Aged PVDF and PSF ultrafiltration membranes restored by functional polydopamine for adjustable pore sizes and fouling control

Aged PVDF and PSF ultrafiltration membranes restored by functional polydopamine for adjustable pore sizes and fouling control

Author’s Accepted Manuscript Aged PVDF and PSF ultrafiltration membranes restored by functional polydopamine for adjustable pore sizes and fouling con...

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Author’s Accepted Manuscript Aged PVDF and PSF ultrafiltration membranes restored by functional polydopamine for adjustable pore sizes and fouling control Fei Gao, Jie Wang, Hongwei Zhang, Hui Jia, Zhao Cui, Guang Yang www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(18)32146-X https://doi.org/10.1016/j.memsci.2018.10.037 MEMSCI16553

To appear in: Journal of Membrane Science Received date: 4 August 2018 Revised date: 17 September 2018 Accepted date: 12 October 2018 Cite this article as: Fei Gao, Jie Wang, Hongwei Zhang, Hui Jia, Zhao Cui and Guang Yang, Aged PVDF and PSF ultrafiltration membranes restored by functional polydopamine for adjustable pore sizes and fouling control, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.10.037 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 galley proof before it is published in its final citable 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.

Aged PVDF and PSF ultrafiltration membranes restored by functional polydopamine for adjustable pore sizes and fouling control Fei Gaoa,b, Jie Wanga,c*, Hongwei Zhanga,c*, Hui Jiaa,c, Zhao Cuia,b, Guang Yanga,b a

State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin

Polytechnic University, Tianjin 300387, China b

School of Environmental Science and Engineering, Tianjin University, Tianjin

300072, China. c

School of Environmental and Chemical Engineering, Tianjin Polytechnic University,

Tianjin 300387, China

[email protected]

[email protected] *

Corresponding author: Tel.: +86 022 8395 5668; fax: +86 022 8395 5451

ABSTRACT Hydrophilic polyvinylidene difluoride (PVDF) and polysulfone (PSF) membranes

modified

by

hydrophilic

additives

such

as

polyvinylpyrrolidone (PVP) are widely used in various industrial fields. However, the degradation and removal of hydrophilic additives caused by chemical cleaning agents can occur in a short-term aging, which impair the integrity and performances of the membranes. A restoration method is 1

essential for improving the performances of initial aged membranes. In this study, the effects of polydopamine (PDA) coatings on the structural/surface characteristics, the filtration performance and fouling behaviors of aged PVDF and PSF membranes were investigated. Results indicate that the surface properties of aged membranes could be improved by PDA modification. However, the PDA coating structures for PSF and PVDF membranes are different, which are regulated by the different aperture ranges of aged membranes. A mechanism of external and internal PDA restoration for aged membranes with different aperture ranges was introduced. PDA deposited on/in aged PVDF membrane that had large aperture could achieve the synchronous repair of membrane external and pore surface, which not only enhance their surface hydrophilicity (contact angle goes from 80.2°to 58.7°) and zeta potential (from -23.7mV to -30.5 mV at pH 7.5) but also these properties of the pore channel surface and narrow the enlarged pore size simultaneously (the average pore size changes from 220 nm to 170 nm), in turn improved the anti-fouling and rejection performances. By contrast, the PDA was formed as an individual layer on the surface of aged PSF membrane with smaller pore sizes and blocked some pores that showed reduced adsorption and accumulation of foulants on membrane surface, but less reduction of fouling was observed during fouling experiments. 2

Graphical Abstract:

Keywords:Aged ultrfiltration membrane; Polydopamine; Performances restoration; Membrane pore size; Membrane fouling 1. Introduction Ultrafiltration (UF) technology has been widely used for water purification and energy regeneration. This is due to their significant advantages such as the high quality water production, small footprint, high efficiency and easy maintenance [1-3]. However, membrane fouling is still considered to be inevitable during practical run, which limits its wide applications, and can result in severe flux decline and deterioration in effluent quality [4, 5]. In order to restore the membrane performance, 3

chemical cleaning is carried out routinely by using cleaning agents, among which sodium hypochlorite (NaClO) is most widely used due to its advantages of chemical stability, high cleaning efficiency against organic foulants and low cost [6-8]. Many studies have been carried out on the optimization of NaClO cleaning scheme in membrane filtration system [9, 10]. It has been well proved that NaClO can remove organic foulants by oxidizing organic foulants to ketonic, aldehydic, and carboxylic groups [6, 7]. However, repeated exposure to sodium hypochlorite can result in accelerated membrane aging, which major changes in pore size, surface and mechanical properties [11-16]. These changes, in turn, impact the filtration performance and fouling behaviors. The inclusion of hydrophilic additives such as polyvinylpyrrolidone (PVP) has become a standard method to obtain hydrophilized membranes with increased antifouling properties compared to additive-free hydrophobic membranes [17, 18]. Many studies have pointed out the effects of sodium hypochlorite on various polymer membranes in the presence of PVP (e.g., polysulfone (PSF) [19], poly(ether sulfone) (PES) [12], polyvinylidene difluoride (PVDF) [13]). Rabiller-Baudry et al. demonstrate that PES membrane aging by NaClO is accelerated in the presence of PVP [20]. Similar results have been found by Ravereau et al. [13] for PVDF membrane modified by hydrophilic additives. Zhang et al. showed that the initial degradation of hydrophilic additives occurrs in a 4

