

DPD

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hydrochloric acid and 1 M sodium hydroxide solutions (AR-grade., Da-Mao Corp.,

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China).

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Membrane ageing assays. The pristine membrane disks were submerged in NaOCl

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solutions (300 ppm total free chlorine, pH at 6, 8 or 10, 25 ± 2 °C) in sealed glass

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bottles in the dark. Exposure time ranged from 0.5 to 30 d, during which NaOCl

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replenishment and pH adjustment were performed periodically to maintain constant

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free chlorine concentration and solution pH. The maximum exposure dose of 216,

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000 ppm·h, corresponding to the 30-d exposure, is below the maximum chlorine

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resistance of the membrane (500,000ppm·h). The NaOCl-exposed membranes were

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thoroughly cleaned with the ultrapure water to remove residual chlorine and then

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stored at 4 °C under humid conditions before further characterization.

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Physicochemical characteristic analysis. The membrane samples were first

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freeze-dried under a vacuum at -57°C for 48 h to remove residual moisture. FTIR

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spectrometry with an attenuated total reflectance accessory (ATR-FTIR) (EQUINOX

133

55, Bruker, Germany) was employed to monitor the changes in functional groups of

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the membranes after NaOCl exposure. Each spectrum was the average of 16 scans

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recorded over 4000-650 cm-1 at a resolution of 2 cm-1. Meanwhile, an XPS

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instrument with Al Kα (1486.6 eV) radiation (ESCALAB 250, Thermo-VG

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Scientific) was used to scan the aged membranes in a broad survey scan with 20-eV

138

pass energy. A high-resolution scan with 80-eV pass energy was applied for the

139

component speciation. Moreover, the surface morphology of aged membranes (after

140

coating with gold) was observed by scanning electron microscopy (SEM) 7



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(JSM-6330F, JEOL, Japan).

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2D-CoS analysis. In this study, the exposure time was used as an external

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perturbation for the functional group changes of the PES/PVP membrane during the

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ageing assays. Based on the work of Noda and Ozaki22, a set of time-dependent

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ATR-FTIR spectra collected above were employed for the 2D correlation analysis.

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Briefly, y(x, t) as the variations of analytical spectra as a function of a spectral

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variable (x, wavenumber in IR spectra) and a perturbation variable (t, exposure time)

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can be used for the definition of dynamic spectra y(x, t) as follows: , − for T ≤ ≤ T ; (x, t)= 0, others;

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(1)

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In Eq. (1), the reference spectrum is typically set as the time-averaged y(x, t),

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i.e., , /T − T , where Tmin and Tmax denote the minimum

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and maximum value of the exposure time, respectively.

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According to the Hilbert-Noda transform method21,

22

, the 2D correlation

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synchronous (Φ(x1, x2)) and asynchronous (Ψ(x1, x2)) spectral maps can be generated

155

from the corresponding dynamic spectra as follows:

156 157

Φ(x1, x2) =

,

· " , ;

(2)

158 159

Ψ(x1, x2) =

,

· #̃ " , ;

(3)

160 " , & %

161

where #̃ " , =

162

" , and & is the integral variable (i.e., time). The value of Φ(x1, x2) represents

· '(

'

& is the Hilbert transform of the signal

8


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the simultaneous or coincidental changes of spectral intensity variations observed at

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x1 and x2 during the interval between Tmin and Tmax. In contrast, the intensity of Ψ(x1,

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x2) represents the asynchrony of spectral intensity variations measured at x1 and x2,

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which can reveal the sequential order of spectral intensity changes by the external

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perturbation. Exact rules of determining such order of the spectral intensity changes

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are present here: (1) If the intensity of the synchronous spectrum is positive (Φ(x1,

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x2) > 0), the positive cross peak (Ψ(x1, x2) > 0) of the asynchronous spectrum reflects

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that the intensity change at x1 occurs before x2 while the negative cross peak (Ψ(x1,

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x2) < 0) reflects that the change at x1 occurs after x2. (2) If the intensity of the

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synchronous spectrum is negative (Φ(x1, x2) < 0), the above rule is reversed. Prior to

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the 2D-CoS analysis, all the ATR-FTIR spectra were normalized, baseline-corrected,

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and denoised by Savitzky-Golay Smoothing. All the calculations above were

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conducted with MatlabR2010a (MathworksInc., USA) and 2Dshige software

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(Shigeaki Morita, Japan).

