Miaomiao Tian

Surface zwitterionic functionalized graphene oxide for a novel loose nanofiltration membrane

Surface zwitterionization of graphene oxide (GO) was firstly conducted by grafting poly(sulfobetaine methacrylate) (PSBMA) onto the GO surface via reverse atom transfer radical polymerization (RATRP). Then, a novel type of GO-PSBMA/polyethersulfone (PES) loose nanofiltration membrane (NFM) was constructed by mixing with modified GO composites via phase inversion. FTIR, XRD, TEM, XPS and TGA were applied to analyze the chemical composition and morphology, confirming a favorable synthesis of GO-PSBMA composites. Besides, the effect of the embedded GO-PSBMA nanoplates on the morphology and overall performance of the hybrid membranes was systematically investigated based on the SEM images, water contact angle, zeta potential, and fouling parameters. It was found that the water flux of the hybrid membrane was greatly enhanced from 6.44 L m−2 h−1 bar−1 to 11.98 L m−2 h−1 bar−1 when the GO-PSBMA content increased from 0 to 0.22 wt%. The antifouling tests revealed that the GO-PSBMA embedded membranes had an excellent antifouling performance: a high flux recovery ratio (ca. 94.4%) and a low total flux decline ratio (ca. 0.18). Additionally, the hybrid membranes exhibited a distinct advance in the mechanical strength due to the addition of highly rigid GO. Notably, compared with unmodified membranes, the hybrid membranes had a higher retention of Reactive Black 5 (99.2%) and Reactive Red 49 (97.2%), and a lower rejection of bivalent salts (10% for Na2SO4) at an operational pressure of 0.4 MPa, rendering the membranes promising for dye/salt fractionation.

Elevated Performance of Thin Film Nanocomposite Membranes Enabled by Modified Hydrophilic MOFs for Nanofiltration

Metal-organic frameworks (MOFs) are studied for the design of advanced nanocomposite membranes, primarily due to their ultrahigh surface area, regular and highly tunable pore structures, and favorable polymer affinity. However, the development of engineered MOF-based membranes for water treatment lags behind. Here, thin-film nanocomposite (TFN) membranes containing poly(sodium 4-styrenesulfonate) (PSS) modified ZIF-8 (mZIF) in a polyamide (PA) layer were constructed via a facile interfacial polymerization (IP) method. The modified hydrophilic mZIF nanoparticles were evenly dispersed into an aqueous solution comprising piperazine (PIP) monomers, followed by polymerizing with trimesoyl chloride (TMC) to form a composite PA film. FT-IR spectroscopy and XPS analyses confirm the presence of mZIF nanoparticles on the top layer of the membranes. SEM and AFM images evince a retiform morphology of the TFN-mZIF membrane surface, which is intimately linked to the hydrophilicity and adsorption capacity of mZIF nanoparticles. Furthermore, the effect of different ZIF-8 loadings on the overall membrane performance was studied. Introducing the hydrophilizing mZIF nanoparticles not only furnishes the PA layer with a better surface hydrophilicity and more negative charge but also more than doubles the original water permeability, while maintaining a high retention of Na2SO4. The ultrahigh retentions of reactive dyes (e.g., reactive black 5 and reactive blue 2, >99.0%) for mZIF-functionalized PA membranes ensure their superior nanofiltration performance. This facile, cost-effective strategy will provide a useful guideline to integrate with other modified hydrophilic MOFs to design nanofiltration for water treatment.

Elevated salt transport of antimicrobial loose nanofiltration membranes enabled by copper nanoparticles via fast bioinspired deposition

Surface functionalization with advanced nanomaterials offers tailored control and targeted design of surface properties, endowing materials with enhanced or new qualities such as high hydrophilicity, excellent selectivity and permeability, and enhanced antimicrobial activity. In this study, we develop two strategies (two-step deposition/co-deposition) that use mussel-inspired polydopamine (PDA) to strongly immobilize copper nanoparticles (CuNPs) onto a porous polymeric membrane, bridging the surface cavities from ultrafiltration (UF) to loose nanofiltration (NF). To confirm the optimization of membrane properties, a series of characterizations were carried out: SEM, EDX analysis, AFM, water contact angle, and zeta potential measurements. The results indicate an overall high performance of surface properties with a homogeneous nanoparticle distribution, low roughness, favorable hydrophilicity, and relatively neutral charge. Co-deposition of PDA and CuNPs exhibits a facile and time-saving process that expedited a higher CuNP loading compared to the two-step strategy, as confirmed by SEM and AFM images. The integration of polyethylenimine (PEI)-modified CuNPs with high density of positive charges plays an important role in fine-tuning the hydrophilicity and compatibility with PDA and in largely neutralizing the negative charge of PDA, thus promoting an outstanding salt permeation (82% Na2SO4, 98% NaCl). In addition, CuNP/PDA-modified membranes show an ultra-high rejection of three types of textile dyes (600–800 Da, >99.0%), demonstrating superior NF performance. Furthermore, the functionalized membranes display distinct bactericidal activity with a great reduction of 93.7% in the number of live Escherichia coli (E. coli) bacteria. This study highlights a fast, facile co-deposition strategy to assemble multifunctional coating onto a UF support, which renders a vast potential for the fractionation of dye/salt mixtures.

