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Fabrication of palygorskite coated membrane for multifunctional oil-in-water emulsions separation
Applied Clay Science 182 (2019) 105295
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Research paper
Fabrication of palygorskite coated membrane for multifunctional oil-inwater emulsions separation
T
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Yueyue Yang, Weidong Liang , Chengjun Wang, Hanxue Sun, Jianli Zhang, Yuan Yu, ⁎ Wenrui Dong, Zhaoqi Zhu, An Li Department of Chemical Engineering, College of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Palygorskite Underwater superoleophobic Oil/water emulsion separation Multifunctional membrane
With the rapid development of industrial growth, the issue of severe water pollution risen from oily waste water, dye waste water, and frequent oil spill accidents has become a bottleneck for hindering the global sustainable development. Herein, we report the facile fabrication of a superhydrophilic/underwater superoleophobic multifunctional separation membrane by vacuum assisted filtration technology using palygorskite (Pal) as raw material. The Pal coated membrane show excellent thermal， chemical stability, and mechanical strength with 300 bending times. As expected, the membrane has excellent separation performance, e.g. it exhibits an enhanced permeation flux of 477.7 ± 5.0 L m−2 h−1 bar−1 and a high separation efficiency of up to 99.6% ± 0.05%. In addition, it can separate different oil-in-water emulsions in harsh environments. Interestingly, the as-prepared membrane can also remove organic dyes in the water phase during the separation process, making it multifunctional and promising membrane for water treatment, by combination with its simple and cost-efficient manufacture, excellent physicochemical properties. More importantly, our work may also open a new way for high-increment utilization of palygorskite mineral with a “green” method.
1. Introduction Nowadays, frequent oil spill accidents and oily wastewater produced from industrial activity have seriously threaten the ecological environment and human health (Birkhead, 2014; Mu et al., 2018; Yong et al., 2018; Jia et al., 2019). Traditional separation techniques such as air flotation (Al-Shamrani et al., 2002), chemical coagulation (Yan et al., 2019) and adsorption (Wang et al., 2019), have been used for oil/ water separation. There methods, however, have their respective drawbacks such as low efficiency and high operating cost, which dramatically limit the wide application of these oil/water separation technologies. Furthermore, these techniques are either ineffective in separating surfactant-stabilized emulsions, or demulsify the emulsions upon applying an electric field or adding chemicals, resulting in high energy consumption and secondary pollution (Ríos et al., 1998; Ren and Kang, 2018; Cui et al., 2019). In recent years, superwetting materials have offered a good platform for oil/water separation, mainly because of their different wettability to water or oil which result in high selectivity and in turn high separation efficiency (Wu et al., 2017; Zhan et al., 2018). Superhydrophobic/superoleophilic materials were firstly reported for oil/water separation (Feng et al., 2004). Since then, all
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kinds of superhydrophobic/superoleophilic materials, including polymer (Li et al., 2015; Das et al., 2018), sponge (Peng et al., 2018), fabric (Zhou et al., 2013), mesh (Guo et al., 2018), ceramic membranes (Samaei et al., 2018) have been developed. Unfortunately, the density of oil phase is less than that of water phase, a water barrier will form between the separation material and the oil layer, which blocks the separation process (Kang and Cao, 2014; Shahabadi and Brant, 2019). Moreover, superhydrophobic materials usually used fluoroalkylsilane with low surface energy, which were harmful to the ecological environment and human health (Liu et al., 2019). A hydrogel-coated mesh was fabricated for oil/water separation, which possessed superhydrophilic/underwater superoleophobic properties (Xue et al., 2011). Inspired by the study, lots of researchers have developed superhydrophilic/underwater superoleophobic materials, which provide a good choice to separate water rich oil/water mixtures (Zhang et al., 2013; Li et al., 2016; Zhou et al., 2016; Tie et al., 2018). However, most of superhydrophilic/underwater superoleophobic materials can noly achieve the oil/water mixtures separation owing to their large pore size. It is difficult to effectively separate surfactants-stabilized emulsions with particle size < 20 μm. Nowadays, various materials with superhydrophilic/underwater superoleophobic properties have
Corresponding authors. E-mail addresses: [email protected] (W. Liang), [email protected] (A. Li).
https://doi.org/10.1016/j.clay.2019.105295 Received 11 July 2019; Received in revised form 4 September 2019; Accepted 4 September 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.
Applied Clay Science 182 (2019) 105295
Y. Yang, et al.
2.3. Preparation of the underwater superoleophobic Pal coated membrane
successfully separated emulsified oil/water mixtures. For example, Liu et al. reported an asymmetric aerogel membrane by one-pot hydrothermal reaction induced polyvinyl alcohol self-crosslinking (Liu et al., 2018). The aerogel membrane showed excellent properties of air superhydrophilic and underwater superoleophobic. Jiang et al. fabricated an original gelatin-based multifunctional aerogel with superhydrophilic and underwater superoleophobic, and successfully separated the stable oil-in-water emulsions (Jiang et al., 2019). However, it is difficult for these materials to maintain their original wettability in harsh environments. But the actual separation of oil/water often occurs in complex environments, such as acid, alkali and salt solutions. Therefore, the chemical stability of separated materials is particularly important in practical application. (Hu et al., 2015; Qu et al., 2017; Zhang et al., 2018c; Contreras Ortiz et al., 2019). In addition to oil pollution, organic dyes in water can also seriously affect human life and the ecological environment (Puri and Sumana, 2018; Gupta et al., 2019). Since most oily wastewater also contains water-soluble dyes. Therefore, it has become increasingly necessary to remove organic dyes in the process of oil/water separation. However, most separation materials developed at present only have the function of separating emulsified oil/water into two phases, and cannot remove pollutants in the water phase during filtration process. Therefore, it is very important to fabricate a kind of separation membrane with low cost, environmental friendly and multifunctional water treatment characteristics. Palygorskite is a kind of silicate clay containing hydrophilic groups, consisting of hydrated magnesium-aluminosilicate with layered chain structure, which has attracted much attention due to its natural nanochannel structure, inexpensive, abundant reserves and adsorption (Liang et al., 2015; Zhang et al., 2019b; Zhou et al., 2019). In continuation of our previous studies in this field (Li et al., 2011; Liang et al., 2014; Wei et al., 2018; Mu et al., 2019; Zhang et al., 2019),and based on the research of the preparation strategy of the membrane (Cheng et al., 2017), this work, we used low-cost palygorskite clay as raw material to explore a kind of superhydrophilic/underwater superoleophobic multifunctional separation membrane via a simple vacuumassisted filtration of palygorskite clay dispersions. The membrane exhibited excellent separation performance, which could not only realize the separation of emulsified oil/water mixtures in harsh environments, but also remove water-soluble dyes during the separation process, showing great potential for a broad variety of applications for wastewater treatment.
