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Improvement in performance of PVDF ultrafiltration membranes by co-incorporation of dopamine and halloysite nanotubes
Journal Pre-proof Improvement in performance of PVDF ultrafiltration membranes by co-incorporation of dopamine and halloysite nanotubes Guangyong Zeng, Ke Wei, Denglei Yang, Jun Yan, Kun Zhou, Tanmoy Patra, Arijit Sengupta, Yu-Hsuan Chiao
PII:
S0927-7757(19)31134-3
DOI:
https://doi.org/10.1016/j.colsurfa.2019.124142
Reference:
COLSUA 124142
To appear in:
Colloids and Surfaces A: Physicochemical and Engineering Aspects
Received Date:
22 August 2019
Revised Date:
15 October 2019
Accepted Date:
18 October 2019
Please cite this article as: Zeng G, Wei K, Yang D, Yan J, Zhou K, Patra T, Sengupta A, Chiao Y-Hsuan, Improvement in performance of PVDF ultrafiltration membranes by co-incorporation of dopamine and halloysite nanotubes, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124142
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Improvement in performance of PVDF ultrafiltration membranes by co-incorporation of dopamine and halloysite nanotubes Guangyong Zenga* Ke Weia Kun Zhoua* Tanmoy Patrab
Denglei Yanga Arijit Senguptac
Jun Yana Yu-Hsuan Chiaod
(a. College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu, Sichuan 610059, P R of China; b. Department of Biomedical Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States;
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c. Radiochemsitry Division, Bhabha Atomic Research Center, Mumbai 400094, India; d. R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan University, Chung Li 32023, Taiwan) *Address correspondence to this author. Email: [email protected];
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[email protected]. Phone and Fax: +8602884078939
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Graphical Abstract
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Abstract A drastic improvement in the overall performance including permeability and antifouling characteristics were observed for polyvinylidene fluoride (PVDF) ultrafiltration (UF) membrane by incorporation of dopamine, halloysite nanotubes (HNTs) and 3-aminopropyltriethoxysilane (APTES) in one pot synthetic scheme. SEM and EDS mapping were used to signature the homogenous distribution of HNTs on membrane surface, whereas surface roughness of membrane was evaluated by AFM. The modified membrane surface was found to be highly hydrophilic as suggested from the water contact angle value (38.2º), while the water flux across the
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membrane was found to be 291.9 Lm-2h-1. The bovine serum albumin (BSA) separation tests indicated that, the modification broke the tradeoff between flux and
rejection ratio of membrane, which reached as high as 235.8 Lm-2h-1 and 92.0%, respectively. Moreover, a significant improvement in antifouling characteristics were
evidenced from static adsorption and dynamic permeation experiments. This study
Keywords:
Ultrafiltration
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practical applications for wastewater treatment. membrane;
Dopamine;
Halloysite
nanotubes;
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Hydrophilicity; Antifouling capacity
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provides a facile and useful guideline to design novel UF membrane and expand its
1.Introduction
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In the past few decades, ultrafiltration (UF) technology has achieved a continuous development, especially in the water treatment and wastewater reclamation[1]. UF is a low-pressure membrane filtration process compared to
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nanofiltration and reverse osmosis membrane, and the pore diameters of UF membrane ranges from several to dozens of nanometers[2]. The contaminants such as suspended solid, organic matters, sorts of oil, turbidity and even virus can be removed
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effectively by UF membrane with an easy automation and relatively low cost[3, 4]. However, the above contaminants are prone to adsorb or deposit onto membrane surface, which can lead to the concentration polarization and subsequent pore blocking[5]. The fouling can result in the increased energy demand, reduced separation efficiency and shorter service life of membrane. Therefore, improvement in antifouling characteristics of the membrane is very important and continuous approach of membrane research. Hydrophilic modification has become an attractive approach for the 2
improvement in membrane fouling resistance due to reduction in nonpolar interaction of organic foulants with the membrane surface[6-8]. It has been demonstrated that, hydrophilic modification leads to the formation of an aqueous layer on membrane surface, thus preventing the accumulation of contaminants. Some physical interactions, i.e. surface coating[9], deposition and chemical interaction, i.e. grafting polymerization[10, 11] have been exploited for modification of membrane surface. However, the surface structure of membrane might be destroyed and the modification effects would gradually reduce or even disappear after several cycles of membrane operation. The effect of additives like hydrophilic polymers and inorganic nanoparticles during membrane fabrication have also been investigated for the
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improvement in the antifouling properties of the membrane[12-14]. These additives are usually blended with raw membrane material into solvent to form casting solution,
and the composite membranes are fabricated via phase inversion method[15]. Although, it can improve the overall performances of membranes, additives are easily prone to aggregate, which has been the major drawbacks for blending
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modification[11].
Recently, the unique property of mussel-inspired chemistry (such as dopamine)
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has attracted interests of many researchers, which become a powerful tool for membrane fabrication. It is not only used as a modifier depositing on membrane surface directly, but also acts as an intermediate layer to further introduce other
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materials to construct composite membrane[16-18]. Zhao and his co-workers reported a dopamine modified polyethersulfone UF membrane. Their results indicated that, the addition of dopamine improved the blood compatibility of membrane. However,
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dopamine also blocked the pores on membrane surface resulting in a significant decrease in water flux. In our previous work[19], the superhydrophilic polyvinylidene fluoride (PVDF) microfiltration membrane was prepared by anchoring functionalized
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multi-wall carbon nanotubes on membrane surface by dopamine, which exhibited excellent performances with ultrahigh flux and oil rejection ratio. Similarly, silver[20],
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titanium dioxide[21] and others nanoparticles were also adhered on membrane surface by self-polymerization of dopamine to achieve high-performance membranes [22-24]. It is generally accepted that, breaking the tradeoff between flux and rejection of
membrane is highly challenging. In view of that, a novel modification method for UF membranes has been reported in the present investigation. Natural hydrophilic material halloysite nanotubes were adhered on membranes surface by dopamine to form a new separation layer, which realized the improvement in hydrophilicity, water flux and bovine serum albumin (BSA) rejection of membranes. Meanwhile, 3-aminopropyltriethoxysilane (APTES) was also used as crosslinker to enhance the 3
interface interaction and the dispersibility of HNTs on membrane surface. Additionally, the antifouling characteristics of modified membranes were investigated in detail. 2. Experimental 2.1. Materials The commercial UF membranes (PVDF400, 100kDa) used in this work were provided by Sepro Membranes, Inc. (Carlsbad, USA). Halloysite nanotubes (HNTs, purity≥95%), dopamine hydrochloride (DA, 98%) and tris (hydroxymethyl) aminomethane (tris, ≥99%) were purchased from Sigma-Aldrich (China). Sodium
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hypochlorite (NaClO), sodium hydroxide (NaOH), bovine serum albumin (BSA, MW=67,000) and ethanol were produced from Kelong Chemical Reagent Factory (Chengdu, China). All the reagents used in this work were of analytical grade and used directly without further purification.
