MIL-53(Fe)@γ-Al<sub>2</sub>O<sub>3</sub> nanocomposites incorporated cellulose acetate for forward osmosis membranes of high desalination performance

Tong Li; Xuelian Ba; Xiuju Wang; Zhongpeng Wang; Jie Yang; Yu Cui; Liguo Wang

doi:10.4491/eer.2021.448

Environ Eng Res > Volume 28(1); 2023 > Article

Li, Ba, Wang, Wang, Yang, Cui, and Wang: MIL-53(Fe)@γ-Al2O3 nanocomposites incorporated cellulose acetate for forward osmosis membranes of high desalination performance

Research

Environmental Engineering Research 2023; 28(1): 210448.

Published online: February 19, 2022

DOI: https://doi.org/10.4491/eer.2021.448

MIL-53(Fe)@γ-Al₂O₃ nanocomposites incorporated cellulose acetate for forward osmosis membranes of high desalination performance

Tong Li^1,^*, Xuelian Ba^1,^*, Xiuju Wang^1,^2,^†

, Zhongpeng Wang¹, Jie Yang¹, Yu Cui¹, Liguo Wang^1,^†

¹School of Water Conservancy and Environment, University of Jinan, Jinan 250022, China

²Shandong Key Laboratory of Water Pollution Control and Resource Reuse, Shandong University, Qingdao 266237, China

^†Corresponding author, E-mail: chm_wangxj@ujn.edu.cn (X.W); lgwang666@163.com (L.W), Tel: +86 19953123057; +86 13698613168

* These authors contributed equally to this work.

Received September 14, 2021 Revised November 25, 2021 Accepted February 16, 2022

(open-access):

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Forward osmosis (FO) membrane is a promising membrane technique. However, its application in desalination is limited. Thus, we synthesized MIL-53(Fe)@γ-Al₂O₃ via solvent-thermal method to fabricate MIL-53(Fe)@γ-Al₂O₃/cellulose acetate (CA) composite membranes of high desalination performance. MIL-53(Fe)@γ-Al₂O₃ and MIL-53(Fe) particles were studied using XRD, FITR, SEM, N₂ adsorption-desorption isotherms, and AFM. The structural features of MIL-53(Fe)@γ-Al₂O₃/CA membrane and MIL-53(Fe)/CA membrane were analyzed using SEM, AFM and water contact angle. The reverse salt flux and water flux of the MIL-53(Fe)@γ-Al₂O₃/CA membrane reached 1.78 g m⁻² h⁻¹ and 37.1 L m⁻² h⁻¹, respectively, when the deionized water and 1 M NaCl solution were the feed solution and traction solution, respectively. Compared to the MIL-53(Fe)/CA membrane, the MIL-53(Fe)@γ-Al₂O₃/CA membrane demonstrated better perviousness and selectivity properties of the membrane. This study shows a potential advantage of MOF composites as additives for improvement in the desalination performance of forward osmosis membranes.

Keywords: Cellulose acetate, Composite membranes, Forward osmosis membranes, MIL-53(Fe)@γ-Al₂O₃

1. Introduction

In the last few decades, membrane separation techniques have become a research hotspot and have a wide range of applications, such as desalination of seawater [1–5], wastewater treatment [6, 7], power generation [8, 9], food processing [10], pharmaceutical industry [11–15], etc. Membrane separation techniques include reverse osmosis (RO), ultrafiltration (UF), nanofiltration (NF), microfiltration (MF) and forward osmosis (FO). Among them, UF, RO, MF and NF belong to pressure-driven techniques, whereas FO is an osmotic-driven technique using osmotic-pressure gradient as the driving force [12, 13, 16]. Hence, FO occurs spontaneously without any external pressure unlike pressure-driven techniques [17]. As an emerging and environment-friendly membrane technology, FO possesses the characteristics of lower energy cost, lower membrane fouling [18, 19], and higher water recovery ratio [20–22]. The principle is that water passes from a lower concentrated feed solution to a higher concentrated extraction solution through a semipermeable membrane under high osmotic pressure [13, 23]. However, internal concentration polarization of forward osmosis membranes limits their properties [24].

