Hydrophilic ePTFE Membranes with Highly Enhanced Water

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Hydrophilic ePTFE membranes with highly enhanced water permeability and improved efficiency for multi-pollutant control Dongyan Li, Jian Hu, Ze-Xian Low, Zhaoxiang Zhong, and Yong Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00086 • Publication Date (Web): 29 Feb 2016 Downloaded from http://pubs.acs.org on March 1, 2016

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Industrial & Engineering Chemistry Research

Hydrophilic ePTFE membranes with highly enhanced water permeability and improved efficiency for multi-pollutant control

Dongyan Lia,b, Jian Hua, Ze-Xian Lowc, Zhaoxiang Zhonga*,Yong Wanga

a

National Engineering Research Center for Special Separation Membrane, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing 210009, P. R. China

b

Chemical Engineering Department, Nanjing Polytechnic Institute, Nanjing 210048, P. R. China

c

Centre for Advanced Separations Engineering, Department of Chemical Engineering, University of Bath, Claverton Down, United Kingdom

*Email: [email protected]; Fax: +86-25-83172292; Tel: +86-25-83172163

1

ACS Paragon Plus Environment


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Abstract Expanded polytetrafluoroethylene (ePTFE) is one of the most common membrane materials, but the intrinsic hydrophobic nature is a major reason for deteriorating performances in water purification. In this work, the influences of atomic layer deposition (ALD) seeding and subsequent ZnO nanorods (NRs) growth on the surface morphologies and water permeability of ePTFE membranes were investigated. ZnO-ALD ePTFE membrane showed an order of magnitude increase in water permeation, owing to the substantial increase in surface hydrophilicity and surface area. The best-performing ZnO NRs membrane had a pure water flux (PWF) of 1764.3 L·m-2·h-1 (compared to 153.8 L·m-2·h-1 of the pristine ePTFE membrane). Suspension filtration experiments indicate that the ZnO NRs filter showed an increase of steady-state flux of more than 5 times and an increase of 29.7% in retention compared to that of the ePTFE membrane. Significantly, the novel ZnO NRs filter exhibited good photo-catalytic performance as demonstrated in the degradation of methyl orange (MO) solution. Therefore, the functionalized membrane can potentially overcome the inherent limitation in the trade-off effect and imply their superiority for controlling water quality. KEYWORDS: ePTFE membranes, ZnO nanorods, hydrophilicity, water permeability, photo-catalytic performance

2


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1. Introduction Membrane separation has been used in several environmental applications to remove nanoparticles due to its high separation efficiency, and ease of operation1-4. Expanded polytetrafluoroethylene (ePTFE) porous membranes, fabricated via a stretching process are effective filters for solvent purification due to its excellent thermal stability and chemical resistance, and high mechanical property 5, 6. However, ePTFE membranes intrinsically exhibit hydrophobic surface property due to the presence of fully fluorinated backbones. When ePTFE membranes are used in water purification, their inherent hydrophobicity impedes water to penetrate into the ePTFE membrane and therefore requires higher pressure and consumes more energy7. Removal efficiency of nanoparticles is another challenge for ePTFE membrane because its pores are often larger than the particles size. Larger pores often have higher initial flux but lower removal efficiency

8-10

. Therefore, a new functional

ePTFE membrane with improved performance is highly demanded in wastewater filtration. It is challenging to modify ePTFE membranes due to their poor compatibility with other polymers. Nevertheless, plasma modification technology is one of many efficient methods to improve the performance of PTFE membranes

11, 12

. It can

change the surface morphology and energy of PTFE. However, the obtained hydrophilicity decreased with time due to various reasons. The grafting of hydrophilic polymer is often combined with the plasma treatment to increase the duration of hydrophilic modification effect

9,13

. However, this approach requires harsh 3



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pretreatment and complex procedures. Atomic layer deposition is commonly used to deposit uniform thin films on substrates with precise layer control. Nucleation and growth of metal oxides such as Al2O3 and TiO2 on various polymer substrates have been fabricated by the ALD method and result in an increased hydrophilicity of the ePTFE membrane

