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Highlights • The fouling behavior of a loose NF membrane was investigated. • The loose NF membrane has an excellent antifouling property due to its strong hydrophilicity. • The charge reduction of membrane surface and humic acid mainly induces the membrane fouling. • Benign hydrodynamic condition can substantially alleviate the fouling of loose NF membrane. Abstract Loose nanofiltration (NF) membranes are an attractive avenue in effective separation of organic matters and salts for resource recovery from highly-loaded wastewater. However, membrane fouling remains an unclear and complex factor in practical applications. In this work, the flux of a loose NF membrane fouled by humic acid at various solution compositions was systematically investigated. The strong hydrophilicity of the loose NF membrane allows for slight deposition of humic acid on the membrane surface, yielding an outstanding antifouling performance. However, a moderate flux decline was observed at low pH and high ionic strength, due to reduction in charge density of membrane surface for formation of a porous foulant layer. At higher ionic strength, cake-enhanced concentration polarization was the fouling mechanism that dominates the membrane flux. The presence of calcium ions induced bridging between humic acid molecules to generate a compact foulant layer, tremendously deteriorating the membrane flux. Based on COMSOL simulation for the membrane module, the hydrodynamics near the membrane surface had a more significant effect on membrane fouling than the solution chemistry, which is consistent with scanning electronic microscopy observation. This indicates benign hydrodynamic condition can be an effective strategy to fouling control for loose NF membranes. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

In digital printing, a high salt content in dye solutions is detrimental, which calls for effective strategy for dye/salt fractionation. In this study, a loose nanofiltration (NF) membrane Sepro 2A was employed to desalinate diverse dye species (2 reactive dyes and 4 cationic dyes). This membrane, with a molecular weight cutoff of 490 Da, showed consistently high rejections to all the reactive and cationic dyes (>97.7%). Operational conditions such as dye concentrations and applied pressures have limited effects on the rejection of the membrane. An integrated NF-diafiltration process, involving a pre-concentration and a diafiltration step, was specifically designed for the fractionation of dye/NaCl mixtures. This loose NF membrane showed >99.7% rejection to reactive dyes (reactive blue 2 and reactive orange 16) and ca. 99.3% salt removal with ca. 2.0% dye loss after 5.0 diavolumes. However, the NF membrane experienced a 15.9% loss to methylene blue due to the lower rejection to this model cationic dye (97.2%). The current study provides important insights into dye/salt fractionation by loose NF membranes for digital ink-jet printing. [ABSTRACT FROM AUTHOR]

Nanofiltration (NF) membranes have been used previously for the recovery of dyes, salts, and water from textile wastewaters with high salinity. However, commercially available NF membranes have a high rejection for divalent salts (i.e., Na 2 SO 4 ), substantially reducing the salt recovery and membrane flux when treating textile wastewater containing Na 2 SO 4 . In this study, a tight ultrafiltration membrane (UH004, Microdyn-Nadir) was proposed to fractionate the dye and Na 2 SO 4 in the textile wastewater. The UH004 membrane with a molecular weight cutoff of 4700 Da provided complete passage of monovalent salts, with little rejection of Na 2 SO 4 . This significantly increases the filtrate flux that can be achieved with high-salinity wastewater since osmotic pressure and concentration polarization effects are minimized. Furthermore, the retention behavior of four different dyes was evaluated to determine the efficiency of this membrane process. This tight ultrafiltration membrane offered the high retention for direct dyes (i.e., direct red 80, direct red 23, and Congo red) and reactive blue 2. For instance, the UH004 membrane yielded >98.9% rejection for all of the dyes at a pressure of 4 bar even in the presence of 60 g L −1 Na 2 SO 4 . Subsequently, an ultrafiltration-diafiltration process was designed to separate a dye/Na 2 SO 4 aqueous mixture with 98% desalination efficiency and greater than 97% dye recovery after 5 diavolumes. These results clearly demonstrate that tight ultrafiltration membranes can be a stand-alone alternative to NF membranes for the effective fractionation of dye and Na 2 SO 4 in the direct treatment of high-salinity textile wastewater. [ABSTRACT FROM AUTHOR]