short exposure to the NaClO solution [14]. Our previous work has shown that the mechanisms of aging process for hydrophilic PVDF membrane are in order of removal of additives, cross-links of the polymers, defluorination and oxygenation reaction and formation of sodium carboxylate [21]. Pellegrin et al. and Zhou et al. showed that the initial degradation and removal of PVP play a critical role in the aging process of PES/PVP membrane, which could trigger or speed up the subsequent degradation of PES [11, 12]. The removal of hydrophilic additives from membrane

matrix could result in changes in membrane surface and

mechanical properties mainly including the decrease in hydrophilicity and tensile yield strength, as well as the enlargement of membrane pores, both leading to a deterioration of membrane performance and serious fouling. Therefore, an easy, efficient, and convenient method is required to restore the surface and structure properties of initial aged membrane, thereby improving antifouling and filtration performances, as well as prolonging the lifetime of membrane. Dopamine, an excellent mussel-inspired coating material, has received significant attention for surface modification of solid substrates, including many types of water purification membranes, to enhance hydrophilicity [22-24]. The hydrophilicity of a polydopamine (PDA) coating can reduce membrane-foulant hydrophobic interactions and enhance the diffusion rate during the filtration process [25]. The formation of a PDA 5

layer on membrane surface occurs via the self-polymerization of dopamine in the presence of oxygen [25, 26]. The PDA coating strongly adheres to the substrates by covalent and coordination interactions, hydrogen bonding and electrostatic interactions [27, 28]. However, many studies indicated that PDA attaches not only on membrane surface, but also in its pores which could result in pore size reduction [29-32]. Changes in membrane pore size may significantly influence membrane water flux and fouling behavior. In addition, Kasemset et al. showed that PDA coating could achieve a tradeoff in hydraulic permeability and selectivity [29]. Hence, we started to be interested in whether PDA coating can be used as a promising restoring method for enhancing the antifouling capacity and rejection property of aged membranes. In this study, PVDF and PSF UF membranes with different aperture ranges, widely used specifications in various industrial fields, were employed in accelerated aging and PDA restoration process. We focused on studying the remediation effect of PDA on these two kinds of membranes. The surface morphology, roughness, pore size distribution, porosity, contact angle and zeta potential of aged and PDA-restored membranes were systematically investigated. In addition, the anti-fouling and rejection performances of aged and PDA-restored membranes were evaluated by two kinds of foulant (bovine serum albumin and sodium alginate), and the distinct effects between PVDF and PSF membranes 6

were investigated separately and comparatively. Furthermore, the fluorescent staining combined with confocal scanning laser microscopy (CLSM) were used for investigating the distribution of BSA and SA fouling layer on different membranes surfaces and the structure of PDA in different membrane pore channel. 2. Materials and methods 2.1. Chemicals All reagents and chemicals were of analytical grade. NaClO, NaOH, NaHCO3, Na2CO3 were purchased from Sinopharm Chemical Reagent Co., Ltd. Bovine serum albumin (BSA), sodium alginate (SA), Dopamine hydrochloride (3-hydroxytyramine hydrochloride), Trizma hydrochloride (Tris-HCl), Fluorescein isothiocyanate (FITC), Rhodamine123, N, N-dimethyl-4-aminopyridine bought

(DMAP),

from

N-hydroxysuccinimide

Sigma-Aldrich.

were The

1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide Hydrochloride (EDC) was bought from TCI Developmemt Co., Ltd (Shanghai, China). Spectra/Pro membranes of 3 kDa and 14 kDa MWCO were supplied by Spectrum Chemicals & Laboratory Products (Shanghai) Co., Ltd. 2.2. Membranes and ageing procedure Commercial polyvinylidene

polysulfone fluoride

(20kDa

(50kDa

MWCO,

MWCO, 7

KOCH)

