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Polyaniline coated membranes for effective separation of oil-in-water emulsions
Accepted Manuscript Polyaniline coated membranes for effective separation of oil-in-water emulsions Mingming Liu, Jing Li, Zhiguang Guo PII: DOI: Reference:
S0021-9797(16)30024-8 http://dx.doi.org/10.1016/j.jcis.2016.01.024 YJCIS 21007
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
10 December 2015 11 January 2016 13 January 2016
Please cite this article as: M. Liu, J. Li, Z. Guo, Polyaniline coated membranes for effective separation of oil-inwater emulsions, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.01.024
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Polyaniline coated membranes for effective separation of oil-in-water emulsions
Mingming Liu a,c, Jing Li a,*, Zhiguang Guo a,b,* a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China b
Ministry of Education Key Laboratory for the Green Preparation and Application of
Functional Materials, Hubei University, Wuhan 430062, People’s Republic of China c
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of
China * Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (Z. Guo).
Abstract: Polyaniline (PANI) decorated commercial filtration membranes, such as stainless steel meshes (SSMs) with 5 µm pore size and polyvinylidene fluoride (PVDF) membranes with 2-0.22 µm pore sizes, were fabricated by a simple one-step dilute polymerization at low temperature. Lots of short PANI nanofibers were firmly and uniformly coated onto the membrane surfaces, forming rough micro- and nanoscale structures and leading to underwater superoleophobicity with low oil-adhesion characteristic. Furthermore, we systematically studied the effect of pore size and pressure difference on oil-water separation ability of the obtained membranes. It was found that the PANI-modified SSMs with 5 µm pore size were suitable for the separation of non-surfactant emulsions with water fluxes of more than 1000 L m-2 h-1 under gravity only. The PANI-modified PVDF membranes were used for the effective separation of surfactant-stabilized emulsions with water fluxes up to 3000 L m-2 h-1 for 2 µm pore size under 0.1 bar or 0.22 µm pore size under 0.6 bar. In addition, the superhydrophilic membranes with PANI coatings were demonstrated for high oil rejection, stable underwater superoleophobic properties after ultrasonic treatment and immersing in oils and various harsh conditions, and high and steady water permeation flux after several cycles.
Keywords:
Polyaniline;
Porous
membranes;
superoleophobic; Oil-in-water emulsion separation
Superhydrophilic;
Underwater
1. Introduction Membrane technologies, especially pressure-driven filtration membranes, are still ceaselessly developed for water treatment applications to satisfy the growing demand for clean water. In the past few decades, various kinds of the pressure-driven filtration membranes have been explored according to their characteristic pore size and relative separation objects. Among them, microfiltration (50-500 nm pore sizes) and ultrafiltration (2-50 nm pore sizes) have been successfully applied for emulsion separation [1]. Generally, the polymeric filtration membranes are intrinsically hydrophobic, which requires a transmembrane pressure of up to several bars to selectively filter water, and leads to a low flux and quick decline due to unavoidable fouling issue. Many surface modification methods have been adopted to reduce membrane fouling, usually attempted by an increase in hydrophilicity [2-6]. However, there are other chemistry parameters rather than hydrophilicity that could affect the membrane fouling [7]. A number of superwetting films have been fabricated to achieve the separation
of
non-emulsified
oil
and
water
mixtures,
including
superhydrophobic or underwater superoleophobic materials, respectively [8-19]. These superwetting films are unsuitable for various emulsions with less than 20 μm droplet size, especially surfactant-stabilized emulsions. To realize emulsion separation by integrating the aforementioned approaches, Jin and coworkers transformed hydrophilic poly(acrylic acid)-grafted polyvinylidene fluoride
(PVDF) membranes to superhydrophilic and underwater superoleophobic surfaces with a rough micro/nanoscale structure by a salt-induced phase inversion [20]. Consequently, underwater superoleophobic membranes with pore sizes less than the oil droplets have been designed and synthesized to deal with emulsified oily wastewater (Table S1) [21-41]. Recently, hydrophilic organic materials have been introduced onto the surface of various commercial porous membranes, providing a strong binding force between hydrophilic coatings and substrates. In order to further enhance the stability and surface roughness of organic coatings, some inorganic materials are widely used to modify the organic coatings. The organic and inorganic materials include for example polydopamine, poly(acrylic acid), poly(sulfobetaine methacrylate), CaCO3, SiO2, and TiO2 [20-32]. Notably, the pressure difference on most of underwater superoleophobic membranes can be less than 1 bar compared to commercial hydrophobic membranes, even down to gravity. In general, filtration membranes with a larger pore size require a lower pressure difference. For example, a 500 mesh membrane with Cu(OH) 2 nanowire-haired coatings can separate oil-in-water emulsions under gravity only [33]. However, with this all-inorganic membrane it is hard to achieve separation of surfactant-stabilized emulsions due to large pore size. Therefore, it is desirable to develop a one-step approach to construct superhydrophilic and underwater superoleophobic membranes with different pore sizes, which can maintain high stability under
various harsh conditions, and effectively treat different emulsified oily wastewaters with high water fluxes under a low pressure difference. In our previous work, we prepared underwater superoleophobic membranes by coating polyaniline (PANI) and polypyrrole (PPy) on the surface of stainless steel meshes (SSMs, 300 and 400 mesh size) to separate non-emulsified oil and water mixtures [18]. After in situ polymerization, PANI and PPy can be strongly coated on the walls of the reaction vessels and the surfaces of hydrophobic SSMs. Because of the rigid structures, conductive polymers show the advantages of inorganic materials, forming stable and rough nanocoatings. Furthermore, it has been demonstrated that PANI nanofibers can be well dispersed in water due to their large number of amino groups and the presence of electric double layers around the PANI chains [18,42,43], resulting in the hydrophilicity and underwater superoleophobicity. We found that the prepared underwater superoleophobic membranes cannot be used to separate emulsions. In order to realize demulsification, we adopted the same oxidation polymerization method to decorate PANI and PPy onto the membrane surfaces with smaller pore size. However, PANI and PPy cannot be uniformly coated on the membrane surfaces and block the small pore channels, which greatly influences the separation efficiency of oil-in-water emulsions. Here we slowed down aniline polymerization to make PANI uniformly and firmly coat onto the surface of commercial hydrophobic materials with different pore sizes, including SSMs (2300 mesh size) and PVDF membranes.
Note that the method is still unsuitable for PPy. After decoration with PANI coatings, the obtained SSMs and PVDF membranes become superhydrophilic and underwater superoleophobic with low oil-adhesion characteristic and small sliding angles. The PANI-modified SSMs with a pore size of about 5 µm can be used to separate non-surfactant emulsions under gravity only. The separation of various emulsions, stabilized by surfactants such as Tween 80 and sodium dodecyl sulfate (SDS), can be effectively achieved by PANI-decorated PVDF membranes with pore sizes ranging from 2 to 0.22 µm employing different external pressures (less than 1 bar). In addition, these PANI-modified filtration membranes are highly stable after ultrasonic treatment and immersing in oils and various harsh conditions, and maintain underwater superoleophobicity and high flux after reusing.
2. Experimental section 2.1. Materials SSMs (2300 mesh) and PVDF membranes (HYXDF, China) with different pore sizes (0.22, 0.45, 0.8, 1.2, 2 µm) are commercially available. Chemical reagents were used as received without further purification.
2.2. Preparation of PANI-modified SSMs and PVDF membranes Aniline (0.02 M) and ammonium persulfate (0.01 M) were dissolved in 10 mL of 1 M perchloric acid solution, respectively. After pre-cooling in an ice bath, the
reaction solution was rapidly mixed under stirring. Meanwhile, commercial SSMs and PVDF membranes were rinsed consecutively by deionized water and ethanol in an ultrasonic cleaner, and dried at 60 °C in an oven. Thereafter, the original SSMs (25 mm × 25 mm) or PVDF membranes (25 mm diameter) were immersed in the above-mentioned reaction solution (20 mL). The aniline polymerization took 12 hours under continuous stirring in an ice bath. Finally, the obtained SSMs and PVDF membranes with PANI coatings were thoroughly washed by deionized water and dried at 60 °C in an oven.