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2.2. Fabrication of modified membranes
The HNTs surface decorated PVDF UF membranes were fabricated by a simple
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one-step method[25], which is schematically presented in Fig.1. HNTs (120 mg), APTES (40 mg) and different amounts of dopamine (from 0 to 40 mg) were dispersed in 20 mL of tris (10 mM) buffer solution at pH=8.5 by ultrasonication. The pH of the
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dispersions was adjusted by NaOH. Every cleaned PVDF UF membrane was completely pre-wetted by ethanol. Then the membrane was immersed in the above dispersions under magnetic stirring for 12 h at room temperature. After that, it was
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rinsed with pure water and then dried in an oven at 60 °C. Finally, the membrane decorated with HNTs nanoparticles was obtained. Table 1 shows the composition of different types of UF membranes prepared in this work.
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2.3. Characterizations
The camera (EOS 750D, Canon) was used to record the appearance photos of UF
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membranes after modification. The transmission electron microscopy (TEM) (Libra 200 FE, ZEISS) was used to observed the morphology and microstructure of HNTs. The surface and cross-sectional morphologies of membranes were obtained by scanning electron microscope (SEM) (JSM-7500F, JEOL). Energy dispersive spectrometer (EDS) (JSM-7500F, JEOL) was used to investigate the element distribution on modified membrane surface. The three-dimensional (3D) images and surface roughness of membranes were obtained by atomic force microscopy (AFM) (SPA300HV, NSK). The hydrophilicity of membrane was evaluated by an instrument 4
of water contact angle (CA) (XED-SPJ, Beijing Hake). The concentration of BSA solution was measured by UV-spectrophotometer (UV-1800, SHIMADZU). 2.4. Membrane performances The permeation and rejection performances play significant roles during membrane application. The permeation property and rejection ratio of as-prepared membranes were tested by a dead-end flow stirred cell (UFSC05001, Millipore) with an effective membrane area of 13.4 cm2. All newly prepared membranes were pre-pressured with pure water at 0.2 MPa over 0.5 h, to make the membrane completely wet in order to obtain stable flux. Then the water flux was recorded under the pressure of 0.1 MPa at room temperature. In subsequent step, BSA solution (1000
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mgL-1) was used to permeate through the above membranes. The water flux (Jw) and rejection ratio (R) of membranes were defined as follows[26, 27]: 𝑉
𝐽𝑤 = 𝐴×∆𝑡
(1)
𝐶𝑝
𝑓
(2)
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𝑅(%) = (1 − 𝐶 ) × 100
where, V represents the volume of permeate water (L), A is the effective permeation
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area (m2), △t is the permeation time (h), and the Cp and Cf are the concentration (mgL-1) of BSA in permeation and feed solution (1000 mgL-1), respectively. As we all know, the fouling resistance of membrane determines the self-life of
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membrane during wastewater treatment. Herein, the static adsorption experiments and dynamic permeation experiments both were carried out to investigate the antifouling characteristics of membrane. The membrane after recording the water flux was
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immersed in 100 mL BSA solution (1000 mgL-1) over 48 h to reach adsorption equilibrium. The adsorption capacity (Q) of membrane was calculated using the
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following equation[28]:
𝑄=
𝑉×(𝐶𝑓 −𝐶𝑎 ) 𝐴
(3)
where, V represents the volume of BSA solution (L), Cf and Ca are the concentration
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(mgL-1) of BSA in feed solution (1000 mgL-1) and after membrane adsorption, respectively.
For dynamic permeation experiments, they included filtration-washing
multicycles[29]. The pure water flux (J1) of membrane was initially recorded at 0.1 MPa after being compacted. Subsequently, the pure water was replaced by BSA solution (1000 mgL-1) to permeate through membrane and BSA flux (JB) was recorded every 10 min. After 50 min filtration, the fouled membrane was taken out and rinsed with NaClO (500 mgL-1) for 30 min. Then the pure water flux was second 5
recorded (J2) at the same pressure. The above steps were further repeated for three times. The flux recovery ratio (FRR) was used to show the antifouling capacity of membrane, which was defined as follows[30]: 𝐽
𝐹𝑅𝑅(%) = (𝐽2 ) × 100
(4)
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3. Results and discussion 3.1 Characterization of HNTs Fig. 2 displays the TEM images of HNTs under different magnifications. Typically, HNTs are presented as hollow tubular nanostructure, which possess smooth
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surface with both ends open[31]. From Fig. 2, it was clearly observed that the lumen, external diameter, and length of HNTs were 10–30 nm, 30–70 nm and hundreds of
nanometers, respectively. These structural properties enable some potential
applications as additives or adsorbents during the process of water treatment[32]. Some reports[33] indicated that, HNTs could be effectively functionalized by loading
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various active chemicals on its inner or outer surfaces, which improved the adsorption capacity of HNTs for the removal of hazardous pollutants, such as dyes, heavy metal ions and phenol. More importantly, some hydrophilic groups exist in HNTs structure
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could promote the permeation of water molecule through the membrane, which increased the separation performances eventually[34].
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3.2 Characterizations of membranes
Fig. 3 shows the appearance of different types of membranes. It was clear that
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the color of membrane top-surface exhibited a drastic change after modification. The pure UF membrane was white, smooth and bright. With the increasing the amounts of dopamine, the membrane gradually became darker, which was attributed the
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self-polymerization of dopamine under an alkaline environment (pH=8.5). According to the reaction mechanism, the amino group in the structure of APTES could act as a bridge and formed the crosslinking between APTES and derivatives of dopamine[25].
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In addition, some irregular substances were adhered on the surface of membrane, resulting in the enhancement of surface roughness. The substances were confirmed to be HNTs from the following characterizations of SEM and EDS mapping. The microstructure and morphologies of membranes were discussed in subsequent section. In order to further investigate the effects of additives (HNTs, dopamine and APTES) on the surface of PVDF UF membrane, SEM and EDS analyses were carried out. Fig. 4 shows the surface SEM images of pure membrane (M0) and modified membrane (M2) under ×10000 magnification. As shown in Fig. 4(a), the pores were 6
uniformly distributed on membrane surface and the pore diameter was dozens of nanometers, which belonged to typical UF membrane. After modification, a large number of HNTs were observed on membrane surface shown in Fig. 4(b). The result was attributed to the following two reasons. Firstly, it has been reported that, dopamine could act as a bio-glue under an alkaline environment, and thus enhance the adhesion of materials on membrane surface[35]. Moreover, the incorporation of APTES would undergo hydrolysis reaction with HNTs. Both of the silicon hydroxyl groups in HNTs and the catechol (derived from dopamine) would bind with HNTs, which made them stably adhered on membrane surface[36]. HNTs was found to be homogeneously dispersed into the membrane and no obvious agglomeration was
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observed. Although, the surface morphology showed a significant change compared to pure membrane, the microstructure was complete and not been destroyed eventually.
