Na2SO4 Rejection Research Articles

Fine-tuning of carbon nanostructures/alginate nanofiltration performance: Towards electrically-conductive and self-cleaning properties

Electrically-conductive membranes became the center of attention owing to their enhanced ion selectivity and self-cleaning properties. Carbon nanostructures (CNS) attain high electrical conductivity, and fast water transport. Herein, we adopt a water-based, simple method to entrap CNS within Alginate network to fabricate self-cleaning nanofiltration membranes. CNS are embedded into membranes to improve the swelling/shrinkage resistivity, and to achieve electrical-conductivity. The CaAlg PEG-formed pores are tuned by organic-inorganic network via silane crosslinking. Flux/rejection profiles of Na2SO4 are studied/optimized in reference to fabrication parameters. 90% Na2SO4 rejection (7 LMH) is achieved for silane-CaAlg200-10% CNS membranes. Membranes exhibit outstanding electrical conductivity (∼2858 S m−1), which is attractive for fouling control. CaAlg/CNS membranes are tested to treat dye/saline water via two-stage filtration, namely, dye/salt separation and desalination. A successful dye/salt separation is achieved at the first stage with a rejection of 100%-RB and only 3.1% Na2SO4, and 54% Na2SO4 rejection in the second stage.

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.

PPSU dual layer hollow fiber mixed matrix membranes with functionalized MWCNT for enhanced antifouling, salt and dye rejection in water treatment

AbstractLoose nanofiltration dual layer hollow fiber membranes (DLHFM) were prepared from polyphenylsulfone (PPSU) with a thin contact mixed matrix membrane (MMM) outer layer containing functionalized multiwalled carbon nanotubes (MWCNT). Two types of MWCNT were used; oxidized MWCNT (OMWCNT) and polyethyleneimine (PEI) functionalized MWCNT (PEI‐MWCNT). Z potential measurements indicate that OMWCNT and PEI‐MWCNT were negatively charged, being OMWCNT most negative. Na2SO4, MgSO4, and methylene blue (MB) rejection was enhanced due to OMWCNT and PEI‐MWCNT incorporation in DLHFM outer layer. Enhanced hydrophilicity and negatively charge mixed matrix DLHFM layer was the key factor for a better mixed matrix DLHFM performance. The best salt and MB rejection was found in those membranes prepared with (OMWCNT) followed by (PEI‐MWCNT) compared with neat PPSU membranes. An increase of 113% and 100% in Na2SO4 and MB rejection respectively was attained in DLHFM with 0.04 wt%, OMWCNT (relative to dope's weight) in comparison with the neat PPSU membrane. Antifouling test for bovine serum albumin (BSA) and MB showed better performance also for the mixed matrix DLHFM containing OMWCNT's. In antifouling tests, a 4.4% and 25% flux recovery increase for MB and BSA respectively was obtained in DLHFM with 0.04 wt% OMWCNT compared with the neat PPSU DLHFM.

Zwitterionic liquid hydrogel sustained-release strategy for high-performance nanofiltration membrane

Controlling the diffusion of amine monomers plays a crucial role in fabricating well-defined architecture and high-performance nanofiltration (NF) membranes through interfacial polymerization (IP). Herein, a sustained-release strategy was proposed to regulate the interfacial polymerization through a zwitterionic liquid hydrogel transition layer. The hydrogel (Gel) was fabricated via in-situ one-step UV photo-initiated free-radical polymerization between 1-vinyl-3-ethylimidazolium chloride ([VEIm][Cl]) and 3-sulfopropyl methacrylate potassium salt (SMP) on the polyethersulfone (PES) substrate. The polyamide (PA) layer was then formed on the Gel layer by mitigating the diffusion of amine monomers. Low-field time-domain nuclear magnetic resonance (TD NMR) analysis demonstrated that the Gel layer significantly enhanced the water-absorbing capacity of the composite membrane. Positron annihilation lifetime spectroscopy (PALS) also confirmed the Gel-PA composite membrane possessed enlarged free volumes. The obtained Gel-PA composite membrane displayed a permeance of 13.22 L m−2 h−1 bar−1, a high Na2SO4 rejection of 98.23 %, a favorable NaCl/Na2SO4 selectivity of 49.75, outstanding long-term operational stability, and membrane antifouling properties. This sustained-release strategy provides a facile and feasible approach to controlling the IP process and obtaining high-performance nanofiltration membranes.

