Rejection Of Heavy Metal Ions Research Articles

Incorporating etched ZnO nanoparticles to fabricate positively charged composite membrane for heavy metal removal

The removal of heavy metal ions by nanofiltration (NF) membranes is attractive, but it is limited by a trade-off effect between permeation and solute rejection. Generally, the permeability of the NF membrane is closely related to its hydrophilicity, while the solute rejection is closely related to size exclusion and the Donnan effect. In this study, porous etched zinc oxide (ZnO) nanoparticles were prepared through alkali etching, and a thin-film nanocomposite (TFN) NF membrane was fabricated by interfacial polymerization (IP) method between PEI and TMC on a polysulfone (PSF) substrate. The incorporation of 0.01 wt% etched ZnO nanoparticles increased the hydrophilicity and positive charge of the NF membrane, resulting in an increase in the permeability (from 6.0 L m−2 h−1 bar−1 to 8.1 L m−2 h−1 bar−1) and rejection of heavy metal ions (Cu2+, Zn2+, and Ni2+ increased from 85 % to over 96 %). Long-term filtration experiments using the simulated wastewater demonstrated that the prepared NF membrane exhibited good stability and had the potential for industrial applications. It opens a new pathway for studying TFN NF membranes, indicating that alkaline etching can transform smooth nanoparticles into nanoparticles with higher specific surface area and porosity, thereby improving the performance of membranes.

Regenerable planting of multifunctional amine: Regeneration-enhanced antifouling/antiscaling properties and performance of thin film composite nanofiltration membrane

The problems of poly(piperazineamide)-based thin film composite (TFC) nanofiltration (NF) membrane are the fouling, scaling and poor separation performance of uncharged organics and heavy metal ions. Permanent surface modification lowers the fouling/scaling problems. However, these problems persist as the separation progresses. We address these issues by the on-demand regenerable planting of polyethyleneimine (PEI) into the polyamide (PA) layer of TFC NF membrane. The regenerable planting is based on the electrostatic interaction among the protonated amine groups of low molecular weight PEI and ionized carboxylic acid groups situated at the relatively inner sites (low dielectric constant) of the PA network. The incorporated PEI chains are stable at the pH range of 3–9. The PEI incorporated membrane exhibits significantly enhanced antifouling/antiscaling property, rejection of heavy metal ions (98–99%) and bisphenol A (BPA, 95%) without much negatively affecting the bivalent anion rejection (Na2SO4, 97.5%) and water permeance (11 L m−2 h−1 bar−1) as compared to the pristine and commercial membranes (55–82%, 50–76% and 98–99% rejections of heavy metal ions, BPA and Na2SO4). Advantageously, PEI chains are removable at pH ≤ 2 or pH ≥ 9.5 without affecting the main PA layer. The multiple removal and re-generation of the membrane have been achieved. The flux recovery of the membrane after antifouling/antiscaling tests significantly improved upon intentional removal and reincorporation of PEI. Inherent properties of the modified membrane channels couple with the on-demand regenerable ability show immense promises of the membrane in the separation applications.

Acid mine drainage (AMD) treatment using a novel tannic acid functionalized Boehmite/polyether sulfone nanofiltration membrane

This study focuses on the fabrication of TA-BM/PES membranes by the incorporation of tannic acid functionalized Boehmite nanoparticles into the PES membrane matrix through the phase inversion technique. The presence of TA-BM nanoparticles in the TA-BM/PES membranes was confirmed by analysis of the FT-IR spectra of the embedded membranes. The hydrophilicity, permeability, surface morphology, and fouling resistance properties of membranes modified with TA-BM nanoparticles were compared to those of the naked membrane. The fouling behavior of the modified membranes was assessed in the presence of a foulant solution containing 200 ppm BSA (bovine serum albumin) and 200 ppm Na2SO4. The flux recovery ratio results showed an improvement in all modified membranes, especially M2 membrane. The modified membranes shown remarkable efficiency in the rejection of heavy metal ions (over 72 %), sulfate (over 80 %), and salt (Na2SO4 > 84 % and NaCl>48 %). The M2 membrane was chosen as the most suitable membrane and shown effective performance in eliminating heavy metals (Fe2+ = 65.5, Mn2+ = 88.8, Mg2+ = 83.5 and Ca2+ = 78.6 %) as well as sulfate (SO42− = 83.5 %) from a synthetic acid mine drainage (AMD) wastewater. Based on the findings presented in this research, it can be concluded that the TA-BM/PES nanofiltration membrane has potential for effective use in the treatment of AMD wastewater.

