Loose Nanofiltration Research Articles

Self-cleaning loose nanofiltration membranes enabled by photocatalytic Cu-triazolate MOFs for dye/salt separation

Although membrane separations have evinced an enormous potential in the treatment of dye wastewater, the ubiquitous membrane fouling leads to a dramatic decline of separation efficacy and life span, impeding the progress of membrane-based applications. The copper-triazolate MOFs (CuTz-1) exhibit robust thermal and chemical stability with a unique feature of degrading organic molecules under the visible light, rendering them highly promising for construction of self-cleaning membrane. This study presents the utilization of photocatalytic MOFs (CuTz-1) for construction of the CuTz-1/graphene oxide (CuTz-1/GO) composite membranes via filtration-assisted assembly followed by chemical crosslinking. The presence of CuTz-1 among the GO nanosheets promotes an enhancement of the GO interlayer distance, thereby promoting a higher water flux without compromising the dye rejections. The best-performing CuTz-1/GO membranes exhibits the high water permeance of 40.2 L m−2 h−1 bar−1 and superior dye rejections (Congo red: 99.4%, direct red: 98.2%, methyl blue: 94.9%), low salt rejections (e.g., NaCl: 0.3% and Na2SO4: 19.6%), as well as long-term nanofiltration stability. Furthermore, after visible light irradiation, the treated membrane was nearly recovered to have the original separation efficacy due to effective photocatalytic removal of the attached dyes from the membrane surface. The CuTz-1/GO membrane also exhibits an ultra-high bacteriostasis rate of 100%, probably to be ascribed to the combined antimicrobial effect of CuTz-1 and GO. This work is expected to give a useful guideline for developing loose nanofiltration membranes with favorable photocatalytic and antimicrobial properties, offering an appropriate solution of using self-cleaning membranes for the treatment of dye wastewater.

Electrospray interfacial polymerization for a loose NF membrane: super-selective dye separation in saline dye wastewater treatment

A loose nanofiltration membrane fabricated by electrospray interfacial polymerization which contained a tiny amount of sulfonated graphene oxide nanoparticles showed excellent water permeance and salt/dye separation.

Scalable Dual-Layer PVDF Loose Nanofiltration Hollow Fiber Membranes for Treating Textile Wastewater

Scalable Dual-Layer PVDF Loose Nanofiltration Hollow Fiber Membranes for Treating Textile Wastewater

High-Throughput Zwitterion-Modified MoS2 Membranes: Preparation and Application in Dye Desalination.

Although loose nanofiltration membranes have been extensively studied for dye desalination, high-throughput membranes with antifouling and antibacterial properties are still highly needed. In this study, a zwitterion-modified molybdenum disulfide (MoS2) dual-layer loose nanofiltration membrane was prepared with the integration of antibacterial, antifouling, and high-flux properties. To be specific, MoS2 nanosheets were loaded on a polyacrylonitrile ultrafiltration membrane through pressure-assisted self-assembly. Then, poly (sulfobetaine methacrylate) (PSBMA) was coated on the surface of the MoS2 membrane via a simple polydopamine (PDA)-assisted one-step codeposition to prepare PSBMA/PDA/MoS2 nanofiltration membranes. Elemental and morphological analyses confirmed the formation of the MoS2 layer and PSBMA/PDA coating. In addition, the effect of the PSBMA amount and codeposition time on surface properties and membrane performances was investigated. Under optimum conditions, the as-prepared membrane showed excellent water permeance of 262 LMH/bar with good dye rejection (99.8% for methylene blue) and salt permeability, as well as excellent antifouling and antibacterial properties benefiting from the synergy of PSBMA/PDA coating layers and MoS2 layers.

