Nanofiltration Membrane Module Research Articles

Influence of graphene oxide additives on the NF separation of triazine-based H2S scavenging compounds using advanced membrane technology

This work proposes an innovative approach for the membrane separation of spent and unspent H2S scavengers (SUS) derived from the application of MEA-triazine in offshore oil and gas production. Modified nanofiltration membranes were fabricated by incorporating graphene oxide (GO) and polyvinyl alcohol (PVA) into a thin film composite (TFC) to obtain a thin film nanocomposite (TFN) with enhanced permeability. In addition, various immobilization strategies for GO were investigated. The performance of the membranes and the effect of the GO loading were evaluated in terms of permeability, fouling propensity, and rejection of key components of the SUS, i.e., MEA-triazine (unspent scavenger), dithiazine (spent scavenger), and monoethanolamine, operating on a sample of SUS wastewater obtained from an offshore oil and gas platform. Various characterization techniques, such as contact angle, FTIR, XRD, SEM, TGA, and AFM, were employed to evaluate the structure, composition, and hydrophilicity of the membrane. The results show a remarkable increase in permeability (from 0.22 Lm-2 h−1 bar−1 for the TFC to 5.8 Lm-2 h−1 bar−1 for the TFN membranes), due to the enhanced hydrophilicity from GO incorporation. The strong interfacial interaction between GO and PVA within the TFN membrane results in negligible nanofiller leaching. The incorporation of GO moderately increases the rejection of the unspent scavenger (63%–73%, 62%–79%, 62%–80%, and 68%–76%), while drastically increasing the rejection of the spent scavenger, which is approximately null for the TFC membrane without GO and increases up to 58% in the TFN membrane with GO. Therefore, while the proposed membranes cannot be used for the selective separation of the unspent form the spent scavenger, they can achieve substantial recovery of all the key components contained in the SUS to avoid their discharge into the sea.

Nanomembrane Approach Reuses Waste, Cleans Produced Water

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 213946, “Nanomembranes From Polymeric Waste for Produced-Water Treatment,” by Anton Manakhov, Iaroslav Rybkin, and Fahd I. AlGhunaimi, Saudi Aramco, et al. The paper has not been peer reviewed. _ In the oil and gas sector, produced water is the most significant waste stream. Among the different materials used for water filtration, including ceramics, polymers, and carbon nanomaterials, polymeric nanofibers can be considered a unique solution that can be used as a membrane or an adsorbent. In the complete paper, the authors write that polymeric nanofibers can be prepared from polystyrene waste. Introduction The management of polystyrene trash and recycling creates a significant environmental problem. While most of this substance ends up in landfills, only a small portion of polystyrene is recycled or burned. By turning this trash into products with added value, recycling will become more appealing economically and will result in lower CO2 emissions and less chemical waste dumped on the ground. At the same time, the use of modified nanofiltration membranes for generated-water purification has been a subject of study for many researchers. The complete paper includes a review of the literature dedicated to advances in this approach. Dissolved polystyrene waste can be used as a feedstock in the electrospinning process to create nanofibrous membranes. Electrospinning is one of the more basic methods for producing nanofibers with dimensions ranging from micrometers to nanometers. Strong electrostatic forces are used to overcome the surface tension of a polymer solution or melt. These pressures produce the ejection of charged polymer jets, which dry as they approach a grounded electrode, where they are gathered as nanofibers. In this work, the nanofibrous polystyrene membranes were prepared from discarded expanded polystyrene waste. Initially, polystyrene waste was collected and dissolved in an organic solvent; then, the nanofibrous membranes were prepared by the electrospinning of nanofibers from this solution. The morphology, pore sizes, and other parameters were controlled and optimized by tuning the concentration of polystyrene, the nature of the solvent, and the spinning voltage to form uniformly sized nanofibrous membranes.

