Abstract
Membrane distillation is an emerging wastewater treatment technology that harnesses low-grade heat as an energy source and exhibits potential for complete desalination. Nonetheless, two notable challenges hinder the practical application of this technology: membrane wetting and fouling. To counter these challenges, an innovative anti-fouling Janus membrane with asymmetric wettability was developed through electrospinning. The hydrophobic layer was formed using tetraethyl orthosilicate/polysulfone (PSF), and the superhydrophilic layer was created using polyvinylpyrrolidone (PVP)/PSF. A sensitive adhesion probe was used to assess the anti-fouling performance of the Janus membrane against oil. Molecular dynamics simulation suggested that PVP reduced the adsorption tendency of the membrane for humic acid (HA). Under experimental conditions involving saline water with HA and a saline oil–water emulsion, the non-Janus membrane suffered severe fouling, resulting in rapid water permeate flux decline. However, the Janus membrane demonstrated consistent permeate flux (26.84 LMH and 24.92 LMH) and an impressive salt rejection rate (> 99.99%). This study suggests that the Janus membrane, with its high permeate fluxes and remarkable resistance to fouling and wetting, could be an effective solution for wastewater treatment, with considerable potential for future application.
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Introduction
Water is a vital resource for life on Earth, and only a small fraction of the total accessible quantity, roughly 2.5%–3%, is currently used for human consumption. However, as the global population and industrial scale escalate, the demand for industrial and domestic water also increases, highlighting an urgent need to address the freshwater crisis1,2,3,4. Membrane technology plays a considerable role in the water treatment field owing to its superior separation performance, operational simplicity and absence of secondary pollution5. Membrane distillation (MD), an advanced water treatment technology, harnesses the selective separation properties of membranes for distillation6,7,8,9 and holds several advantages over traditional distillation, including spatial efficiency, energy conservation and ease of operation10,11. Moreover, the theoretical removal rate of non-volatile substances using this method approaches 100, meeting the high-quality water requirements of the industry12,13. The MD process is typically divided into four types: direct contact membrane distillation (DCMD), vacuum membrane distillation (VMD), scavenging membrane distillation (SGMD), and air gap membrane distillation (AGMD), of which DCMD has garnered considerable attention owing to its straightforward operational mode14,15. In the DCMD process, the flow of hot saline water (feed) and cold pure water (permeate) on either side of a hydrophobic membrane induces a transmembrane temperature gradient (∆T) that propels the transport of water vapour across the membrane16,17,18.
Although the use of MD is advantageous in many respects, it is crucial to be mindful of potential problems such as contamination with organic compounds such as oil and humic acid (HA) in the feed solution19,20. Oil, which is a low-surface tension pollutant, is being increasingly released into the environment due to the rapid expansion of oil industries, such as the petroleum, food, textile, leather, steel and metal-finishing industries21. Oil droplets readily adsorb onto and accumulate on the surface of the hydrophobic membrane, diminishing its hydrophobicity and decreasing its liquid entry pressure (LEP), with accelerated membrane wetting the general consequence22,23. HA, a widespread natural organic matter in the natural water environment, is derived from complex macromolecules that are produced by the chemical and biological degradation of plants and microorganisms24,25. Owing to hydrophobic-hydrophobic and electrostatic interactions, both oil and HA adhere to the MD membrane surface and intrude its pores, interrupting the transfer of water vapour across the membrane22,26,27,28,29. The presence of natural organic matter in HA can compromise membrane performance and consequently reduce its lifespan30,31,32. Considering these challenges, the development of a new type of membrane that boasts a high permeate flux along with resistance to organic compound contamination is of considerable importance.
To address the challenges associated with membrane fouling and wettability, a novel Janus membrane featuring asymmetric wetting properties has been proposed33,34,35. This innovative solution draws inspiration from the structures of mussel and fish scales, in which a hydrophilic surface with a rough structure establishes a dense hydration layer, thereby effectively counteracting organic pollutants36,37. A variety of preparation strategies have been suggested for Janus membranes, including electrospinning38,39, chemical deposition40,41, grafting42,43 and interfacial polymerization44,45. For instance, Yang et al. 34 prepared a Janus hollow fibre membrane in which a hydrophilic polydopamine (PDA)/polyethylenimide (PEI) coating was deposited onto the surface of a hydrophobic polypropylene (PP) membrane. Similarly, Huang et al. 28 desalinated highly saline water with hydrophobic contaminants and amphiphilic wetting agents by associating a fully hydrophobic substrate with a hydrophilic layer. Tang et al. 46 prepared a composite membrane with asymmetric wettability by electrospinning a hydrophobic polytetrafluoroethylene (PTFE) membrane coated with polyacrylonitrile (PAN), and then hydrolyzing with ethylenediamine and sodium hydroxide to achieve better hydrophilicity. However, the preparation methods of Janus membranes require some excessive late or pre-modified treatment to achieve better hydrophobicity or hydrophilicity, which makes their preparation methods complicated or or reduce the transmittance of water vapour, compromising the flux.
Among the diverse Janus membrane preparation techniques, electrospinning emerges as an excellent technique owing to its ability to create nanofiber membranes with a large surface area, selective wettability and an interconnecting pore structure. This method allows precise control over the microstructure of the membrane during preparation while allowing the incorporation of other functional materials47,48. When compared to membranes that are prepared through stretching or phase inversion, the electrospun nanofiber membrane, which is composed of stacked ultrafine fibres and interconnected pores, exhibits superior water flux49,50. Li et al. 51 showcased the potential of this technique via electrospinning to fabricate a high performance membrane from poly N-isopropylacrylamide (PNIPAM)/polystyrene (PS). However, their preparation process also requires post-processing such as high-temperature calcination, which affects the continuity of the preparation process.
