Hydroxyl functionalized polytriazole-co-polyoxadiazole as substrates for forward osmosis membranes

Research progress in external field intensification of forward osmosis process for water treatment: A critical review

Forward osmosis (FO) is an emerging permeation-driven membrane technology that manifests advantages of low energy consumption, low operating pressure, and uncomplicated engineering compared to conventional membrane processes. The key issues that need to be addressed in FO are membrane fouling, concentration polarization (CP) and reverse solute diffusion (RSD). They can lead to problems about loss of draw solutes and reduced membrane lifetime, which not only affect the water treatment effectiveness of FO membranes, but also increase the economic cost. Current research has focused on FO membrane preparation and modification strategies, as well as on the selection of draw solutions. Unfortunately, these intrinsic solutions had limited success in unraveling these phenomena. In this paper, we provide a brief review of the current state of research on existing external field-assisted FO systems (including electric-, pressure-, magnetic-, ultrasonic-, light- and flow-assisted FO system), analyze their mitigation mechanisms for the above key problems, and explore potential research directions to aid in the further development of FO systems. This review aims to reveal the feasibility of the development of external field-assisted FO technology to achieve a more economical and efficient FO treatment process.

Design of microstructure for hollow fiber loose nanofiltration separation layer and its compactness-tailoring mechanism

In order to promote the application of membrane technology in the treatment of textile wastewater containing small molecule dye, fabricating a hollow fiber loose nanofiltration (LNF) with a thin and compact separation layer and deepening the understanding of compactness-tailoring mechanism in chemical crosslinking are essential. Firstly, the mechanisms of synergistic crosslinking of PEI-70K and PEI-10K, along with a weakening of the PEI hydration by ethanol, were expounded in primary crosslinking. Then, some LNF separation layers with different compactness were prepared through crosslinking with different crosslinkers to further reduce pore size, which resulted in the efficient removal (~100%) of a small molecular dye (methyl orange (MO), M = 327 g mol^-1). The removal of methyl orange is mainly caused by size sieving. The relationship among the pore size, the M_w of the secondary crosslinkers, and the pore size reduction rate was interpreted by comparing the pore size reduction rate of three secondary crosslinkers with different molecular weights. In addition, the as-prepared separation layer exhibited excellent dimensional stability and solvent resistance. This paper not only provides a reference for fabricating hollow fiber LNF with better purification performance, but also shows their potential in developing solvent resistant nanofiltration.

Reduction of the Structure Parameter of Forward Osmosis Membranes by Using Sodium Bicarbonate as Pore-Forming Agent

The forward osmosis (FO) process suffers from unfavorable internal concentration polarization (ICP) of the solute within the support layer of thin-film composite forward osmosis (TFC-FO) membranes. To lower the ICP effect, a support layer with low tortuosity, high porosity, and interconnected pores is necessary. In the present investigation, sodium bicarbonate has been presented as a simple pore-forming agent to decline the ICP within a poly(ethersulfone) substrate. In particular, the porous poly(ethersulfone) support layer was fabricated by embedding sodium bicarbonate into the casting solution to form CO₂ gas bubbles in the substrate during phase inversion in an acidic nonsolvent. Experimental results revealed that the separation performance of the TFC-FO membranes significantly improved. The most water-permeable membrane was prepared in the acidic nonsolvent (TFC-SB.3) and it demonstrated a water flux of 26.6 LMH and a reverse salt flux of 3.6 gMH in the FO test. In addition, the TFC-SB.3 membrane showed an 85% increase in water permeability (2.13 LMH/bar) with negligible change in salt rejection (94.3%). Such observations were based on the increase of substrate porosity and the improved connectivity of the finger-like channels through in situ CO₂ gas bubbling that alleviate the ICP phenomena. Therefore, the current study presents a simple, scalable method to design a high-performance TFC-FO membrane.