Fig. 5 displays the EDS mapping of modified membrane surface, indicating the
presence of O, F, C, N, Si and Al. It is well known that, PVDF is semi-crystalline polymer[37], which has a repeated unit of -(CH2CF2)n-. The commercial membranes
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used in this work were PVDF400, which were pure PVDF membranes without any additives. The presence of F, C in EDS spectra was attributed from the virgin PVDF
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structure. After incorporation of HNTs, APTES and dopamine; the signature of uniformly distributed new elements, such as O, N, Si and Al were obtained on the surface of M2. The above result further confirmed the successful modification of the
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membrane.
The cross-sectional SEM images of membrane before and after surface modification are shown in Fig. 6. Both the samples were obtained after being
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glaciated and fractured in liquid N2. As is vividly depicted in the images, pure UF membrane was composed of two different parts. The top surface was PVDF layer, which prepared by phase-inversion method. It determined the separation
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performances of membrane directly. The bottom surface was support layer made up of non-woven fabrics, which improved the mechanical property of membrane. The
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overall thicknesses of M0 and M2 were ~200 μm. However, there was a slight difference in terms of the thickness of PVDF skin layer. Under higher magnification of SEM images, it could be clearly seen that, the skin layer of M2 was thicker than that of M0 (marked with red line) after surface modification, which were 2.73 μm and 3.64 μm, respectively. It was generally acknowledged that, thicker separation layer could prolong the filtration path and thus contributed to enhancing the rejection ability of membrane[38, 39]. Fig. 7 presents the 2D and 3D AFM images of M0 and M2, respectively. The tests were manipulated in a tapping-mode with a scan area of 2 μm×2 μm. It could be 7
observed that plenty of pores were existed on membrane surface, and thus caused the black valleys on AFM images. However, the black valleys and bright peaks increased evidently after surface modification, which was due to the adhesion of HNTs. The average roughness (Ra) and root-mean-square roughness (Rq) of M0 were found to be 9.5±0.1 and 13.0±0.4 nm, whereas the Ra and Rq of M2 were evaluated as 22.5±3.5 and 29.5±5.0 nm, respectively. The result indicated that the surface roughness of membrane was enhanced dramatically after surface modification. Although some pores were blocked, which increased the penetration resistances of water, the introduction of hydrophilic HNTs contributed to roughening of the membrane surface resulting enhancement in the effective filtration area. The above result can lead to the
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improvement in water flux of membrane. On the contrary, pollutants are easy to embed into the black valleys of membrane surface at a higher surface roughness, and
thus would increase the irreversible fouling. Hence, it would cause the decrease of antifouling characteristic of the membrane as discussed in the subsequent section.
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3.3 The hydrophilicity of membranes
The hydrophilicity of membrane surface plays a significant role in membrane performance. The hydrophilicity of membrane surface was reflected by contact angles
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(CA), which were recorded in a dynamic mode. 5 μL water droplet was dripped on membrane surface and the data were obtained after the membrane being wetted in
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three different areas. As shown in Fig. 8, the pure PVDF membrane had a CA of 71.6±1.8°at around 30 s, due to the natural hydrophobicity of PVDF polymer. Compared to M5, we could draw the conclusion that, the improvement of membrane
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hydrophilicity was mainly due to the incorporation of hydrophilic groups on HNTs surface. The CA decreased drastically with increasing the content of dopamine. Hence, it was related to the synergism of dopamine and HNTs. It has been reported[40, 41]
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that, HNTs is a typical hydrophilic material with many hydrophilic groups (-OH) on its inner and outer surface. The higher the dopamine concentration (from 0 to 10 mg), the more HNTs were adhered on the membrane surface. But the amount of HNTs
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became saturated at a dopamine concentration of 10 mg. Moreover, the self-polymerization of dopamine (concentration from 20 to 40 mg) was less hydrophilic than HNTs, and they also replaced and covered part of the hydrophilic layer formed by HNTs. In particular, M2 presented the best hydrophilicity and the corresponding value of CA was as low as 38.2±0.8°. The result clearly indicated that modified membrane exhibited the remarkable hydrophilicity at a suitable content of dopamine and HNTs. 3.4 The water flux of membranes 8
The pure water flux of different membranes was tested under 0.1 MPa at room temperature. As is vividly depicted in Fig. 9, the variation in water flux was in accordance with the above hydrophilicity data in Fig. 8. The water flux of pristine PVDF membrane was 171.6±2.7 Lm-2h-1, which was similar to the value reported in literature. With the addition of HNTs and dopamine, the water flux of membrane was gradually improved. In particular, the water flux of M2 reached a peak value of 291.9±6.2 Lm-2h-1, which increased by 70.1% compared to M0. However, further addition of dopamine led to an afterwards decline in water flux. It was attributed to the enhancement of separation layer thickness with the addition of dopamine concentration, and it also caused the higher resistances of water penetration. Moreover,
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the flux was lower than M0, when there was no HNTs on membrane surface (M5). The incorporation of HNTs and dopamine can improve the hydrophilicity of
membrane, which accelerated the permeation of water molecules across it[42]. The modification can also block the pores on membrane surface, enhancing the permeation resistances and decreased water flux. The optimized amount of HNTs and
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dopamine can be very effective for water permeation property as well as antifouling
3.5 The BSA rejection of membranes
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characteristics.
The rejection capacity of membrane was investigated by the rejection of BSA.
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The BSA flux and BSA rejection ratio of different membranes are depicted in Fig.10 (a) and Fig.10 (b), respectively. As could be seen in Fig.10 (a), the trend in BSA flux was similar to pure water flux shown in Fig. 9. M2 was superior to other membranes
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and displayed the greatest BSA flux value of 235.8±3.6 Lm-2h-1. The enhancement in permeation resistances caused by BSA molecules leads to reduction in BSA flux compared to water flux. More importantly, the BSA rejection ratio of membrane
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increased obviously after modification. The molecular weight of BSA used in this work was 67000 Da. The dopamine and adhered HNTs increased the thickness of membrane surface as shown in cross-sectional SEM images, which was equivalent to
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the formation of a new separation layer coated on membrane surface. Herein, the BSA rejection ratios of all modified membranes were found to be more than 90%, whereas for the pure PVDF membrane, it was ~69.3±1.2%. The results clearly indicated that the one-step method for membrane modification can simultaneously increase the flux and rejection ratio of membrane, which is very unique in nature. 3.6 The antifouling property of membranes In order to investigate the anti-fouling property of membranes, static adsorption 9
experiments were carried out. The results are shown in Fig.11 (a). M0 and M2 were immersed in 1000 mgL-1 of BSA solution for 48 h. The adsorption capacity of M0 was found to be higher than M2, which were 25.4±2.4μgcm-2 and 17.7±1.1μgcm-2, respectively. It was demonstrated that, there were less BSA adsorbed on the modified membrane surface. The results of dynamic permeation experiments are presented in Fig. 11(b). With the filtration of BSA solution, an increasing number of BSA molecules accumulated on the membrane surface and even embedded in the membrane pores. It drastically increased permeation resistances and caused the decrease in flux. However, after being washed by 500 mgL-1 of NaClO solution for 30 min, both the membranes
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showed excellent recovery of flux. Consecutive three fouling-cleaning dynamic tests were performed. The flux recovery ratio (FRR) is displayed in Table 2. The result
further demonstrated that modified membrane exhibited better antifouling capacity.