Rapid transport of water and monovalent ions through ultrathin polyamide nanofilms for highly efficient mono/bivalent ions separation

Defect-free ultrathin polyamide nanofilms enable superior separation performance, which can be prepared by interfacial polymerization (IP) at free oil–water interface (also known as support-free IP). However, the current free IP methods usually suffer from complicated process, or complex equipment. Herein, an easily-manufactured plate-and-frame assembly for free IP was designed for more efficient fabrication of ultrathin membranes. Piperazine (PIP)-based negatively-charged NF membranes were fabricated by this method, combining with introduction of a rigidly-contorted spirobisindane monomer that simultaneously enhanced the mechanical strength, microporosity, and electronegativity of membrane. The resulting defect-free ultrathin membranes not only exhibit super-high Na2SO4 rejection (∼99.7 %), and allow rapid transport of water and monovalent ions, thus leading to a Cl-/SO42- selectivity of up to 240. Furthermore, the polymerization of acyl chloride and macromolecule polyethyleneimine (PEI) at free oil–water interface was also investigated. The resulting positively-charged ultrathin membranes show outstanding calculated monovalent/bivalent cation selectivity (54.8 for Li+/Mg2+, 43.2 for Na+/Mg2+), further demonstrating the effectiveness of ‘thickness reduction’ strategy to enhance ions selectivity. In addition, the cause of different surface charge properties for PIP and PEI membrane system was revealed by molecular simulation.

Ultrathin-film composite (uTFC) membranes based on amorphous perfluoropolymers for liquid separations

Thin-film composite membranes based on ∼100 nm cross-linked polyamides (PAs) are state-of-the-art membranes for liquid separations such as desalination. However, the PA layer shows poor antifouling properties and instability in chlorine solutions. Herein, we show for the first time that glassy amorphous perfluoropolymers (PFPs, such as Teflon AF and Hyflon AD) can be fabricated into ultrathin film composite (uTFC) membranes with a selective layer of <20 nm for desalination. Increasing the polymer concentration in the coating solutions from 0.05 mass% to 0.3 mass% increases the selective layer thickness from 9 to 25 nm. When Teflon AF2400 was fabricated into 16-nm selective layers on various porous supports, the water permeance decreases from 0.37 LMH/bar for PES10k to 0.07 LMH/bar for PES1k, and the Na2SO4 rejection increases from 86.6% to 95.2%. While the water permeance is lower than the commercial PA-based NF membranes, the rejection in these PFP-based membranes is comparable. The effect of the porous support on the water permeance can be ascribed to the geometric restriction and satisfactorily described using an empirical model. Interestingly, the membranes also exhibit good Na+/Li+ separation performance and stable performance for organic solvent nanofiltration, showcasing the versatility of the PFP-based uTFC membranes for various liquid separations.

Tailored design of highly permeable polyamide-based nanofiltration membrane via a complex-dissociation regulated interfacial polymerization