Loose nanofiltration (NF) membranes can be used to treat textile wastewater effectively, providing an attracted avenue for resource recovery ( i.e., dye purification and salt reuse) at the premise of the integration of high dye retention and salt permeation. However, the issue of membrane fouling has to be adequately addressed in view of its practical application. In this study, superhydrophilic loose NF membranes (Sepro NF 6 and NF 2A, Ultura) were applied for textile wastewater treatment. Synthetic solutions containing dyes and NaCl were used as the feed stream of a NF unit. It was found that two factors, namely cake-enhanced concentration polarization and the formation of a dye cake layer, dramatically deteriorated the flux of NF membranes with a synergic effect. In viewpoint of realistic application, diafiltration of a binary dye/salt mixture indicates that cake-enhanced concentration polarization plays a dominant role for the low membrane flux. As the diafiltration continued, cake-enhanced concentration polarization was alleviated with a decreasing concentration of salt in the feed. At the subsequent post-concentration procedure, the formation of a dye cake layer slightly compromised the membrane flux, but the negative impact of cake-enhanced concentration polarization was negligible due to the small quantity of salt remained in the feed. [ABSTRACT FROM AUTHOR]

In the textile industry, high salinity waste streams are a challenge urging for the recovery and purification of dyes and salts (e.g., NaCl), requiring a treatment going beyond the classical filtration by e.g., reverse osmosis to produce pure water. In this work, two commercial loose nanofiltration (NF) membranes (Sepro NF 6 and NF 2A, Ultura) are proposed to fractionate dye/salt aqueous mixtures. It was observed that both NF membranes have a salt rejection <33.3% in solutions with 0.1–40.0 g L −1 of NaCl at 6 bar. Furthermore, both membranes have >99.6% retention of direct dyes (direct red 80, direct red 23, and congo red), even though 40.0 g L −1 NaCl is present, indicating salt addition has no obvious impact on the dye retention. The combination of a low salt rejection and a high dye rejection indicates the feasibility for the reuse of salt from fractionation in forward osmosis and bipolar membrane electrodialysis. Application of diafiltration for an aqueous mixture containing direct red 80 (1000 ppm) and NaCl (~20 g L −1 ) by both membranes demonstrates that above 95% of NaCl is removed from aqueous mixture, and <0.9 g L −1 NaCl remains after the addition of pure water with a volume factor of 4.0 in the feed solution. At the premise of excellent diafiltration performance, concentration as the post-treatment for dye recovery expectedly indicates direct red 80 is concentrated by a factor of 4.0 for both membranes while keeping the salt concentration with a very slight increase. Over 99.9% dye retention in both diafiltration and concentration procedures yields a very high recovery since <0.045% of dye is permeated. These results indicate that loose nanofiltration membranes have potential for dye/salt fractionation. [ABSTRACT FROM AUTHOR]

• Metal-organic coordination complex coating effectively tailors surface property of nanofiltration membrane. • The nanofiltration membrane yields a molecular weight cutoff of 297 Da after 15-s coating. • The coated nanofiltration membrane shows impressive fractionation of humic substances and salts. • Nanofiltration-diafiltration can enrich and purify humic substances from landfill leachate concentrate. Sustainable treatment of the highly saline landfill leachate concentrate for application as green fertilizer calls for effective fractionation of the existing humic substances and inorganic salts; advanced selective nanofiltration membranes are proposed for this. One-step, rapid assembly of a tannic acid-Fe3+ coordination complex is a promising strategy to endow the membranes with an enhanced nanofiltration performance. In this study, a robust and homogeneous tannic acid-Fe3+ coordination complex layer was effectively coated onto the surface of a loose nanofiltration substrate in an extremely short time (15 s). After the coating of the tannic acid-Fe3+ coordination complex layer, the nanofiltration membrane showed a significantly reduced molecule weight cutoff (i.e., reduction from 601 to 279 Da) and thus enhanced selectivity towards humic substances. Specifically, the rejection to humic substances of the coated nanofiltration membrane increased from 95.31±0.54% to 99.32±0.18% with negligible rise in salt rejection, demonstrating an enhanced fractionation efficacy for humic substances and salts. Assisted by a diafiltration operation with the coated nanofiltration membrane, humic substances in the landfill leachate concentrate were effectively purified and extracted with 96.60% recovery. Particularly, the humic substances were linearly enriched by ca. 7.8 folds (i.e., from 1837 to 13970 mg·L−1) with a purity of 98.91% for potential application as liquid fertilizer. The one-step rapid tannic acid-Fe3+ coordination complex coating exhibits an impressive efficacy to engineer advanced nanofiltration membranes that could be applied at a large scale for sustainable resource extraction from landfill leachate concentrate. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract Nanofiltration (NF) has attracted increasing interest for dye and NaCl fractionation in textile industry. However, it typically shows high rejections to divalent salts (i.e., Na 2 SO 4), compromising to effectively purify dye from dye/Na 2 SO 4 mixtures, which is crucial in digital ink-jet printing. This study used a tight ceramic ultrafiltration (UF) membrane (MWCO of 2410 Da) to desalinate diverse reactive dye species with molecular weights from 626.6 to 1205.4 Da. The UF membrane showed considerably high rejections to all the reactive dyes (> 98.12%) and nearly complete salt transmission (rejections < 0.1% and < 1.5% with 60 g L−1 NaCl and Na 2 SO 4 , respectively). Operational conditions, i.e., dye concentration, applied pressure and salt addition, have insignificant impacts on the dye rejection, due to dye aggregation. An integrated UF-diafiltration process, including pre-concentration, diafiltration and post-concentration, was designed to fractionate reactive dyes and Na 2 SO 4. The model dye, reactive blue 19, can be concentrated to 142.60 g L−1 with a purity of 99.83% in the process, since low salt rejection and high dye retention of the membrane allows for 99.88% salt removal and 2.3% dye loss after ca. 9 diavolumes. These results indicate that tight ceramic UF membranes have high potential as a state-of-the-art technology in reactive dyes and Na 2 SO 4 fractionation for digital ink-jet printing applications. Graphical abstract fx1 Highlights • Tight UF membrane is used in efficient dye purification for digital printing. • Tight UF membrane has consistently high rejection (> 98.12%) for 7 reactive dyes. • Integrated UF-diafiltration process removes 99.88% of Na 2 SO 4 with ~9 diavolumes. • Reactive blue 19 is concentrated to 142.60 g L−1 with 99.83% purity in diafiltration. • Complete flux recovery for UF membrane is obtained after NaOH/HCl cleaning. [ABSTRACT FROM AUTHOR]

