Construction of superhydrophilic and under-water superoleophobic carbon-based membranes for water purification

Abstract

Although the rapid development of membrane separation materials has provided a solution for the treatment of oil pollution, new membranes with functionality are also recommended for oil/water separation. Based on the micro-/nano-sized pore of carbon nanotubes (CNTs) membranes, polyacrylic acid (PAA) brushes were grafted via free radical polymerization, then silver nanoparticles (Ag NPs) were loaded, and Ag/PAA-CNTs hybrid membranes have thus been constructed. Such membranes are capable of separating oil-in-water emulsions because of their hydrophilicity and underwater oleophobicity, moreover, the loaded Ag NPs provide antibacterial function, which maybe open a new window to achieve multifunctional materials for water treatment.

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Introduction

Oil/water separation has become a worldwide problem because of the increasing industrial oily wastewater, as well as the frequent oil spill accidents.^1,2 Due to the different interfacial effects of oil and water, utilizing the wetting behavior of solid surfaces to design oil/water separation materials has been considered to be an effective approach. A series of superhydrophobic or superoleophobic materials in combination with surface chemistry and roughness have been fabricated to realize separation of oils from water selectively and effectively,^3,4 such as carbon-based materials,⁵ Cu(OH)₂ nanowire-haired copper mesh,⁶ graphene sponge,⁷ core/shell/satellite composite particles,⁸ silicone nanofilaments,⁹ and so on. Nevertheless, these materials always have drawbacks such as ease of fouling, difficulty to clean and poor reusability, which may limit their practical applications.^10,11 Recently, superhydrophilic and under-water superoleophobic materials are expected to be the ideal material to the separation of all types of oil/water mixtures, in which water permeates through the membranes whereas oil is retained.^12–14 Jiang et al. has reported nanowire-haired superhydrophilic inorganic membranes, which can effectively separate oil/water mixtures solely driven by gravity.¹⁵ Some other materials including photo-responsive ZnO mesh films,¹⁶ pH triggered poly[(2-dimethylamino)ethyl methacrylate] brushes,¹⁷ potential-modulated polyaniline nanowires and polypyrrole films¹⁸ have been developed. Notably, these materials have high separation efficiency, but most of them are still not effective for separating oil/water emulsions, especially for surfactant-stabilized emulsions with droplet sizes below 20 μm, restricting their real potential applications in oil/water separation.^19,20 Therefore, it is of great significance to develop materials that are more effective and scalable to realize the separation of oil/water emulsions.

Carbon nanotubes (CNTs) have attracted tremendous interests because of their low density, high porosity, extraordinary mechanical strength and hydrophobic properties.^21–24 For instance, Shi et al. have reported that single-walled CNTs network films could be used for the separation of emulsified oil/water mixtures with a higher flux than commercial filtration membranes.²⁵ We have reported previously that polymers could be covalently attached onto the surface of CNTs to achieve polymer/CNTs hybrid membranes, which can effectively separate various surfactant-stabilized oil/water emulsions with high separating efficiency.^26–28 However, water pollution has presented particularly serious due to the presence of variety of harmful bacteria, the resulting water phase is not appropriate to drain into water system even after oil/water separation. It is generally known that Ag nanoparticles (Ag NPs) have good bactericidal effect.^29,30 By taking the advantage of Ag NPs and carbon-based materials, herein, we have presented a method towards fabricating Ag/PAA-CNTs hybrid membranes for effective separation of various surfactant-stabilized oil-in-water emulsions. The obtained Ag/PAA-CNTs membranes show superhydrophilicity and under-water superoleophobicity in oil/water/solid three-phase system. Moreover, the obtained membranes can effectively separate surfactant-stabilized water-in-oil emulsions with a promising flux (3000 L m⁻² h⁻¹ bar⁻¹). Additionally, the Ag/PAA-CNTs membranes inherit the intrinsic bactericidal property of Ag NPs. It is a new attempt to use special wettability to design next-generation materials for oil/water separation, which will suggest attractive potential applications in industrial oily waste water treatments and oil spill cleanup.