This study is an investigation of Janus MD membranes that were fabricated through electrospinning. The developed membranes comprise a superhydrophilic layer in which PSF is blended with polyethylpyrrolidone (PVP) and a hydrophobic substrate that is created by mixing PSF with tetraethyl orthosilicate (TEOS). The low surface energy substance TEOS introduced by electrospinning creates a rough hydrophobic layer to improve the resistance of wetting, and PVP forms a hydration layer on the membrane surface via hydrogen bonding, enhancing the anti-pollution performance of the Janus membrane. The nanofiber membranes with asymmetric wetability fabricated by electrospinning can be directly used for DCMD operation after hot pressing without excessive post-processing or pre-modification, which can improve the preparation efficiency of the membrane. The morphology and wettability of the Janus membranes are characterised, and the water flux of the commercial polytetrafluoroethylene membrane (C-PTFE) is compared to that of the modified PSF membrane during DCMD. DCMD experiments involving oil and HA-containing brine are also conducted to assess the anti-fouling performance of the Janus membrane. The findings reveal excellent flux and salt rejection rates for the developed Janus membrane when exposed to saltwater containing oil and organic pollutants.
Results
Optimization and stability of Janus membrane hydrophilic layer
Two key components are required for the formation of superhydrophilic and underwater superoleophobic surfaces: a rough micro–nano structure and a hydrophilic chemical composition52,53. Herein, the hydrophilic layer of the Janus membrane is formed via the introduction of PVP, which carries a high number of hydrophilic groups. To obtain the most appropriate anti-fouling hydrophilic layer, it is crucial to investigate the effect of PVP content on membrane hydrophilicity, which is assessed via the WCA in air. A lower WCA indicates superior hydrophilicity, and a higher underwater OCA indicates a stronger resistance of the membrane to hydrophobic pollutants. As depicted in Fig. 1a, the WCA of the modified PSF membrane gradually decreases and the underwater OCA increases with increasing PVP content. This trend indicates an improvement in the hydrophilicity of the PVP-modified PSF membrane. With a PVP content of 6 wt%, the WCA of the modified PSF membrane is 67.77°, demonstrating a hydrophilic surface. At the PVP content of 7.5 wt%, the modified PSF membrane exhibits superhydrophilicity (WCA = 0°) and underwater superoleophobicity (OCA > 150°). Thus, a modified PSF membrane with a PVP content of 7.5 wt% was chosen as the hydrophilic layer of the Janus membrane.
a Effect of PVP content on WCA and underwater OCA of modified PSF membrane. Rinse with water for 24 h, (b) WCA and OCA, and (c, d) surface morphology of the Janus membrane’s hydrophilic layer. The error bar shows the standard deviation (n = 5 independent replicates).
The stability of the hydrophilic layer is crucial for the anti-pollution performance of Janus membrane. As shown in Fig. 1b, after 24 h-rinsing, a slight decrease in underwater OCA was observed, while the superhydrophilic side of the Janus membrane maintained its superhydrophilicity (WCA = 0°) and superoleophobicity (OCA > 150°). The stability of the superhydrophilic layer can be attributed to the entwined ultra-high molecular weight PVP and polysulfone molecules introduced during preparation process. Furthermore, the surface morphology (Fig. 1c, d) revealed that there was no dissolution or collapse of fibres on the hydrophilic side, demonstrating the stability of PVP/PSF superhydrophilic layer prepared by electrospinning.
Membrane morphology
Figure 2a–f displays the representative SEM images of the pristine PSF membrane, PSF membranes infused with varying concentrations of TEOS and Janus membrane. The images reveal that the nanofibers haphazardly form a comprehensive network structure. The exceptional spinnability of the prepared electrospun suspension contributes to the fibrous structure of the resultant electrospun membrane, as illustrated in Fig. 2a–f, with the randomly-stacked fibre structure yielding a membrane characterised by high porosity, an interconnected open-hole structure and considerable surface roughness, collectively resulting in a membrane with superior water flux and energy efficiency. Figure 2g1–g3 portrays the surface structure and cross-section of C-PTFE. The PTFE membrane, which was fabricated using the stretching method, exhibits a solidified pore size structure and a considerably lower porosity than the nanofiber membrane. In comparison to C-PTFE, the porous nanofiber membranes that were prepared through electrospinning set the foundation for achieving higher flux in MD applications.
SEM showing the surface morphology and cross sections of pristine PSF (a1–a3), 10%TEOS/PSF (b1–b3), 30%TEOS/PSF (c1–c3) and 50%TEOS/PSF (d1–d3), the superhydrophilic side of the Janus membrane (e1–e3) and the hydrophobic side of the Janus membrane (f1–f2) and C-PTFE (g1–g3). (a4–e4, f3) Fibre diameter distribution.