High perm-selectivity and performance of tuned nanofiltration membranes by merging carbon nitride derivatives as interphase layer for efficient water treatment

The support characteristics under polyamide thin films have proven significant impact on the performance of nanofiltration thin film composite (TFC) membranes. In this study, polyethersulphone (PES) support was initially furnished with an interphase layer of different graphitic carbon nitride derivatives and nanocomposites (g-C3N4, oxidized g-C3N4, and g-C3N4/CuFe2O4) to improve the specifications of TFC membranes. Then, the polyamide layer was fabricated by interfacial polymerization on top of the interphase layer. The interlayer helped fine-tune the thickness and surface characteristics such as roughness, wettability, and zeta potential of the polyamide layer, in the way of improving the separation and antifouling capabilities of the TFC membranes. The interlayered thin film nanocomposite (ITFN) membranes considerably indicated better water permeability and rejection of methylene blue (MB), methyl orange (MO), Cadmium nitrate, and sodium sulfate (Na2SO4). We showed that ITFNs loaded with 0.01 wt% g-C3N4/CuFe2O4 had a permeability of ~9.2 LMH/bar, upper from ~3.8 LMH/bar in the pristine form, exhibiting outstanding water flux. The rejection data showed nearly two-fold increase of rejection of Cadmium nitrate, sodium sulfate, and dyes rather than the pristine nanofiltration membrane. The modified nanofiltration membranes remarkably exhibited better antifouling properties and higher flux recovery ratios as well.

Modulating interfacial polymerization via 1-methylimidazole as reactive additive for nanofiltration membrane with high-performance

Nanofiltration (NF) membrane technology has become the most critical alternative to resolve fresh water resources shortage and water pollution. Preparing polyamide (PA) NF membrane with excellent water permeance and mono/multivalent salts selectivity is highly desired. Herein, we introduced 1-methylimidazole (1-MI) as a reactive additive into interfacial polymerization (IP) to prepare NF membrane and attempted to improve the NF membrane performance. 1-MI modified NF membranes exhibited enlarged pore size and thickness of PA layer, as well as a smooth and hydrophilic membrane surface, the water permeance improved from 7.7 L/(m2·h·bar) of Virgin-NF to 13.1 L/(m2·h·bar) of 1-MI-0.05-NF. The Na2SO4 rejection was maintained at 95.01% while NaCl rejection dramatically reduced from 25.79% to 13.29%, and the Cl−/SO42− selectivity slightly increased from 21.47 to 23 under the mixed salts solution as feed. The enhancement of water permeance was ascribed to the looser PA layer and enhanced hydrophilicity. The retaining Na2SO4 rejection was attributed to electrostatic repulsion caused by more carboxylic acid groups, and the decreased NaCl rejection was ascribed to the looser PA layer. Besides, the modified membrane had an acceptable pH stability. In summary, 1-MI modified NF membrane was suitable for the separation of monovalent/divalent anionic salts.

Enhancing Nanofiltration Selectivity of Metal-Organic Framework Membranes via a Confined Interfacial Polymerization Strategy.

Development of well-constructed metal-organic framework (MOF) membranes can bring about breakthroughs in nanofiltration (NF) performance for water treatment applications, while the relatively loose structures and inevitable defects usually cause low rejection capacity of MOF membranes. Herein, a confined interfacial polymerization (CIP) method is showcased to synthesize polyamide (PA)-modified NF membranes with MOF nanosheets as the building blocks, yielding a stepwise transition from two-dimensional (2D) MOF membranes to polyamide NF membranes. The CIP process was regulated by adjusting the loading amount of piperazine (PIP)-grafted MOF nanosheets on substrates and the additional content of free PIP monomers distributed among the nanosheets, followed by the reaction with trimesoyl chloride in the organic phase. The prepared optimal membrane exhibited a high Na2SO4 rejection of 98.4% with a satisfactory water permeance of 37.4 L·m-2·h-1·bar-1, which could be achieved by neither the pristine 2D MOF membranes nor the PA membranes containing the MOF nanosheets as the conventional interlayer. The PA-modified MOF membrane also displayed superior stability and enhanced antifouling ability. This CIP strategy provides a novel avenue to develop efficient MOF-based NF membranes with high ion-sieving separation performance for water treatment.