PVA was stirred and dissolved in deionized water at 90 °C to prepare PVA solution 0.5% (W/V). When 0.25 g Pal was dispersed in deionized water, the homogeneous dispersion was obtained by magnetic stirring about 30 min and ultrasonic for half an hour. Then, 2 mL PVA (0.5% W/ V) was added to the Pal dispersed solution. The mixed dispersion was stirred for at least 2 h at room temperature. Subsequently, Pal coated membrane was prepared by vacuum filtration (0.8 bar) and dried at room temperature. 2.4. Stable oil/water emulsions separation To prepare the corresponding oil-in-water emulsions. Tween-80 was firstly added to deionized water in a three-necked flask. Afterwards, oils (kerosene, n-heptane, petroleum ether and diesel) were added to the above solution at a ratio of 1:50 mL/mL, and stirred vigorously for 12 h, until stable white emulsions were formed. The preparation of kerosenein-NaCl, kerosene-in-HCl and kerosene-in-NaOH emulsions were described above. Pal coated membrane of the prewetted by water was placed on the separation device. The above emulsions were slowly poured onto the vacuum filter device. The whole separation process was carried out under the vacuum condition of 0.08 MPa. The obtained filtrate was preserved for further performance testing. The flux (F) for Pal coated membrane was obtained by calculating the volume of water collected per unit time as follows:
F = V /(S × t × ∆P )
(1)
where V is the volume of the filtrate, t is the testing time, S is the effective filtration area of the membrane, ΔP is the suction pressure across the membrane. The separation efficiency (R1) was calculated as:
R1 (%) = (1 − Cp/ C0) × 100
(2)
where Cp and C0 are the concentration of oil in filtrate and the prepared emulsion. 2.5. Stability of Pal coated membrane The stability was evaluated by the mass loss of the membrane (ML) after the sample was exposed to harsh environments for 48 h and different temperatures for 24 h. The calculation formula was as follows:
ML = (mi − mt )/ mi × 100%
(3)
Where mi is the initial mass of the membrane and mt is the remaining mass of the membrane.
2. Experimental 2.1. Materials
2.6. Removal of organic dyes Palygorskite [Mg5Si8O20(OH)2(OH2)4·4H2O] was purchased from Jiangsu Jiuchuan Nanomaterials Technology Co., Ltd. Microfiltration membrane (MF, 0.45 μm) was obtained from Xinya purification Co., Ltd. Poly (vinyl alcohol) (PVA, size 1788) was obtained from Aladdin Industrial Corporation. Hydrochloric acid (HCl) and Sodium hydroxide was supplied by Shanghai Aladdin Biochemical Technology Co., Ltd. Kerosene and diesel were purchased from the local store. Organic solvents (n-heptane, petroleum ether) were provided by Tianjin Fuyu chemical reagent Co., Ltd. Tween-80 was obtained from Laiyang Shuangshuang chemical Co., Ltd. The other chemical reagents were analytical grade and used without further treatment.
Dyes methylene blue (MB) and malachite green (MG) were evaluated the adsorption properties of Pal coated membrane. Oil/water mixtures containing pollutants were prepared with kerosene as the oil phase and dyes with different concentrations as the water phase. The dyes removal rate R2(%) of Pal coated membrane was calculated according to equation:
R2 (%) = (1 − Cm/ Cn ) × 100
(4)
where Cn (mg/L) is the initial concentration of dyes and Cm (mg/L) is the concentration of dyes in filtrate.
2.2. Purification of Pal
2.7. Characterization
For the sake of remove the impurities, the Pal (10 g) was added to HCl (1 M) and stirred vigorously for about 4 h at room temperature, then washed with deionized water until the pH was 7. The residue was dried at 60 °C in the blast drying oven and the purified Pal was obtained.
Fourier transform infrared (FT-IR) spectroscopy was performed using a Nexus 680 spectrum instrument. The microstructure of the asprepared membrane and MF surfaces were characterized by scanning electron microscopy (SEM, JSM-6701F, JEOL, Ltd.). X-ray photoelectron spectroscopy (XPS) analyses were performed using a PHI2
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Fig. 1. Illustration of the preparation of Pal coated membrane and its application of oil-in-water emulsions separation and dyes removal.
underwater superoleophobic property (Zhang et al., 2019a). In addition, pore size distribution of Pal coated membrane and MF was measured by nitrogen sorption using a BET method. As shown in Fig. 2f, the highest peak of Pal coated membrane is mainly concentrated at 47 nm, indicating that it is a mesoporous material (He et al., 2019a, 2019b). The average pore diameter was 103.5 nm, which was much smaller than that of the substrate membrane (average pore diameter 0.45 μm). Therefore, the above the hierarchical micro-nanoscale roughness structure is beneficial for preparing superwetting materials. To verify the valid formation of Pal/PVA composite layer on the surface, the elements of the prepared membrane were tested by XPS method. Fig. 3a shows full-scan spectrum XPS data for the elements of Pal coated membrane and MF. New characteristic signals of Si2s, Si2p and MgKLL elements were observed on Pal coated membrane in contrast with MF. These emerging elements were believed to originate from the typical groups in the palygorskite components. From the narrow scan of C1s XPS spectrum, the peak intensity of C1s decreased after coating, as illustrated in Fig. 3b. The above results indicated that Pal coated membrane was successfully prepared. In order to further characterize the chemical structure of the as-prepared membrane, the functional groups of Pal, PVA and Pal coated membrane were analyzed by FT-IR spectroscopy. As noted in Fig. 3c, the peak of 3600–3400 cm−1 was related to the hydroxyl groups of PVA, Pal and Pal coated membrane. The peak at 1655 cm−1 could be assigned to the bend vibration of H2O in Pal and Pal coated membrane (Yahia et al., 2019). The 1741 cm−1 absorption peak of PVA at was attributed to C]O (Shi et al., 2016). The peak at 2927 cm−1 was associated with the stretching of –CH of PVA. The 1256 cm−1 absorption peak of PVA was associated with the asymmetric stretching vibration of the C-O-C bond (Wang et al., 2017). The characteristic peak at 1032 cm−1 belongs to Si-O of Pal. At the same time, Si-O also appears in Pal coated membrane. And the FTIR spectrum of Pal coated membrane is basically the same as Pal. Therefore, Pal coated membrane is mainly composed of Pal. Above results indicated that Pal coated membrane had a structure of amphiphilic groups, showing amphiphilic properties (Long et al., 2018). To evaluate the wetting behavior of Pal coated membrane towards water and oil by CAs method. As highlighted in Fig. 4a and b, the asprepared Pal coated membrane shows superamphiphilic behavior in air. In other words, when water and oil droplets contacted with the surface of Pal coated membrane, the water and oil droplets instantaneously spread out and CAs were about 0°. However, the as-prepared Pal coated membrane exhibited superoleophobicity in underwater. When the droplet of kerosene was dropped on the surface of the membrane underwater, the oil CA was 153.1° ± 3.0°, which was displayed in Fig. 4c. To further verify the wetting behavior of Pal coated membrane in underwater, different types of oils were tested: n-heptane, petroleum ether, and diesel (Fig. 4d). All these oils exhibited nearly stable
5300ESCA spectrometer (Perkin-Elmer). The contact angles (CAs) and sliding angles (SAs) were measured using a SL200KB apparatus. The oil content in the filtrate was obtained using an infrared spectrometer oil content analyzer (OIL 480). The pore size distribution of the sample were measured by N2 adsorption and desorption at 120 °C using a volumetric sorption analyzer (micromeritics ASAP 2020). Droplet sizes of emulsions were measured using a Malvern Zetasizer Nano ZS (Malvern, UK). The microscopic image of the emulsion was taken on an optical microscope. The rough surface structure of the samples was acquired Atomic force microscopy (AFM) under ambient conditions. The absorbance of dyes was determined by using a UV–vis spectrophotometer (7230G) from Shanghai Precision Scientific Instruments Co., Ltd. The tensile strength of the membrane was tested by a universal material testing machine at an elongation rate of 2 mm min−1 from French A.D company. 3. Result and discussion Fig. 1 shows the preparation steps of Pal coated membrane and its application of oil-in-water emulsions separation and dyes removal. Pal coated membrane was prepared by filtering the mixed dispersion of PVA and Pal onto microfiltration membrane substrate. Among them, the existence of PVA can enhance the strength between the coating and the substrate membrane. At the same time, the addition of PVA is beneficial to the realization of membrane formation of palygorskite. The further experimental results and discussion were provided (Fig. S1). It is obvious that the surface of Pal coated membrane without PVA dropped off obviously, while the surface of Pal coated membrane with PVA remained intact. Pal coated membrane can not noly efficiently separate a variety of oil-in-water emulsions under harsh conditions, but also remove water-soluble dyes. The morphologies of MF and Pal coated membrane were characterized by scanning electron microscopy (SEM). As illustrated in Fig. 2, MF has a relatively smooth surface with an average pore diameter of 0.45 μm (Fig. 2a). After being coated with the mixed dispersion, due to the random distribution of Pal, rough structure was formed on the surface of Pal coated membrane. Fig. 2b illustrates the magnified image of Pal coated membrane, which can more intuitively show many micro-nanostructures. Pal coated membrane surface has the hierarchical micro-nanoscale roughness, which is benefit to the acquisition of underwater superoleophobicity. The cross section of Pal coated membrane was also tested (Fig. 2c), and the thickness of the Pal layer was approximated 74.44 μm. Meanwhile, the surface roughness feature of MF and Pal coated membrane were measured (Fig. 2d and e). It was revealed that the surface roughness also increased from 232.7 nm (MF) to 351.0 nm (Pal coated membrane) after leaching. The increase of surface roughness makes it easier for the material to realize the 3
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Fig. 2. FE-SEM images of (a) MF and (b) Pal coated membrane (magnified image of Pal coated membrane), respectively. (c) Cross-sectional image of Pal coated membrane. AFM images of (d) MF and (e) Pal coated membrane. (f) Pore size distribution of MF and Pal coated membrane.
