- Email: [email protected]
salt separation
Journal of Membrane Science 581 (2019) 321–330
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
In-situ grown covalent organic framework nanosheets on graphene for membrane-based dye/salt separation
T
Xuke Zhanga, Hui Lia,b, Jing Wanga,∗, Donglai Pengc, Jindun Liua, Yatao Zhanga,∗∗ a
School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, 450001, PR China Research Department of New Materials, Zhengzhou Institute of Emerging Industrial Technology, Zhengzhou, 450000, PR China c School of Material & Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou, 450001, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Covalent organic framework Graphene oxide In-situ growth Membrane Dye/salt separation
Covalent organic framework-1 (COF-1) with ordered channel structure and precise pore size covalently attaching the surface of graphene oxide (GO) was synthesized by in-situ growth. The prepared GO/COF-1 nanocomposite improves dispersivity and stability in water over COF-1, allowing GO/COF-1 nanocomposite to be used as membrane material for water treatment. Here, high-flux membranes-based hybrid 2D-2D GO/COF-1 nanocomposites were constructed on the support of Polyacrylonitrile (PAN) ultrafiltration membrane using a deadend filtration device. The prepared membranes exhibit an excellent water permeation over 310 L m-2 h−1 MPa−1, high rejection rate for water soluble dyes (> 99%) and high permeability for ion salts. The outstanding dye rejection and water permeation performance can be attributed to the physical size sieving of COF-1 (the layer distance of near 0.33 nm) and the suitably interlayer spacing between the adjacent GO sheets. Moreover, electrostatic interaction also plays an important role for the rejection process. This approach presents an enormous potential for purification of dye wastewater.
1. Introduction Two-dimensional (2D) covalent organic frameworks (COFs) are a novel class of crystalline materials with the layered structure and permanent porosity. The crystal structures in COFs were entirely linked built by strong covalent bonds between light elements (B, C and O). Because of its high thermal, chemical stability, low density and ordered channel structure, COFs is regards to be a promising material in the field of gas storage, energy storage, catalysis and membrane separation material [1–3]. Beyond that, suitable monomers design may endow COFs with the tunable porosities and variable structure, making COFs to be a desirable material for molecular sieving and separation [4–9]. However, the internal defects could severely restrict the crystallization growth of COFs into nanodomains. The conventional synthetic approaches of 2D COFs are unable synthesize microcrystalline powders with controllable morphology [10–13], thus the weakness limits the application of COFs. Currently, many approaches have been proposed to use COFs into membrane, namely self-standing COFs membranes and mixed matrix membranes (MMMs). But self-standing COFs membranes are difficult to fabrication due to internal defects of COFs could impede the formation of a complete continuous membrane structure during
∗
membrane preparation. Poly/COFs mixed matrix membranes was obtained via uniformly dispersing COFs into polymeric matrices. Although such membranes are easy to process, their separation performance is lower than self-standing COFs membranes because of the interface voids and COFs pore blockage [14–16]. In brief, poor membraneforming properties of COFs restrict its application in the field of membrane separation technology. Consequently, it is still a great challenge to finely combine insoluble COFs into membrane. Several strategies have been developed to solve this issue. For example, singlelayered graphene could be obtained by exfoliation, but it is difficult to implement for COF due to the large size crystals fail to obtain. Meanwhile, COFs could grow directly on the surface, even though there are different types of organic coupling reactions in the synthesis of COFs, which is regarded as a promising strategy [17,18]. Graphene oxide (GO) with hydrophilicity, easy to function ability, nearly frictionless surface and unprecedented fast water-transport channels [19,20] have attracted increasing interest for desalination, purification and molecular separation [21–27]. However, it is difficult to reasonably regulate the interlayer spacing between the adjacent GO nanosheets to achieve molecular sieving, sealing solvent [28–31]. Therefore, it is of great value to developed a facile strategy to control
Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. Wang), [email protected] (Y. Zhang).
∗∗
https://doi.org/10.1016/j.memsci.2019.03.070 Received 23 December 2018; Received in revised form 21 February 2019; Accepted 23 March 2019 Available online 25 March 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
Scientific Ltd (China). 1,4-dioxane (98%) and mesitylene (99%) were purchased from Shanghai Macklin Biochemical Co., Ltd. Polyvinyl alcohol (PVA) (Mw-70,000 Da) was supplied by Sigma-Aldrich (St. Louis, MO). Poly (vinylamine) (PVAm, Mw-100,000 Da) was procured by Mitsubishi Chemical (Shanghai) Co., Ltd. sodium chloride (NaCl, 99%), Sodium sulfate (Na2SO4, 99%), magnesium sulfate (MgSO4, 99%), magnesium chloride (MgCl2, 99%) were obtained from, Congo red (Mw-696.66 Da), Direct red 23 (Mw-813.72 Da), Methylene blue (Mw99.80 Da) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd, Reactive Black 5 (Mw-991.8 Da) were obtained from Sunwell Chemicals Co., Ltd., China. PAN membrane (Mw-50 kDa) was provided by Spero Membranes, Beijing. Deionized water (DI water) was used in all experiments.
the interlayer spacing of GO based membrane. Recently, it is reported that the interlayer spacing of GO could be tailored by partially chemical reduction or intercalating with nanocrystalline materials (ca: MOFs) to increase the water permeability of the GO membrane [32–34]. This method was useful for increasing permeation flux, but the stability was not good enough after the long-time operation, the flux inevitably decreases in water permeability. Therefore, it is of great value to developed a facile strategy to control the interlayer spacing of GO based membrane. As a typical member of COFs family, COF-1 was synthesized by the condensation reaction of 1, 4-benzenediboronic acid in mesitylene/dioxane solution in a sealed pyrextube [1]. The structure of COF-1 is similar to expanded porous graphitic layers, along with an interlayer spacing similarly to graphite [3.328 Å versus 3.348 Å] [35]. Moreover, the capture size of COF-1 is 1.5 nm, which have been estimated by the van der Waals distance between phenyl and B3O3 rings of adjacent layers [1]. These features make it possible to become a new candidates acting as size exclusion for membrane. However, COF-1 is considered to have a weak dispersion, inevitably producing agglomeration in the process of membrane preparation. Recently, solvothermal reaction of GO with 1, 4-benzene diboronic acid (DBA) were reported to form graphene oxide framework (GOF) materials [36–38]. COF-1 can be grown in the surface of GO because of BeO bonding between DBA and oxygen functional groups on the GO sheets (Fig. S1). This method maybe solves the agglomeration issue of COF-1. Besides that, GO membranes suffer from the poor stability issue in aqueous solution [39,40]. The chemical reduction of the oxidation groups could improve the stability of GO membrane in aqueous solution, while the narrow interlayer space between reduced GO layers could lead to the drastic decreases of water permeability [41]. The intercalated COF-1 provides an extra passageway to improve water penetration. Additionally, the covalent bonds between COF-1 and GO are stronger than metal-organic frameworks (MOFs) due to the composition of COF-1 with non-metallic element (B, C). Besides, COF-1 reduced the oxidation groups on GO surface to increase the stability of GO under the long water pressure operation, resulting in GO sheets not easy to collapse. Therefore, GO/ COF-1 nanocomposites were expected to be used in membrane. Furthermore, we assume that molecular sieve membranes-based GO/COF-1 nanocomposites (GO/COF-1 membranes) would exhibit excellent selective water permeation (ca. 2.8 Å) and water-soluble salt (ca. 6.6–10 Å) permeation compared with water soluble dye (> 20 Å) [42]. Herein, hybrid GO/COF-1 nanocomposites were synthesized by insitu growth of COF-1 on the surface of GO, which has an outstanding thermal stability and water stability. To study the effects of GO/COF-1 nanocomposites on membrane structural and separation properties, a series of GO/COF-1 membranes were prepared using a dead-end filtration device (Scheme 1). The flux and separation efficiency were investigated through water flux and salt, dyes rejection tests. This work seeks to provide an approach which improves the dispersion of COFs, tunes the interlayer spacing between adjacent GO nanosheets and improve the stability of GO membrane in water solution, emphasizing the synergistic effect between COF-1 and GO for water purification and dye desalination.
2.2. Synthesis of graphite oxide (GO) GO was prepared via using the modified Hummers method [43]. Typically, graphite powder (3.0 g, 1 wt%) and KMnO4 (18.0 g, 6 wt%) was added to the mixture composed of 360 mL of concentrated H2SO4 and 40 mL of H3PO4, and then the mixture was stirred at 50 °C for 22 h. After that, 1200 mL ice water (DI water) was added into the mixture, together with 2 mL of 30% H2O2 to a graphite oxide suspension. Then, the suspension was sonicated for 1 h and centrifuged at 1000 rpm for 10 min to attain the graphite oxide supernatant. Finally, the graphite oxide supernatant was centrifuged at 8000 rpm for 7 min. The remaining product was washed repeatedly and continuously with DI water, 30 wt% HCl, and ethanol for 3 times respectively. A yellowbrown graphene oxide (GO) was attained and further dried using a vacuum freeze-drying device. 2.3. Synthesis of COF-1 COF-1 was prepared by the same method as described in a previous paper [44]. In brief, DBA (25 mg, 0.15 mmol) was dissolved into 1 mL of mesitylene: dioxane (1:1 in volume). Then, the reaction mixture was heated at 120 °C for 72 h standing at the nitrogen environment. After that, a white solid at bottom of the stainless steel reactor which was separated by centrifugation and washed repeatedly 3 times with acetone. Finally, the attained products were dried at 50 °C for 24 h under vacuum. 2.4. Synthesis of GO/COF-1 nanocomposites
2. Experimental
GO powder (160 mg) was sonicated in methanol (32 mL) to obtain GO dispersion. And then, 1, 4-benzenediboronic acid (DBA, 480 mg) was added. The mixture was transferred to a stainless steel reactor to make a solvothermal reaction at 90 °C for 12 h. The resultant was repeatedly washed with methanol to ensure that the unreacted DBA was removed, and then washed 3 times by using mesitylene/dioxane solution (1:1 in volume). The resulting products was directly re-dispersed in 32 mL mesitylene/dioxane solution (1:1 in volume), and DBA (720 mg) were added to acquire homogeneous solution. At last, the mixture was transferred to a stainless steel reactor to conduct a solvothermal reaction at 120 °C for 72 h. The sample was thoroughly washed with acetone and then dried under vacuum at 50 °C.
2.1. Chemicals and materials
2.5. Preparation of PVAm/PVA polymeric matrix
All chemicals (analytical grade) were used without further purification. A natural graphite powder (ca. 45 μm) was obtained from Sinopharm Chemical Regent. Concentrated sulfuric acid (H2SO4, 98 wt %), hydrochloric acid (HCl), phosphoric acid (H3PO4, 85 wt%), and hydrogen peroxide (H2O2, 30 wt%) of analytical grade were provided by Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd (Tianjin, China). Potassium permanganate (KMnO4), sodium hydroxide (NaOH) and 1,4-benzenediboronic acid (DBA, 98%) was purchased from J&K
To further strengthen the interaction between the support and GO/ COF-1 nanosheets, the crosslinked PVA/PVAm matrix was used as cross-linker. In brief, 2 g PVA was dissolved in 98 g DI water and refluxed at 95 °C for 2 h, solution A. And 2 g PVAm was added into DI water and stirred for 30 min, solution B. After that, solution B was mixed with solution A and stirred at 25 °C for 24 h (the mass ratio of PVA and PVAm aqueous solution is 1:4). Finally, the crosslinked PVA/ PVAm matrix was diluted to 0.1 wt%. 322
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
Scheme 1. Schematic representation of GO/COF-1 membrane fabrication.