SEPRO)

and

ultrafiltration

membranes were used in this study. To accelerate aging process, membranes were soaked in sodium hypochlorite solutions of 1000 ppm of total free chlorine at pH 10-10.5, and the degree of ageing was presented as concentration×time (c×t=gh/L). Exposure times were 200 hours. Previous studies indicate that when ct=200 gh/L, both membranes are aged to a certain extent, induced by the initial hydrolysis of hydrophilic additives [14, 21]. Sodium hypochlorite solution was changed every 24 hours. After aging procedure, membranes were rinsed and stored in ultrapure water until used. 2.3 Polydopamine (PDA) restoration of aged membranes Both aged PVDF and PSF membranes were coated via PDA self-polymerization [33, 34]. Dopamine solution (2 mg/ml) was prepared by dissolving dopamine hydrochloride in 15 mM Tris-HCl buffer aqueous solution (pH 8.8). Aged membranes were fixed on a glass plate and its selective side faced up. Then these membranes were immersed in the dopamine solution, in contact with atmospheric oxygen, at room temperature (25±2℃) for 1 h with gentle rocking motion to agitate. After 1 h immersion, the membranes were soaked in ethanol for 30 min to remove weakly-bound polydopamine and then washed with running ultrapure water to remove the ethanol. The PDA-restored membranes were stored in ultrapure water prior to use. (Scheme1) 8

Scheme 1. Illustration for the preparation of aged and restored membranes.

2.4 Fluorescence labeling of PDA coating and BSA and SA FITC was chosen as a fluorescent probes to be loaded onto the aged and PDA-coated membranes. As-prepared membranes were dipped in FITC solution (0.1 g L-1). After labeling, the membranes were incubated for 30 min in the dark, and then were rinsed 3 times with phosphate buffered saline (PBS) solution to remove unreacted FITC. BSA and SA were selected as model organic foulants at a concentration of 100 mg/L for feed solutions in this study. Based on existing literatures [35, 36], FITC and Rhodamine123 were applied to label the BSA and SA, respectively. The specific fluorescence labeling procedures have been described in detail in our previous work [36]. 2.5 Characterizations of Membranes The surface morphology of membranes was observed using a field 9

emission scanning electron microscope (FESEM, Hitachi, S-4800) operating at 10 kV. The membrane samples were immersed in liquid nitrogen and carefully fractured to obtain the cross-section of the membrane, and then observed using FESEM (ZEISS, SEM500). Surface roughness of membranes was measured by atomic force microscopy (AFM, Multimode 3, Bruker Co.). The mean roughness (R a), the root mean square (Rq) and the difference between the highest points and lowest valleys (Rz) of the height data were measured at a scan size of 5 µm×5 µm. The porosity was evaluated by gravimetric method with a wetting agent (Porefil, IB-GT GmbH, Germany) of a low surface tension of 16 mN/m [37]. The pore size distribution for membranes was measured by a capillary flow porometer (Poroflux 1000, IBFT GmbH, Germany) following the method described in previous literature [38]. The gas permeation velocity for dried membranes and wetted membranes by isopropyl alcohol (IPA) was measured at different transmembrane pressure difference (ΔP). Surface hydrophilicity of the membranes was evaluated by water contact angle using a Micro Drop Shape Analyzer (DSA100m, KRUSS, Germany). Ten random locations of water contact angles were measured after exactly 5 s for all samples. A SurPass electrokinetic analyzer (Anton Parr Gmbh, Graz, Austria) was used to measure the zeta potential of membranes using the tangential streaming potential method. For each membrane, all above measurements were 10

conducted at least three times and the reported average and standard deviation were calculated from the mean values. 2.6 Filtration experiments All filtration experiments were carried out using a stirred dead-end cell at room temperature (25℃±1). Prior to BSA and SA fouling filtration, the membranes were precompacted with pure water under 0.15 MPa until stable flux was achieved. Then the pure water flux of membranes was determined at 0.1 MPa. Water permeability of all the samples, summarized in Table S1, has hardly changed after a long time and repeatedly measurement. Finally, the BSA and SA solution was filtered through the membranes under 0.1 MPa for 120 min and the stirring speed was set at 300 rpm for all fouling experiments. The permeate flux data continuously recorded using an electronic balance connected to a computer. Membrane rejection was measured by TOC analyzer (TOC-VCPH, SHIMADZU, Japan). The rejection ratio R(%), was calculated using equation (1), where CF and CP are the concentration of the solute in the feed and permeate, respectively. R(%) = [1-(CP/CF)]*100

(1)

2.7 CLSM analysis 11

The stained membranes were imaged by CLSM (TCS SP8, Leica Microsystems CMS GmbH, Germany) following the previous procedures [36]. The excitation wavelength of FITC and rhodamine123 was 488 nm. The emission wavelengths ranges of FITC and rhodamine123 were 500-530 nm and 520-540 nm, respectively. The aged and PDA-coated membranes were vertically fixed and the cross-section of these membranes were observed using a plan apochromatic 20×objective. For fouled membrane samples, reflection mode was set to identify the membrane surface at the position of z-axis and then observed in fluorescence mode. Membranes were imaged in the xyz mode and which can scan the xy plane along z-axis. Images of fouled membranes were captured from the outside of the cake layer to the membrane surface and then to the inside of membrane. 120 slices were recorded for a total depth of 60 µm. All fouled membrane samples were observed using a plan apochromatic 10×objective, and the zoom magnification was set at 1. Images were scanned at 400 Hz, with a resolution of 512×512 pixels. Three dimensional reconstruction of image stacks was carried out using LAS AF confocal software. In addition, in order to calculate the distribution of foulants in different depths of cake layer formed on membrane surfaces, the images were analyzed by Image J software (NIH, Betheada, MD, USA) using ‘mean intensity’ method. 3. Results and Disscussion 12