The rapid emergence of two-dimensional nanomaterials for high-performance separation membranes

Membrane-based nanotechnologies have gained growing attention in various separation fields due to advantages such as high energy efficiency and ecofriendliness. However, the limited and empirical design parameters of conventional membranes greatly slow down research progress on rapid and efficient molecular separations. Two-dimensional materials (2DMs) – e.g., graphene, MoS2, MXene, metal–organic frameworks, and covalent organic framework nanosheets – are rapidly emerging in the development of membranes with high selectivity and permeability. The rational tunability and precise control of interlayer distances and/or nanoporous apertures of 2DMs enable 2DM-based membranes to achieve highly specific separations. Furthermore, the ultrathin structure of 2DMs offers minimal transport resistance and thus leads to extraordinary flux rates. This review briefly outlines recent approaches in the use of various 2DMs for membrane designs. Some remarkable performances of 2DM-based membranes and their separation mechanisms are also noted. Challenges and future directions toward molecular-level designs of membranes with 2DMs are additionally discussed.

Mussel-Inspired Architecture of High-Flux Loose Nanofiltration Membrane Functionalized with Antibacterial Reduced Graphene Oxide–Copper Nanocomposites

Graphene-based nanocomposites have a vast potential for wide-ranging antibacterial applications due to the inherently strong biocidal activity and versatile compatibility of such nanocomposites. Therefore, graphene-based functional nanomaterials can introduce enhanced antibiofouling and antimicrobial properties to polymeric membrane surfaces. In this study, reduced graphene oxide-copper (rGOC) nanocomposites were synthesized as newly robust biocides via in situ reduction. Inspired by the emerging method of bridging ultrafiltration membrane surface cavities, loose nanofiltration (NF) membranes were designed using a rapid (2 h) bioinspired strategy in which rGOC nanocomposites were firmly codeposited with polydopamine (PDA) onto an ultrafiltration support. A series of analyses (SEM, EDS, XRD, XPS, TEM, and AFM) confirmed the successful synthesis of the rGO-Cu nanocomposites. The secure loading of rGOC composites onto the membrane surfaces was also confirmed by SEM and AFM images. Water contact angle results display a high surface hydrophilicity of the modified membranes. The PDA-rGOC functionalization layer facilitated a high water permeability (22.8 L m-2 h-1 bar-1). The PDA-rGOC modification additionally furnished the membrane with superior separation properties advantageous for various NF applications such as dye purification or desalination, as ultrahigh (99.4% for 0.5 g L-1 reactive blue 2) dye retention and high salt permeation (7.4% for 1.0 g L-1 Na2SO4, 2.5% for 1.0 g L-1 NaCl) was achieved by the PDA-rGOC-modified membranes. Furthermore, after 3 h of contact with Escherichia coli (E. coli) bacteria, the rGOC-functionalized membranes exhibited a strong antibacterial performance with a 97.9% reduction in the number of live E. coli. This study highlights the use of rGOC composites for devising loose NF membranes with strong antibacterial and separation performance.

A rapid deposition of polydopamine coatings induced by iron (III) chloride/hydrogen peroxide for loose nanofiltration

Mussel-inspired polydopamine (PDA) coatings have received widespread concern due to the advantages of eco-friendliness, adhesion nature, and film-forming feasibility. However, self-polymerization of dopamine assisted by air-oxidation under alkaline condition is time-consuming, and the ensuing uneven PDA coatings restrict their applications. In this study, we proposed a rapid PDA deposition triggered by a facile system of iron (III) chloride/hydrogen peroxide (FeCl3/H2O2) under acidic condition. The oxygen-radical species generated by FeCl3/H2O2 largely promote covalent polymerization and deposition rate of dopamine. This not only considerably shortens the deposition time of PDA, but also improves the stability of PDA coatings, combined with the chelation of Fe ions in PDA matrices. SEM, AFM, XPS, zeta potential and water contact angle analyses confirmed the formation of a hydrophilic, smooth, and negatively charged PDA layer onto several membrane substrates. Herein, PDA-coated hydrolyzed polyacrylonitrile membranes yield a remarkable NF performance with superior dye retentions (direct red 23: 98.6%, Congo red: 99.0%, reactive blue 2: 98.2%) and a high water permeability (17.5 L m-2 h-1 bar-1). Furthermore, a low salt rejection (NaCl: 5.6%) of PDA-modified membranes demonstrates their great potential in fractionation of dye/salt mixtures. Meanwhile, the PDA-modified membranes show an excellent organic fouling resistance and a long-term stability. This facile, environmental-friendly method provides a rapid PDA deposition onto various substrates for a wide range of applications, including filtration membranes.