After three cycles, the FRR of M2 was as high as 80.0%, which increased 5.3% compared to pure PVDF membrane.
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In general, the overall properties of membrane were improved after dopamine
and HNTs modification. Fig. 12 shows the mechanism of separation and antifouling
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capacity of modified membrane. With the addition of dopamine and the crosslink APTES, more and more HNTs were adhered on the membrane surface, which formed a new hydrophilic separation layer. The improvement in hydrophilicity brought a
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higher water flux and BSA flux. Meanwhile, the new separation layer prevented the filtration of BSA molecules and caused the enhancement in BSA rejection. Furthermore, although the surface of membrane became rougher, the membrane
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surface was more hydrophilic. Hence, there was a water layer between BSA and membrane surface during the filtration of BSA solution. It prevented the adsorption and accumulation of BSA and increased the antifouling capacity eventually.
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4. Conclusions
In this work, a simple one-step approach was used to decorate PVDF commercial
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UF membrane based on the self-polymerization of dopamine, crosslink of APTES and adhesion of HNTs. The contents of dopamine played a key role in the performances of membrane including hydrophilicity, rejection ratio and antifouling characteristics. The uniform distribution of HNTs on membrane surface were confirmed by SEM and EDS mapping. The incorporation of additives was found to increase the surface roughness of as shown in AFM images. The results revealed that, the modified membrane exhibited better hydrophilicity as evident from the CA mesurement and thus caused the improvement in water flux. At optimized condition (10mg dopamine, 120mg 10
HNTs and 40mg APTES), M2 possessed a pure water flux of 291.9 Lm-2h-1 and BSA rejection ratio of 92.0%, which were both higher than the virgin membrane. In addition, M2 presented superior antifouling behavior with an FRR of 80.0% after three consecutive cycles. Hence, it might provide a new method to break the tradeoff between flux and rejection ratio, which was one of the major problems for membrane separation. The modified membrane can be employed as a potential candidate for practical application in the field of wastewater treatment.
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Acknowledgments
Research
Start-up
Fund
of
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Financial support of this work is acknowledged to the Teacher Development Chengdu
University
of
Technology
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“Ceshigo” (www.ceshigo.com).
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(10912-2019KYQD-07276). We are grateful for the characterizations provided by
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[27] Y. Zhan, X. Wan, S. He, et al., Design of durable and efficient poly (arylene ether nitrile)/bioinspired polydopamine coated graphene oxide nanofibrous composite membrane for anionic dyes separation, Chem. Eng. J. 333 (2018) 132-145.
[28] R. S. Vieira, M. M. Beppu, Dynamic and static adsorption and desorption of Hg (II) ions on chitosan membranes and spheres, Water Res. 40 (2006) 1726-1734.
[29] X. Yi, S. Yu, W. Shi, et al., The influence of important factors on ultrafiltration of oil/water emulsion using PVDF membrane modified by nano-sized TiO2/Al2O3, Desalination 281 (2011) 13
179-184. [30] V V. Vatanpour, S. S. Madaeni, R. Moradian., Novel antibifouling nanofiltration polyethersulfone membrane fabricated from embedding TiO 2 coated multiwalled carbon nanotubes,
Sep. Purif. Technol. 90 (2012) 69-82. [31] R. S. Hebbar, A. M. Isloor, K. Ananda, et al., Fabrication of polydopamine functionalized halloysite nanotube/polyetherimide membranes for heavy metal removal , J. Mater. Chem. A 4 (2016)
764-774. [32] L. Yu, H. Wang, Y. Zhang, et al., Recent advances in halloysite nanotube derived composites for water treatment, Environ. Sci.: Nano 3 (2016) 28-44.
[33] X. Wan, Y. Zhan, Z. Long, et al., Core@ double-shell structured magnetic halloysite nanotube
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nano-hybrid as efficient recyclable adsorbent for methylene blue removal , Chem. Eng. J. 330 (2017)
491-504.
[34] G. Zeng, Y. He, Y. Zhan, et al., Novel polyvinylidene fluoride nanofiltration membrane blended
with functionalized halloysite nanotubes for dye and heavy metal ions removal, J. Hazard. Mater. 317
(2016) 60-72.
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[35] W. O. Yah, H. Xu, H. Soejima, W. Ma, Y. Lvov, et al., Biomimetic dopamine derivative for
selective polymer modification of halloysite nanotube lumen, J. Am. Chem. Soc. 134 (2012)
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[36] Y. Zhan, S. He, X. Wan, et al., Thermally and chemically stable poly (arylene ether nitrile)/halloysite nanotubes intercalated graphene oxide nanofibrous composite membranes for highly
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efficient oil/water emulsion separation in harsh environment , J. Membr. Sci. 567 (2018) 76-88.
[37] T. Otitoju, A. Ahmad, B. Ooi, Polyvinylidene fluoride (PVDF) membrane for oil rejection from oily wastewater: A performance review, J. Water. Process. Eng. 14 (2016) 41-59.
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[38] L.Yu, Y. Zhang, Y. Wang,et al., High flux, positively charged loose nanofiltration membrane by blending with poly (ionic liquid) brushes grafted silica spheres , J. Hazard. Mater. 287 (2015)
373-383.
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[39] L. Jiang, Y. Huang, T. Liu, Enhanced visible-light photocatalytic performance of electrospun carbon-doped TiO2/halloysite nanotube hybrid nanofibers, J. Colloid. Interf. Sci 439 (2015) 62-68.
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[40] Y. Zhang, Y. Shen, J. Hou, et al., Ultraselective pebax membranes enabled by templated microphase separation, ACS Appl. Mater. Interfaces 10 (2018) 20006-20013.