The modification of FO membranes is a promising approach to improve membrane performance by incorporating nanomaterials [20]. Metal-organic frameworks (MOFs) are new types of hybrid materials comprising inorganic metal ions or metal clusters coordinated to organic linkers [25, 26]. MOFs have been receiving increasing interest due to various ranges of organic ligands and metal cations that can be used. They possess the characteristics of high surface area, tunable pore sizes, and well-defined structures [27–29]. They are widely applied in gas storage [30], adsorption [31], catalysis [32], drug delivery [33], separation [34], sensing [35], and photonics [36] among other diverse applications. Lately, MOFs-based mixed matrix membranes (MMMs) have been successfully developed and employed [20, 37, 38]. MMMs are composed of microparticles or nanoparticles of MOFs incorporated into a polymeric matrix [39], As the MMMs, organic linkers in MOFs improve the compatibility with organic polymers by increasing the interfacial interactions between embedded particles and the surrounding polymer, promoting improvement in performance. Moreover, a series of MOFs with porous structure and varied pore sizes can be synthesized choosing appropriate inorganic metal ions and linkers. Hence, MOFs are promising additives for FO membranes [40, 41].

The characteristics of MMM can be ameliorated by tuning the physical and chemical properties of the materials ⁴¹. Nano-alumina is cheap, non-toxic, large pore volume, and specific properties with good mechanical/thermal stability. It is one of the most stable inorganic materials, and thus it is used as a film-forming additive [42–44]. Wang et al. [45] fabricated the polyethersulfone (PES)/Al₂O₃ composite hollow fiber nanofiltration membranes, and found excellent chemical resistance and good stability of Al₂O₃ inorganic hollow fiber NF membranes. Yan et al. [46] prepared the polyvinylidene fluoride (PVDF)/Al₂O₃ composite ultrafiltration membranes via phase-inversion method, and the hybrid membranes showed a great improvement in anti-fouling and permeation-flux performance. Lately, researchers have been synthetizing and studying MOF/γ-Al₂O₃ composite materials. For instance, Aguado et al. [47] fabricated SIM-1@γ-Al₂O₃, and investigated that the composite containing alumina had characteristics of higher catalytic activity and easy recovery. Górka et al. [48] prepared HKUST-1@Al₂O₃ composite, and found that the composite containing alumina was more flexible in terms of tuning their porosity by structural and adsorption parameters.

In this work, MIL-53(Fe)@γ-Al₂O₃ was compounded via solvent-thermal method, and added into CA membranes as an additive to fabricate MIL-53(Fe)@γ-Al₂O₃/CA composite membranes. The properties of MIL-53(Fe)@γ-Al₂O₃ particles and MIL-53(Fe) particles were analyzed and studied firstly. Moreover, the influence of MIL-53(Fe)@γ-Al₂O₃ on FO membranes was researched compared to MIL-53(Fe)/CA FO membranes.

2. Experimental

2.1. Materials

Iron chloride hexahydrate (FeCl₃·6H₂O) and 1,4-benzenedicarboxylic acid (H₂BDC) were purchased from Sinopharm Chemical Reagent Co., Ltd. γ-Al₂O₃ was used from Shanghai Macklin Biochemical Co. N,N-dimethylformamide (DMF), Ethanol, polyethylene glycol (PEG-400) and 1,4-dioxane were obtained from Tianjin Damao Chemical Co., Ltd. Cellulose acetate(CA) was purchased from Aladdin (Shanghai, China). Sodium chloride (NaCl) was obtained from Tianjin Jinbei Fine chemical Co., Ltd. Polyester non-woven fabric with 100 meshes obtained from Suzhou Renaissance Weaving Co. The membrane support supplied from LTD. All experimental reagents used were of analytical grade. None of the applied chemicals were further purified in this work.

2.2. Preparation of MOFs and Composite Membranes

2.2.1. Preparation of MOFs

On the basis of previous studies, the synthetic way of MIL-53(Fe) nanoparticles was improved [49]. In general, 0.674 g FeCl₃·6H₂O, 0.415 g H₂BDC, and 56 mL DMF (with the mole ratio of 1:1:280) were fully stirred to be clarified at room temperature, then they were transferred to a 100 mL autoclave. It was heated at 170°C for 17 h. The product synthesized was washed with DMF and C₂H₅OH, sequentially, and then dried at 100°C for 10 h under vacuum. The yellow powder was the synthesized MIL-53(Fe) materials.

