Channelization of water pathway and encapsulation of DS in the SL of the TFC FO membrane as a novel approach for controlling dilutive internal concentration polarization

Mehrzad Arjmandi ab, Majid Peyravi *c, Mahdi Pourafshari Chenar ab and Mohsen Jahanshahi c
aChemical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
bResearch Center of Membrane Processes and Membrane, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
cDepartment of Chemical Engineering, Babol Noshirvani University of Technology, P.O. Box: 484, Shariati Av., Babol, 47148-7116, Iran. E-mail: majidpeyravi@nit.ac.ir; majidpeyravi@gmail.com; Fax: +98 1132320342; Tel: +98 1132320342

Received 8th March 2019 , Accepted 10th June 2019

First published on 10th June 2019


In this study, we explored the use of hydrophobic (ZIF-8) and hydrophilic (UiO-66) water-stable metal–organic-frameworks (MOFs) as well as their mixture for the fabrication of high-performance mixed-matrix membrane (MMM)-based thin film composite (TFC) forward osmosis (FO) membranes for controlling internal concentration polarization (ICP). According to the characteristic curve, the use of UiO-66 inside the support layer (SL) resulted in a negative effect on FO selectivity and positive effect on FO water permeability. Moreover, after the incorporation of ZIF-8 into the SL, the FO selectivity and FO water permeability increased and decreased, respectively. However, the incorporation of the mixture of ZIF-8 and UiO-66 (ZIF-8@UiO-66) into the SL (TFC-ZIF-UiO) caused positive effects on both FO water permeability and selectivity. Channelization and fast water transport through the UiO-66 cavities and the encapsulation of draw solution (DS) into the ZIF-8 cavities are the main reasons for increasing the water flux in the TFC-ZIF-UiO FO membrane. However, the long-time experiment showed that this phenomenon requires time. After replacing DI water with Caspian seawater, the more positive impact of using the TFC-ZIF-UiO membrane is more evident. Therefore, the strategy of using a mixture of hydrophobic and hydrophilic MOFs inside the SL of the TFC FO membranes is of great interest and potential for advancing membrane performance in water and wastewater treatment.



Water impact

Global water crisis refers to the problems caused by water scarcity. Therefore, our research has been focused on water and wastewater treatment. Among various methods applied to water and wastewater treatment, forward osmosis (FO) has emerged as an alternative to conventional pressure-driven membrane processes for various applications, especially seawater desalination, because of its low-cost, high selectivity and fewer tendencies to fouling compared to other membrane techniques. In practice, the obtained water flux for FO membranes differs dramatically from the theoretical equation due to four undesirable phenomena, termed CECP, DECP, CICP, and DICP. It has been shown that ICP plays a major role in FO processes and is considered as the main cause for the lower-than-expected water flux in the FO membranes. To reduce the impact of ICP, the hydrophilicity of the SL needs to be improved by hydrophilic NPs. However, there is still a major problem that includes the loss of effective driving force in the TFC FO membranes with hydrophilic NPs. To overcome this challenge, we explored the use of hydrophilic (UiO-66) and hydrophobic (ZIF-8) water-stable MOFs, as well as their mixture, for the fabrication of high-performance MMM-based TFC FO membranes with mitigated DICP. According to the characteristic curve, the incorporation of a mixture of ZIF-8 and UiO-66 (ZIF-8@UiO-66) into the SL (TFC-ZIF-UiO) caused positive effects in both FO selectivity and water flux. Channelization and fast water transport through the UiO-66 cavities and increased driving force by the encapsulation of DS into the ZIF-8 cavities are the main reasons for the increased water flux in the TFC-ZIF-UiO FO membrane. After replacing the DI water with Caspian seawater, the more positive impact of using the TFC-ZIF-UiO membrane became more evident. The strategy of using a mixture of hydrophobic and hydrophilic MOFs inside the SL of the TFC FO membranes can help scientists to fabricate high-performance membranes for water and wastewater treatment.

1. Introduction

Saline water desalination and wastewater treatment can be considered as solutions to providing freshwater for countries that face water shortage problems and have access to sea and wastewater.1 Forward osmosis (FO) has been given special attention as a pretreatment stage in desalination systems due to its advantages, such as low energy requirement, high water recovery and low membrane fouling tendency.2–4 The spontaneous movement of water through a semi-permeable membrane from a solution of lower solute concentration (as feed solution (FS)) into a solution of higher solute concentration (as draw solution (DS)) is the main concept of the FO process.5 A major type of FO membrane is thin film composite (TFC) membranes, which consist of an ultra-thin active layer (AL) on top of a porous support layer (SL). In general, the AL of TFC membranes can be made through the interfacial polymerization (IP) technique on an SL.6,7 The membrane can either be used with AL facing the FS (AL-FS or FO mode) or with AL facing the DS (AL-DS or PRO mode). Although the driving force in the PRO mode is higher than that in the FO mode, the PRO mode is more prone to fouling.8 The standard equation for FO water flux (Jw) is given by the following equation:
 
image file: c9ew00201d-t1.tif(1)
where A is the water permeability of the membrane, π is the osmotic pressure, Am is the effective membrane area, and ΔVfeed is the absolute volume change of the FS over a predestined time (Δt). However, according to experimental results, the FO water flux differs dramatically from the theoretical equation.9–14 Four undesirable phenomena termed concentrative external concentration polarization (CECP), dilutive external concentration polarization (DECP), concentrative internal concentration polarization (CICP), and dilutive internal concentration polarization (DICP) were the main reasons for the severe drop in Jw.3,15,16 If the solutes in the FS (FO-mode) accumulate on the AL, the CECP accrued. Also for both FO and PRO modes, the DECP takes place due to the displacement and dilutive of dissolved DS solute near the membrane surface in the permeate side. Until the total dissolved solids (TDS) in the FS side and Jw in the DS side are low, CECP and DECP are respectively negligible. In contrast, ICP is exclusive to the FO and is the most important factor for decreasing Jw.15–21 CICP and DICP occur at the PRO and FO modes, respectively. In CICP, the FS solutes penetrate the SL via the Jw and direct diffusion. For DICP, the Jw decreased due to dilution of the DS in the SL pores by water diffusion. Among the four major types of CP, DICP is exclusive to the FO and is the most important factor in reducing FO water flux (Jw). In this case, the Jw decreases from dilution of the DS in the SL pores by water diffusion. In other words, DICP decreases the effective driving force across the semipermeable membrane. The intensity of DICP can be determined by the structural parameter (S). This parameter is mainly dependent on the physicochemical properties of the SL. According to scientific reports, to reduce DICP in FO membranes, the SL should be fabricated with high porosity (ε), low tortuosity (τ), and low thickness (t) (S = t × τ/ε).22–24 In other words, the optimization of the SL is the main step to reduce the severity of ICP.25–28 A different technique, such as the incorporation of functionalized nanoparticles (NPs) in the SL structure, was used as one of the methods.29–31 It is believed that the hydrophilicity of the SL needs to be improved by hydrophilic NPs to decrease DICP. According to this opinion, after the incorporation of hydrophilic NPs in the structure of the SL, the DS moves towards the substrate and diffuses into the SL structure. This will increase the driving force and reduce DICP. However, there is still a major problem, which includes the loss of effective driving force in the TFC FO membranes with hydrophilic NPs.