Nanofiltration membrane with CM-β-CD tailored polyamide layer for high concentration cephalexin solution separation

Nanofiltration (NF) is a promising technique to achieve sustainable and efficient separation of antibiotics with high industrial value. In this work, carboxymethyl-β-cyclodextrin (CM-β-CD) modified nanofiltration membrane was fabricated through interfacial polymerization (IP) for high concentration cephalexin solution separation, which is a classic antibiotic pharmaceutical wastewater. In principle, CM-β-CD can slow down the diffusion of amine monomers via electrostatic attraction, steric effect, hydrogen bonding and adsorption of the piperazine (PIP) monomer onto the hydrophilic surface, which renders amine monomer the high local concentration. The fabricated NF membranes possessed a larger proportion of dense primary layer on the surface. In addition, the accumulation of CM-β-CD between the substrate membrane and polyamide (PA) layer played the role as a barrier with specific cavity structure and bulk carboxylate to enhance the rejection of cephalexin. The cavity structure and stacking gap of CM-β-CD simultaneously rendered the membrane interlayer highly permeable. The optimal CM-β-CD modified membrane exhibited high rejection of cephalexin (94.7%) and water permeability of 122.4 L m−2 h−1 MPa−1 when the concentration of cephalexin was 3473.9 ppm (10 mmol/L). This work furnished an available way to prepare a novel nanofiltration membrane with efficient separation performance for relevant pharmaceutical production wastewater.

Advanced Mg2+/Li+ separation nanofiltration membranes by introducing hydroxypropyltrimethyl ammonium chloride chitosan as a co-monomer

In recent years, the global demand for lithium resources has been constantly rising thanks to the rapid development of the new energy industry. Nanofiltration (NF) membranes with positive charge could achieve high efficiency separation performance of magnesium ion (Mg2+)/ lithium ion (Li+), which provides a useful solution for the extraction and recovery of lithium resources from brine. In this work, hydroxypropyltrimethyl ammonium chloride chitosan (HACC) was introduced into the interfacial polymerization (IP) as co-monomer with piperazine (PIP) to react with trimesoyl chloride (TMC) for preparing positively charged NF membranes. Based on the modulation of IP process by HACC, the thickness of the modified NF membrane separation layer reduced and the hydrophilicity increased, while the pore size had no obvious variation. In addition, the introduction of the quaternary ammonium group in HACC also increased the antibacterial properties of the NF membrane. The optimized NF-HACC-0.3 not only improved the separation factor significantly, but also increased the flux by 2 times compared with the original membrane. This strategy provides a new valuable idea and insight for the preparation and development of Mg2+/Li+ separation NF membranes.

Improving Mg2+/Li+ separation performance of polyamide nanofiltration membrane by swelling-embedding-shrinking strategy

Extraction of lithium from salt lakes is an important route for lithium sources. Due to the unique structure and separation mechanism, i.e., size exclusion and Donnan effect, nanofiltration (NF) is considered as a promising way to separate magnesium (Mg2+) and lithium (Li+) ions. However, the Mg2+/Li+ separation factor of NF membrane fabricated by traditional interfacial polymerization (IP) is usually dissatisfactory. Here, a swelling-embedding-shrinking strategy was proposed to modify polyethyleneimine (PEI)/trimesoyl chloride (TMC) NF membranes using diethylenetriamine (DETA) to improve the performance in Mg2+/Li+ separation. The pristine PEI/TMC NF membrane was swollen with methanol to increase the polyamide chain spacing followed by embedding electropositive DETA as a modifier. The charge properties and pore structure of the active layer could be regulated by optimizing the modification conditions. Within comparison to the pristine membrane (TFC-0), the pure water flux of modified NF membrane (TFC-3) increased by 63.6% because of the rougher surface and the larger pore size. Meanwhile, the embedding of electropositive DETA reduced the electronegative property of membrane surface. In consequence, TFC-3 membrane exhibited higher rejection of MgCl2 (93.9% against 88.4%) and lower rejection of LiCl (31.0% against 32.1%), thus showing a twofold improvement in Mg2+/Li+ separation factor (SLi, Mg = 11.38 against 5.87). The swelling-embedding-shrinking strategy in this article provided a good reference for the modification of NF membranes in the separation of multivalent and monovalent ions.