Experimental section

Materials

General chemicals in chemical reagent grade were used as received from Sinopharm Chemical Reagent. Ethanol and deionized water were used as rinsing solvents. Acrylic acid (AA, 99%) and benzoyl peroxide (BPO) were obtained from Alfa Aesar China (Tianjin) Co., Ltd, which was purified by neutral Al₂O₃ column and dried with 0.4 nm molecular sieves at room temperature for 3 days. Sodium dodecyl sulphate (SDS), silver nitrate (AgNO₃) and formaldehyde were obtained from J&K China Co., Ltd. The raw CNTs (diameter about 10–30 nm and length about 30 μm, –OH% about 3%) with a purity of over 90% was purchased from Chengdu Organic Chemistry Co., Ltd, which was rinsed thoroughly with anhydrous ethanol, dried in a stream of nitrogen before use. PVDF membranes were purchased from millipore Industrial & Lab Chemicals with the aperture about 0.45 μm and the thickness about 125 μm.

Preparation of Ag/PAA-CNTs hybrid membranes

The preparation process includes two steps: (1) 0.1 g acrylic acid and 0.01 g CNTs were mixed with 100 mL acetone in a three-neck round bottom flask. After being treated by ultrasonic for 10 min, oxygen was removed by bubbling of nitrogen for 30 min, and then 0.05 g free radical initiator benzoyl peroxide (BPO) was added into the mixture. The reaction was kept at 75 °C for 6 h. The polymer-modified CNTs were carefully filtered to remove the excess PAA, and then dried at 80 °C for 14 h. (2) 100 mL of 1% silver nitrate solution were placed in a 250 mL beaker and then 5% ammonia was added dropwise to the beaker under stirring until the brown precipitate dissolved to obtain the Tollens reagent. PAA-CNTs powders (200 mg) were dispersed in 50 mL of 0.1% sodium dodecyl sulfate (SDS) aqueous solution under sonication. The resultant PAA/CNTs suspension was introduced to the Tollens reagent under gentle stirring. Formaldehyde (0.5 g) was added to the system and the system was kept stirring at 60 °C for 0.5 h. The final Ag/PAA-CNTs products were collected by centrifugation and washed with water and ethanol for several times, then filtrated on the hydrophilic PVDF membrane with the vacuum degree at 0.09 MPa to get Ag/PAA-CNTs hybrid membrane.

Test of antibacterial activity

E. coli was inoculated in Luria Bertani (LB) medium and cultured at 37 °C in a shaking water bath until the culture reached optical density of 0.5 at 600 nm. To prepare the bacterial solution for disinfection experiments, the cells were harvested by centrifugation and then diluted with sterile phosphate saline buffer solution (PBS, pH 7, 0.1 mol L⁻¹). PBS solution was spiked with E. coli at concentrations of 10³ colony forming units (CFU) mL⁻¹. E. coli was incubated for 15 h under 37 °C, and then the Ag/PAA-CNTs membrane with 0.5 × 0.5 cm² was placed in the bacterial solution to observe the growth of the bacteria. Notice: the LB medium and the Ag/PAA-CNTs membrane were sterilization with UV (λ = 100 W) beforehand.

Preparation of oil-in-water emulsions

Three kinds of surfactant-stabilized water-in-oil emulsions labelled as S1 for toluene-in-water emulsion, S2 for hexadecane-in-water emulsion and S3 for dichloromethane-in-water emulsion were prepared. The procedure is as follows: for S1 and S3, Tween 80 (HLB = 15, an emulsifier of the oil-in-water type, 1.5 g) was added into water (1.2 L), and then oil (2.5 mL) was added, for S2 span80 (2.0 g) was added into water (1.2 L), and then oil (3 mL) was added. The mixture was stirred for 6 h to obtain surfactant-stabilized emulsions, and all the emulsions were stable for more than 2 weeks without demulsification or precipitation was observed.

Separation of oil-in-water emulsions

The separation experiments were carried out with a vacuum driven filtration system with the vacuum degree at 0.09 MPa. The diameter of the filter is 25 mm.

Characterizations

Transmission electronic microscopy (TEM) measurements were performed with a JEOL JEM-2100F microscope. Scanning electronic microscopy (SEM) measurements were carried out using a JEOL JMS-6700F scanning microscope equipped with an energy-dispersive X-ray (EDX) Oxford ISIS 300 micro analytical system. Fourier transform infrared (FTIR) spectra were performed on a Varian Scimitar1000 Fourier transform IR spectrophotometer. TGA measurements were conducted with a Netzsch TG 209F1 instrument with a heating rate of 10 °C min⁻¹ under N₂ atmosphere. Static water contact angles (WCA) measurements were measured at room temperature using an OCA-20, Dataphysics instrument. The water (Milli-Q) droplet volume was 3 μL, and average of three measurements was made to determine the surface wettability. Optical microscopy images were taken on Nikon AZ100 by dropping emulsion solution on biological counting board. The water contents in collected filtrates were determined using a Karl Fischer moisture titrator (KF831). Dynamic light scattering (DLS) measurement was performed on a Zetasizer Nano ZS.