To overcome this challenge, the use of functionalized NPs in the SL with a new approach was investigated in this study. Contrary to traditional views that commonly used hydrophilic NPs, the combination of hydrophilic and hydrophobic NPs of different ratios, was applied and studied in this work. The achievement of the expected results depends on the selection of suitable polymer and NPs for the fabrication of MMM as the SL. First of all, note that all analyses presented in this study are valid only for polymers (as SL) with moderate hydrophilic property. In fact, highly hydrophobic polymers are never used in TFC FO membranes (as SL) due to intensive DICP. Also, highly hydrophilic polymers will not be investigated in this study due to the negligible impact of the hydrophilic–hydrophobic property of NPs (filler). Although such polymers are usually rare, accordingly, polyethersulfone (PES) as a moderately hydrophilic polymer was used as the polymer matrix due to its good mechanical and chemical properties.32,33 Also, high-performance MMM requires a uniform dispersion of NPs in the polymeric matrix, which is important for impeding the formation of non-selective defects. In this study, metal–organic frameworks (MOFs), as porous NPs, were selected due to the organic linkers present in MOF structure providing better affinity between MOF and organic polymers.34–37 On the other hand, the physicochemical property of MOFs is very important. For example, most MOFs are water unstable.38–40 Thus, choosing MOFs with excellent water stability is very important. Also, the structure and size of the MOF pores are very important for the passage or encapsulation of the DS. In recent years, the use of UiO-66 with ∼6 Å windows has yielded acceptable results in the field of water and wastewater treatment.41–43 UiO-66 belongs to the family of zirconium(IV)–carboxylate MOFs. Its exceptional chemical and thermal resistance, hydrophilic nature and also the intrinsic micro-porosity of UiO-66 introduce it as the best candidate for the fabrication of MMM-based TFC FO membranes.44,45 Also, a selection of MOFs with the hydrophobic property and specific pore structure is vital. Zeolitic imidazolate frameworks (ZIFs) have recently gained considerable attention because of their potential applications, tunable porosity, structural flexibility, as well as their thermal and chemical stability.46,47 ZIFs are a subclass of MOFs that are composed of imidazolate organic linkers and tetrahedral transition metal ions, which show high water stability in aqueous solution.48 Among various ZIFs, specifically, the ZIF-8 has angstrom-sized pore windows of ∼3.4 Å and nanometer-sized pore cavities of ∼11.6 Å.49 Some recent studies have confirmed ZIF-8 as an attractive candidate for water-related separations.50–53

In this point of view, the NPs, MMMs as the SL, and MMM-based TFC FO membranes were fabricated and characterized comprehensively. In particular, the effects of the hydrophilic–hydrophobic properties of NPs on the performance of MMM-based TFC FO membranes were investigated as well. This strategy has been studied in Caspian seawater desalination. This is the first scientific report to study the use of the mixture of hydrophilic and hydrophobic NPs in the structure of the SL for improving the separation performance of TFC FO membranes as well as reducing DICP.

2. Materials and methods

2.1. Materials and chemicals

Zirconium chloride (ZrCl4, >99.5%), benzene-1,4-dicarboxylic acid (BDC, 98%), acetic acid (>99%), N,N-dimethylformamide (DMF, >99.8%), Zn(NO3)2·6H2O (>99%), 2-methylimidazole (Hmim, 99%), methanol (MeOH, 98%) N-methyl-2-pyrrolidone (NMP, 99%), lithium chloride (LiCl, 99.9%), polyvinylpyrrolidone (PVP), m-phenylenediamine (MPD, 99.5%) and trimesoyl chloride (TMC, 98%) were supplied from Sigma-Aldrich Co., USA. All the mentioned chemicals were used without further purification. PES (Radel®A) was provided by Arkema Inc., France. Before use, PES was dried at 110 °C overnight. Sodium chloride (NaCl, 99% purity) and deionized (DI) water were also purchased from Merck-Millipore, Germany. For the water treatment experiment, Caspian seawater in Northern Iran was selected as the FS (Table S1).

2.2. Fabrication of ZIF-8 and UiO-66

Both ZIF-8 and UiO-66 NPs were prepared based on previously reported procedures.54,55

For the synthesis of ZIF-8 crystals, 1.27 g of Zn(NO3)2·6H2O was first dissolved in 20 ml of methanol (as the inorganic solution). Then, 0.97 g of 2-methylimidazole was dissolved in another 20 ml of methanol (as the organic solution). In the next step, both organic and inorganic solutions were mixed in a Teflon-lined autoclave and heated at 95 °C for 20 h. Finally, the as-synthesized ZIF-8 was collected and washed with methanol.

In the case of the UiO-66 crystals, 0.114 g of H2BDC (as an organic ligand) and 0.16 g of ZrCl4 (as an inorganic cluster) were dissolved in 20 mL of the solvent containing a mixture of DMF (90 v/v%) and acetic acid (10 v/v%). After sonication of the solution for 20 min, the reaction vessel was placed in an oven at 125 °C. After 40 h, the reaction vessel was removed from the oven, and the white solids were recovered by centrifuge, washed with DMF three times and heated at 80 °C for 15 h. Finally, the as-synthesized UiO-66 was washed via a solvent exchange process using methanol.