2.3. Characterization All optical images were taken by a digital camera (Sony, DSC-HX200). The surface morphology of membranes was observed on a field emission scanning electron microscope with Au-sputtered specimens (FESEM, JEOL JSM-6701F). The accelerating voltage was 5 kV and the current was 10 μA. The surface chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB250Xi) using Mg-Kα radiation and Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet iS10). Optical microscope images were obtained by OLYMPUS BX51 microscope. Effective water contact angles (WCAs) and underwater effective oil contact angles (OCAs) were measured on a JC20001 contact angle system (Zhongchen digital equipment Co., Ltd. Shanghai, China). The average CA value was obtained by measuring the sample at five different positions. The oil content in the collected filtrate was calculated by measuring
chemical oxygen demand (COD) according to U.S. Environmental Protection Agency method 8000 (HACH, DRB 200). Electrochemical measurements were performed in a three electrode system at room temperature, containing a platinum slice as counter electrode, a saturated calomel electrode as reference electrode and PANI-modified SSMs (25 mm in length and 15 mm in width) as working electrode. The electrolyte was 70 mL of 0.1 M H2SO4. The mechanical properties were measured using an electrical universal material testing machine with a 500 N load cell (SHIMADZU, EZ-Test). The crosshead velocity was kept at 10 mm/min for tensile measurement. The PVDF membranes were cut into 20 mm × 3 mm pieces.
2.4. Preparation of different types of oil-in-water emulsions Non-surfactant oil-in-water emulsions were prepared by mixing oil (including hexane, petroleum ether and hexadecane) and water in a volume ratio of 1:9, and sonicating the mixtures for 1 hour to produce white emulsions. The non-surfactant emulsions were stable for 1 hour. Tween 80 and SDS stabilized oil-in-water emulsions were obtained by mixing oil (including hexane, petroleum ether and hexadecane) and water in a volume ratio of 1:99 with dissolving 0.02~1 mg Tween 80 and 0.1 mg SDS per ml, respectively. Afterward, the mixtures were intensively stirred for 6 hours. All the surfactant-stabilized emulsions were stable for 24 hours.
2.5. Emulsion separation experiments The emulsion separation tests were carried out under gravity or a vacuum driven
filtration system, as shown in Fig. 1. A vacuum pump (Millipore, WP6122050) was used to supply adjustable pressure difference from 0.1 to 0.6 bar. The height of pure water or emulsions kept 10 cm. Water fluxes were determined by calculating the volume of water permeating per unit time using the following equation: flux = V/St, where V is the volume of water permeation, S is the area of the films, and t is the testing time. All measurements were repeated 3-5 times and the results were reproducible with relative errors less than ±5%.
Fig. 1. Photographs of the instrument for the separation of oil-in-water emulsion using PANI-modified SSMs and PVDF membranes. (a, b) Separation instrument; (c) PANI-modified SSMs (up) and PVDF membranes (down).
3. Results and discussion 3.1. Preparation of PANI-coated SSMs and PVDF membranes PANI has intrinsic nanofibrillar morphology due to its rigid chain structures and positive charge rich surface [43,44]. It has been verified that homogeneous nucleation of PANI in dilute solution or at the early stage of aniline
polymerization
preferentially
forms
nanofibers
rather
than
heterogeneous nucleation and growth [43,45-47]. In order to prepare uniform PANI coatings on the surface of hydrophobic membranes with small pore size,
aniline polymerization was performed in dilute solution (0.01 M) and at low temperature (0 °C). This reaction condition can suppress nucleation and growth of PANI in the bulk of the solution at the early stage of aniline polymerization, resulting in competitive nucleation of PANI nanostructures on the surface of porous membranes. As shown in Fig. 1c, PANI has been successfully coated on the surface of commercial SSMs and PVDF membranes. Based on the electrochemical characterisation of PANI-modified SSMs, it is indicated that the PANI coatings are in emeraldine state (Fig. S1).
Fig. 2. SEM images of original and PANI-modified SSMs. (a, b) Original SSMs; (c, d) PANI-modified SSMs.
Commercial 2300 mesh SSMs were firstly selected to prepare underwater superoleophobic filtration membranes. Fig. 2, S2 and S3 show SEM images of the top and cross-section surfaces of original and PANI-modified SSMs. As seen in SEM images, this Dutch weave SSMs are woven using two sizes of wires, showing different weave densities in the warp and weft directions. The
smaller wires (about 22 µm) are woven tightly together, determining the pore size of the prepared filtration membranes (an average of 5 µm). The larger wires (about 36 µm) provide strength to the SSMs beneficial for the practical applications. Under observation at large magnification, the surface of the original SSMs is clean and smooth. After dilute polymerization of aniline at low temperature, the nucleation and growth of PANI result in short nanofibers that uniformly coat the smooth surface of the original SSMs, forming microand nanoscale hierarchical structures. The enlarged roughness has been demonstrated to be one of the crucial chemical parameters for superwetting behavior based on the Cassie model [48]. Furthermore, slowing down aniline polymerization appears to be particularly important to provide enough time for the diffusion of reacting species in the narrower hydrophobic pore channels of commercial PVDF membranes, leading to a uniform PANI nanocoating on their surfaces. In the same manner, micro- and nanoscale rough surfaces constructed by short PANI nanofibers were also obtained by using commercial PVDF membranes with different pore sizes (2, 1.2, 0.8, 0.45, and 0.22 µm) as substrates (Fig. 3 and S4).
Fig. 3. SEM images of original and PANI-modified PVDF membranes. (a, c, e) Original PVDF membranes; (b, d, f) PANI-modified PVDF membranes.
It is well known that not only the surface roughness but also the chemical composition has a great impact on the wettability of substrates. Therefore, XPS and FTIR measurements were used to investigate the chemical composition of SSMs and PVDF membranes before and after PANI modification. Fig. S5 shows the XPS spectra of pristine and PANI-modified SSMs. For the pristine SSMs, the observed XPS peaks are mainly assigned to iron, carbon and oxygen. In contrast, the PANI-modified SSMs display two new peaks at about 400 and 208 eV corresponding to nitrogen of the PANI chains and chlorine of the HClO4, respectively. In addition, the iron-related peak disappears due to the presence of the PANI coatings. For the pristine PVDF membranes, XPS peaks
belong to fluorine, carbon and oxygen, as shown in Fig. 4a. After the PANI modification, the fluorine-related peak vanishes. Similarly, two new peaks attributed to nitrogen and chlorine appear in the XPS curve of the PANI-modified PVDF membranes. Moreover, in the FTIR spectra as shown in Fig. 4b, strong peaks are detected at about 1400 cm-1, 1300-1000 cm-1, and 900-800 cm-1 assigned to C-H, C-F and =C-H bond, respectively. Weak IR signals around 3000 cm-1 due to C-H bonds are also observed. All signals are typical IR characteristic of the PVDF membranes. After PANI nanofibers are coated on the surface of the PVDF membranes, new peaks at around 3240 cm-1, 1563 cm-1, 1487 cm-1, 1294 cm-1, and about 800 cm-1 are observed and ascribed to N-H stretching vibration, benzene rings, C-N stretching vibration, and N-H deformation vibration, respectively. Both XPS and FTIR results demonstrate the successful modification of SSMs and PVDF membranes with PANI coatings. Because of the presence of polar components, such as nitrogen and doping acids, the PANI coatings possess inherent hydrophilicity.
Fig. 4. XPS and FTIR spectra of original and PANI-modified PVDF membranes. (a) XPS; (b) FTIR.
3.2. Wetting properties of PANI-coated SSMs and PVDF membranes Based on the micro- and nanoscale rough surface of PANI coatings and the large amount of hydrophilic amino function groups of PANI chains, the wettability of commercial porous materials (SSMs and PVDF membranes) becomes
more
hydrophilic,
even
superhydrophilic
and
underwater
superoleophobic. The measurements of WCAs and underwater OCAs were used to systematically assess the wettability of original and PANI-modified SSMs and PVDF membranes. Fig. S6a exhibits the underwater OCAs for a series of oils on the pristine and PANI-modified SSMs. The underwater OCAs of
the
pristine
SSMs
are
approximately
130°,
showing
underwater
oleophobicity. As expected, the hydrophobic SSMs with a WCA of about 100° become superhydrophilic and underwater superoleophobic after coating with PANI nanofibers. The modified SSMs have a WCA of 0° and the underwater OCAs of above 150° (Fig. S6b). Meanwhile, to examine the oil adhesion characteristics in water, we took an about 5 µL oil droplet (dichloroethane) to contact the SSM surface with PANI coatings. The oil droplet is distorted under pressure when it contacts with the SSM surface. After lifting up, the oil droplet can easily detach from the SSM surface without any residues and deformation (Fig. S6c). Furthermore, an oil droplet easily rolls along the inclined SSM surface with PANI coatings (Fig. S6d). The sliding angle is 2.4±0.4° (Fig. 5). In contrast, an oil droplet adheres strongly to the surface of the nascent SSMs, and even is difficult to detach from a face-down surface (Fig. S7). These results indicate that the PANI coatings make the hydrophobic SSMs underwater superoleophobic with low oil-adhesion ability. Fig. S8a shows the WCAs of original PVDF membranes with different pore sizes. All original PVDF membranes are hydrophobic with the WCAs of about 120°, and float on water surface even after pushing it into the water by external force. After the PANI modification, the white and hydrophobic PVDF membranes change to dark green, and can be easily wetted by water and sink into the bottom of water (Fig. S8b and S8c). Moreover, it is observed that a water droplet can fully spread on the surface of PANI-modified PVDF membranes in 10 seconds, showing excellent hydrophilic properties (Fig. S9).