separation, the filtrate collected was completely transparent compared with the milky white kerosene-in-water emulsion. In sharp contrast, when MF was used as a separation material, the filtrate after separation was still translucent (Fig. 5c), showing that MF is not effective for separating the surfactant-stabilized oil-in-water emulsions. Moreover, there was a visible difference between the microscopic images of the emulsion and the filtrate collected, as demonstrated by the optical microscopy images (Fig. 6a and c). A large number of oil droplets were distributed in the emulsion before separation while no kerosene droplets were seen in the filtrate collected. It showed that Pal coated membrane could be effectively separated kerosene-in-water emulsion. The reason is that Pal coated membrane has superhydrophilic/underwater superoleophobic properties and suitable pore size, which can effectively prevent oil droplets in emulsion from passing through the membrane. Furthermore, the droplet size distributions of the kerosenein-water emulsion and collected filtrate were analyzed as illustrated in Fig. 6a1 and c1. The droplet size of the kerosene-in-water emulsion ranged from 200 nm to 800 nm. However, no droplets were found in the filtrate within these ranges, and the droplet sizes of the filtrates for various oil-in-water emulsions were lower than the instrument threshold, revealing excellent emulsion separation effect. And Pal coated membrane can also separate other kinds of oil-in-water emulsions, including n-heptane-in-water, petroleum ether-in-water and
spherical shapes with OCAs > 150°, revealing significant superoleophobicity. Meanwhile, corresponding SAs of various oils (kerosene, n-heptane, petroleum ether and diesel) underwater were evaluated, as demonstrated in Fig. 4d. The result clearly showed that the SAs were lower than 5°. Pal coated membrane was shown a lower surface adhesion, which can effectively prevent the as-prepared membrane to be polluted or blocked up during oil/water separation process (Li et al., 2017). This special wettability is due to the synergistic effect of the hydrophilicity of Pal and the rough micro-nanoscale structure on the surface of Pal coated membrane, Pal coated membrane would be ideal candidates for oil-in-water emulsions separation. Based on this special wettability and suitable pore size, Pal coated membrane would be naturally an ideal material in the treatment of oily wastewater. The stability of various emulsions was tested (Fig. 5a). A week later, the prepared oil/water emulsions remained stable. The emulsion separation system is shown in Fig. 5b. The kerosene-in-water emulsion as a typical sample was researched. During the separation process, due to the superhydrophilic/underwater superoleophobic property of Pal coated membrane, the water molecules were absorbed by the membrane, forming a water barrier on the membrane surface to prevent oil from permeating the membrane. And the emulsion separation is realized under the pressure drive. As shown in Fig. 6b, the digital photos kerosene-in-water emulsion before and after separation. After
Fig. 3. (a) XPS full-scan spectrum of MF and Pal coated membrane. (b) High-resolution XPS C1s narrow scans as a functional electron binding energy. (c) FT-IR spectra of Pal, PVA and Pal coated membrane. 4
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Fig. 4. Wettability of Pal coated membrane: (a) water in air (b) oil in air (c) oil in water. (d) CAs and SAs of oil droplets on Pal coated membrane underwater.
Fig. 5. (a) Stability tests of the various types of emulsions. (b) The vacuum driven filtration system. (c) Digital photograph of kerosene-in-water emulsion before and after MF separation.
Fig. 6. (a-l) Optical microscopy images and digital photographs of different oil-in-water emulsions before and after separation. (a1-l1) Size distribution of the oil droplets in various emulsions before and after separation. 5
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Fig. 7. (a) Fluxes for various surfactant-stabilized oil-in-water emulsions. (b) Separation efficiency and oil content of Pal coated membrane for various surfactantstabilized oil-in-water emulsions. (c) The separation efficiencies and fluxes for kerosene-in-water emulsion as a representative of cycle times.
Fig. 8. (a) Mass loss (ML) of Pal coated membrane after acidic or alkaline corrosion for 48 h. (b) Mass loss (ML) of Pal coated membrane under different temperatures. (c) A typical stress-strain curve of a dried Pal coated membrane. (d) Optical images of Pal coated membrane before and after bending 300 times. (e) Schematic view of the devices of the sandpaper abrasion test. (f) Underwater kerosene CA of Pal coated membrane after 100 scratch cycles.
2019). In this work, the separation performance of Pal coated membrane was tested by a series of emulsions. Pal coated membrane shows higher separation fluxes of various oil-in-water emulsions. As exhibited in Fig. 7a, the fluxes for oil-in-water emulsions > 298.6 ± 3.0 L m−2 h−1 bar−1. In order to test the separation efficiency of Pal coated membrane, the oil rejection (R1%) of Pal coated membrane was measured by an infrared spectrophotometric oil analyzer. As Fig. 7b
diesel-in-water emulsions. Similarly, optical microscope images and droplet sizes were measured (shown in Fig. 6d1-l1). The above results showed that Pal coated membrane can be completely successful in separating various oil-in-water emulsions. In order to further explore the separation effect of Pal coated membrane, the separation performances were evaluated by separation flux and efficiency (Zhang et al., 2018a; Zhang et al., 2018b; Zin et al.,
6
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Fig. 9. (a-f) Optical microscopy images and digital photographs of kerosene-in-NaCl, kerosene-in-NaOH and kerosene-in-HCl emulsions before and after separation, respectively. (j, k) Flux and separation efficiency of kerosene-in-NaOH, kerosene-in-HCl and kerosene-in-NaCl emulsions on Pal coated membrane.