Eventually, after activation of the as-synthesized ZIF-8 and UiO-66 by drying under vacuum conditions, to ensure the stability of the structure in aqueous solution, both MOFs were immersed in water for 30 h.

2.3. Fabrication of FO substrates

Pure PES membrane substrate (control, denoted as PES-C) was prepared via a non-solvent induced phase separation technique.22 Briefly, dried PES (15.5 wt%), LiCl (3 wt%), and PVP (0.5 wt%) were dissolved in NMP and mixed at 70 °C for 24 h. Afterward, the homogeneous dope solution was placed in the vessel overnight to remove all entrained bubbles. The obtained solution was then cast on a smooth plate using a casting knife in order to achieve a final thickness of around 50 μm. Finally, the plate was immersed in a water bath over 48 h to prepare the PES-C membrane substrates.

The preparation of MMM substrates was similar to that of PES-C with the additional step of compositing MOF NPs. The fabricated MMM substrates were labeled as MMM-ZIF-1, MMM-ZIF-10, MMM-UiO-1, MMM-UiO-10, and MMM-ZIF-UiO corresponding to the MOF nanoparticle types and loadings of 1.0 wt% ZIF-8, 10.0 wt% ZIF-8, 1.0 wt% UiO-66, 10.0 wt% UiO-66, and a mixture of 5.0 wt% ZIF-8 and 5.0 wt% UiO-66 (based on polymer content) in the membrane substrates, respectively.

2.4. Preparation of TFC-FO membranes

Thin film PA layer was made via the IP method on top of the MMM substrates. For this purpose, initially, the MMM substrates were immersed in an aqueous MPD solution (2.0 wt%) for 130 s. After removing excess MPD solution from the substrate surface with filter paper, the membrane was directly immersed into 0.1 wt% TMC/n-hexane solution for 50 s. The complete reaction between MPD and TMC on top of the substrates formed an ultra-thin PA layer. Finally, the resultant TFC membranes were dried at 80 °C for 5 minutes and stored in DI water. These membranes were labeled as TFC-C, TFC-ZIF-1, TFC-ZIF-10, TFC-UiO-1, TFC-UiO-10, and TFC-ZIF-UiO.

2.5. Measurement of the transport and structural parameters of the TFC membrane

The mass transport properties of TFC membranes such as pure water permeability (A), salt flux (B), and salt rejection (R) were measured using a crossflow reverse osmosis (RO) filtration test unit.56 All experiments were performed at a constant crossflow velocity (20 cm s−1) at 22 ± 0.5 °C. The A value was evaluated using DI water (as FS) at three pressures 50, 100 and 150 psi. According to the obtained volumetric pure water flux (J) for each applied pressure (ΔP), the A value for each TFC membrane was evaluated according to following equation.16
 
image file: c9ew00201d-t2.tif(2)

Also for salt rejection (R%) experiments, a feed solution of 20 mmol L−1 of NaCl was used under 2.5 bar for 5 h. The R-value was calculated using the following equation:16

 
image file: c9ew00201d-t3.tif(3)
where CP and CF are the concentrations of salt in the permeate and feed side, respectively. The B value of the TFC membrane was determined based on the A and R values using the following equation:16
 
image file: c9ew00201d-t4.tif(4)
where Δπ is the osmotic pressure difference across the membrane.

Also, the performance of the TFC FO membranes was evaluated by a cross-flow laboratory-scale unit. The membrane area was approximately 30 cm2 in both the FS and DS sides. In order to better mix the channel to reduce CECP, a thin spacer was used. NaCl solutions at concentrations of 0.5, and 2 M and DI water were used as the DS and FS, respectively. The initial amounts of DS and FS were 2.5 L and 2 L, respectively. Two diaphragm pumps (Headon, 2.2 LPM) were used to pump both DS and FS. To fix the velocity at 21 cm s−1, two adjustable flow meters were applied in both FS and DS sides. For more accuracy, the temperature of both FS and DS streams was fixed at 25 °C. The tank containing DS was placed on a digital balance (Mettler Toledo), and the changes were carefully recorded. During testing, the conductivity was monitored and recorded for both the FS and DS. Also in this study, the experiments were performed in both FO and PRO modes. Based on the absolute volume change of the FS (ΔVfeed) over a predestined time (Δt) during the FO experiments and effective membrane area (Am), the FO water flux was determined using eqn (1). Also based on the conductivity increment in the FS, the FO reverse salt flux (Js) was calculated by the following equation:

 
image file: c9ew00201d-t5.tif(5)
where C0 and Ct are the initial and final (at time) salt concentrations of the FS, respectively, and V0 and Vt are the initial and final (at time t) volumes of the FS, respectively.

The structural parameter (S) is an intrinsic parameter of the FO membrane that is associated with the ICP phenomenon inside the membrane SL and is determined by the tortuosity (τ), support thickness (t), and porosity (ε) (S = /ε). According to that, the SL in FO membranes creates a resistance to DS diffusion; there is an agreement that the ICP is affected by the S parameter. The S parameter of the TFC FO membranes was determined by using the following equations:21

 
image file: c9ew00201d-t6.tif(6)
 
image file: c9ew00201d-t7.tif(7)
where D is the solute diffusion coefficient and πfeed and πdraw are the osmotic pressures of the bulk FS and DS, respectively. Also, the DS diffusion resistivity (K) within the SL can be calculated by following equation.20
 
image file: c9ew00201d-t8.tif(8)

2.6. Characterization

The physicochemical properties of both ZIF-8 and UiO-66 particles were characterized by powder X-ray diffraction (PXRD) (PW3710, Philips), Fourier-transform infrared (FTIR) spectroscopy (Thermo Nicolet Avatar 370), field emission scanning electron microscopy (FESEM) (TM3000, Hitachi), and dynamic light scattering (DLS) (Shimadzu SALD-2101).