14-17

. In our previous studies, a new form of membranes with a

hierarchal fibrous structure by growing ZnO NRs onto a high porous ePTFE matrix via the ALD-seeded hydrothermal growth method. The new filters are exceptional in several ways including high air filtration efficiencies, low pressure drops and easy pulse cleaning of surface-filtered dusts18. They demonstrate promising applications in ultrafine pollutant particles and organic chemical pollutants removal from water. In the present study, aiming at the potential applications in water filtration, the influences of ZnO-ALD seeding and ZnO NRs growth on the surface morphologies and water permeability of ePTFE membranes were investigated. The performance of functional ePTFE membrane for nanoparticles capture and degradation of methyl orange (MO) were evaluated.

2. Experimental 2.1 Materials ePTFE membranes discs (Φ25 mm ╳ 0.65 mm) were provided by Sartorius (Germany). The ALD reactants were deionized water (used as O source) and diethylzinc (Zn (C2H5)2, DEZ, Aladdin) (used as Zn source). High-purity of N2 (99.99%) was used as the purging and carrier gas. Besides, hexamethylenetetramine 4


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(HMTA, C6H12N4) and zinc nitrate hydrate (Zn(NO3)2·6H2O) were both provided by Shanghai Lingfeng Chemical Co. (China). All chemical reagents were analytical grade and used as received without further purification. SiO2 nanoparticles with a mean diameter of 152 nm (Fig.1) were chosen as pollutants19 and synthesized by Stoeber method.

Fig.1 Particle size distribution of SiO2 spheres.

2.2 ALD ZnO layer and growth of NRs on PTFE membrane ALD ZnO seeding was performed in a holder of ALD reactor (S100, Cambridge Nano-Tech). ePTFE membranes were placed in the ALD chamber whose temperature is 130 oC. The ALD reaction was initiated after the vacuum of the chamber was 1 Torr. The precursor DEZ and H2O were heated to 40 °C and 60 °C, respectively. An ALD cycle included (i) DEZ pulse for 1 s; (ii) exposure for 10 s; (iii) purge for 35 s; (iv) H2O pulse for 0.1 s; (v) exposure for 10 s; and (vi) purge for 35 s. The membranes were seeded for different cycles at the same temperature with a N2 flow rate of 20 sccm16. 5



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Hydrothermal growth of ZnO NRs was performed by immersing ALD ZnO seeded ePTFE substrate in a 35 mL water solution with equimolar (5mM) Zn(NO3)2 and HMTA. The reaction was carried out at 90 °C for different time. Subsequently, the sample was rinsed repeatedly with deionized H2O and then dried in air.

2.3 Characterizations of membranes SEM images were obtained by a field emission scanning electron microscope (FESEM, HitachiS-4800). Energy dispersive x-ray system (EDX, Oxford INCA 350) was used to characterize the distribution of the Zn element across the ZnO-ALD ePTFE membrane with the operating voltage of 20 kV. X-ray diffraction patterns were obtained by an X-ray diffractometer (XRD, Mini Flex 600) using CuKα radiation (40 kV, 15 mA) at a scanning rate of 2°/min, from 10° to 80° with step size of 0.05°. To evaluate the thermal stability of filter, thermogravimetric (TG) analysis was conducted using a thermogravimetric analyzer (TGA, Netzsch, Germany) operated within the temperature scope of 30–1000 °C under nitrogen with a heating rate of 10 °C/min. The pore size of the membrane were measured with the gas bubble pressure (GBP) method. Before measuring, all the membranes were dipped in alcohol for two hours under a low pressure. Then the pore size could be calculated by the flow rate and transmembrane pressure of nitrogen across the membrane. A Dropmeter A-100P contact angle goniometer was used to measure the water contact angles. The reported results were the mean value of the three measurements of each sample. The UV-vis diffuse reflection spectra (DRS) of the samples are recorded on a 6


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spectrophotometer of Perkin Elmer Lambda 950 (USA). The gas adsorption properties were carried out by N2 sorption at 77 K with a sorptometer (Micromeritics ASAP 2460) and the surface area was calculated by Brunauer-Emmett-Teller (BET) method.