Positively charged poly (N-vinyl imidazole) gel-filled loose nanofiltration membranes: Performances and modelling analysis

Loose nanofiltration (NF) membrane with pore radius approaching 1 nm and positive charges can be valuable alternative to commercial available negatively charged polyamide NF membrane in specific applications, such as removal of hardness and heavy metal ions and selective separation of dyes/salts mixture. In this work, positively charged poly(N-vinyl imidazole) (PVI) gel-filled loose NF membranes were developed by asymmetric gel-filling upon an asymmetric ultrafiltration (UF) support via UV-initiated surface grafting polymerization and post-quaternization. Compared to the polyamide NF membranes, these membranes displayed a comparable salt rejection ability due to their stronger Donnan exclusion effect. A rigorous three-parameter transport model (TPM) considering steric, Donnan and dielectric exclusion on ions partitioning via effective pore radius (rp), membrane charge density (Xd) and averaged pore dielectric constant (εp), was used to analyze the membrane parameters. Then the gel conformation model (GM) treating confined gel as semi-dilute polymer solution with a characteristic correlation length (ξ) was applied to analyze these gel composites. The results suggest that the gel polymer volume fraction (φ) is decisive in the gel mesh size (viewed as membrane pore size) and gel permeability. Surface porosity of the asymmetric UF base is a limiting factor in membrane permeability. The joint modelling analysis can clarify the membrane structure-properties relationship, performance limits and potentials of the gel-filled NF membrane and hence facilitate structure design and optimization.

Compactness-tailored hollow fiber loose nanofiltration separation layers based on “chemical crosslinking and metal ion coordination” for selective dye separation

In this work, three types of hollow fiber loose nanofiltration membranes (LNFs) with different compactness in an “outside-in” mode were prepared via “chemical crosslinking and metal ion coordination”. The formation and evolution in the microstructural regularity of three types of separation layers (G-PEI, M-PEI and C-PEI) are expounded. The compactness of a separation layer can be effectively tailored by combining chemical cross-linking with metal ion coordination on the membrane surface (achieving mean pore sizes as small as 1.2 nm). The three as-prepared membranes can effectively reject dyes with different molecular weights, 99.5% acid red 27 for G-PEI, 99.3% sunset yellow for M-PEI and 96.1% crystal violet for C-PEI maintaining high flux (41.8, 31.2, and 28.7 L m–2 h–1 bar–1, respectively). The size sieving decolorization mechanism of the membranes was demonstrated from the relationship between the pore sizes of the LNFs and the “hydration, aggregation and absorption” process of dye molecules. In addition, the as-prepared membrane displayed excellent anti-bacterial performance and good stability in the long-term separation of dye/salt system. This work provides a new idea in fabricating hollow fiber LNFs with the separation layer with adjustable microstructure.

Fabrication of highly permeable PVDF loose nanofiltration composite membranes for the effective separation of dye/salt mixtures

The separation of dyes and salts is very challenging in regards to treating high salinity textile wastewater. To effectively dispose dye/salt mixtures, it has become urgent to develop highly permeable and antifouling nanofiltration membranes. Herein, a novel loose nanofiltration membrane was fabricated by combining blending with interfacial polymerisation. Polyvinylidene fluoride (PVDF)/styrene-maleic anhydride (SMA) copolymer membranes with uniform microstructures were achieved via phase inversion by controlling the miscibility. Numerous carboxyl groups were gathered on the surface due to the amphiphilic SMA segregation during the membrane formation. Carboxylated carbon nanotubes (CNT-COOH) were used to further tune the membrane microstructures and promote the carboxyl groups segregation onto the surface. Ultimately, polyethylenimine (PEI) and triethanolamine (TEA) were co-deposited on the membrane surface by the amidation reaction and hydrogen bonding to form a loose structure. The optimized membrane exhibited high dye rejections to Congo red (99.9%), methyl blue (96.7%), acid fuchsin (96.1%), and low salt rejections to NaCl (1.7%), MgCl2 (4.5%), MgSO4 (1.2%). Significantly, it possessed a permeate flux of up to 251.6 L/m2·h·bar during the dye/salt mixtures filtration. Moreover, the optimized membrane also performed excellent antifouling properties toward proteins and dyes. Overall, this work provides a promising loose nanofiltration membrane for textile wastewater treatment.