Zwitterionic cyclodextrin membrane with uniform subnanometre pores for high-efficient heavy metal ions removal

Polymer nanofiltration (NF) membranes have been widely applied in the desalination, resource substances purification and separation, and wastewater treatment. Nevertheless, direct construction of subnanometre channel polymer membranes for achieving high-efficient metal ions separation is still challenging because of the complicated membrane preparation and difficultly manipulating the membrane pore-size in sub-nanometer. Herein, we exploited a novel subnanometre channel membrane with zwitterionic β-cyclodextrins (ZCDs) by interfacial polymerization. Wherein ZCDs act as building blocks with hydrophobic inner subnanometre cavities and hydrophilic outer rims, allowing for simultaneously improved water permeability and metal ions selectivity. By modulating the zwitterionic monomer grafting ratio of ZCDs and their loading content in the membrane, the ZCDM membrane exhibits high water permeance (J = 21.7 L m−2 h−1∙bar−1) and heavy metal ions rejection, e.g., RCr3+ = 99.8%, RFe3+ = 98.9%, respectively, these values outperform the state-of-the-art NF membranes. Moreover, the ZCDM membrane has good stability and anti-fouling/-bacterial properties, e.g., the water flux recovery ratio and anti-bacterial ratio is >97% and >99%, in the long-term running process. This work offers a feasible strategy for constructing polymer membranes with uniform subnanometre pore, and demonstrates their potential for the heavy metal separation and wastewater treatment.

Photoelectrocatalytic modification of nanofiltration membranes with SrF2/Ti3C2Tx to simultaneously enhance heavy metal ions rejection and permeability

Heavy metal pollution can significantly harm water systems and human health. Combining photoelectrocatalytic (PEC) and nanofiltration (NF) membrane separation technologies can effectively remove heavy metal ions from wastewater. In this study, a water bath method was used to form SrF2/Ti3C2Tx (ST) nanoparticles on the surface of polyvinylidene fluoride (PVDF) membranes and an additional polyamide (PA) functional layer was formed at the interface by crosslinking. ST@PA composite NF membranes (STPP) with good photocatalytic performance were obtained. The separation and catalytic properties of the STPP membranes were controlled by the ST content, which modifies the surface structure and properties of the membranes. The membrane with optimal ST crosslinking exhibited a water contact angle of 50.8°, pure water flux of 24.6 L·m−2·h−1·bar−1, and rejection rates of Mn2+, Ni2+, Cu2+, and Zn2+ of 98.8%, 95.3%, 95.7%, and 97.3%, respectively, under PEC-assisted separation with visible light illumination from a Xe lamp (300 W) and an applied voltage (2 V). The STPP membranes showed improved rejection rates of heavy metal ions under PEC-assisted operation. The mechanism for the improved membrane performance under PEC conditions was preliminarily clarified considering the relationship between the photocatalytic and filtration properties of STPP membranes along with the influence of light irradiation and an external voltage on the heavy metal ions. The generation of electrons, holes, superoxide radicals, and hydroxyl radicals during membrane operation enhances the rejection rates of heavy metal ions. Based on these results, STPP membranes are considered a promising technology for industrial applications in heavy metal removal.

Surface modification of thin film composite nanofiltration membrane with graphene oxide by varying amine linkers: Synthesis, characterization, and applications