Membrane Surface Functionalization with Imidazole Derivatives to Benefit Dye Removal and Fouling Resistance in Forward Osmosis

Water contaminated with low concentrations of pollutants is more difficult to clean up than that with high pollutant content levels. Membrane separation provides a solution for removing low pollutant content from water. However, membranes are prone to fouling, losing separation performances over time. Here we synthesized neutral (IM-NH₂) and positively charged (IL-NH₂) imidazole derivatives to chemically functionalize membranes. With distinct properties, these imidazole grafts could tailor membrane physicochemical properties and structures to benefit forward osmosis (FO) processes for the removal of 20-100 ppm of Safranin O dye-a common dye employed in the textile industry. The water fluxes produced by IM-NH₂- and IL-NH₂-modified membranes increased by 67% and 122%, respectively, with DI water as the feed compared to that with the nascent membrane. A 39% flux increment with complete dye retention (∼100%) was achieved for the IL-NH₂-modified membrane against 100 ppm of Safranin O dye. Regardless of the dye concentration, the IL-NH₂-modified membrane exhibited steadily higher permeation performance than the original membrane in long-term experiments. Reproducible experimental results were obtained with the IL-NH₂-modified membrane after cleaning with DI water, demonstrating the good antifouling properties and renewability of the newly developed membrane.

Polymer Having Dicationic Structure in Dumbbell Shape for Forward Osmosis Process

The thermal-responsive polymers, poly(alkane-1,#-diylbis(tri-n-butylphosphonium) 4-vinylbenzenesulfonate) (PSSBP#, # = 8, 6, and 4), where # is the number of carbon atoms in the central bridge structure of the dicationic phosphonium moiety, were synthesized to examine their potential application as draw solutes in forward osmosis (FO). The polymers exhibited low critical solution temperature (LCST) characteristics in aqueous solutions, which is essential for recovering a draw solute from pure water. The LCSTs of the 20 wt% aqueous solutions of PSSBP8, PSSBP6, and PSSBP4 were confirmed to be approximately 30, 38, and 26 °C, respectively, which is advantageous in terms of energy requirements for the recovering draw solute. When the concentration of the PSSBP4 draw solution was 20 wt%, water flux and reverse solute flux were approximately 1.61 LMH and 0.91 gMH, respectively, in the active layer facing the draw solution (AL-DS) system when the feed solution was distilled water. The PSSBP# thermal-responsive draw solute has considerable potential for use as a next-generation draw solute because of its excellent osmotic performance and efficient recovery. Therefore, this study provides inspiration for novel ideas regarding structural transformations of polymers and their applicability as draw solutes.

Tackle reverse solute flux in forward osmosis towards sustainable water recovery: reduction and perspectives

Forward osmosis (FO) has emerged as a potentially energy-efficient membrane treatment technology to yield high-quality reusable water from various wastewater/saline water sources. A key challenge remained to be solved for FO is reverse solute flux (RSF), which can cause issues like reduced concentration gradient and loss of draw solutes. Yet no universal parameters have been developed to compare RSF control performance among various studies, making it difficult to position us in this "battle" against RSF. In this paper, we have conducted a concise review of existing RSF reduction approaches, including operational strategies (e.g., pressure-, electrolysis-, and ultrasound-assisted osmosis) and advanced membrane development (e.g., new membrane fabrication and existing membrane modification). We have also analyzed the literature data to reveal the current status of RSF reduction. A new parameter, mitigation ratio (MR), was proposed and used together with specific RSF (SRSF) to evaluate RSF reduction performance. Potential research directions have been discussed to help with future RSF control. This review intends to shed more light on how to effectively tackle solute leakage towards a more cost-effective and environmental-friendly FO treatment process.

A DNA-mimic contact-active functional group for antifouling ultrafiltration membranes

Despite advanced materials and techniques to reduce the fouling issue of membranes, 10-30% of the cost of ultrafiltration (UF) processes have been spent on membrane cleaning. Particularly in water treatment, the traditional heavy metal-based method is challenged due to its environmental pollution risk and increasing public health awareness. Here, we report the synthesis of a metal-free contact-active antifouling and antimicrobial membrane by covalently functionalizing a commercial polyacrylonitrile (PAN) UF membrane with 2,4-diamino-1,3,5-triazine (DAT) via a one-step catalyst-free hydrothermal [4 + 2] cyclization of dicyandiamide reaction. The proposed mechanism of the antimicrobial activity of the DAT-functionalized membrane is through strong attraction between the DAT groups and the microbial membrane protein via strong hydrogen bonding, leading to microbial membrane disruption and thus microbe death. A high water flux and good reusability of the membrane against protein in a UF experiment were achieved. The low cost, easy availability of the compounds, as well as the facile reaction offer a high potential of the membrane for real applications in ultrafiltration.