3.1 Characteristics of membranes 3.1.1 Surface morphology of the membranes The SEM images of the membranes surfaces were taken to study the morphology changes in the membrane surface caused by NaClO aging and PDA restoration. As shown in Fig. 1, for PVDF membranes, the pristine membrane surface was rough, which might be attributed to the highly crystalline structure of PVDF [39, 40]. After exposed to NaClO solution, the surface morphology of aged membrane changed significantly, the irregular macro-voids were observed in the membrane surface, which might caused by the degradation and leakage of the PVP in PVDF matrix [21]. But interestingly, after the PDA coating, the macro-voids were not shown in the membrane surface. In terms of PSF membranes, it can be observed that the pristine membrane surface was much smoother than PVDF membrane surface, and relatively uniform pores were observed. After aged by NaClO, the membrane became rugged. However, after treated with PDA, the surface was covered by PDA layer and became less porous. These phenomena suggested that the deposition of PDA aggregates on the surface of aged membrane might have great influence on its pore size and porosity. Kasemset et al indicated that thicker PDA could accumulate on the surface of the membranes and at the same time within its pores, resulting in pore 13

restriction [29]. Our previous studies demonstrated that the macro-voids formed in aged membrane surface could affect the permeability and selectivity of the membrane, foulants were more likely to enter into the membrane pores which could accelerate membrane fouling and decrease the sieving properties of the membrane [36]. Thus the changing of membrane pore size based on PDA coating might be a feasible method to improve the performances of different aged membranes in a certain degree.

14

Fig. 1. SEM images of the surface of membranes: (a) untreated PVDF membrane; (b) aged PVDF membrane; (c) PDA-restored PVDF membrane; (d) untreated PSF membrane; (e) aged PSF membrane; (f) PDA-restored PSF membrane

The surface roughness is an important parameter for membrane and 15

may affect membrane intrinsic wetting property and fouling behaviors [30]. AFM was applied to measure membranes surface roughness as presented in Table1. It was found that all PVDF membranes have a larger surface roughness than PSF membranes. The surface average roughness (Ra) of pristine PVDF membrane is about 172 nm before aging and PDA coating. However, Ra values of aged PVDF membranes were decreased to 120 nm and further decreased to 116 nm after restored by PDA. In contrast, the surface roughness values of the pristine, aged and PDA-coated PSF membrane were 36 nm, 70.2 nm and 49.8 nm, respectively. The pristine PSF membranes were relative smooth, while the value of Ra increased after exposed to NaClO, and then decreased after PDA coating. These results suggested that the roughness of aged membranes could be changed through PDA modification. Table 1. Measured membrane roughness parameters

PVDF

PSF

Membrane Untreated Membrane area (μm2)

100

Aged 100

PDA-restored 100

Untreated 100

Aged 100

PDA-restored 100

Ra (nm)

172±3.5

120±9.8

116±6.5

36±1.2

70.2±5.5

49.8±8.6

Rq (nm)

209±12

150±21

137±18

43.6±3.5

84.7±10.6

60.7±12

Rz (nm)

911±33

891±42

571±38

250±17

433±35

372±28

16

3.1.2 Porosity and pore size distribution of the membranes The bubble-point and gas permeation tests, usually called the wet and dry flow method, were carried on membrane samples, presenting a surface of approximately 5×10-4 m2, to estimate the mean pore size and the pore size distribution. For PVDF membranes, Fig.2(a) shows the shift of the initial pore size distribution and the formation of a second population of larger pores after aging. The average pore size of pristine membrane is 180±20 nm and that of the aged membrane become 220± 23 nm. Importantly, the shift of pore size distribution of aged PVDF membrane to small pores was significantly after PDA coating. The PDA remediation appears to reduce the size of the larger pores, which narrows the pore size distribution. The average pore size of PDA-coated aged PVDF membrane was similar to the pristine PVDF membrane. In terms of PSF membranes, shown in Fig.2(b), the pore size distribution was not significantly changed by NaClO aging, with the average pore size of pristine membrane being 78.56±8.6 nm and that of the aged membrane being 82.25±9.5 nm. The emergence of the macro-voids in aged PSF membrane skin structure might be partially attributed to the “fusion” of present pores, as suggested by Pellegrin et al. [11]. Only a few macro-voids formed with the pore size at around 120 nm. After the PDA coating, the pore size distribution of aged PSF membrane did not narrow and the macro-voids still exist, which is different from PDA-coated aged 17

PVDF membrane. We assumed that the structures of PDA coating in/on the membranes with different aperture ranges were varied. The deposited PDA could adhere to the inner wall of aged PVDF membrane larger pores and thus narrow the enlarged pore size. However, for aged PSF membrane, PDA coating might cover their small pores and detach from the external of pores under increasing transmembrane pressure due to no hydrogen bonding with PSF backbone.