We report surface modifications of thin film composite nanofiltration (TFC NF) membrane with amine and the characterization of the resulting membrane for anti-biofouling properties. Graphene oxide (GO) was grafted on the surface of the TFC NF membrane using three linkers, ethylene diamine, diethylene triamine and triethylene tetraamine. Grafting of GO on the surface was successfully validated through Raman Spectroscopy and XPS. Ring-like structures were observed on the surface of the dense polyamide layer of TFC NF membranes under SEM after modification with GO. A slight increase in the roughness, and hydrophilicity was noticed in the membranes modified with GO. GO-grafted membranes demonstrated enhanced chlorine stability, lower biofouling tendency, lower flux decline during filtration of seawater and improved hexavalent chromium rejection in the basic pH. The improved chlorine stability is attributed to intermolecular hydrogen bonding between GO and polyamide structure, while the enhanced biofouling resistance and lower flux decline is ascribed to hydrophilic GO. The membranes were further evaluated for rejection of heavy metal ions, in particular hexavalent chromium, which showed about 97% rejection in basic pH attributed to the rise in negative character of the membrane surface due to the grafting of GO. Na2SO4 rejection for control TFC was reduced by 9.6%, while a reduction between 1 and 5.5% was found in fGO grafted membranes after 1 h exposure to 1000 ppm NaOCl.

Double-charged self-assembled rGO/g-C3N4 membrane prepared by “functional group substitution” for heavy metal ions rejection at low pressure

Heavy metals are still the critical pollutants in industrial wastewater and there is an urgent need for efficient and environmentally friendly treatment technologies. Reduced graphene oxide (rGO) is widely used for preparations of nanofiltration (NF) membranes but suffers from poor hydrophilicity and electronegativity. In this work, a double-charged rGO/g-C3N4-P membrane was prepared for removal of heavy metals at low pressure. Graphitic carbon nitride (g-C3N4) assisted reduction of GO membranes under ultraviolet (UV) irradiation, and the modification of functional groups with high polarity improved the hydrophilicity of membrane surface. The filtration performance for heavy metals of rGO/g-C3N4-P membrane was evaluated under low pressure (1–2 bar). The rejection rates of Cu2+, Cr3+, Mn2+, Cd2+, and Pb2+ by membranes reduced by UV for 18 h (rGO/g-C3N4–18-P membrane) reached 94.72 %, 98.05 %, 82.32 %, 88.2 % and 77.15 %, respectively. In the experiment of mixed simulated wastewater, the rejection rates of NO3− and SO42− both reached >95 %. Outstanding rejection rates were attributed to the interaction and the synergy effect of double-charged layers as well as steric effects. Meanwhile, the water flux of rGO/g-C3N4–18-P membrane was as high as 37.14–50.16 L m−2h−1bar−1 (1–2 bar). The high flux was due to the reduced degree of oxidation so that water molecules transported between GO nanochannels more smoothly and the transport path was shortened through the nanopores of g-C3N4. Obviously, flux and heavy metal rejection of rGO/g-C3N4–18-P membrane were simultaneously improved, breaking “trade-off” effect. Furthermore, rGO/g-C3N4–18-P membrane showed excellent antifouling ability and the potential for heavy metal wastewater filtration in comparison with other NF membranes reported in literature.

Foaming of oxidized nanocellulose for the preparation of high-flux water filters

Treatment of polluted water is an important task to secure access to clean water also for future generations. Filters are an efficient means to reject various pollutants on a wide range of size scales either by size-exclusion or electrostatic interaction, respectively. Commonly, filters and membranes from various synthetic materials are employed for these applications. Recently, filters based on renewable (nano) cellulose papers and coatings emerged as sustainable alternative to synthetic materials usually utilized. However, fabrication of such paper network structures from aqueous suspension by filtration processes is a time-consuming process caused by the high water holding capacity of highly hydrophilic and negatively charged nanocellulose fibrils. To optimize the preparation of nanocellulose coated filters, substitution of water by air and thus generating nanocellulose foams that are collapsed onto a substrate would be an appealing approach. Here we present the development of foams from negatively charged TEMPO-oxidized nanocellulose by screening various surfactants and concentrations to generate a foam stable enough to be transferred onto a viscose substrate. Foams were collapsed by oven consolidation, positive pressure filtration, or hot-pressing, respectively. Consolidated filters were tested for their water permeance and rejection of heavy metal ions using copper ions as model system. Very high permeances competitive to commercial filters based on synthetic polymers were achieved. Furthermore, adsorption capacities for copper of up to 70 mg/g were found. This is close to adsorption capacities reported for negatively charged TEMPO-oxidized nanocellulose in conventional batch-wise static adsorption. However, in the current process adsorption takes place during filtration of water through filters in a continuous process which constitutes a tremendous advantage.Graphical

Fabrication of superhydrophilic/underwater superoleophobic functionalized-nanoparticles/PVDF-supported thin film composite polyamide membranes for efficient water purification

