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Photothermal membrane distillation (PMD) achieves localized temperature increase by absorbing and converting light energy to drive evaporation and phase change processes. It is expected to solve the challenges of low energy conversion efficiency and fouling of traditional membrane distillation (MD). A dual-structured photothermal composite membrane was constructed by oxidatively polymerizing photothermal functional materials on an electrospinning substrate membrane. The superhydrophobic Polytetrafluoroethylene/Polyimide (PTFE/PI) substrate membranes offer a spacious pore structure and excellent mechanical properties with low mass transfer resistance and low heat loss. The presence of the densely connected region of the Polyaniline (PANI) in the photothermal layer accelerates the local photothermal conversion. The porous structure formed by interconnecting fibers loaded with PANI is conducive to the repeated refraction of light beams and thus enhances the absorption of light energy. Under 1 kW·m−2 light intensity, the permeate flux of the PTFE/PI-PI/PANI membrane is stable at 1.44 ± 0.02 L·m−2·h−1 within 480 h PMD. With the outstanding salt rejection performance of 99.99 % and high photothermal conversion efficiency of 90.3 %, the photothermal membrane can be applied in areas where fresh water and fuel resources are scarce but solar energy resources are abundant. [Display omitted] • The superhydrophobic layer prevents membrane wetting for durable PMD operation. • The hydrophilic photothermal layer mitigates the temperature polarization effect. • The dual heating mode of composite membrane boots the mass transfer driving force. • The overall porous structure expands efficient water vapor transmission channels. [ABSTRACT FROM AUTHOR]

Among the various desalination technologies, membrane distillation (MD) stands out for its high salt rejection, low operating temperature and operating pressure. Photothermal membrane distillation (PMD) represents an innovative approach to membrane distillation (MD), wherein the process harnesses plentiful and renewable solar energy to achieve localized heating at the interface between the membrane and the feed solution. This is achieved by employing photothermal materials, eliminating the need for heating a substantial volume of the feed solution. This study has prepared membranes with micro/nano fiber interwoven structure based on polystyrene (PS) and polyvinylidene fluoride hexafluoropropylene copolymer (PVDF-HFP) by electrospinning technology, and constructed multi-stage MoS 2 photothermal fiber membranes by hydrothermal growth process. The PS/PVDF-HFP micro/nano fiber membrane possesses a spacious pore structure and high porosity, serving as a robust framework that facilitates swift transport of water vapor. The MoS 2 nanoflowers uniformly grown on the micro/nano fibers have both metal 1T and semiconductor 2H phases, which facilitate the absorption of sunlight, reaching an average absorbance of 90.9 % in the240-2600 nm wavelength range. Under 1 kW m−2 solar illumination, the surface temperature of the PS/PVDF-HFP/MoS 2 membrane could be raised to 74.2 °C after 120 s. The permeate flux was stable at 1.21 ± 0.02 L m−2 h−1 within 20 h PMD. The membrane showcased a salt rejection rate of 99.96 % and achieved an energy efficiency of 72.08 %. This suggests that the multi-stage MoS 2 photothermal fiber membrane, characterized by remarkable stability and high photothermal conversion efficiency, lays a solid foundation for advancing research in the field of PMD applications. [Display omitted] • Construction of electrospinning multi-stage fiber membranes loaded with MoS 2. • Membranes feature large pore size, high porosity and high photothermal efficiency. • Multi-stage fiber membranes are used for brine purification. • Photothermal membrane distillation process runs with high flux and salt rejection. [ABSTRACT FROM AUTHOR]