The wettability of a membrane is mainly the result of its microstructure and surface chemical composition. The high hydrophobicity of the fibrous membrane is mainly attributed to the high level of roughness and narrow fibre diameter54. As demonstrated in Fig. 4b–d, the inclusion of TEOS alters the average fibre diameter, which decreases from 902.89 ± 286.19 to 678.85 ± 264.51 nm. This reduced fibre diameter is a result of the decreased viscosity of the spinning solution upon the addition of TEOS, which in turn impacts the morphology of the nanofiber membrane (The viscosity of the polymer solution can be seen in the Supplementary Discussion 1). Previous studies have shown that the decrease of solution spinning viscosity is related to the decrease of fibre diameter55,56,57. As the TEOS concentration escalates, the fibre surface becomes noticeably rougher; however, the impact it has on the porous fibre structure of the membrane is limited. The introduction of TEOS enhances the roughness of the membrane, and the reduced fibre diameter results in denser fibre packing, which leads to a decrease in the pore size of the membrane, improving its wetting resistance.
Figure 2e–g portrays the surface morphology and cross-section of a Janus membrane with asymmetric wetting characteristics. Despite the presence of a superhydrophilic PVP/PSF layer covering, the porous structure and fibre diameter of the hydrophobic base layer remain unaffected. As demonstrated in Fig. 2e1–e3, the PVP/PSF nanofibers, characterised by a smooth fibre surface, entwine continuously, forming membranes with high porosity and impressive interconnectivity. The reduced fibre diameter and increased specific surface area of the PVP/PSF membrane offers a more extensive reaction space for water vapour penetration as compared to the original PSF membrane. One interesting outcome of the PVP addition is the emergence of a nanoscale spindle junction structure on the nanofibers, which is depicted in Fig. 2e2. This structure not only retains the connectivity and porosity of the membrane but also triggers the movement of the oil droplets, which eventually float off the surface. As a result, the PVP/PSF membrane manifests high anti-fouling performance58. The surface morphologies of modified PSF membranes with different PVP content are shown in the Supplementary Fig. 2. As can be seen in Fig. S2 under the same preparation conditions, increasing the PVP content leads to a decrease in the number of beads in the PVP/PSF fibre, leading to a more complete fibrous structure for the nanofiber membrane. This indicates that the addition of PVP improves the spinnability of the PSF.
Surface roughness is a notable determinant of the hydrophobicity of a particular material. The surface roughness of the membranes was investigated using AFM. As imllustrated in Fig. 3a–f, the Ra of the PSF nanofiber membrane surpasses that of C-PTFE following modification with TEOS. This is attributed to the uneven surface that forms due to the random stacking of nanofibers. Further, it is observed that the roughness of the modified PSF membrane escalates in tandem with the increased TEOS content. This phenomenon is caused by the addition of TEOS as a dopant to the polymer solution, which affects the evaporation of the solvent and elasticity of the polymer jet during electrospinning. As a result, the jet surface is ejected and split into smaller jets, reducing the local charge balance per unit surface area, which leads to an uneven accumulation of nanofibers on the collector59. These results align with those obtained through FE-SEM. As the TEOS concentration increases, the roughness of the nanofiber membrane follows suit, enhancing the hydrophobicity of the modified membrane. Compared to C-PTFE, the electrospun nanofiber membrane exhibits greater roughness, thereby attributing a larger specific surface area to the fibre membrane60,61, which leads to a larger area for contact with the water vapour during MD, resulting in a more substantial water permeate flux. Supplementary Figure 3 presents the AFM images of the modified PSF membranes with varying PVP content, which show that increasing the PVP content leads to an increase in the surface roughness of the PVP/PSF membrane, with the roughness and introduction of hydrophilic groups leading to the excellent hydrophilicity observed in the PVP/PSF membranes.
a–f AFM image of the pristine PSF membrane, modified PSF composite membrane and C-PTFE.
Membrane surface chemical analysis
The surface chemical composition of the pristine PSF membrane and modified PSF composite membrane were analysed through XPS. The typical spectra are provided in Supplementary Fig. 4, and the related elemental compositions are outlined in Table 1. Comparison of the elemental composition of the pristine PSF membrane with that of the 50%TEOS/PSF membrane indicates the presence of significant quantities of silicon, which suggests changes in the elemental composition of the membrane surface due to TEOS addition. As illustrated in Fig. 4a, the Si peaks on the 50% TEOS/PSF surface at 101.6 and 102.2 eV are attributable to the presence of Si–C and Si–O bonds in the TEOS-treated modified PSF, respectively62. This implies that the introduction of TEOS leads to the emergence of low-surface-energy substances on the modified membrane, which is a key factor that enhances the hydrophobicity of the modified PSF composite membrane.
a XPS Si 2p of the 50%TEOS/PSF membrane. b XPS C1s of the superhydrophilic layer in the Janus membrane.
The hydrophilicity of the Janus membrane contributes considerably to its anti-fouling capabilities. As seen in Table 1, the N element in the hydrophilic layer is linked to the C–N bond of PVP. The inclusion of PVP is therefore essential to enhance the hydrophilicity of the modified PSF membrane. Figure 4b reveals the C 1 s nuclear class spectrum of the PVP/PSF composite membrane, in which four peak components with chemical energies of 287.2, 285.5, 284.6 and 283.9 eV corresponding to C = O, C–H, C–N and C–C/C = C, respectively, are observed46,63. Notably, C = O and C–N, which come from the amide group, are specific chemical components of PVP, and the integration of the hydrophilic amide groups enhances the anti-pollution capabilities of the composite membrane.