For water treatment, membrane-based separation methods are very essential owing to long-term efficiency, high mechanical stability and durability. The filtration process with polymer-inorganic and organic composite is an outstanding strategy that enhances the properties of both inorganic materials such as thermal and mechanical stability and organic material including flexibility and processability of the membranes. Therefore, the surface wettability of the membranes is vital for pollutants separation. Here, the high hydrophobic behavior of polyvinylidene fluoride (PVDF) membranes is altered to superhydrophilicity and underwater superoleophobicity via single-step coating. This PVDF membrane has high molecular separation due to the ultrathin thickness and good controllable interlayer. In this study, the PVDF membrane was by interfacial polymerization coated with a polyamide (PAm) layer, modified alumina nanoparticles (Al-NPs), (aminopropyl)triethoxysilane (APTES), and folic acid (FA) through a chemical grafting reaction. The influence of the PAm and Al-APTES-FA content on the morphology, structure, antifouling, and superhydrophilicity of the PVDF-g-PAm@Al-APTES-FA membrane has been investigated which has enhanced the oil and salt rejection as well as filtration performance. Fourier Transform Infrared (FTIR) spectroscopy showed the specific functional groups from the obtained results indicating the successful preparation. Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray (EDX), and AFM (Atomic Force Microscope) techniques were conducted to observe the membrane surface morphology, roughness, and elemental adsorption after the filtration process. Furthermore, this modified PVDF membrane exhibited an excellent superoleophobicity with underwater oil contact angle (OCA~145°), and superhydrophilic visualization with water contact angle (WCA ~ 0°) after being functionalized. Additionally, the parameters of membrane performance such as permeate flux, heavy metal ions, oil and salts rejection, were assessed. The modified material on the PVDF membrane enhanced the membrane efficiency under high operating pressure of 10 bar with 53 LMH (L.m−2 h−1, while it is ≈0 for PVDF membrane), permeate flux, over 90 % salt rejection, and almost 100 % hydrocarbons and heavy metal ions rejection, owing to membrane surface changes, hydrophilicity, and roughness. This research will provide new insights on porous surface transformation to improve the separation and antifouling properties of PVDF membranes used in water purification.

Fabrication of hollow fiber nanofiltration separation layer with highly positively charged surface for heavy metal ion removal

Efficient concentration and separation of heavy metal ions from wastewater are crucial for environmental protection and resource recovery. In this research, a double highly positively charged nanofiltration (NF) separation layer was constructed on the outer surface of hollow fiber membranes for the first time via surface grafting and iodomethane (CH3I) quaternization. The separating properties for Ni2+, Cu2+ and Cr3+ were measured at different concentrations and pressures, and the long-term stability and anti-fouling property of the membrane were investigated. The relationship between the positively charged degree of surface, pore size, and rejection of heavy metal ions were interpreted by comparing the discrepancies in microstructures and separating properties of membranes. The membrane can efficaciously remove Ni2+ (99.4%), Cu2+ (99.6%), and Cr3+ (99.9%) while maintaining a high permeability (30.9, 29.6 and 28.5 L m−2 h−1 bar−1). The high permeability (about 27.9 L m−2 h−1 bar−1) and rejection (>99%) were retained even after 72 h of operation. Moreover, the membrane exhibited a good anti-fouling property for heavy metal ions. This study offers the reference for preparing hollow fiber NF membrane with stable microstructure, highly positively charged surface and high efficiency for the removal of heavy metal ions.

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.

Rejection Capacity of Nanofiltration Membranes for Nickel, Copper, Silver and Palladium at Various Oxidation States.