[41] S. Hashemifard, A. Ismail, T. Matsuura, Mixed matrix membrane incorporated with large pore size halloysite nanotubes (HNT) as filler for gas separation: experimental, J. Colloid. Interf. Sci 359 (2011) 359-370. [42] G. Zeng, Z. Ye, Y. He, et al., Application of dopamine-modified halloysite nanotubes/PVDF blend membranes for direct dyes removal from wastewater, Chem. Eng. J. 323 (2017) 572-583.
[43] S. Muchtar, M. Wahab, L. Fang, et al., Polydopamine-coated poly(vinylidene fluoride) membranes with high ultraviolet resistance and antifouling properties for a photocatalytic 14
membrane reactor, J. Appl. Polym. Sci (2018) DOI: 10.1002/APP.47312. [44] Y.-H. Chiao, S.-T. Chen, T. Patra, et al., Zwitterionic forward osmosis membrane modified by fast second interfacial polymerization with enhanced antifouling and antimicrobial properties for
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produced water pretreatment, Desalination 469 (2019) 114090.
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Figure captions: Fig.1. Schematic illustration of the fabrication of HNTs decorated PVDF UF membrane. Fig.2. TEM images of HNTs under different magnifications. Fig.3. Appearances photos of different modified UF membranes.
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Fig.4. Surface SEM images of M0 (a) and M2 (b). Fig.5. Surface EDS mapping of M2.
Fig.6. Cross-sectional SEM images of M0 (a, b) and M2 (c, d).
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Fig.7. 3D AFM images of M0 and M2.
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Fig.8. Contact angle data and images of different membranes.
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Fig.9. Water flux variation of different membranes.
Fig.10. BSA flux and rejection ratio of different membranes.
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Fig.11. Antifouling performances of M0 and M2: (a) static adsorption experiments and (b) dynamic permeation experiments
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Fig.12. Mechanism of separation and antifouling capacity of modified membrane.
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Fig.8. 100
80 70 60 50 40 30 20 10 M0
M1
M2
M3
M4
M5
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0
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Contact Angle ()
90
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Table 1. The composition of different types of modified PVDF UF membranes. of
dopamine
HNTs
APTES
membrane
(mg)
(mg)
(mg)
M0
0
0
0
M1
5
120
40
M2
10
120
40
M3
20
120
40
M4
40
120
40
M5
40
0
40
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Type
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Table 2．FRR data of membranes in fouling and washing cycle. of
1st cycle
2nd cycle
3rd cycle
membrane
FRR (%)
FRR (%)
FRR (%)
M0
87.5
81.4
74.7
M2
90.8
85.6
80.0
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Type
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PII:
S0927-7757(19)31134-3
DOI:
https://doi.org/10.1016/j.colsurfa.2019.124142
Reference:
COLSUA 124142
To appear in:
Colloids and Surfaces A: Physicochemical and Engineering Aspects
Received Date:
22 August 2019
Revised Date:
15 October 2019
Accepted Date:
18 October 2019
Please cite this article as: Zeng G, Wei K, Yang D, Yan J, Zhou K, Patra T, Sengupta A, Chiao Y-Hsuan, Improvement in performance of PVDF ultrafiltration membranes by co-incorporation of dopamine and halloysite nanotubes, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124142
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier.
Improvement in performance of PVDF ultrafiltration membranes by co-incorporation of dopamine and halloysite nanotubes Guangyong Zenga* Ke Weia Kun Zhoua* Tanmoy Patrab
Denglei Yanga Arijit Senguptac
Jun Yana Yu-Hsuan Chiaod
(a. College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu, Sichuan 610059, P R of China; b. Department of Biomedical Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States;
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c. Radiochemsitry Division, Bhabha Atomic Research Center, Mumbai 400094, India; d. R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan University, Chung Li 32023, Taiwan) *Address correspondence to this author. Email: [email protected];
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[email protected]. Phone and Fax: +8602884078939
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Graphical Abstract
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Abstract A drastic improvement in the overall performance including permeability and antifouling characteristics were observed for polyvinylidene fluoride (PVDF) ultrafiltration (UF) membrane by incorporation of dopamine, halloysite nanotubes (HNTs) and 3-aminopropyltriethoxysilane (APTES) in one pot synthetic scheme. SEM and EDS mapping were used to signature the homogenous distribution of HNTs on membrane surface, whereas surface roughness of membrane was evaluated by AFM. The modified membrane surface was found to be highly hydrophilic as suggested from the water contact angle value (38.2º), while the water flux across the
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membrane was found to be 291.9 Lm-2h-1. The bovine serum albumin (BSA) separation tests indicated that, the modification broke the tradeoff between flux and
rejection ratio of membrane, which reached as high as 235.8 Lm-2h-1 and 92.0%, respectively. Moreover, a significant improvement in antifouling characteristics were
evidenced from static adsorption and dynamic permeation experiments. This study
Keywords:
Ultrafiltration
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practical applications for wastewater treatment. membrane;
Dopamine;
Halloysite
nanotubes;
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Hydrophilicity; Antifouling capacity
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provides a facile and useful guideline to design novel UF membrane and expand its
1.Introduction
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In the past few decades, ultrafiltration (UF) technology has achieved a continuous development, especially in the water treatment and wastewater reclamation[1]. UF is a low-pressure membrane filtration process compared to
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nanofiltration and reverse osmosis membrane, and the pore diameters of UF membrane ranges from several to dozens of nanometers[2]. The contaminants such as suspended solid, organic matters, sorts of oil, turbidity and even virus can be removed
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effectively by UF membrane with an easy automation and relatively low cost[3, 4]. However, the above contaminants are prone to adsorb or deposit onto membrane surface, which can lead to the concentration polarization and subsequent pore blocking[5]. The fouling can result in the increased energy demand, reduced separation efficiency and shorter service life of membrane. Therefore, improvement in antifouling characteristics of the membrane is very important and continuous approach of membrane research. Hydrophilic modification has become an attractive approach for the 2
improvement in membrane fouling resistance due to reduction in nonpolar interaction of organic foulants with the membrane surface[6-8]. It has been demonstrated that, hydrophilic modification leads to the formation of an aqueous layer on membrane surface, thus preventing the accumulation of contaminants. Some physical interactions, i.e. surface coating[9], deposition and chemical interaction, i.e. grafting polymerization[10, 11] have been exploited for modification of membrane surface. However, the surface structure of membrane might be destroyed and the modification effects would gradually reduce or even disappear after several cycles of membrane operation. The effect of additives like hydrophilic polymers and inorganic nanoparticles during membrane fabrication have also been investigated for the
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improvement in the antifouling properties of the membrane[12-14]. These additives are usually blended with raw membrane material into solvent to form casting solution,
and the composite membranes are fabricated via phase inversion method[15]. Although, it can improve the overall performances of membranes, additives are easily prone to aggregate, which has been the major drawbacks for blending
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modification[11].