PH-regulated interfacially polymerized nanofiltration membranes to achieve high separation of NOM and moderate desalination for purifying ground water

Highly permeable nanofiltration (NF) membranes with efficient natural organic matter (NOM) removal yet preserving essential minerals are of great significance for purifying ground water. In this work, a novel pH-regulated interfacial polymerization (PRIP) strategy was proposed to fabricate ultra-low pressure NF membranes. This strategy was achieved by changing the aqueous solution pH (5, 7, 9, and 11) during IP process. The effects of pH-regulated processes on the structure, chargeability, and physicochemical properties of NF membranes were systematically investigated. Tailored NF membranes under acidic conditions (TFC-pH 5) achieved the ultrahigh permeability of 42.9 L m−2 h−1 bar−1 and 83.3 % moderate rejection of Na2SO4. For the NF membranes prepared under alkaline conditions (TFC-pH 11), the sieving capacity was obviously improved with fewer defects, and 99.4 % rejection of Na2SO4 was observed. In addition, the purification performance of NF membranes in ground water treatment was further studied. Modified NF membranes showed significant removal of NOM and retain moderate amounts of mineral salts. Finally, the mechanism of the pH-regulated influence on the polyamide active layer formation process was proposed. This study provides a feasible and facile design idea for ultralow-pressure NF membranes in purifying ground water.

Enhancing the NaCl/Na2SO4 separation selectivity and chlorine resistance of nanofiltration membranes by incorporating novel designed starch nanoparticles

The challenge of poor compatibility between nanoparticles and separation layers in thin film nanocomposite (TFN) nanofiltration (NF) membranes has always plagued the researchers. In this work, a carboxylated starch nanoparticles (CSNPs) were introduced into the polyamide (PA) separation layer of NF membrane by the IP reaction between trimesoyl chloride (TMC) and piperazine (PIP). The agglomerated or large size CSNPs could transfer the reaction interface above the particles through the formation of a water template by adsorption of aqueous phase monomers, which shaped the defect-free separation layer. Furthermore, the reduced thickness and increased density of the separation layer greatly enhanced the separation selectivity of the nanofiltration membrane thanks to the introduction of hydrophilic groups of nanoparticles. The modified membrane achieved a nearly 5-fold improvement in separation selectivity for NaCl and Na2SO4 (SNaCl/Na2SO4 = 180) and a 2-fold increase in the water permeance. In addition, the modified NF membrane also possessed better chlorine-resistance property based on the formation of polyarylester between the reaction of hydroxyl groups in CSNPs and acyl chloride groups in TMC. Such novel aqueous-phase additives provide new insights to effectively solve the compatibility problem between the nanoparticles and separation layers, which is favorable for the commercial production of NF membrane.

Natural deep eutectic solvent modified nanofiltration membranes with superior antifouling properties for pharmaceutical wastewater treatment

Herein, the L-menthol/10-camphorsulfonic acid (L-M/CSA) natural deep eutectic solvent (NDES) was introduced as an innovative modifier for polyethersulfone (PES) nanofiltration (NF) membrane. Four membranes with different L-M/CSA NDES loadings (0, 0.2, 0.6, and 1 wt%) were prepared and characterized to evaluate the influence of L-M/CSA NDES on the properties of the membranes. The modified membrane with 0.2 wt% L-M/CSA NDES (M-0.2NDES) showed the pure water flux (PWF) of 111.5 L/m2h, which was about 7 times higher than that of the control membrane (M-0NDES). Meanwhile, the mean pore radius was increased from 0.7 nm for the M-0NDES membrane to 0.85 nm for the M-0.2NDES membrane, indicating the good pore forming capability of L-M/CSA NDES. Benefiting from L-M/CSA NDES blending, the modified membranes showed excellent removal efficiency to ceftriaxone (CTX) and amoxicillin (AMOX) solutions. Particularly, the M-0.2NDES membrane possessed 99.6 and 99.2% rejection to CTX and AMOX, respectively, with permeated flux up to about 111 L/m2h, which was far better than the data reported in the literature. M-0.2NDES membrane showed 89.1% chemical oxygen demand (COD), 100% turbidity, and 51.6% total dissolved solids (TDS) removal from the industrial pharmaceutical wastewater. Blending with L-M/CSA NDES was also confirmed to be crucial to ameliorate the membrane antifouling properties. The related results revealed that the flux recovery ratio (FRR) of the M-0.2NDES membrane for milk solution reached 90.2%. The facile fabrication method of L-M/CSA NDES from green materials, good pore forming capability, enhanced antifouling properties, together with the efficient pharmaceuticals removal indicated the good prospect of L-M/CSA NDES for membrane separation applications.

