Congjie Gao

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.

Stiff metal–organic framework–polyacrylonitrile hollow fiber composite membranes with high gas permeability

Metal–organic framework (MOF) membranes used for gas separation have attracted considerable attention recently. Although there are studies focusing on their continuous growth on inorganic substrates, MOF–polymer membranes are in demand because of their low cost, high processing ability and large membrane area. To achieve this purpose, the flexibility of the polymer must be reduced and the MOF-to-substrate adhesion strength should be greatly enhanced. Herein, a continuous and well inter-grown Cu3(BTC)2–PAN composite membrane has been successfully fabricated by directly growing via a covalent linker. Dehydrogenation, cyclization and crosslinking reactions of the PAN hollow fiber by solvothermal treatment can greatly improve the stiffness and compression strength of the composite membrane. To increase the adhesion strength effectively, a covalent linker between the MOF layer and PAN substrate is provided by chemical modification. The prepared membrane achieves a high H2 permeance of 7.05 × 10−5 mol (m−2 s−1 Pa−1) and a good separation factor of 7.14 for binary H2–CO2 mixtures with high thermal and pressure stability. Our strategy has also been developed for preparing a continuous and well inter-grown ZIF-8–PAN membrane. A very thin ZIF-8 layer is finally synthesized due to the large number of nucleation sites on the substrate. All of these distinguished properties suggest that MOF–PAN composite membranes fabricated by chemical modification are promising candidates for gas separation.

Mixed matrix membranes containing MIL-53(Al) for potential application in organic solvent nanofiltration

Aromatic poly(m-phenyleneisophthalamide) (PMIA) and the metal-organic framework (MOF) MIL-53(Al) were employed as the polymer matrix and additive, respectively, to develop mixed matrix membranes (MMMs) via non-solvent induced phase separation for potential application in organic solvent nanofiltration. The prepared membranes were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and water contact angle measurements. The membrane water permeance enhanced when MIL-53(Al) was incorporated into the membrane structure while the rejection had no significant change. The optimum MMM (with 0.5 wt% MOF concentration) passes mono and bivalent inorganic salts but rejects larger charged organic molecules and has a mean effective pore size of 0.7 nm. The influence of organic solvents on MMM performance was also investigated and the result shows that the performance shifts towards a lower pure water permeance and a higher rejection after exposure to organic solvents (ethyl acetate or methanol). The membrane performance in organic solvent nanofiltration was evaluated on the basis of the permeance and rejection of brilliant blue G in ethanol, and the result showed that the permeance of MMMs significantly increased (by 289%) while the rejection slightly reduced by 4% in contrast to the pure membrane.

Enhanced conductivity of monovalent cation exchange membranes with chitosan/PANI composite modification

The application of electrodialysis (ED) for desalination requires the use of natural seawater or river water, in which the presence of multivalent ions is inevitable. This currently limits the process performance. Membranes with selectivity for monovalent ions may overcome this limitation. This study used the method of electro-deposition with chitosan/aniline polymer as a modification material to coat a commercial anion exchange membrane in view of obtaining selectivity for monovalent ions. Chitosan was grafted with polyaniline through copolymerization using ammonium persulfate as an initiator. FTIR spectra of the composites revealed that there was a strong interaction between substituted polyanilines and chitosan. The method was used to prepare a series of membranes by varying the aniline ratio and polymerization time. The chemical composition and surface properties of the membranes were characterized by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM), respectively. Current–voltage curves and resistance were measured to characterize the transport properties of the membranes and the membrane conductivity. The results show that the membrane conductivity increases with the aniline ratio; the selectivity initially increases with the aniline ratio, and then decreases again. The optimum modification condition was an electrodeposition time of 4 h with an aniline ratio of 0.4. Using the modified membrane in concentrated sea water, it was demonstrated that the modified membrane has an excellent selectivity towards monovalent cations.