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Membrane performance tests. The side-effect of chemical changes of membranes

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on their performance was investigated with a stirred dead-end filtration cell (an

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effective area of 37.4 cm2, MSC300, Mosu Corp., Shanghai, China). During the

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filtration, the rotating speed was maintained at 350 rpm and the pressure was fixed at

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69 kPa. The permeate amount was recorded automatically by an electronic scale

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connecting with a personal computer. The aged membranes were first soaked in the

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ultrapure water for 24 h, and then their water permeability was measured with 300

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mL ultrapure water. Afterwards, humic acid (HA, Sigma-Aldrich) with a 9



185

concentration of 10 mg DOC/L was used as a model foulant to perform fouling tests

186

for the aged membranes. Unless specified otherwise, all parameters in the fouling

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and permeability tests are the same. Finally, the unified membrane fouling index

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(UMFI) was calculated to evaluate the fouling propensity of the aged membranes30.

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RESULT AND DISCUSSION

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ATR-FTIR spectra of NaOCl-aged PES/PVP membranes. As shown in Fig. 1, the

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variations of IR absorption peaks were mainly concentrated in the region of

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1800-1000 cm-1. The characteristic bands of PES (i.e., 1580, 1486, 1320, 1292, 1241,

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1150, and 1105 cm-1) exhibited high absorption intensities in the IR spectra of

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pristine membranes (0 day)

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related to aromatic ring groups; ii) bands located at 1320/1292 and 1150/1105 cm-1

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are attributed to the asymmetric and symmetric stretching vibrations of sulfone

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groups (Ar-SO2-Ar), respectively; and iii) strong peaks at 1241 cm-1 are due to the

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presence of aromatic ether structures (Ar-O-Ar). Moreover, a number of

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characteristic peaks (i.e., 1662, 1463, 1440 and 1410 cm-1) representing PVP were

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observed in the IR spectra20. For instance, the bands located at 1463, 1440 and 1410

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cm-1 are associated with the cyclic C-H structures of pyrrolidine of PVP33.

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Additionally, the band at 1662 cm-1 is assigned to the C=O vibration of amide groups,

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which was further confirmed by the XPS data that the pristine membrane contained

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3.45 % nitrogen atom (Table S1). Previous studies also reported that the pure PES

205

membranes had no absorption band at 1662 cm-1 12, 34. These findings confirm that

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the commercial PES membrane employed in this study is blended with some PVP.

10, 31, 32

: i) two typical peaks at 1580 and 1486 cm-1 are

10


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207 208

Figure 1. ATR-FTIR spectra (1000-1800 cm-1) of PES/PVP membranes upon exposure to

209

NaOCl at three different pHs of 6 (a), 8 (b), and 10 (c) as a function of exposure time (0-30

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days). 11



211

Moreover, the ATR-FTIR spectra also showed that the variation of membrane

212

functional groups was impacted significantly by the pH of NaOCl. The decreasing

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degree of spectral peak intensities between 1800-1000 cm-1 became weaker with

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increasing pHs. Particularly, the treatment of NaOCl at pH 6 induced drastic

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degradation of the amide group (N-C=O) at 1662 cm-1 as well as the generation of a

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new shoulder peak at 1700 cm-1 representing the formation of succinimide groups20.

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In contrast, the shoulder peak at 1700 cm-1 was much weak at pH 8 and nearly absent

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at pH 10, which is consistent to the finding by Kourde-Hanafi et al.16. A possible

219

explanation is that the succinimide groups formed could be further degraded or

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released at higher pHs35. Moreover, the two small peaks at 1440 and 1463 cm-1

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involving in the cyclic C-H structures of PVP almost disappeared at pH 6 and 8 with

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increasing exposure time, further indicating that the characteristic structures

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involving in PVP were destructed. However, the 30-day exposure of NaOCl at pH 10

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just led to a slight decrease in the band intensity of the IR spectra. These results

225

imply that the ageing rate of PES/PVP membranes strongly depends on the pH of

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NaOCl solutions36-38. Unfortunately, the traditional one-dimensional FTIR spectra

227

analysis suffers from giving limited information on the dynamic ageing process at

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different pHs, and especially fails to probe the change order of functional groups of

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NaOCl-aged PES/PVP membranes.