In Situ Chemical Modification with Zwitterionic Copolymers of Nanofiltration Membranes: Cure for the Trade-Off between Filtration and Antifouling Performance.

Breaking the trade-off between filtration performance and antifouling property is critical to enabling a thin-film nanocomposite (TFC) nanofiltration (NF) membrane for a wide range of feed streams. We proposed a novel design route for TFC NF membranes by grafting well-defined zwitterionic copolymers of [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SBMA) and 2-aminoethyl methacrylate hydrochloride (AEMA) on the polyamide surfaces via an in situ surface chemical modification process. The successful grafting of a zwitterionic copolymer imparted the modified NF membranes with better surface hydrophilicity, a larger actual surface area (i.e., nodular structures), and a thinner polyamide layer. As a result, the water permeability of the modified membrane (i.e., TFC-10) was triple that of the pristine TFC membrane while maintaining high Na2SO4 rejection. We further demonstrated that the TFC-10 membrane possessed exceptional antifouling properties in both static adsorption tests and three cycles of dynamic protein and humic acid fouling tests. To recap, this work provides valuable insights and strategies for the fabrication of TFC NF membranes with simultaneously enhanced filtration performance and antifouling property.

Novel antibacterial and antifouling PES nanofiltration membrane incorporated with green synthesized nickel-bentonite nanoparticles for heavy metal ions removal

• The nickel-bentonite nanoparticles (NBNPs) were synthesized using plant extract. • The NBNPs were used to modifying the polyethersulfone membrane. • The modified membranes exhibited significant water flux and antifouling properties. • The NBNPs/PES membranes showed high rejection of heavy metal ions than PES. • The NBNPs/PES membranes exhibited desirable antibacterial activity. Lack of water resources and water pollution by heavy metal ions and microorganisms has made it necessary to utilize efficient water treatment methods. In this study, for the first time, nickel-bentonite nanoparticles (NBNPs) were synthesized using Scrophularia striata extract. NBNPs were used as novel nanofillers to fabricate antifouling and antibacterial NBNPs/polyethersulfone (PES) nanofiltration membranes via the phase inversion method for heavy metal ions rejection. NBNPs were characterized by FT-IR, XRD, zeta potential, EDS, FESEM, and TEM. The fabricated membranes were characterized using FT-IR, XRD, EDS, SEM, water contact angle, zeta potential, and porosity analyses. The results showed improvement of hydrophilicity, surface charge, and porosity for the modified membranes. The blended membranes had substantially less fouling and higher heavy metal ions rejection than the unmodified membrane. Pure water flux of the unfilled PES membrane was enhanced from 15 to 34.4 and 58.8 kg/m2h for 0.1 and 0.5 wt% of NBNPs, respectively. The flux recovery ratio was increased from 38.3% for the unmodified membrane to 98.2% for 0.5 wt% NBNPs/PES membrane. Also, 0.5 wt% NBNPs/PES membrane exhibited the best rejection performance (Zn2+: 98.62%, Cu2+: 97.88% and Pb2+: 97.03%). Evaluation of antibacterial properties of the membranes against Escherichia coli and Staphylococcus aureus was performed. The modified membranes had significant antibacterial capability and 1 wt% NBNPs/PES membrane exhibited the highest antibacterial rate. According to the findings, NBNPs could be used as nanofillers to fabricate modified nanofiltration membranes for heavy metal ions rejection and antibacterial ability.