Recovery of chemically degraded polyethyleneimine by a re-modification method: prolonging the lifetime of cation exchange membranes

Selectivity for monovalent cations is an important property of cation exchange membranes (CEMs). The cation exchange membranes of the CSO modified with polyethyleneimine type have a higher selectivity for monovalent cations than the multivalent cations. Unfortunately, the loss of selectivity for these kinds of CSO seems to be unavoidable due to fouling and degradation of polyethyleneimine groups. In this situation, a “re-modification” technique was developed for recovery of fouled CSO, activating the fouled CSO by methanol and a sulfuric acid solution with ultrasonic vibration, followed by a layered surfacial electro-deposition method to prolong the lifetime of cation exchange membranes. A series of electrodialysis experiments for Na+/Ca2+ separation was performed for evaluating and comparing the monovalent cation selectivity of the samples. The restoration of the surface and cross section morphology after “re-modification” was demonstrated by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). As a result of the re-modification method the membranes with chemically degraded polyethyleneimine were again made functional. The ion exchange groups of the CSO modified with polyethyleneimine were successfully recovered, giving the membrane a high permselectivity again.

Mimicking the cell membrane: bio-inspired simultaneous functions with monovalent anion selectivity and antifouling properties of anion exchange membrane

A new bio-inspired method was applied in this study to simultaneously improve the monovalent anion selectivity and antifouling properties of anion exchange membranes (AEMs). Three-layer architecture was developed by deposition of polydopamine (PDA) and electro-deposition of N-O-sulfonic acid benzyl chitosan (NSBC). The innermost and outermost layers were PDA with different deposition time. The middle layer was prepared by NSBC. Fourier transform infrared spectroscopy and scanning electron microscopy confirmed that PDA and NSBC were successfully modified on the surfaces of AEMs. The contact angle of the membranes indicated an improved hydrophilicity of the modified membranes. A series of electrodialysis experiments in which Cl−/SO42− separation was studied, demonstrating the monovalent anion selectivity of the samples. The Cl−/SO42− permselectivity of the modified membranes can reach up to 2.20, higher than that of the commercial membrane (only 0.78) during 90 minutes in electrodialysis (ED). The increase value of the resistance of the membranes was also measured to evaluate the antifouling properties. Sodium dodecyl benzene sulfonate (SDBS) was used as the fouling material in the ED process and the membrane area resistance of modified membrane increase value of was only 0.08 Ωcm2 30 minutes later.

Charge-Gated Ion Transport through Polyelectrolyte Intercalated Amine Reduced Graphene Oxide Membranes

Charge-gated channels are natures solutions for transport of water molecules and ions through aquaporins in biological membranes while excluding undesired substances. The same mechanism has good potentials to be adopted in pressure or electrically driven membrane separation processes. Herein, we report highly charged nanochannels created in polyelectrolyte (PE) intercalated amine reduced graphene oxide membrane (PE@ArGO membrane). The PE@ArGO membrane, with a rejection layer of ∼160 nm in thickness, features a laminate structure and a smooth top surface of a low roughness (typically ∼17.2 nm). Further, a modified PE@ArGO membrane (mPE@ArGO membrane) was developed in situ using free chlorine scavenging post-treatment method, which was designed to alter the charge while keeping alteration to the layered structure minimal. The surface charge of the PE@ArGO and mPE@ArGO membrane was +4.37 and -4.28 mC/m2 respectively. In pressure driven processes, the pure water permeability for PE@ArGO and mPE@ArGO was 2.9 and 10.8 L m-2 h-1 bar-1, respectively. Salt rejection is highly dependent on the charge density of the membrane surface, the valence of the co-ions and the size of ions in hydrated form. For example, in the positively charged PE@ArGO membranes, the rejection of the salts follows the order of: R(MgCl2), 93.0% > R(NaCl), 88.2% ≈ R(MgSO4), 88.1% > R(Na2SO4), 65.1%; while in the negatively charged mPE@ArGO membranes, the rejection of the salts follows the order of: R(Na2SO4), 90.3% > R(NaCl), 85.4% > R(MgSO4), 68.3% > R(MgCl2), 42.9%. To the best knowledge of the authors, this is the first study to report graphene oxide based membranes (GOBMs) with high density positive/negative charge gated ion transport behavior. Whats more, the high rejection rate along with high water permeability of the PE@ArGO and mPE@ArGO membranes has not been achieved by other types of GOBMs.

