Membrane distillation (MD), a membrane-based thermal separation process driven by a vapor pressure gradient, is a promising method for treating complex high salinity industrial wastewater or desalination. Compared to other desalination technologies, MD has broad application prospects in small-scale desalination applications due to its outstanding performance in terms of relatively small footprint, highly compact configuration and expected utilization of low-grade waste heat energy. Interconnected porous membranes prepared by electrospinning technology receive great attention due to their excellent permeability. However, poor hydrophobic durability due to pore deformation limits its desalination performance.
In view of this, Professor Wang Wei's team of Harbin Institute of Technology used electrospinning technology to build a new type of nanofiber membrane, which showed excellent durability in seawater desalination.
Environ. Sci. Technol.: In situ 3D welded nanofiber membranes for membrane distillation treatment of concentrated seawater
This article reports an in situ three-dimensional (3D) soldering method for emulsion electrospinning for the preparation of robust nanofiber membranes. The method is simple and effective, and can be soldered at the intersection of nanofibers, enhancing the uniform distribution of membrane pores in 3D space. The results show that compared with the non-soldered (superhydrophobic) nanofiber film and the post-welded (superhydrophobic) nanofiber film, the stability of in situ 3D welded nanofiber film is 170h, the water recovery rate is 76.9%, and it has good desalination performance.
In addition, the current work also studies the stability mechanism of in situ 3D welded nanofiber membranes and two different wetting mechanisms of non-soldered and post-welded nanofiber membranes. What's more, in situ 3D welded nanofiber membranes can further concentrate actual concentrated seawater (121°E, 37°N) to crystallization, demonstrating its potential for application in challenging concentrated seawater desalination. The research was published in the journal Environmental Science & Technology under "In Situ Three-Dimensional Welded Nanofibrous Membranes for Robust Membrane Distillation of Concentrated Seawater."
Figure 1. (a) Schematic diagram of the preparation of PH/PDMS emulsion, (b) three stages of in situ 3D soldering nanofiber film formation during electrospinning. (c) ATR-FTIR spectroscopy of in situ 3D welded nanofiber membranes.
Figure 2. (a) Surface SEM image of in situ 3D welded nanofiber film, (b) cross-section SEM image and (c) photograph. (d) Surface SEM images of non-soldered nanofiber films, (e) cross-sectional SEM images, and (f) photographs. (g) Surface SEM image of the nanofiber film after welding, (h) cross-section SEM image and (i) photograph.
Figure 3.(a) Water flux and distillate conductivity trend of in situ 3D welded nanofiber membrane, non-soldered nanofiber membrane and post-welded nanofiber membrane, using DCMD process to feed on synthetic concentrated seawater. Note that the concentration of the feed solution is constant. (b) Show photos of in situ 3D welded nanofiber membranes after experiments, WCAs of in situ 3D welded nanofiber membranes before and after experiments, non-soldered nanofiber membranes after experiments, and post-experimental post-welded nanofiber membranes. (c) The mechanism diagram of the excellent hydrophobic durability of in situ 3D welded nanofiber membranes, the wettability of non-soldered nanofiber membranes, and the water bag phenomenon of post-welded nanofiber membranes.
Figure 4. (a) In situ 3D welded nanofiber film and (b) C-PVDF film water flux and distillate conductivity changes with time and water recovery rate, where synthetic concentrated seawater is fed. (c) Photos of feed bottles before and after concentration experiments using in situ 3D welded nanofiber membranes. (d) The change trend of water flux and distillate conductivity of non-soldered superhydrophobic nanofiber membranes and post-welded superhydrophobic nanofiber membranes over time, of which synthetic concentrated seawater is fed.
Fig. 5.(a) In the DCMD process, the water flux and distillate conductivity of the nanofiber membrane are soldered in situ 3D using the actual concentrated seawater as the feed, and the water flux and distillate conductivity change with time and water recovery rate. The actual seawater is taken from the Yellow Sea of China (121° E, 37° N). (b) Photographs of feed bottles before and after concentration experiments using in situ 3D welded nanofiber membranes. (c) 3D fluorescence spectra of actual seawater and distilled water (obtained from actual seawater using in situ 3D welded nanofiber membranes).
Desalination: Design of robust superhydrophobic fiber membranes and study of their membrane distillation durability
A major challenge in desalting high salinity wastewater using electrospinn fiber distillation membranes (EFDM) is pore wetting caused by fouling. Here, Professor Wang Wei's team designed a solid superhydrophobic fiber membrane through a simple and scalable electrospinning technology combined with the dip coating method, which showed a high water flux of about 28.5 L/m2·h (ΔT=40°C) and unexpected durability (>312h) during the long-term operation of direct contact membrane distillation.
It is worth noting that the authors also systematically studied the effects of surface wettability and pore structure on the durability of EFDM. Experiments and simulations show that the formation of hard pores in EFDM has an important impact on the durability of the membrane when desalting high salinity wastewater, because the effect of water supply turbulence on the surface of EFDM is avoided on pore deformation. At the same time, the superhydrophobic surface also helps to reduce the contact area between the crystal and the salt, further improving the durability of the membrane. This theoretical simulation provides new insights into the design of novel structural fiber membranes with excellent MD stability. The research was published in the journal Desalination under the title "Design of firm-pore superhydrophobic fibrous membrane for advancing the durability of membrane distillation".
Figure 1.(a) The working mechanism of the MD process. (b) Illustration of phase conversion membranes during MD. (c) Illustration of electrospinning film in the MD process. (d) Assumption that hard-bore superhobular hydrospinning fibers have excellent water flux and durability.
Figure 2. Schematic diagram of the synthesis process of superhydrophobic PVDF FM, illustrated in this illustration showing the morphological structure of (a) PVDF FM, (b) PVDF@PDMS FM, and (c) PVDF@PDMS/SNP FM, respectively.
Figure 3. PVDF@PDMS/SNP NFM compared with the moisture resistance and durability of the associated membrane, the inlet water was 3.5wt% NaCl solution. (a) Water flux and (b) permeability.
Figure 4.The deformation resistance of membrane pores of different nanofiber networks is simulated by ABAKQUS software: (a) raw PVDF FM and (b) PVDF@PDMS FM.