2.4 Filtration experiments The PWF of the membranes was measured using a stirred filtration cell (Fig. 2, Amicon 8010, Millipore Co., Billerica, MA) under a transmembrane pressure (TMP) of 16.7 kPa pressure and the temperature was fixed at 25°C. The performances of the pristine ePTFE membranes and ZnO-ALD ePTFE membranes were characterized by filtrating SiO2 nanospheres with average diameter of 152 nm. The concentration of the feeding solution (Cf) of silica nanoparticles was 1 g.L-1. The concentrations of silica nanospheres in the permeate side (Cp) were obtained by a NanoDROP 2000C UV-vis spectrophotometer at wavelengths of 210 nm. The retention (R) was defined as R= (1-Cp/Cf)×100%. The permeate was collected and a stopwatch was used to measure the time to determine the permeate flux. The flux (J) is defined as the volume of permeate per unit area and time, as shown in the following equation:

=

(1)

·

In which J is the flux(L·m-2·h-1), Q is the volume of permeate (L), A is the membrane area (m-2), and ∆t is the measuring time (h).

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Fig. 2. Schematic diagram of experimental set-up.1. Agitator; 2.Mixing tank; 3. Elevator; 4. Ball valve; 5. Shutoff valve; 6.Filter; 7. Magnetic stirrer; 8. Pressure gauge; 9. Rotor flow meter; 10. Collecting tank.

2.5 Photocatalytic performance tests The photocatalytic activity of the samples (pristine, ZnO-ALD and ZnO NRs ePTFE membranes) were evaluated by measuring the photodegradation efficiency of MO aqueous solution under the irradiation of UV. The photocatalytic reactions in the solution were performed in a closed system at room temperature using a UV-lamp (Nanjing Huaqiang electronic co., LTD, 24W, with the main emission wavelength of 365 nm). A simple home-made photo-reactor device is shown in Fig. 3. After different irradiation intervals, 5 mL of the MO solution (10mg/L) was withdrawn and analyzed by the UV-Vis spectrophotometer (Lambda-35) at room temperature under the fixed wavelength of 463 nm. The degradation efficiency of MO was calculated according to Lambert-Beer equation: =

100% =

100%

(2)

where C0 is the original concentration, A0 is absorbance of MO solution; and Ct and At are the dye concentration and absorbance after degradation for a period of time, 8


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respectively.

Fig. 3 The schematic of photo-reactor device.

3. Results and discussion 3.1 Surface morphologies evolution of ePTFE membranes with ZnO-ALD deposition Pristine ePTFE membranes fabricated via a stretching process have a porous structure composed of interconnected and smooth nanosized fibrils without obvious particulate matter (Fig. 4a). Pores are formed among these fibrils, and these fibrils are parallel to the stretching direction. During the ALD seeding process, the precursor vapor diffuses into the 3D net-like structure and adsorbs onto ePTFE nanofibrils. ALD reactions then take place throughout the interconnected and smooth nanofibrils of the membrane, and ZnO nucleates and grows on the surface of the ePTFE nanofibrils. Small particulates were deposited on the surface of the nanofibrils. It seemed that the globular zinc oxide particulates grew randomly on the surface and increased in size with increasing ALD number of ALD (Fig.4b–f). Zinc oxide particulates grown below 100 ALD cycles are relatively small and the particles are far 9



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apart. Beyond 500 cycles, the particles started to fuse into continuous layers (Fig. 4g). The membrane subjected to 700 ALD cycles has a relatively smooth zinc oxide layer (Fig. 4h) which could be due to the conformal feature of ALD films, i.e filling of cracks between the adjacent particles. The deposition of ZnO on the ePTFE membrane can also be investigated via EDX analysis. As shown in Figure 5, Zn elements are present in the entire cross section of the ePTFE membrane under 300 cycles, indicating complete coverage of ZnO on PTFE membranes.

Fig. 4 SEM images of the surface morphologies of (a) the pristine ePTFE membrane, the inset is a low magnification image of the same sample, and ALD zinc oxide-coated PTFE membranes with varying number of ALD cycles (b) 50 cycles, (c)100 cycles, (d) 200 cycles, (e) 300 cycles, diffuse(f) 400 cycles, (g) 500 cycles, and (h)700 cycles. 10


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Fig. 5 the cross section SEM image of an ePTFE membrane subjected to 300 cycles of ZnO ALD (a) and the corresponding EDX map (b) showing the distribution of Zn element.

