Shushan Yuan

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.

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.

Super-hydrophobic 3D printed polysulfone membranes with a switchable wettability by self-assembled candle soot for efficient gravity-driven oil/water separation

The development of super-hydrophobic surfaces for water/oil separation has attracted much interest in fundamental research and industrial applications in recent years. This article proposes a facile method to fabricate super-hydrophobic surfaces on 3D printed polysulfone (PSU) membranes via the coating of candle soot. A 3D printed PSU membrane fabricated by selective laser sintering was applied for the oil/water separation and showed a different wettability on its top surface and bottom surface. The self-assembled candle soot loose network endowed the 3D printed PSU membrane with super-hydrophobicity with a water contact angle of 161° and a sliding angle of 5°, preserving an outstanding mechanical stability under sonication and chemical stability in 1 M HCl, 1 M NaOH, 1 M NaCl and hot water. Interestingly, this super-hydrophobic surface could dramatically switch to a super-oleophobic state after being prewetted by water. Ten cycles of switchable oil/water separation were performed, demonstrating a high oil/water separation stability of the dry candle soot coated PSU membrane and the water prewetted candle soot coated PSU membrane. Additionally, the effect of selective laser sintering processing parameters on the structure and performance and the influence of the immersion time on the deposition of candle soot were investigated. Overall, this study provides an efficient, simple and reliable fabrication method for super-hydrophobic surfaces with switchable wettability.

High flux organic solvent nanofiltration membrane from Kevlar aramid nanofibers with in-situ incorporation of microspheres

Membranes with unparalleled solvent permeance and excellent selectivity are needed to reduce the energy input by molecular separations in organic liquids. A novel organic solvent nanofiltration (OSN) membrane with high flux was prepared from aramid nanofibers (ANFs) with in situ incorporation of microspheres. The ANF was dissociated from Kevlar aramid fibers in dimethyl sulfoxide (DMSO)/potassium hydroxide (KOH) solution and interacted with polyethyleneimine (PEI) through non-covalent interactions, forming ANF/PEI microparticles. The ANF/PEI OSN membrane was successfully fabricated by non-solvent induced phase separation and post-treatment. Dead-end filtration experiments showed that the permeance of the ANF/PEI membrane increased by a factor of over four (6.0 ± 0.5 L m−2 h−1 bar−1vs. 1.4 ± 0.1 m−2 h−1 bar−1). Meanwhile, the unique polymeric layered nano/micro structure of the ANF/PEI thin film membrane contributed to the ultrafast organic solvent permeance for tetrahydrofuran (THF) (20.5 m−2 h−1 bar−1) and acetone (11.2 m−2 h−1 bar−1). Finally, the ANF/PEI membrane significantly promotes ethanol permeance, without compromising its high selectivity when it is treated with methanol. The highest permeability was 9.1 L m−2 h−1 bar−1 with an RB rejection of 98.4% after 6 d of methanol treatment. The ANF/PEI OSN membrane showed a good stability in a long-term experiment. This facile construction of nanofiber membranes with in situ modification provides a new direction for high performance OSN membranes.