Similar to the PANI-modified SSMs, the superhydrophilic PVDF membranes decorated by PANI coatings also show underwater superoleophobicity with the OCAs of above 150° and low oil-adhesion ability (Fig. 5). The sliding angles are 3.7±1.2°, 4.9±0.8°, 4.9±1.1°, 4.8±0.7°, and 5.7±0.2° for 2, 1.2, 0.8, 0.45, and 0.22 µm pore sizes, respectively. The underwater superoleophobic and low oil-adhesion characteristics of the PANI-modified SSMs and PVDF membranes with different pore sizes (5-0.22 µm) offer the possibility of realizing oil and water separation, especially for oil-in-water emulsions.
Fig. 5. Contact angle and sliding angle (1,2-dichloroethane) of PANI-coated SSMs and PVDF membranes with different pore sizes.
3.3. The separation of non-surfactant emulsions using PANI-coated SSMs Low cost, high flux, energy saving, good selectivity, excellent antifouling and stability are desired objectives of pressure-driven filtration membranes for oily wastewater treatment. Generally, the selection of filtration membranes with larger pore size and the use of a higher pressure difference bring about a higher flux, but could result in lower separation efficiency. In order to solve this
dilemma, underwater superoleophobic filtration membranes can be used in terms of their pore size to separate various types of oil-water mixtures under an applicable external pressure. Hence, we systematically studied the effect of pore size and pressure difference on the separation ability of the prepared SSMs and PVDF membranes with hydrophilic PANI coatings. To investigate this structure-performance relationship, we prepared different oil-water mixtures, such as non-emulsified oil and water mixtures, and non-surfactant and surfactant-stabilized oil-in-water emulsions. Hexane, petroleum ether and hexadecane were selected to prepare different oily wastewaters, in which Tween 80 and SDS as surfactants were used to form stable emulsions. The as-prepared oil-water mixtures were poured onto the surface of PANI-coated SSMs and PVDF membranes to perform oil-water separation under the drive of different transmembrane pressures (0.1-0.6 bar) or gravity only. In the past five years, commercial metallic meshes (300 and 400 mesh sizes) have been widely used to prepare underwater superoleophobic films for the separation of non-emulsified oil and water mixtures under gravity only. For such oily wastewater, the water permeating flux does not depend on the type of oils, and is comparable to the pure water flux. For example, water fluxes through zeolite-coated SSMs with about 10 and 20 µm pore sizes were 36000 and 108000 L m-2 h-1, respectively [11]. In our group, water fluxes through graphene oxide- and PANI-coated SSMs (300 and 400 mesh sizes) were in the range of 720-3024000 L m-2 h-1 [12,18]. However, all obtained SSMs (300 and
400 mesh sizes) cannot separate non-surfactant emulsions, possibly owing to the large pore size of selected commercial metallic meshes. Based on this, Dutch weave SSMs (2300 mesh size) were used to be decorated by PANI coatings.
Similarly,
oils
cannot
permeate
through
the
pre-wetted
PANI-modified SSMs by gravity only due to their underwater superoleophobic properties. On the contrary, water quickly permeates with a flux of 16652±1120 L m-2 h-1. Importantly, all non-surfactant emulsions (including hexane, petroleum ether and hexadecane) can be separated with high fluxes of more than 1000 L m-2 h-1 solely by gravity utilizing the PANI-modified SSMs with 5 µm pore size (Fig. 6). In contrast, a water flux of about 500 L m -2 h-1 was obtained by separating non-surfactant emulsions using a 500 mesh membrane with Cu(OH)2 nanowire-haired coatings, in which the pore size was less than 1 µm [33]. Note that this Cu(OH)2 nanowire coated meshes were only suitable for use under gravity. To examine oil rejection of the PANI-modified SSMs, the original non-surfactant emulsions and the collected filtrates were observed by digital camera and optical microscope (Fig. S10). It is clearly seen that all oil-in-water emulsions are milky white turbid mixtures with abundant oil droplets that are micrometer and sub-micrometer size. After oil-water separation, the collected filtrates are clear and transparent, and no oil droplets are observed. Meanwhile, COD was measured to analyze the oil content in the collected filtrates. As shown in Fig. 6, all oil contents in the collected filtrates are below 40 mg/L,
demonstrating that the non-surfactant emulsions have been successfully separated with high efficiency. In addition, the PANI-modified SSMs cannot separate the non-surfactant emulsions just by utilizing a very low pressure difference (such as less than 0.1 bar), and cannot realize the effective separation of surfactant-stabilized emulsions due to their smaller drop size. Therefore, the commercial PVDF membranes with smaller pore size were chosen as the candidate to achieve the separation of different surfactant-stabilized emulsions.
Fig. 6. Water fluxes and COD in the collected filtrates of non-surfactant emulsions using PANI-modified SSMs under gravity.
3.4. The separation of surfactant-stabilized emulsions using PANI-coated PVDF membranes We tested the water permeability of original PVDF membranes by exerting different pressures. The original PVDF membrane with 2 µm pore size prohibits water from penetrating the membrane under 0.1 bar. Further decreasing the pore size down to 0.22 µm, water cannot pass through the membrane even under 0.6 bar due to their intrinsically hydrophobic properties (Fig. S11). In contrast, the PANI coatings not only render all PVDF membranes
to have outstanding hydrophilicity but also greatly enhance the water flux. As shown in Fig. 7a, under gravity only, the pure water fluxes of the PANI-modified PVDF membranes with 2, 1.2, 0.8, 0.45, and 0.22 µm pore sizes are 2128±19, 1560±37, 595±130, 238±37, and 119±19 L m -2 h-1, respectively. Membranes with larger pore size allow higher water flux. In other words, under gravity only, the superhydrophilic filtration membranes with larger pore size, such as the PANI-coated SSMs, are suitable for the separation of non-emulsified oil-water mixtures and non-surfactant emulsions with a high water flux. If energy saving is no great concern, the use of external pressures greatly enhances pure water fluxes. For example, the pure water fluxes of the PANI-modified PVDF membranes with 0.22 µm pore size under 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 bar are 1771±187, 2881±411, 3833±374, 4692±654, 6476±825, and 7533±934 L m-2 h-1, respectively. More importantly, these superhydrophilic PVDF membranes have good capability of separating surfactant-stabilized emulsions under an applicative pressure difference. For the separation of 0.02 mg/mL Tween 80 stabilized emulsions containing hexane, Fig. 7b shows the dependence of water flux on the applied pressure across the PANI-coated PVDF membranes with different pore sizes. In general, the increase of the applied pressure enhances the water flux, which is obviously lower than the pure water flux. When the applied pressure exceeds the breakthrough pressure, the collected filtrates become more or less turbid. Taking 2 µm pore size as an
example, the PANI-coated PVDF membranes can effectively separate 0.02 mg/mL Tween 80 stabilized emulsions under gravity and 0.1 bar, but cannot under 0.2 bar or higher.
Fig. 7. Fluxes of PANI-modified PVDF membranes with different pore sizes under different external pressures. (a) Pure water; (b) 0.02 mg/mL Tween 80 stabilized hexane-in-water emulsion.
We systematically studied the separation capability of the PANI-coated PVDF membranes with 2 µm pore size for various oil-in-water emulsions under gravity only and 0.1 bar. Tween 80 (0.02 mg/mL) and SDS (0.1 mg/mL) were used to stabilize emulsions, and oils included hexane, petroleum ether and hexadecane (Fig. 8). For the emulsions stabilized by Tween 80, water fluxes of
1718±187, 1427±150, 1850±523 L m -2 h-1 under gravity only and 3833±561, 3304±187, 4493±374 L m -2 h-1 under 0.1 bar for hexane, petroleum ether, and hexadecane, respectively, are obtained. These values are one order of magnitude higher than those of conventional microfiltration and ultrafiltration membranes under high pressure [2-6], and are also superior to those of most superwetting membranes reported (Table S1).
Fig. 8. Fluxes of Tween 80 and SDS stabilized emulsions using PANI-modified PVDF membranes with 2 µm pore size under gravity or 0.1 bar.