A novel method of fabricating anti-biofouling nanofiltration membrane with almost no potential to induce antimicrobial resistance in bacteria

The fabrication of anti-biofouling nanofiltration (NF) membrane is one of the effective methods to improve the vulnerability of membranes to bacteria in the feed solution. The prevalent approach to prepare anti-biofouling NF membranes is to graft biotoxic substances onto the NF membrane surface to kill the attached bacteria, inducing antimicrobial resistance in bacteria and water safety problem. In this study, non-biotoxic methyl 2-aminobenzoate (MA) was employed to prepare the anti-biofouling NF membranes. Instead of killing bacteria, the modified NF membrane could effectively prevent proliferation of bacteria on the membrane surface by blocking the quorum sensing in bacteria, thereby avoiding the generation of antimicrobial resistance. On the condition that the bacteria are alive, the biofilm formed by Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) on the modified NF membrane were significantly reduced by 94.5% and 65.1%, respectively. Additionally, the modified membrane showed superior anti-adhesion property for proteins and polysaccharides, attributing from the improved hydrophilicity of membrane surface. The optimally modified membrane exhibited the enhanced water permeability (from 6.72 L m-2h−1 bar−1 to 12.12 L m-2h−1 bar−1) and un-degraded divalent salt rejection (98.4% of Na2SO4). This work provided an efficient and non-biotoxic approach for the fabrication of environmental friendly anti-biofouling NF membranes, which would eliminate the worries about the water safety and the development of antimicrobial resistance in bacteria.

Fabrication of modified nanofiltration membranes by functionalized cellulose nanocrystals with high anti-fouling capability in removing dye from water and wastewater

Fabrication of modified nanofiltration membranes by functionalized cellulose nanocrystals with high anti-fouling capability in removing dye from water and wastewater

A Study to Enhance the Nitrate-Nitrogen Removal Rate without Dismantling the NF Module by Building a PFSA Ionomer-Coated NF Module.

Water resource pollution by nitrate-nitrogen, mainly caused by anthropogenic causes, induces eutrophication of water resources, and indicates the degree of organic pollution. Therefore, this study devised a method for coating PFSA ionomer with excellent chemical resistance without disassembling the module to improve the removal rate of nitrate-nitrogen in water by using a cyclic coating method on a commercially available nanofiltration membrane (NF membrane) module. Nafion was prepared as a supercritical fluid dispersion using a high-temperature and high-pressure reactor, and the particle size and the degree of dispersion of the dispersion were analyzed by DLS. The crystallinity was confirmed through XRD by drying the dispersion in the liquid state. After the dispersion was prepared as a membrane according to the heat treatment conditions, the characteristics according to the particle size were analyzed by tensile strength and TEM. The nitrate-nitrogen removal rate of the NF membrane module coated with the dispersion was increased by 93% compared to that before coating. Therefore, the result showed that the cycle coating method devised in this study could efficiently coat the already commercialized module and improve performance.

An attempt to enhance water flux of hollow fiber polyamide composite nanofiltration membrane by the incorporation of hydrophilic and compatible PPTA/PSF microparticles

Hollow fiber composite (HFC) nanofiltration (NF) membrane was prepared on the inner surface of porous hollow fiber polysulphone (PSF) support membrane through the interfacial polymerization (IP) of piperazine (PIP) and trimesoyl chloride (TMC). Hydrophilic and compatible poly(p-phenylene terephthalamide) (PPTA)/PSF blend particles were sythesized by the in situ polycondensation and cloud-point precipitation method and embedded in the polyamide (PA) functional layer via the incorporation into PIP aqueous phase to enhnace membrane water flux. The crystal structure and size distribution of the particles were characterized by X-ray diffraction (XRD) and dynamic light scattering (DLS). The effects of blend particle content on membrane morphology, surface hydrophilicity, water permeability and separation performance were investigated through the Attenuated Total Reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) analysis, scanning electron microscope (SEM) observation, dynamic water contact angle (WCA) and separation performance measurements. The embedding of blend particles effectively improves membrane hydrophilicity, negative charge and water flux. The modified NF membrane with a blend particle concentration of 0.5 wt% has an enhanced water flux (5.46 L.m−2.h−1.bar−1) which is increased by 57.2 % meanwhile the salt rejection of MgSO4 can be well maintained (98.1 %) at 0.7 MPa. NF membrane exhibits an excellent NF performance with a high selectivity factor of 6 for MgSO4/NaCl and a high rejection (>98 %) of the selected dyes with different charge types. Appropriate membrane cleaning can effectively recover the water flux up to 90 %. The model of interfacial polymerization process was also proposed. PPTA blend particle was proved to be an excellent modifier for the separation performance improvement of NF membrane.

