Yan-Li Ji

High-Flux Positively Charged Nanocomposite Nanofiltration Membranes Filled with Poly(dopamine) Modified Multiwall Carbon Nanotubes

The poor dispensability of pristine carbon nanotubes in water impedes their implications in thin-film nanocomposite membranes for crucial utilities such as water purification. In this work, high-flux positively charged nanocomposite nanofiltration membranes were exploited by uniformly embedding poly(dopamine) modified multiwall carbon nanotubes (PDA-MWCNTs) in polyamide thin-film composite membranes. With poly(dopamine) modification, fine dispersion of MWCNTs in polyethyleneimine (PEI) aqueous solutions was achieved, which was interracially polymerized with trimesoyl chloride (TMC) n-hexane solutions to prepare nanocomposite membranes. The compatibility and interactions between modified MWCNTs and polyamide matrix were enhanced, attributed to the poly(dopamine) coatings on MWCNT surfaces, leading to significantly improved water permeability. At optimized conditions, pure water permeability of the PEI/PDA-MWCNTs/TMC nanofiltration membrane (M-4) was 15.32 L m(-2) h(-1) bar(-1), which was ∼1.6 times increased compared with that of pristine PEI/TMC membranes. Salt rejection of M-4 to different multivalent cations decreased in the sequence ZnCl2 (93.0%) > MgCl2 (91.5%) > CuCl2 (90.5%) ≈ CaCl2, which is well-suited for water softening and heavy metal ion removal.

Bio-inspired fabrication of high perm-selectivity and anti-fouling membranes based on zwitterionic polyelectrolyte nanoparticles

Nanofiltration membranes featuring high permeability, selectivity and anti-fouling properties represent a focal point of advanced membrane technologies for clean water production and purification. Inspired by “water channel” structures and fouling resistance characteristics of biological membranes, we fabricated a novel thin-film nanocomposite (TFN) membrane containing zwitterionic polyelectrolyte nanoparticles (ZPNPs) by interfacial polymerization, wherein ZPNPs act as building blocks allowing for simultaneously improved permeability, selectivity and anti-fouling properties. By modulating the zwitterionic group content and ionic cross-linking degree of ZPNPs, the TFN-ZPNP membrane showed high water permeability (109.7 L m−2 h−1 MPa−1) and enhanced NaCl/Na2SO4 selectivity (28.4), respectively; these values were 191% and 125% of those for the pristine polyamide membrane. It was also demonstrated that the incorporation of ZPNPs can increase the surface hydrophilicity, electronegativity and reduce the surface roughness, leading to an improved anti-fouling performance against the bovine serum albumin protein foulant.

Novel separation membranes based on zwitterionic colloid particles: tunable selectivity and enhanced antifouling property

Zwitterionic colloid particles (ZCPs), a novel type of membrane material endowed with tunable selectivity and improved antifouling properties, were synthesized from 3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate (DMAPS) and 2-hydroxyethyl acrylate (HEA) by means of aqueous free-radical polymerization. Atomic force microscopy and dynamic light scattering analyses indicated that the water dispersed ZCPs nanoparticles (300–600 nm) were formed owing to the competitive balance between the electrostatic attraction of DMAPS and the hydrophilic dispersion effect of HEA. As such, when the DMAPS content in ZCPs was selected in the range of 19.4 to 37.1 mol%, ZCP membranes (ZCPMs) were prepared via surface coating, followed by glutaraldehyde crosslinking at 50 °C for 3 h. The ZCPMs have a good separation performance to organic molecules (polyethylene glycol, PEG) and inorganic salts (Na2SO4). For example, the average rejection of ZCPMs to PEG1000, PEG600, PEG200 and Na2SO4 is about 96%, 88%, 60%, and 20%, respectively. Noteworthy, the nanostructures of ZCPs and their membrane performances could be conveniently adjusted by tuning the monomer ratio in copolymerization, the H2SO4 concentration in the casting solution and the pH value of the feed solution. Furthermore, by using humic acid and bovine serum album as model organic foulant molecules, the antifouling properties of ZCPMs proved to be excellent as exemplified by the stable water flux and salt rejection versus filtration time.

Engineering novel polyelectrolyte complex membranes with improved mechanical properties and separation performance

Polyelectrolyte complexes (PECs) commonly suffer from poor processability owing to their ionic crosslinking nature, a problem which spurs increasing interest in processable PECs. New processing technologies have been exploited to render PECs processable, but usually at the expense of compromising their mechanical properties. Through a conceptually novel strategy of “complexation–sulfation”, here we engineer solution-processable PECs derived from a strong polyacid, in pursuit of high mechanical strength combined with exceptional separation performance. Effects of chemical structures and compositions on mechanical properties of these PEC membranes were studied. It was found that the mechanical properties of these PEC membranes based on strong “ion-pairs” were substantially enhanced, with their tensile strength and elongation at break reaching as high as 108.3 MPa and 5.0%, respectively. In addition, PEC membranes exhibited a high performance in separating water–ethanol mixtures. For example, the flux and water content in the permeate for PEC membranes were 2100 g m−2 h−1 and 99.58 wt%, respectively, in dehydrated 10 wt% water–ethanol mixture at 70 °C.

Polyelectrolyte complex nanofiltration membranes: performance modulation via casting solution pH

Nanofiltration (NF) membranes were prepared from a solution processable polyelectrolyte complex (PEC) between sodium carboxymethyl cellulose (CMCNa) and poly(2-methacryloyloxy ethyl trimethylammonium chloride) (PDMC). Electrostatic complexation structures of the PEC were studied by light transmittance and field emission electron scanning microscopy (FESEM). It is found that the electrostatic complexation structure of the PEC membranes determines their NF performance, which is conveniently tailored via the pH values of casting solutions. For membranes prepared at the optimum solution pH 2.1, their water flux and salt rejection to K2SO4 (1 g L−1) are 18 g m−2 h−1 and 97%, respectively, when the operation pressure and temperature are 0.6 MPa and 25 °C. Both the water permeability and the salt selectivity are substantially improved compared to the pristine CMCNa NF membrane. In addition, antifouling performance of the PEC membrane is improved, coupled with an exceptional stability versus the operation time.

Tailoring the structure of polyamide thin film composite membrane with zwitterions to achieve high water permeability and antifouling property

A series of carefully designed polyamide thin film composite nanofiltration membranes (TFCMs) were prepared via interfacial polymerization of piperazine (PIP), N-aminoethyl piperazine (AEP) or N-aminoethyl piperazine propane sulfonate (AEPPS) with trimesoyl chloride (TMC) on top of hydrolysed polyacrylonitrile ultrafiltration supporting membranes (hPAN-UF). Chemical structures of the TFCMs were evaluated by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). The cross section and surface characteristics of the TFCMs were examined by scanning electron microscopy (SEM), atomic force microscopy (AFM), water contact angle measurements and zeta potential. Membrane fouling characteristics were studied by the adsorption of bovine serum albumin (BSA) and lysozyme (LYZ). Varying the diamine affected the structure of the resulting membranes, which ultimately defined separation performance and antifouling property. The results of water contact angle indicated zwitterionic membranes were more hydrophilic, which incorporated polyamide selective layer thickness to account for their high water flux (80.3 L m−2 h−1). Compared with other TFCMs, zwitterionic membranes showed an improved antifouling property due to their high hydrophilicity, low surface electrical charges and smooth surface roughness. These results provide important insights into the high water permeability and low fouling characteristics of zwitterionic nanofiltration membranes from molecular structure and interfacial polymerization process.