When the pore size of PANI-modified PVDF membrane is decreased to 0.22 µm, oils cannot pass through the pre-wetted superhydrophilic PVDF membranes under 0.6 bar. Furthermore, we tested the separation capability of the PANI-coated PVDF membranes with 0.22 µm pore size for various oil-in-water emulsions (Fig. 9). The water fluxes of the emulsions stabilized by Tween 80 are enhanced by increasing the pressure difference, and are 3304±187, 3216±202, 3978±206 L m -2 h-1 under 0.6 bar for hexane, petroleum ether, and hexadecane, respectively. Note that the water fluxes of the emulsions
stabilized by SDS are enhanced but thereafter remain constant, when the applied pressure is increased. Under 0.6 bar, the water fluxes are 3238±280, 2841±280, 1533±150 L m -2 h-1 for hexane, petroleum ether, and hexadecane, respectively, which are lower than those of the emulsions stabilized by Tween 80. These differences are probably due to the electrostatic interaction between the PANI-modified PVDF membranes with positive charges and SDS with opposite charges from the sulfate group. In addition, it is found that high-concentration emulsifier (such as 1 mg/mL Tween 80) has a great impact on the separation capability of the PANI-coated PVDF membranes. For example, the PANI-coated PVDF membranes with 2 µm pore size cannot effectively separate the emulsions stabilized by 1 mg/mL Tween 80 under 0.1 bar. Only under gravity, this separation process can be achieved with a water flux of 978 L m-2 h-1, suggesting that lower applied pressures favor the effective separation of the emulsions stabilized by a high-concentration emulsifier. Fig. 10 shows the dependence of the water flux through the PANI-modified PVDF membranes with 0.22 µm pore size on the concentration of Tween 80 under 0.1 bar, and the photographs before and after the separation of the emulsion stabilized by 1 mg/mL Tween 80. Under a low applied pressure (0.1 bar), water selectively passes through the PANI-decorated PVDF membranes with 0.22 µm pore size, and its flux gradually decreases by increasing the concentration of Tween 80.
Fig. 9. Fluxes of PANI-modified PVDF membranes with 0.22 µm pore size under different external pressures. (a) Tween 80 stabilized emulsions; (b) SDS stabilized emulsions.
Fig. 10. Separation of emulsions stabilized by high-concentration emulsifier. (a) Fluxes of hexane-in-water emulsions stabilized by Tween 80 at different concentrations using PANI-modified PVDF membranes with 0.22 µm pore size under 0.1 bar; (b) optical photographs of hexane-in-water emulsion stabilized by 1 mg/mL Tween 80 (left) and the collected filtrates (right).
The high-efficiency separation of the PANI-modified PVDF membranes with different pore sizes can be confirmed by the investigation of the original surfactant-stabilized emulsions and the collected filtrates, as shown in Fig. S12-S14. Moreover, the separation efficiency of different surfactant-stabilized emulsions was also characterized by detecting the COD values in the collected filtrates (Fig. 11). For example, the COD values in the collected filtrates of the emulsions stabilized by 0.02 mg/mL Tween 80 through the PANI-coated PVDF membranes with 2 µm pore size under gravity are 240±14, 298±28, and 320±26
mg/L for hexane, petroleum ether, and hexadecane, respectively. These values reveal the high-efficiency separation of the obtained membranes, in spite of the higher oil content than that obtained for non-surfactant emulsions. The COD values contain the permeated oil and dissolved surfactant. Note that higher applied pressures would lead to the increased COD values.
Fig. 11. COD in the collected filtrates. (a) Tween 80 stabilized emulsions using PANI-modified PVDF membranes with 2 µm pore size under gravity or 0.1 bar; (b) Tween 80 and SDS stabilized emulsions using PANI-modified PVDF membranes with 0.22 µm pore size under 0.6 bar.
3.5. Antifouling and stability of PANI-coated SSMs and PVDF membranes
Fig. 12. Change of water flux and flux recovery and underwater OCA in the separation of oil and water mixtures over five cycles. The oil droplet is 5 µl 1,2-dichloroethane. (a) Non-surfactant emulsions separated by PANI-modified SSMs under gravity; (b) Tween 80 and (c) SDS stabilized emulsions separated by PANI-modified PVDF membranes with 0.22 µm pore size under 0.6 bar.
For all membranes, fouling or plugging by oil droplets is the main problem which decreases the water flux and separation efficiency subsequently. Therefore, the antifouling performance of all membranes is very important for oily wastewater treatment. To test the antifouling properties of PANI-modified SSMs and PVDF membranes, cycling tests were carried out. For each cycle, the PANI-modified filtration membranes were used to continuously separate the emulsions for three times, and then were rinsed with deionized water for next cycle. Fig. 12 shows the variation of water flux during the separation of non-surfactant emulsions, 0.02 mg/mL Tween 80 and 0.1 mg/mL SDS stabilized emulsions. The water flux gradually decreases with separation time in one cycle. After rinsing, the prepared filtration membranes completely recover to the initial flux and still maintain underwater superoleophobic properties without visible morphology variation (Fig. S15-17), demonstrating that the PANI-modified SSMs and PVDF membranes have outstanding antifouling performance. After oil-water separation, we did not observe PANI in the collected filtrates. To examine the stability of the PANI coatings, we designed ultrasonic treatment of PANI-coated PVDF membranes with 0.22 µm pore size in water. After 30 min sonication, the PANI-coated PVDF membranes keep underwater superoleophobicity (Fig. S18) and do not change according to the optical images and SEM observation (Fig. S19). The PANI coatings also have no obvious influence on the mechanical strength of the original PVDF membranes
(Fig. S20). In addition, we immersed the PANI-coated PVDF membranes in various oils (1,2-dichloroethane, hexane and petroleum ether) for 12 hours. It is found that the obtained membranes also show underwater superoleophobicity (Fig. S21), indicating the good stability of the PANI coatings under oils. The stability under harsh conditions is another significant indicator for the practical applications of filtration membranes. Hence, we studied the corrosion resistance of the as-prepared PANI-modified PVDF membranes (such as 0.22 µm pore size) by immersing these membranes into the corrosive solutions including 1 M HCl, saturated NaCl and 1 M NaOH for 12 hours (Fig. 13). It can be seen that the PANI-modified membranes show outstanding stability without any change of the underwater superoleophobic properties after immersion
for
12
hours,
which
is
superior
to
other
polymer
or
polymer/inorganic hybrid filtration membranes. For example, Xu and coworkers reported that polydopamine-coated filtration membranes returned to hydrophobic state after immersing in alkaline aqueous solution (pH = 12) for 12 hours due to the polydopamine degradation [25]. Some inorganic coatings such as CaCO3 and SiO2 will react with the acidic or alkaline solution [22,26,30]. Because of the excellent antifouling and the outstanding stability under harsh corrosion conditions, PANI-coated commercial membranes are promising candidates for the practical oily wastewater treatment. As shown in Fig. S22 and S23, we used PANI-modified PDVF membranes with 0.22 µm pore size to successfully separate 0.1 mg/mL Tween 80 stabilized hexane-in-water
emulsions (containing 0.1 M HCl, 0.5 M NaCl or 0.1 M NaOH) under 0.6 bar, in which the water fluxes were 2088±112, 2260±206, and 2392±56 L m -2 h-1 for 0.1 M HCl, 0.5 M NaCl, and 0.1 M NaOH, respectively.
Fig. 13. Stability of PANI-modified membranes. (a) Photographs of PANI-modified PVDF membranes before and after immersing in 1 M HCl, saturated NaCl solution and 1 M NaOH for 12 hours; (b) OCAs in water of PANI-modified PVDF membranes after immersing in 1 M HCl, saturated NaCl solution and 1 M NaOH for 12 hours. The oil droplet is 5 µl 1,2-dichloroethane.
4. Conclusions
In summary, PANI nanostructures have been coated on the surface of commercial hydrophobic SSMs and PVDF membranes with different pore sizes (5-0.22 µm) by a one-step dilute polymerization at low temperature. The PANI-modified
SSMs
and
PVDF
membranes
exhibit
underwater
superoleophobic properties with small sliding angles. Additionally, we have systematically studied the effect of pore size and pressure difference on oil-water separation ability of the obtained membranes. Taking advantages of firm coating ability, high stability, intrinsic fibrillar nanostructures and hydrophilic properties of PANI, it has been demonstrated that the obtained superhydrophilic membranes can be effectively used for oily wastewater treatment with high water flux and oil rejection. Moreover, the PANI-coated membranes show stable underwater superoleophobic properties after ultrasonic treatment and immersing in oils and various harsh conditions, and a steady water permeation flux after several cycles. Therefore, we suggest that nanostructured conductive polymers with high-content nitrogen possess unique wetting properties and can be promising candidates to construct superwetting porous membranes for the separation of oil-in-water emulsions.
Acknowledgements This work was financially supported by the National Nature Science Foundation of China (no. 21203217 and 11172301) and the “Top Hundred Talents” Program of Chinese Academy of Sciences.
Appendix A. Supplementary Information Supplementary data associated with this article can be found in the online version at
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Graphical abstract
Polyaniline-coated filtration membranes with different pore sizes are used to effectively separate various oil-in-water emulsions with high water flux and oil rejection under gravity or a low pressure difference.