Fig. 2. Pore size distribution of membranes

According to Table 2, the measured porosity of the aged PVDF membrane was lower than that of the pristine PVDF membrane, but it was significantly increased after PDA coating. These results may be attributed to the PDA coating inside the pores, which restored the macro-voids in aged PVDF membrane surface. Another plausible reason is the increase of the hydrophilicity of the pore walls due to PDA coating, which would increase membrane porosity [29, 41]. In contrast, the porosity of aged PSF membrane was slightly lower than that of the 18

pristine PSF membrane, and further decreased after PDA coating. This phenomenon could be ascribed to a limited number of pores completely blocked by PDA coating. Table 2. Porosity of the membranes.

PVDF

PSF

Membrane Untreated

Aged

PDA-restored

Untreated

Aged

PDA-restored

Thickness (μm)

184±3.5

183.3±1.7

184±2.6

150±0.5

149.3±3.3

150±2.2

Porosity (%)

59.56±4.3

54.22±0.3

63.13±2.5

64.40±0.5

62.87±3.3

54.13±2.8

Based on the study above, results indicate that the membrane surface structure changed when the aged membranes were coated with PDA. More interestingly, the changes of surface coating morphology and internal structure of the PDA-restored aged PVDF and PSF membranes are different. To give this study more comprehensive and credibility, further experiments are ongoing to evaluate the surface characteristics and filtration performance of aged membranes restored by PDA. 3.1.3 Hydrophilicity and charge characteristics Fig. 3 shows the water contact angle of PVDF and PSF membranes after NaClO aging and PDA restoration. As clearly seen, the contact angle for both aged PVDF and aged PSF membranes were increased, indicating the removal of hydrophilic additives dominates the evolution of surface 19

hydrophilicity of membranes at this aging degree [42]. After PDA coating, the contact angle of aged membranes were reduced, suggesting that the PDA coating enhanced the hydrophilicity of both aged PVDF and aged PSF membranes surfaces.

Fig. 3. Water contact angle for untreated, aged and PDA-restored membranes : (a) PVDF membranes; (b) PSF membranes

When PVDF and PSF membranes aged by NaClO and then repaired using PDA, the evolution of surface zeta potential with pH value was represented in Fig.4. All membranes have similar zeta potential trends that transit to negative charge with the increasing of pH values and remain a pseudo plateau for pH higher than ~7. As it is clearly shown in Fig.4(a), the negative surface zeta potential values for aged PVDF membrane were lower than that of the pristine PVDF membrane, which could be due to the formation of macro-voids caused by the loss of degraded PVP from aged membrane (less weak acid groups on the membrane surface) [18]. After PDA coating, the PDA-coated aged PVDF 20

membrane became more negatively charged in the neutral and alkaline environment due to the amphoteric nature of PDA [43], and its zeta potential is similar to that of pristine PVDF membrane. For PSF membrane, the charge of the aged membrane became more negative, which could be attributed to the formation of a sulfonic group [44]. Another reasonable reason is the oxidation of PVP, leading to the formation of carboxylic acid groups through a ring opening mechanism [18]. After PDA deposition, no zeta potential change was observed in pH from 6 to 9. In conclusion, membrane surface characterizations indicated that the surface properties of aged membrane could be improved by PDA modification. To further reveal the applicability of the PDA restoration, the filtration performance and fouling behavior of PDA-coated aged PVDF and PDA-coated aged PSF membranes were investigated separately and in detail, as shown below.

Fig. 4. Membrane surface zeta potential as a function of pH value and the membrane zeta potential evolution after NaClO aging and PDA coating: (a) PVDF membranes; (b) PSF membranes