Developing helical carbon functionalized chitosan-based loose nanofiltration membranes for selective separation and wastewater treatment

• Alternative TFC membrane using unique multi-functional helical carbon. • Study achieves precise molecular separation of >90%/<10% rejection with 40–60 L.m −2 .h −1 flux. • Separation performance for series of real wastewater resulted in flux of 43.12 L.m −2 .h −1 at 3 bar operating pressure. • Membrane interfacial stability was determined to be stable even after long test run of >100 h. This study reports an effective method for preparing oxygenated helical carbon impregnated chitosan-based loose nanofiltration membrane, cross-linked with sodium dodecyl sulfate (SDS) for selective solute permeation and various industrial wastewater treatment. The structural and functional properties of as prepared thin-film composite (TFC) membranes were thoroughly characterized using ATR-IR, FE-SEM, contact angle, and zeta potential measurements. Performance testing using various feed streams under different testing conditions revealed that the optimized membrane showed >90.0% rejection for dyes (both cationic and anionic) and humic acid, and <8.0% rejection for MgSO 4 with 40.0–60.0 L.m −2 .h −1 flux at 3 bar operating pressure. The study was extended using four different real industrial wastewaters as feed solutions. In all cases, the rejection values for salts resembled with that of simulated wastewater with significant removal of organic pollutants (reduced COD, BOD and TOC). Thereafter, long term (>100 h) studies were conducted using simulated wastewater spiked with multiple foulants as feed constituents and the results showed >90.0% rejection for organic matters and foulants with a long-term stable flux profile. The membranes were stable even after repeated operating cycles without any significant reduction in flux or rejection, demonstrating their potential to be used as an effective pre-treatment media for various industrial wastewaters.

Fine-tuning the architecture of loose nanofiltration membrane for improved water flux, dye rejection and dye/salt selective separation

Herein, loose nanofiltration membranes were prepared using non-solvent induced phase separation (NIPS) method. In NIPS, the non-solvent controls the morphology of the skin layer of the asymmetric membranes formed, while the solvent tunes the morphology of the lower layer. The membranes were made from polyethersulfone in n-methyl pyrrolidone (NMP) with the addition of dicarboxylic acids (glutaric acid, azelaic acid). Aliphatic alcohols (1-butanol, 1-octanol) were added to the solvent to tailor the skin layer pore morphology. Cross-sectional electron microscope images revealed the acids created nano-voids and shorter micro-voids. The addition of alcohols extended the micro-voids and improved symmetry. Pore size distribution results showed that the addition of the alcohols did indeed reduce the pore size as hypothesised. The performance of the modified polyethersulfone membranes was greatly improved in terms of pure water flux and salt passage without sacrificing the dye rejection. The loose nanofiltration membranes prepared in this study showed good dye/salt separation capability compared to most of the results reported in literature. The membranes were compacted with de-ionised water at an applied pressure of 4 bar for 5 h and rejections tests were done at 2 bar. It is hereby reported that the membranes achieved a high salt passage (Na2SO4 rejection varying between <3% and ~12%), as well as sustainable high Congo red rejection of 97%.

Bioinspired modification of molybdenum disulfide nanosheets to prepare a loose nanofiltration membrane for wastewater treatment

Loose nanofiltration membrane (NFM) is suitable for wastewater treatment especially for dye solution desalination due to its high rejection of dyes and high permeance of salts. In this study, MoS2@PDA was prepared by modifying molybdenum disulfide (MoS2) nanosheets with polydopamine (PDA). Then, the loose nanofiltration membrane was prepared by the deposition of MoS2@PDA on hydrolyzed polyacrylonitrile (HPAN) ultrafiltration membrane with pressure-assisted self-assembly. The morphology and properties of MoS2@PDA and MoS2@PDA NFMs were analyzed by SEM, TEM, TGA, FT-IR, AFM, WCA, XRD, and ζ-potentials. The PDA nanoaggregates on the surface of MoS2 nanosheets interfered with the regular stacking of MoS2 nanosheets parallel to the membrane surface, so the resultant membranes showed irregular loose structure which could provide transmembrane channels along with the impermeable nanosheet surface for molecule diffusion. When the MoS2 loading was 0.116 mg/cm2 and the PDA modification time was 5 h, the pure water flux of MoS2@PDA NFM could reach 204.05 LMH/bar, the rejection of methylene blue was 99 %, and rejection of salts (NaCl, Na2SO4, MgCl2, MgSO4) were less than 12 %. And MoS2@PDA NFM exhibited satisfactory long-term operational stability. This study provides a new strategy for the design of loose nanofiltration membrane with high performance.