S0021-9797(16)30024-8 http://dx.doi.org/10.1016/j.jcis.2016.01.024 YJCIS 21007
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
10 December 2015 11 January 2016 13 January 2016
Please cite this article as: M. Liu, J. Li, Z. Guo, Polyaniline coated membranes for effective separation of oil-inwater emulsions, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.01.024
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Polyaniline coated membranes for effective separation of oil-in-water emulsions
Mingming Liu a,c, Jing Li a,*, Zhiguang Guo a,b,* a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China b
Ministry of Education Key Laboratory for the Green Preparation and Application of
Functional Materials, Hubei University, Wuhan 430062, People’s Republic of China c
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of
China * Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (Z. Guo).
Abstract: Polyaniline (PANI) decorated commercial filtration membranes, such as stainless steel meshes (SSMs) with 5 µm pore size and polyvinylidene fluoride (PVDF) membranes with 2-0.22 µm pore sizes, were fabricated by a simple one-step dilute polymerization at low temperature. Lots of short PANI nanofibers were firmly and uniformly coated onto the membrane surfaces, forming rough micro- and nanoscale structures and leading to underwater superoleophobicity with low oil-adhesion characteristic. Furthermore, we systematically studied the effect of pore size and pressure difference on oil-water separation ability of the obtained membranes. It was found that the PANI-modified SSMs with 5 µm pore size were suitable for the separation of non-surfactant emulsions with water fluxes of more than 1000 L m-2 h-1 under gravity only. The PANI-modified PVDF membranes were used for the effective separation of surfactant-stabilized emulsions with water fluxes up to 3000 L m-2 h-1 for 2 µm pore size under 0.1 bar or 0.22 µm pore size under 0.6 bar. In addition, the superhydrophilic membranes with PANI coatings were demonstrated for high oil rejection, stable underwater superoleophobic properties after ultrasonic treatment and immersing in oils and various harsh conditions, and high and steady water permeation flux after several cycles.
Keywords:
Polyaniline;
Porous
membranes;
superoleophobic; Oil-in-water emulsion separation
Superhydrophilic;
Underwater
1. Introduction Membrane technologies, especially pressure-driven filtration membranes, are still ceaselessly developed for water treatment applications to satisfy the growing demand for clean water. In the past few decades, various kinds of the pressure-driven filtration membranes have been explored according to their characteristic pore size and relative separation objects. Among them, microfiltration (50-500 nm pore sizes) and ultrafiltration (2-50 nm pore sizes) have been successfully applied for emulsion separation [1]. Generally, the polymeric filtration membranes are intrinsically hydrophobic, which requires a transmembrane pressure of up to several bars to selectively filter water, and leads to a low flux and quick decline due to unavoidable fouling issue. Many surface modification methods have been adopted to reduce membrane fouling, usually attempted by an increase in hydrophilicity [2-6]. However, there are other chemistry parameters rather than hydrophilicity that could affect the membrane fouling [7]. A number of superwetting films have been fabricated to achieve the separation
of
non-emulsified
oil
and
water
mixtures,
including
superhydrophobic or underwater superoleophobic materials, respectively [8-19]. These superwetting films are unsuitable for various emulsions with less than 20 μm droplet size, especially surfactant-stabilized emulsions. To realize emulsion separation by integrating the aforementioned approaches, Jin and coworkers transformed hydrophilic poly(acrylic acid)-grafted polyvinylidene fluoride
(PVDF) membranes to superhydrophilic and underwater superoleophobic surfaces with a rough micro/nanoscale structure by a salt-induced phase inversion [20]. Consequently, underwater superoleophobic membranes with pore sizes less than the oil droplets have been designed and synthesized to deal with emulsified oily wastewater (Table S1) [21-41]. Recently, hydrophilic organic materials have been introduced onto the surface of various commercial porous membranes, providing a strong binding force between hydrophilic coatings and substrates. In order to further enhance the stability and surface roughness of organic coatings, some inorganic materials are widely used to modify the organic coatings. The organic and inorganic materials include for example polydopamine, poly(acrylic acid), poly(sulfobetaine methacrylate), CaCO3, SiO2, and TiO2 [20-32]. Notably, the pressure difference on most of underwater superoleophobic membranes can be less than 1 bar compared to commercial hydrophobic membranes, even down to gravity. In general, filtration membranes with a larger pore size require a lower pressure difference. For example, a 500 mesh membrane with Cu(OH) 2 nanowire-haired coatings can separate oil-in-water emulsions under gravity only [33]. However, with this all-inorganic membrane it is hard to achieve separation of surfactant-stabilized emulsions due to large pore size. Therefore, it is desirable to develop a one-step approach to construct superhydrophilic and underwater superoleophobic membranes with different pore sizes, which can maintain high stability under
various harsh conditions, and effectively treat different emulsified oily wastewaters with high water fluxes under a low pressure difference. In our previous work, we prepared underwater superoleophobic membranes by coating polyaniline (PANI) and polypyrrole (PPy) on the surface of stainless steel meshes (SSMs, 300 and 400 mesh size) to separate non-emulsified oil and water mixtures [18]. After in situ polymerization, PANI and PPy can be strongly coated on the walls of the reaction vessels and the surfaces of hydrophobic SSMs. Because of the rigid structures, conductive polymers show the advantages of inorganic materials, forming stable and rough nanocoatings. Furthermore, it has been demonstrated that PANI nanofibers can be well dispersed in water due to their large number of amino groups and the presence of electric double layers around the PANI chains [18,42,43], resulting in the hydrophilicity and underwater superoleophobicity. We found that the prepared underwater superoleophobic membranes cannot be used to separate emulsions. In order to realize demulsification, we adopted the same oxidation polymerization method to decorate PANI and PPy onto the membrane surfaces with smaller pore size. However, PANI and PPy cannot be uniformly coated on the membrane surfaces and block the small pore channels, which greatly influences the separation efficiency of oil-in-water emulsions. Here we slowed down aniline polymerization to make PANI uniformly and firmly coat onto the surface of commercial hydrophobic materials with different pore sizes, including SSMs (2300 mesh size) and PVDF membranes.
Note that the method is still unsuitable for PPy. After decoration with PANI coatings, the obtained SSMs and PVDF membranes become superhydrophilic and underwater superoleophobic with low oil-adhesion characteristic and small sliding angles. The PANI-modified SSMs with a pore size of about 5 µm can be used to separate non-surfactant emulsions under gravity only. The separation of various emulsions, stabilized by surfactants such as Tween 80 and sodium dodecyl sulfate (SDS), can be effectively achieved by PANI-decorated PVDF membranes with pore sizes ranging from 2 to 0.22 µm employing different external pressures (less than 1 bar). In addition, these PANI-modified filtration membranes are highly stable after ultrasonic treatment and immersing in oils and various harsh conditions, and maintain underwater superoleophobicity and high flux after reusing.
2. Experimental section 2.1. Materials SSMs (2300 mesh) and PVDF membranes (HYXDF, China) with different pore sizes (0.22, 0.45, 0.8, 1.2, 2 µm) are commercially available. Chemical reagents were used as received without further purification.
2.2. Preparation of PANI-modified SSMs and PVDF membranes Aniline (0.02 M) and ammonium persulfate (0.01 M) were dissolved in 10 mL of 1 M perchloric acid solution, respectively. After pre-cooling in an ice bath, the
reaction solution was rapidly mixed under stirring. Meanwhile, commercial SSMs and PVDF membranes were rinsed consecutively by deionized water and ethanol in an ultrasonic cleaner, and dried at 60 °C in an oven. Thereafter, the original SSMs (25 mm × 25 mm) or PVDF membranes (25 mm diameter) were immersed in the above-mentioned reaction solution (20 mL). The aniline polymerization took 12 hours under continuous stirring in an ice bath. Finally, the obtained SSMs and PVDF membranes with PANI coatings were thoroughly washed by deionized water and dried at 60 °C in an oven.
2.3. Characterization All optical images were taken by a digital camera (Sony, DSC-HX200). The surface morphology of membranes was observed on a field emission scanning electron microscope with Au-sputtered specimens (FESEM, JEOL JSM-6701F). The accelerating voltage was 5 kV and the current was 10 μA. The surface chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB250Xi) using Mg-Kα radiation and Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet iS10). Optical microscope images were obtained by OLYMPUS BX51 microscope. Effective water contact angles (WCAs) and underwater effective oil contact angles (OCAs) were measured on a JC20001 contact angle system (Zhongchen digital equipment Co., Ltd. Shanghai, China). The average CA value was obtained by measuring the sample at five different positions. The oil content in the collected filtrate was calculated by measuring
chemical oxygen demand (COD) according to U.S. Environmental Protection Agency method 8000 (HACH, DRB 200). Electrochemical measurements were performed in a three electrode system at room temperature, containing a platinum slice as counter electrode, a saturated calomel electrode as reference electrode and PANI-modified SSMs (25 mm in length and 15 mm in width) as working electrode. The electrolyte was 70 mL of 0.1 M H2SO4. The mechanical properties were measured using an electrical universal material testing machine with a 500 N load cell (SHIMADZU, EZ-Test). The crosshead velocity was kept at 10 mm/min for tensile measurement. The PVDF membranes were cut into 20 mm × 3 mm pieces.