Nanofiltration (NF) that can separate neutral/charged solutes is considered as a key industrial technology in water softening and wastewater treatment. However, commercial polyamide (PA) NF membranes, with undesirable surface properties and excessive mass transfer resistance, seriously restrict the permeability and are easily attached by pollutants. In this study, a novel complex-dissociation regulated interfacial polymerization (CDRIP) strategy was proposed to prepare highly permeable polyamide-based (PA-based) NF membranes, in which tannic acid (TA)/Ca(II) complex was in situ formed on substrate surface and then dissociated by sodium citrate (SC) after interfacial polymerization. Interestingly, SC can not only adjust surface properties of PA, such as hydrophilicity and surface charge, but also form water-soluble SC/Ca(II) complexes for constructing additional transport channels within the separation layer to reduce mass transfer resistance. After the dissociation of TA/Ca(II) complex, TA remained on the membrane surface to further regulate the surface properties of separation layer. The results showed that the PA/TA/Ca(II)-SC NF membrane maintained a high Na2SO4 rejection (98.5 %) and a remarkable water permeability of 31.7 L·m−2·h−1·bar−1, which is 2-fold that of the pristine PA NF membrane. Moreover, the prepared NF membrane exhibits favorable operation stability and anti-fouling ability. Therefore, by employing the CDRIP method, the permeability of NF membrane can be further improved almost without sacrificing the rejection of Na2SO4, which provides a green and efficient strategy for the fabrication of high-performance PA-based NF membranes.

Finely regulated polyamide membranes with rapid water transport for low-pressure precise nanofiltration

Low-pressure nanofiltration (NF) has evinced huge application prospects for ion/molecule separation. However, current polyamide-based membranes made by highly reactive monomers offer less control in precisely tuning of their pore apertures, leading to a permeability-selectivity trade-off limit. Herein, the usage of amino-piperidine (AP, 4-aminopiperidine: 4AP; 3-aminoapiperidine: 3AP) is proposed as new building block that interfacially polymerizes with trimesoyl chlorides (TMC) onto a nanofibrous hydrogel to fabricate high-performance thin film composite (TFC) membranes. The resultant amino-piperidine thin film composite (AP-TFC) membranes with nodular and bubble-like surface have high hydrophilicity, electronegativity, and moderate crosslinking degree. Introducing an alkane bond length based on the piperidine structure allows to finely augment the free-volume elements of polyamide-based membranes, combined with varying the monomer concentrations. Furthermore, the 4AP-TFC membranes showed an excellent water permeability of 35.0 L m−2 h−1 bar−1 with Na2SO4 rejection above 98.5% when the 4AP concentration was 0.05 w/v%. The 4AP-TFC membrane was also found having a superior sieving capability for antibiotics, attributed to the synergy of size exclusion by well-defined ultra-microporous structure (<0.7 nm) and electrostatic repulsion due to the abundance of surface negative charges. This work provides the guideline of using AP molecules for the fabrication of high-crosslinked TFC membranes with potential applications in low-pressure NF, organic solvent NF, and related water purification.

Polyvinyl alcohol and sodium alginate hydrogel coating with different crosslinking procedures on a PSf support for fabricating high-flux NF membranes

Polyvinyl alcohol (PVA) and sodium alginate (SA) hydrogel-coated nanofiltration (NF) membranes with high-flux and permselectivity were prepared. The coating of PVA and SA hydrogel selective layer on a porous polysulfone (PSf)/non-woven fabric ultrafiltration substrate membrane was conducted through different three procedures including pre-crosslinking, in-situ crosslinking, and immersing crosslinking and the use of glutaraldehyde as a crosslinking agent. The properties and performances of all types of the prepared membranes were evaluated through ATR-FTIR spectroscopy, AFM, SEM, zeta potential, contact angle, and cross-flow permeation tests. The immersing technique resulted in the formation of TFC membranes with higher hydrophilicity, smoother surface layer, higher negative charge, higher permeation flux, higher salt rejection and better anti-fouling performance. Also, the higher negative surface charge of the immersing coated TFC membranes due to dissociation of hydrophilic functional groups of the PVA and SA hydrogel selective layer resulted in higher As(III) rejection. SA coated NF membrane through immersing method exhibited a higher pure water permeability of 11.2 L m−2 h−1 bar−1, NaCl, MgSO4, and Na2SO4 rejection of 38.2%, 55.1%, and 70.4%, respectively with As(III) rejection of 60.6%. All types of the PVA and SA hydrogel-coated PSf membranes possessed improved fouling resistance to BSA aqueous solution, superior anti-fouling performance was obtained with SA hydrogel coating through immersing method. Such optimum membranes indicated high stability in the long-term experiments. This study showed that the coating of the SA hydrogel layer on a PSf support through immersing method could be a promising candidate for fabricating high-flux NF membranes.