Recently, it was shown that the high reactivity of DEZ precursors can result in Zn infiltration into PTFE, locating in the defect sites and chain-end parts with –CF3 14, 20

. Subsequently, the absorbed DEZ will react with water vapor and produce ZnO

nucleation clusters. A homogeneous ALD surface nucleation was least expected as some preferential nucleation sites will help to form ZnO clusters. The clusters will further grow to globular particulates during progressive exposures of DEC and H2O. Fig. 6 are the XRD patterns of pristine ePTFE membranes and ZnO-ALD ePTFE membrane. An intense peak centered was observed for the pristine membrane at 18.42°, which corresponds to the diffraction from (100) planes with a spacing of 4.9 Å in PTFE20. After 200 ALD cycles, the PTFE diffraction peak shows a detectable left-shift. This indicates that the interlayer space of PTFE was slightly increased after the ALD deposition.

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Fig. 6 XRD patterns of (a) pristine ePTFE membranes and (b) ZnO-ALD ePTFE membrane.

3.2 Surface wettability improvement of the ZnO-ALD PTFE membranes Contact angle measurement of water droplets on the membrane surfaces were used to investigate the changes in hydrophilicity of the membrane surfaces. As shown in Fig. 7, water contact angle of the pristine ePTFE membrane is about 136º.The porous structure of PTFE nanofibrils contributed to the higher hydrophobicity than that of the flat PTFE surfaces (about 105º)

21

. When ALD cycles were increased to

200 cycles, the water contact angle decreased quickly to about 111º.This was because the fibril surface of the ePTFE membranes was mostly wrapped by ZnO. As ZnO has a much higher surface energy than PTFE and abundant surface hydrophilic groups22, the ZnO-deposited ePTFE membranes showed enhanced hydrophilicity. Improved coverage of the ZnO on the ePTFE membrane was observed with increasing ALD cycles (Fig. 4) as indicated by the slower decrease of the water contact angles at higher ALD cycles. When the ALD number was more than 600 cycles, there was no 12


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clear changes to the hydrophilicity and the water contact angle remained ~105º because the PTFE fibrils of membrane were fully wrapped by ZnO film.

Fig. 7 Water contact angles of ePTFE membranes seeded with different cycles. Inset figures were the pictures of the water droplet on corresponding membranes.

Pure water permeability tests were conducted on the ePTFE filters after different ALD ZnO cycles. As shown in the Fig. 8, the flux of pristine membrane was about 153.8 L·m-2·h-1, and then it increased quickly with the ALD cycles due to the improvement of hydrophilicity. With ALD cycles increased to 200 cycles, the pure water flux was increased to 1546.3 L·m-2·h-1, which is large higher than pristine ePTFE membrane. When the ALD number was more than 200 cycles, the flux increased slowly with ALD cycles and reached 1725.1 L·m-2·h-1 at 700 ALD cycles. The variation trend of the flux is consistent to that of the average water contact angles obtained from ePTFE membranes with different ALD cycles. The results strongly demonstrated that the ZnO deposition had an important effect on the water permeability of ePTFE membrane. 13



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Fig. 8

PWF of membranes deposited with different ALD ZnO cycles. (TMP=16.7 kPa, 25°C).

3.3 Effect of ZnO NRs on the pore structure and surface wettability of ePTFE membrane SEM images of ePTFE membranes after different hydrothermal growth periods using 300 cycles ZnO-ALD seeding layer are shown in Fig. 9a. It can be seen that short hydrothermal treatment periods cannot bring about the uniform ZnO NRs. However, after 3 h hydrothermal growth of ZnO, continuous and uniform ZnO nanorods were fabricated on the 3D fibrils support. The closely packed ZnO nanorods are roughly perpendicular to the fibrils axis, which reduced the free spaces between the nanofibrils that define the pores in the ePTFE membrane for water transmission. But with further increase of the hydrothermal growth time to 5 h, the length of ZnO nanorods increased quickly which led to substantial pore space shrinkage (Fig. 9a). Fig. 9b shows the pore size distributions of aforementioned pristine ePTFE membrane and ZnO NRs functionalized membranes. For the pristine membrane, the peak pore 14