Results and discussion

The procedure for fabricating the CNTs hybrid membranes as described in the Experimental section was schematically shown in Fig. 1. Firstly, PAA brushes was grafted onto CNTs via free radical polymerization, then Ag NPs was in situ reduced on the surface of PAA-CNTs, the Ag/PAA-CNTs membrane was achieved by filtrating through a hydrophilic PVDF membrane.

Fig. 1 Schematic illustration of the fabrication of Ag/PAA-CNTs membrane and the application of Ag/PAA-CNTs membrane for water/oil separation and antibacterial effect.

In order to find out whether there was a successful attachment of PAA brushes and Ag NPs onto CNTs, WCA measurement was performed to investigate the surface properties of CNTs membrane before and after modification. Movie S1† shows the wettability of the as-prepared Ag/PAA-CNTs hybrid membranes to water and oil in air, and under water and oil, respectively. In the liquid/air/solid three-phase system, when a water droplet (3 μL) makes contact with the membrane surface, it immediately spreads out and permeates into the membrane within 5 s, and the WCA is nearly 0°. A similar result is obtained when water is replaced by oil and the oil droplet spreads out and permeates into the membrane within 3 s (Movie S2†). In air, the as-prepared Ag/PAA-CNTs hybrid membrane is superamphiphilic due to its high surface energy and large number of carboxyl groups on the CNTs surface. However, in the oil/water/solid three-phase system, the as-prepared membranes cannot be wetted by oil (chloroform) (Fig. 2A–C, Movie S3 and S4†). Moreover, the membranes present unique under-water superoleophobic characteristics for various organic solvents, including dichloromethane, carbon tetrachloride (Fig. S1†), which indicates that the Ag/PAA-CNTs hybrid membrane exhibits superoleophobicity under water. The wetting behavior under toluene (Fig. 2D–F) further confirms the superhydrophilicity of the Ag/PAA-CNTs hybrid membrane. The under-water superoleophobicity to various “oils”, and under-oil superhydrophilicity of Ag/PAA-CNTs hybrid membrane make it a good candidate for oil/water separation.

Fig. 2 Photographs of (A) an oil droplet (5 μL) on the surface of as-prepared Ag/PAA-CNTs membrane under water, (B) a Ag/PAA-CNTs membrane under water, (C) under-water superoleophobicity of the as-prepared Ag/PAA-CNTs membrane in the oil/water/solid three-phase system for chloroform, inset, contact angle picture of chloroform; (D) a water droplet (3 μL) on the surface of as-prepared Ag/PAA-CNTs membrane under toluene, (E) a Ag/PAA-CNTs membrane under toluene, (F) under-oil superhydrophilicity of the as-prepared Ag/PAA-CNTs membrane in the water/oil/water/solid three-phase system for water. Inset, contact angle picture of water.

SEM and TEM were applied to characterize the surface morphology of the CNTs before and after the modification. The cross profile of the Ag/PAA-CNTs hybrid membrane is shown in Fig. S2.† The thickness of the as-prepared hybrid membrane is about 1 μm. Though the presence of Ag NPs and PAA brushes cannot be seen clearly, the attachment of Ag NPs and PAA brushes onto CNTs can be observed by further amplification of the surface of the hybrid membrane. As shown in Fig. 3, after modified with Ag NPs and PAA, the microstructures of the CNTs hybrid membrane are different from that of the original CNTs network membrane. The average diameter of the modified CNTs (Fig. 3B–C) was about 40 nm, which are larger than that of CNTs (∼25 nm, Fig. 3A). The TEM results are consistent with the SEM results (Fig. 3D–F). These results illustrate that Ag NPs and PAA brushes were uniformly decorated onto the surfaces of CNTs membrane.

Fig. 3 SEM and TEM images of (A and D) CNTs membrane, (B and E) PAA-CNTs membrane, (C and F) Ag/PAA-CNTs membrane.

The adsorption of Ag NPs on the PAA-modified CNTs was also confirmed by XPS and EDS. As shown in Fig. 4 and S3,† the strong signals of O 1s indicating the successful modification of PAA on the CNTs and the Ag 3d_5/2 and Ag 3d_3/2 are observed at binding energies of 368.7 and 374.6 eV, respectively, which are in accordance with the previous report.³¹ EDS was further verified the presence of Ag NPs on CNTs. Both the XPS and EDS results provide direct evidence of the successful attachment Ag NPs to the PAA-modified CNTs.