shows, the oil content in each filtrate is not > 90 ppm, and the R1 values of each filtrate were > 99.3%. Therefore, Pal coated membrane has excellent separation performance for various oil-in-water emulsions. Moreover, to study the reusability of Pal coated membrane, which testing the cycle flux and efficiency of kerosene-in-water emulsion. The separation efficiency remained above 99.5% after the membrane had been reused for 10 separation cycles in Fig. 7c, confirming that the separation membrane has excellent reusability. To evaluate the stability of the membrane, Pal coated membrane was immersed in different environments. As exhibited in Fig. 8a, in the range of pH 1–11, Pal coated membrane still remained intact and maintained a low mass loss after corrosion for 48 h. At the same time, the wettability of Pal coated membrane under different corrosive environments was tested. Under the conditions of acid, alkali and saturated salt, the underwater CAs of Pal coated membrane were 151.9° ± 2.5°, 150.2° ± 2.0° and 153.6° ± 2.5°, suggesting that Pal coated membrane could overcome acidic, salty, and alkaline environments and maintain stable underwater superoleophobicity. As shown in
Fig. 8b, CAs of the underwater kerosene were still > 150° when the membrane was soaked into aqueous solutions at different temperatures (20-50 °C). The results indicated that Pal coated membrane still maintained the underwater superoleophobicity. And the mass loss of the membrane is very small. In addition, the tensile strength of Pal coated membrane was 1.23 MPa. What's more important, the membrane has good flexibility. As demonstrated in Fig. 8d, there were still no cracks in Pal coated membrane after bending for 300 times. In addition, in order to further study the mechanical property of Pal coated membrane under abrasion, 1500 mesh sandpaper was used for abrasion test and shown in Fig. 8e. Drag Pal coated membrane back and forth on the surface of the sandpaper. Pulling the sandpaper for 10 cm was defined as a cycle. Fig. 8f shows the underwater kerosene CA of after 100 cycles, indicating that after 100 cycles, the underwater kerosene CA is still > 150°. The above results show that Pal coated membrane not only has excellent chemical stability and heat resistance, but also has good the mechanical stability, which provides a great possibility for practical oil/water separation. 7
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Fig. 10. (a) Photographs of filtration device. UV–vis absorption spectra of (b) methylene blue and (c) malachite green solutions. (d) Adsorption efficiency of dyes at different concentrations. (e) Adsorption efficiency of dyes with concentration of 60 mg/L on Pal coated membrane and MF. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
higher removal efficiency than that of MF. The insert of Fig. 10e shows digital photographs of Pal coated membrane and MF after filtering dyes with concentration of 60 mg/L, in which the left part of the insert shows the photo of the dyes filtered by Pal coated membrane, which is darker than the color of the dyes filtered by the MF on the right. It shows that Pal coated membrane has strong dye adsorption capacity, which is due to the fact that the adsorption of Pal coated membrane is mainly controlled by electrostatic force and hydrogen bond. In order to clearly explain the electrostatic adsorption mechanism of Pal coated membrane, the Zeta potential of Pal coated membrane is provided (Fig. S2). It can be seen that the surface of Pal coated membrane shows negative charge and the cationic dye can be adsorbed by electrostatic action. The above results revealed that Pal coated membrane can not only effectively separate the emulsions, but also remove organic dyes in the water, thus achieving multifunctional water treatment.
To further demonstrate the corrosion resistance of Pal coated membrane, surfactant-stabilized corrosive emulsions were separated. The separation result of the kerosene-in-NaCl emulsion was showed in Fig. 9a and c. After the kerosene-in-NaCl emulsion was filtrated by Pal coated membrane, the numerous oil droplets were not observed, and the digital photograph that the milky white emulsion became transparent. In addition, Pal coated membrane can also achieve the separation of kerosene-in-HCl and kerosene-in-NaOH emulsions. The corresponding optical microscope images and digital photographs were displayed (Fig. 9d-f). As shown in Fig. 9j, Pal coated membrane exhibited high fluxes with 382.2 ± 4.0 L m−2 h−1 bar−1, −2 −1 −1 394.1 ± 6.0 L m h bar , and 716.6 ± 4.0 L m−2 h−1 bar−1 for kerosene-in-NaOH, kerosene-in-HCl, and kerosene-in-NaCl emulsions, respectively. Moreover, the separation efficiency of emulsions was > 99.4% (Fig. 9k). The result suggests that Pal coated membrane can effectively separate a variety of surfactants-stabilized emulsions under harsh conditions. Due to the adsorption of Pal on dyes, Pal coated membrane can not only achieve the separation of oil-in-water emulsions, but also remove the organic dyes from water phase. The steps of dye adsorption in the separation process were recorded (Fig. 10a). The oil/water mixtures are composed of kerosene dyed by Red oil O and MB aqueous solution. After the oil/water mixtures containing methylene blue solution were filtered by Pal coated membrane, the filtrate collected was almost colorless, demonstrating that MB was effectively removed by Pal coated membrane. There were the UV–vis absorption spectra of methylene blue and malachite green solutions. As shown in Fig. 10b and c, the UV–vis spectra of the methyl blue and malachite green water solutions before filtration showed obvious peaks. After filtration with Pal coated membrane, the absorption spectra of methylene blue and malachite green solutions were almost a straight line while the spectra peaks of MF did not have obvious change. Apparently, all these two curves illustrated that the adsorption efficiency increased with the increase of dyes initial concentration, but the adsorption rate slowed down. The reason is that with the increase of concentration, the active sites on the composite were decreased, leading to the decrease of the adsorption rate. The removal rates of methylene blue and malachite green could be achieved 99.7% ± 0.3% and 97.0% ± 0.3% on Pal coated membrane. Furthermore, adsorption efficiency was performed at the initial concentration of 60 mg/L of two dyes on Pal coated membrane and MF (Fig. 10e). It can be seen from the figure that Pal coated membrane has
4. Conclusion In summary, we have demostrated the fabrication of a multifunctional membrane using inexpensive Pal clay as raw materials by a facile vacuum assisted filtration technology. Pal coated membrane with superhydrophilicity/underwater superoleophobicity properties shows excellent separation efficiency and flux for a variety of oil-in-water emulsions. Meanwhile, it shows better cyclability, e.g., Pal coated membrane still maintains excellent separation performance after 10 separation cycles. Moreover, Pal coated membrane can also effectively separate oil-in-water emulsions under harsh environments. In addition, Pal coated membrane could remove organic dyes during oil/water separation. The removal rates of methylene blue could be achieved 99.7% ± 0.3%. Based on the merits mentioned above, such Pal coated membrane may hold great potential as a novel multifunctional and promising membrane for water treatment by combination with its simple and cost-efficient manufacture, excellent physicochemical properties. More importantly, our work may also open a new way for high-increment utilization of palygorskite mineral with a “green” method. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant No. 51962018, 21975113, 51663012), Project of 8
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Collaborative Innovation Team, Gansu Province, China (Grant No. 052005), Support Program for Hongliu Young Teachers of LUT, 2019 Key Talent Project of Gansu, and Innovation and Entrepreneurship Talent Project of Lanzhou (Grant No. 2017-RC-33).