2.4. Preparation of different types of oil-in-water emulsions Non-surfactant oil-in-water emulsions were prepared by mixing oil (including hexane, petroleum ether and hexadecane) and water in a volume ratio of 1:9, and sonicating the mixtures for 1 hour to produce white emulsions. The non-surfactant emulsions were stable for 1 hour. Tween 80 and SDS stabilized oil-in-water emulsions were obtained by mixing oil (including hexane, petroleum ether and hexadecane) and water in a volume ratio of 1:99 with dissolving 0.02~1 mg Tween 80 and 0.1 mg SDS per ml, respectively. Afterward, the mixtures were intensively stirred for 6 hours. All the surfactant-stabilized emulsions were stable for 24 hours.
2.5. Emulsion separation experiments The emulsion separation tests were carried out under gravity or a vacuum driven
filtration system, as shown in Fig. 1. A vacuum pump (Millipore, WP6122050) was used to supply adjustable pressure difference from 0.1 to 0.6 bar. The height of pure water or emulsions kept 10 cm. Water fluxes were determined by calculating the volume of water permeating per unit time using the following equation: flux = V/St, where V is the volume of water permeation, S is the area of the films, and t is the testing time. All measurements were repeated 3-5 times and the results were reproducible with relative errors less than ±5%.
Fig. 1. Photographs of the instrument for the separation of oil-in-water emulsion using PANI-modified SSMs and PVDF membranes. (a, b) Separation instrument; (c) PANI-modified SSMs (up) and PVDF membranes (down).
3. Results and discussion 3.1. Preparation of PANI-coated SSMs and PVDF membranes PANI has intrinsic nanofibrillar morphology due to its rigid chain structures and positive charge rich surface [43,44]. It has been verified that homogeneous nucleation of PANI in dilute solution or at the early stage of aniline
polymerization
preferentially
forms
nanofibers
rather
than
heterogeneous nucleation and growth [43,45-47]. In order to prepare uniform PANI coatings on the surface of hydrophobic membranes with small pore size,
aniline polymerization was performed in dilute solution (0.01 M) and at low temperature (0 °C). This reaction condition can suppress nucleation and growth of PANI in the bulk of the solution at the early stage of aniline polymerization, resulting in competitive nucleation of PANI nanostructures on the surface of porous membranes. As shown in Fig. 1c, PANI has been successfully coated on the surface of commercial SSMs and PVDF membranes. Based on the electrochemical characterisation of PANI-modified SSMs, it is indicated that the PANI coatings are in emeraldine state (Fig. S1).
Fig. 2. SEM images of original and PANI-modified SSMs. (a, b) Original SSMs; (c, d) PANI-modified SSMs.
Commercial 2300 mesh SSMs were firstly selected to prepare underwater superoleophobic filtration membranes. Fig. 2, S2 and S3 show SEM images of the top and cross-section surfaces of original and PANI-modified SSMs. As seen in SEM images, this Dutch weave SSMs are woven using two sizes of wires, showing different weave densities in the warp and weft directions. The
smaller wires (about 22 µm) are woven tightly together, determining the pore size of the prepared filtration membranes (an average of 5 µm). The larger wires (about 36 µm) provide strength to the SSMs beneficial for the practical applications. Under observation at large magnification, the surface of the original SSMs is clean and smooth. After dilute polymerization of aniline at low temperature, the nucleation and growth of PANI result in short nanofibers that uniformly coat the smooth surface of the original SSMs, forming microand nanoscale hierarchical structures. The enlarged roughness has been demonstrated to be one of the crucial chemical parameters for superwetting behavior based on the Cassie model [48]. Furthermore, slowing down aniline polymerization appears to be particularly important to provide enough time for the diffusion of reacting species in the narrower hydrophobic pore channels of commercial PVDF membranes, leading to a uniform PANI nanocoating on their surfaces. In the same manner, micro- and nanoscale rough surfaces constructed by short PANI nanofibers were also obtained by using commercial PVDF membranes with different pore sizes (2, 1.2, 0.8, 0.45, and 0.22 µm) as substrates (Fig. 3 and S4).
Fig. 3. SEM images of original and PANI-modified PVDF membranes. (a, c, e) Original PVDF membranes; (b, d, f) PANI-modified PVDF membranes.
It is well known that not only the surface roughness but also the chemical composition has a great impact on the wettability of substrates. Therefore, XPS and FTIR measurements were used to investigate the chemical composition of SSMs and PVDF membranes before and after PANI modification. Fig. S5 shows the XPS spectra of pristine and PANI-modified SSMs. For the pristine SSMs, the observed XPS peaks are mainly assigned to iron, carbon and oxygen. In contrast, the PANI-modified SSMs display two new peaks at about 400 and 208 eV corresponding to nitrogen of the PANI chains and chlorine of the HClO4, respectively. In addition, the iron-related peak disappears due to the presence of the PANI coatings. For the pristine PVDF membranes, XPS peaks
belong to fluorine, carbon and oxygen, as shown in Fig. 4a. After the PANI modification, the fluorine-related peak vanishes. Similarly, two new peaks attributed to nitrogen and chlorine appear in the XPS curve of the PANI-modified PVDF membranes. Moreover, in the FTIR spectra as shown in Fig. 4b, strong peaks are detected at about 1400 cm-1, 1300-1000 cm-1, and 900-800 cm-1 assigned to C-H, C-F and =C-H bond, respectively. Weak IR signals around 3000 cm-1 due to C-H bonds are also observed. All signals are typical IR characteristic of the PVDF membranes. After PANI nanofibers are coated on the surface of the PVDF membranes, new peaks at around 3240 cm-1, 1563 cm-1, 1487 cm-1, 1294 cm-1, and about 800 cm-1 are observed and ascribed to N-H stretching vibration, benzene rings, C-N stretching vibration, and N-H deformation vibration, respectively. Both XPS and FTIR results demonstrate the successful modification of SSMs and PVDF membranes with PANI coatings. Because of the presence of polar components, such as nitrogen and doping acids, the PANI coatings possess inherent hydrophilicity.
Fig. 4. XPS and FTIR spectra of original and PANI-modified PVDF membranes. (a) XPS; (b) FTIR.
3.2. Wetting properties of PANI-coated SSMs and PVDF membranes Based on the micro- and nanoscale rough surface of PANI coatings and the large amount of hydrophilic amino function groups of PANI chains, the wettability of commercial porous materials (SSMs and PVDF membranes) becomes
more
hydrophilic,
even
superhydrophilic
and
underwater
superoleophobic. The measurements of WCAs and underwater OCAs were used to systematically assess the wettability of original and PANI-modified SSMs and PVDF membranes. Fig. S6a exhibits the underwater OCAs for a series of oils on the pristine and PANI-modified SSMs. The underwater OCAs of
the
pristine
SSMs
are
approximately
130°,
showing
underwater
oleophobicity. As expected, the hydrophobic SSMs with a WCA of about 100° become superhydrophilic and underwater superoleophobic after coating with PANI nanofibers. The modified SSMs have a WCA of 0° and the underwater OCAs of above 150° (Fig. S6b). Meanwhile, to examine the oil adhesion characteristics in water, we took an about 5 µL oil droplet (dichloroethane) to contact the SSM surface with PANI coatings. The oil droplet is distorted under pressure when it contacts with the SSM surface. After lifting up, the oil droplet can easily detach from the SSM surface without any residues and deformation (Fig. S6c). Furthermore, an oil droplet easily rolls along the inclined SSM surface with PANI coatings (Fig. S6d). The sliding angle is 2.4±0.4° (Fig. 5). In contrast, an oil droplet adheres strongly to the surface of the nascent SSMs, and even is difficult to detach from a face-down surface (Fig. S7). These results indicate that the PANI coatings make the hydrophobic SSMs underwater superoleophobic with low oil-adhesion ability. Fig. S8a shows the WCAs of original PVDF membranes with different pore sizes. All original PVDF membranes are hydrophobic with the WCAs of about 120°, and float on water surface even after pushing it into the water by external force. After the PANI modification, the white and hydrophobic PVDF membranes change to dark green, and can be easily wetted by water and sink into the bottom of water (Fig. S8b and S8c). Moreover, it is observed that a water droplet can fully spread on the surface of PANI-modified PVDF membranes in 10 seconds, showing excellent hydrophilic properties (Fig. S9).