21

3.2 Filtration performances and fouling mechanisms BSA and SA, typical foulants in surface water, were chosen to evaluate the membranes fouling tendency in ultrafiltration of the individual foulant. Fig. 5 (a) and (b) showed time-dependent declines of the flux of PVDF membranes during the filtration of BSA and SA, respectively. According to the results, the original flux for the aged membranes was increased due to the enlarged pore diameter, and the flux decline of these membranes was the most rapid in the initial stage due to the enhanced surface hydrophobicity and decreased negative charge for aged PVDF membrane surface. With the increase of filtration time, the BSA permeate flux decline trends for aged PVDF membranes became slow and obtained a balanced states gradually, however the SA permeate flux declined continuously. After PDA coating, for both BSA and SA, the initial flux of PDA-coated PVDF membranes was lower than those of aged PVDF membranes, but it is worth noting that these restored membranes exhibited significantly lesser flux decline trends than aged membranes and even the pristine membranes during the whole filtration process. One reason is that the hydrophilicity and zeta potential played important roles in the membrane filtration performances, the improved anti-fouling capacity for BSA and SA was partly attributed to the significantly enhanced hydrophilicity and zeta potential (more negative) for both membrane surface and channel surfaces of pores after PDA restoration. 22

Qiu et al. included that the thin coating into the membrane pores channel provides a protective layer and reduces membrane pore defects which could improve the anti-fouling performance [45]. On the other hand, the narrowed membrane pores might cause fewer foulants to accumulate in the membrane. Zeng et al. suggested that membrane with small pore size experienced slow fouling rate and low fouling extent [46].

Fig. 5. Flux reduction curves for untreated, aged and PDA-restored membranes in filtration runs with different foulant solution: (a) filtration of BSA solution for PVDF membranes ; (b) filtration of SA solution for PVDF membranes ; (c) filtration of BSA solution for PSF membranes ; (d) filtration of SA solution for PSF membranes. Experimental conditions: the concentration and pH of the BSA and SA solutions were 100 mg/L and 7.5± 0.2, respectively; the applied pressure was 23

0.1 MPa.

For PSF membranes, shown in Fig.5 (c) and (d), aged PSF membranes exhibited decreased initial flux and larger flux decline than the pristine PSF membranes during the treatment of BSA and SA. This result can be interpreted by the increased hydrophobicity and the greater roughness of aged PSF membrane, which provide a larger surface area for foulants on the membrane surface. Similar to the results of aged PVDF membrane, the BSA permeate flux was decreased rapidly in a short period of filtration time, however, the permeate flux for SA continued to decline throughout the filtration process. More importantly, the evolutions of flux decline for PDA-coated aged PSF membranes were different from those of PDA-coated aged PVDF membranes. When BSA existed, the fouling rate of PDA-coated aged PSF membrane decreased compared to aged membrane, however, was still higher than pristine PSF membrane. In addition, the permeate flux continued to decline in long-term stage .When only SA existed, the flux of PDA-restored membrane remains almost unchanged compared to aged PSF membrane. Similar results were obtained from Miller et al. Long-term filtration experiments for PDA-modified PS20 polysulfone ultrafiltration membrane and TS80 nanofiltration membrane showed that no reduction of biofouling was observed. Short-term static BSA adhesion test could not predict ultimate membrane biofouling behavior [47]. We think that these results may be 24

explained in terms of the blocking and accumulation of foulants in/on membrane pores. Single PDA surface coating is inefficient in control membrane fouling during filtration process. In addition, the different fouling mechanisms of BSA and SA might affect the flux decline of PDA-coated membrane. Compared to the loose BSA cake layer, the absorbance of BSA on membrane surface or in membrane pores was the main reason for the decline of flux. In contrast, the pore blocking and formation of SA gel layer resulted in a significant flux decline. PDA surface coating could reduce BSA binding to the aged PSF membrane to a certain extent, but could not prevent SA to plug the membrane pores and form the gel layer on membrane surface during filtration process. Based on the analysis of membrane properties and performances, we suspect that the effect of PDA restoration on aged membranes with different aperture range is varied. The PDA deposition on aged PSF membranes with small aperture was ineffectual, however the PDA-restored PVDF membrane with enlarged pores prepared in this study exhibited improvement during filtration process. To a more thorough understanding of fouling mechanisms, four typical fouling models, complete blocking, standard blocking, intermediate blocking and cake formation, were fitted [48] to the filtration curves and the regression results are listed in Table S2. According to the R2 values of the four fitting sets, in term of BSA, the four models have low fitting 25

degree of PVDF membrane (R2< 0.9) because their large pore size were not enough to cause the blockage of the membrane, and compared to the loose BSA cake layer, the absorbance of BSA on membrane surface or in membrane pore was the main reason for the decline of flux. However, the model of cake layer has a high fitting degree of PSF membrane. For SA, cake formation was the major reason for flux decline of the pristine PVDF and PSF membranes. After NaClO aging, cake layer formation played an important role in aged PVDF and PSF membranes fouling for both BSA and SA, which was mainly due to the deterioration of surface properties of aged membranes. After PDA restoration, intermediate blocking dominated the BSA fouling process for PDA-coated aged PVDF membrane due to its decreased pore sizes and the enhanced hydrophilicity. However, standard blocking and cake layer was the important reason for the flux decline of the PDA-coated aged PSF membrane. This may be due to the fact that PDA coating can not form on the pore channel walls but block some pores, which lead to the adsorption and accumulation of BSA in the hydrophobic pores and on the membrane surface. For the SA filtration experiments, fouling mechanisms did not change when aged PVDF and PSF membranes were coated by PDA. This may be reason for the larger size and good colloidal properties of SA particles than that of BSA, hence, most SA molecular solution are likely to form a gel layer on the 26