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2D-CoS maps on the ageing processof PES/PVP membranes. As seen in Fig. 2a,

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c,e, the synchronous maps at three pHs were almost similar in the distribution of

232

peaks. Twelve major autopeaks were identified at 1105, 1150, 1241, 1292, 1320, 12


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1410, 1440, 1463, 1486, 1580, 1662, and 1700 cm-1 along the diagonal line.

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Regardless of the pH, peaks at 1105/1150 and 1241 cm-1 involving in the sulfone

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groups and aromatic ether had the highest intensity, followed by the peaks at 1662

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and 1486/1580 cm-1 (the amide and aromatic structures, respectively) while the

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cyclic C-H structures at 1410, 1440 and 1463 cm-1 had the lowest intensity.

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Regarding the controversial peak at around 1030 cm-1 likely attributed to sulphonic

239

acids groups or phenolic groups, there was no autopeak found in this work, which

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could be related to the low amount of PVP in the membrane used in this study (less

241

than 5%). Prulho et al. only observed the formation of the weak band at 1030 cm-1 in

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the aged PES membrane with a high PVP up to 50%18. Moreover, these autopeaks at

243

pH 6 and 8 had a higher intensity than those at pH 10 (Fig. 2a, c, e), clearly

244

revealing that the PES/PVP membranes are more susceptible to be degraded by

245

NaOCl at pH 6 and 838. Notably, the intensity of the autopeak at 1662 cm-1at pH 6

246

and 8 was around ten times stronger than that at pH 10. This demonstrates that the

247

amide structures of PVP are much more sensitive to NaOCl compared to the

248

characteristic bands of PES36. Majority of cross peaks in the synchronous maps at

249

three pHs had a positive sign (Table 1), suggesting that almost all the functional

250

groups underwent synchronously changes (i.e., decreases in the intensity of bands)

251

during the ageing process of PES/PVP membranes. Nevertheless, the negative sign

252

of all cross peaks (Φ(x1, 1700) < 0) at pH 6 demonstrated that the change direction of

253

peak at 1700 cm-1 was opposite to that of other groups in the range of 1000-1662

254

cm-1, as a result of the increasing intensity of peak at 1700 cm-1 with increasing 13



255

exposure time. It is of great interest that the cross peaks (Φ(x1, 1700)) at pH 8

256

showed negative signs for x1 at 1292, 1320, 1410, 1440, 1463, and 1662 cm-1,but no

257

sign for x1 at 1105, 1150, 1241, 1486, and 1580 cm-1. Apparently, these above results

258

give direct evidences on the formation of succinimide groups at 1700 cm-1, occurring

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with the transformation of cyclic C-H structures (1410-1463 cm-1) and amide groups

260

at 1662 cm-1of PVP39. Meanwhile, it further confirmed that the oxidization or

261

hydrolysis of succinimide groups occurred at pH 8 in contrast to the accumulation of

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this band at pH 616, 18. However, at pH 10, the autopeak at 1700 cm-1was no longer

263

observable and all the cross peaks had no sign (Table 1). This could be related to

264

two reasons: i) NaOCl solutions at pH 10 are mainly composed of the ion of ClO- (>

265

90 %), which has a lower production potential of·OH and ·C1O radicals causing a

266

weaker oxidative capacity40; ii) Moreover, the ring opening of pyrrolidine occurs in

267

alkaline solutions (pH 8 and 10)18,

268

ATR-FTIR spectra, the synchronous maps of 2D-CoS analysis can provide more

269

deep insights into the changes in membrane functional groups induced by NaOCl,

270

and especially on the relationships among molecular structure variations.

35

. Overall, in comparison to the general

271

Based on the Noda’s rules21 described before, asynchronous correlation spectra

272

can reveal the sequential order of specific chemical reactions during the ageing

273

process of PES/PVP membranes. The asynchronous map is antisymmetric with

274

respected to the diagonal line and thus displayed no autopeak. In Fig. 2b, d and f,

275

distinctive differences in the sign of cross peaks were found among the three pHs. As

276

shown in Table 1, at pH 6, most of the cross peaks (Ψ(x1, 1700)) showed a positive 14


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sign apart from those at 1410, 1440, 1463 and 1662 cm-1, indicating that the

278

increasing intensity of the peak at 1700 cm-1 occurred after the decreasing intensity

279

of the peak at 1410, 1440, 1463 and 1662 cm-1 but before others (i.e., 1105, 1150,

280

1241, 1292, 1320, 1486, and 1580 cm-1). Moreover, negative signs were observed at