Firstly, FeCl₃·6H₂O (0.674 g), H₂BDC (0.415 g), γ-Al₂O₃ (8.2 mg) and DMF (56 mL) were fully stirred to be clarified at room temperature, then they were transferred to a 100 mL autoclave. It was heated at 170°C for 17 h. The product was washed with DMF and C₂H₅OH for three times, respectively, and dried at 100°C for 10 h under vacuum. The yellow powder was the synthesized MIL-53(Fe)@γ-Al₂O₃ materials.

2.2.2. FO membranes preparation

MIL-53(Fe)@γ-Al₂O₃/CA flat sheet membranes were fabricated via phase-inversion method. Firstly, MIL-53(Fe)@γ-Al₂O₃ (0.5 wt.%) was mixed with DMF and sonicated for 10 min to ensure a homogeneous spread of MIL-53(Fe)@γ-Al₂O₃. Next, PEG-400 (6.0 wt.%), 1,4-dioxane (7.0 wt.%), and CA (12.0 wt.%) were put into the mixed solution, sequentially. The mixed solution was stirred continuously at 60 °C for 8 h. Afterwards, the casting solution was kept for 12 h to remove air bubbles at room temperature. The polyester gauze fabric was laid on the horizontal glass plate as a support layer, and the mixed solution was scraped on a dry glass plate with a scraper. After the casting solution evaporated in the air for 30 s, the glass plate was immediately immersed in deionized water at a temperature of 30°C for 15 min until the membrane fell off the glass plate. At the same time, the solvent and non-solvent exchanged. Then the membranes were taken away from the water bath (30°C) and immersed in a bath of deionized water for 24 h at room temperature to wipe off the residual organic solvent. At last, the membrane was heated in deionized water at 60°C for 15 min before testing.

2.3. Characterization

The water contact angle indicated the hydrophilic property of the fabricated membranes by contact angle goniometer (Germany). X-ray diffraction (XRD, Bruker D8 Advance) analyzed crystal phase composition of the samples prepared. The diffractograms were recorded at the scanning angle (2θ) range from 5° to 50° with the step size of 0.02° and the scanning velocity of 6 °/min. Scanning electron microscopy (SEM, QUANTA 250 FEG) reflected the morphological properties of the membranes and nanoparticles prepared. The membranes fabricated were fractured in liquid nitrogen and sprayed a uniform gold layer before testing. Energy dispersive spectroscopy (EDS, INCA) characterized the elemental analysis of the membranes. Atomic force microscopy (AFM, DimensionEdge) reflected the membranes’ surface roughness. Fourier-transform infrared spectroscopy (FTIR, Nicolet iS5) evaluated the functional groups of the samples fabricated and the scanning range was 500–4,000 cm⁻¹. N₂ adsorption-desorption isotherms observed the BET surface area and pore structure of the samples fabricated using Micromeritics ASAP 2020 surface area analyzer. MIL-53(Fe)@γ-Al₂O₃ and MIL-53(Fe) prepared were vacuum degassed at 150 °C for 6 h before the measurement.

2.4. Performance Testing of FO Membranes

At room temperature, with 1.0 M NaCl as DS and DI water as the FS, the performance test was carried out on the self-made forward osmosis evaluation device. The effective membrane area of forward osmosis membrane was 27 cm². FS and DS had the same cross-current rate. The speed of the peristaltic pump (WT600-2J) was 100 rpm. The data were recorded after 5 min of the FO system becoming stabilized. Use a digital weight balance (AR4202CN) to measure the weight change of DS. The conductivity change of FS for DI water was measured using a conductivity meter (DDSJ-308). The weight change and conductivity change were recorded every 1 min. The experiment runs well in a total of 65 min.

The water flux (Jw, L m⁻² h⁻¹) was calculated by measuring the increase in the mass of the suction side over a period of time using the formula.

(1)

J_{w} = \frac{Δ m}{ρ A t}

Where Δm is the mass increase of the extraction solution side, and the unit is kg; the effective membrane surface area is represented by A (m²); ρ (kg m⁻³) represents the water density; t is the running time of the system, the unit is h.