2.6. Modification of the support surface
Cp ⎞ R = ⎜⎛1 − ⎟ × 100% Cf ⎠ ⎝
To make membrane surface possess abundant carboxy groups, Polyacrylonitrile (PAN) ultrafiltration membranes were immersed in NaOH aqueous solution (1 mol/L) at 50 °C for 1 h. After that, PAN membranes were washed repeatedly using DI water until pH values of the soaked water reached about 7.0.
Where Cp (mg/L) and Cf (mg/L) are the concentrations of permeate of dyes and salts in permeate and the feed sides, respectively. 2.9. Membrane characterization The morphologies (surface and cross-section) and elemental distribution of the samples was investigated using Focused ion beam scanning electron microscopy (Zeiss/Auriga FIB SEM) energy dispersive X-ray spectroscopy (EDX, Oxford), respectively. All the samples were coated with gold before analysis. The thermal stabilities of the neat GO, COF-1, and GO/COF-1 nanocomposite samples were studied by a Simultaneous Thermal Analyzer (STA 409 PC/STA 409 PC). Samples (∼200 mg) were heated under a flowing Ar atmosphere from 25 °C to 800 °C at a heating rate of 5 °C/min. The crystalline structures of samples were analyzed in an X-ray Diffraction and Scattering (D8ADVANCE) with Cu-Kα radiation for 2θ from 5 to 60 °C with a scan step of 0.026 °C. The surface roughness analysis of the membranes was measured by using Atomic-force microscopy (AFM, Bruker Dimension Fastscan, USA). All the membranes were dried before the AFM analysis was performed. The gas adsorption isotherm was carried out with Micromeritic Tristar 3000 analyzer. At least 200 mg sample was loaded into the sample vial and dried at 60 °C under vacuum for 3 h. The samples were degassed using Micromeritics Degassing system at 150 °C in nitrogen with the pressure range of 0–1.06 bar. To research the hydrophilic nature of the membrane surface, water contact angles of the membrane surfaces were measured using an EasyDrop contact angle instrument and software (Kruss, Germany) at room temperature. All membranes were dried at room temperature before measuring their contact angles. Contact angles of DI water were measured by using a sessile drop (2 μL) on the samples. All tests were performed on different locations of the each membrane at least 5 times.
2.7. Fabrication of GO/COF-1 membranes Firstly, GO/COF-1 suspension was prepared by dispersing GO/COF1 powder into DI water and sonicated for 1 h at room temperature. Then, the mixture composed of 1 mL of PVA/PVAm (0.1 wt%) and a certain amount of GO/COF-1 suspension (0.05–0.75 mg/mL) was sonicated by the ultrasonic washer for 10 min. After that, the PAN membranes were placed in a dead-end filtration device which was filled with solution A. and before that we should pre-pressure PAN membrane with N2 (0.01 MPa) for 20 min at room temperature. At last, the substrates were rinsed in DI water and dried using an oven at 50 °C. 2.8. Dyes and salt rejections for GO/COF-1 membranes The rejection performance of the membranes, including the water fluxes and rejections to dye/salt, was evaluated by a typically a crossflow circulation system with an effective membrane area of 7.1 cm2. To make the membranes reach a steady state before testing the performance, pre-pressure was employed for 30 min under 0.4 MPa. Feed solutions involving pure water, saline solutions (1 g/L) include NaCl, MgCl2, Na2SO4 and MgSO4 and dyes solution (0.2 g/L) contain Congo red, Direct red, Methylene blue, Reactive Black 5, Rhodamine B. The experiments were conducted under a pressure of 0.4 MPa at room temperature. The salt concentrations were measured with an electrical conductivity meter and the concentration of dyes was analyzed by a UV–Vis detector at the maximal absorption wavelength of the organic dyes. Pure water flux (J, L m−2 h−1) was calculated using the following equation:
J=
3. Results and discussion 3.1. Morphologies analysis of GO/COF-1 nanocomposites
V AΔt
As shown in Fig. S2, COF-1 nanosheets were synthesized, which are in good agreement with reported COF-1 powder [1]. SEM images of GO/COF-1 in different magnification are shown in Fig. 1a–b, it could be seen that COF-1 nanosheets are uniformly distributed in the surface of GO. Thus, we speculated that the massive crystallization of COF-1
Where V(L), A(m2) and Δt are the volume of the permeated water (L), the effective area of membrane (m2) and the permeation time (h). The rejection ratios (R) of the membranes were defined by the equation: 323
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
Fig. 1. (a, b) SEM image of GO/COF-1 as-synthesized, (c, d) AFM peak force images of GO/COF-1.
crystallites on the surface of GO. Meanwhile, GO/COF-1 was detected by AFM in Fig. 1c, and a uniform distribution structure similar to that of Fig. 1b is shown in Fig. 1d. The XRD of GO, COF-1 and GO/COF-1 powder were characterized shown in Fig. 2. GO shows a single diffraction peak in 2θ of 9.5°, corresponding to the interlayer spacing of 0.93 nm, this could be attributed to the hydration effect originated from water molecules trapped inside the stacked GO sheets and the oxygen-containing functional groups [45,46]. The diffraction peaks of COF-1 are consistent with the literature [1]. As expected, the results show that the peak position is exactly alike COF-1 except for the difference of the intensity of each peak. Besides, a small diffraction peak in 2θ of 8.6° was observed, and we speculated that it is the diffraction peak of GO. The introduction of COF-1 could cause the diffraction peak to the lower angle, corresponding to the interlayer spacing of 1.03 nm. These results
demonstrated that COF-1 is formed into adjacent GO sheets acted as spacer increasing the interlamellar spacing. Furthermore, the specific surface area and the pore size distribution of GO, COF-1 and GO/COF-1 were measured respectively by analyzing the N2 adsorption isotherm. As shown in Fig. 3 and Fig. 4, the specific surface area of GO/COF-1 nanocomposite is 692.62 m2 g−1, which is higher than both GO (55.33 m2 g−1) and COF-1 (425.16 m2 g−1). 50.95% of pore volume for GO/COF-1 nanocomposite can be assigned to mesopore is accepted that high specific surface area, ordered channel structure and precise pore size are in favor of increasing the permeability of membranes [47,48]. Thus, we speculate that GO/COF-1 nanocomposite is a promising membrane material for fabricating highflux and energy-efficient membranes. COF-1 is based on the molecular dehydration reaction, in which
Fig. 2. X-ray analysis of GO, COF-1 and GO/COF-1.
Fig. 3. Nitrogen gas adsorption isotherms for GO, COF-1 and GO/COF-1. 324
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
Fig. 4. (a–c) Pore size distribution and cumulative pore volume of GO, COF-1 and GO/COF-1. (d) Specific surface area of GO, COF and GO/COF-1.
Fig. 5. (a) XRD pattern of GO/COF-1 as-synthesized after stability test in water at 25 °C for 7 days. (b) Thermogravimetric analysis result of GO, COF-1 and GO/COF1.
reverse reaction is easy to occur under humid environment, finally causes the collapse of COF-1 microstructure. In order to verify the water stability of GO/COF-1, it was dispersed in aqueous solution for 7 days. As shown in Fig. S3, slightly changed could be observed for the surface morphology of GO/COF-1 samples, XRD pattern (Fig. 5a) shows that the good crystallinity of GO/COF-1 samples can be still obtained, together with a slightly different in the intensity of peak. Hence, we could believe that GO/COF-1 has fairly stable stability in water environment. The thermal stability of GO/COF-1 is also further confirmed by TGA analysis. It is clearly that GO/COF-1 samples exhibit the decomposition process containing four steps (Fig. 5b). The loss of GO/COF-1 with 9.65% in the first step under 25–150 °C, which was mainly due to the loss of adsorbed water. The second step with a loss of 14.92% from 150 to 280 °C, largely because of the decomposition of GO. Interestingly, the
in-suit growth of COF-1 on the surface of GO slowed down the decomposition rate of GO to a certain extent compared with the TGA of GO, this result suggest that besides the decomposition of GO from 150 to 230 °C, the decomposition of GO also occurred from 230 to 280 °C. It can be considered as the partially exposed GO is preferentially decomposed, and the part covered by COF-1 will then be decomposed. The three steps was the removal of the organic molecules with a loss of 22.33% from 280 to 590 °C. The final step decomposition temperature from 590 to 680 °C was mainly due to the collapse of COF-1. The TGA results indicate that there is a high amount of COF-1 (75.43%) on the surface of GO sheets and GO/COF-1 samples also exhibit high thermal stability. Briefly, these results reveal that the prepared GO/COF-1 nanocomposite have superior water stability and thermal stability. This further illustrates its potential as a membrane material.
325
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
Fig. 6. (a, b) Top-view and cross-sectional SEM images of GO/COF-1 membranes, (c, d) Top-view SEM images of COF-1 membranes and GO membranes, respectively, (e, f) The TEM of GO/COF-1 membranes, (g–i) AFM peak force images and three-dimensional AFM images of COF-1 membranes, GO membranes and GO/COF-1 membranes, respectively. (EDX mapping and optical physical map inserted in up-left corner).
Fig. 7. The water (0.021 mg cm−2).
contract
angles
of
different
membrane
Fig. 8. Pure water flux and retention measured for four different salt solutions.
samples
membranes were carried out as seen from Fig. 6c–d and Fig. S6. COF-1 membranes have obvious aggregation of COF-1 crystals compared with GO membranes and GO/COF-1 membranes. For this phenomenon, we speculate that homogeneous in-situ crystallization of COF-1 crystals on the surface of GO sheets make the superior dispersivity of GO/COF-1 nanocomposite in water (Fig. S7), which circumvent effectively the aggregation phenomenon of the single COF-1 crystal. The pictures of the bottom right corner at Fig. 6b–d were the material object of GO/ COF-1 membrane. Meanwhile, compared with the tightly stacked lamellar structure of GO membranes (Fig. S8), intuitively, GO/COF-1 membranes have a relatively loose lamellar structure (Fig. 6b). This conclusion can be also proved by TEM of GO/COF-1 membrane (Fig. 6e–f). For the structure information of prepared GO/COF-1 membranes, the corresponding intensity peaks for the membrane are recorded in
3.2. Morphologies, chemical, physical and structural properties of GO/COF1 membranes In view of these properties, GO/COF-1 nanocomposite based membranes (GO/COF-1 membranes) were prepared. The morphologies and structure characteristics of GO/COF-1 membranes with different content are presented in Fig. 6a–b and Figs. S4–5. A smooth surface could be observed and the top layer with the thickness of 100 nm–250 nm (additional to the thickness of supports) is evaluated by the cross-sectional view using ImageJ. Moreover, Energy-dispersive X-ray spectroscopy (EDX) was performed, which visibly indicates that distributions of elemental C and B. Element B originates from the B3O3 boroxine unit of COF-1 phase, which are uniformly distributed in the top layer of GO/COF-1 membranes, corresponding to SEM of GO/COF-1 powder. In order to make a contrast, SEM of COF-1 membrane and GO 326
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
Table 1 Dye rejection rates of GO/COF-1 membrane with different loading amount of GO/COF-1. The loading mount GO/COF-1 (mg cm−2)
0.007 0.014 0.021 0.035 0.049
Operation Pressure (MPa)
0.4 MPa
Rejection (%) Congo red
Methylene blue
Reactive Black 5
Direct red
Chrome black T
98.72 99.08 99.62 99.79 99.65
96.55 98.46 99.04 99.39 99.34
63.59 90.19 99.49 99.40 99.10
97.00 97.56 98.95 98.99 98.97
100 100 100 100 100
Fig. 9. Schematic for molecular sieving mechanism through GO/COF-1 membranes.