The membrane morphology was examined by field emission scanning electron microscopy (FESEM) (JEOL JSM-6700F). The surface morphology of the membranes in terms of mean roughness parameter was examined by atomic force microscopy (AFM) (EasyScan II, Swiss). The membrane thickness was determined by a digital micrometer (Carl Mahr D7300, Germany). The surface hydrophilicity of the membranes was measured with DI water using a contact angle (CA) measurement (G10, KRUESS). The elemental composition of the polyamide layer was identified by X-ray photoelectron spectroscopy (XPS) (Bestec, Germany) using a monochromatized Al Kα X-ray source. The following equation was used in order to estimate the cross-linking degree of the PA layer:57

 
image file: c9ew00201d-t9.tif(9)
where m and n are the relative fractions of the cross-linked and linear parts, respectively.

The developed SHN1 method based on linear regularization theory was used to evaluate the average pore size of substrates from the wet-flow state curve.58,59 The membrane bulk porosity (ε) was measured by weight change between the wet and dried membrane according to the following equation.60

 
image file: c9ew00201d-t10.tif(10)
where ρw and ρm are the densities of the water and polymer, respectively, while mdry and mwet are the weights of the dry and wet membranes.

3. Results and discussion

3.1. Characterization of ZIF-8 and UiO-66

The PXRD, FTIR spectrum, DLS, and FESEM results of lab-synthesized ZIF-8 and UiO-66 particles are shown in Fig. 1. As shown in Fig. 1(a and b), the PXRD patterns of the ZIF-8 and UiO-66 particles are identical with those of the literature, which indicates a highly crystalline MOF structure without any impurities.45,54 Also according to Fig. 1(c and d), the FTIR spectra show that all main peak positions for ZIF-8 and UiO-66 in this study are in good agreement with those of the previously reported ZIF-8 and UiO-66 crystals.45,61 This indicates that the MOF particles were synthesized with high accuracy. According to Fig. 1(e and f), the average sizes of the ZIF-8 and UiO-66 particles are around 95 and 250 nm, respectively. Also, the morphologies of the ZIF-8 and UiO-66 particles shows that the ZIF-8 and UiO-66 NPs reveal spherical- and octagonal-shaped crystals, respectively (Fig. 1(e and f)). The spherical structure of ZIF-8 is due to morphological changes after immersion in water for 30 h.62
image file: c9ew00201d-f1.tif
Fig. 1 Characterization of MOF samples: (a) PXRD pattern of ZIF-8, (b) PXRD pattern of UiO-66, (c) FTIR analysis of ZIF-8, (d) FTIR analysis of UiO-66, (e) DLS measurement and FESEM image (insert) of ZIF-8, and (f) DLS measurement and FESEM image (insert) of UiO-66.

3.2. Characterization of MMM substrates

3.2.1. Physicochemical properties. The water contact angle (CA) of the PES membrane made with different concentrations of MOFs is shown in Fig. 2. The water CA of the pure PES membrane is ∼59°, which is consistent with the reported values in the literature.32,33,63,64 Compared to PES-C, the water CA of MMM-UiO-1 and MMM-UiO-10 decreased due to the addition of hydrophilic UiO-66 framework. In contrast, when the ZIF-8 is incorporated into the PES matrix, the water CA of the MMM-ZIF-1 and MMM-ZIF-10 increased due to the introduction of the hydrophobic nature of ZIF-8. This is while, with the addition of ZIF-8@UiO-66 into the PES matrix, the water CA did not change due to hydrophobic and hydrophilic nature of the fillers.
image file: c9ew00201d-f2.tif
Fig. 2 Water contact angle measurement of six substrates with different types of MOFs.

Table 1 also shows the structural properties of all MMMs, such as thickness (t), porosity (ε), the viscosity of dope solutions, mean pore size (davg) and the εdavg2/4t factor. As shown in Table 1, the average thickness of all MMMs ranged from 49–51 μm. According to the results obtained, there was no significant difference in the thicknesses of the MMM substrates, while the overall porosity and mean pore size improved when MOFs were incorporated into the PES matrix (especially under high loading).

Table 1 The general properties of all substrates with different types of MOFs
Membrane code Thickness t (μm) Porosity ε (%) Viscosity (mPa s) Mean pore size davg (nm)

image file: c9ew00201d-t11.tif

PES-C 50 ± 1 63 ± 1 872.08 36.39 4.17
MMM-ZIF-1 49 ± 2 64 ± 1 870.33 37.77 4.57
MMM-ZIF-10 51 ± 3 69 ± 2 860.17 38.33 4.87
MMM-UiO-1 49 ± 2 65 ± 1 863.12 37.91 4.67
MMM-UiO-10 51 ± 2 70 ± 2 816.49 38.99 5.02
MMM-ZIF-UiO 51 ± 3 69 ± 3 841.90 38.63 4.95


The morphological images of cross-section, top and bottom surfaces of the neat PES and the corresponding MMM substrates induced by ZIF-8, UiO-66 and ZIF-8@UiO-66 were characterized by FESEM and presented in Fig. 3. A common asymmetric structure could be clearly observed for the neat PES membrane (Fig. 3(a and e)). As shown in Fig. 3(b–d), by adding the MOF NPs in PES structure, the finger-like pores have been changed. As listed in Table 1, by incorporating the MOF NPs into the casting solution, the polymer dope viscosity decreases as a result of decreased polymer entanglement. The reduction in viscosity causes a change in phase inversion condition as a result of relatively faster-slower solvent/non-solvent exchange rate during membrane fabrication.65 This can be due to the hydrophilic–hydrophobic nature of the MOFs. Also by close examination of the cross-sections of MMM substrates in high magnification, the presence of MOF NPs inside the membrane structure was observable and marked with circles (Fig. 3(f–h)). The FESEM images of the top surfaces of the corresponding substrates are illustrated in Fig. 3(i–l). According to Table 1 and Fig. 3(i–l), all of the substrates have porous surfaces. Also, the pores are evenly distributed throughout all the membranes surfaces. Moreover, the presence of MOF particles on the top surfaces of the MMMs and some modifications are evident. Finally, the images of the bottom surfaces of the prepared substrates show a number of large macro voids that help to facilitate water transport in the FO process (Fig. 3(m–p)). Given the differences observed between neat PES and MMM substrates, the presence of MOF NPs in the MMMs plays a decisive role in the structure and consequent performance changes of the corresponding membranes.