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Graphene-like porous carbon nanostructure from bengal gram bean husk and its application for fast and efficient adsorption of organic dyes. Appl. Surf. Sci. 476, 647–657. He, J., Zhao, G., Mu, P., Wei, H., Su, Y., Sun, H., Zhu, Z., Liang, W., Li, A., 2019a. Scalable fabrication of monolithic porous foam based on cross-linked aromatic polymers for efficient solar steam generation. Sol. Energy Mater. Sol. C. 201 110111. He, J., Zhao, G., Mu, P., Wei, H., Su, Y., Sun, H., Zhu, Z., Liang, W., Li, A., 2019b. Scalable fabrication of monolithic porous foam based on cross-linked aromatic polymers for efficient solar steam generation. Sol. Energy Mater. Sol. Cells 201. Hu, L., Gao, S., Ding, X., Wang, D., Jiang, J., Jin, J., Jiang, L., 2015. Photothermal-responsive single-walled carbon nanotube-based ultrathin membranes for on/off switchable separation of oil-in-water nanoemulsions. ACS Nano 9, 4835–4842. Jia, J., Liang, W., Sun, H., Zhu, Z., Wang, C., Li, A., 2019. 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A prewetting induced underwater superoleophobic or underoil (super) hydrophobic waste potato residue-coated mesh for selective efficient oil/water separation. Green Chem. 18, 541–549. Li, Y., Zhang, H., Fan, M., Zheng, P., Zhuang, J., Chen, L., 2017. A robust salt-tolerant superoleophobic alginate/graphene oxide aerogel for efficient oil/water separation in marine environments. Sci. Rep. 7 46379. Liang, W., Liu, Y., Sun, H., Zhu, Z., Zhao, X., Li, A., Deng, W., 2014. Robust and allinorganic absorbent based on natural clay nanocrystals with tunable surface wettability for separation and selective absorption. RSC Adv. 4, 12590–12595. Liang, W., Wang, Y., Sun, H., Chen, P., Zhu, Z., Li, A., 2015. Superhydrophobic attapulgite-based films for the selective separation of oils and organic solvents from water. RSC Adv. 5, 105319–105323. Liu, Y., Su, Y., Guan, J., Cao, J., Zhang, R., He, M., Jiang, Z., 2018. Asymmetric aerogel membranes with ultrafast water permeation for the separation of oil-in-water emulsion. ACS Appl. Mater. Inter. 10, 26546–26554. Liu, W., Cui, M., Shen, Y., Zhu, G., Luo, L., Li, M., Li, J., 2019. Waste cigarette filter as nanofibrous membranes for on-demand immiscible oil/water mixtures and emulsions separation. J. Colloid Interf. Sci. 549, 114–122. Long, Y., Shen, Y., Tian, H., Yang, Y., Feng, H., Li, J., 2018. Superwettable coprinus comatus coated membranes used toward the controllable separation of emulsified oil/water mixtures. J. Membr. Sci. 565, 85–94. Mu, P., Bai, W., Zhang, Z., He, J., Sun, H., Zhu, Z., Liang, W., Li, A., 2018. Robust aerogels based on conjugated microporous polymer nanotubes with exceptional mechanical strength for efficient solar steam generation. J. Mater. Chem. A 6, 18183–18190. Mu, P., Zhang, Z., Bai, W., He, J., Sun, H., Zhu, Z., Liang, W., Li, A., 2019. Superwetting Monolithic Hollow-Carbon-Nanotubes Aerogels with Hierarchically Nanoporous
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Research paper
Fabrication of palygorskite coated membrane for multifunctional oil-inwater emulsions separation
T
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Yueyue Yang, Weidong Liang , Chengjun Wang, Hanxue Sun, Jianli Zhang, Yuan Yu, ⁎ Wenrui Dong, Zhaoqi Zhu, An Li Department of Chemical Engineering, College of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Palygorskite Underwater superoleophobic Oil/water emulsion separation Multifunctional membrane
With the rapid development of industrial growth, the issue of severe water pollution risen from oily waste water, dye waste water, and frequent oil spill accidents has become a bottleneck for hindering the global sustainable development. Herein, we report the facile fabrication of a superhydrophilic/underwater superoleophobic multifunctional separation membrane by vacuum assisted filtration technology using palygorskite (Pal) as raw material. The Pal coated membrane show excellent thermal， chemical stability, and mechanical strength with 300 bending times. As expected, the membrane has excellent separation performance, e.g. it exhibits an enhanced permeation flux of 477.7 ± 5.0 L m−2 h−1 bar−1 and a high separation efficiency of up to 99.6% ± 0.05%. In addition, it can separate different oil-in-water emulsions in harsh environments. Interestingly, the as-prepared membrane can also remove organic dyes in the water phase during the separation process, making it multifunctional and promising membrane for water treatment, by combination with its simple and cost-efficient manufacture, excellent physicochemical properties. More importantly, our work may also open a new way for high-increment utilization of palygorskite mineral with a “green” method.
1. Introduction Nowadays, frequent oil spill accidents and oily wastewater produced from industrial activity have seriously threaten the ecological environment and human health (Birkhead, 2014; Mu et al., 2018; Yong et al., 2018; Jia et al., 2019). Traditional separation techniques such as air flotation (Al-Shamrani et al., 2002), chemical coagulation (Yan et al., 2019) and adsorption (Wang et al., 2019), have been used for oil/ water separation. There methods, however, have their respective drawbacks such as low efficiency and high operating cost, which dramatically limit the wide application of these oil/water separation technologies. Furthermore, these techniques are either ineffective in separating surfactant-stabilized emulsions, or demulsify the emulsions upon applying an electric field or adding chemicals, resulting in high energy consumption and secondary pollution (Ríos et al., 1998; Ren and Kang, 2018; Cui et al., 2019). In recent years, superwetting materials have offered a good platform for oil/water separation, mainly because of their different wettability to water or oil which result in high selectivity and in turn high separation efficiency (Wu et al., 2017; Zhan et al., 2018). Superhydrophobic/superoleophilic materials were firstly reported for oil/water separation (Feng et al., 2004). Since then, all
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kinds of superhydrophobic/superoleophilic materials, including polymer (Li et al., 2015; Das et al., 2018), sponge (Peng et al., 2018), fabric (Zhou et al., 2013), mesh (Guo et al., 2018), ceramic membranes (Samaei et al., 2018) have been developed. Unfortunately, the density of oil phase is less than that of water phase, a water barrier will form between the separation material and the oil layer, which blocks the separation process (Kang and Cao, 2014; Shahabadi and Brant, 2019). Moreover, superhydrophobic materials usually used fluoroalkylsilane with low surface energy, which were harmful to the ecological environment and human health (Liu et al., 2019). A hydrogel-coated mesh was fabricated for oil/water separation, which possessed superhydrophilic/underwater superoleophobic properties (Xue et al., 2011). Inspired by the study, lots of researchers have developed superhydrophilic/underwater superoleophobic materials, which provide a good choice to separate water rich oil/water mixtures (Zhang et al., 2013; Li et al., 2016; Zhou et al., 2016; Tie et al., 2018). However, most of superhydrophilic/underwater superoleophobic materials can noly achieve the oil/water mixtures separation owing to their large pore size. It is difficult to effectively separate surfactants-stabilized emulsions with particle size < 20 μm. Nowadays, various materials with superhydrophilic/underwater superoleophobic properties have
Corresponding authors. E-mail addresses: [email protected] (W. Liang), [email protected] (A. Li).
https://doi.org/10.1016/j.clay.2019.105295 Received 11 July 2019; Received in revised form 4 September 2019; Accepted 4 September 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.
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2.3. Preparation of the underwater superoleophobic Pal coated membrane
successfully separated emulsified oil/water mixtures. For example, Liu et al. reported an asymmetric aerogel membrane by one-pot hydrothermal reaction induced polyvinyl alcohol self-crosslinking (Liu et al., 2018). The aerogel membrane showed excellent properties of air superhydrophilic and underwater superoleophobic. Jiang et al. fabricated an original gelatin-based multifunctional aerogel with superhydrophilic and underwater superoleophobic, and successfully separated the stable oil-in-water emulsions (Jiang et al., 2019). However, it is difficult for these materials to maintain their original wettability in harsh environments. But the actual separation of oil/water often occurs in complex environments, such as acid, alkali and salt solutions. Therefore, the chemical stability of separated materials is particularly important in practical application. (Hu et al., 2015; Qu et al., 2017; Zhang et al., 2018c; Contreras Ortiz et al., 2019). In addition to oil pollution, organic dyes in water can also seriously affect human life and the ecological environment (Puri and Sumana, 2018; Gupta et al., 2019). Since most oily wastewater also contains water-soluble dyes. Therefore, it has become increasingly necessary to remove organic dyes in the process of oil/water separation. However, most separation materials developed at present only have the function of separating emulsified oil/water into two phases, and cannot remove pollutants in the water phase during filtration process. Therefore, it is very important to fabricate a kind of separation membrane with low cost, environmental friendly and multifunctional water treatment characteristics. Palygorskite is a kind of silicate clay containing hydrophilic groups, consisting of hydrated magnesium-aluminosilicate with layered chain structure, which has attracted much attention due to its natural nanochannel structure, inexpensive, abundant reserves and adsorption (Liang et al., 2015; Zhang et al., 2019b; Zhou et al., 2019). In continuation of our previous studies in this field (Li et al., 2011; Liang et al., 2014; Wei et al., 2018; Mu et al., 2019; Zhang et al., 2019),and based on the research of the preparation strategy of the membrane (Cheng et al., 2017), this work, we used low-cost palygorskite clay as raw material to explore a kind of superhydrophilic/underwater superoleophobic multifunctional separation membrane via a simple vacuumassisted filtration of palygorskite clay dispersions. The membrane exhibited excellent separation performance, which could not only realize the separation of emulsified oil/water mixtures in harsh environments, but also remove water-soluble dyes during the separation process, showing great potential for a broad variety of applications for wastewater treatment.