Fig. 10. (a) Water flux versus pressure applied on GO/COF-1 membrane with a GO/COF-1 loading of 0.021 mg cm−2 (b) Stability test of GO/COF-1 membrane with a GO/COF-1 loading of 0.021 mg cm−2 in a long-time of chrome black T under 0.4 MPa at room temperature.
explain GO/COF-1 membrane keeps the smooth surface. Membrane surface chemistry was measured by the contact angle test. Generally, it is generally considered that the smaller contact angle represents higher hydrophilicity, lower surface roughness, and higher surface energy [51]. Fig. 7 and Fig. S10 show the water contact angle results of GO/COF-1 membranes as a function of GO/COF-1 nanocomposite loading amount. All of prepared GO/COF-1 membranes (Contact angle of 55.1–79.9°) show better hydrophilic property compared to the GO membrane (Contact angle of 85.5°) and COF-1 membrane (Contact angle of 93.2°), which can be contributed to the changes
XRD pattern (Fig. S9). There is no obvious the intensity peak of GO/ COF-1 (additional to the peaks of supports), which can be attributed to the trace amounts of GO/COF-1 nanocomposites in the system or the completely dried of GO/COF-1 coating forming a more ordered compact microstructure [49,50]. In addition, to further demonstrate membrane surface structure, AFM images of COF-1 membrane, the GO membrane and GO/COF-1 membrane were also investigated (Fig. 6g–i). We can clearly see that GO/COF-1 membrane has a relatively low surface roughness compared with COF-1 membrane and the GO membrane. These results consistent with the results obtained by SEM, which 327
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
the rejection rate of exceed 98% for direct red (Mw = 769.76; ca. 2.52 nm × 1.08 nm × 0.26 nm) [42,57]. Surprisingly, Reactive black 5 with a maximum molecular weight and a middle molecular size, has a poor rejection rate compared to other tested compounds, we speculate that the completely clear connection between the molecular shape of Reactive black 5 and rejection rate results in a lower rejection rate [20,56]. In addition to these negative dyes, we find that the membrane exhibits a poor retention rate for positively charged rhodamine B. For these experimental results, the one reason is that the synergistic effect between the intergallery channels (ca. 0.33 nm) of COF-1 and the narrow interlayer spacing of adjacent GO sheets, can reject these dye molecules (ca. exceeding 1.5 nm) (Fig. 9). Another possible reason can be that the electrostatic interaction between negatively charged dye molecules and negatively charged GO, which results in the high rejection rates of negatively charged dye molecules. To further emphasize the practical value of prepared GO/COF-1 membranes in dye wastewater system, the rejection performance of multivalent salt solution (MgSO4, MgCl2) and monovalent salt solution (NaCl, Na2SO4) were evaluated (Fig. 8). The membrane with the loading amount of 0.021 mg cm−2 reveal the low rejection (Na2SO4 for 4.46%, NaCl for 6.54%, MgSO4 for 7.88%, MgCl2 for 11.30%), and other membrane also widely show the low rejection below 22%. Thus, GO/COF-1 membrane with a high dye permeability of 284 L m-2 h−1 MPa−1, high dye rejection (99.9%) and poor salt rejection (< 12%) exhibits excellent selectivity for the separation of dye/salt. Furthermore, the change of water flux was recorded with the increase of pressure when GO/COF-1 loading content was 0.021 mg cm−2 (Fig. 10a and Fig. S12). The water permeation almost increase linearly with the increase of applied pressure ranging 1 bar–8 bar, which indicates that prepared GO/COF-1 membranes show an outstanding mechanical properties. And the stability of the membrane was also evaluated by a long time operation of 48 h, the result demonstrated that a steady dye rejection (99.9%) and dye flux (∼285.3 L m-2 h−1 MPa−1) performance for Chrome black T owing to the excellent stability of GO/ COF-1 nanocomposite (Fig. 10b). SEM of GO/COF-1 membranes after long-term operation (3 days) also reveal that micro-morphology of membranes did not change (Fig. S12). In addition, compared with the most of commercial membranes, GO/COF-1 membrane remains the excellent dye permeability under the similar rejection rate (Fig. 11). Therefore, GO/COF-1 membrane not only has high permeation flux, but also has excellent operation stability and mechanical stability. We could believe that GO/COF-1 nanocomposite as the selective material of membrane for dye/salt is very promising in industry.
Fig. 11. Dye rejection rates of GO/COF-1 membrane with a GO/COF-1 loading of 0.021 mg cm−2 compared with those of other membranes (S1–S10).
in the membrane surface microstructure owing to the introduction of GO/COF-1 nanocomposite. Furthermore, the analysis results from Fig. 7 and Fig. S10 also show that the contact angles of prepared GO/COF-1 membranes increase initially and then decrease with the increase of GO/COF-1 nanocomposite content, which is good agreed with the surface morphology properties of prepared GO/COF-1 membranes as shown in Fig. S11. 3.3. The dyes and salt rejections performance Currently, the output of commercially available dyes in China has reached 90 thousand tons per year, ranking first in the word. However, approximately 10–20% of the produced dyes annually is discharged into the environment in the process of using [52–54]. These discharged dyes not only can cause serious effects on the environment, but also damage human body a lot owing to the carcinogenicity and mutagenicity of them [55]. In view of dye wastewater system containing inorganic salts and water-soluble dyes, the dye rejection performance of prepared GO/COF-1 membranes were evaluated using a typically crossflow filtration system (a transmembrane pressure of 0.4 MPa) at room temperature. The performance of the prepared GO/COF-1 membranes was shown in Fig. 8 and Table 1. The results reveal the water permeation decrease with the increase of GO/COF-1 loading amount. It is worth noting that GO/COF-1 membrane show the higher water flux (310.9 L m-2 h−1 MPa−1) when the loading amount of GO/COF-1 nanocomposite is 0.021 mg cm−2, but the GO membrane with the water flux of 67.3 L m-2 h−1 MPa−1 at the same loading capacity as seen from Table S1. A great increase in water flux may be explained from the structure of GO/COF-1 nanocomposite. It is usually considered that the hydrated diameter of the water is 0.28 nm. We speculate that COF-1 with various shape was embedded in the interlayer of GO, expanding the inner spacing of GO, and the inner spacing of COF-1 is about 0.33 nm simultaneously (Fig. 9). Thus, water molecules can easily pass through GO/COF-1 nanocomposite. Certainly, the water permeation does not depend strongly on molecular size sieving, it also depend strongly on the thickness of GO/COF-1 coating. Theoretically, the thicker GO/COF-1 coating can cause the lower water permeation owing to its longer channel tortuosity [20,56]. The dyes (Congo red, Methylene blue, Reactive black 5, Chrome black T) rejection of GO/COF-1 membranes were evaluated as shown in Table 1. For example, when the loading amount of GO/COF-1 nanocomposite exceeds 0.021 mg cm−2, the prepared membrane can reject Congo red (Mw = 696.68; dimensions of ca. 2.56 nm × 0.73 nm), Methylene blue (Mw = 799.80; ca. 2.36 nm × 1.74 nm), Reactive black 5 (Mw = 991.82; ca. 2 nm) with high retention (> 99%) while exhibit
4. Conclusion Summarily, we have loaded successfully COF-1 crystals onto the GO surface by in-situ growth method. GO/COF-1 nanocomposite avoids the insoluble of COF-1 and has the outstanding water stability. Subsequently, GO/COF-1 membrane was prepared by a dead-end filtration device on the support of PAN ultrafiltration membrane. Prepared membrane exhibits an excellent water permeation over 310 L m-2 h−1 MPa−1, high rejection rate for water soluble dyes (> 99%) and lower rejection (< 12%) for ion salts. This work means to emphasize that nanocomposites combining 2D COF-1 and 2D GO into a material (GO/COF-1) acting as membrane material to prepare the high permeance and energy-efficient membranes for the treatment of dye wastewater. This provides us a potential opportunity to seek excellent hybrid membrane materials through combining the different dimension of nano-materials (0D-2D, 1D-2D and 2D-2D). Acknowledgments This work was supported by the National Natural Science Foundation of China (No. U1704139), Key Science and Technology 328
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
Program of Henan Province (182102310013) and Training Plan for Young Backbone Teachers in Universities of Henan Province (2017GGJS002). The authors gratefully thank Center of Advanced Analysis & Computational Science, Zhengzhou University for help with the characterization.