Membrane structural properties and wettability
The contact angle serves to evaluate the surface wettability of the fabricated membrane. As depicted in Fig. 5a, the WCA of the original PSF membrane was 132° and the underwater OCA was registered at 49°. The WCA of the modified PSF membrane shows a gradual increase as the TEOS concentration increases, a phenomenon that is attributed to the enhanced surface roughness and introduction of low-surface-energy substances that are a result of TEOS addition64,65,66. Because hydrophobic membranes should demonstrate superior wetting resistance to facilitate long-term MD operation, 50% TEOS/PSF was selected as the hydrophobic layer for the Janus membrane (The stability analysis of the hydrophobic layer can be seen in the Supplementary Discussion 2). However, interaction between the hydrophobic substances allows the oil droplets to spread easily across the hydrophobic membrane, prompting consideration of the underwater OCA as a critical index for assessing membrane fouling. The subsequent investigation of the underwater OCA indicted a tendency for the TEOS-modified PSF membrane to be easily wetted by oil, contributing to membrane contamination in the actual MD process. On the contrary, the Janus membrane, which is overlaid with a hydrophilic layer, exhibits both superhydrophilic and superoleophobic characteristics underwater. These traits are attributed to the formation of a hydration layer on the Janus membrane, which is due to the action of the PVP-borne hydrophilic groups and hydrogen bonds that can effectively resist organic pollutants such as oil or HA. Moreover, as shown in Fig. 5a, the combination of hydrophobic and superhydrophilic layers exert minimal influence on each other in terms of wettability. This observation signifies the successful preparation of a Janus membrane with asymmetric wetting properties (The asymmetric wetting properties of Janus membrane can be seen in Supplementary Fig. 6).
a Wetting properties of original PSF and modified PSF composite membranes. Changes in the adhesion of oil droplets to the (b) 50%TEOS/PSF membrane and (c) Janus membrane during advancing and receding. d Pore size diameter distribution of the original and modified PSF composite membranes. The error bar shows the standard deviation (n = 5 independent replicates).
The interaction between pollutants and the membrane was assessed by monitoring the changes in the adhesion force during the interaction between corn oil and membrane. This approach allowed measurement of the susceptibility of the membrane to pollution via the adhesion curve. Figure 5b, c illustrates the changes in adhesion observed in the 50%TEOS/PSF and Janus membranes due to the interaction between oil droplets and their surfaces. The process involving the interaction of oil with the membrane is divided into four distinct stages: contact, compression, retraction and separation, with contact and compression forming the advancing stage and retraction and separation forming the receding stage. Figure 5b reveals a stark increase in the adhesion curve when corn oil contacts the 50%TEOS/PSF membrane. This surge indicates that the surface of the material is immediately drenched upon contact due to the interaction between the hydrophobic substances. This finding is in line with the results of the underwater OCA. On the contrary, the Janus membrane, with its asymmetric wetting properties, repels oil droplets upon contact with its superhydrophilic layer, as depicted in Fig. 5c. This behaviour is consistent with the superoleophobicity observed via the underwater OCA.
LEP, which represents the minimum transmembrane pressure necessary for a feed solution to infiltrate the membrane pores, is a key component in maintaining the long-term stability of MD. The LEP values for the unmodified PSF membrane and modified PSF composite membrane were ascertained using the Young–Laplace equation. In particular, these values are directly proportional to the surface tension (γ) and contact angle (θ) of the feed solution and are inversely proportional to the maximum pore size diameter (D) of the membrane. It should be noted that increasing the pore diameter leads to a decreased LEP, which triggers severe problems such as scaling and wetting of a membrane. Several factors can influence the pore size diameter of the electrospun membrane, including the choice of polymer and the type of collector, with the fibre diameter also playing a significant role. Generally speaking, the pore size diameter of a nanofibrous membrane tends to decrease as the fibre diameter decreases67. As depicted in Fig. 5d, increasing the TEOS concentration leads to a decrease in the pore diameter of the modified PSF membrane via a gradual reduction in the average diameter of the nanofibers. Consequently, the TEOS-modified PSF membrane exhibits a smaller average pore diameter and hence a larger LEP. It can be inferred that the original PSF membrane has a lower salt rejection rate as compared to the TEOS-modified PSF membrane during MD, as salt migration occurs concurrently with water migration. Other properties of the membrane are listed in Table 2, including the excellent resistance to water pressure as observed for the 50%TEOS/PSF membrane, which is a crucial attribute for withstanding robust MD processes. In comparison to C-PTFE, the electrospun membrane has a porosity of more than 80%, which enhances the water vapour transport channel, increasing the permeability.
Simulation of membrane pollution resistance to HA
Molecular dynamics simulation was used to analyse the experimental results and explain the anti-pollution mechanism of the Janus membranes. Figure 6a, b respectively shows the initial moment of the two systems and the atomic trajectory diagram after 800 ps. The simulation results of the two systems showed that the HA molecule did not penetrate either the PSF or the Janus membranes. Figure 6a2 shows that the HA molecules are adsorbed onto the surface of the PSF membrane and are nested within the PSF molecules. However, it is obvious from Fig. 6b2 that the HA molecules are blocked by a hydrophilic layer composed of PVP.
Initial and final images of Systems 1 (a1–a2) and 2 (b1–b2) in the molecular dynamics simulation (with the HA molecule in blue for easy observation).