Electroplating and metalworking industries produce enormous amounts of waste containing heavy metals in their effluents, leading to potential threats to biotic and abiotic life. According to regulation, heavy metal contamination must be kept within the regulated standard of a few parts per million, which has led to a recent pique in interest in the utilization of nanofiltration technology for metal recovery. The effect of feed pH, pressure, metal concentration, and oxidation of metal on the rejection of heavy metal ions using three commercial nanofiltration membranes (NF, NF90, and NF270) were explored. To begin, studies of electrolyte salts, contact angle, and water permeability were employed to characterize the nanofiltration membranes. A dead-end module was used to test the permeation and retention capacities of the nanofiltration membranes. The results showed an increase in salt rejection for all metals examined irrespective of the membrane, at a pH below the isoelectric point. For divalent cations, the NF90 membrane achieved recovery capacities of 97% and 85% at 200 ppm and 20 ppm respectively, as compared to the recovery observed for Ni2+, Cu2+, and Pd2+ ions by NF and NF270. At a pH 2, 20 ppm and 5 bar, the NF90 membrane had the highest percent recovery, but at a pH 3, the recovery was at 95%. Mono and divalent stable Ag+ and Ni2+ ions showed a comparatively high percent recovery as compared to Pd2+ and Cu2+, which have high molecular weight and charge effect. In the presence of chelating agents, the membrane surface area is increased, resulting in high divalent ion recovery capacities due to favourable interaction with the polyamide functional group of the membranes. This study establishes the significance of oxidation in high removal efficiency cation in varying experimental conditions.

Highly selective heavy metal ions membranes combining sulfonated polyethersulfone and self-assembled manganese oxide nanosheets on positively functionalized graphene oxide nanosheets

This study presents the synthesis of a novel graphene oxide-manganese oxide (GO-MnO2) nanohybrid and its incorporation into sulfonated polyethersulfone (SPES) ultrafiltration (UF) membranes for wastewater treatment applications. The nanohybrid was first prepared by grafting ethylenediamine (ED) onto the edge of GO followed by direct mixing with the MnO2 nanosheets. The composite membranes were prepared via the phase inversion method and optimized by varying the concentration of the GO-MnO2 nanohybrid (0–6 wt%). The membranes were characterized using scanning electron microscopy, surface zeta potential, water flux, porosity, among others. The results showed that the pure water flux increased from 59.5 ± 2.5 L·m−2·h−1 in the pristine SPES membrane to 129.7 ± 4.1 L·m−2·h−1 in the SPGM4 (4 wt% GO-MnO2). Furthermore, the heavy metal ions rejection improved from 70.1, 49.1, and 55.8% for Cu2+, Zn2+, and Ni2+ ions, respectively, in the pristine SPES membrane to 81.1, 64.0, and 67.4% in the SPGM4 membrane. Also, the composite membranes revealed improvement in the anti-fouling properties over the pristine SPES membrane. For example, the SPGM4 membrane recovered 90.5 ± 2.9% of its initial flux compared to only 72.6 ± 3.1% in the pristine membrane after 4 cycles of heavy metal filtration and simple acid cleaning steps. Overall, the addition of GO-MnO2 nanohybrid enhanced the properties of the pristine SPES, creating new potential for such composite membranes in the wastewater treatment industry.

Impact of graphitic carbon nitride nanosheets in mixed- matrix membranes for removal of heavy metals from water

Removal of heavy metal ions from water is being a challenge and Polysulfone (PSf) membranes have shown great potential to remove them from contaminated solutions. In this work, the introduction of Graphitic carbon nitride nanosheets (g-C3N4) into PSf membranes was implemented to improve the permeability and separation performance of PSf membranes. g-C3N4 was incorporated into the membrane matrix via nonsolvent induced phase inversion method. The prepared mixed matrix membranes showed enhanced performances towards water filtration. The incorporation of g-C3N4 into the membrane matrix caused an increase in the desired physicochemical properties like hydrophilicity and surface charge density. This in turn enhanced the rejection of heavy metal ions. The presence of free –NH2 groups on the surface of the membrane helps in increasing the water permeability of the mixed matrix membranes. At 1 bar pressure, the mixed matrix membranes showed rejections higher than 95, 80, and 70 % for lead, cadmium, and arsenic, respectively. Anti-fouling properties of PSf membrane were also enhanced by g-C3N4, which was due to a notable increase in hydrophilicity and the formation of hydration layer on the membrane, preventing interactions between membrane surface and Bovine Serum Albumin (BSA). These properties could be favourable in fulfilling the need of a potential candidate for filtration of waters contaminated by toxic ions.