Similar to the PANI-modified SSMs, the superhydrophilic PVDF membranes decorated by PANI coatings also show underwater superoleophobicity with the OCAs of above 150° and low oil-adhesion ability (Fig. 5). The sliding angles are 3.7±1.2°, 4.9±0.8°, 4.9±1.1°, 4.8±0.7°, and 5.7±0.2° for 2, 1.2, 0.8, 0.45, and 0.22 µm pore sizes, respectively. The underwater superoleophobic and low oil-adhesion characteristics of the PANI-modified SSMs and PVDF membranes with different pore sizes (5-0.22 µm) offer the possibility of realizing oil and water separation, especially for oil-in-water emulsions.
Fig. 5. Contact angle and sliding angle (1,2-dichloroethane) of PANI-coated SSMs and PVDF membranes with different pore sizes.
3.3. The separation of non-surfactant emulsions using PANI-coated SSMs Low cost, high flux, energy saving, good selectivity, excellent antifouling and stability are desired objectives of pressure-driven filtration membranes for oily wastewater treatment. Generally, the selection of filtration membranes with larger pore size and the use of a higher pressure difference bring about a higher flux, but could result in lower separation efficiency. In order to solve this
dilemma, underwater superoleophobic filtration membranes can be used in terms of their pore size to separate various types of oil-water mixtures under an applicable external pressure. Hence, we systematically studied the effect of pore size and pressure difference on the separation ability of the prepared SSMs and PVDF membranes with hydrophilic PANI coatings. To investigate this structure-performance relationship, we prepared different oil-water mixtures, such as non-emulsified oil and water mixtures, and non-surfactant and surfactant-stabilized oil-in-water emulsions. Hexane, petroleum ether and hexadecane were selected to prepare different oily wastewaters, in which Tween 80 and SDS as surfactants were used to form stable emulsions. The as-prepared oil-water mixtures were poured onto the surface of PANI-coated SSMs and PVDF membranes to perform oil-water separation under the drive of different transmembrane pressures (0.1-0.6 bar) or gravity only. In the past five years, commercial metallic meshes (300 and 400 mesh sizes) have been widely used to prepare underwater superoleophobic films for the separation of non-emulsified oil and water mixtures under gravity only. For such oily wastewater, the water permeating flux does not depend on the type of oils, and is comparable to the pure water flux. For example, water fluxes through zeolite-coated SSMs with about 10 and 20 µm pore sizes were 36000 and 108000 L m-2 h-1, respectively [11]. In our group, water fluxes through graphene oxide- and PANI-coated SSMs (300 and 400 mesh sizes) were in the range of 720-3024000 L m-2 h-1 [12,18]. However, all obtained SSMs (300 and
400 mesh sizes) cannot separate non-surfactant emulsions, possibly owing to the large pore size of selected commercial metallic meshes. Based on this, Dutch weave SSMs (2300 mesh size) were used to be decorated by PANI coatings.
Similarly,
oils
cannot
permeate
through
the
pre-wetted
PANI-modified SSMs by gravity only due to their underwater superoleophobic properties. On the contrary, water quickly permeates with a flux of 16652±1120 L m-2 h-1. Importantly, all non-surfactant emulsions (including hexane, petroleum ether and hexadecane) can be separated with high fluxes of more than 1000 L m-2 h-1 solely by gravity utilizing the PANI-modified SSMs with 5 µm pore size (Fig. 6). In contrast, a water flux of about 500 L m -2 h-1 was obtained by separating non-surfactant emulsions using a 500 mesh membrane with Cu(OH)2 nanowire-haired coatings, in which the pore size was less than 1 µm [33]. Note that this Cu(OH)2 nanowire coated meshes were only suitable for use under gravity. To examine oil rejection of the PANI-modified SSMs, the original non-surfactant emulsions and the collected filtrates were observed by digital camera and optical microscope (Fig. S10). It is clearly seen that all oil-in-water emulsions are milky white turbid mixtures with abundant oil droplets that are micrometer and sub-micrometer size. After oil-water separation, the collected filtrates are clear and transparent, and no oil droplets are observed. Meanwhile, COD was measured to analyze the oil content in the collected filtrates. As shown in Fig. 6, all oil contents in the collected filtrates are below 40 mg/L,
demonstrating that the non-surfactant emulsions have been successfully separated with high efficiency. In addition, the PANI-modified SSMs cannot separate the non-surfactant emulsions just by utilizing a very low pressure difference (such as less than 0.1 bar), and cannot realize the effective separation of surfactant-stabilized emulsions due to their smaller drop size. Therefore, the commercial PVDF membranes with smaller pore size were chosen as the candidate to achieve the separation of different surfactant-stabilized emulsions.
Fig. 6. Water fluxes and COD in the collected filtrates of non-surfactant emulsions using PANI-modified SSMs under gravity.
3.4. The separation of surfactant-stabilized emulsions using PANI-coated PVDF membranes We tested the water permeability of original PVDF membranes by exerting different pressures. The original PVDF membrane with 2 µm pore size prohibits water from penetrating the membrane under 0.1 bar. Further decreasing the pore size down to 0.22 µm, water cannot pass through the membrane even under 0.6 bar due to their intrinsically hydrophobic properties (Fig. S11). In contrast, the PANI coatings not only render all PVDF membranes
to have outstanding hydrophilicity but also greatly enhance the water flux. As shown in Fig. 7a, under gravity only, the pure water fluxes of the PANI-modified PVDF membranes with 2, 1.2, 0.8, 0.45, and 0.22 µm pore sizes are 2128±19, 1560±37, 595±130, 238±37, and 119±19 L m -2 h-1, respectively. Membranes with larger pore size allow higher water flux. In other words, under gravity only, the superhydrophilic filtration membranes with larger pore size, such as the PANI-coated SSMs, are suitable for the separation of non-emulsified oil-water mixtures and non-surfactant emulsions with a high water flux. If energy saving is no great concern, the use of external pressures greatly enhances pure water fluxes. For example, the pure water fluxes of the PANI-modified PVDF membranes with 0.22 µm pore size under 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 bar are 1771±187, 2881±411, 3833±374, 4692±654, 6476±825, and 7533±934 L m-2 h-1, respectively. More importantly, these superhydrophilic PVDF membranes have good capability of separating surfactant-stabilized emulsions under an applicative pressure difference. For the separation of 0.02 mg/mL Tween 80 stabilized emulsions containing hexane, Fig. 7b shows the dependence of water flux on the applied pressure across the PANI-coated PVDF membranes with different pore sizes. In general, the increase of the applied pressure enhances the water flux, which is obviously lower than the pure water flux. When the applied pressure exceeds the breakthrough pressure, the collected filtrates become more or less turbid. Taking 2 µm pore size as an
example, the PANI-coated PVDF membranes can effectively separate 0.02 mg/mL Tween 80 stabilized emulsions under gravity and 0.1 bar, but cannot under 0.2 bar or higher.
Fig. 7. Fluxes of PANI-modified PVDF membranes with different pore sizes under different external pressures. (a) Pure water; (b) 0.02 mg/mL Tween 80 stabilized hexane-in-water emulsion.
We systematically studied the separation capability of the PANI-coated PVDF membranes with 2 µm pore size for various oil-in-water emulsions under gravity only and 0.1 bar. Tween 80 (0.02 mg/mL) and SDS (0.1 mg/mL) were used to stabilize emulsions, and oils included hexane, petroleum ether and hexadecane (Fig. 8). For the emulsions stabilized by Tween 80, water fluxes of
1718±187, 1427±150, 1850±523 L m -2 h-1 under gravity only and 3833±561, 3304±187, 4493±374 L m -2 h-1 under 0.1 bar for hexane, petroleum ether, and hexadecane, respectively, are obtained. These values are one order of magnitude higher than those of conventional microfiltration and ultrafiltration membranes under high pressure [2-6], and are also superior to those of most superwetting membranes reported (Table S1).
Fig. 8. Fluxes of Tween 80 and SDS stabilized emulsions using PANI-modified PVDF membranes with 2 µm pore size under gravity or 0.1 bar.