The reverse salt flux (Js) was calculated as follows, and the unit is g m⁻² h⁻¹.

(2)

J_{s} = \frac{Δ C Δ V}{A t}

Where ΔV (m³) is the volume change of FS; ΔC (mol L⁻¹) is the concentration change of FS; the effective membrane surface area is represented by A (m²); and t is the time interval (h).

3. Results and Discussion

3.1. Analysis of Synthesized MIL-53(Fe)@γ-Al2O3

3.1.1. XRD analysis

Fig. 1 displays the XRD spectra of synthesized MIL-53(Fe), γ-Al₂O₃ and MIL-53(Fe)@γ-Al₂O₃. Apparently, the characteristic peaks of MIL-53(Fe)@γ-Al₂O₃ composites were consistent with MIL-53(Fe). It implied that the well-defined framework units existed in the materials synthesized, and the incorporation of γ-Al₂O₃ did not destroyed the crystal structure of MIL-53(Fe) [50]. Particularly, the peak of γ-Al₂O₃ was usually around 46–47°.

3.1.2. FTIR analysis

The FTIR spectra identify functional groups of MIL-53(Fe) and MIL-53(Fe)@γ-Al₂O₃ prepared. In Fig. 2, the peak at 533 cm⁻¹ corresponded to Fe-O stretching vibrations, indicating that there was a metal-oxygen bond between the terephthalic acid (BDC) and Fe³⁺. The peak at 745 cm⁻¹ was assigned to C-H bending vibrations of benzene in the organic linker. The two intense peaks of the asymmetric and symmetric vibrations of the carboxyl groups were observed at 1,388 cm⁻¹ and 1,535 cm⁻¹, illustrating presence of the diformate anion cross-linking agent in the samples. The O-H stretching vibration of the water molecules adsorbed on the material surface was surveyed at 3432 cm⁻¹. The infrared spectra of the synthesized MIL-53(Fe)@γ-Al₂O₃ was similar to the synthesized MIL-53(Fe), indicating the formation of MIL-53(Fe)@γ-Al₂O₃ without influence of γ-Al₂O₃.

3.1.3. Morphology analysis

The SEM images of γ-Al₂O₃, MIL-53(Fe) and MIL-53(Fe)@γ-Al₂O₃ can be seen in Fig. 3. Fig. 3(a) indicated that γ-Al₂O₃ was composed of a large quantity of aggregated small nanoparticles. As shown in Fig. 3(b) and (c), MIL-53(Fe) was a smooth particle with spindle and irregular polyhedron shape [51]. Fig. 3(d) and (e) were the morphologies of MIL-53(Fe)@γ-Al₂O₃. It can be observed that γ-Al₂O₃ spread on the surface of MIL-53(Fe)@γ-Al₂O₃ densely and tightly, and the morphology features of MIL-53(Fe)@γ-Al₂O₃ had few changes, indicating γ-Al₂O₃ attaching well with MIL-53(Fe). However, the surface of MIL-53(Fe)@γ-Al₂O₃ was rough due to encirclement of γ-Al₂O₃.

3.1.4. N₂ adsorption-desorption isotherms analysis

Fig. S1 reflects the N₂ adsorption-desorption isotherms of MIL-53(Fe) synthesized and MIL-53(Fe)@γ-Al₂O₃ synthesized at 150°C. As shown in Fig. S1, the sorption isotherms of the bifunctional composites inclined to the middle between type I and IV of the referred International Union of Pure and Applied Chemistry (IUPAC) classification [52]. It can be seen that two secondary up-takes were at near P/P0 = 0.1 and P/P0 = 0.2, reflecting the existence of two nano-porous windows in MIL-53(Fe)@γ-Al₂O₃ composites [53]. The BET test results for the MIL-53(Fe) and MIL-53(Fe)@γ-Al₂O₃ composites were shown in Table S1. The BET surface areas of the MIL-53(Fe)@γ-Al₂O₃ and MIL-53(Fe) reached 1,698 m²g⁻¹ and 222 m²g⁻¹, respectively. The average pore diameters of the MIL-53(Fe) and the MIL-53(Fe)@γ-Al₂O₃ were 5.38 nm and 2.63 nm, respectively.