Development of the tetrathioterephthalate filler incorporated PES nanofiltration membrane with efficient heavy metal ions rejection and superior antifouling properties

The novel antifouling nanofiltration membranes were developed for efficient heavy metal ions rejection from water. These membranes were fabricated by the introduction of the tetrathioterephthalate (TTTP) filler into Polyethersulfone (PES) matrix via the phase inversion. Surface roughness, pore radius distribution, average pore radius, surface charge, overall porosity, and water contact angle were also characterized. FTIR and XPS analysis were performed to confirm the presence of TTTP filler in the TTTP embedded PES (T-PES) membranes. As confirmed by FESEM images, more finger-like voids with higher connectivity were formed in the presence of TTTP fillers in the membrane matrix. Compared with the PES membrane, the T-PES membranes exhibited higher hydrophilicity, porosity, and mean pore radius. Besides, these membranes exhibited higher pure water permeation flux and superior antifouling properties. For 0.1 wt.% TTTP embedded PES membrane the FRR value stayed at a high level during five repeated filtration cycles. This improvement was mainly ascribed to the hydrophilic quiddity of TTTP fillers resulting from its functionalities like C = S and C–S bonds. Furthermore, these membranes showed high rejection towards heavy metal ions (Zn2+, 99.2%; Cu2+, 98.5%; Pb2+, 97.4%). Moreover, the metal ion removal experiments in the presence of foreign ions of Fe3+, Mg2+, and Na+ indicated acceptable results.

Polydopamine nanoparticles modified nanofiber supported thin film composite membrane with enhanced adhesion strength for forward osmosis

To minimize the internal polarization concentration (ICP) in FO process and improve the structural stability of thin film composite (TFC) forward osmosis (FO) membrane, a reactable hydrophilic nanofiber substrate was prepared by vacuum filtrating an interlayer composed of polydopamine nanoparticles (PDA NPs) onto an electrospun polyacrylonitrile nanofiber substrate. Then a TFC FO membrane with dense selective layer, low structure parameter, high water permeability and low salt permeability was prepared by the interfacial polymerization thereon. According to the T-peel test, the resulting TFC FO membrane showed greatly improved adhesion strength between selective layer and substrate due to the chemical bonding and entanglement between polyamides and the PDA NPs, and the strong adhesion between PDA NPs and the nanofibers. The ICP in FO process for the resulting membranes were minimized due to the application of the nanofiber substrates with low structure parameter (in the range of 290 ± 36 μm–354 ± 20 μm). These FO membranes performed well in the heavy metal ions contained water treatment, showed 1–2 times higher water flux (29.2 ± 0.3 L/m2h) compared with commercial TFC membrane (HTI-TFC) and high heavy metal ions rejection (99.5% ± 0.4%, 99.1% ± 0.5%, 98.1% ± 0.7% for Cr3+, Cu2+ and Ni2+, respectively) when using 2.0 M NaCl as draw solution. The findings provide the pathway for the design of nanofiber based FO membrane with improved structure stability and high performance by interfacial polymerization.

Voltage-enhanced ion sieving and rejection of Pb2+ through a thermally cross-linked two-dimensional MXene membrane

Highly stable and anti-swelling two-dimensional (2D) MXene (Ti2C3Tx) membranes can be applied for ion sieving and water purification under external voltage. This paper shows the effective ion sieving of mixed K+/Pb2+ pairs and the high rejection of the heavy metal ions (Pb2+) through a thermal cross-linked 2D MXene membrane by exerting external voltage. Due to the dehydration and cross-linking of hydroxyls during the drying process, the interlayer spacing of MXene membrane is tuned to be 6.7 ~ 6.92 Å, which lies in the selective ion sieving size of the mixed K+ and Pb2+. The rejection of hydrated Pb2+ is up to 99%, while the hydrated K+ can smoothly transport the channels of MXene membrane under external voltage. For 365-nm-thick MXene membrane, the separator factor of mixed K+ and Pb2+ reaches up to 78 under an applied voltage of 16.5 V. Furthermore, the application of external voltage doesn’t result in swelling of MXene membrane, showing stable separation of K+/Pb2+ pairs with effective rejection of Pb2+. The high rejection of Pb2+ and good anti-swelling demonstrate the stability of the resultant MXene membranes. Our work indicates that the thermal cross-linked MXene membrane is a promising alternative for ion sieving and the rejection of heavy metal ions.

How to fabricate a negatively charged NF membrane for heavy metal removal via the interfacial polymerization between PIP and TMC?