Fig. 4 (A) XPS spectrum of Ag/PAA-CNTs membrane, (B) EDS spectrum of Ag/PAA-CNTs membrane.

The observed distinct stable under-water superoleophobic, under oil superhydrophilic indicates this membrane is a good candidate for industrial oily water treatment. A series of surfactant-stabilized oil-in-water emulsions were prepared to evaluate the separation capability of the membrane (Table S1†). Fig. 5 gives the separating results of using Tween 80-stabilized toluene-in-water (S-1) emulsions as example. Emulsion droplets could be de-emulsified once touching the Ag/PAA-CNTs membrane, the oil droplets were repelled, assembled together and retained above the membrane due to the under-water superoleophobicity of the membrane and the water permeated through the membrane due to its superhydrophilicity (Movie S5†). As shown in Fig. 5B and C, the collected filtrate (down) is transparent compared with the original milky white feed emulsion (upon). Optical microscopy was used to examine the separation effectiveness by comparing the feed with its collected filtrate. In the feed solution, there are densely-packed droplets flood the entire view and the size of the droplets is about 180 nm (Fig. 5D–F), and droplets size greater than 20 nm are not observed in the collected filtrate in the whole view (Fig. 5E–G), indicating the excellent separating properties of the Ag/PAA-CNTs membranes. Similar effective separations are also achieved for other emulsion systems including chloroform-in-water (S-2), hexane-in-water (S-3) (Fig. S5 and S6†). Membrane permeability was comprehensively investigated. Since our membrane only enables continuous water phase to pass through, if the density of oil is higher than that of water, the rejected oil droplets will accumulate to form a barrier layer on the membrane surface and impede water permeation. Therefore the permeate flux of the membrane heavily depends on the oil's density. Since toluene is the most viscous oils studied in this work (Table S2†), the membrane presents the lowest flux of 2930 ± 448 L m⁻² h⁻¹ bar⁻¹ for water-in-toluene emulsions, and flux of 3220 ± 434, 3480 ± 555 L m⁻² h⁻¹ bar⁻¹ for water-in-chloroform and water-in-hexane emulsions respectively (Table S3†).

Fig. 5 (A) A photograph of separating toluene-in-water emulsion where toluene selectively permeates through the Ag/PAA-CNTs membrane. Photographs and DLS data of toluene-in-water emulsion before (B, D and F) and after (C, E and G) filtration.

Ag nanoparticles have been known to be able to inactivate microorganism cells by destroying cell membrane and DNA replication.^32,33 The antibacterial ability of the as-prepared Ag/PAA-CNTs membrane was assessed by disc diffusion method. As shown in Fig. 6, the inhibition is clear, which indicates the Ag/PAA-CNTs membrane exhibits to a certain extent bacterial inactivation capability. Antibacterial ability was further proven by the morphological studies of the bacteria before and after incubating with the Ag/PAA-CNTs membrane using TEM and SEM. Fig. 6B and E show the internal structure of the untreated E. coli cells. It is clear that the cells have unanimous electron density, suggesting the cells are in a normal condition with intact cell walls and membranes. Morphological changes occurred in E. coli cells after incubating with the Ag/PAA-CNTs membrane (Fig. 6C and F), indicating Ag/PAA-CNTs membrane has antibacterial ability.

Fig. 6 Photographs of colonies formed by E. coli cells (A) in water samples and (D) being incubated with Ag/PAA-CNTs membrane. TEM and SEM images of (B and E) E. coli cells before and (C and F) E. coli cells damaged after filtrating from the as-prepared Ag/PAA-CNTs membrane.

Conclusions

In summary, we have presented a facile strategy for constructing hybrid Ag/PAA-CNTs membranes for oil/water separation. The as-prepared Ag/PAA-CNTs membranes can effectively separate a wide range of surfactant-stabilized oil-in-water emulsions with a high flux (3000 L m⁻² h⁻¹ bar⁻¹). Moreover, the hybrid membranes also inherit the antibacterial effect of Ag NPs. The effective oil/water separation performance and antibacterial functions make the Ag/PAA-CNTs membranes a promising candidate in practical oil/water separation applications.

Acknowledgements

We thank the Ningbo Natural Science Foundation (2015A610022), Open Research Fund of Key Laboratory of Marine Materials and Related Technologies (2016Z01), Zhejiang Nonprofit Technology Applied Research Program (2015C33031) and Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 2016268).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14310e

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