PVA was stirred and dissolved in deionized water at 90 °C to prepare PVA solution 0.5% (W/V). When 0.25 g Pal was dispersed in deionized water, the homogeneous dispersion was obtained by magnetic stirring about 30 min and ultrasonic for half an hour. Then, 2 mL PVA (0.5% W/ V) was added to the Pal dispersed solution. The mixed dispersion was stirred for at least 2 h at room temperature. Subsequently, Pal coated membrane was prepared by vacuum filtration (0.8 bar) and dried at room temperature. 2.4. Stable oil/water emulsions separation To prepare the corresponding oil-in-water emulsions. Tween-80 was firstly added to deionized water in a three-necked flask. Afterwards, oils (kerosene, n-heptane, petroleum ether and diesel) were added to the above solution at a ratio of 1:50 mL/mL, and stirred vigorously for 12 h, until stable white emulsions were formed. The preparation of kerosenein-NaCl, kerosene-in-HCl and kerosene-in-NaOH emulsions were described above. Pal coated membrane of the prewetted by water was placed on the separation device. The above emulsions were slowly poured onto the vacuum filter device. The whole separation process was carried out under the vacuum condition of 0.08 MPa. The obtained filtrate was preserved for further performance testing. The flux (F) for Pal coated membrane was obtained by calculating the volume of water collected per unit time as follows:
F = V /(S × t × ∆P )
(1)
where V is the volume of the filtrate, t is the testing time, S is the effective filtration area of the membrane, ΔP is the suction pressure across the membrane. The separation efficiency (R1) was calculated as:
R1 (%) = (1 − Cp/ C0) × 100
(2)
where Cp and C0 are the concentration of oil in filtrate and the prepared emulsion. 2.5. Stability of Pal coated membrane The stability was evaluated by the mass loss of the membrane (ML) after the sample was exposed to harsh environments for 48 h and different temperatures for 24 h. The calculation formula was as follows:
ML = (mi − mt )/ mi × 100%
(3)
Where mi is the initial mass of the membrane and mt is the remaining mass of the membrane.
2. Experimental 2.1. Materials
2.6. Removal of organic dyes Palygorskite [Mg5Si8O20(OH)2(OH2)4·4H2O] was purchased from Jiangsu Jiuchuan Nanomaterials Technology Co., Ltd. Microfiltration membrane (MF, 0.45 μm) was obtained from Xinya purification Co., Ltd. Poly (vinyl alcohol) (PVA, size 1788) was obtained from Aladdin Industrial Corporation. Hydrochloric acid (HCl) and Sodium hydroxide was supplied by Shanghai Aladdin Biochemical Technology Co., Ltd. Kerosene and diesel were purchased from the local store. Organic solvents (n-heptane, petroleum ether) were provided by Tianjin Fuyu chemical reagent Co., Ltd. Tween-80 was obtained from Laiyang Shuangshuang chemical Co., Ltd. The other chemical reagents were analytical grade and used without further treatment.
Dyes methylene blue (MB) and malachite green (MG) were evaluated the adsorption properties of Pal coated membrane. Oil/water mixtures containing pollutants were prepared with kerosene as the oil phase and dyes with different concentrations as the water phase. The dyes removal rate R2(%) of Pal coated membrane was calculated according to equation:
R2 (%) = (1 − Cm/ Cn ) × 100
(4)
where Cn (mg/L) is the initial concentration of dyes and Cm (mg/L) is the concentration of dyes in filtrate.
2.2. Purification of Pal
2.7. Characterization
For the sake of remove the impurities, the Pal (10 g) was added to HCl (1 M) and stirred vigorously for about 4 h at room temperature, then washed with deionized water until the pH was 7. The residue was dried at 60 °C in the blast drying oven and the purified Pal was obtained.
Fourier transform infrared (FT-IR) spectroscopy was performed using a Nexus 680 spectrum instrument. The microstructure of the asprepared membrane and MF surfaces were characterized by scanning electron microscopy (SEM, JSM-6701F, JEOL, Ltd.). X-ray photoelectron spectroscopy (XPS) analyses were performed using a PHI2
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Fig. 1. Illustration of the preparation of Pal coated membrane and its application of oil-in-water emulsions separation and dyes removal.
underwater superoleophobic property (Zhang et al., 2019a). In addition, pore size distribution of Pal coated membrane and MF was measured by nitrogen sorption using a BET method. As shown in Fig. 2f, the highest peak of Pal coated membrane is mainly concentrated at 47 nm, indicating that it is a mesoporous material (He et al., 2019a, 2019b). The average pore diameter was 103.5 nm, which was much smaller than that of the substrate membrane (average pore diameter 0.45 μm). Therefore, the above the hierarchical micro-nanoscale roughness structure is beneficial for preparing superwetting materials. To verify the valid formation of Pal/PVA composite layer on the surface, the elements of the prepared membrane were tested by XPS method. Fig. 3a shows full-scan spectrum XPS data for the elements of Pal coated membrane and MF. New characteristic signals of Si2s, Si2p and MgKLL elements were observed on Pal coated membrane in contrast with MF. These emerging elements were believed to originate from the typical groups in the palygorskite components. From the narrow scan of C1s XPS spectrum, the peak intensity of C1s decreased after coating, as illustrated in Fig. 3b. The above results indicated that Pal coated membrane was successfully prepared. In order to further characterize the chemical structure of the as-prepared membrane, the functional groups of Pal, PVA and Pal coated membrane were analyzed by FT-IR spectroscopy. As noted in Fig. 3c, the peak of 3600–3400 cm−1 was related to the hydroxyl groups of PVA, Pal and Pal coated membrane. The peak at 1655 cm−1 could be assigned to the bend vibration of H2O in Pal and Pal coated membrane (Yahia et al., 2019). The 1741 cm−1 absorption peak of PVA at was attributed to C]O (Shi et al., 2016). The peak at 2927 cm−1 was associated with the stretching of –CH of PVA. The 1256 cm−1 absorption peak of PVA was associated with the asymmetric stretching vibration of the C-O-C bond (Wang et al., 2017). The characteristic peak at 1032 cm−1 belongs to Si-O of Pal. At the same time, Si-O also appears in Pal coated membrane. And the FTIR spectrum of Pal coated membrane is basically the same as Pal. Therefore, Pal coated membrane is mainly composed of Pal. Above results indicated that Pal coated membrane had a structure of amphiphilic groups, showing amphiphilic properties (Long et al., 2018). To evaluate the wetting behavior of Pal coated membrane towards water and oil by CAs method. As highlighted in Fig. 4a and b, the asprepared Pal coated membrane shows superamphiphilic behavior in air. In other words, when water and oil droplets contacted with the surface of Pal coated membrane, the water and oil droplets instantaneously spread out and CAs were about 0°. However, the as-prepared Pal coated membrane exhibited superoleophobicity in underwater. When the droplet of kerosene was dropped on the surface of the membrane underwater, the oil CA was 153.1° ± 3.0°, which was displayed in Fig. 4c. To further verify the wetting behavior of Pal coated membrane in underwater, different types of oils were tested: n-heptane, petroleum ether, and diesel (Fig. 4d). All these oils exhibited nearly stable