The potential toxicity of heavy metals arising from the discharge of industrial wastewater has become a serious public concern. Fabrication of a thin film composite (TFC) NF membrane with desirable rejection of heavy metal ions is an attracting route to solve this problem. Hereon, we proposed a facile strategy for fabricating a TFC NF membrane with excellent heavy metal removal. The ratio of PIP and TMC was greatly enlarged by decreasing the concentration of TMC to an exceptional low extent. Thus a polyamide film with weak electronegativity and small pore size was successfully constructed on an ultrafiltration substrate membrane. Comprehensive characterizations were adopted to reveal the micro-variation of the resulted NF membranes when regulating TMC concentration. The optimized membrane prepared only with PIP and TMC can exhibit a high pure water permeability of 14.6 L m−2 h−1 bar−1, desirable rejection of over 98% against Ni2+, Zn2+, Cu2+, and >92.7% against Cd2+. This performance could go beyond most of the reported candidates which were prepared with other complicated methods. The simplicity, low cost and scaling up feasibility of the current strategy is expected to well meet the requirements of industrial production of TFC NF membrane for heavy metal removal.

Polysulfone membranes with CNTs/Chitosan biopolymer nanocomposite as selective layer for remarkable heavy metal ions rejection capacity

Chitosan (CHIT) biopolymer has a remarkable adsorption capacity for heavy metal ions. The incorporation of multiwall carbon nanotubes (CNTs) into the CHIT matrix should enhance the water permeability and separation performance of the CHIT. The use of carboxylic acid functionalised multiwall carbon nanotubes (CNTS-COOH) as an additive is aimed at adding the COOH functional group to the CHIT matrix without blocking its adsorption sites. Therefore, two nanocomposites CNTs-COOH/CHIT and CNTs/CHIT are used as a selective layer on top of polysulfone (PS) to form CNTs-COOH/CHIT/PS and CNTs/CHIT/PS membranes. To the best of our knowledge, the efficiency of the CNTs-COOH/CHIT nanocomposite as a selective layer on polysulfone sheet to enhance the membrane’s rejection for metal ions is reported for the first time. Both fabricated membranes are investigated in terms of morphology, wettability, surface charge, porosity and water permeability. CNTs-COOH/CHIT/PS membrane recorded water flux rate 64.4 L m−2 h−1 at transmembrane pressure 22 bar, where CNTs/CHIT/PS membrane recorded 57.15 L m−2 h−1 at transmembrane pressure 22 bar. Heavy metal ions rejection capabilities of both membranes are evaluated at various pHs and transmembrane pressures. Both CNTs-COOH/CHIT/PS and CNTs/CHIT/PS membranes effectively reject Cu+2, Ni+2, Pb+2, Cd+2, and Co+2 ions from aqueous solutions. At pH 3, CNTs-COOH/CHIT/PS and CNTs/CHIT/PS membranes presented significant rejection ranged 71–92.2% for examined metal ions. Where at pH range (6–8), Co+2 rejection reached up to 92.2% and 89.6% by CNTs /CHIT/PS and CNTs-COOH/CHIT/PS membranes, respectively. Overall, the CNTs-COOH/CHIT/PS membrane shows metal ions rejection superior to that of the CNTs/CHIT/PS membrane.

Photothermal-Responsive Graphene Oxide Membrane with Smart Gates for Water Purification.

In this work, photothermal-responsive graphene oxide (GO)-based membranes intercalated by multiwalled carbon nanotubes (MWCNTs) were prepared by polydopamine (PDA) modification and 2-ureido-4[1H]-pyrimidinone (UPy) assembly process. Benefited from the high photothermal conversion ability of PDA and reversible UPy dimer, the permeation flux of the GO-based membrane can be easily modulated with a high gating ratio under near-infrared light switched off and on. Nanochannels created by the intercalation of MWCNTs into GO layers greatly increase water permeation without sacrificing the rejection of dye molecules in water (96.2% for methyl orange, 98.9% for rhodamine B, and 99.7% for Coomassie brilliant blue). The composite membranes also exhibit trade-off between rejection of heavy metal ions (Cu2+ and Fe3+). Moreover, such GO-based membranes show high pH stabilities, which show great potential in the water purification applications.