281

the cross peaks of Ψ(x1, 1292/1320/1410/1440/1463), implying that the spectral

282

decreases at 1292, 1320, 1410, 1440, and 1463 cm-1 occurred prior to the decreasing

283

peaks at 1105, 1150, and 1241 cm-1. In addition, six negative cross peaks at Ψ(1105,

284

1486/1580), Ψ(1150, 1486/1580), and Ψ(1241, 1486/1580) were observed, indicating

285

that the change sequence of bands at 1486/1580 cm-1 occurred earlier than those at

286

1241, 1150 and 1105 cm-1. In conclusion, the ageing process of PES/PVP

287

membranes at pH 6 could occur in the following sequence: 1463, 1440, and 1410

288

(cyclic C-H structures) >1662 (amide groups)> 1700 (succinimide groups)> 1320,

289

1292 (S=O asymmetric) >1486, 1580 (aromatic structures) > 1241 (aromatic ether

290

bands) > 1105, 1150cm-1 (O=S=O symmetric).

15



291 292

Figure 2. Synchronous (a, c, e) and asynchronous (b, d, f) 2D correlation maps constructed

293

from time-dependent ATR-FTIR spectra of PES/PVP membranes in the treatment of

294

NaOCl at three different pHs of 6, 8 and 10. Red color denotes positive correlation, and blue

295

color means negative correlation; darker color demonstrates a higher intensity and thus a

296

stronger positive or negative correlation. In 2D-CoS maps, the peaks located at the diagonal line

297

are called auto-peaks while cross peaks are located off-diagonal positions.

298 16


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Likewise, according to the sign of 2D-CoS maps at pH 8, the sequential order

300

of band changes followed 1463, 1440, and 1410 > 1662 > 1292, 1320 > (1700) >

301

1486, 1580 > 1241 > 1150, 1105 cm-1. It demonstrated that the degradation of groups

302

involving in PVP also occurred earlier than the scission of Ph-SO2-Ph chains,

303

followed by the variation of aromatic ether of PES. However, some cross peaks

304

(Φ(x1, 1700) and Ψ(x1, 1700)) in the 2D-CoS maps showed no sign at pH 8. As such,

305

it was unable to identify the change order of band at 1700 cm-1 except for probing

306

that the formation of succinimide groups at 1700 cm-1 followed the changes of cyclic

307

structures and amide groups of PVP (1410/1440/1463 and 1662 cm-1). At pH 10, the

308

sequential order of band changes was identified as 1463, 1440, and 1410 > 1662 >

309

1320, 1292 > 1486, 1580, 1241, 1150, 1105 cm-1. Expectedly, the variations of the

310

bands (1463, 1440, 1410 and 1662 cm-1) involving in the PVP material still occurred

311

initially whereas the C-H structure of aromatic ring (1486 and 1580 cm-1) and the

312

aromatic ether (1241 cm-1) of PES was attacked less and hardly changed at pH 10.

313

Accordingly, the XPS high-resolution C s1, N s1, O s1 and Cl 2p data (Fig. S2) only

314

showed slight changes with the exposure time at pH 10. Several previous studies

315

also reported that the NaOCl solution under a stronger alkaline condition had less

316

impact on membrane functional groups and properties (e.g., surface charges,

317

elongation at break and tensile strength)10, 36, 38.

318

Generally, regardless of the pH, the dynamic ageing process of PES/PVP

319

membranes by NaOCl is almost similar except for the diverse degradation rate of

320

functional groups. As a first step, the C-H vibration of the cyclic structure of PVP 17



Page 18 of 33

321

was attacked readily by unstable ·OH radicals, especially in the α-position of the

322

amide group41 (Fig.S4). Through mass spectrometry analysis, Foutquet et al.39 also

323

observed the hydrogen abstraction of the PVP exposed to NaOCl. Subsequently, the

324

amide groups (1662 cm-1) of PVP were greatly impacted, and then the succinimide

325

groups formed at 1700 cm-1 were observed18, 41. These reactions could be further

326

revealed by the XPS spectra (Fig. S1 and S2), e.g., with increasing exposure time,

327

the proportion of C-(C, H) decreased while the functional group of C=O increased

328

(Table S1). Additionally, increasing proportion of protonated nitrogen as a function

329

of exposure time further confirmed the destruction of pyrrolidine structures. In

330

particular, the percentage of protonated nitrogen increased from 0 to 26.16 % at pH 8

331

as the ring-opening or hydrolysis of succinimide groups could occur more easily in

332

alkaline solutions18, 35. The XPS data also showed that the atomic percentage of N

333

elements reduced to the lowest level at pH 8 (3.45 to 1.55 %) compared with the

334

cases of pH 6 (3.45 to 2.48 %) and pH 10 (3.45 to 3.19 %) (Table S1), indicating

335

that the aged PVP material could be leached or released from the membranes.