membrane surface [49, 50]. BSA and SA retention of membranes are presented in Fig. 6. In terms of PVDF membrane, the retention ability of aged membranes for both BSA and SA was dropped. After PDA restoration, both BSA and SA rejection was enhanced, which is attributed to the reduction in pore size. In addition, BSA retention of the PDA-modified membrane was still lower than that of the untreated membrane, because the hydrophilicity of the membrane pores was enhanced so that BSA were more likely to pass through the membrane into the effluent than to adsorb to the pore channel. For PSF membrane, a very small change was observed, and the membranes keep its good retention properties after aging and PDA coating. Therefore, considering the evolution on the antifouling ability and selectivity, we suggest that the filtration performances of aged PVDF membrane could be improved better by PDA coating.

27

Fig. 6. BSA and SA retention of PVDF and PSF membranes before and after PDA restoration

3.3 The development and structure of the cake layer on the membrane surface The structure and distribution of fouling layer on the membranes surfaces were researched by CLSM observation. The views of 3D structures of fouled membranes surfaces are showed in Fig. 7-10. In addition, in order to quantify the distribution of BSA and SA along membrane depths, the method of ‘mean intensity’ was applied, which is based on the fluorescence intensity values of images from cross-sectional in an area of 1.10 × 1.10 mm2 measured by Image J software.

28

(a)

(b)

(c)

(d)

Fig. 7. (a-c) CLSM images of FITC-BSA fouling layer on PVDF membranes surface before and after PDA restoration: (a) untreated membrane; (b) aged membrane; (c) PDA-restored membrane. (d) Mean fluorescence intensity profile in different depths of FITC-BSA fouling layer on PVDF membranes surface

(a)

(b)

29

(c)

(d)

Fig. 8. (a-c) CLSM images of rhodamine-SA fouling layer on PVDF membranes surface before and after PDA restoration: (a) untreated membrane; (b) aged membrane; (c) PDA-restored membrane. (d) Mean fluorescence intensity profile in different depths of rhodamine-SA fouling layer on PVDF membranes surface

Fig. 7 and 8 show the development of BSA and SA fouling on PVDF membranes surfaces. As it was seen clearly, less abundance of BSA and SA formed on PDA-coated aged membrane surface. It could be seen from Fig. 7(d) that the stronger fluorescence intensity of BSA was observed on aged membrane surface compared to that of pristine membrane and the intensity decreased significantly when aged membrane coated by PDA. However, as shown in Fig. 8(d), the intensity of SA for aged membrane was weaker than that of pristine membrane, which might be explained by the formation of macro-voids on the membrane surface and that lead to more SA particles into membrane, leading to dramatically flux decline [36]. Similar to the CLSM images, the weaken fluorescence intensity of 30

SA was observed on PDA-coated aged membrane surface.

(a)

(b)

(c)

(d)

Fig. 9. (a-c) CLSM images of FITC-BSA fouling layer on PSF membranes surface before and after PDA restoration: (a) untreated membrane; (b) aged membrane; (c) PDA-restored membrane. (d) Mean fluorescence intensity profile in different depths of FITC-BSA fouling layer on PSF membranes surface

31

(a)

(b)

(c)

(d)

Fig. 10. (a-c) CLSM images of rhodamine-SA fouling layer on PSF membranes surface before and after PDA restoration: (a) untreated membrane; (b) aged membrane; (c) PDA-restored membrane. (d) Mean fluorescence intensity profile in different depths of rhodamine-SA fouling layer on PSF membranes surface

For PSF membranes, as presented in the Fig. 9 and 10, it could be observed that compared to the pristine membranes, the fluorescence intensities of BSA and SA were increased for aged membranes. Importantly, the fluorescence intensities of BSA and SA were decreased for PDA-coated aged membrane surface, but still greater than that of the pristine membranes. This should be attributed to the hydrophilic PDA coating layer on the aged PSF membrane surface, and thus reduce the adsorption of partial foulants on the membrane surface. These phenomena are inconsistent with the results of filtration performances. We think that single surface modification by PDA can not effective for membrane 32

fouling control, owing to the absorption and blockage of foulants in the membrane pores leading to a decrease in flux.

Fig. 11. Cross-section SEM images of untreated membranes and cross-section CLSM images of aged and PDA-restored membranes labeled with FITC excited at 488nm

33

Fig. 12 Schematic diagram showing the external and internal repair mechanisms for aged membranes with different aperture ranges. Synchronous repair of membrane external and pore surface were achieved in PVDF aged membrane due to the occurrence of dopamine oxidation in enlarged pores wall, however, for aged PSF membrane, only individual PDA layer formed on the external surface and blocked some pores because of the small pore size.