[24] K. Celebi, J. Buchheim, R.M. Wyss, A. Droudian, P. Gasser, I. Shorubalko, J.I. Kye, C. Lee, H.G. Park, Ultimate permeation across atomically thin porous graphene, Science 344 (2014) 289–292. [25] T. Huang, L. Zhang, H. Chen, C. Gao, Sol–gel fabrication of a non-laminated graphene oxide membrane for oil/water separation, J. Mater. Chem. A 3 (2015) 19517–19524. [26] L. Zhang, Y. Lu, Y.L. Liu, M. Li, H. Zhao, L. Hou, High flux MWCNTs-interlinked GO hybrid membranes survived in cross-flow filtration for the treatment of strontiumcontaining wastewater, J. Hazard. Mater. 320 (2016) 187–193. [27] J. Wang, T. Huang, L. Zhang, Q.J. Yu, L.A. Hou, Dopamine crosslinked graphene oxide membrane for simultaneous removal of organic pollutants and trace heavy metals from aqueous solution, Environ. Technol. 39 (2018) 3055–3065. [28] H. Huang, Z. Song, N. Wei, S. Li, Y. Mao, Y. Ying, L. Sun, Z. Xu, X. Peng, Ultrafast viscous water flow through nanostrand-channelled graphene oxide membranes, Nat. Commun. 4 (2013) 2979. [29] W.S. Hung, C.H. Tsou, M.D. Guzman, Q.F. An, Y.L. Liu, Y.M. Zhang, C.C. Hu, K.R. Lee, J.Y. Lai, Cross-linking with diamine monomers to prepare composite graphene oxide-framework membranes with varying d-spacing, Chem. Mater. 26 (2014) 2983–2990. [30] Y. Su, V.G. Kravets, S.L. Wong, J. Waters, A.K. Geim, R.R. Nair, Impermeable barrier films and protective coatings based on reduced graphene oxide, Nat. Commun. 5 (2014) 4843. [31] P. Sun, K. Wang, H. Zhu, Recent developments in graphene-based membranes: structure, mass-transport mechanism and potential applications, Adv. Mater. 28 (2016) 2287–2310. [32] K.H. Thebo, X. Qian, Q. Zhang, L. Chen, H.M. Cheng, W. Ren, Highly stable graphene-oxide-based membranes with superior permeability, Nat. Commun. 9 (2018) 1486. [33] A. Akbari, P. Sheath, S.T. Martin, D.B. Shinde, M. Shaibani, P.C. Banerjee, R. Tkacz, D. Bhattacharyya, M. Majumder, Large-area graphene-based nanofiltration membranes by shear alignment of discotic nematic liquid crystals of graphene oxide, Nat. Commun. 7 (2016) 10891. [34] H. Liu, H. Wang, X. Zhang, Facile fabrication of freestanding ultrathin reduced graphene oxide membranes for water purification, Adv. Mater. 27 (2014) 249–254. [35] R.C. Tatar, S. Rabii, Electronic properties of graphite: a unified theoretical study, Phys. Rev. B Condens. Matter 25 (1982) 4126–4141. [36] J.W. Burress, S. Gadipelli, J. Ford, J.M. Simmons, W. Zhou, T. Yildirim, Graphene oxide framework materials: theoretical predictions and experimental results, Angew. Chem. Int. Ed. 49 (2010) 8902–8904. [37] G. Srinivas, J.W. Burress, J. Ford, T. Yildirim, Porous graphene oxide frameworks: synthesis and gas sorption properties, J. Mater. Chem. 21 (2011) 11323–11329. [38] G. Mercier, A. Klechikov, M. Hedenström, J. Dan, I.A. Baburin, G. Seifert, R. Mysyk, A.V. Talyzin, Porous graphene oxide/diboronic acid materials: structure and hydrogen sorption, J. Phys. Chem. C 119 (2016) 27179–27191. [39] G. Liu, W. Jin, N. Xu, Graphene-based membranes, Chem. Soc. Rev. 44 (2015) 5016–5030. [40] Y. Zhang, S. Zhang, T.S. Chung, Nanometric graphene oxide framework membranes with enhanced heavy metal removal via nanofiltration, Environ. Sci. Technol. 49 (2015) 10235–10242. [41] K. Goh, W. Jiang, H.E. Karahan, S. Zhai, L. Wei, D. Yu, A.G. Fane, R. Wang, Y. Chen, All‐carbon nanoarchitectures as high‐performance separation membranes with superior stability, Adv. Funct. Mater. 25 (2016) 7348–7359. [42] X. Liu, N.K. Demir, Z. Wu, K. Li, Highly water-stable zirconium metal–organic framework UiO-66 membranes supported on alumina hollow fibers for desalination, J. Am. Chem. Soc. 137 (2015) 6999–7002. [43] J. Geng, H.T. Jung, Porphyrin functionalized graphene sheets in aqueous suspensions: from the preparation of graphene sheets to highly conductive graphene films, J. Phys. Chem. C 114 (2010) 8227–8234. [44] J. Sun, A. Klechikov, C. Moise, M. Prodana, M. Enachescu, A.V. Talyzin, A molecular pillar approach to grow vertical covalent organic framework nanosheets on graphene: hybrid materials for energy storage, Angew. Chem. Int. Ed. 57 (2018) 1–6. [45] R.R. Nair, H.A. Wu, P.N. Jayaram, I.V. Grigorieva, A.K. Geim, Unimpeded permeation of water through helium-leak-tight graphene-based membranes, Science 335 (2012) 442–444. [46] K. Huang, G. Liu, J. Shen, Z. Chu, H. Zhou, X. Gu, W. Jin, N. Xu, High-efficiency water-transport channels using the synergistic effect of a hydrophilic polymer and graphene oxide laminates, Adv. Funct. Mater. 25 (2015) 5809–5815. [47] G. Liu, W. Jin, N. Xu, Two‐dimensional‐material membranes: a new family of high‐performance separation membranes, Angew. Chem. Int. Ed. 55 (2016) 13384–13397. [48] C. Tan, X. Cao, X.J. Wu, Q. He, Y. Jian, Z. Xiao, J. Chen, Z. Wei, S. Han, G.H. Nam, Recent advances in ultrathin two-dimensional nanomaterials, Chem. Rev. 117 (2017) 6225–6331. [49] Z. Kang, S. Wang, L. Fan, M. Zhang, W. Kang, J. Pang, X. Du, H. Guo, R. Wang, D. Sun, In situ generation of intercalated membranes for efficient gas separation, Commun. Chem. 1 (2018) 1–8. [50] J. Zhu, G. Zeng, F. Nie, X. Xu, S. Chen, Q. Han, X. Wang, Decorating graphene oxide with CuO nanoparticles in a water-isopropanol system, Nanoscale 2 (2010) 988–994. [51] B. Van der Bruggen, M. Mänttäri, M. Nyström, Drawbacks of applying nanofiltration and how to avoid them: a review, Separ. Purif. Technol. 63 (2008) 251–263. [52] S. Sansuk, S. Srijaranai, S. Srijaranai, A new approach for removing anionic organic dyes from wastewater based on electrostatically driven assembly, Environ. Sci. Technol. 50 (2016) 6477–6484. [53] M. Barbosa, N.F.F. Moreira, A.R. Ribeiro, M.F.R. Pereira, A.M.T. Silva, Occurrence
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.03.070. References [1] A.P. Côté, A.I. Benin, N.W. Ockwig, M. O'Keeffe, A.J. Matzger, O.M. Yaghi, Porous, crystalline, covalent organic frameworks, Synfacts 310 (2005) 1166–1170. [2] Q. Fang, Z. Zhuang, S. Gu, R.B. Kaspar, J. Zheng, J. Wang, S. Qiu, Y. Yan, Designed synthesis of large-pore crystalline polyimide covalent organic frameworks, Nat. Commun. 5 (2014) 4503. [3] B.J. Smith, A.C. Overholts, N. Hwang, W.R. Dichtel, Insight into the crystallization of amorphous imine-linked polymer networks to 2D covalent organic frameworks, Chem. Commun. 52 (2016) 3690–3693. [4] Z. Kang, Y. Peng, Y. Qian, D. Yuan, M.A. Addicoat, T. Heine, Z. Hu, L. Tee, Z. Guo, D. Zhao, Mixed matrix membranes (MMMs) comprising exfoliated 2D covalent organic frameworks (COFs) for efficient CO2 separation, Chem. Mater. 28 (2016) 1277–1285. [5] C.J. Doonan, D.J. Tranchemontagne, T.G. Glover, J.R. Hunt, O.M. Yaghi, Exceptional ammonia uptake by a covalent organic framework, Nat. Chem. 2 (2010) 235–238. [6] S. Chandra, D.R. Chowdhury, M. Addicoat, T. Heine, A. Paul, R. Banerjee, Molecular level control of the capacitance of two-dimensional covalent organic frameworks: role of hydrogen bonding in energy storage materials, Chem. Mater. 29 (2017) 2074–2080. [7] G. Lin, H. Ding, D. Yuan, B. Wang, C. Wang, A pyrene-based, fluorescent threedimensional covalent organic framework, J. Am. Chem. Soc. 138 (2016) 3302–3305. [8] B.P. Biswal, H.D. Chaudhari, R. Banerjee, U.K. Kharul, Chemically stable covalent organic framework (COF)‐Polybenzimidazole hybrid membranes: enhanced gas separation through pore modulation, Chem. A Eur J. 22 (2016) 4695–4699. [9] S. Kandambeth, B.P. Biswal, H.D. Chaudhari, K.C. Rout, H.S. Kunjattu, S. Mitra, S. Karak, A. Das, R. Mukherjee, U.K. Kharul, Selective molecular sieving in selfstanding porous covalent-organic-framework membranes, Adv. Mater. 29 (2017) 1603945. [10] X.H. Liu, C.Z. Guan, S.Y. Ding, W. Wang, H.J. Yan, D. Wang, L.J. Wan, On-surface synthesis of single-layered two-dimensional covalent organic frameworks via solidvapor interface reactions, J. Am. Chem. Soc. 135 (2013) 10470–10474. [11] X. Feng, X. Ding, D. Jiang, Covalent organic frameworks, Chem. Soc. Rev. 41 (2012) 6010–6022. [12] M. Dogru, A. Sonnauer, S. Zimdars, M. Döblinger, P. Knochel, T. Bein, Facile synthesis of a mesoporous benzothiadiazole-COF based on a transesterification process, CrystEngComm 15 (2013) 1500–1502. [13] B.P. Biswal, C. Suman, K. Sharath, L. Binit, H. Thomas, B. Rahul, Mechanochemical synthesis of chemically stable isoreticular covalent organic frameworks, J. Am. Chem. Soc. 135 (2013) 5328–5331. [14] B.P. Biswal, H.D. Chaudhari, R. Banerjee, U.K. Kharul, Chemically stable covalent organic framework (COF)-Polybenzimidazole hybrid membranes: enhanced gas separation through pore modulation, Chem. A Eur J. 22 (2016) 4695–4699. [15] E.S. Cho, N.E. Coates, J.D. Forster, A.M. Ruminski, B. Russ, A. Sahu, N.C. Su, F. Yang, J.J. Urban, Engineering synergy: energy and mass transport in hybrid nanomaterials, Adv. Mater. 27 (2015) 5744–5752. [16] C.Z. Guan, D. Wang, L.J. Wan, Construction and repair of highly ordered 2D covalent networks by chemical equilibrium regulation, Chem. Commun. 48 (2012) 2943–2945. [17] X.H. Liu, C.Z. Guan, S.Y. Ding, W. Wang, H.J. Yan, D. Wang, L.J. Wan, On-surface synthesis of single-layered two-dimensional covalent organic frameworks via solidvapor interface reactions, J. Am. Chem. Soc. 135 (2013) 10470–10474. [18] D.D. Medina, V. Werner, F. Auras, R. Tautz, M. Dogru, J. Schuster, S. Linke, M. Döblinger, J. Feldmann, P. Knochel, Oriented thin films of a benzodithiophene covalent organic framework, ACS Nano 8 (2014) 4042–4052. [19] R.K. Joshi, P. Carbone, F.C. Wang, V.G. Kravets, Y. Su, I.V. Grigorieva, H.A. Wu, A.K. Geim, R.R. Nair, Precise and ultrafast molecular sieving through graphene oxide membranes, Science 343 (2014) 752–754. [20] Y. Han, Z. Xu, C. Gao, Ultrathin graphene nanofiltration membrane for water purification, Adv. Funct. Mater. 23 (2013) 3693–3700. [21] P. Sun, F. Zheng, M. Zhu, Z. Song, K. Wang, M. Zhong, D. Wu, R.B. Little, Z. Xu, H. Zhu, Selective trans-membrane transport of alkali and alkaline earth cations through graphene oxide membranes based on Cation−π interactions, ACS Nano 8 (2014) 850–859. [22] S.P. Surwade, S.N. Smirnov, I.V. Vlassiouk, R.R. Unocic, G.M. Veith, S. Dai, S.M. Mahurin, Water desalination using nanoporous single-layer graphene, Nat. Nanotechnol. 10 (2015) 459–464. [23] L.C. Lin, J.C. Grossman, Atomistic understandings of reduced graphene oxide as an ultrathin-film nanoporous membrane for separations, Nat. Commun. 6 (2015) 8335.
329
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
[56] H. Fan, J. Gu, H. Meng, A. Knebel, J. Caro, High‐flux membranes based on the covalent organic framework COF‐LZU1 for selective dye separation by nanofiltration, Angew. Chem. Int. Ed. Int. Ed. 57 (2018) 4083–4087. [57] L. Huang, J. Chen, T. Gao, M. Zhang, Y. Li, L. Dai, L. Qu, G. Shi, Reduced graphene oxide membranes for ultrafast organic solvent nanofiltration, Adv. Mater. 28 (2016) 8669–8674.
and removal of organic micropollutants: an overview of the watch list of EU Decision 2015/495, Water Res. 94 (2016) 257–279. [54] C. Allègre, P. Moulin, M. Maisseu, F. Charbit, Treatment and reuse of reactive dyeing effluents, J. Membr. Sci. 269 (2016) 15–34. [55] S. Karan, Z. Jiang, A.G. Livingston, Sub-10 nm polyamide nanofilms with ultrafast solvent transport for molecular separation, Science 348 (2015) 1347–1351.