This work is focused on the application of a novel antibacterial nanofiltration (NF) membrane formed by interfacial polymerization on a polyethersulfone supporting membrane for dye removal. Lysozyme, a ubiquitous and cheap enzyme with antibacterial activity was deployed as an aqueous monomer to react with 1,3,5-benzenetricarbonyl trichloride (TMC) to construct separation membranes. The formation of an ultrathin lysozyme-polymer active layer was verified by ATR-FTIR and FESEM. The degree of crosslinking, controlled by the concentration of lysozyme and TMC, had a significant influence on the physicochemical properties of the resultant membrane, as well as its separation performance. The optimum membranes show a high water flux (58.04 L m −2 h −1 ), distinguished rejection for low molecular weight reactive dyes (600–800 Da, >98.0%) and a high permeation of salts (>95%). Furthermore, the membranes with higher lysozyme concentration exhibited a sufficient antibacterial activity (81.9%) for E. coli bacteria. This facile strategy of enzyme immobilization not only allows for an in-situ preparation of enzyme-polymer membranes, but also maintains the native enzyme activity despite the high degree of covalent bonding between proteins. The resultant lysozyme membranes prove their potential in dyes removal, while also corroborating the value of interfacial polymerization in the field of enzyme immobilization and protein-polymer film construction. [ABSTRACT FROM AUTHOR]

The hybrid membrane bioreactor - nanofiltration treatment process can be an effective approach for treating the landfill leachate, but the residual leachate concentrate highly loaded with the humic substance and salts remains an environmental concern. Herein, a loose nanofiltration membrane (molecular weight cut-off of 860 Da) was used to recover the humic substance, which can act as a key component of organic fertilizer, from the leachate concentrate. The loose nanofiltration membrane showed the high permeation fluxes and high transmissions (>94.7%) for most inorganic ions (i.e., Na+, K+, Cl−, and NO 3 −), while retaining 95.7 ± 0.3% of the humic substance, demonstrating its great potential in effective fractionation of humic substance from inorganic salts in the leachate concentrate. The operation conditions, i.e., cross-flow rates and temperatures, had more pronounced impacts on the filtration performance of the loose nanofiltration membrane. Increasing cross-flow rates from 60 to 260 L h−1 resulted in an improvement of ca. 7.3% in the humic substance rejection, mainly due to the reduced concentration polarization effect. In contrast, the solute rejection of the nanofiltration membrane was negatively dependent on the temperature. The rejection of humic substance decreased from 96.3 ± 0.3% to 92.0 ± 0.4% with increasing the temperature from 23 to 35 °C, likely due to the enlargement of the membrane pore size and enhancement in solute diffusivity. The humic substance was enriched from 1735 to 15,287 mg L−1, yielding a 91.2% recovery ratio with 85.7% desalination efficiency at a concentration factor of 9.6. The recovered HS had significantly stimulated the seed germination and growth of the green mungbean plants with no obvious phytotoxicity. These results demonstrate that loose nanofiltration can be an effective promising technology to recover the humic substance as a valuable fertilizer component towards sustainable management of the landfill leachate concentrate. Image 1 • Loose nanofiltration (NF) membrane was used for sustainable treatment of leachate concentrate. • Loose NF can effectively separate humic substance (HS) and salts in leachate concentrate. • NF concentration process can significantly enrich and harvest HS with 85.7% salt removal. • The recovered HS stimulated the plant growth, presenting a potential as water-soluble fertilizer. [ABSTRACT FROM AUTHOR]