Using loose nanofiltration membrane for lake water treatment: A pilot study

Nanofiltration (NF) using loose membranes has a high application potential for advanced treatment of drinking water by selectively removing contaminants from the water, while membrane fouling remains one of the biggest problems of the process. This paper reported a seven-month pilot study of using a loose NF membrane to treat a sand filtration effluent which had a relatively high turbidity (∼0.4 NTU) and high concentrations of organic matter (up to 5 mg/L as TOC), hardness and sulfate. Results showed that the membrane demonstrated a high rejection of TOC (by<90%) and a moderately high rejection of two pesticides (54%–82%) while a moderate rejection of both calcium and magnesium (∼45%) and a low rejection of total dissolved solids (∼27%). The membrane elements suffered from severe membrane fouling, with the membrane permeance decreased by 70% after 85 days operation. The membrane fouling was dominated by organic fouling, while biological fouling was moderate. Inorganic fouling was mainly caused by deposition of aluminum-bearing substances. Though inorganic foulants were minor contents on membrane, their contribution to overall membrane fouling was substantial. Membrane fouling was not uniform on membrane. While contents of organic and inorganic foulants were the highest at the inlet and outlet region, respectively, the severity of membrane fouling increased from the inlet to the outlet region of membrane element with a difference higher than 30%. While alkaline cleaning was not effective in removing the membrane foulants, the use of ethylenediamine tetraacetate (EDTA) at alkaline conditions could effectively restore the membrane permeance.

High-Performance Polyacrylic Acid-Grafted PVDF Nanofiltration Membrane with Good Antifouling Property for the Textile Industry.

In the textile industry, a high-efficiency dye removal and low-retention of salt is demanded for recycling wastewater. In this study, polyvinylidene fluoride (PVDF) ultrafiltration membrane was transformed to a negatively charged loose nanofiltration (NF) membrane through UV-grafting of acrylic acid. At the optimal exposure of PVDF membrane in UV light for 5 min, the membrane had a high dye recovery above 99% (Congo red and Eriochrome® Black T) and a low sodium chloride (NaCl) rejection of less than 15% along with pure water flux of 26 L∙m−2∙h−1∙bar−1. Its antifouling and oleophobicity surface properties were verified using fluorescent- bovine serum albumin (BSA) and underwater mineral oil contact angle, respectively. According to the fluorescent microscopic images, the modified membrane had ten times lower adhesion of protein on the surface than the unmodified membrane. The underwater oil contact angle was raised from 110° to 155°. Moreover, the salt rejection followed this sequence: Na2SO4 > MgSO4 > NaCl > MgCl2, which agreed with the typical negatively charged NF membrane. In addition, the physicochemical characterization of membranes was further investigated to understand and link to the membrane performance, such as surface functional group, surface elements analysis, surface roughness/morphology, and surface hydrophilicity.

Low-pressure loose GO composite membrane intercalated by CNT for effective dye/salt separation

Low-pressure loose nanofiltration membranes were facilely fabricated by intercalating CNTs into graphene oxide (GO) laminates supported by cellulose acetate (CA) ultrafiltration substrates. The CNTs could loosen the GO laminar film and enlarge the interlayer spacing of GO laminates, thus reducing the resistance of water flux and salt permeation. Crosslinking bonds between GO layers were generated to create hydro-stable size-sieving channel necks of the tortuous pathway for selective molecular permeation. It was found that the GO composite membrane with 80 wt% of CNT content and 0.5 mg of GO loading exhibited the optimal separation performance in the application of textile wastewater treatment. It had the highest pure water permeability of 26.3 L·m−2·h−1·bar−1 and almost complete rejection to dyes (e.g. 98.7% for Congo red, 94.1% for Methyl blue) as well as the lowest rejections to salts (e.g. 3.1% for NaCl, 6.3% for Na2SO4) under a low transmembrane pressure of 1.0 bar. This work provides a feasible approach for designing GO-based membrane with improved dye/salt separation performance.

Fractionation of Dye/NaCl Mixtures Using Loose Nanofiltration Membranes Based on the Incorporation of WS2 in Self-Assembled Layer-by-Layer Polymeric Electrolytes

Salt accumulation often occurs in conventional processes for textile wastewater treatment, which affects the dye removal efficiency and water recovery rate. Nanofiltration is a feasible way to frac...