Recently, the unique property of mussel-inspired chemistry (such as dopamine)
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has attracted interests of many researchers, which become a powerful tool for membrane fabrication. It is not only used as a modifier depositing on membrane surface directly, but also acts as an intermediate layer to further introduce other
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materials to construct composite membrane[16-18]. Zhao and his co-workers reported a dopamine modified polyethersulfone UF membrane. Their results indicated that, the addition of dopamine improved the blood compatibility of membrane. However,
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dopamine also blocked the pores on membrane surface resulting in a significant decrease in water flux. In our previous work[19], the superhydrophilic polyvinylidene fluoride (PVDF) microfiltration membrane was prepared by anchoring functionalized
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multi-wall carbon nanotubes on membrane surface by dopamine, which exhibited excellent performances with ultrahigh flux and oil rejection ratio. Similarly, silver[20],
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titanium dioxide[21] and others nanoparticles were also adhered on membrane surface by self-polymerization of dopamine to achieve high-performance membranes [22-24]. It is generally accepted that, breaking the tradeoff between flux and rejection of
membrane is highly challenging. In view of that, a novel modification method for UF membranes has been reported in the present investigation. Natural hydrophilic material halloysite nanotubes were adhered on membranes surface by dopamine to form a new separation layer, which realized the improvement in hydrophilicity, water flux and bovine serum albumin (BSA) rejection of membranes. Meanwhile, 3-aminopropyltriethoxysilane (APTES) was also used as crosslinker to enhance the 3
interface interaction and the dispersibility of HNTs on membrane surface. Additionally, the antifouling characteristics of modified membranes were investigated in detail. 2. Experimental 2.1. Materials The commercial UF membranes (PVDF400, 100kDa) used in this work were provided by Sepro Membranes, Inc. (Carlsbad, USA). Halloysite nanotubes (HNTs, purity≥95%), dopamine hydrochloride (DA, 98%) and tris (hydroxymethyl) aminomethane (tris, ≥99%) were purchased from Sigma-Aldrich (China). Sodium
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hypochlorite (NaClO), sodium hydroxide (NaOH), bovine serum albumin (BSA, MW=67,000) and ethanol were produced from Kelong Chemical Reagent Factory (Chengdu, China). All the reagents used in this work were of analytical grade and used directly without further purification.
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2.2. Fabrication of modified membranes
The HNTs surface decorated PVDF UF membranes were fabricated by a simple
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one-step method[25], which is schematically presented in Fig.1. HNTs (120 mg), APTES (40 mg) and different amounts of dopamine (from 0 to 40 mg) were dispersed in 20 mL of tris (10 mM) buffer solution at pH=8.5 by ultrasonication. The pH of the
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dispersions was adjusted by NaOH. Every cleaned PVDF UF membrane was completely pre-wetted by ethanol. Then the membrane was immersed in the above dispersions under magnetic stirring for 12 h at room temperature. After that, it was
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rinsed with pure water and then dried in an oven at 60 °C. Finally, the membrane decorated with HNTs nanoparticles was obtained. Table 1 shows the composition of different types of UF membranes prepared in this work.
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2.3. Characterizations
The camera (EOS 750D, Canon) was used to record the appearance photos of UF
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membranes after modification. The transmission electron microscopy (TEM) (Libra 200 FE, ZEISS) was used to observed the morphology and microstructure of HNTs. The surface and cross-sectional morphologies of membranes were obtained by scanning electron microscope (SEM) (JSM-7500F, JEOL). Energy dispersive spectrometer (EDS) (JSM-7500F, JEOL) was used to investigate the element distribution on modified membrane surface. The three-dimensional (3D) images and surface roughness of membranes were obtained by atomic force microscopy (AFM) (SPA300HV, NSK). The hydrophilicity of membrane was evaluated by an instrument 4
of water contact angle (CA) (XED-SPJ, Beijing Hake). The concentration of BSA solution was measured by UV-spectrophotometer (UV-1800, SHIMADZU). 2.4. Membrane performances The permeation and rejection performances play significant roles during membrane application. The permeation property and rejection ratio of as-prepared membranes were tested by a dead-end flow stirred cell (UFSC05001, Millipore) with an effective membrane area of 13.4 cm2. All newly prepared membranes were pre-pressured with pure water at 0.2 MPa over 0.5 h, to make the membrane completely wet in order to obtain stable flux. Then the water flux was recorded under the pressure of 0.1 MPa at room temperature. In subsequent step, BSA solution (1000
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mgL-1) was used to permeate through the above membranes. The water flux (Jw) and rejection ratio (R) of membranes were defined as follows[26, 27]: 𝑉
𝐽𝑤 = 𝐴×∆𝑡
(1)
𝐶𝑝
𝑓
(2)
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𝑅(%) = (1 − 𝐶 ) × 100
where, V represents the volume of permeate water (L), A is the effective permeation
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area (m2), △t is the permeation time (h), and the Cp and Cf are the concentration (mgL-1) of BSA in permeation and feed solution (1000 mgL-1), respectively. As we all know, the fouling resistance of membrane determines the self-life of
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membrane during wastewater treatment. Herein, the static adsorption experiments and dynamic permeation experiments both were carried out to investigate the antifouling characteristics of membrane. The membrane after recording the water flux was
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immersed in 100 mL BSA solution (1000 mgL-1) over 48 h to reach adsorption equilibrium. The adsorption capacity (Q) of membrane was calculated using the
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following equation[28]:
𝑄=
𝑉×(𝐶𝑓 −𝐶𝑎 ) 𝐴
(3)
where, V represents the volume of BSA solution (L), Cf and Ca are the concentration
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(mgL-1) of BSA in feed solution (1000 mgL-1) and after membrane adsorption, respectively.
For dynamic permeation experiments, they included filtration-washing
multicycles[29]. The pure water flux (J1) of membrane was initially recorded at 0.1 MPa after being compacted. Subsequently, the pure water was replaced by BSA solution (1000 mgL-1) to permeate through membrane and BSA flux (JB) was recorded every 10 min. After 50 min filtration, the fouled membrane was taken out and rinsed with NaClO (500 mgL-1) for 30 min. Then the pure water flux was second 5
recorded (J2) at the same pressure. The above steps were further repeated for three times. The flux recovery ratio (FRR) was used to show the antifouling capacity of membrane, which was defined as follows[30]: 𝐽
𝐹𝑅𝑅(%) = (𝐽2 ) × 100
(4)
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3. Results and discussion 3.1 Characterization of HNTs Fig. 2 displays the TEM images of HNTs under different magnifications. Typically, HNTs are presented as hollow tubular nanostructure, which possess smooth
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surface with both ends open[31]. From Fig. 2, it was clearly observed that the lumen, external diameter, and length of HNTs were 10–30 nm, 30–70 nm and hundreds of
nanometers, respectively. These structural properties enable some potential
applications as additives or adsorbents during the process of water treatment[32]. Some reports[33] indicated that, HNTs could be effectively functionalized by loading
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various active chemicals on its inner or outer surfaces, which improved the adsorption capacity of HNTs for the removal of hazardous pollutants, such as dyes, heavy metal ions and phenol. More importantly, some hydrophilic groups exist in HNTs structure
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could promote the permeation of water molecule through the membrane, which increased the separation performances eventually[34].