Metal-polyphenol cross-linked titanium carbide membranes with stable interlayer spacing for efficient wastewater treatment

Membranes based on transition metal carbides/nitrides (MXenes) have significant water treatment potential because of their unique molecular sieving properties and excellent permeation performance. However, hydrophilic MXenes swell upon water immersion, and improving their stability remains challenging. In this study, a Fe3+–tannic acid (TA) complex was used as a cross-linker and surface modifier to prepare high-performance titanium carbide (Ti3C2Tx) MXene laminar membranes. Fe3+–TA formation on the nanosheets increased the interlayer spacing and stabilized the laminar structure. The membrane with the highest performance among the as-prepared membranes exhibited a high water permeance of 90.5 L/m−2(−|-)h−1 bar−1 (which is twice that of the pristine Ti3C2Tx membrane) and good separation efficiency (methyl blue rejection rate: ∼99.8 %; Na2SO4 rejection rate: ∼5.0 %). Furthermore, the Fe3+–TA complex enhanced the membrane hydrophilicity, resulting in excellent antifouling properties. This study provides an environmentally friendly and facile method for fabricating two-dimensional loose nanofiltration membranes for textile wastewater treatment.

Polyamide membranes with a ZIF-8@Tannic acid core-shell nanoparticles interlayer to enhance nanofiltration performance

Polyamide nanofiltration membranes with an interlayer of either a plant polyphenol tannic acid (TA) coating to improve polyamide formation or zeolitic imidazolate framework-8 (ZIF-8) nanoparticles (NPs) as a sacrificial template have been recently attracting great interest, although none has reported such a dual-functional interlayer. In this study, we demonstrated a facile and controllable one-pot synthesis of novel ZIF-8@TA core-shell NPs. Mixing Zn2+, Hmim, and TA in an aqueous solution could readily give rise to these NPs. Microscopy characterizations showed that composition and morphology of NPs could be easily controlled by the amount of TA used. Comparing with the thin film nanocomposite (TFN) membrane prepared with ZIF-8 NPs as an interlayer, which had significant defects in its polyamide layer due to the aggregation of hydrophobic ZIF-8 NPs, the homogeneous and hydrophilic ZIF-8@TA NPs interlayered TFN membranes demonstrated intact polyamide layers. It was found that when 0.5 mg ZIF-8@TA1.0 NPs were used, the obtained TFN membrane showed a high water permeance of 20.7 L m−2 h−1 bar−1 and Na2SO4 rejection of 96.6 %, typically outperforming other interlayer-based PA membranes reported elsewhere. This work provided not only a novel interlayer material but also deep insights into design of high-performance polyamide membranes.

Rapid Upcycling of End-of-Life Microfiltration Membrane Mediated by the Healing of Metal–Organic Complex

Membrane separation is widely adopted in many industries, and the membranes reach their end of life (EOL) after long-term use. However, the conventional approach of replacing EOL membranes has a high carbon footprint, hindering the development of a green, low-carbon, and carbon-neutral process. In this study, we developed a cleaning–tannic acid-iron (TA-Fe)-healing–interfacial polymerization strategy with a reaction time of less than 20 min for the upcycling of an EOL microfiltration (MF) membrane. The cleaning step could remove most of the foulants from the EOL membrane. The TA-Fe healing step decreased the surface pore size and increased the hydrophilicity of the cleaned membrane, offering a more favorable platform for the storage of amine monomers required for interfacial polymerization. The upcycled NF-Healed membrane possessed a Na2SO4 rejection of 96.9 ± 0.7% and high pure water permeance of 23.7 ± 1.4 L m–2 h–1 bar–1. An economic analysis indicated that the chemical cost for upcycling was ∼$4.9/m2 membrane, which was lower than the estimated cost to replace the EOL MF membrane. This study provides a rapid and cost-effective method for the upcycling of the EOL MF membrane, which is a promising strategy to close the sustainable loop in the membrane materials.

