Xiao-Hua Ma

Superhydrophobic modification of PVDF–SiO2 electrospun nanofiber membranes for vacuum membrane distillation

Electrospun nanofiber membranes having a hierarchical structure with multilevel roughness were generated via electrospinning of poly(vinylidene fluoride) (PVDF)–SiO2 blend solutions. The composite PVDF–SiO2 nanofiber membranes were then endowed with superhydrophobicity by the fluorosilanization of the surface with low surface energy fluoroalkylsilane (FAS). The results showed that when the SiO2 content in the dope solutions increased from 0 wt% to 8 wt%, the water contact angles of the FAS modified nanofiber membranes increased significantly from 130.4° to 160.5°. The increment of the silica content in the dope solutions decreased the fiber diameters and pore sizes of the modified membranes, while the mechanical properties were enhanced with the silica addition. The liquid entry pressures of the membranes increased gradually from 84 kPa to 195 kPa with silica addition due to the increased contact angles and decreased pore size. Vacuum membrane distillation experiments were carried out for the modified nanofiber membranes to evaluate the anti-wetting properties. The optimal superhydrophobic nanofiber membrane maintained a stable flux of 31.5 kg m−2 h−1 with a permeate conductivity approximately 10 μs cm−1 over the entire test, while the fluxes and conductivities of the nanofiber membranes without superhydrophobicity showed a significant decrease and increase, respectively. The results indicated that the superhydrophobic modification process rendered the nanofiber membrane anti-wetting properties without compromising its excellent permeability.

FAS Grafted Electrospun Poly(vinyl alcohol) Nanofiber Membranes with Robust Superhydrophobicity for Membrane Distillation

This study develops a novel type of electrospun nanofiber membranes (ENMs) with high permeability and robust superhydrophobicity for membrane distillation (MD) process by mimicking the unique unitary microstructures of ramee leaves. The superhydrophobic ENMs were fabricated by the eletrospinning of poly(vinyl alcohol) (PVA), followed by chemical cross-linking with glutaraldehyde and surface modification via low surface energy fluoroalkylsilane (FAS). The resultant FAS grafted PVA (F-PVA) nanofiber membranes were endowed with self-cleaning properties with water contact angles of 158° and sliding angles of 4° via the modification process, while retaining their high porosities and interconnected open structures. For the first time, the robust superhydrophobicity of the ENMs for MD was confirmed by testing the F-PVA nanofiber membranes under violent ultrasonic treatment and harsh chemical conditions. Furthermore, vacuum membrane distillation experiments illustrated that the F-PVA membranes presented a high and stable permeate flux of 25.2 kg/m2 h, 70% higher than those of the commercial PTFE membranes, with satisfied permeate conductivity (<5 μm/cm) during a continuous test of 16 h (3.5 wt % NaCl as the feed solution, and feed temperature and permeate pressure were set as 333 K and 9 kPa, respectively), suggesting their great potentials in myriad MD processes such as high salinity water desalination and volatile organiccompounds removal.

A Novel Seeding Method of Interfacial Polymerization-Assisted Dip Coating for the Preparation of Zeolite NaA Membranes on Ceramic Hollow Fiber Supports

A novel seeding method combining interfacial polymerization (IP) technique with dip-coating operation was designed for directly coating nanosized NaA seed crystals (150 nm) onto the micrometer-sized α-Al2O3 hollow fiber support, in which the polyamide (PA) produced by IP acted as an effective medium to freeze and fix seed crystals at the proper position so that the controlled seed layer could be accomplished. While a coating suspension with only 0.5 wt % seed content was used, a very thin seed layer with high quality and good adhesion was achieved through dip coating twice without drying between, and the whole seeding process was operated at ambient conditions. The resulting zeolite NaA membranes not only exhibited high pervaporation (PV) performance with an average separation factor above 10000 and flux nearly 9.0 kg/m(2)·h in dehydration of 90 wt % ethanol aqueous solution at 348 K but also demonstrated great reproducibility by testing more than eight batches of zeolite membranes. In addition, this seeding strategy could be readily extended to the preparation of other supported zeolite membranes for a wide range of separation applications.

A self-cleaning TiO2 coated mesh with robust underwater superoleophobicity for oil/water separation in a complex environment

Porous materials with underwater superoleophobicity have recently become attractive candidates for oil/water separation, but are restricted in practical applications because of their easy contamination in air and poor resistance in a complex environment. Herein, we present a facile, efficient, and environmentally friendly approach to fabricating a TiO2 coated stainless mesh with both a photocatalytic self-cleaning property and excellent chemical resistance via a sol–gel method. The uniform, crack-free TiO2 nanoparticle coating layer endows the mesh with underwater superoleophobicity and corrosion resistance, which enables the effective separation of oil–water mixtures not only in a gentle environment but also in a complex environment such as acidic, alkaline and salty solutions. In addition, once these meshes were contaminated by organic molecules and lost their unique surface wettability, they could readily recover their origin wettability and separation performance by ultraviolet (UV) illumination owing to the photocatalysis of TiO2. With respect to its exceptional stability, self-cleaning property and ease fabrication, the TiO2 coated mesh holds promising potential for practical oil/water separation applications.

A one-step rapid assembly of thin film coating using green coordination complexes for enhanced removal of trace organic contaminants by membranes

We report a fast, simple, and green coating method using the coordination complex of tannic acid (TA) and ferric ion (Fe3+) to enhance the removal of trace organic contaminants (TrOCs) by polyamide membranes. The entire coating process can be completed in less than 2 min; quartz crystal microbalance characterization revealed that a TA-Fe thin film formed in merely 10-20 s. Coating this TA-Fe thin film on a commercial nanofiltration membrane (NF270) reduced its effective pore size from 0.44 to 0.40 nm. The TA-Fe-coated NF270 showed significantly increased rejection of both NaCl and trace organic contaminants. In comparison with the more-time-consuming polydopamine coating (e.g., 0.5 h), the TA-Fe coating presented greater resistance to TrOC permeation (i.e., lower permeability of TrOCs). The advantages of the fast coating process, greatly improved rejection performance, and use of green accessible materials make TA-Fe a highly promising coating material for large-scale applications.