image file: c9ew00201d-f3.tif
Fig. 3 The cross-section (a–h), top surface (i–l), and bottom surface (m–p) morphologies of the prepared substrates.
3.2.2. Intrinsic separation property. The effects of loading MOFs on pure water permeability (PWP) and the total porosity of the MMMs are shown in Fig. 4. As shown in Fig. 4, the PWP and porosity of neat PES increases with the incorporation of MOF NPs (even for low loading). The results show that there is a similar trend between the increase of PWP and total porosity. According to the εdavg2/4δ factor presented in Table 1, the increment in PWP for all MMMs is expected.32 Also, the comparison of Fig. 2 and 4 shows that the alteration of the PWP relative to a change in MOF hydrophilicity is not sensitive. There was also no significant difference between the obtained results of water permeability for MMMs reinforced with different hydrophilic and hydrophobic NPs. This behavior can be interpreted in two ways; (1) fast water transport through the hydrophilic paths by increasing the membrane affinity to water,66 and (2) fast water transport through the hydrophobic paths without direct contact with hydrophobic pore walls that provide a frictionless transmission.32,67–73
image file: c9ew00201d-f4.tif
Fig. 4 Pure water permeability and total porosity of all prepared substrates.

3.3. Characterization and performance of MMM-based TFCs

The thickness (L), contact angle (θ), intrinsic water permeability (A), salt flux (B), ratio of the transport parameters (B/A), salt rejection (R), and roughness parameters (Ra) of all TFC membranes are reported in Table 2 and Fig. 5. As shown in Fig. 5, a non-monotonic behavior of water permeability was observed when MOF NPs were incorporated inside the SL. The use of MMM-UiO-1(0) instead of the neat PES membrane as the SL caused an increase in water permeability (especially for MMM-UiO-10). This trend is similar to the PWP results, as presented in Fig. 4. Thus, it seems that the enhancement of water permeability was associated with the improved PWP of MMM-UiO-1(0). Also, in contrast to the results of the PWP, the water permeability of TFC-ZIF-1(0) was lower than the corresponding TFC-UiO-1(0) and also the neat PES. This is while, compared to TFC-UiO-10, with the incorporation of ZIF-8@UiO-66 into the SL, the water permeability increased, as shown in Fig. 4. The authors believe that differences in the PA layers may be the main reasons for such non-monotonic behavior.
Table 2 Summary of the thickness, contact angle, ratio of transport parameters, and salt rejection of all TFC membranes
Membrane type Thickness, L (μm) Contact angle, θ (°) Mean roughness, Ra (nm) B/A (kPa) Salt rejection, R (%)
TFC-C 51 ± 2 47 10.04 14.40 97.20
TFC-ZIF-1 50 ± 1 36 13.78 97.32
TFC-ZIF-10 52 ± 1 34 9.85 10.95 97.86
TFC-UiO-1 50 ± 2 37 14.43 97.19
TFC-UiO-10 53 ± 1 39 10.92 16.58 96.79
TFC-ZIF-UiO 52 ± 3 35 10.39 13.43 97.38



image file: c9ew00201d-f5.tif
Fig. 5 Water permeability (A) and solute flux (B) of all TFC membranes.

In order to investigate the structure of the AL, XPS analysis was conducted from the surface region to evaluate the degree of cross-linking of the PA layer on each substrate. The elemental composition of the PA layer for each selected TFC membrane was listed in Table 3. According to the obtained results, it can be noted that by adding ZIF-8 or UiO-66 (or also their mixture) into the PES matrix, the PA cross-linking degree changed. During the IP reaction, the MPD diffuses from the substrate surface cavities to cross the aqueous/organic interface and react with TMC to form the PA active layer.6,32,74,75 The diffusion rate of the MPD monomer (DMPD) is related to two physical and chemical factors, each of which is discussed separately. In the case of the physical factor, the DMPD is proportional to the εdavg2/4δ factor.32 In this study, this factor decreased in the order of MMM-UiO-10 > MMM-ZIF-UiO > MMM-ZIF-10 > MMM-UiO-1 > MMM-ZIF-1 > PES-C. On the other hand, the hydrophilicity of the substrates also affects DMPD and the formation of PA active layer (chemical factor). According to contact angle data (Fig. 2), the hydrophilicity of substrates decreased in the order of MMM-UiO-10 > MMM-UiO-1 > MMM-ZIF-UiO ≥ PES-C > MMM-ZIF-1 > MMM-ZIF-10. This is while the results presented in Table 3 are not consistent with any of the stated changes. In other words, although MMM-UiO-10 has the highest physical factor, it has the lowest degree of cross-linking due to the high hydrophilic nature of UiO-66. In fact, hydrogen bonding between MPD and UiO-66 limits the diffusion of MPD to the surface of MMM-UiO-10. It is possible that some TMC diffuses into the cavities and form PA deep within the cavities. This belief says that, according to the increase of the overall path length for water and solute transport, the PA composite membrane formed over the MMM-UiO-10 substrate was less permeable.76 This phenomenon depended on several factors, such as the time and size of the substrate cavities. On the other hand, the formation of the initial PA layers in a limited and closed space can also act as a barrier layer and the growth rate of the PA layer would be limited. Therefore, only by examining the results of pure water permeability can it be commented as to what hypothesis had occurred. In contrast, although the physical factor of the MMM-ZIF-10 substrate is lower than that of the MMM-UiO-10 substrate, the TFC-ZIF-10 has the highest degree of cross-linking due to the hydrophobic nature of ZIF-8. However, compared to MMM-ZIF-10, by incorporating ZIF-8@UiO-66 into the PES, the physical factor of MMM-ZIF-UiO had increased by up to 1.6%. Also in comparison with MMM-UiO-10 and MMM-ZIF-10 substrates, the chemical factor for the MMM-ZIF-UiO substrate was moderate.