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3.2 Characterizations of membranes
Fig. 3 shows the appearance of different types of membranes. It was clear that
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the color of membrane top-surface exhibited a drastic change after modification. The pure UF membrane was white, smooth and bright. With the increasing the amounts of dopamine, the membrane gradually became darker, which was attributed the
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self-polymerization of dopamine under an alkaline environment (pH=8.5). According to the reaction mechanism, the amino group in the structure of APTES could act as a bridge and formed the crosslinking between APTES and derivatives of dopamine[25].
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In addition, some irregular substances were adhered on the surface of membrane, resulting in the enhancement of surface roughness. The substances were confirmed to be HNTs from the following characterizations of SEM and EDS mapping. The microstructure and morphologies of membranes were discussed in subsequent section. In order to further investigate the effects of additives (HNTs, dopamine and APTES) on the surface of PVDF UF membrane, SEM and EDS analyses were carried out. Fig. 4 shows the surface SEM images of pure membrane (M0) and modified membrane (M2) under ×10000 magnification. As shown in Fig. 4(a), the pores were 6
uniformly distributed on membrane surface and the pore diameter was dozens of nanometers, which belonged to typical UF membrane. After modification, a large number of HNTs were observed on membrane surface shown in Fig. 4(b). The result was attributed to the following two reasons. Firstly, it has been reported that, dopamine could act as a bio-glue under an alkaline environment, and thus enhance the adhesion of materials on membrane surface[35]. Moreover, the incorporation of APTES would undergo hydrolysis reaction with HNTs. Both of the silicon hydroxyl groups in HNTs and the catechol (derived from dopamine) would bind with HNTs, which made them stably adhered on membrane surface[36]. HNTs was found to be homogeneously dispersed into the membrane and no obvious agglomeration was
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observed. Although, the surface morphology showed a significant change compared to pure membrane, the microstructure was complete and not been destroyed eventually.
Fig. 5 displays the EDS mapping of modified membrane surface, indicating the
presence of O, F, C, N, Si and Al. It is well known that, PVDF is semi-crystalline polymer[37], which has a repeated unit of -(CH2CF2)n-. The commercial membranes
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used in this work were PVDF400, which were pure PVDF membranes without any additives. The presence of F, C in EDS spectra was attributed from the virgin PVDF
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structure. After incorporation of HNTs, APTES and dopamine; the signature of uniformly distributed new elements, such as O, N, Si and Al were obtained on the surface of M2. The above result further confirmed the successful modification of the
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membrane.
The cross-sectional SEM images of membrane before and after surface modification are shown in Fig. 6. Both the samples were obtained after being
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glaciated and fractured in liquid N2. As is vividly depicted in the images, pure UF membrane was composed of two different parts. The top surface was PVDF layer, which prepared by phase-inversion method. It determined the separation
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performances of membrane directly. The bottom surface was support layer made up of non-woven fabrics, which improved the mechanical property of membrane. The
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overall thicknesses of M0 and M2 were ~200 μm. However, there was a slight difference in terms of the thickness of PVDF skin layer. Under higher magnification of SEM images, it could be clearly seen that, the skin layer of M2 was thicker than that of M0 (marked with red line) after surface modification, which were 2.73 μm and 3.64 μm, respectively. It was generally acknowledged that, thicker separation layer could prolong the filtration path and thus contributed to enhancing the rejection ability of membrane[38, 39]. Fig. 7 presents the 2D and 3D AFM images of M0 and M2, respectively. The tests were manipulated in a tapping-mode with a scan area of 2 μm×2 μm. It could be 7
observed that plenty of pores were existed on membrane surface, and thus caused the black valleys on AFM images. However, the black valleys and bright peaks increased evidently after surface modification, which was due to the adhesion of HNTs. The average roughness (Ra) and root-mean-square roughness (Rq) of M0 were found to be 9.5±0.1 and 13.0±0.4 nm, whereas the Ra and Rq of M2 were evaluated as 22.5±3.5 and 29.5±5.0 nm, respectively. The result indicated that the surface roughness of membrane was enhanced dramatically after surface modification. Although some pores were blocked, which increased the penetration resistances of water, the introduction of hydrophilic HNTs contributed to roughening of the membrane surface resulting enhancement in the effective filtration area. The above result can lead to the
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improvement in water flux of membrane. On the contrary, pollutants are easy to embed into the black valleys of membrane surface at a higher surface roughness, and
thus would increase the irreversible fouling. Hence, it would cause the decrease of antifouling characteristic of the membrane as discussed in the subsequent section.
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3.3 The hydrophilicity of membranes
The hydrophilicity of membrane surface plays a significant role in membrane performance. The hydrophilicity of membrane surface was reflected by contact angles
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(CA), which were recorded in a dynamic mode. 5 μL water droplet was dripped on membrane surface and the data were obtained after the membrane being wetted in
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three different areas. As shown in Fig. 8, the pure PVDF membrane had a CA of 71.6±1.8°at around 30 s, due to the natural hydrophobicity of PVDF polymer. Compared to M5, we could draw the conclusion that, the improvement of membrane
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hydrophilicity was mainly due to the incorporation of hydrophilic groups on HNTs surface. The CA decreased drastically with increasing the content of dopamine. Hence, it was related to the synergism of dopamine and HNTs. It has been reported[40, 41]
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that, HNTs is a typical hydrophilic material with many hydrophilic groups (-OH) on its inner and outer surface. The higher the dopamine concentration (from 0 to 10 mg), the more HNTs were adhered on the membrane surface. But the amount of HNTs
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became saturated at a dopamine concentration of 10 mg. Moreover, the self-polymerization of dopamine (concentration from 20 to 40 mg) was less hydrophilic than HNTs, and they also replaced and covered part of the hydrophilic layer formed by HNTs. In particular, M2 presented the best hydrophilicity and the corresponding value of CA was as low as 38.2±0.8°. The result clearly indicated that modified membrane exhibited the remarkable hydrophilicity at a suitable content of dopamine and HNTs. 3.4 The water flux of membranes 8
The pure water flux of different membranes was tested under 0.1 MPa at room temperature. As is vividly depicted in Fig. 9, the variation in water flux was in accordance with the above hydrophilicity data in Fig. 8. The water flux of pristine PVDF membrane was 171.6±2.7 Lm-2h-1, which was similar to the value reported in literature. With the addition of HNTs and dopamine, the water flux of membrane was gradually improved. In particular, the water flux of M2 reached a peak value of 291.9±6.2 Lm-2h-1, which increased by 70.1% compared to M0. However, further addition of dopamine led to an afterwards decline in water flux. It was attributed to the enhancement of separation layer thickness with the addition of dopamine concentration, and it also caused the higher resistances of water penetration. Moreover,
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the flux was lower than M0, when there was no HNTs on membrane surface (M5). The incorporation of HNTs and dopamine can improve the hydrophilicity of
membrane, which accelerated the permeation of water molecules across it[42]. The modification can also block the pores on membrane surface, enhancing the permeation resistances and decreased water flux. The optimized amount of HNTs and
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dopamine can be very effective for water permeation property as well as antifouling
3.5 The BSA rejection of membranes
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characteristics.