336

Previous studies denoted that the initial degradation/release of the PVP material

337

might accelerate the subsequent degradation of PES polymers and worsen the

338

enlargement of membrane pores36, 38. Actually, the amide groups (O=C-N 10)36. In the future, more detailed work need to be focused on the fouling

419

process of aged membranes as a function of operating period to aid engineers in

420

adjusting the cleaning method so that the membrane life time could be extended

421

operationally 56, 57. In addition, the combined use of 2D-CoS analysis and previously

422

reported methods, such as electrokinetic characterizations, are suggested in order to

423

obtain complementary data.

424 425 27



426

ASSOCIATED CONTENT

427

Supporting Information

428

Table S1. Atomic fraction variations for elemental composition obtained from XPS

429

high resolution C1s, N1s, O1s, S2p and Cl2p spectra of NaOCl-aged PES/PVP

430

membranes as a function of exposure time (i.e., 1, 6, 30 days) at pH 6, 8, and 10,

431

respectively; Figure S1. XPS wide survey scans of NaOCl-aged PES/PVP

432

membranes as a function of exposure time (i.e., 1, 6, 30 days) at pH 6, 8, and 10,

433

respectively; Figure S2. XPS high resolution C1s, N1s, O1s, S2p and Cl2p spectra

434

of NaOCl-aged PES/PVP membranes as a function of exposure time (i.e., 1, 6, 30

435

days) at pH 6, 8, and 10, respectively; Figure S3. SEM images of NaOCl-aged

436

PES/PVP membranes as a function of exposure time (i.e., 1, 6, 30 days) at pH 6, 8,

437

and 10, respectively. Figure S4. Ageing process of PVP during the NaOCl exposure;

438

Figure S5. Ageing process of PES during the NaOCl exposure.

439 440 441 442

AUTHOR INFORMATION

443

Corresponding authors

444

Email: [email protected] (Fangang MENG)

445

Notes

446

The authors declare no competing financial interest

447 448

ACKNOWLEDGEMENTS

449

This study was supported by the National Natural Science Foundation of China (No.

450

51622813 and 51608546), the Natural Science Foundation of Guangdong Province

451

(No. 2014A030306002), and the Science and Technology Planning Project of

452

Guangdong Province (No. 2015A020215014). 28


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617 618

Table caption

619

Table 1. 2D-CoS data on the assignment and sign of each cross-peak in synchronous (Φ)

620

and asynchronous (Ψ, in the brackets) maps of PES/PVP membranes upon exposure to

621

NaOCl at three different pHs of 6, 8 and 10 with increasing exposure time.

622 623

Figure caption

624

Figure 1.ATR-FTIR spectra (1000-1800 cm-1) of PES/PVP membranes upon exposure to

625

NaOCl at three different pHs of 6 (a), 8 (b), and 10 (c) as a function of exposure time (0-30

626

days).

627 628

Figure 2. Synchronous (a, c, e) and asynchronous (b, d, f) 2D correlation maps constructed

629

from time-dependent ATR-FTIR spectra of PES/PVP membranes in the treatment of

630

NaOCl at three different pHs of 6, 8 and 10.

631 632

Figure 3. Changes in pure water permeability (a) and membrane pore sizes (b) of PES/PVP

633

membranes exposed to NaOCl as a function of exposure time at three pHs (i.e., 6, 8 and 10).

634 635

Figure4. Filtration behavior of PES/PVP membranes exposed to NaOCl as a function of

636

exposure time at three pHs (i.e., (a) 6, (b) 8 and (c) 10) with HA solutions (10 mg-TOC/L)

637

and the vibrations of corresponding UMFI in each case (d).

638 639

Figure5. Correlations between UMFI (Hollow icons)/water permeability (Solid icons) and

640

the decrease of IR intensity of the band at 1662 cm-1 (amide groups of PVP) as a function of

641

exposure time.

33


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