To further evaluate the resistance of the Janus membranes to pollution, the formation of the hydration layers is explained by the interaction between molecules. The interaction energy can reflect the ability of the components to mix with each other, with smaller interaction energies indicating a more stable system. The interaction energy between the HA and water molecules on the PSF and PVP layers was therefore obtained through simulation, and the results are shown in Fig. 7a1–a2. Figure 7a1 shows that the interaction energy of the water molecules on PVP is much lower than it is on PSF. The main reason for this phenomenon is the strong polarity of the amide groups in PVP. The stronger the polarity effect, the more easily hydrogen bonds form between polar groups and water molecules. The addition of PVP enhances the hydrophilicity of the membrane, which is also consistent with the measured WCA results. However, the large number of hydrophobic groups (benzene rings) on the PSF lead to an increase in the interaction energy between water and PSF. Compared with PVP, HA showed a lower interaction energy for PSF, indicating that it was more likely to adsorb onto the PSF surface. This suggests that introducing PVP can reduce the tendency of HA to adsorb onto the membrane surface.
a1–a2 The interaction energy of water and HA molecules with PSF and PVP, respectively. b1–b2 Over the simulation period and distance from the HA and water molecules to the membrane surface as a function of simulation time in Systems 1 and 2. c MSD of the water molecule during simulation.
As shown in Fig. 7b1–b2, the simulation inferred that the distance between the water molecules and HA molecules and the membrane decreases over time in System 1, indicating that PSF cannot resist HA adsorption. On the contrary, Fig. 7b2 shows that water molecules were adsorbed onto the surface of the membrane at the beginning of the simulation and that the distance between the molecules and membrane did not change during the whole simulation process, with the HA molecule only touching the surface of the membrane in the final stage. This shows that the added PVP quickly forms a hydration layer on the membrane surface, isolating it from the HA. The effect of PVP on the water molecules was further quantified by MSD. As shown in Fig. 7c, comparison of Systems 1 and 2 showed considerably lower water molecule diffusivity in System 2 as compared to System 1. This is caused by the strong interaction between the membrane and water. Moreover, more hydrogen bonds and electrostatic interactions are observed between the PVP and water molecules, which also reduce the diffusion of the water molecules. This phenomenon indicates that the molecules in the hydration layer are tightly bound to those on the surface of the polymer membrane.
MD desalination test
The original PSF, modified PSF composite, Janus and commercial C-PTFE membranes were each subjected to MD operation, with a 35-g/L NaCl solution used to assess their desalting performance. Figure 8a, b illustrates the flux and salt rejection of the membranes. In comparison to the other composite membranes, the original PSF membrane was observed to wet quickly, causing a rapid decrease in salt resistance during the DCMD process. The experiment was terminated when the desalting rate dropped below 80% or the water permeation flux disappeared. However, the flux and salt removal rates of the modified composite membranes to which TEOS was added remained relatively stable. This suggests that the incorporation of TEOS enhances the wetting resistance of the modified membrane, which was consistent with the WCA and LEP characterization results of the modified membrane. The average permeate flux of the C-PTFE, 10%TEOS/PSF, 30%TEOS/PSF, 50%TEOS/PSF, and Janus membranes were found to be 10.67 LMH, 28.57 LMH, 30.45LMH, 33.79LMH, and 30.39LMH, respectively. After 24 h, the desalting rates were recorded as 99.99%, 99.57%, 99.86%, 99.99% and 99.99%, respectively. These results indicate that the inclusion of TEOS equips the PSF membrane to endure long-term, robust MD operation.
MD performance of the commercial PTFE (C-PTFE), PSF, modified PSF and Janus membranes. a Salt rejection; (b) permeate flux. The error bar shows the standard deviation (n = 3 independent replicates).
The large pore size and heightened porosity of the electrospun nanofiber membrane results in the modified PSF membrane showing a high water flux68. Consequently, the C-PTFE membrane, which was prepared through the stretching process, exhibits a considerably lower flux than the modified PSF composite membrane. The average permeate flux of the 50%TEOS/PSF membrane is 3.2 times that of the C-PTFE membrane. As the concentration of TEOS increases, a corresponding rise is observed in the flux and salt rejection rate of the modified PSF composite membrane, with an 18.27% increase observed in the flux, from 28.57 to 33.79 LMH. The increase in the salt rejection rate is due to the improved hydrophobicity of the modified PSF composite membrane, which better restricts the transmembrane transportation of salt. This improvement means that the salt rejection rate of the 50%TEOS/PSF membrane reached commercial membrane levels. The decline in the flux of the Janus membrane is primarily due to the coverage provided by the superhydrophilic layer. This increased membrane thickness to some extent escalates the mass transfer resistance and obstructs the transmission channel on the base membrane. Nonetheless, the high roughness of the superhydrophilic layer offers a larger specific surface for the transmembrane transportation of water vapour, limiting the average flux decrease from 33.97 to 30.39 LMH, a decrease of only 10.06%. Furthermore, the hydrophilic layer coating exerts a limited impact on the salt rejection rate of the modified PSF composite membrane. Table 3 provides a summary of the DCMD test results for some of the developed Janus membranes, which demonstrate good performance in terms of high permeability and desalting efficiency.