The rejection capacity of membrane was investigated by the rejection of BSA.
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The BSA flux and BSA rejection ratio of different membranes are depicted in Fig.10 (a) and Fig.10 (b), respectively. As could be seen in Fig.10 (a), the trend in BSA flux was similar to pure water flux shown in Fig. 9. M2 was superior to other membranes
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and displayed the greatest BSA flux value of 235.8±3.6 Lm-2h-1. The enhancement in permeation resistances caused by BSA molecules leads to reduction in BSA flux compared to water flux. More importantly, the BSA rejection ratio of membrane
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increased obviously after modification. The molecular weight of BSA used in this work was 67000 Da. The dopamine and adhered HNTs increased the thickness of membrane surface as shown in cross-sectional SEM images, which was equivalent to
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the formation of a new separation layer coated on membrane surface. Herein, the BSA rejection ratios of all modified membranes were found to be more than 90%, whereas for the pure PVDF membrane, it was ~69.3±1.2%. The results clearly indicated that the one-step method for membrane modification can simultaneously increase the flux and rejection ratio of membrane, which is very unique in nature. 3.6 The antifouling property of membranes In order to investigate the anti-fouling property of membranes, static adsorption 9
experiments were carried out. The results are shown in Fig.11 (a). M0 and M2 were immersed in 1000 mgL-1 of BSA solution for 48 h. The adsorption capacity of M0 was found to be higher than M2, which were 25.4±2.4μgcm-2 and 17.7±1.1μgcm-2, respectively. It was demonstrated that, there were less BSA adsorbed on the modified membrane surface. The results of dynamic permeation experiments are presented in Fig. 11(b). With the filtration of BSA solution, an increasing number of BSA molecules accumulated on the membrane surface and even embedded in the membrane pores. It drastically increased permeation resistances and caused the decrease in flux. However, after being washed by 500 mgL-1 of NaClO solution for 30 min, both the membranes
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showed excellent recovery of flux. Consecutive three fouling-cleaning dynamic tests were performed. The flux recovery ratio (FRR) is displayed in Table 2. The result
further demonstrated that modified membrane exhibited better antifouling capacity.
After three cycles, the FRR of M2 was as high as 80.0%, which increased 5.3% compared to pure PVDF membrane.
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In general, the overall properties of membrane were improved after dopamine
and HNTs modification. Fig. 12 shows the mechanism of separation and antifouling
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capacity of modified membrane. With the addition of dopamine and the crosslink APTES, more and more HNTs were adhered on the membrane surface, which formed a new hydrophilic separation layer. The improvement in hydrophilicity brought a
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higher water flux and BSA flux. Meanwhile, the new separation layer prevented the filtration of BSA molecules and caused the enhancement in BSA rejection. Furthermore, although the surface of membrane became rougher, the membrane
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surface was more hydrophilic. Hence, there was a water layer between BSA and membrane surface during the filtration of BSA solution. It prevented the adsorption and accumulation of BSA and increased the antifouling capacity eventually.
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4. Conclusions
In this work, a simple one-step approach was used to decorate PVDF commercial
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UF membrane based on the self-polymerization of dopamine, crosslink of APTES and adhesion of HNTs. The contents of dopamine played a key role in the performances of membrane including hydrophilicity, rejection ratio and antifouling characteristics. The uniform distribution of HNTs on membrane surface were confirmed by SEM and EDS mapping. The incorporation of additives was found to increase the surface roughness of as shown in AFM images. The results revealed that, the modified membrane exhibited better hydrophilicity as evident from the CA mesurement and thus caused the improvement in water flux. At optimized condition (10mg dopamine, 120mg 10
HNTs and 40mg APTES), M2 possessed a pure water flux of 291.9 Lm-2h-1 and BSA rejection ratio of 92.0%, which were both higher than the virgin membrane. In addition, M2 presented superior antifouling behavior with an FRR of 80.0% after three consecutive cycles. Hence, it might provide a new method to break the tradeoff between flux and rejection ratio, which was one of the major problems for membrane separation. The modified membrane can be employed as a potential candidate for practical application in the field of wastewater treatment.
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Acknowledgments
Research
Start-up
Fund
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Financial support of this work is acknowledged to the Teacher Development Chengdu
University
of
Technology
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“Ceshigo” (www.ceshigo.com).
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(10912-2019KYQD-07276). We are grateful for the characterizations provided by
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Figure captions: Fig.1. Schematic illustration of the fabrication of HNTs decorated PVDF UF membrane. Fig.2. TEM images of HNTs under different magnifications. Fig.3. Appearances photos of different modified UF membranes.
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Fig.4. Surface SEM images of M0 (a) and M2 (b). Fig.5. Surface EDS mapping of M2.
Fig.6. Cross-sectional SEM images of M0 (a, b) and M2 (c, d).
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Fig.7. 3D AFM images of M0 and M2.
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Fig.8. Contact angle data and images of different membranes.
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Fig.9. Water flux variation of different membranes.
Fig.10. BSA flux and rejection ratio of different membranes.
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Fig.11. Antifouling performances of M0 and M2: (a) static adsorption experiments and (b) dynamic permeation experiments
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Fig.12. Mechanism of separation and antifouling capacity of modified membrane.
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Fig.8. 100
80 70 60 50 40 30 20 10 M0
M1
M2
M3
M4
M5
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Contact Angle ()
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Table 1. The composition of different types of modified PVDF UF membranes. of
dopamine
HNTs
APTES
membrane
(mg)
(mg)
(mg)
M0
0
0
0
M1
5
120
40
M2
10
120
40
M3
20
120
40
M4
40
120
40
M5
40
0
40
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Type
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Table 2．FRR data of membranes in fouling and washing cycle. of
1st cycle
2nd cycle
3rd cycle
membrane
FRR (%)
FRR (%)
FRR (%)
M0
87.5
81.4
74.7
M2
90.8
85.6
80.0
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Type
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