3.2. Optimization of MIL-53(Fe)@γ-Al₂O₃/CA FO Membrane

A series of MIL-53(Fe)@γ-Al₂O₃/CA FO membranes with different concentration of MIL-53(Fe)@γ-Al₂O₃ (0.1–0.9 wt.%, take 0.2 wt.% as the increment) were fabricated to investigate the influence of MIL-53(Fe)@γ-Al₂O₃ content on the performance of MIL-53(Fe)@γ-Al₂O₃/CA mixed matrix FO membrane. The other parameters such as mixing temperature (60ºC), coagulation bath temperature (30ºC) and annealing temperature (60ºC) were maintained the same. As shown in Fig. 4, water permeability of the series membranes improved obviously from 29.7 to 35.3 L m⁻² h⁻¹ with MIL-53(Fe)@γ-Al₂O₃ content increasing to 0.5 wt.%, and then decreased to 33.5 L m⁻² h⁻¹ with MIL-53(Fe)@γ-Al₂O₃ content further increasing to 0.9 wt.%. Meanwhile, reverse salt flux obeyed the same trend as MIL-53(Fe)@γ-Al₂O₃ content increasing. This might be related to the influence of MIL-53(Fe)@γ-Al₂O₃ on membrane pore size. When the proportion of MIL-53(Fe)@γ-Al₂O₃ was less than 0.5 wt.%, the addition of MIL-53(Fe)@γ-Al₂O₃ was beneficial to construct the membrane channels, increase the membrane surface and inside pore volume of the supporting layer. Besides, a large amount of carboxyl functional groups in the MIL-53(Fe)@γ-Al₂O₃ help to enhance the hydrophilicity of the membrane. However, with MIL-53(Fe)@γ-Al₂O₃ content further increasing and higher than 0.5 wt.%, both water and reverse salt flux were found to be reduced simultaneously. This might be related to the incomplete dispersion of MIL-53(Fe)@γ-Al₂O₃ in the mixed solution, which instead clogged the membrane pores and affected the formation of thin membrane. Overall, for comprehensive consideration, the CA/MIL-53(Fe)@γ-Al₂O₃ content of 0.5 wt.% was selected as the optimal addition amount and for the following experiments.

The MIL-53(Fe)@γ-Al₂O₃/CA membranes were manufactured at different mixing temperature of 40–80°C (take 10°C as increment), and the other operation conditions were maintained the same. When the mixing solution temperature was lower than 60ºC, the water permeability and reverse salt flux were enhanced to 38.3 L m⁻²h⁻¹ and 1.95 g m⁻²h⁻¹ with temperature increasing, while they both decreased with temperature further increasing to 80°C (Fig. 5). When the mixing temperature was increased from 40 to 60°C, it was benefit not only for the well dissolution of membrane material and the uniform dispersion of MIL-53(Fe)@γ-Al₂O₃, but also for the compatibility of MIL-53(Fe)@γ-Al₂O₃ and CA, which was conducive to form a uniform and stable casting solution and to promote the construction of membrane pores. When the temperature exceeded 60°C, the pore size inside the membrane was like to shrink, resulting in a decrease in the permeability and an increase in the selection performance [54]. From the above results, considering the water permeability and the reverse salt flux of MIL-53(Fe)@γ-Al₂O₃/CA membrane, 60°C was selected as the optimal sating solution temperature.

The influence of coagulation bath temperature on the performance of MIL-53(Fe)@γ-Al₂O₃/CA FO membrane was investigated via fabricating membranes at different coagulation bath temperatures, including 20–40°C (take 5°C as increment). As shown in Fig. 6, the water permeability of MIL-53(Fe)@γ-Al₂O₃/CA FO membrane increased from 30.2 to 38.3 L m⁻²h⁻¹ with temperature increasing from 20 to 30°C, while the reverse salt flux also increased from 1.68 to 1.95 g m⁻²h⁻¹. As the coagulation bath temperature further increasing to 35 and 40°C, both the water permeability and reverse salt flux were all decreased. This was because that higher coagulation bath temperature would enhance the mass transfer of organic and inorganic solvents, reduce the time required for liquid membrane to complete phase separation, and promote the formation of membrane with macroporous. However, too much high coagulation bath temperature would destroy the structure of CA, causing membrane pore size shrinkage [55–58]. Taken together, the optimal coagulation bath temperature should be maintained at 30°C.