Removal mechanisms of perfluorinated compounds (PFCs) by nanofiltration: Roles of membrane-contaminant interactions

Perfluorinated compounds (PFCs)-membrane interactions play important roles in the removal of PFCs by membrane separation, especially in their adsorption onto membrane interfaces and membrane rejection. In this work, a loose nanofiltration (NF) membrane and a tight NF membrane were used to remove six PFCs that have different head groups and different C-F chains. The roles of typical PFCs-membrane interactions, including electrostatic repulsion, hydrophobic interactions and hydrogen bonding, on membrane adsorption and rejection were quantitatively evaluated. The membrane adsorption capacities to the six PFCs were determined by static adsorption experiments. Influences of the PFCs-membrane interactions mentioned above were quantified with six negatively charged PFCs by comparing the adsorption and rejection at neutral pH with those at the isoelectric points (IEPs) of the two membranes. Results showed that electrostatic repulsion caused that the rejections by the loose membrane and tight membrane increased by 2.2%–36.0%, and 0.8%–7.4%, respectively. Non-electrostatic interactions (hydrophobic interactions and hydrogen bonding) decreased rejections of the tight membrane and the loose membrane by 9.5%–23.6% and 1.2%–10.3%, respectively. Interactions between the PFCs and the membranes were further calculated by density functional theory (DFT). DFT analysis and Gibbs free energy changes suggest that PFCs could interact with membranes in thermodynamics. The head groups are away from the membrane surface because of electrostatic repulsion and the tail groups close to the membrane surface due to hydrophobic interactions and hydrogen bonding.

Erythritol-based polyester loose nanofiltration membrane with fast water transport for efficient dye/salt separation

Loose nanofiltration membranes with a remarkable water permeability are highly promising for the fractionation of dyes and salts in the treatment of textile wastewater. In this study, a novel polyester membrane with unprecedented water permeability was developed via interfacial polymerization (IP), in which meso-erythritol (ME) was utilized as an aqueous monomer. Instead of using toxic catalysts, sodium hydroxide was incorporated to the aqueous solution to catalyze the ester reaction between hydroxyl and acyl chloride. A series of characterizations demonstrated that the formed polyester film is hydrophilic and negatively charged, with microsphere structures on the surface. Due to the low-active hydroxyl groups of the aqueous monomers, the participation of ME could yield a coherent polyester film with slightly loose structure atop the polyethersulfone (PES) membrane. The resultant membrane with high water permeability exhibited both a high rejection of dyes and a high transmission of salts. The rejection of the LNFM-2 membrane with an excellent water permeability of 53.23 LMH bar−1 for Congo red (CR), direct red 23 (DR23), reactive blue 2 (RB2), Na2SO4, NaCl was 99.6%, 95.2%, 99.6%, 11.0%, and 5.6% respectively. Furthermore, LNFM-1 has a water permeability of up to 87.13 LMH bar−1 while maintaining a competitive dye/salt separation performance. In addition, this type of polyester membrane has a superior antifouling performance and long-term stability during the filtration of dye/salt mixtures.The newly developed polyester LNF membranes have great application potential in the treatment of textile wastewater. This study paves the way for applying hydroxyl monomers like polyols for constructing TFC membranes for diverse separations.

Green preparation of polyvinylidene fluoride loose nanofiltration hollow fiber membranes with multilayer structure for treating textile wastewater

In this work, polyvinylidene fluoride (PVDF) loose nanofiltration (NF) hollow fiber membranes with multilayer structure were prepared successfully based on a solvent-free process. Graphene oxide (GO) was used to cover the interface pores of the pristine PVDF membranes via vacuum filtration, and polypyrrole (PPy) was polymerized on the surface to further decorate the membrane structure. Interestingly, the modified membranes exhibited a multilayer structure due to synergistic effect of GO and PPy. The structure and property of PVDF loose NF membranes were investigated in detail. After modifying by GO and PPy, the hydrophilicity improved obviously. Moreover, the molecular weight cut off (MWCO) was about 3580 Da, and the smallest pore size of skin layer decreased to 2.5–4 nm. Furthermore, the PVDF loose NF hollow fiber membranes presented a high dye rejection (˃98.5%) for negative dyes, whereas a low salt rejection for NaCl (about 4%), showing a great potential for separating dye/salt accurately. Specifically, there were not any solvent used in all the preparation processes. The work offered a novel strategy for green preparation of loose NF membranes.