Membrane bioreactor (MBR) and nanofiltration (NF) hybrid process has dominated in the treatment of leachate, however, NF concentrate generated from the process remains a challenge to be addressed. Given the high content of humic substances (HS) in NF concentrate, a promising approach by using a two-stage tight ultrafiltration (TUF) process for recovering HS as a water-soluble fertilizer was proposed. The effect of operating factors on membrane flux and rejection of organic matters, and the concentration and desalination performance of this TUF process were investigated on pilot scale. The pressure and temperature posed significant influence on the flux, while slight impact on the rejection of organics, and pH had negligible effect on the flux and the retention of organics. During the concentration and desalting process, the chemical oxygen demand (COD) retention increased slightly from 91% to 94%, and the rejections of divalent and monovalent ions had the respective range of 21–62% and 10–26%, except chloride ion with a negative rejection. The results demonstrated that HS dominated in organics could be effectively separated and refined from inorganic salts. The end concentrate achieved with the two-stage TUF process can meet the requirements of Chinese nation professional standards for water-soluble fertilizers containing HS. This study illustrates the feasibility of recycling HS from NF concentrate. [ABSTRACT FROM AUTHOR]

• 2D MXene nanosheets were supplied as nanofillers on membranes surface. • The d-spacing of MXene were turned by PEI molecular chains. • Traditional IP was used to hold the MXene nanosheets with enhanced stability. • The membranes exhibit excellent separation performance for dye/salt mixtures. Thin film nanocomposite (TFN) membrane, which consists of nanofiller embedded polyamide layer and a porous substrate, have been deemed as a promising candidate to achieve multifunctionality, property enhancement and controllable separation process. Being different from traditional interfacial polymerization synthesis process, this work attempts to use MXene nanosheets as nanofillers on membranes surface while polyamide constructed by interfacial polymerization is served as intercalate reagent and stabilizer. The positively charged polyethyleneimine (PEI) and negatively charged MXene nanosheets guaranteed a favorable compatibility, which finally endowed the obtained membranes with high rejection toward different dyes. The optimal membrane decorated by 0.1 mg PEI and 0.15 mg MXene possesses a pure water permeability of 20.9 LMH bar−1 and dyes rejection of 99.42%, 99.02% and 98.84% for Congo red, Reactive blue 19 and Methyl blue, respectively. Meantime, the membrane exhibited impressive rejections towards Na 2 SO 4 and NaCl (65.7% and 23.9%). The enhanced performance of the MXene-based membrane is mainly due to the amplified interlayer distance and the appropriate crosslinking degree, which increases the water transport and retain the rejection simultaneously. Overall, this work suggests that the employment of MXene nanosheets for the construction of high-performance nanofiltration membranes can be a successful choice to treat dye wastewater. [ABSTRACT FROM AUTHOR]

Thin film nanocomposite (TFN) polyamide (PA) membranes has drawn increasing attention due to the improved physicochemical properties of the top layer for versatile separation performance. However, the commonly used fabrication approach, lacking a controllable loading of nanofillers, leads to a random and insufficient distribution of nanofillers into the PA layer and generates interfacial defects that deteriorate the membrane performance. Herein, a rational strategy for incorporating metal-organic frameworks (i.e., ZIF-8) into the PA layer via electrophoretic deposition (EPD) was proposed. Driven by an external direct current field, the ZIF-8 nanoparticles were uniformly assembled onto a porous ultrafiltration (UF) substrate for fabrication of high-performance TFN nanofiltration membranes by vacuum-assisted interfacial polymerization (IP). The resultant TFN nanofiltration membrane yielded an enhanced water permeability of 22.4 ± 1.2 L·m−2·h−1·bar−1, due to incorporation of ZIF-8 nanoparticles which created extra nano-channels for fast water permeation. Additionally, such an EPD process effectively tailored the membrane surface properties (e.g., zeta potential) and conferred a considerably high rejection for Na 2 SO 4 (96.9 ± 0.7%) but a low rejection for NaCl (18.9 ± 2.5%), demonstrating an excellent selectivity between SO 4 2− and Cl− ions. This study highlights the impressive efficacy of the EPD process in precisely anchoring nanomaterials to design high-performance TFN nanofiltration membranes for target separations. [Display omitted] • Electrophoretic deposition (EPD) is used to assemble ZIF-8 into polyamide membrane. • Uniform and sufficient assembly of ZIF-8 is achieved by precisely controlling the ageing and EPD times. • Uniform ZIF-8 assembly into the polyamide layer creates extra nano-channels for enhanced flux. • ZIF-8 assembly by EPD tailors the surface charge of TFN membranes, yielding an enhanced selectivity between SO 4 2− and Cl−. [ABSTRACT FROM AUTHOR]