Table 3 XPS analysis of TFC membranes fabricate by various types of substrates
Membrane type O (%) N (%) C (%) O/N O/C N/C Degree of cross-linking (%)
TFC-C 18.36 12.08 69.56 1.520 0.264 0.174 38
TFC-ZIF-10 17.75 12.83 69.42 1.384 0.256 0.185 52
TFC-UiO-10 18.39 11.85 69.76 1.552 0.264 0.170 35
TFC-ZIF-UiO 18.01 12.41 69.58 1.451 0.259 0.178 45


The degree of crosslinking is the main factor influencing the rejection of the salts.77 The B/A ratio is an important parameter that illustrates membrane selectivity.20 The relationship between the B/A ratio and the degree of cross-linking for four selected TFC membranes is shown in Fig. S1. The results show that as the cross-linking degree increases, the B/A ratio changes almost linearly (R2 = 0.94). Judgment as to which TFC membrane shows acceptable intrinsic transports properties will be possible by a tradeoff between the salt rejection parameter (R) and water permeability (A), as shown in Fig. S2. According to Fig. S2, depending on the final purpose of separation, the definition of the best membrane will be different. However, the moderate and defensible amounts of the “R” and “A” parameters for the TFC-ZIF-UiO membrane are quite evident.

For further investigation, the top surface and cross-sectional morphology of the corresponding MMM-based TFC membranes were characterized by FESEM observation (Fig. 6). The top surface of all TFC membranes (Fig. 6(a–c)) showed typical ridge and valley structures and confirmed that the PA layer was successfully formed. According to Fig. 6(a), a compact structure of the PA layer on the top surface of the MMM-ZIF-10 substrate was evident. Also, as shown in Fig. 6(b), the PA layer of the TFC-UiO-10 membrane was less compact. The surface roughness results presented in Table 2 confirm these observations. These were attributed to the higher and lower cross-linking degree in the TFC-ZIF-10 and TFC-UiO-10 membranes, respectively (see Table 3). Also, it seems that the AL compaction for TFC-ZIF-UiO was less than TFC-ZIF-10 and more than TFC-UiO-10. This phenomenon could be due to the hydrophilicity and hydrophobicity nature of UiO-66 and ZIF-8, respectively. On the other hand, the cross-section images in Fig. 6(d–f) indicated that the TFC FO membrane based on the MMM-ZIF-10 substrate showed a more uniform and regular PA layer. Compared to TFC-ZIF-10, the irregular structure of the PA layer was observed for TFC-UiO-10. This may be due to the increased hydrophilicity of the SL, which allows a deeper interaction between the MPD and the TMC in the SL cavities.


image file: c9ew00201d-f6.tif
Fig. 6 Top surface (a–c) and cross-sectional (d–f) FESEM images of the prepared TFC membranes.

3.4. Experimental trends of FO performance

In the following, the water flux (Jw) and also the specific reverse solute flux (Js/Jw) of the six TFC membranes under the FO process were measured in terms of DS concentration in both AL-FS and AL-DS modes using DI water as the FS and 0.5 and 2.0 M NaCl as the DS. Fig. 7 compares the Jw and Js/Jw of the TFC membranes in the AL-FS mode.
image file: c9ew00201d-f7.tif
Fig. 7 FO water flux and FO reverse solute flux of the TFC membranes in the FO mode.

As shown in Fig. 7, after incorporation of ZIF-8 NPs inside the PES matrix (as SL), the Jw decreased, especially under high loading. In this case, the hydrophobic nature of ZIF-8 causes the hydrophilicity of the SL to decrease, and as a result, the DS will have a low tendency to move and diffuse into the SL. This causes the driving force to decrease and the subsequent reduction of the Jw. In contrast, the addition of UiO-66 NPs inside the PES SL increased the Jw. Also, for both DS concentrations, high Jw was obtained for the high loading of UiO-66. The enhanced hydrophilicity of the SL membranes due to the addition of UiO-66 is the key reason for the enhancement of Jw. Comparing between Fig. 5 and 7, it is clear that for these five TFC membranes (TFC-C, TFC-ZIF-1(0), and TFC-UiO-1(0)), the trend of Jw change is in accordance with the trend of the water permeability (A) alteration. At first glance and as a traditional analysis, it can be said that considering that the trend of changes in Jw is not in accordance with changes in the εdavg2/4δ factor, the degree of cross-linking of the PA layer and the hydrophilicity of the SL are, thus, the main reasons for such results. But the inverse trend of A and Jw for TFC-ZIF-UiO indicates the importance of deeper analysis.

As shown in Fig. 7, after using various ratios of hydrophilic and hydrophobic MOFs in the PES matrix, the A value of the TFC membranes increased in the order of TFC-ZIF-10 < TFC-ZIF-1 < TFC-C < TFC-UiO-1 < TFC-ZIF-UiO < TFC-UiO-10. This can be attributed to the degree of cross-linking of the PA layer. However, in both DS concentrations, the TFC-ZIF-UiO FO membrane exhibits a higher Jw than other TFC membranes. A detailed study of this phenomenon will be possible by analyzing parameters such as structural parameter (S) and DS diffusion resistivity (K). These parameters, as characteristics of the intensity of ICP in FO SL, can describe the trends of Jw. The results of the S parameter and the tortuosity of all six TFC membranes are listed in Table 4.

Table 4 Summary of the structural parameters of the TFC membranes
Membrane type S (μm) Changing S (%) τ
TFC-C 614.59 7.59
TFC-ZIF-1 675.85 +9.97 8.65
TFC-ZIF-10 779.23 +26.79 10.34
TFC-UiO-1 496.39 −19.23 6.45
TFC-UiO-10 307.59 −49.95 4.06
TFC-ZIF-UiO 235.99 −61.60 3.13


The S value of all prepared membranes was higher than that of the TFC-ZIF-UiO membrane (235.99 μm). Also, the resistance behavior of DS relative to diffusion into the SL (K) is shown in Fig. S3. These results have a direct impact on the driving force. Fig. 8 shows the values of the effective driving force (ΔCeff) for all TFC FO membranes.


image file: c9ew00201d-f8.tif
Fig. 8 Values of ΔCeff for all TFC membranes.

According to Table 4 and Fig. S3, the diffusion and durability of the DS inside the TFC-ZIF-UiO cavities were greater than that of other TFC FO membranes. Also, according to Table 4 and Fig. 8, due to the highest S (and K) value calculated for TFC-ZIF-10, the ΔCeff is lowest for this TFC membrane as a result of the lowest diffusion of DS into the MMM-ZIF-10 pores. According to the above-mentioned, this phenomenon is quite logical. On the other hand, as shown in Table 1 and Fig. 2, although the highest εdavg2/4δ factor and the lowest contact angle belong to TFC-UiO-10, the obtained driving force was not the maximum (see Fig. S4). Investigating the diffusion and durability of DS in the SL is the key to solving this puzzle.