5300ESCA spectrometer (Perkin-Elmer). The contact angles (CAs) and sliding angles (SAs) were measured using a SL200KB apparatus. The oil content in the filtrate was obtained using an infrared spectrometer oil content analyzer (OIL 480). The pore size distribution of the sample were measured by N2 adsorption and desorption at 120 °C using a volumetric sorption analyzer (micromeritics ASAP 2020). Droplet sizes of emulsions were measured using a Malvern Zetasizer Nano ZS (Malvern, UK). The microscopic image of the emulsion was taken on an optical microscope. The rough surface structure of the samples was acquired Atomic force microscopy (AFM) under ambient conditions. The absorbance of dyes was determined by using a UV–vis spectrophotometer (7230G) from Shanghai Precision Scientific Instruments Co., Ltd. The tensile strength of the membrane was tested by a universal material testing machine at an elongation rate of 2 mm min−1 from French A.D company. 3. Result and discussion Fig. 1 shows the preparation steps of Pal coated membrane and its application of oil-in-water emulsions separation and dyes removal. Pal coated membrane was prepared by filtering the mixed dispersion of PVA and Pal onto microfiltration membrane substrate. Among them, the existence of PVA can enhance the strength between the coating and the substrate membrane. At the same time, the addition of PVA is beneficial to the realization of membrane formation of palygorskite. The further experimental results and discussion were provided (Fig. S1). It is obvious that the surface of Pal coated membrane without PVA dropped off obviously, while the surface of Pal coated membrane with PVA remained intact. Pal coated membrane can not noly efficiently separate a variety of oil-in-water emulsions under harsh conditions, but also remove water-soluble dyes. The morphologies of MF and Pal coated membrane were characterized by scanning electron microscopy (SEM). As illustrated in Fig. 2, MF has a relatively smooth surface with an average pore diameter of 0.45 μm (Fig. 2a). After being coated with the mixed dispersion, due to the random distribution of Pal, rough structure was formed on the surface of Pal coated membrane. Fig. 2b illustrates the magnified image of Pal coated membrane, which can more intuitively show many micro-nanostructures. Pal coated membrane surface has the hierarchical micro-nanoscale roughness, which is benefit to the acquisition of underwater superoleophobicity. The cross section of Pal coated membrane was also tested (Fig. 2c), and the thickness of the Pal layer was approximated 74.44 μm. Meanwhile, the surface roughness feature of MF and Pal coated membrane were measured (Fig. 2d and e). It was revealed that the surface roughness also increased from 232.7 nm (MF) to 351.0 nm (Pal coated membrane) after leaching. The increase of surface roughness makes it easier for the material to realize the 3
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Fig. 2. FE-SEM images of (a) MF and (b) Pal coated membrane (magnified image of Pal coated membrane), respectively. (c) Cross-sectional image of Pal coated membrane. AFM images of (d) MF and (e) Pal coated membrane. (f) Pore size distribution of MF and Pal coated membrane.
separation, the filtrate collected was completely transparent compared with the milky white kerosene-in-water emulsion. In sharp contrast, when MF was used as a separation material, the filtrate after separation was still translucent (Fig. 5c), showing that MF is not effective for separating the surfactant-stabilized oil-in-water emulsions. Moreover, there was a visible difference between the microscopic images of the emulsion and the filtrate collected, as demonstrated by the optical microscopy images (Fig. 6a and c). A large number of oil droplets were distributed in the emulsion before separation while no kerosene droplets were seen in the filtrate collected. It showed that Pal coated membrane could be effectively separated kerosene-in-water emulsion. The reason is that Pal coated membrane has superhydrophilic/underwater superoleophobic properties and suitable pore size, which can effectively prevent oil droplets in emulsion from passing through the membrane. Furthermore, the droplet size distributions of the kerosenein-water emulsion and collected filtrate were analyzed as illustrated in Fig. 6a1 and c1. The droplet size of the kerosene-in-water emulsion ranged from 200 nm to 800 nm. However, no droplets were found in the filtrate within these ranges, and the droplet sizes of the filtrates for various oil-in-water emulsions were lower than the instrument threshold, revealing excellent emulsion separation effect. And Pal coated membrane can also separate other kinds of oil-in-water emulsions, including n-heptane-in-water, petroleum ether-in-water and
spherical shapes with OCAs > 150°, revealing significant superoleophobicity. Meanwhile, corresponding SAs of various oils (kerosene, n-heptane, petroleum ether and diesel) underwater were evaluated, as demonstrated in Fig. 4d. The result clearly showed that the SAs were lower than 5°. Pal coated membrane was shown a lower surface adhesion, which can effectively prevent the as-prepared membrane to be polluted or blocked up during oil/water separation process (Li et al., 2017). This special wettability is due to the synergistic effect of the hydrophilicity of Pal and the rough micro-nanoscale structure on the surface of Pal coated membrane, Pal coated membrane would be ideal candidates for oil-in-water emulsions separation. Based on this special wettability and suitable pore size, Pal coated membrane would be naturally an ideal material in the treatment of oily wastewater. The stability of various emulsions was tested (Fig. 5a). A week later, the prepared oil/water emulsions remained stable. The emulsion separation system is shown in Fig. 5b. The kerosene-in-water emulsion as a typical sample was researched. During the separation process, due to the superhydrophilic/underwater superoleophobic property of Pal coated membrane, the water molecules were absorbed by the membrane, forming a water barrier on the membrane surface to prevent oil from permeating the membrane. And the emulsion separation is realized under the pressure drive. As shown in Fig. 6b, the digital photos kerosene-in-water emulsion before and after separation. After
Fig. 3. (a) XPS full-scan spectrum of MF and Pal coated membrane. (b) High-resolution XPS C1s narrow scans as a functional electron binding energy. (c) FT-IR spectra of Pal, PVA and Pal coated membrane. 4
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Fig. 4. Wettability of Pal coated membrane: (a) water in air (b) oil in air (c) oil in water. (d) CAs and SAs of oil droplets on Pal coated membrane underwater.
Fig. 5. (a) Stability tests of the various types of emulsions. (b) The vacuum driven filtration system. (c) Digital photograph of kerosene-in-water emulsion before and after MF separation.
Fig. 6. (a-l) Optical microscopy images and digital photographs of different oil-in-water emulsions before and after separation. (a1-l1) Size distribution of the oil droplets in various emulsions before and after separation. 5
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Fig. 7. (a) Fluxes for various surfactant-stabilized oil-in-water emulsions. (b) Separation efficiency and oil content of Pal coated membrane for various surfactantstabilized oil-in-water emulsions. (c) The separation efficiencies and fluxes for kerosene-in-water emulsion as a representative of cycle times.
Fig. 8. (a) Mass loss (ML) of Pal coated membrane after acidic or alkaline corrosion for 48 h. (b) Mass loss (ML) of Pal coated membrane under different temperatures. (c) A typical stress-strain curve of a dried Pal coated membrane. (d) Optical images of Pal coated membrane before and after bending 300 times. (e) Schematic view of the devices of the sandpaper abrasion test. (f) Underwater kerosene CA of Pal coated membrane after 100 scratch cycles.