As above mentioned, with MIL-53(Fe)@γ-Al₂O₃ content of 0.5 wt.%, mixing temperature of 60°C, and coagulation bath temperature of 30°C, series of membranes were fabricated at different annealing temperature (40–80°C, take 10°C as increment) to investigate its effect on membrane performance. It could be analyzed from Fig. 7 that when the annealing temperature was below 70°C, the water permeability and the reverse salt flux of the prepared FO membrane increased with the temperature increasing. This might be ascribed to the reason that the residual solvent in the membrane pores would be largely released into the pores, leading to the membrane pore volume and water permeability obviously increased [58]. However, an excessively high annealing temperature would break the intramolecular hydrogen bonds in the cellulose acetate molecules to generate intermolecular hydrogen bonds, causing sharp shrinkage of the membrane pores [59, 60]. Therefore, when the annealing temperature reached 80°C, the water flux was 31.2 L m⁻²h⁻¹ and the reverse salt flux reduced to 1.76 g m⁻²h⁻¹. The overall consideration indicated that 60°C was the best annealing temperature to the prepared MIL-53(Fe)@γ-Al₂O₃/CA FO membrane with high performance.

3.3. Analysis of Membranes

3.3.1. Morphology of the prepared membranes

Fig. 8 displays the representative surface and cross section morphology of MIL-53(Fe)/CA membrane and MIL-53(Fe)@γ-Al₂O₃/CA membrane. As can be seen, the upper surfaces of all the prepared membranes are dense and have no open pores. Compared to the MIL-53(Fe)/CA membrane, the MIL-53(Fe)@γ-Al₂O₃/CA membrane was more porous. Moreover, membrane pore of the MIL-53(Fe)@γ-Al₂O₃/CA membrane was even-distributed and slightly small. The exchange rate between the solvent and non-solvent determines the pore characteristics of the membranes. The addition of the hydrophilic MIL-53(Fe)@γ-Al₂O₃ intensified the thermal instability between the polymers and solvents, accelerating the exchange rate between the solvent and non-solvent. This favored the formation of porous membranes [61–63]. Moreover, the presence of activity and the concentration gradient of each component accelerated solidification of the polymer at the interface between the polymer and non-solvent. The faster the polymer solution solidifies, the greater the unrelieved stress on the surface of solid polymers, resulting in the formation of weak spots on the surface of solidified polymers. The weak spots led to the formation of the fractured spots, which improved the formation of large pores [64]. However, membrane pore of the MIL-53(Fe)@γ-Al₂O₃/CA membrane was slightly small in comparison with the MIL-53(Fe)/CA membrane. It may be caused by a smaller average pore diameter of MIL-53(Fe)@γ-Al₂O₃.

3.3.2. EDS of the MIL-53(Fe)@γ-Al₂O₃/CA membrane

Fig. S2 exhibits the distribution of Al and Fe elements in the MIL-53(Fe)@γ-Al₂O₃/CA hybrid membranes, indicating the participation of the MOF-polymer chains in the structure. The good compatibility of MOF and the organic polymer is interrelated to the formation of the non-covalent bond, such as hydrogen bonding, or even covalent bonding. This is beneficial to enhance the performance of active layer of the membrane without affecting its selectivity [65].

3.3.3. AFM and water contact angle of the prepared membranes

The three-dimensional surface AFM images assess the surface roughness of the prepared membranes. As shown in Fig. S3, the surface of MIL-53(Fe)@γ-Al₂O₃/CA membranes (Fig. S3(b)) was coarser than that of MIL-53(Fe)/CA membranes (Fig. S3(a)). Membrane surface roughness influences the contact angle of the membranes, which affects the hydrophilicity of the membranes. Generally speaking, the rougher the membrane surface is, the larger the contact angle is, due to the hysteresis of wetting increasing with increasing roughness [66]. However, the water contact angle decreased with increasing in the membrane surface roughness in Table S2. Moreover, the surface roughness and the water flux are related. The available area of the membrane transport enlarges with the increase of surface roughness [67, 68]. Therefore, the greater the surface roughness is, the better the permeability is.