As you know, the hydrated ionic diameters of metal ions (dH-ion) and dehydrated ionic diameters (dion) are the key parameters that affect the diffusion of DS into the SL and subsequently the driving force. Since NaCl was used as the DS in this study, dH–Na, dH–Cl, dNa and dCl are 7.16, 6.64, 1.9 and 3.62 Å, respectively.78 Thus, it is also expected that, because the mean pore size of UiO-66 is larger than dion and close to dH-ion the higher driving force belongs to the TFC-UiO-10 membrane. However, the results are contrary to this analysis. In justifying the lower driving force for TFC-UiO-1(0) compared to TFC-ZIF-UiO, it can be said that, although after the incorporation of hydrophilic NPs in the structure of the SL, the DS moves towards the substrate and diffuses into the SL structure (Fig. 9(a)), but this is not the end of the story. After the diffusion of DS into the SL, due to the increased driving force, the Jw from the FS to the DS will increase. In this case, it is likely that, because the NPs are hydrophilic (UiO-66 in this study), the Jw from the FS will pass through the hydrophilic pores and path. This causes the DS to be washed from the SL structure to DS bulk (Fig. 9(b)). The result of this phenomenon is the aggravation of two negative phenomena: DECP and especially DICP.


image file: c9ew00201d-f9.tif
Fig. 9 Proposed mechanism of the diffusion of water and DS within the hydrophilic cavities of the NP in the early moments (left), and after a certain amount of time (right).

In the case of TFC-ZIF-UiO, due to the use of both hydrophilic and hydrophobic NPs in the SL structure, the final hydrophilicity of the SL did not change compared to the pure polymer (see Fig. 2). This means that the tendency of the DS to move near the SL surface will not change significantly (compared to neat PES). The mechanism of DS diffusion into the UiO-66 pores was described in the previous paragraph. In the case of DS diffusion into the ZIF-8 pores, Zhang et al. indicated that ions (i.e., Na+ and Cl) must undergo multiple dehydrating–hydrating processes, when they diffuse into the ZIF-8 pores.78 Firstly, when the hydrated ions try to enter into the window regions of the ZIF-8 structure, they will be dehydrated partially. Then, when the dehydrated ions exit the windows of the ZIF-8 and enter the cavities of the ZIF-8, they will be hydrated again.78 The dehydrating–hydrating processes of ions into the cavities of ZIF-8 are illustrated in Fig. 10.


image file: c9ew00201d-f10.tif
Fig. 10 Encapsulation mechanism of DS ions into the cavities of ZIF-8.

Accordingly, at the beginning of the process, the DS moves towards the SL structure and diffuses into SL pores, as shown in Fig. 11(a). By increasing the driving force (as a result of the effective diffusion of DS into the SL) and following the increase in Jw, a different behavior (compared to TFC-UiO-1(0)) can be expected. In this case, it is likely that the flowing water from the FS to the DS preferably passes through easier paths. These easier paths are related to the hydrophilic pores. In other words, according to Fig. 11(b), the water flux through the passage of hydrophilic pores will be channelized after a certain time, and so, the trapped or encapsulated DS will remain in the hydrophobic pores and will not be washed (removed) by the water flux.


image file: c9ew00201d-f11.tif
Fig. 11 Proposed mechanism of the diffusion of water and DS within the hydrophilic and hydrophobic cavity of NPs in the early moments (left), and after a certain amount of time (right).

Hypothesized mechanisms for the increased water flux with the TFC FO membranes fabricated by a mixture of hydrophilic and hydrophobic NPs in SL include: (i) good membrane affinity to water; (ii) channelization and fast water transport through the hydrophilic pores; (iii) increased driving force by the encapsulation of DS into the hydrophobic pores; and (iv) increased degree of cross-linking of PA layer because of the change in the physicochemical property of the SL. Therefore, it can be expected that the highest water flux will be achieved due to the achievement of the maximum driving force (see Fig. 8). This was more obvious for higher DS concentrations (i.e., DS = 2.0 M).

The investigation on the performance of the TFC membranes can be complemented by studying the Js/Jw factor. This ratio is an important tool used to estimate how much salt is lost during the FO process. Low values of Js/Jw indicate high membrane selectivity. In Fig. 7, the results of Js/Jw for all TFC FO membranes were compared in both DS concentrations. The ratio of Js/Jw for TFC-ZIF-10 was lower than for other TFCs. This indicates that the TFC-ZIF-10 membrane has a high power of rejection due to the highest degree of cross-linking. On the other hand, considering that the highest Jw does not belong to TFC-UiO-10 FO membrane, the highest ratio of Js/Jw for this TFC membrane reflects the highest Js in TFC-UiO-10. The lowest degree of cross-linking can be considered the main reason for this behavior (see Table 2).

Fig. S4 shows the values of Jw and Js/Jw for all six TFC FO membranes in the AL-DS mode. According to Fig. 7 and S4, it is clear that regardless of the orientation modes, after increasing the DS concentration from 0.5 to 2.0 M, the Jw increased because of the larger osmotic pressure created across the FO membranes. According to the obtained results for the AL-DS mode, a higher Jw was observed in this mode due to the insignificant effect of the DICP phenomenon. In accordance with the trend for water permeability coefficient A, the results of Jw in the AL-DS mode show that the highest Jw was achieved for the TFC-UiO-10 FO membrane. Also, for each DS concentration, the ratio of Js/Jw was relatively similar for both orientation modes, which is in agreement with the theory of the FO process.13,20 In contrast to the AL-FS mode, the same trend of A and Jw (for all TFCs) in AL-DS mode, as well as the similar trends of changes in the Js/Jw factor (for both modes), is a confirmation that the mutation of ΔCeff in Fig. 8 for the TFC-ZIF-UiO membrane is due to the theory outlined in Fig. 11.