2019). In this work, the separation performance of Pal coated membrane was tested by a series of emulsions. Pal coated membrane shows higher separation fluxes of various oil-in-water emulsions. As exhibited in Fig. 7a, the fluxes for oil-in-water emulsions > 298.6 ± 3.0 L m−2 h−1 bar−1. In order to test the separation efficiency of Pal coated membrane, the oil rejection (R1%) of Pal coated membrane was measured by an infrared spectrophotometric oil analyzer. As Fig. 7b
diesel-in-water emulsions. Similarly, optical microscope images and droplet sizes were measured (shown in Fig. 6d1-l1). The above results showed that Pal coated membrane can be completely successful in separating various oil-in-water emulsions. In order to further explore the separation effect of Pal coated membrane, the separation performances were evaluated by separation flux and efficiency (Zhang et al., 2018a; Zhang et al., 2018b; Zin et al.,
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Fig. 9. (a-f) Optical microscopy images and digital photographs of kerosene-in-NaCl, kerosene-in-NaOH and kerosene-in-HCl emulsions before and after separation, respectively. (j, k) Flux and separation efficiency of kerosene-in-NaOH, kerosene-in-HCl and kerosene-in-NaCl emulsions on Pal coated membrane.
shows, the oil content in each filtrate is not > 90 ppm, and the R1 values of each filtrate were > 99.3%. Therefore, Pal coated membrane has excellent separation performance for various oil-in-water emulsions. Moreover, to study the reusability of Pal coated membrane, which testing the cycle flux and efficiency of kerosene-in-water emulsion. The separation efficiency remained above 99.5% after the membrane had been reused for 10 separation cycles in Fig. 7c, confirming that the separation membrane has excellent reusability. To evaluate the stability of the membrane, Pal coated membrane was immersed in different environments. As exhibited in Fig. 8a, in the range of pH 1–11, Pal coated membrane still remained intact and maintained a low mass loss after corrosion for 48 h. At the same time, the wettability of Pal coated membrane under different corrosive environments was tested. Under the conditions of acid, alkali and saturated salt, the underwater CAs of Pal coated membrane were 151.9° ± 2.5°, 150.2° ± 2.0° and 153.6° ± 2.5°, suggesting that Pal coated membrane could overcome acidic, salty, and alkaline environments and maintain stable underwater superoleophobicity. As shown in
Fig. 8b, CAs of the underwater kerosene were still > 150° when the membrane was soaked into aqueous solutions at different temperatures (20-50 °C). The results indicated that Pal coated membrane still maintained the underwater superoleophobicity. And the mass loss of the membrane is very small. In addition, the tensile strength of Pal coated membrane was 1.23 MPa. What's more important, the membrane has good flexibility. As demonstrated in Fig. 8d, there were still no cracks in Pal coated membrane after bending for 300 times. In addition, in order to further study the mechanical property of Pal coated membrane under abrasion, 1500 mesh sandpaper was used for abrasion test and shown in Fig. 8e. Drag Pal coated membrane back and forth on the surface of the sandpaper. Pulling the sandpaper for 10 cm was defined as a cycle. Fig. 8f shows the underwater kerosene CA of after 100 cycles, indicating that after 100 cycles, the underwater kerosene CA is still > 150°. The above results show that Pal coated membrane not only has excellent chemical stability and heat resistance, but also has good the mechanical stability, which provides a great possibility for practical oil/water separation. 7
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Fig. 10. (a) Photographs of filtration device. UV–vis absorption spectra of (b) methylene blue and (c) malachite green solutions. (d) Adsorption efficiency of dyes at different concentrations. (e) Adsorption efficiency of dyes with concentration of 60 mg/L on Pal coated membrane and MF. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
higher removal efficiency than that of MF. The insert of Fig. 10e shows digital photographs of Pal coated membrane and MF after filtering dyes with concentration of 60 mg/L, in which the left part of the insert shows the photo of the dyes filtered by Pal coated membrane, which is darker than the color of the dyes filtered by the MF on the right. It shows that Pal coated membrane has strong dye adsorption capacity, which is due to the fact that the adsorption of Pal coated membrane is mainly controlled by electrostatic force and hydrogen bond. In order to clearly explain the electrostatic adsorption mechanism of Pal coated membrane, the Zeta potential of Pal coated membrane is provided (Fig. S2). It can be seen that the surface of Pal coated membrane shows negative charge and the cationic dye can be adsorbed by electrostatic action. The above results revealed that Pal coated membrane can not only effectively separate the emulsions, but also remove organic dyes in the water, thus achieving multifunctional water treatment.
To further demonstrate the corrosion resistance of Pal coated membrane, surfactant-stabilized corrosive emulsions were separated. The separation result of the kerosene-in-NaCl emulsion was showed in Fig. 9a and c. After the kerosene-in-NaCl emulsion was filtrated by Pal coated membrane, the numerous oil droplets were not observed, and the digital photograph that the milky white emulsion became transparent. In addition, Pal coated membrane can also achieve the separation of kerosene-in-HCl and kerosene-in-NaOH emulsions. The corresponding optical microscope images and digital photographs were displayed (Fig. 9d-f). As shown in Fig. 9j, Pal coated membrane exhibited high fluxes with 382.2 ± 4.0 L m−2 h−1 bar−1, −2 −1 −1 394.1 ± 6.0 L m h bar , and 716.6 ± 4.0 L m−2 h−1 bar−1 for kerosene-in-NaOH, kerosene-in-HCl, and kerosene-in-NaCl emulsions, respectively. Moreover, the separation efficiency of emulsions was > 99.4% (Fig. 9k). The result suggests that Pal coated membrane can effectively separate a variety of surfactants-stabilized emulsions under harsh conditions. Due to the adsorption of Pal on dyes, Pal coated membrane can not only achieve the separation of oil-in-water emulsions, but also remove the organic dyes from water phase. The steps of dye adsorption in the separation process were recorded (Fig. 10a). The oil/water mixtures are composed of kerosene dyed by Red oil O and MB aqueous solution. After the oil/water mixtures containing methylene blue solution were filtered by Pal coated membrane, the filtrate collected was almost colorless, demonstrating that MB was effectively removed by Pal coated membrane. There were the UV–vis absorption spectra of methylene blue and malachite green solutions. As shown in Fig. 10b and c, the UV–vis spectra of the methyl blue and malachite green water solutions before filtration showed obvious peaks. After filtration with Pal coated membrane, the absorption spectra of methylene blue and malachite green solutions were almost a straight line while the spectra peaks of MF did not have obvious change. Apparently, all these two curves illustrated that the adsorption efficiency increased with the increase of dyes initial concentration, but the adsorption rate slowed down. The reason is that with the increase of concentration, the active sites on the composite were decreased, leading to the decrease of the adsorption rate. The removal rates of methylene blue and malachite green could be achieved 99.7% ± 0.3% and 97.0% ± 0.3% on Pal coated membrane. Furthermore, adsorption efficiency was performed at the initial concentration of 60 mg/L of two dyes on Pal coated membrane and MF (Fig. 10e). It can be seen from the figure that Pal coated membrane has
4. Conclusion In summary, we have demostrated the fabrication of a multifunctional membrane using inexpensive Pal clay as raw materials by a facile vacuum assisted filtration technology. Pal coated membrane with superhydrophilicity/underwater superoleophobicity properties shows excellent separation efficiency and flux for a variety of oil-in-water emulsions. Meanwhile, it shows better cyclability, e.g., Pal coated membrane still maintains excellent separation performance after 10 separation cycles. Moreover, Pal coated membrane can also effectively separate oil-in-water emulsions under harsh environments. In addition, Pal coated membrane could remove organic dyes during oil/water separation. The removal rates of methylene blue could be achieved 99.7% ± 0.3%. Based on the merits mentioned above, such Pal coated membrane may hold great potential as a novel multifunctional and promising membrane for water treatment by combination with its simple and cost-efficient manufacture, excellent physicochemical properties. More importantly, our work may also open a new way for high-increment utilization of palygorskite mineral with a “green” method. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant No. 51962018, 21975113, 51663012), Project of 8
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Collaborative Innovation Team, Gansu Province, China (Grant No. 052005), Support Program for Hongliu Young Teachers of LUT, 2019 Key Talent Project of Gansu, and Innovation and Entrepreneurship Talent Project of Lanzhou (Grant No. 2017-RC-33).
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