As mentioned above, the surface properties of aged membranes could be improved by PDA modification. However, the aperture size range of aged membrane would be especially important, which could affect whether PDA layer can be coated on the pore channel of the membrane but not cover and plug the membrane pores. As shown in Fig. 11, the SEM images of PVDF and PSF membrane, the cross-sectional morphologies of different membranes revealed different structure. PVDF membrane showed typical asymmetrical structures with a dense skin-layer on the top surface supported by a finger-like structure and a porous architecture. PSF membrane has typical symmetric cellular-like porous structure, which may be attributed to the additives used in membrane preparation process. More importantly, Fig. 11 showed cross-sectional CLSM images of the PVDF and PSF membrane after aging and PDA restoration using FITC as a fluorescent dye. FITC could react with the uncyclized amine group on the PDA. For PVDF membrane, the skin layer of PDA-restored membrane present obvious green fluorescence compared with aged membrane, suggesting the presence of 34

PDA in the membrane pore channel. In terms of PSF membrane, the porous layer inside the aged membrane show fluorescence that maybe due to the reaction between FITC and additives found in the membrane. More importantly, there was no difference in thickness and fluorescence intensity when aged membrane modified by PDA. The possible explanations for this result are that enlarged pore size of aged PVDF membrane promotes the permeation, oxidation and self-polymerization of dopamine in pore wall of membrane, However, PDA layer cannot be coated on the inner wall of PSF aged membrane because of the smaller pore size (as a schematic figure shown in Fig. 12). The mechanism of dopamine polymerization on the surface of the substrate is proposed to

start

via

dopamine

5,6-dihydroxyindole

and

oxidation,

then

the

formation

of

5,6-indolequinone

via

intramolecular

cyclization, oxidation, and rearrangement that give rise to molecular stacking based on hydrogen bonding and π−πstacking interactions [51, 52]. As such, the dopamine oxidation is crucial and the large membrane pore sizes are essential for adequate oxidation reaction and a homogenous surface deposition that could easily access the pore walls. Correspondingly, PDA deposited on aged PVDF membrane showed excellent improvement that not only enhanced the hydrophilicity and zeta potential of aged membranes but also narrowed their enlarged pore size, which in turn improved the antifouling capacity and rejection 35

properties. In contrast, the PDA restoration employed in aged PSF membrane does not demonstrate a significant reduction in membrane fouling during fouling experiments. The results provide new insights into the application and development of PDA coatings. We envision that this simple strategy will be applied to periodic repair and optimize the membrane performance in engineering practice, such as wastewater purification, for which hydrophilized membrane surface and pore wall are essential. Further study is needed in this area. 4. Conclusions We proposed an efficient and convenient method to restore the properties and performances of aged membranes via the deposition of PDA. The effects of PDA restoration on the structural/surface characteristics, the filtration performance and fouling behaviors of PVDF and PSF aged membranes were investigated. It was found that the aperture size range of membrane was especially important which would lead to an external and internal restoring mechanism based the different membranes structures. When applied to aged PVDF membranes that had large aperture, PDA could coat on external surface and internal surface of enlarged pores synchronously. PDA-restored membranes had lower water flux reduction, higher retention, and lower protein adsorption and polysaccharide accumulation. In contrast, the PDA restoration employed in aged PSF 36

membrane does not demonstrate an obvious restoration in membrane structure properties or performances. The porosity of PDA-coated membrane decreased compared wtih that of the aged membrane, possibly due to blocking of some pores by PDA. There were no significant changes in the pore size distribution of restored membrane. Perhaps the PDA coating only attached on membrane surface and could not be formed in most of the inner wall of aged PSF membranes with smaller aperture. Besides, the aged PSF membrane coated by PDA showed less adsorption and accumulation of foulants than pristine PSF membrane, however, less improvement in anti-fouling and retention properties were observed during long-term fouling experiments. The different fouling behaviors for PDA coated aged PVDF and PSF membrane indicated that the coating structure could affect the recovery of membrane filtration performance, and the internal modification of membrane pore played a key role in the reduction of membrane fouling during continuous filtration process. Acknowledgments This study was financially supported by the National Natural Science Foundation of China (No.51638011, No.51578375), China Postdoctoral Science Foundation (2017M621081), and program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (Grand No. IRT-17R80). The authors would thank for 37

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47

Highlights 1. Polydopamine was used to repair aged PVDF and PSF membrane to some extent. 2. The exteral and internal repair mechanism is regulated by the aperture range of aged membrane. 3. Synchronous repair of membrane external and pore surface can reduce membrane fouling.

48