Anti-fouling performance
Hydrophobic interactions between the surface of hydrophobic membranes and organic pollutants play a crucial role in membrane fouling, and resolving this issue is key for successful MD application. Given that the 50%TEOS/PSF membrane demonstrates an outstanding salt rejection rate and a considerably high permeate flux, this membrane was selected as a control for anti-fouling MD testing. As illustrated in Fig. 9a, the 50%TEOS/PSF membrane was fouled in the MD experiment, wherein HA served as an organic pollutant in the feed solution. Post 12 h, the water permeate flux fell from 30.42 to 0 LMH, indicating a severe membrane fouling phenomenon. The decline in the membrane flux during MD may be attributed to the adsorption of HA onto or within the membrane pores, HA deposition, and the concentration polarisation of HA during MD69. Contrastingly, the Janus membrane exhibited exceptional anti-scaling performance when exposed to HA. Post 24 h, the Janus membrane maintained a stable permeate flux and salt rejection rate, with an average flux of 28.15 LMH. After exposure to saline containing HA for 24 hours, the water permeation flux of the Janus membrane exhibited a slight decrease (26.84 LMH), representing a 12.30% reduction compared to the average flux observed with pure saline treatment. This indicates that the loaded superhydrophilic layer effectively reduces the tendency of HA to foul the membrane. The results of the DCMD experiment, in which the membrane activity was investigated using saline and HA, were also consistent with the results of the molecular dynamics simulation, and the hydration layer that formed after the introduction of PVP successfully resisted HA adsorption.
Assessment of the anti-fouling properties of the prepared composite membranes via a feed solution comprising (a) 100-mg/L HA in 35-g/L NaCl brine. Utilisation of an oil-in-water emulsion containing (b) 35-g/L NaCl and 1-g/L corn oil as feed. The error bar shows the standard deviation (n = 3 independent replicates).
To assess the resistance of the fabricated composite membrane to oil pollution during DCMD, an oil-in-water emulsion was utilised as the feed. As depicted in Fig. 9b, the 50%TEOS/PSF membranes encountered severe fouling, with the hydrophobic membrane water flux precipitously dropping to 0 LMH within the first hour. This decline in the flux can be ascribed to the adherence of oil droplets onto the underwater superoleophilic modified PSF membrane through hydrophobic interactions. When corn oil droplets attach to the membrane surface, the membrane pores are obstructed, leading to a decrease in the permeate flux. The fouling intensifies as corn oil continues to accumulate on the membrane surface, ultimately blocking the pores completely70,71. On the contrary, the permeate flux of the Janus composite membrane is sustained at 24.92 LMH, with the salt rejection rate remaining steady at ~99.99%. The oil fouling resistance of the Janus composite membrane is attributed to the additional superhydrophilic PVP/PSF layer, with the inclusion of the hydrophilic PVP material leading to the formation of a hydration layer underwater, which resists the adherence of oil droplets. This results in a membrane with a robust anti-oil capability72. The results of the DCMD experiments studying salt-containing emulsions are also consistent with the characterisation of the high-sensitivity force probes. The superhydrophilic layer can resist the adhesion of low-surface-tension pollutants such as oil droplets. The aforementioned anti-fouling MD process verifies that, in comparison to non-Janus composite membranes, the Janus membrane displays superior fouling and scaling resistance.
Discussion
The traditional hydrophobic MD membrane showed remarkable efficiency for desalination. However, the wetting and fouling problems of the membrane during real operation limit its practical application for MD. A Janus membrane was thus fabricated through electrospinning to address these problems. The membrane is constructed by covering a superhydrophilic layer on a hydrophobic nanofiber substrate that can effectively resist contamination from substances with low surface energy. Molecular dynamics simulations describe the formation of the hydration layer in the Janus membrane and its resistance to HA, an organic pollutant that is widely found in nature. The highly sensitive adhesion probe proves that the superhydrophilic layer can effectively prevent the adhesion of hydrophobic pollutants such as oil. Moreover, the prepared Janus membranes exhibited high permeate flux and stable salt rejection rate during DCMD. The anti-wetting and anti-pollution properties of the prepared membrane are expected to better solve the difficult problem of treating water that contains salt and organic pollutants in a more scalable, environmentally friendly and cost-effective manner.
Methods
Material
N, N-dimethylacetamide (DMAC, 99.0%), tetraethyl silicate (TEOS, >99%), 1-butanol (98%), corn oil (reagent grade) and HA were acquired from Shanghai Aladdin Co., Ltd., China. PSF (P-3500 LCD, average MW = 75,000–81,000, industrial grade) was obtained from Solvay (St Louis, MO, USA). Polyvinylpyrrolidone (PVP, average MW = 1,300,000) was purchased from Macklin Biochemical Co., Ltd., Shanghai, China. Acetone (Analytical Reagent) was supplied by Chengdu Colon Chemical Co., Ltd., China. C-PTFE prepared by the stretching method was purchased from China Zhongli Filtration Equipment Factory (The parameters of C-PTFE can be seen in the Supplementary Table 1). Corn oil was used in all the processes.
Preparation of polymer solution
Preparation of the hydrophobic layer by spinning is referred to in a previous study73. Briefly, 20 wt% PSF was dissolved in DMAC and acetone solvents (mass ratio of 2:1), and the mixture was stirred at 60 °C until homogenous. TEOS was then added until mass ratios of 0%, 10%, 30% and 50% were obtained for TEOS relative to PSF (labelled PSF, 10%TEOS/PSF, 30%TEOS/PSF and 50%TEOS/PSF, respectively). Then, the samples were stirred at 50 °C for 3 h and the resulting mixture was labelled solution A.
To obtain solution B, PVP and PSF were mixed with DMAC in different proportions at a total concentration of 15 wt% and stirred at 70 °C until completely miscible (resulting in 0 wt%, 1.5 wt%, 3 wt%, 4.5 wt%, 6 wt% and 7.5 wt% PVP).