Molecularly microporous polyarylate-polyamide nanofiltration membrane patched by tris(2-aminoethyl)amine for ionic sieving

Macrocyclic molecules with intramolecular cavities have shown great potential in membrane science. However, the nanofiltration (NF) membranes fabricated by macrocyclic monomers often suffer from inferior ionic rejection due to the commonly formed intermolecular defects among these large molecules. Herein, a patching strategy was employed to fabricate polyarylate-polyamide hybrid NF membrane through interfacial polymerization (IP) by introducing tris(2-aminoethyl)amine (TAEA) with low concentration into Noria solution as the aqueous phase. The polyamide patch could repair the defects of polyarylate film formed by Noria with trimesoyl chloride and endow it with irregularly reticulate morphology. Specially, the Na2SO4 rejection of hybrid NF membrane fabricated with a TAEA doping concentration of 0.03 wt% was improved from 28.1 % to 96.8 %, while it maintained high permeance up to 15.3 L·m−2·h−1·bar−1. In order to compare the intrinsic permeability of fabricated polyarylate-polyamide membrane with other state-of-the-art NF membranes reported recently, the separation layer thickness was normalized. Benefitting from the inner cavity of Noria, its permeability was calculated to be 2.85 L·m−2·h−1·bar−1·μm, which was among the most permeable NF membranes. Additionally, the hybrid NF membrane exhibited excellent anti-compression ability and long-term stability. This patching method as a simple and effective strategy would further promote the application of various macrocyclic molecules in membrane science.

Development of Composite Thin-Film Nanofiltration Membranes Based on Polyethersulfone for Water Purification

The development of thin-film composite membranes has gained more importance as they exhibit higher permeability, higher solute rejection and improved antifouling behavior. The incorporation of hydrophilic nanomaterials within the composite thin film has reported to effectively increase the hydrophilicity and improve the membrane performance. In this study novel thin-film composite nanofiltration membranes were prepared by interfacial polymerization of trimesoyl chloride, polyvinylpyrolidone (PVP) and montmorillonite (MMT) composite on the porous polyethersulfone membrane. The effect of preparation parameters was studied as; reaction time, PVP concentration and MMT content. The properties of the prepared composite thin-film membranes were analyzed in terms of their chemical structure, surface morphological features, pure water contact angle, zeta potential, pure water permeation flux (PWP) and solutes rejection. The prepared composite thin-film membranes showed smoother surfaces, different surfaces functionality and amphoteric surfaces with points of zero charge at pH 7–7.5. The PWP was found to increase with increasing MMT content up to 0.10 wt%. The rejection of crystal violet dye (CV) was studied using the prepared membranes. The membrane with 0.5% PVP, 0.10% MMT and 6 min reaction time showed PWP of 18.1 L/m2 h bar and CV rejection of 80.01%. The rejection of Na2SO4 and MgCl2 increased with increasing MMT content and reached 96% and 35.4%, respectively. The steady state flux was decreased by 5% with increasing MMT content from 0.10 to 0.12 wt% indicating improved antifouling behavior with MMT.

Ultrafast Interfacial Self-Assembly toward Supramolecular Metal-Organic Films for Water Desalination.

Supramolecular metal‐organic materials are considered as the ideal candidates for membrane fabrication due to their excellent film forming characteristics, diverse metal centers and ligand sources, and designable structure and function. However, it remains challenging to rapidly construct highly permeable supramolecular metal‐organic membranes with high salt rejection. Herein, a novel ultrafast interfacial self‐assembly strategy to prepare supramolecular metal‐organic films through the strong coordination interaction between highly active 1,3,5‐triformylphloroglucinol (TFP) ligands and Fe3+, Sc3+, or Cu2+ at the organic–aqueous interface is reported. Benefiting from the self‐completing and self‐limiting characteristics of this interfacial self‐assembly, the new kind of supramolecular membrane with optimized composition can be assembled within 3.5 min and exhibits ultrathin, dense, defect‐free features, and thus shows an excellent water permeance (21.5 L m–2 h–1 bar–1) with a high Na2SO4 rejection above 95%, which outperforms almost all of the non‐polyamide membranes and commercially available nanofiltration membranes. This strong‐coordination interfacial self‐assembly method will open up a new way for the development of functional metal‐organic supramolecular films for high‐performance membrane separation and beyond.