3.5. Characteristic curve of the TFC FO membranes

After an extensive investigation on structural and transport parameters of all TFC FO membranes, it is very important to use the characteristic curve for selecting the best TFC FO membrane,2 since it presents a trade-off between Jw/Js and Jwπtheoretical, as shown in Fig. 12. As shown in Fig. 12, at higher DS concentrations, the FO efficiency decreased as a result of the increased ICP. According to the characteristic curve, using MMM-ZIF-1(0) and MMM-UiO-1(0) as the SL caused to positive and negative effects in FO selectivity, respectively. They also had negative and positive effects on FO water permeability (especially for high loading, respectively). However, using MMM-ZIF-UiO as the SL caused a positive effect in both FO selectivity and FO water permeability, as shown in Fig. 12. These results show the superiority of incorporating both hydrophilic and hydrophobic NPs in the substrates of TFC FO membranes.
image file: c9ew00201d-f12.tif
Fig. 12 Comparisons of the FO performance of all TFC membranes (orientation: FO mode, FS: DI water, DS: NaCl 0.5 and 2 M).

Fig. 13 compares the FO performance of the TFC-C, TFC-ZIF-10, TFC-UiO-10, and TFC-ZIF-UiO membranes with some results presented in the literature.2,4,79 A greater circle signifies a higher overall performance in terms of both Jw and Jw/Js. Although compared to many recent FO membranes, the TFC-UiO-10 membrane presents favorable results, nonetheless, the extraordinary efficiency of the TFC-ZIF-UiO membrane is undeniable. However, Zhou et al. prepared a unique TFC membrane with outstanding FO performance.80 In contrast to traditional TFC FO membranes, this unique membrane showed a high water flux of 274.2 L m−2 h−1 and a low reverse salt flux of 1.65 g m−2 h−1 using 1 M NaCl DS.


image file: c9ew00201d-f13.tif
Fig. 13 Comparisons of the FO performance of MMM-based TFC FO membranes with other TFC membranes presented in the literature2,4,79 (orientation: AL-FS mode, FS: DI water, DS: NaCl 0.5 M, T: 20–25 °C).

3.6. Caspian seawater desalination

In order to verify the opportunity of our proposed strategy in the enhancement of the desalination performance for the FO membranes, the fabricated MMM-based TFC membranes were examined using Caspian seawater and 2 M NaCl solution as the FS and DS, respectively (in the FO mode). Fig. 14 shows the corresponding results of DI water flux. After replacing DI water with Caspian seawater, the water flux of the TFC-C membrane decreased (28.73%). A reduction in water flux was attributed to the drop in the driving force and/or membrane fouling. After the incorporation of the MOF NPs into the SL, the FO water flux decreased by about 30.59, 24.14 and 17.89% for TFC-ZIF-10, TFC-UiO-10, and TFC-ZIF-UiO, respectively. The more positive impact of using the mixture of hydrophilic and hydrophobic NPs inside the SL is more evident.
image file: c9ew00201d-f14.tif
Fig. 14 A comparison between selected TFC FO water flux for DI water and Caspian seawater as the FS (orientation: FO mode, DS: NaCl 2 M).

Finally, these TFC membranes were continuously examined under Caspian seawater desalination, and the results of FO water flux decline during the time were presented in Fig. 15. During the FO experiment, the TFC-C, TFC-ZIF-10, TFC-UiO-10 and TFC-ZIF-UiO membranes showed about 18.48, 20.01, 17.18 and 14.70% reduction in water flux, respectively. These reductions were attributed to the membrane fouling and DS dilution and/or FS concentration. According to obtained results, a greater reduction in the water flux of the fouling experiment was found for the TFC-ZIF-10, TFC-C, and TFC-UiO-10 membranes compared with the TFC-ZIF-UiO membrane. This indicates an increase in FO efficiency due to the use of the mixture of hydrophilic and hydrophobic NPs in the SL. Also the inset of Fig. 15 highlights the change in FO water flux in the first five hours. In contrast to the other three TFC membranes, the water flux increases slightly in the first three hours. This phenomenon shows that the channelization of water inside the hydrophilic cavities, as well as the encapsulation of DS into the hydrophobic cavities, requires time.


image file: c9ew00201d-f15.tif
Fig. 15 Normalized selected TFC FO water flux (Jw/Jw0) decline over a long period of time, (orientation: FO mode, FS: Caspian seawater, DS: NaCl 2 M).

4. Conclusions

High-performance MMM-based TFC membranes comprising various types of water-stable MOFs in the SL have been successfully fabricated for FO applications. Contrary to the traditional view that commonly used hydrophilic NPs as filler, in this study, UiO-66 and ZIF-8 (as hydrophilic and hydrophobic NPs, respectively), as well as their mixture, were investigated. PES was used as a polymer matrix. The results showed that the PWP and porosity of neat PES increased by the incorporation of MOF NPs. According to the εdavg2/4δ factor, there is a similar trend in the increase of PWP and total porosity. Due to fast water transport through the hydrophilic paths and fast water transport through the hydrophobic paths without direct contact (frictionless) with the hydrophobic pore walls, the sensitivity of the PWP relative to the changes in MOF hydrophilicity is not very significant. On the other hand, the use of MMM-UiO-1(0) (TFC-ZIF-1(0)) as an SL caused an increase (decrease) in water permeability. However, when compared to TFC-UiO-10, by increasing the percentage of ZIF-8 and decreasing the loading of UiO-66, the TFC-ZIF-UiO water permeability increased. According to characteristic curve, the use of ZIF-8 (UiO-66) inside the support layer (SL) caused a positive (negative) effect on FO selectivity and a negative (positive) effect on FO water permeability. However, the incorporation of the mixture of ZIF-8 and UiO-66 into the SL (TFC-ZIF-UiO) caused positive effects in both FO selectivity and water permeability. Channelization and fast water transport through the hydrophilic cavities of UiO-66 increased the driving force by encapsulating of DS into the hydrophobic cavities of ZIF-8 and the increased degree of cross-linking of the PA layer are the main reasons for the increased water flux in the TFC-ZIF-UiO FO membrane. After replacing DI water with Caspian seawater, the more positive impact of using the TFC-ZIF-UiO membrane is more evident. This study has provided a new concept on the fabrication of high-performance TFC membranes by using a mixture of hydrophilic and hydrophobic NPs for water and wastewater treatment.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the funding support of Babol Noshirvani University of Technology through Grant program No. BNUT/389026/97.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ew00201d

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