When the pore size of PANI-modified PVDF membrane is decreased to 0.22 µm, oils cannot pass through the pre-wetted superhydrophilic PVDF membranes under 0.6 bar. Furthermore, we tested the separation capability of the PANI-coated PVDF membranes with 0.22 µm pore size for various oil-in-water emulsions (Fig. 9). The water fluxes of the emulsions stabilized by Tween 80 are enhanced by increasing the pressure difference, and are 3304±187, 3216±202, 3978±206 L m -2 h-1 under 0.6 bar for hexane, petroleum ether, and hexadecane, respectively. Note that the water fluxes of the emulsions
stabilized by SDS are enhanced but thereafter remain constant, when the applied pressure is increased. Under 0.6 bar, the water fluxes are 3238±280, 2841±280, 1533±150 L m -2 h-1 for hexane, petroleum ether, and hexadecane, respectively, which are lower than those of the emulsions stabilized by Tween 80. These differences are probably due to the electrostatic interaction between the PANI-modified PVDF membranes with positive charges and SDS with opposite charges from the sulfate group. In addition, it is found that high-concentration emulsifier (such as 1 mg/mL Tween 80) has a great impact on the separation capability of the PANI-coated PVDF membranes. For example, the PANI-coated PVDF membranes with 2 µm pore size cannot effectively separate the emulsions stabilized by 1 mg/mL Tween 80 under 0.1 bar. Only under gravity, this separation process can be achieved with a water flux of 978 L m-2 h-1, suggesting that lower applied pressures favor the effective separation of the emulsions stabilized by a high-concentration emulsifier. Fig. 10 shows the dependence of the water flux through the PANI-modified PVDF membranes with 0.22 µm pore size on the concentration of Tween 80 under 0.1 bar, and the photographs before and after the separation of the emulsion stabilized by 1 mg/mL Tween 80. Under a low applied pressure (0.1 bar), water selectively passes through the PANI-decorated PVDF membranes with 0.22 µm pore size, and its flux gradually decreases by increasing the concentration of Tween 80.
Fig. 9. Fluxes of PANI-modified PVDF membranes with 0.22 µm pore size under different external pressures. (a) Tween 80 stabilized emulsions; (b) SDS stabilized emulsions.
Fig. 10. Separation of emulsions stabilized by high-concentration emulsifier. (a) Fluxes of hexane-in-water emulsions stabilized by Tween 80 at different concentrations using PANI-modified PVDF membranes with 0.22 µm pore size under 0.1 bar; (b) optical photographs of hexane-in-water emulsion stabilized by 1 mg/mL Tween 80 (left) and the collected filtrates (right).
The high-efficiency separation of the PANI-modified PVDF membranes with different pore sizes can be confirmed by the investigation of the original surfactant-stabilized emulsions and the collected filtrates, as shown in Fig. S12-S14. Moreover, the separation efficiency of different surfactant-stabilized emulsions was also characterized by detecting the COD values in the collected filtrates (Fig. 11). For example, the COD values in the collected filtrates of the emulsions stabilized by 0.02 mg/mL Tween 80 through the PANI-coated PVDF membranes with 2 µm pore size under gravity are 240±14, 298±28, and 320±26
mg/L for hexane, petroleum ether, and hexadecane, respectively. These values reveal the high-efficiency separation of the obtained membranes, in spite of the higher oil content than that obtained for non-surfactant emulsions. The COD values contain the permeated oil and dissolved surfactant. Note that higher applied pressures would lead to the increased COD values.
Fig. 11. COD in the collected filtrates. (a) Tween 80 stabilized emulsions using PANI-modified PVDF membranes with 2 µm pore size under gravity or 0.1 bar; (b) Tween 80 and SDS stabilized emulsions using PANI-modified PVDF membranes with 0.22 µm pore size under 0.6 bar.
3.5. Antifouling and stability of PANI-coated SSMs and PVDF membranes
Fig. 12. Change of water flux and flux recovery and underwater OCA in the separation of oil and water mixtures over five cycles. The oil droplet is 5 µl 1,2-dichloroethane. (a) Non-surfactant emulsions separated by PANI-modified SSMs under gravity; (b) Tween 80 and (c) SDS stabilized emulsions separated by PANI-modified PVDF membranes with 0.22 µm pore size under 0.6 bar.
For all membranes, fouling or plugging by oil droplets is the main problem which decreases the water flux and separation efficiency subsequently. Therefore, the antifouling performance of all membranes is very important for oily wastewater treatment. To test the antifouling properties of PANI-modified SSMs and PVDF membranes, cycling tests were carried out. For each cycle, the PANI-modified filtration membranes were used to continuously separate the emulsions for three times, and then were rinsed with deionized water for next cycle. Fig. 12 shows the variation of water flux during the separation of non-surfactant emulsions, 0.02 mg/mL Tween 80 and 0.1 mg/mL SDS stabilized emulsions. The water flux gradually decreases with separation time in one cycle. After rinsing, the prepared filtration membranes completely recover to the initial flux and still maintain underwater superoleophobic properties without visible morphology variation (Fig. S15-17), demonstrating that the PANI-modified SSMs and PVDF membranes have outstanding antifouling performance. After oil-water separation, we did not observe PANI in the collected filtrates. To examine the stability of the PANI coatings, we designed ultrasonic treatment of PANI-coated PVDF membranes with 0.22 µm pore size in water. After 30 min sonication, the PANI-coated PVDF membranes keep underwater superoleophobicity (Fig. S18) and do not change according to the optical images and SEM observation (Fig. S19). The PANI coatings also have no obvious influence on the mechanical strength of the original PVDF membranes
(Fig. S20). In addition, we immersed the PANI-coated PVDF membranes in various oils (1,2-dichloroethane, hexane and petroleum ether) for 12 hours. It is found that the obtained membranes also show underwater superoleophobicity (Fig. S21), indicating the good stability of the PANI coatings under oils. The stability under harsh conditions is another significant indicator for the practical applications of filtration membranes. Hence, we studied the corrosion resistance of the as-prepared PANI-modified PVDF membranes (such as 0.22 µm pore size) by immersing these membranes into the corrosive solutions including 1 M HCl, saturated NaCl and 1 M NaOH for 12 hours (Fig. 13). It can be seen that the PANI-modified membranes show outstanding stability without any change of the underwater superoleophobic properties after immersion
for
12
hours,
which
is
superior
to
other
polymer
or
polymer/inorganic hybrid filtration membranes. For example, Xu and coworkers reported that polydopamine-coated filtration membranes returned to hydrophobic state after immersing in alkaline aqueous solution (pH = 12) for 12 hours due to the polydopamine degradation [25]. Some inorganic coatings such as CaCO3 and SiO2 will react with the acidic or alkaline solution [22,26,30]. Because of the excellent antifouling and the outstanding stability under harsh corrosion conditions, PANI-coated commercial membranes are promising candidates for the practical oily wastewater treatment. As shown in Fig. S22 and S23, we used PANI-modified PDVF membranes with 0.22 µm pore size to successfully separate 0.1 mg/mL Tween 80 stabilized hexane-in-water
emulsions (containing 0.1 M HCl, 0.5 M NaCl or 0.1 M NaOH) under 0.6 bar, in which the water fluxes were 2088±112, 2260±206, and 2392±56 L m -2 h-1 for 0.1 M HCl, 0.5 M NaCl, and 0.1 M NaOH, respectively.
Fig. 13. Stability of PANI-modified membranes. (a) Photographs of PANI-modified PVDF membranes before and after immersing in 1 M HCl, saturated NaCl solution and 1 M NaOH for 12 hours; (b) OCAs in water of PANI-modified PVDF membranes after immersing in 1 M HCl, saturated NaCl solution and 1 M NaOH for 12 hours. The oil droplet is 5 µl 1,2-dichloroethane.
4. Conclusions
In summary, PANI nanostructures have been coated on the surface of commercial hydrophobic SSMs and PVDF membranes with different pore sizes (5-0.22 µm) by a one-step dilute polymerization at low temperature. The PANI-modified
SSMs
and
PVDF
membranes
exhibit
underwater
superoleophobic properties with small sliding angles. Additionally, we have systematically studied the effect of pore size and pressure difference on oil-water separation ability of the obtained membranes. Taking advantages of firm coating ability, high stability, intrinsic fibrillar nanostructures and hydrophilic properties of PANI, it has been demonstrated that the obtained superhydrophilic membranes can be effectively used for oily wastewater treatment with high water flux and oil rejection. Moreover, the PANI-coated membranes show stable underwater superoleophobic properties after ultrasonic treatment and immersing in oils and various harsh conditions, and a steady water permeation flux after several cycles. Therefore, we suggest that nanostructured conductive polymers with high-content nitrogen possess unique wetting properties and can be promising candidates to construct superwetting porous membranes for the separation of oil-in-water emulsions.
Acknowledgements This work was financially supported by the National Nature Science Foundation of China (no. 21203217 and 11172301) and the “Top Hundred Talents” Program of Chinese Academy of Sciences.
Appendix A. Supplementary Information Supplementary data associated with this article can be found in the online version at
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Graphical abstract
Polyaniline-coated filtration membranes with different pore sizes are used to effectively separate various oil-in-water emulsions with high water flux and oil rejection under gravity or a low pressure difference.