TEMPO-mediated oxidation as surface modification for cellulosic ultrafiltration membranes: Enhancement of ion rejection and permeability

In membrane technology, rejection of dissolved salts often requires the use of nanofiltration or reverse osmosis membranes due to their sufficiently dense structure and favourable charge effects. In this work, the concept of charge-based salt rejection is extended to the more open ultrafiltration class. Surface charge of a commercial regenerated cellulose acetate ultrafiltration membrane is enhanced with TEMPO-mediated oxidation. With the modification, a membrane whose properties lie between the traditional nanofiltration and ultrafiltration classes is obtained. Moreover, a purely charge-based salt rejection mechanism is introduced to an ultrafiltration membrane.Increasing hypochlorite exposures are found to induce successively increasing surface carboxylate densities, negative zeta potentials, permeabilities and molecular weight cut-off values. In the low exposure region, the treatment mostly affects surface charge, whereas high exposures begin to open the membrane pores. Ultimately, the pH 7 zeta potential is increased from −30 up to −100 mV, and 500 ppm Na2SO4 rejections increase from an initial 4% to 36%. At the same time, pure water permeability is doubled, and cut-off value increased from 2.8 to 8.0 kDa. The reactions causing the change in membrane performance seem to take place almost exclusively on the membrane surface, while the bulk material appears unchanged.

Cucurbit[n]uril-rotaxanes functionalized membranes with heterogeneous channel and regenerable surface for efficient and sustainable nanofiltration

Nanofiltration membranes with high permeability and antifouling ability is favor of efficient and sustainable separation. However, simultaneous enhancement of above performances faces formidable challenges. Herein, we present a facile method to obtain ultrapermeable antifouling nanofiltration membranes via surface binding binary complex of dihydroxyl viologen (HV2+) axis molecule and cucurbit[n]uril (CB[n]) on the freshly prepared polyamide (PA) thin-film composite membranes in a rotaxane manner. CB[n]-rotaxanes feature three-dimensional hydrophilic structure and create heterogeneous channel in PA selective layer, which favors water transport resistance decrease. Meanwhile, CB[n]-rotaxanes as host molecules enhance superficial grafting ability of membranes towards guest-labelled antifouling materials through supramolecular recognition. Inspiringly, adopting stimuli-responsive guest, such as azobenzene, endows grafted antifouling layer with photo-activated regeneration capability, which further improves fouling reversibility of membranes. The optimal membrane exhibits a 3-fold increase of permeance, maintained Na2SO4 rejection and enhanced antifouling properties compared to the pristine, which overmatches most of the state-of-the-art nanofiltration membranes. The proposed construction strategy may pave a novel avenue to develop ultrapermeable antifouling membranes for treating complex wastewater.

Enhancing the flux and salt rejection of thin-film composite nanofiltration membranes prepared on plasma-treated polyethylene using PVA/TS-1 composite