Effect of surface morphology on the mesoporous silicon modified nanofiltration membranes for high rejection performance

A new method to prepare composite nanofiltration membrane directly and efficiently by controlling the pore structure and surface morphology of the membrane is proposed in this paper. The fabrication of mesoporous silicon modified polysulfone blend membranes is via a phase inversion method. The structural morphology, surface functional group analysis, elemental analysis, hydrophilicity, chargeability, and nitrogen pollutant (ammonia nitrogen, nitrate nitrogen, total nitrogen) rejection properties of the modified membranes were found to be dependent on the amount of mesoporous silicon incorporated. The combination of the mesoporous silicon framework layer can not only effectively improve the surface structure of the modified membrane with a narrow pore size distribution but also increase the rejection of nitrogen pollutant compared with the pure NF membranes. The mesoporous material can absorb and storage the nitrogenous solution to facilitate the following interfacial polymerization, as well as induce the change of pore radius and surface structure. Compared to those pure NF composite membranes, the modified blend membranes exhibit increased water permeation flux as high as 29.09 L m−2 h−1 at 0.2 MPa. The results show that the optimum doping amount of mesoporous silicon is in the range of 0.5%-1.0%. Characterization studies demonstrated that the addition of the mesoporous silicon leads to decreased membrane pore size. Then, due to the enhanced hydrophilicity of carboxyl and hydroxyl groups on the surface, the electrostatic repulsion between the functional groups and nitrogen pollutant molecules in the membrane results in the enhanced rejection of nitrogen pollutants.

Effect and regulation mechanism of oxidation degrees on the O–MoS2 structure and separation performance of nanofiltration membrane

Oxidized molybdenum disulfide (O–MoS2) as a two-dimensional (2D) functional nanosheets material, has been used to fabricate high separation performance nanofiltration (NF) membranes for water treatment. However, the effect and regulation mechanism of the degree of oxidation on structure and separation performance of O–MoS2 modified NF membrane has not yet been fully understood. In this work, O–MoS2 with different oxidation degrees was synthesized by adjusting the oxidation time and KMnO4 oxidant content via Hummers’ method. The results demonstrated that the O–MoS2 synthesized under with an oxidation time of 3 h and MoS2:KMnO4 ratio of 1:2 had excellent stability under acidic, neutral and alkaline conditions. The pure water flux increased 2.5 times and the Na2SO4 salt rejection increased to 97.6% with this O–MoS2 incorporation compared to the control membrane. The influence of different oxidation degrees on the performance of O–MoS2 is a breakthrough finding of this work to provide a theoretical basis for the application of O–MoS2 in the field of membrane separations.

Surface functionalization of nanofiltration membrane by catechol-amine codeposition for enhancing antifouling performance

Nanofiltration (NF) technology is promising for treatment of wastewater containing micropollutants (MPs) because it can retain both MPs and small organics while permeate inorganic salts. However, both MPs adsorption and organic fouling formation deteriorate NF membrane performance. In this work, a novel surface functionalization strategy is proposed to tackle both two issues. Based on catechol-amine chemistry, a commercially available NF membrane is modified by tannic acid-3-aminopropyltriethoxysilane (TA–APTES) coating and subsequent horseradish peroxidase (HRP) immobilization. Taking aflatoxin B1 (AFB1) as an example of MPs, its adsorption on the modified NF membrane is greatly inhibited by coating induced pore narrowing (enhancing size exclusion to MPs), surface barrier effect (alleviating the interaction between MPs and membrane) and enzymatic self-cleaning ability (degrading MPs adsorbed on the membrane), resulting in higher and more stable AFB1 rejection (~90%). The flux decline and irreversible fouling are also reduced by the amphiphilic property of the immobilized HRP on the membrane surface when treating corn soaking solution. Although the on-line self-cleaning operation in the presence of hydrogen peroxide can alleviate concentration polarization by interfacial catalytic reaction, the flux decline and irreversible fouling become severer possibly due to the pore blocking by the degraded products. The off-line self-cleaning after the filtration enables to remove loose fouling layer, but the dense fouling layer impedes the accessibility of hydrogen peroxide to the immobilized HRP, limiting its self-cleaning ability. Therefore, catalytic intensity, enzymatic products and substrate accessibility to enzyme should be considered when constructing a self-cleaning NF membrane to deal with combined fouling.