Preparation of Janus nanofiber membrane
As depicted in Fig. 10, the Janus membrane was fabricated through electrospinning and heat-press. Before spinning, a loose and porous release paper is wrapped on the aluminium drum to facilitate better stripping of the fibre membrane. Firstly, the hydrophobic layer was prepared by spinning solution A under a high voltage of 18 kV, flow rate of 1.5 mL/h, and clearance distance of 15 cm. Then, hydrophilic layer was electrospun onto the hydrophobic layer by spinning solution B at a voltage of 13.5 kV, a flow rate of 1.0 mL/h, and a clearance distance of 12 cm. Once spun, the composite membrane was subjected to heat-pressing at 200 °C for 60 min (The pressure is 0.12 × 10−3MPA), enhancing the density of the Janus membrane while simultaneously ensuring that the hydrophilic and hydrophobic layers are tightly bonded. Non-Janus single-layer hydrophobic/hydrophilic PSF membranes were prepared under the same conditions used for solutions A and B.
Schematic diagram of preparation process and anti-pollution mechanism of Janus composite membrane.
Characterisation
The contact angle can be used to reflect the wettability of a membrane and was therefore measured using a contact angle analyser (DJ-SDJS, Yifeng, China). The water contact angle (WCA) and underwater oil contact angle (OCA) were investigated. Five tests were conducted at random locations on each sample, with 2 μL of deionized water and 5 μL of oil droplets used as the test liquids. The underwater OCA was measured by placing the oil droplet on the downward-facing surface of the membrane, as the density of oil is less than that of water.
The surface and cross-sectional morphologies of the membranes were observed through field emission scanning electron microscopy (FE-SEM, Merlin, Zeiss, German). The roughness and surface morphology of the membranes were analysed through atomic force microscopy (AFM, Multimode 8, Bruker, German) at a scanning size of 50 μm × 50 μm, and the average surface roughness (Ra) was determined from the results. X-ray photoelectron spectroscopy (XPS) was performed using an electron spectrometer (K-Alpha, Thermo Scientific, U.S.A), and the elements in the membrane were quantified. A universal testing machine (5967, Instron, USA) was used to measure the mechanical properties of the membranes (The mechanical properties of the membranes can be seen in the Supplementary Discussion 3). A rotary rheometer (MCR 702e, Anton Paar, Austria) was used to evaluate the viscosity of the polymer solutions at 25 °C and shear rates ranging from 0.1 to 100 s–1.
The pore size and pore distribution of the membranes were determined using a capillary flow metre (CFP-1500AE, PMI, U.S.A). The liquid entry pressure (LEP) of the membrane can be obtained according to the Young-Laplace Eq. (1)74:
where γ is the interfacial tension (γwater = 71.97 mN m−1 at 25 °C), θ is the contact angle and D is the max pore size diameter of the membrane.
The porosity of the membrane was evaluated by gravimetry. The weight of the membrane samples (3 cm × 3 cm) were measured under dry condition and fully wetted by 1-butanol. The porosity of the membrane was determined using Eq. (2)12:
Where where m1 and m2 represent the dry weight and wet weights (g) of the membrane, respectively, δ and S represent the thickness (cm) and area (cm2) of the membrane, respectively, and ρ represents the density of 1-butanol (g·cm−3).
To assess the stability of the superhydrophilic layer, the Janus membrane was immersed underwater and pure water was allowed to flow through it for 24 hours. After drying the Janus membrane in the oven, the WCA and underwater OCA were investigated, and the surface morphology of the hydrophilic layer was detected by FE-SEM.
In the MD process, the stability of the hydrophobic layer is essential for long-term operation. 50%TEOS/PSF membranes were immersed in pure water for 0, 1, 3, 5, and 7 days. To identify the chemical groups on the membrane surface during this period, attenuated total reflection fourier transform infrared spectroscopy (ATR-FTIR) analysis was performed using an FTIR spectrophotometer (iS50, Thermo Scientific, U.S.A). In addition, the WCA of the modified membrane was also measured during this period.
A highly sensitive adhesion measurement system (K100, Kruss, Germany) was used to analyse the interaction of contaminants on the membrane surface. The system included fixing the membrane sample on the bottom of a beaker and placing it on a vertically moving sample table. A platinum-iridium ring suspended on the oil droplet (about 15 μL) was used as a force probe and pre-immersed in a beaker filled with deionized water. The platform moves upward at a constant speed of 0.6 mm/min, and after contacting the sample surface at a constant speed, the sample platform continues to be extruded at a distance of 0.8 mm. The sample platform was then released to fall at a constant speed until the oil droplet separated completely from the surface of the membrane. The adhesive force between the oil droplets and the membrane was recorded to evaluate the anti-fouling ability of the membrane.
Humic acid fouling simulations
The anti-fouling performance of the Janus membrane was studied using classical molecular dynamics simulation (the related complex molecular structures and chains can be seen in Supplementary Fig. 8) Molecular dynamics can not only determine the trajectory of atoms but also observe the microscopic details of atomic motion; thus, it serves as a complementary tool for analysing and validating experimental results. To simplify the simulation process, the hydrophobic PSF membrane system (System 1) and PVP and PSF constituting the hydrophilic and hydrophobic layers of the Janus membrane (System 2) were simulated, allowing comparison between the two. In both the simulation systems, 1000 water molecules, 11 NaCl molecules and 1 HA molecule were used as the feed solution for MD and 1000 water molecules were used as the infiltration solution. The initial configurations of both the solvent systems were built using Material Studio.