330
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
In-situ grown covalent organic framework nanosheets on graphene for membrane-based dye/salt separation
T
Xuke Zhanga, Hui Lia,b, Jing Wanga,∗, Donglai Pengc, Jindun Liua, Yatao Zhanga,∗∗ a
School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, 450001, PR China Research Department of New Materials, Zhengzhou Institute of Emerging Industrial Technology, Zhengzhou, 450000, PR China c School of Material & Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou, 450001, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Covalent organic framework Graphene oxide In-situ growth Membrane Dye/salt separation
Covalent organic framework-1 (COF-1) with ordered channel structure and precise pore size covalently attaching the surface of graphene oxide (GO) was synthesized by in-situ growth. The prepared GO/COF-1 nanocomposite improves dispersivity and stability in water over COF-1, allowing GO/COF-1 nanocomposite to be used as membrane material for water treatment. Here, high-flux membranes-based hybrid 2D-2D GO/COF-1 nanocomposites were constructed on the support of Polyacrylonitrile (PAN) ultrafiltration membrane using a deadend filtration device. The prepared membranes exhibit an excellent water permeation over 310 L m-2 h−1 MPa−1, high rejection rate for water soluble dyes (> 99%) and high permeability for ion salts. The outstanding dye rejection and water permeation performance can be attributed to the physical size sieving of COF-1 (the layer distance of near 0.33 nm) and the suitably interlayer spacing between the adjacent GO sheets. Moreover, electrostatic interaction also plays an important role for the rejection process. This approach presents an enormous potential for purification of dye wastewater.
1. Introduction Two-dimensional (2D) covalent organic frameworks (COFs) are a novel class of crystalline materials with the layered structure and permanent porosity. The crystal structures in COFs were entirely linked built by strong covalent bonds between light elements (B, C and O). Because of its high thermal, chemical stability, low density and ordered channel structure, COFs is regards to be a promising material in the field of gas storage, energy storage, catalysis and membrane separation material [1–3]. Beyond that, suitable monomers design may endow COFs with the tunable porosities and variable structure, making COFs to be a desirable material for molecular sieving and separation [4–9]. However, the internal defects could severely restrict the crystallization growth of COFs into nanodomains. The conventional synthetic approaches of 2D COFs are unable synthesize microcrystalline powders with controllable morphology [10–13], thus the weakness limits the application of COFs. Currently, many approaches have been proposed to use COFs into membrane, namely self-standing COFs membranes and mixed matrix membranes (MMMs). But self-standing COFs membranes are difficult to fabrication due to internal defects of COFs could impede the formation of a complete continuous membrane structure during
∗
membrane preparation. Poly/COFs mixed matrix membranes was obtained via uniformly dispersing COFs into polymeric matrices. Although such membranes are easy to process, their separation performance is lower than self-standing COFs membranes because of the interface voids and COFs pore blockage [14–16]. In brief, poor membraneforming properties of COFs restrict its application in the field of membrane separation technology. Consequently, it is still a great challenge to finely combine insoluble COFs into membrane. Several strategies have been developed to solve this issue. For example, singlelayered graphene could be obtained by exfoliation, but it is difficult to implement for COF due to the large size crystals fail to obtain. Meanwhile, COFs could grow directly on the surface, even though there are different types of organic coupling reactions in the synthesis of COFs, which is regarded as a promising strategy [17,18]. Graphene oxide (GO) with hydrophilicity, easy to function ability, nearly frictionless surface and unprecedented fast water-transport channels [19,20] have attracted increasing interest for desalination, purification and molecular separation [21–27]. However, it is difficult to reasonably regulate the interlayer spacing between the adjacent GO nanosheets to achieve molecular sieving, sealing solvent [28–31]. Therefore, it is of great value to developed a facile strategy to control
Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. Wang), [email protected] (Y. Zhang).
∗∗
https://doi.org/10.1016/j.memsci.2019.03.070 Received 23 December 2018; Received in revised form 21 February 2019; Accepted 23 March 2019 Available online 25 March 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
Scientific Ltd (China). 1,4-dioxane (98%) and mesitylene (99%) were purchased from Shanghai Macklin Biochemical Co., Ltd. Polyvinyl alcohol (PVA) (Mw-70,000 Da) was supplied by Sigma-Aldrich (St. Louis, MO). Poly (vinylamine) (PVAm, Mw-100,000 Da) was procured by Mitsubishi Chemical (Shanghai) Co., Ltd. sodium chloride (NaCl, 99%), Sodium sulfate (Na2SO4, 99%), magnesium sulfate (MgSO4, 99%), magnesium chloride (MgCl2, 99%) were obtained from, Congo red (Mw-696.66 Da), Direct red 23 (Mw-813.72 Da), Methylene blue (Mw99.80 Da) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd, Reactive Black 5 (Mw-991.8 Da) were obtained from Sunwell Chemicals Co., Ltd., China. PAN membrane (Mw-50 kDa) was provided by Spero Membranes, Beijing. Deionized water (DI water) was used in all experiments.
the interlayer spacing of GO based membrane. Recently, it is reported that the interlayer spacing of GO could be tailored by partially chemical reduction or intercalating with nanocrystalline materials (ca: MOFs) to increase the water permeability of the GO membrane [32–34]. This method was useful for increasing permeation flux, but the stability was not good enough after the long-time operation, the flux inevitably decreases in water permeability. Therefore, it is of great value to developed a facile strategy to control the interlayer spacing of GO based membrane. As a typical member of COFs family, COF-1 was synthesized by the condensation reaction of 1, 4-benzenediboronic acid in mesitylene/dioxane solution in a sealed pyrextube [1]. The structure of COF-1 is similar to expanded porous graphitic layers, along with an interlayer spacing similarly to graphite [3.328 Å versus 3.348 Å] [35]. Moreover, the capture size of COF-1 is 1.5 nm, which have been estimated by the van der Waals distance between phenyl and B3O3 rings of adjacent layers [1]. These features make it possible to become a new candidates acting as size exclusion for membrane. However, COF-1 is considered to have a weak dispersion, inevitably producing agglomeration in the process of membrane preparation. Recently, solvothermal reaction of GO with 1, 4-benzene diboronic acid (DBA) were reported to form graphene oxide framework (GOF) materials [36–38]. COF-1 can be grown in the surface of GO because of BeO bonding between DBA and oxygen functional groups on the GO sheets (Fig. S1). This method maybe solves the agglomeration issue of COF-1. Besides that, GO membranes suffer from the poor stability issue in aqueous solution [39,40]. The chemical reduction of the oxidation groups could improve the stability of GO membrane in aqueous solution, while the narrow interlayer space between reduced GO layers could lead to the drastic decreases of water permeability [41]. The intercalated COF-1 provides an extra passageway to improve water penetration. Additionally, the covalent bonds between COF-1 and GO are stronger than metal-organic frameworks (MOFs) due to the composition of COF-1 with non-metallic element (B, C). Besides, COF-1 reduced the oxidation groups on GO surface to increase the stability of GO under the long water pressure operation, resulting in GO sheets not easy to collapse. Therefore, GO/ COF-1 nanocomposites were expected to be used in membrane. Furthermore, we assume that molecular sieve membranes-based GO/COF-1 nanocomposites (GO/COF-1 membranes) would exhibit excellent selective water permeation (ca. 2.8 Å) and water-soluble salt (ca. 6.6–10 Å) permeation compared with water soluble dye (> 20 Å) [42]. Herein, hybrid GO/COF-1 nanocomposites were synthesized by insitu growth of COF-1 on the surface of GO, which has an outstanding thermal stability and water stability. To study the effects of GO/COF-1 nanocomposites on membrane structural and separation properties, a series of GO/COF-1 membranes were prepared using a dead-end filtration device (Scheme 1). The flux and separation efficiency were investigated through water flux and salt, dyes rejection tests. This work seeks to provide an approach which improves the dispersion of COFs, tunes the interlayer spacing between adjacent GO nanosheets and improve the stability of GO membrane in water solution, emphasizing the synergistic effect between COF-1 and GO for water purification and dye desalination.
2.2. Synthesis of graphite oxide (GO) GO was prepared via using the modified Hummers method [43]. Typically, graphite powder (3.0 g, 1 wt%) and KMnO4 (18.0 g, 6 wt%) was added to the mixture composed of 360 mL of concentrated H2SO4 and 40 mL of H3PO4, and then the mixture was stirred at 50 °C for 22 h. After that, 1200 mL ice water (DI water) was added into the mixture, together with 2 mL of 30% H2O2 to a graphite oxide suspension. Then, the suspension was sonicated for 1 h and centrifuged at 1000 rpm for 10 min to attain the graphite oxide supernatant. Finally, the graphite oxide supernatant was centrifuged at 8000 rpm for 7 min. The remaining product was washed repeatedly and continuously with DI water, 30 wt% HCl, and ethanol for 3 times respectively. A yellowbrown graphene oxide (GO) was attained and further dried using a vacuum freeze-drying device. 2.3. Synthesis of COF-1 COF-1 was prepared by the same method as described in a previous paper [44]. In brief, DBA (25 mg, 0.15 mmol) was dissolved into 1 mL of mesitylene: dioxane (1:1 in volume). Then, the reaction mixture was heated at 120 °C for 72 h standing at the nitrogen environment. After that, a white solid at bottom of the stainless steel reactor which was separated by centrifugation and washed repeatedly 3 times with acetone. Finally, the attained products were dried at 50 °C for 24 h under vacuum. 2.4. Synthesis of GO/COF-1 nanocomposites
2. Experimental
GO powder (160 mg) was sonicated in methanol (32 mL) to obtain GO dispersion. And then, 1, 4-benzenediboronic acid (DBA, 480 mg) was added. The mixture was transferred to a stainless steel reactor to make a solvothermal reaction at 90 °C for 12 h. The resultant was repeatedly washed with methanol to ensure that the unreacted DBA was removed, and then washed 3 times by using mesitylene/dioxane solution (1:1 in volume). The resulting products was directly re-dispersed in 32 mL mesitylene/dioxane solution (1:1 in volume), and DBA (720 mg) were added to acquire homogeneous solution. At last, the mixture was transferred to a stainless steel reactor to conduct a solvothermal reaction at 120 °C for 72 h. The sample was thoroughly washed with acetone and then dried under vacuum at 50 °C.
2.1. Chemicals and materials
2.5. Preparation of PVAm/PVA polymeric matrix
All chemicals (analytical grade) were used without further purification. A natural graphite powder (ca. 45 μm) was obtained from Sinopharm Chemical Regent. Concentrated sulfuric acid (H2SO4, 98 wt %), hydrochloric acid (HCl), phosphoric acid (H3PO4, 85 wt%), and hydrogen peroxide (H2O2, 30 wt%) of analytical grade were provided by Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd (Tianjin, China). Potassium permanganate (KMnO4), sodium hydroxide (NaOH) and 1,4-benzenediboronic acid (DBA, 98%) was purchased from J&K
To further strengthen the interaction between the support and GO/ COF-1 nanosheets, the crosslinked PVA/PVAm matrix was used as cross-linker. In brief, 2 g PVA was dissolved in 98 g DI water and refluxed at 95 °C for 2 h, solution A. And 2 g PVAm was added into DI water and stirred for 30 min, solution B. After that, solution B was mixed with solution A and stirred at 25 °C for 24 h (the mass ratio of PVA and PVAm aqueous solution is 1:4). Finally, the crosslinked PVA/ PVAm matrix was diluted to 0.1 wt%. 322
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
Scheme 1. Schematic representation of GO/COF-1 membrane fabrication.