[Display omitted] • Electrospinning hydrophilic/hydrophobic photothermal membranes for SDMD is proposed. • The membrane shows strong photothermal conversion properties and anti-fouling properties. • The membrane exhibits superstable and high flux in long-term desalination of SDMD. Low permeate flux and membrane fouling are major limiting factors for solar-driven membrane distillation (SDMD) industrial applications. Here, we report a Janus nanofiber membrane with electrospinning CB/PVA as hydrophilic photothermal layer and electro-blown spinning PTFE nanofiber membrane as hydrophobic layer, which exhibits excellent photothermal conversion performance, anti-fouling performance and durability. The hydrophilic photothermal layer simultaneously captures solar energy and performs photothermal conversion, repels chemical/oil-based pollutants and promotes steam flow. At the same time, the low surface of PTFE layer can guarantee the long-term durability of the composite nanofiber membrane. Under the light intensity of 1 kW·m−2, the Janus nanofiber membrane can obtain a permeate flux of 1.05 L·m−2·h−1 and a salt rejection of > 99.99 %. And the photothermal conversion efficiency reached 71.4 %. More importantly, the permeate flux of the Janus nanofiber membrane was still stable at 1.035 L·m−2·h−1 after continuous treatment of pollutant-containing feed solutions for 60 h, and the salt rejection was > 99.98 %. This indicates that the efficient photothermal conversion performance and anti-fouling properties make the CB-PVA/PTFE photothermal nanofiber membrane promising for generating fresh water from feed solutions containing various pollutants in SDMD systems. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is an attractive membrane process for seawater and brine water desalination. One of the main obstacles for MD application is the absence of an effective MD membrane with high water permeability and an excellent salt rejection. This work proposes a novel design strategy of hydrophobic/hydrophilic bi-component membranes for MD by electrospinning and calcination. Unlike the conventional dual-layer hydrophobic/hydrophilic membranes, the bi-component membrane developed in this work is composed of multiple layers of hydrophilic polyimide (PI) nanofibers and hydrophobic polytetrafluoroethylene (PTFE) microclusters, which interweave with each other in the entire membrane. The optimal hydrophobic/hydrophilic bi-component membrane #PTFE/PI-2 shows robust three-dimensional (3D) superhydrophobic and self-cleaning properties. It exhibits a highly competitive permeation flux of 42 ± 3 L m−2 h−1 and an excellent salt rejection above 99.9% in a 400-h continuous DCMD operation when the feed solution was synthetic seawater with a NaCl concentration of 3.5 wt% at 60 °C. Moreover, it is able to achieve an outstanding water flux of 40 ± 2 L m−2 h−1 and a salt rejection over 99.9% when the feed solution was brine water with a NaCl concentration of 25 wt%. The high vapor permeability of #PTFE/PI-2 should be attributed to the high membrane porosity, negligible mass transfer resistance of PI nanofibers and more effective water evaporation area on its superhydrophobic surface. Meanwhile, the excellent anti-wetting property should be due to a unique 3D superhydrophobicity constructed by multiple layers of PTFE microclusters. Image 1 • Hydrophobic/hydrophilic bi-component membrane is proposed. • It composes of PTFE microclusters and PI nanofibers. • The resultant membrane exhibits robust 3D superhydrophobicity. • #PTFE/PI-2 shows a water flux of 40 ± 2 L/m2·h and a salt rejection above 99.9% in MD. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is an attractive thermally-driven membrane process for desalination, brine and wastewater treatment. One of the main obstacles impeding MD application is the absence of effective MD membranes with sufficient water permeability and long-term stability. Attributed to the outstanding hydrophobicity, excellent thermal and chemical resistance, the most ideal polymer to develop MD membrane is polytetrafluoroethylene (PTFE). However, the fabrication of PTFE membranes is challenging due to its poor processability. This work describes a novel strategy to develop superhydrophobic and highly porous PTFE membranes via electrospinning. The hydrophobic nanoparticles 8 vinyl-grafted polyhedral oligosilicone (vinyl-POSS) were firstly incorporated into PTFE nanofibers to promote the crystallization of PTFE nanofiber, enhance nanofiber roughness, and improve membrane mechanical robustness and porosity. The effects of vinyl-POSS concentration, temperature and salt concentration of feed solutions on MD performance of PTFE/POSS nanofibrous membranes were investigated systemically in this study. The optimized PTFE/POSS nanofibrous membrane #POSS-2 has a three-dimensional (3D) superhydrophobic property with a water contact angle of 151 ± 4°. It exhibited a competitive water flux of 40 ± 2 Lm−2 h−1 in the direct contact membrane distillation (DCMD) process when the feed and permeate temperatures were 60 and 20 °C, respectively. Moreover, #POSS-2 possessed an excellent long-term stability in a 200-h continuous DCMD operation. Unlabelled Image • Vinyl-POSS has been incorporated into PTFE nanofibers to promote PTFE crystallization. • PTFE/POSS nanofiber shows a highly rough surface with valleys and ridges. • #POSS-2 possesses 3D superhydrophobicity, high porosity, and sufficient mechanical and chemical robustness. • #POSS-2 shows a competitive water flux and a stable rejection above 99.99% in a 200-h long-term DCMD operation. [ABSTRACT FROM AUTHOR]

Permeability and selectivity are the two most important parameters in membrane distillation process. The membrane with high porosity and superhydrophobicity could increase the mass transfer coefficient, decrease the heat losses and improve the resistance to wettability. To satisfy this purpose, a novel superhydrophobic polyvinylidene fluoride (PVDF)/tetrabutylammonium hexafluorophosphate (TBAHP)/polystyrene (PS) multi-scale nanofiber membrane (MNM) with high porosity and low thermal conductivity was successfully designed and developed via one-step electrospinning. The formation of multi-scale structures was caused by the addition of TBAHP. PS owns low thermal conductivity and low surface energy which can enhance the thermal efficiency and improve the hydrophobicity of MNMs. Besides, the addition of PS can amplify the classifying effect, construct the nano-micron structure and endow the membrane with superhydrophobicity. The results showed that the thermal conductivity of membranes decreased from 0.03361 W/mK to 0.02777 W/mK and the water contact angle increased from 138.2° to 151.7°. The PVDF/TBAHP/35%PS MNMs exhibited a stable MD performance with an average permeation flux of 50 ± 3 L/m2 h and a satisfactory salt rejection higher than 99.9% during 72-h continuous DCMD operation (3.5 wt% NaCl solution, the feed/permeate temperatures were 60/20 °C, respectively). Unlabelled Image • A multi-scale nanofiber membrane (MNM) was developed by electrospinning. • The MNMs exhibited high porosity and narrow pore size distribution. • The MNMs possessed low thermal conductivity and superhydrophobicity. • The MNMs showed high permeate flux (51.96 L/m2 h) and salt rejection (99.99%). [ABSTRACT FROM AUTHOR]