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size is between the scopes of 3.8–5.4 µm. The functionalized membranes show bimodal pore size distributions (3.4–4.6 µm and 2.1–3.2 µm). The different size pores arise from the free spaces between the nanofibrils and between ZnO nanrods. The stability of functionalized membranes was determined in our previous work. The membrane remained stable after an ultrasonication oscillation treatment18. The ZnO content of the filters was further assessed by TG analysis. As shown in Fig. 9c, pristine ePTFE starts to decompose at ~550 °C and completely degrades at ~610 °C. ePTFE membranes subjected to ZnO-ALD process have a more residue mass of ca. 30.7 % at 800°C. After hydrothermal growth, the residual mass of ca. 38.5 % at 800°C was achieved due to the ZnO seeds growing into nanorods. Furthermore, there was a slight weight loss after ~610 °C for the ZnO-ALD and nanorod-growth membranes, which may be attributed to the formation of Zn–PTFE hybrid material after the ZnO-ALD process. It was found that the high reactivity of the ALD precursors resulted in the penetration of Zn into PTFE which influences its molecular structure and mechanical properties20. After 900 °C, another weight loss was observed. This is likely due to the carbothermal reduction of ZnO into Zn and vaporization of Zn (boiling point of Zn=908 °C) at higher temperatures23. Therefore, the TG curves suggest that ZnO growth does not reduce the thermal stability of the ePTFE membranes.

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Fig. 9 (a) SEM image of membranes after hydrothermal growth of ZnO NRs for different time; (b) pore size distribution of pristine membrane and ZnO NRs membrane for 3h hydrothermal growth, (c) TG curves of membranes in different periods.

3.4 Permeation and retention properties of the ZnO NRs membranes The wettability test results and pure water flux of ZnO NRs membrane are shown in Fig. 10. It was observed that the average water contact angle decreased quickly and then slowly with the hydrothermal time, indicating a good improvement of hydrophilicity. This is because NRs had more specific area with high surface energy18, which increased the interfacial adhesion of water drop with membrane. PWF of the membrane increased with hydrothermal time but after 3 h, it decreased drastically. The growth of ZnO NRs influenced the water permeability of the membrane through enhancing the hydrophilicity and reducing the pore sizes. When hydrothermal treatment was performed in less than 3 h, the increased hydrophilicity could offset the 16


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effect caused by the pore reduction by ZnO NRs growth. Further increase in the hydrothermal time causes more significant pore size reduction induced by the overgrowth of ZnO NRs on the pore surface which decreased the water flux through the membrane despite the enhanced hydrophilicity. Therefore, the optimal hydrothermal time to improve the PWF of the PTFE membranes following the ALD seeding is 3 h.

Fig. 10 (a) Average water contact angles and pure water flux; (b) measured on ZnO NRs membranes with different hydrothermal growth time.

The ZnO NRs membrane with highest flux was used to filtrate SiO2 nanoparticles suspension and the results are shown in the Fig. 11. The ZnO NRs membrane had a high initial flux, which resulted in increased particle deposition on the membrane surface, leading to a drastic flux decline in a short time24, but a much higher steady-state flux was obtained as compared to the pristine membrane, implying that the pristine ePTFE filter is progressively clogged, while the functionalized filter maintains a good porous cake structure with continuous filtration. For the pristine filter, the initial removal efficiency was 11.8% for 152 nm SiO2 particles and reached 70.3% 17



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after 90 min filtration. The removal efficiencies increase with filtration time due to the formation of filter cake, which acts as an additional filtration barrier. For the ZnO NRs membrane, the removal efficiencies increased from the initial ~96.5% to maximum efficiency of 100% after 45 min filtration. It is notable that ZnO NRs membrane achieves high retention while maintaining its high flux. These behaviors indicate the membrane functionalized with ZnO nanorods can enhance both the permeability and selectivity.

Fig.11. Flux and retention to SiO2 nanoparticles with a diameter of 152 nm.