• Rejection of smaller solutes more sensitive to pH and NaCl concentration. • Increasing pH from 3 to 7 gave 30% higher rejection of neutral boric acid. • Increasing pH from 7 to 11 led to 15% higher rejection of charged acetate. • NaCl decreased rejection of charged borate by 15% and acetate by 50%. • Permeation of solutes smaller than membrane pore differed from similar-sized ones. Nanofiltration (NF) is promising for replacing reverse osmosis for boron removal more effectively and at a higher energy efficiency. However, the interplay of steric and charge effects underlying rejection behaviors of solutes smaller than the membrane pore size remained incompletely understood, which impedes implementation. This study investigated the influence of pH and NaCl concentration on boric acid/borate rejection by the Desal 5DK NF membrane, with acetic acid/acetate (comparable size and charge nature) and glucose (larger and neutral) serving as benchmarks. Filtration experiments demonstrate that varying pH in the range of 3–11 and NaCl concentration up to 100 mM had little effect on glucose rejection (consistently greater than 95%), but affected the two smaller solutes significantly. Increasing pH from 7 to 11 led to 15% higher rejection of charged acetate because of the increased charges of both the membrane and solute, while increasing pH from 3 to 7 gave 30% higher rejection of neutral boric acid due to steric effects stemming from pore-size reduction. The rejections of the neutral boric acid and acetic acid were negligibly affected by NaCl concentration, but rejection decreased by 15% and 50% for the charged borate and acetate, respectively, due to salt-induced pore-swelling. [ABSTRACT FROM AUTHOR]

• Mussel-inspired co-deposition was used to effectively tailor the surface property of NF membrane. • The modified NF membrane yielded a molecular weight cut-off of 305 Da after 90-min co-deposition. • The modified NF membrane showed enhanced fractionation of humic substances (HS) and salts. • NF-based diafiltration enriched and purified the HS from landfill leachate concentrate for sustainability. During the disposal of highly saline wastewaters, i.e., landfill leachate concentrate, a conceptual and paradigmatic transformation from contaminant elimination to sustainable resource extraction urges for novel selective membranes. Mussel-inspired modification through dopamine self-polymerization can be a competitive alternative to tailor the membrane surface properties for enhanced permselectivity to target compounds. Herein, co-deposition of polydopamine-polyethylenimine (PEI) complex onto loose nanofiltration (NF) membranes was conducted using persulfate as trigger, which resulted in a significant alteration in membrane surface properties. Particularly, the pore size of bio-inspired NF membranes decreased, along with a reduction in molecular weight cut-off (MWCO) from 640 to 298 Da after 120-min bio-inspired coating, which yielded an enhancement in rejection of humic substances from 95.1% to 98.9% in filtration of landfill leachate concentrate. Furthermore, the charge of membrane surface was tailored to be more positive due to incorporation of positively-charged PEI, which markedly undermined the electrostatic repulsion force for enhanced salt permeation. These bio-inspired NF membranes showed a superior selectivity between humic substances and salts for their potential fractionation from the landfill leachate concentrate. Through an integrated NF-based diafiltration procedure, the concentration of humic substances was remarkably enriched from 1779.4 to 17247.1 mg·L−1 with 96.0% recovery. Additionally, 99.5% desalination efficiency was achieved, resulting in a high purity (i.e., 98.3%) for humic substances as potential organic fertilizer application. These findings suggest that the bio-inspired co-deposition is an effective strategy to tailor the surface properties of NF membranes for efficient fractionation of humic substances and salts, in view of sustainable resource recovery from the landfill leachate concentrate. [ABSTRACT FROM AUTHOR]