The complex and variable structures of HA renders its simulation challenging. However, several HA models are available, and the Temple–Northwestern–Birmingham (TNB) model meets the criteria of a low Mw HA with multiple functional groups, and a molecular weight and chemical composition close to that of the HA that is likely to become attached to the hydrophobic membrane during an actual operation75. Therefore, the TNB model was selected as the HA molecule model in the systems.
Energy minimisation was used to relax the simulation chamber for each simulation, and the structure was optimised using a regular system at a time step of 0.1 fs, with the temperature difference (ΔT) set to 30 °C and the temperature maintained using a nose-Hoover thermostat. To allow sufficient time to obtain a stable structure, the NVT (N-Constant particle number, V-Constant volume and T-Constant temperature) optimisation time was set to 800.0 ps. The atomic motion was described using the classical Newton equation and solved using the Velity–Verlet algorithm. The simulation program is executed using LAMMPS 1.
The molecular dynamics simulation can verify that PVP can form a hydration layer on the membrane surface by calculating the interaction energy, and reduce the adsorption trend of HA on the membrane surface. The interaction energy can be obtained using Eq. (3)76:
where Etotal, Epolymer and Etarget represent the total energy of a system, the energy of the polymer and the energy of the target molecule (water and HA molecule), respectively.
The molecular dynamics model was used to analyse the motion trajectories of water and pollutants in Systems 1 and 2 and obtain the distances among the water, pollutants and membrane during simulation.
The mean square displacement (MSD), which is a measure of the position deviation of a particle relative to its initial reference position after it has been moving for some time, was used to measure the motion amplitude change of the water molecules during simulation and can be obtained using Eq. (4)77:
Where T is the total time of the simulation. MSD is a function of the time interval Δt.
Membrane distillation performance
The performances of the pristine PSF, modified PSF and C-PTFE membranes were evaluated using a laboratory-scale DCMD system (A schematic of DCMD’s experimental setup can be seen in the Supplementary Fig. 10) with an effective area of 12 cm2. The membrane module is made of PTFE and has internal dimensions of 20 mm × 60 mm × 2 mm. In the experiment, the temperature difference (ΔT) of 30 °C and 40 °C maintained between the feed and distillate solution. (The DCMD and anti-fouling performance of the membrane at a temperature difference of 40 °C can be seen in Supplementary Discussion 4). The temperature of the distillate solution is controlled at 25 ± 1 °C by the ice bath. In the DCMD tests, the superhydrophilic layer of the Janus membrane, faced the feed solution, while the hydrophobic layer faced the distillate solution. The feed and distillate solutions were circulated simultaneously through a dual-channel peristaltic pump operating at a controlled flow velocity of 0.11 m/s, with a 35-g/L NaCl solution and deionized water used as the feed and distillate solution, respectively. Each test was operated for 24 h during the experiment. The feed solution was supplemented with pure water every 3 h to maintain a constant volume. Two conductivity metres with temperature recording function (DDSJ-307F, Leici, China) record the temperature and conductivity of the feed and distillate solution every 30 s. The electronic balance records the mass change of the distillate solution every 30 s. The ongoing recording of the mass and conductivity of the distillate flow allowed for real-time calculation of salt rejection rate and water permeate flux. The salt rejection rate (R) and permeate flux (J) can be calculated using Eqs. (5)78 and (6)79:
where Cp is the concentration of the permeate solution, and Cf is the initial concentration of the feed solution (The relationship between conductivity and salt concentration can be seen in the Supplementary Fig. 11). Δm (kg) is the mass change of the distillate, A is the effective membrane surface area (m2), and ∆T is the time interval (h).
Evaluation of fouling resistance
Fouling resistance tests were conducted using a salt feed that incorporated oil and HA, respectively. To create a saline oil–water emulsion, 1 g of corn oil was thoroughly mixed with 1 L of saline water (35-g/L NaCl) using a homogenizer operating at 2000 rpm for 2 hours (the size distribution of oil droplets in the emulsion is given in Supplementary Fig. 12). Remarkably, no phase separation was observed in the prepared emulsion over a span of 7 days. A feed solution containing 35-g/L of NaCl and 100-mg/L of HA was used for the anti-scaling test. Both oil and HA were subjected to MD operations for a duration of 24 hours. The fouling resistance of the modified PSF membrane was assessed by comparing the MD performance.
Data availability
All data generated or analyzed during this study are available on request to the corresponding author.
Code availability
All software and code used in this study is open access.
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Acknowledgements
We gratefully acknowledge the financial support from the Ministry of Science and Technology of China for State Key Research and Development Project (2022YFC3705003-3), Guangdong Basic and Applied Basic Research Foundation (2021A1515110369 and 2019A1515110937), and Guizhou Provincial Basic Research Program (Natural Science, ZK[2024]005).
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Z W: Conceptualization, Methodology, Investigation, Resources, Data curation, Writing - original draft. K Z: Investigation, Resources, Writing - review & editing, Supervision, Funding acquisition. G Z: Validation. L H: Validation. S Z: Resources, Project administration, Supervision, Funding acquisition.
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Wu, Z., Zheng, K., Zhang, G. et al. Preparation of polysulfone-based nanofiber Janus membrane for membrane distillation containing organic pollutants. npj Clean Water 7, 51 (2024). https://doi.org/10.1038/s41545-024-00342-5
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DOI: https://doi.org/10.1038/s41545-024-00342-5