Solvent treatment of CTA hollow fiber membrane and its pervaporation performance for organic/organic mixture

Cellulose triacetate (CTA) hollow fiber membrane used to separate methanol/methyl tert-butyl ether (MTBE) by pervaporation (PV) has been prepared from CTA hollow fiber reverse osmosis (RO) membrane for desalination of brackish water with high salinity. Acetone was selected as a modification agent of CTA membrane. PV performance depended on the solvent concentration, the treatment time and modification temperature of CTA RO hollow fiber membrane soaked in the aqueous acetone. The results show that CTA hollow fiber membrane modified with the solvent has a superior performance both the separation factor and the permeate flux in the PV experiment conditions.

Preparation of Cu2O nanowire-blended polysulfone ultrafiltration membrane with improved stability and antimicrobial activity

Polysulfone (PSF) membranes have been widely applied in water and wastewater treatment, food-processing and biomedical fields. In this study, we report the preparation of modified PSF membranes by blending PSF with Cu2O nanowires (NWs) to improve their stability and antifouling activity. Synthesis of novel Cu2O NWs/PSF-blended ultrafiltration membrane was achieved via phase inversion method by dispersing one-dimensional Cu2O nanowires in PSF casting solutions. Various techniques such as XRD, SEM, TEM, and EDS were applied to characterize and investigate the properties of nanowires and membranes. The introduced Cu2O nanowires can firmly be restricted into micropores of PSF membranes, and therefore, they can effectively prevent the serious leaking problem of inorganic substances in separation process. The blended PSF membranes also provided enhanced antimicrobial activity and superior permeation property compared to pure PSF membrane. The overall work can not only provide a new way for preparation of novel blended membranes with multidimensional nanomaterials, but can also be beneficial to solve the annoying problem of biofouling.Graphical Abstract

A novel semi-aromatic polyamide TFC reverse osmosis membrane fabricated from a dendritic molecule of trimesoylamidoamine through a two-step amine-immersion mode

In this work, a novel semi-aromatic polyamide RO membrane was fabricated by using a new self-made dendritic molecule trimesoylamidoamine (TMAAM) as a key functional amine monomer that combined 1,3-diamino-2-propanol (DAP) to react with trimesoyl chloride (TMC) through interfacial polymerization technology. By adjusting the TMAAM concentration and amine-immersion mode, this new TMAAM-based semi-aromatic polyamide RO membrane exhibits simultaneously improved water permeability, antifouling and chlorine-tolerant properties. The introduced TMAAM units in polyamide chains can enhance the TMAAM-based membranes water flux due to its dendritic structure as well as rich hydrophilic groups, and the DAP–TMAAM–TMC membrane prepared via the new two-step amine-immersion mode has 1.9 times more water flux without loss of salt rejection than the pristine DAP–TMC membrane and also shows higher water flux than the hand-cast conventional aromatic polyamide MPD–TMC membrane, respectively. At the same time, the regularly distributed hydroxyl groups and aliphatic amide bonds in TMAAM units contribute to the improved hydrophilicity and chlorine-tolerant property of the resultant DAP–TMAAM–TMC membrane, respectively. It is also demonstrated that the new two-step amine-immersion mode leads to a much smoother surface which endows the resultant DAP–TMAAM–TMC membrane with a favorable antifouling ability. This research provides us with a promising functional amine monomer and a new membrane formation method to fabricate a high performance RO membrane.