PIP/TMC Interfacial Polymerization with Electrospray: Novel Loose Nanofiltration Membrane for Dye Wastewater Treatment.

A loose nanofiltration (NF) membrane with excellent dye rejection and high permeation of inorganic salt is required to fractionate dye/salt mixture in dye wastewater treatment. In this study, we fabricated the loose NF membrane by using the electrospray interfacial polymerization (EIP) method. It is a novel and facile interfacial polymerization method, which controls the thickness of the poly(piperazine-amide) (PPA) layer in nanometers (1 nm/min) and changes cross-linking degree of PPA layer and pore size by varying the electrospray time; consequently, water permeance and dye/salt rejection ratio can be handled. The fabricated EIP membrane with an optimized fabrication condition (M30, electrospray time was 30 min) possessed excellent pure water permeance (20.2 LMH/bar), high dye rejection (e.g., 99.6% for congo red (CR)), and low salt rejection (e.g., 6.3% for NaCl). Moreover, the EIP membrane exhibited enhanced antifouling property than commercial NF membrane (NF90) with a high flux recovery rate (FRR) of 87.1% and low irreversible fouling (Rir) of 12.9% after fouled by bovine serum albumin (BSA) due to its great smooth surface (average roughness (Ra) is 12.2 nm), hydrophilicity property, enhanced zeta potential, and low protein adsorption. The results indicate that the EIP loose NF membrane had a high potential for dye wastewater treatment.

Preparation and characterization of loose antifouling nanofiltration membrane using branched aniline oligomers grafted onto polyether sulfone and application for real algal dye removal

This research presents a simple and versatile procedure (diazonium-induced grafting) for in situ surface functionalization of Polyethersulfone (PES) membrane through covalent attachment of aniline oligomers. The process involves gradual reduction of oligomer diazonium salts which yields a superhydrophilic layer of branched aniline oligomers (BAO) grafted on the membrane surface. With this procedure, the PES nanofiltration membrane, alter facilely from hydrophobicity to superhydrophilicity. The structural characteristic, nanofiltration performance, antifouling behavior and chemical, mechanical and long-term permeation stability of prepared membranes were evaluated. The modified membrane showed great performance with high water flux (93.6 L/m2 h bar), high flux recovery ratio 99% and after fouling with powdered milk protein for 90 min. The novel BAO-PES modified membrane had long-term structural stability in an acidic and basic environment that it’s a critical property for real operation in industry. Its application in algal dye removal was also assessed using a real algal wastewater from a working stabilization pond in a cross-flow filtration system for a 62-h period. The modified membrane showed a smooth behavior in flux with only 11.45% reduction after 62 h, indicating outstanding stable permeability and anti-fouling function provided by the applied modification. This modification demonstrates great potential for convenient practical application.

Bio-inspired co-deposited preparation of GO composite loose nanofiltration membrane for dye contaminated wastewater sustainable treatment

The fully separation of dye/salt through loose nanofiltration membranes is of great significance for the sustainable development paradigm of textile wastewater. However, the current loose nanofiltration membranes suffer low separation efficiency and complex preparation. Herein, by one-step co-deposition, we develop graphene oxide (GO) composite loose nanofiltration membranes with low negatively charged surface. Our membrane possesses unconventional high pure water permeation of 71.7 LMH/bar, 92.9 % rejection for Methyl blue (MB) and 98.8 % rejection for Congo red (CR). Benefiting from the large interlayer distance of GO nanosheets and low negatively charged surface, membrane achieves high dyes/salts separation with satisfactory permeation to salts (94.3 % of Na2SO4, 97.6 % of MgSO4, 98.3 % of MgCl2 and 99.0 % of NaCl). The CR/salt mixed solutions exhibit similar removal rates to their constituents’ single dye or salt solutions (CR rejection is up to more than 97 % and the permeations of all salts are above 93 %). At the same time, binary dyes mixtures (Congo red and Methyl orange) can also be effectively separated. Furthermore, the membrane shows a relatively desirable antifouling property. The flux recovery still remains at 85.9 % after three cycling filtrations. This study provides a facile approach to prepare highly-efficient loose nanofiltration membranes for wastewater sustainable remediation.