2.6. Modification of the support surface
Cp ⎞ R = ⎜⎛1 − ⎟ × 100% Cf ⎠ ⎝
To make membrane surface possess abundant carboxy groups, Polyacrylonitrile (PAN) ultrafiltration membranes were immersed in NaOH aqueous solution (1 mol/L) at 50 °C for 1 h. After that, PAN membranes were washed repeatedly using DI water until pH values of the soaked water reached about 7.0.
Where Cp (mg/L) and Cf (mg/L) are the concentrations of permeate of dyes and salts in permeate and the feed sides, respectively. 2.9. Membrane characterization The morphologies (surface and cross-section) and elemental distribution of the samples was investigated using Focused ion beam scanning electron microscopy (Zeiss/Auriga FIB SEM) energy dispersive X-ray spectroscopy (EDX, Oxford), respectively. All the samples were coated with gold before analysis. The thermal stabilities of the neat GO, COF-1, and GO/COF-1 nanocomposite samples were studied by a Simultaneous Thermal Analyzer (STA 409 PC/STA 409 PC). Samples (∼200 mg) were heated under a flowing Ar atmosphere from 25 °C to 800 °C at a heating rate of 5 °C/min. The crystalline structures of samples were analyzed in an X-ray Diffraction and Scattering (D8ADVANCE) with Cu-Kα radiation for 2θ from 5 to 60 °C with a scan step of 0.026 °C. The surface roughness analysis of the membranes was measured by using Atomic-force microscopy (AFM, Bruker Dimension Fastscan, USA). All the membranes were dried before the AFM analysis was performed. The gas adsorption isotherm was carried out with Micromeritic Tristar 3000 analyzer. At least 200 mg sample was loaded into the sample vial and dried at 60 °C under vacuum for 3 h. The samples were degassed using Micromeritics Degassing system at 150 °C in nitrogen with the pressure range of 0–1.06 bar. To research the hydrophilic nature of the membrane surface, water contact angles of the membrane surfaces were measured using an EasyDrop contact angle instrument and software (Kruss, Germany) at room temperature. All membranes were dried at room temperature before measuring their contact angles. Contact angles of DI water were measured by using a sessile drop (2 μL) on the samples. All tests were performed on different locations of the each membrane at least 5 times.
2.7. Fabrication of GO/COF-1 membranes Firstly, GO/COF-1 suspension was prepared by dispersing GO/COF1 powder into DI water and sonicated for 1 h at room temperature. Then, the mixture composed of 1 mL of PVA/PVAm (0.1 wt%) and a certain amount of GO/COF-1 suspension (0.05–0.75 mg/mL) was sonicated by the ultrasonic washer for 10 min. After that, the PAN membranes were placed in a dead-end filtration device which was filled with solution A. and before that we should pre-pressure PAN membrane with N2 (0.01 MPa) for 20 min at room temperature. At last, the substrates were rinsed in DI water and dried using an oven at 50 °C. 2.8. Dyes and salt rejections for GO/COF-1 membranes The rejection performance of the membranes, including the water fluxes and rejections to dye/salt, was evaluated by a typically a crossflow circulation system with an effective membrane area of 7.1 cm2. To make the membranes reach a steady state before testing the performance, pre-pressure was employed for 30 min under 0.4 MPa. Feed solutions involving pure water, saline solutions (1 g/L) include NaCl, MgCl2, Na2SO4 and MgSO4 and dyes solution (0.2 g/L) contain Congo red, Direct red, Methylene blue, Reactive Black 5, Rhodamine B. The experiments were conducted under a pressure of 0.4 MPa at room temperature. The salt concentrations were measured with an electrical conductivity meter and the concentration of dyes was analyzed by a UV–Vis detector at the maximal absorption wavelength of the organic dyes. Pure water flux (J, L m−2 h−1) was calculated using the following equation:
J=
3. Results and discussion 3.1. Morphologies analysis of GO/COF-1 nanocomposites
V AΔt
As shown in Fig. S2, COF-1 nanosheets were synthesized, which are in good agreement with reported COF-1 powder [1]. SEM images of GO/COF-1 in different magnification are shown in Fig. 1a–b, it could be seen that COF-1 nanosheets are uniformly distributed in the surface of GO. Thus, we speculated that the massive crystallization of COF-1
Where V(L), A(m2) and Δt are the volume of the permeated water (L), the effective area of membrane (m2) and the permeation time (h). The rejection ratios (R) of the membranes were defined by the equation: 323
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
Fig. 1. (a, b) SEM image of GO/COF-1 as-synthesized, (c, d) AFM peak force images of GO/COF-1.
crystallites on the surface of GO. Meanwhile, GO/COF-1 was detected by AFM in Fig. 1c, and a uniform distribution structure similar to that of Fig. 1b is shown in Fig. 1d. The XRD of GO, COF-1 and GO/COF-1 powder were characterized shown in Fig. 2. GO shows a single diffraction peak in 2θ of 9.5°, corresponding to the interlayer spacing of 0.93 nm, this could be attributed to the hydration effect originated from water molecules trapped inside the stacked GO sheets and the oxygen-containing functional groups [45,46]. The diffraction peaks of COF-1 are consistent with the literature [1]. As expected, the results show that the peak position is exactly alike COF-1 except for the difference of the intensity of each peak. Besides, a small diffraction peak in 2θ of 8.6° was observed, and we speculated that it is the diffraction peak of GO. The introduction of COF-1 could cause the diffraction peak to the lower angle, corresponding to the interlayer spacing of 1.03 nm. These results
demonstrated that COF-1 is formed into adjacent GO sheets acted as spacer increasing the interlamellar spacing. Furthermore, the specific surface area and the pore size distribution of GO, COF-1 and GO/COF-1 were measured respectively by analyzing the N2 adsorption isotherm. As shown in Fig. 3 and Fig. 4, the specific surface area of GO/COF-1 nanocomposite is 692.62 m2 g−1, which is higher than both GO (55.33 m2 g−1) and COF-1 (425.16 m2 g−1). 50.95% of pore volume for GO/COF-1 nanocomposite can be assigned to mesopore is accepted that high specific surface area, ordered channel structure and precise pore size are in favor of increasing the permeability of membranes [47,48]. Thus, we speculate that GO/COF-1 nanocomposite is a promising membrane material for fabricating highflux and energy-efficient membranes. COF-1 is based on the molecular dehydration reaction, in which
Fig. 2. X-ray analysis of GO, COF-1 and GO/COF-1.
Fig. 3. Nitrogen gas adsorption isotherms for GO, COF-1 and GO/COF-1. 324
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
Fig. 4. (a–c) Pore size distribution and cumulative pore volume of GO, COF-1 and GO/COF-1. (d) Specific surface area of GO, COF and GO/COF-1.
Fig. 5. (a) XRD pattern of GO/COF-1 as-synthesized after stability test in water at 25 °C for 7 days. (b) Thermogravimetric analysis result of GO, COF-1 and GO/COF1.
reverse reaction is easy to occur under humid environment, finally causes the collapse of COF-1 microstructure. In order to verify the water stability of GO/COF-1, it was dispersed in aqueous solution for 7 days. As shown in Fig. S3, slightly changed could be observed for the surface morphology of GO/COF-1 samples, XRD pattern (Fig. 5a) shows that the good crystallinity of GO/COF-1 samples can be still obtained, together with a slightly different in the intensity of peak. Hence, we could believe that GO/COF-1 has fairly stable stability in water environment. The thermal stability of GO/COF-1 is also further confirmed by TGA analysis. It is clearly that GO/COF-1 samples exhibit the decomposition process containing four steps (Fig. 5b). The loss of GO/COF-1 with 9.65% in the first step under 25–150 °C, which was mainly due to the loss of adsorbed water. The second step with a loss of 14.92% from 150 to 280 °C, largely because of the decomposition of GO. Interestingly, the
in-suit growth of COF-1 on the surface of GO slowed down the decomposition rate of GO to a certain extent compared with the TGA of GO, this result suggest that besides the decomposition of GO from 150 to 230 °C, the decomposition of GO also occurred from 230 to 280 °C. It can be considered as the partially exposed GO is preferentially decomposed, and the part covered by COF-1 will then be decomposed. The three steps was the removal of the organic molecules with a loss of 22.33% from 280 to 590 °C. The final step decomposition temperature from 590 to 680 °C was mainly due to the collapse of COF-1. The TGA results indicate that there is a high amount of COF-1 (75.43%) on the surface of GO sheets and GO/COF-1 samples also exhibit high thermal stability. Briefly, these results reveal that the prepared GO/COF-1 nanocomposite have superior water stability and thermal stability. This further illustrates its potential as a membrane material.
325
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
Fig. 6. (a, b) Top-view and cross-sectional SEM images of GO/COF-1 membranes, (c, d) Top-view SEM images of COF-1 membranes and GO membranes, respectively, (e, f) The TEM of GO/COF-1 membranes, (g–i) AFM peak force images and three-dimensional AFM images of COF-1 membranes, GO membranes and GO/COF-1 membranes, respectively. (EDX mapping and optical physical map inserted in up-left corner).
Fig. 7. The water (0.021 mg cm−2).
contract
angles
of
different
membrane
Fig. 8. Pure water flux and retention measured for four different salt solutions.
samples
membranes were carried out as seen from Fig. 6c–d and Fig. S6. COF-1 membranes have obvious aggregation of COF-1 crystals compared with GO membranes and GO/COF-1 membranes. For this phenomenon, we speculate that homogeneous in-situ crystallization of COF-1 crystals on the surface of GO sheets make the superior dispersivity of GO/COF-1 nanocomposite in water (Fig. S7), which circumvent effectively the aggregation phenomenon of the single COF-1 crystal. The pictures of the bottom right corner at Fig. 6b–d were the material object of GO/ COF-1 membrane. Meanwhile, compared with the tightly stacked lamellar structure of GO membranes (Fig. S8), intuitively, GO/COF-1 membranes have a relatively loose lamellar structure (Fig. 6b). This conclusion can be also proved by TEM of GO/COF-1 membrane (Fig. 6e–f). For the structure information of prepared GO/COF-1 membranes, the corresponding intensity peaks for the membrane are recorded in
3.2. Morphologies, chemical, physical and structural properties of GO/COF1 membranes In view of these properties, GO/COF-1 nanocomposite based membranes (GO/COF-1 membranes) were prepared. The morphologies and structure characteristics of GO/COF-1 membranes with different content are presented in Fig. 6a–b and Figs. S4–5. A smooth surface could be observed and the top layer with the thickness of 100 nm–250 nm (additional to the thickness of supports) is evaluated by the cross-sectional view using ImageJ. Moreover, Energy-dispersive X-ray spectroscopy (EDX) was performed, which visibly indicates that distributions of elemental C and B. Element B originates from the B3O3 boroxine unit of COF-1 phase, which are uniformly distributed in the top layer of GO/COF-1 membranes, corresponding to SEM of GO/COF-1 powder. In order to make a contrast, SEM of COF-1 membrane and GO 326
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
Table 1 Dye rejection rates of GO/COF-1 membrane with different loading amount of GO/COF-1. The loading mount GO/COF-1 (mg cm−2)
0.007 0.014 0.021 0.035 0.049
Operation Pressure (MPa)
0.4 MPa
Rejection (%) Congo red
Methylene blue
Reactive Black 5
Direct red
Chrome black T
98.72 99.08 99.62 99.79 99.65
96.55 98.46 99.04 99.39 99.34
63.59 90.19 99.49 99.40 99.10
97.00 97.56 98.95 98.99 98.97
100 100 100 100 100
Fig. 9. Schematic for molecular sieving mechanism through GO/COF-1 membranes.