The high hydrophobicity of polyethylene (PE) film restricts its application as a thin-film composite (TFC) support layer. Therefore, in this study, oxygen plasma treatment was used to generate hydrophilic groups on PE and facilitate the formation of the TFC membrane. The difference in the performance of the plasma treated-PE (P-PE) membrane compared to untreated-PE indicated the significant effect of plasma pretreatment. In the following, to enhance the hydrophilicity of the P-PE membrane, titanium silicate-1 (TS-1) was synthesized as a zeotype through the hydrothermal method and added to the aqueous solution of the interfacial polymerization process. Due to the decrease in the agglomeration of TS-1 in the membrane matrix, PVA/TS-1 composites with distinct concentrations of polymer were prepared and applied for modifying membrane properties. Membranes were characterized by FESEM, ATR-FTIR, AFM, zeta potential, and water contact angle analyses. Permeability, rejection of monovalent and divalent salts, and antifouling ability of the fabricated nanofiltration membranes were studied and the optimal polymer concentration was determined. Comparison of P-PE-TFC 0.1 membrane with P-PE showed an increase of ~42% in pure water flux. Also, the rejection of Na2SO4, MgSO4, MgCl2, and NaCl salts were 90.28%, 81.77%, 74.79%, and 70.94% for P-PE-TFC 0.1 membrane, respectively, which showed a notable improvement compared to the P-PE membrane. Evaluation of antifouling ability showed that P-PE-TFC 0.1 membrane had a higher resistance to fouling compared to other membranes. Flux recovery ratio of this membrane after three cycles of bovine serum albumin (BSA) filtration was higher than other membranes resulting from the reduced roughness due to the combined effect of PVA and TS-1.

Elucidating ion transport mechanism in polyelectrolyte-complex membranes

Polyelectrolyte-complex (PEC) nanofiltration (NF) membranes attract much attention, however, the mechanisms governing ion separation in PEC films is not well understood. Here, we elucidate the ion transport in PECs using a recently reported Nafion-polyvinylamine (PVAm) membrane prepared via double-coating approach tuned to “rejection neutrality”, i.e., similar rejection of MgCl2 and Na2SO4 as single salts. New insights are gained by examining ion rejection for single- and mixed-salt solutions of NaCl, MgCl2 and Na2SO4 of varying concentrations and pH. The single salt permeability was found to vary with concentration, obeying a power law with an exponent around 0.4, matching neither the Donnan-dielectric nor a proposed PEC dissociation model. This is explained by progressive dissociation of the complex, which raises membrane swelling and dissociation constants, and weakenis dielectric exclusion, when salt concentration increases. Nevertheless, the membrane remains highly stable in all conditions, which is ascribed to the insolubility of Nafion in water. The results also indicate that “rejection-neutral” PEC still possesses a net negative charge, affecting ion selectivity at low salinities. The insights and physical picture proposed here may help understand and tune separation performance of PEC NF membranes and facilitate their implementation in applications such as purification and reuse of contaminated waters, resource recovery, and ion separations.

Highly permeable composite nanofiltration membrane via γ-cyclodextrin modulation for multiple applications

The treatment of pharmaceutical wastewater has become a worldwide problem. Highly efficient treatment for wastewater is extremely critical for ecological environment. Herein, a thin-film nanofibrous composite (TFNC) nanofiltration (NF) membrane modulated by γ-cyclodextrin (γ-CD) was prepared via interfacial polymerization. The γ-CD and piperazine (PIP) were adopted and acting as the mixed reaction monomers in aqueous phase for the fabrication of the separation layer. Not only PIP, γ-CD units would also react with organic phase trimesoyl chloride (TMC) at the interface and participate into the polymerization network. Compared with pure polyamide TFNC membranes, the γ-CD modulated TFNC membranes demonstrated thinner skin thickness. Owing to the 3D hollow structure of γ-CD, the self-contained through cavities would provide additional direct nanochannels to facilitate the water transport. Under 0.5 MPa, the optimized PIP0.5γCD0.5 and PIP0.5γCD1.5 TFNC membranes showed much higher permeate flux of 23.27 and 26.34 L m-2h−1 bar−1, respectively, than that of pure polyamide membranes (9.13 and 9.85 L m-2h−1 bar−1) and the Na2SO4 rejection remained high (>98.7%). Moreover, both TFNC membranes exhibited extremely high rejection (>99%) for several pharmaceuticals (doxycycline hydrochloride, tetracycline, and amoxicillin), and the PIP0.5γCD0.5 TFNC membrane exhibited a good separation factor (31.8) for NaCl/tetracycline solution. In addition, both optimized TFNC membranes showed good pressure resistance and long-term operating stability. This research revealed a facile modulation strategy for the fabrication of high-performance NF membrane and indicated the great promise for desalination, pharmaceuticals wastewater treatment and antibiotics concentration.