3.5 Photocatalytic degradation properties of ZnO NRs membranes The photocatalytic efficiencies of pristine, ZnO-ALD and ZnO NRs membranes are shown in Fig.12. It is clear from the figure that pristine ePTFE membrane shows no obvious degradation of MO solution with increasing time, but only a small drop before 0.5 h which is attributed to the adsorption of MO into porous ePTFE matrix. However, ZnO-ALD and ZnO NRs membranes present good photocatalytic 18


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degradation efficiency. The photocatalytic efficiency of ZnO NRs membrane is higher than ZnO-ALD membrane because of the higher loading of ZnO and high crystallinity of ZnO NRs

18

. The surface areas of ZnO NRs, ZnO-ALD and pristine membranes

were 7.4, 4.6 and 4.3 m2.g-1, respectively. The apparent densities of three membranes were 0.37, 0.28 and 0.21 g.cm-3, respectively. Therefore, we can roughly estimate that the surface-area-to-volume ratio of the ZnO NRs membrane increases to 2.74× 106 m2.m-3 from 1.29× 106 m2.m-3 of the ZnO-ALD membrane and 0.90× 106 m2.m-3 of the pristine membrane. With the highest surface-area-to-volume ratios, the ZnO NRs membrane has the largest catalytic area, which increases the photocatalytic degradation efficiency.

Figure 12. Photocatalytic efficiency of degradation of MO solution for pristine, ZnO-ALD and ZnO NRs membranes.

These

results

also

can

be

reflected

from

the

analysis

of

UV-vis

diffuse-reflectance spectra (DRS), which are shown in Fig. 13. There is nearly no photo response in the spectrum line of pristine ePTFE membrane. However, almost 19



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the same sharp absorption edge at ~375 nm belonging to ZnO-ALD and ZnO NRs membranes indicates that ZnO possesses the UV absorption, and the absorbance of ZnO NRs is a little larger than ZnO-ALD seeds in the UV region. Hydrothermal growth of ZnO NRs produced a functional ePTFE membrane filter with excellent photocatalytic performance inherited from ZnO NRs. In the case of UV irradiation, the electrons transfer from the bands of valence to conduction on the surface of ZnO NRs when the energy adsorbed exceeds the energy band gap of ZnO (3.20 eV), which produces a lot of light electrons and holes. Furthermore, the holes could oxidize OHand produce strongly oxidizing .OH radicals while the electrons could restore O2 to the strongly oxidizing superoxide anion (·O2-). These active groups could efficiently degrade the MO solution into CO2 and H2O 25, 26.

Fig.13. UV-vis diffuse-reflectance spectra (DRS) of pristine, ZnO-ALD and ZnO NRs membranes within the wavelength range of 200–800 nm.

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4 Conclusions In this work, we demonstrated that growing aligned ZnO NRs onto the fibrous matrix of ePTFE membranes can transform the membrane surface from hydrophobic to hydrophilic, which endowed the membrane with high water flux, high nanoparticles removal efficiency and photocatalytic functionality. ZnO-ALD ePTFE membrane showed an order of magnitude higher water permeation than that of the pristine membrane. The best-performing ZnO NRs membrane had a pure water flux of 1764.3 L·m-2·h-1. According to suspension filtration experiments, the stable membrane flux of ZnO NRs membrane showed 5 times more than the pristine PTFE membrane, and at the same time, ZnO NRs membrane possessed an increase of retention of 29.7%. In addition, the presence of ZnO NRs endowed the ePTFE with good photocatalytic performance, which helped the membrane to not only remove pollutant particles but also degrade organic chemical pollutants. The results strongly suggested that the ALD-seeded hydrothermal growth strategy is an effective and versatile way to functionalize ePTFE membranes toward diverse environmental applications. The novel functionalized ePTFE may be a promising membrane coupled with photocatalytic properties suitable for applications in water purification.

Acknowledgements Financial support was provided by the National Natural Science Foundation of China (21276124), Jiangsu Province scientific supporting project (BE2014717), and Jiangsu Province Six Talent Peaks Project. 21



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TOC Graphic

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Hydrophilic ePTFE Membranes with Highly Enhanced Water

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