Fig. 10. (a) Water flux versus pressure applied on GO/COF-1 membrane with a GO/COF-1 loading of 0.021 mg cm−2 (b) Stability test of GO/COF-1 membrane with a GO/COF-1 loading of 0.021 mg cm−2 in a long-time of chrome black T under 0.4 MPa at room temperature.
explain GO/COF-1 membrane keeps the smooth surface. Membrane surface chemistry was measured by the contact angle test. Generally, it is generally considered that the smaller contact angle represents higher hydrophilicity, lower surface roughness, and higher surface energy [51]. Fig. 7 and Fig. S10 show the water contact angle results of GO/COF-1 membranes as a function of GO/COF-1 nanocomposite loading amount. All of prepared GO/COF-1 membranes (Contact angle of 55.1–79.9°) show better hydrophilic property compared to the GO membrane (Contact angle of 85.5°) and COF-1 membrane (Contact angle of 93.2°), which can be contributed to the changes
XRD pattern (Fig. S9). There is no obvious the intensity peak of GO/ COF-1 (additional to the peaks of supports), which can be attributed to the trace amounts of GO/COF-1 nanocomposites in the system or the completely dried of GO/COF-1 coating forming a more ordered compact microstructure [49,50]. In addition, to further demonstrate membrane surface structure, AFM images of COF-1 membrane, the GO membrane and GO/COF-1 membrane were also investigated (Fig. 6g–i). We can clearly see that GO/COF-1 membrane has a relatively low surface roughness compared with COF-1 membrane and the GO membrane. These results consistent with the results obtained by SEM, which 327
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
the rejection rate of exceed 98% for direct red (Mw = 769.76; ca. 2.52 nm × 1.08 nm × 0.26 nm) [42,57]. Surprisingly, Reactive black 5 with a maximum molecular weight and a middle molecular size, has a poor rejection rate compared to other tested compounds, we speculate that the completely clear connection between the molecular shape of Reactive black 5 and rejection rate results in a lower rejection rate [20,56]. In addition to these negative dyes, we find that the membrane exhibits a poor retention rate for positively charged rhodamine B. For these experimental results, the one reason is that the synergistic effect between the intergallery channels (ca. 0.33 nm) of COF-1 and the narrow interlayer spacing of adjacent GO sheets, can reject these dye molecules (ca. exceeding 1.5 nm) (Fig. 9). Another possible reason can be that the electrostatic interaction between negatively charged dye molecules and negatively charged GO, which results in the high rejection rates of negatively charged dye molecules. To further emphasize the practical value of prepared GO/COF-1 membranes in dye wastewater system, the rejection performance of multivalent salt solution (MgSO4, MgCl2) and monovalent salt solution (NaCl, Na2SO4) were evaluated (Fig. 8). The membrane with the loading amount of 0.021 mg cm−2 reveal the low rejection (Na2SO4 for 4.46%, NaCl for 6.54%, MgSO4 for 7.88%, MgCl2 for 11.30%), and other membrane also widely show the low rejection below 22%. Thus, GO/COF-1 membrane with a high dye permeability of 284 L m-2 h−1 MPa−1, high dye rejection (99.9%) and poor salt rejection (< 12%) exhibits excellent selectivity for the separation of dye/salt. Furthermore, the change of water flux was recorded with the increase of pressure when GO/COF-1 loading content was 0.021 mg cm−2 (Fig. 10a and Fig. S12). The water permeation almost increase linearly with the increase of applied pressure ranging 1 bar–8 bar, which indicates that prepared GO/COF-1 membranes show an outstanding mechanical properties. And the stability of the membrane was also evaluated by a long time operation of 48 h, the result demonstrated that a steady dye rejection (99.9%) and dye flux (∼285.3 L m-2 h−1 MPa−1) performance for Chrome black T owing to the excellent stability of GO/ COF-1 nanocomposite (Fig. 10b). SEM of GO/COF-1 membranes after long-term operation (3 days) also reveal that micro-morphology of membranes did not change (Fig. S12). In addition, compared with the most of commercial membranes, GO/COF-1 membrane remains the excellent dye permeability under the similar rejection rate (Fig. 11). Therefore, GO/COF-1 membrane not only has high permeation flux, but also has excellent operation stability and mechanical stability. We could believe that GO/COF-1 nanocomposite as the selective material of membrane for dye/salt is very promising in industry.
Fig. 11. Dye rejection rates of GO/COF-1 membrane with a GO/COF-1 loading of 0.021 mg cm−2 compared with those of other membranes (S1–S10).
in the membrane surface microstructure owing to the introduction of GO/COF-1 nanocomposite. Furthermore, the analysis results from Fig. 7 and Fig. S10 also show that the contact angles of prepared GO/COF-1 membranes increase initially and then decrease with the increase of GO/COF-1 nanocomposite content, which is good agreed with the surface morphology properties of prepared GO/COF-1 membranes as shown in Fig. S11. 3.3. The dyes and salt rejections performance Currently, the output of commercially available dyes in China has reached 90 thousand tons per year, ranking first in the word. However, approximately 10–20% of the produced dyes annually is discharged into the environment in the process of using [52–54]. These discharged dyes not only can cause serious effects on the environment, but also damage human body a lot owing to the carcinogenicity and mutagenicity of them [55]. In view of dye wastewater system containing inorganic salts and water-soluble dyes, the dye rejection performance of prepared GO/COF-1 membranes were evaluated using a typically crossflow filtration system (a transmembrane pressure of 0.4 MPa) at room temperature. The performance of the prepared GO/COF-1 membranes was shown in Fig. 8 and Table 1. The results reveal the water permeation decrease with the increase of GO/COF-1 loading amount. It is worth noting that GO/COF-1 membrane show the higher water flux (310.9 L m-2 h−1 MPa−1) when the loading amount of GO/COF-1 nanocomposite is 0.021 mg cm−2, but the GO membrane with the water flux of 67.3 L m-2 h−1 MPa−1 at the same loading capacity as seen from Table S1. A great increase in water flux may be explained from the structure of GO/COF-1 nanocomposite. It is usually considered that the hydrated diameter of the water is 0.28 nm. We speculate that COF-1 with various shape was embedded in the interlayer of GO, expanding the inner spacing of GO, and the inner spacing of COF-1 is about 0.33 nm simultaneously (Fig. 9). Thus, water molecules can easily pass through GO/COF-1 nanocomposite. Certainly, the water permeation does not depend strongly on molecular size sieving, it also depend strongly on the thickness of GO/COF-1 coating. Theoretically, the thicker GO/COF-1 coating can cause the lower water permeation owing to its longer channel tortuosity [20,56]. The dyes (Congo red, Methylene blue, Reactive black 5, Chrome black T) rejection of GO/COF-1 membranes were evaluated as shown in Table 1. For example, when the loading amount of GO/COF-1 nanocomposite exceeds 0.021 mg cm−2, the prepared membrane can reject Congo red (Mw = 696.68; dimensions of ca. 2.56 nm × 0.73 nm), Methylene blue (Mw = 799.80; ca. 2.36 nm × 1.74 nm), Reactive black 5 (Mw = 991.82; ca. 2 nm) with high retention (> 99%) while exhibit
4. Conclusion Summarily, we have loaded successfully COF-1 crystals onto the GO surface by in-situ growth method. GO/COF-1 nanocomposite avoids the insoluble of COF-1 and has the outstanding water stability. Subsequently, GO/COF-1 membrane was prepared by a dead-end filtration device on the support of PAN ultrafiltration membrane. Prepared membrane exhibits an excellent water permeation over 310 L m-2 h−1 MPa−1, high rejection rate for water soluble dyes (> 99%) and lower rejection (< 12%) for ion salts. This work means to emphasize that nanocomposites combining 2D COF-1 and 2D GO into a material (GO/COF-1) acting as membrane material to prepare the high permeance and energy-efficient membranes for the treatment of dye wastewater. This provides us a potential opportunity to seek excellent hybrid membrane materials through combining the different dimension of nano-materials (0D-2D, 1D-2D and 2D-2D). Acknowledgments This work was supported by the National Natural Science Foundation of China (No. U1704139), Key Science and Technology 328
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
Program of Henan Province (182102310013) and Training Plan for Young Backbone Teachers in Universities of Henan Province (2017GGJS002). The authors gratefully thank Center of Advanced Analysis & Computational Science, Zhengzhou University for help with the characterization.