Water contact angle reflects the hydrophilicity of the membranes. Normally, water contact angle is in relation to the membrane pore and surface roughness of the prepared membranes [69]. As can be seen from Fig. 8, the MIL-53(Fe)@γ-Al₂O₃/CA membranes were more porous. Hence, as seen in Table S2, water contact angle of the MIL-53(Fe)@γ-Al₂O₃/CA membrane was 52.4°, whereas one of the MIL-53(Fe)/CA membrane was 53.6°. It indicated that the addition of hydrophilic nano-filler MIL-53(Fe)@γ-Al₂O₃ enhanced the hydrophilicity of the membrane surface.

3.3.4. FTIR of the prepared membranes

The FTIR spectra of the prepared membranes are shown in Fig. S4. The absorption peak of the stretching of primary alcohol was observed at 1,040 cm⁻¹. The peak at 1,228 cm⁻¹ was related to the expansion of ester group C-O. The peak at 1,370 cm⁻¹ represented C-H stretching mode. The absorption peak of the stretching of C=O was surveyed at 1,745 cm⁻¹. There was no free acetic anhydride in all prepared membranes, as no absorption peak was observed between 1,745 cm⁻¹ and 2,885 cm⁻¹.

3.4. Performance of the Prepared Membranes

Fig. S5 exhibits the water flux and the reverse salt flux of MIL-53(Fe)/CA composite membrane and the MIL-53(Fe)@γ-Al₂O₃/CA composite membrane. The property of composite FO membranes was tested with 1 M NaCl solution as DS and DI water as FS. In Fig. S5, the water flux and the reverse salt flux of the MIL-53(Fe)@γ-Al₂O₃/CA composite membrane were 37.1 L m⁻²h⁻¹ and 1.78 g m⁻²h⁻¹, respectively. Whereas the water flux of the MIL-53(Fe)/CA composite membrane was 34.9 L m⁻²h⁻¹, and reverse salt flux was 2.02 g m⁻²h⁻¹. Compared to the MIL-53(Fe)/CA composite membrane, incorporation of MIL-53(Fe)@γ-Al₂O₃ decreased water contact angle and increased roughness and porosity of composite FO membranes. Therefore, the water permeability and salt selectivity of composite FO membranes rose.

4. Conclusions

In this paper, we studied morphology, specific surface area, pore volume, and average pore size of prepared MIL-53(Fe) and MIL-53(Fe)@γ-Al₂O₃. Compared with the two materials, the surface area and pore volume of the composite increased and the average pore size decreased after γ-Al₂O₃ modification. Moreover, we synthesized the MIL-53(Fe)@γ-Al₂O₃/CA hybrid membranes via the phase inversion method. We studied and found that presence of MIL-53(Fe)@γ-Al₂O₃ changed the properties of the pore structure, roughness, and hydrophilicity of mixed membranes. When MIL-53(Fe)@γ-Al₂O₃ particles are added, the reverse salt flux of the composite membrane is less than that of MIL-53(Fe)/CA composite membrane. It can be concluded that the presence of MIL-53(Fe)@γ-Al₂O₃ enhanced the desalination performance of FO membranes. This shows MOF particles as potential additive materials can play a role in application and development of forward osmosis membranes. The research results indicate that MMMs can be promising and potential membrane separation techniques for FO membranes with high selectivity.

Supplementary Information

eer-2021-448-suppl.pdf

Acknowledgments

This research was funded by Shandong Province Natural Science Foundation (ZR2020MB120), Shandong Key Laboratory of Water Pollution Control and Resource Reuse (Grant No. 2019KF09), Shandong Province Key Research and Development Plan (2018GGX102032), The National Natural Science Foundation of China (21777055) and Project of Shandong Province Higher Educational Science and Technology Program (No. 2018LS007)

Notes

Conflict-of-Interest

The authors declare that they have no conflict of interest.

Author Contributions

T.L. (Graduate student) analyzed the data, wrote the manuscript and contributed to experiments. X.B. (Graduate student) conceived and designed the experiments, analyzed the data and wrote the manuscript. X.W. (Professor), Y.C. (Professor) and J.Y. (Graduate student) helped in result analysis and the design of the study. Z.W. (Professor) and L.W. (Professor) modified the manuscript.

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