[24] K. Celebi, J. Buchheim, R.M. Wyss, A. Droudian, P. Gasser, I. Shorubalko, J.I. Kye, C. Lee, H.G. Park, Ultimate permeation across atomically thin porous graphene, Science 344 (2014) 289–292. [25] T. Huang, L. Zhang, H. Chen, C. Gao, Sol–gel fabrication of a non-laminated graphene oxide membrane for oil/water separation, J. Mater. Chem. A 3 (2015) 19517–19524. [26] L. Zhang, Y. Lu, Y.L. Liu, M. Li, H. Zhao, L. Hou, High flux MWCNTs-interlinked GO hybrid membranes survived in cross-flow filtration for the treatment of strontiumcontaining wastewater, J. Hazard. Mater. 320 (2016) 187–193. [27] J. Wang, T. Huang, L. Zhang, Q.J. Yu, L.A. Hou, Dopamine crosslinked graphene oxide membrane for simultaneous removal of organic pollutants and trace heavy metals from aqueous solution, Environ. Technol. 39 (2018) 3055–3065. [28] H. Huang, Z. Song, N. Wei, S. Li, Y. Mao, Y. Ying, L. Sun, Z. Xu, X. Peng, Ultrafast viscous water flow through nanostrand-channelled graphene oxide membranes, Nat. Commun. 4 (2013) 2979. [29] W.S. Hung, C.H. Tsou, M.D. Guzman, Q.F. An, Y.L. Liu, Y.M. Zhang, C.C. Hu, K.R. Lee, J.Y. Lai, Cross-linking with diamine monomers to prepare composite graphene oxide-framework membranes with varying d-spacing, Chem. Mater. 26 (2014) 2983–2990. [30] Y. Su, V.G. Kravets, S.L. Wong, J. Waters, A.K. Geim, R.R. Nair, Impermeable barrier films and protective coatings based on reduced graphene oxide, Nat. Commun. 5 (2014) 4843. [31] P. Sun, K. Wang, H. Zhu, Recent developments in graphene-based membranes: structure, mass-transport mechanism and potential applications, Adv. Mater. 28 (2016) 2287–2310. [32] K.H. Thebo, X. Qian, Q. Zhang, L. Chen, H.M. Cheng, W. Ren, Highly stable graphene-oxide-based membranes with superior permeability, Nat. Commun. 9 (2018) 1486. [33] A. Akbari, P. Sheath, S.T. Martin, D.B. Shinde, M. Shaibani, P.C. Banerjee, R. Tkacz, D. Bhattacharyya, M. Majumder, Large-area graphene-based nanofiltration membranes by shear alignment of discotic nematic liquid crystals of graphene oxide, Nat. Commun. 7 (2016) 10891. [34] H. Liu, H. Wang, X. Zhang, Facile fabrication of freestanding ultrathin reduced graphene oxide membranes for water purification, Adv. Mater. 27 (2014) 249–254. [35] R.C. Tatar, S. Rabii, Electronic properties of graphite: a unified theoretical study, Phys. Rev. B Condens. Matter 25 (1982) 4126–4141. [36] J.W. Burress, S. Gadipelli, J. Ford, J.M. Simmons, W. Zhou, T. Yildirim, Graphene oxide framework materials: theoretical predictions and experimental results, Angew. Chem. Int. Ed. 49 (2010) 8902–8904. [37] G. Srinivas, J.W. Burress, J. Ford, T. Yildirim, Porous graphene oxide frameworks: synthesis and gas sorption properties, J. Mater. Chem. 21 (2011) 11323–11329. [38] G. Mercier, A. Klechikov, M. Hedenström, J. Dan, I.A. Baburin, G. Seifert, R. Mysyk, A.V. Talyzin, Porous graphene oxide/diboronic acid materials: structure and hydrogen sorption, J. Phys. Chem. C 119 (2016) 27179–27191. [39] G. Liu, W. Jin, N. Xu, Graphene-based membranes, Chem. Soc. Rev. 44 (2015) 5016–5030. [40] Y. Zhang, S. Zhang, T.S. Chung, Nanometric graphene oxide framework membranes with enhanced heavy metal removal via nanofiltration, Environ. Sci. Technol. 49 (2015) 10235–10242. [41] K. Goh, W. Jiang, H.E. Karahan, S. Zhai, L. Wei, D. Yu, A.G. Fane, R. Wang, Y. Chen, All‐carbon nanoarchitectures as high‐performance separation membranes with superior stability, Adv. Funct. Mater. 25 (2016) 7348–7359. [42] X. Liu, N.K. Demir, Z. Wu, K. Li, Highly water-stable zirconium metal–organic framework UiO-66 membranes supported on alumina hollow fibers for desalination, J. Am. Chem. Soc. 137 (2015) 6999–7002. [43] J. Geng, H.T. Jung, Porphyrin functionalized graphene sheets in aqueous suspensions: from the preparation of graphene sheets to highly conductive graphene films, J. Phys. Chem. C 114 (2010) 8227–8234. [44] J. Sun, A. Klechikov, C. Moise, M. Prodana, M. Enachescu, A.V. Talyzin, A molecular pillar approach to grow vertical covalent organic framework nanosheets on graphene: hybrid materials for energy storage, Angew. Chem. Int. Ed. 57 (2018) 1–6. [45] R.R. Nair, H.A. Wu, P.N. Jayaram, I.V. Grigorieva, A.K. Geim, Unimpeded permeation of water through helium-leak-tight graphene-based membranes, Science 335 (2012) 442–444. [46] K. Huang, G. Liu, J. Shen, Z. Chu, H. Zhou, X. Gu, W. Jin, N. Xu, High-efficiency water-transport channels using the synergistic effect of a hydrophilic polymer and graphene oxide laminates, Adv. Funct. Mater. 25 (2015) 5809–5815. [47] G. Liu, W. Jin, N. Xu, Two‐dimensional‐material membranes: a new family of high‐performance separation membranes, Angew. Chem. Int. Ed. 55 (2016) 13384–13397. [48] C. Tan, X. Cao, X.J. Wu, Q. He, Y. Jian, Z. Xiao, J. Chen, Z. Wei, S. Han, G.H. Nam, Recent advances in ultrathin two-dimensional nanomaterials, Chem. Rev. 117 (2017) 6225–6331. [49] Z. Kang, S. Wang, L. Fan, M. Zhang, W. Kang, J. Pang, X. Du, H. Guo, R. Wang, D. Sun, In situ generation of intercalated membranes for efficient gas separation, Commun. Chem. 1 (2018) 1–8. [50] J. Zhu, G. Zeng, F. Nie, X. Xu, S. Chen, Q. Han, X. Wang, Decorating graphene oxide with CuO nanoparticles in a water-isopropanol system, Nanoscale 2 (2010) 988–994. [51] B. Van der Bruggen, M. Mänttäri, M. Nyström, Drawbacks of applying nanofiltration and how to avoid them: a review, Separ. Purif. Technol. 63 (2008) 251–263. [52] S. Sansuk, S. Srijaranai, S. Srijaranai, A new approach for removing anionic organic dyes from wastewater based on electrostatically driven assembly, Environ. Sci. Technol. 50 (2016) 6477–6484. [53] M. Barbosa, N.F.F. Moreira, A.R. Ribeiro, M.F.R. Pereira, A.M.T. Silva, Occurrence
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.03.070. References [1] A.P. Côté, A.I. Benin, N.W. Ockwig, M. O'Keeffe, A.J. Matzger, O.M. Yaghi, Porous, crystalline, covalent organic frameworks, Synfacts 310 (2005) 1166–1170. [2] Q. Fang, Z. Zhuang, S. Gu, R.B. Kaspar, J. Zheng, J. Wang, S. Qiu, Y. Yan, Designed synthesis of large-pore crystalline polyimide covalent organic frameworks, Nat. Commun. 5 (2014) 4503. [3] B.J. Smith, A.C. Overholts, N. Hwang, W.R. Dichtel, Insight into the crystallization of amorphous imine-linked polymer networks to 2D covalent organic frameworks, Chem. Commun. 52 (2016) 3690–3693. [4] Z. Kang, Y. Peng, Y. Qian, D. Yuan, M.A. Addicoat, T. Heine, Z. Hu, L. Tee, Z. Guo, D. Zhao, Mixed matrix membranes (MMMs) comprising exfoliated 2D covalent organic frameworks (COFs) for efficient CO2 separation, Chem. Mater. 28 (2016) 1277–1285. [5] C.J. Doonan, D.J. Tranchemontagne, T.G. Glover, J.R. Hunt, O.M. Yaghi, Exceptional ammonia uptake by a covalent organic framework, Nat. Chem. 2 (2010) 235–238. [6] S. Chandra, D.R. Chowdhury, M. Addicoat, T. Heine, A. Paul, R. Banerjee, Molecular level control of the capacitance of two-dimensional covalent organic frameworks: role of hydrogen bonding in energy storage materials, Chem. Mater. 29 (2017) 2074–2080. [7] G. Lin, H. Ding, D. Yuan, B. Wang, C. Wang, A pyrene-based, fluorescent threedimensional covalent organic framework, J. Am. Chem. Soc. 138 (2016) 3302–3305. [8] B.P. Biswal, H.D. Chaudhari, R. Banerjee, U.K. Kharul, Chemically stable covalent organic framework (COF)‐Polybenzimidazole hybrid membranes: enhanced gas separation through pore modulation, Chem. A Eur J. 22 (2016) 4695–4699. [9] S. Kandambeth, B.P. Biswal, H.D. Chaudhari, K.C. Rout, H.S. Kunjattu, S. Mitra, S. Karak, A. Das, R. Mukherjee, U.K. Kharul, Selective molecular sieving in selfstanding porous covalent-organic-framework membranes, Adv. Mater. 29 (2017) 1603945. [10] X.H. Liu, C.Z. Guan, S.Y. Ding, W. Wang, H.J. Yan, D. Wang, L.J. Wan, On-surface synthesis of single-layered two-dimensional covalent organic frameworks via solidvapor interface reactions, J. Am. Chem. Soc. 135 (2013) 10470–10474. [11] X. Feng, X. Ding, D. Jiang, Covalent organic frameworks, Chem. Soc. Rev. 41 (2012) 6010–6022. [12] M. Dogru, A. Sonnauer, S. Zimdars, M. Döblinger, P. Knochel, T. Bein, Facile synthesis of a mesoporous benzothiadiazole-COF based on a transesterification process, CrystEngComm 15 (2013) 1500–1502. [13] B.P. Biswal, C. Suman, K. Sharath, L. Binit, H. Thomas, B. Rahul, Mechanochemical synthesis of chemically stable isoreticular covalent organic frameworks, J. Am. Chem. Soc. 135 (2013) 5328–5331. [14] B.P. Biswal, H.D. Chaudhari, R. Banerjee, U.K. Kharul, Chemically stable covalent organic framework (COF)-Polybenzimidazole hybrid membranes: enhanced gas separation through pore modulation, Chem. A Eur J. 22 (2016) 4695–4699. [15] E.S. Cho, N.E. Coates, J.D. Forster, A.M. Ruminski, B. Russ, A. Sahu, N.C. Su, F. Yang, J.J. Urban, Engineering synergy: energy and mass transport in hybrid nanomaterials, Adv. Mater. 27 (2015) 5744–5752. [16] C.Z. Guan, D. Wang, L.J. Wan, Construction and repair of highly ordered 2D covalent networks by chemical equilibrium regulation, Chem. Commun. 48 (2012) 2943–2945. [17] X.H. Liu, C.Z. Guan, S.Y. Ding, W. Wang, H.J. Yan, D. Wang, L.J. Wan, On-surface synthesis of single-layered two-dimensional covalent organic frameworks via solidvapor interface reactions, J. Am. Chem. Soc. 135 (2013) 10470–10474. [18] D.D. Medina, V. Werner, F. Auras, R. Tautz, M. Dogru, J. Schuster, S. Linke, M. Döblinger, J. Feldmann, P. Knochel, Oriented thin films of a benzodithiophene covalent organic framework, ACS Nano 8 (2014) 4042–4052. [19] R.K. Joshi, P. Carbone, F.C. Wang, V.G. Kravets, Y. Su, I.V. Grigorieva, H.A. Wu, A.K. Geim, R.R. Nair, Precise and ultrafast molecular sieving through graphene oxide membranes, Science 343 (2014) 752–754. [20] Y. Han, Z. Xu, C. Gao, Ultrathin graphene nanofiltration membrane for water purification, Adv. Funct. Mater. 23 (2013) 3693–3700. [21] P. Sun, F. Zheng, M. Zhu, Z. Song, K. Wang, M. Zhong, D. Wu, R.B. Little, Z. Xu, H. Zhu, Selective trans-membrane transport of alkali and alkaline earth cations through graphene oxide membranes based on Cation−π interactions, ACS Nano 8 (2014) 850–859. [22] S.P. Surwade, S.N. Smirnov, I.V. Vlassiouk, R.R. Unocic, G.M. Veith, S. Dai, S.M. Mahurin, Water desalination using nanoporous single-layer graphene, Nat. Nanotechnol. 10 (2015) 459–464. [23] L.C. Lin, J.C. Grossman, Atomistic understandings of reduced graphene oxide as an ultrathin-film nanoporous membrane for separations, Nat. Commun. 6 (2015) 8335.
329
Journal of Membrane Science 581 (2019) 321–330
X. Zhang, et al.
[56] H. Fan, J. Gu, H. Meng, A. Knebel, J. Caro, High‐flux membranes based on the covalent organic framework COF‐LZU1 for selective dye separation by nanofiltration, Angew. Chem. Int. Ed. Int. Ed. 57 (2018) 4083–4087. [57] L. Huang, J. Chen, T. Gao, M. Zhang, Y. Li, L. Dai, L. Qu, G. Shi, Reduced graphene oxide membranes for ultrafast organic solvent nanofiltration, Adv. Mater. 28 (2016) 8669–8674.
and removal of organic micropollutants: an overview of the watch list of EU Decision 2015/495, Water Res. 94 (2016) 257–279. [54] C. Allègre, P. Moulin, M. Maisseu, F. Charbit, Treatment and reuse of reactive dyeing effluents, J. Membr. Sci. 269 (2016) 15–34. [55] S. Karan, Z. Jiang, A.G. Livingston, Sub-10 nm polyamide nanofilms with ultrafast solvent transport for molecular separation, Science 348 (2015) 1347–1351.
330