Thermo-responsive draw solute for forward osmosis process; poly(ionic liquid) having lower critical solution temperature characteristics

A poly(ionic liquid) having lower critical solution temperature characteristics was synthesized to investigate its suitability as a draw solute for forward osmosis.

A low-cost and high-performance thin-film composite forward osmosis membrane based on an SPSU/PVC substrate

A low-cost sulfonated polysulfone (SPSU)/poly(vinyl chloride) (PVC) substrate based high-performance thin-film composite (TFC) forward osmosis (FO) membrane was fabricated in this work. The results showed that the morphologies of the substrates were looser and more porous, and the porosity, pure water permeability, surface hydrophilicity, and average pore size of the substrates significantly improved after the SPSU was introduced into the PVC substrates. Furthermore, the SPSU/PVC-based TFC membranes exhibited rougher, looser and less crosslinked polyamide active layers than the neat PVC-based TFC membrane. The water permeability obviously increased, and the structure parameter dramatically declined. Moreover, the FO performance significantly improved (e.g. the water flux of TFC2.5 reached 25.53/48.37 LMH under FO/PRO mode by using 1.0 M NaCl/DI water as the draw/feed solution, while the specific salt flux exhibited a low value of 0.10/0.09 g/L). According to the results, it can be concluded that 2.5% of SPSU was the optimal blend ratio, which exhibited the lowest sulfonated material blend ratio compared to the data reported in the literature. Hence, this is a feasible and low-cost fabrication approach for high-performance FO membrane by using the cheap PVC and low blend-ratio SPSU as the membrane materials.

Antifouling enhancement of polyimide membrane by grafting DEDA-PS zwitterions

In order to improve the water flux and antifouling property of polyimide (PI) membrane, zwitterions are grafted on PI membrane surface via a two-step modification route by reactions with N,N-diethylethylenediamine (DEDA) and 1,3-propane sultone (PS) sequentially. The reaction mechanism and physicochemical properties of membranes are confirmed via various characterization techniques. The anti-biofouling performance of the zwitterion-grafted PI membranes is evaluated by bacterial suspension immersion tests in Escherichia coli (E. coli) and staphylococcus aureus (S. aureus) solutions. The antifouling property is assessed via the filtration test using the bovine serum albumin (BSA) and dodecyl trimethyl ammonium bromide (DTAB) aqueous feed solutions. The effect of the reaction time with DEDA in the zwitterion-grafted process on the antifouling property is further investigated systematically. The results show that both the anti-biofouling and antifouling performances of zwitterion-grafted PI membranes are significantly improved.

Mesh-Embedded Polysulfone/Sulfonated Polysulfone Supported Thin Film Composite Membranes for Forward Osmosis

In this work, mesh-embedded polysulfone (PSU)/sulfonated polysulfone (sPSU) supported thin film composite (TFC) membranes were developed for forward osmosis (FO). The robust mesh integrated in PSU/sPSU sublayer imparts impressive mechanical durability. The blending of hydrophilic sPSU in PSU sublayer affects the hydrophilicity, porosity, pore structure, and pore size of mesh-embedded PSU/sPSU substrates, and the total thickness, cross-linking degree, and roughness of the corresponding TFC-FO membrane active layers. An appropriate incorporation of sPSU not only significantly decreases the structural parameter, S of the mesh-embedded substrate to 220 μm, which is the lowest reported value for fabric backed FO membrane, but also optimizes the permselectivity of the formed active layer. Regarding the osmosis performance, TFC membranes with sPSU modified substrates gain a higher water flux (J_w) while keeping the specific reverse salt flux (J_s/J_w) low. The optimal TFC-FO membrane has a J_w of 31.76 LMH with J_s/J_w of 0.19 g/L in FO mode when using deionized water feed and 1 M NaCl draw solution. This paper is practical for developing TFC-FO membrane on hydrophilic support membrane materials.

Synthetic polymer materials for forward osmosis (FO) membranes and FO applications: a review

Forward osmosis (FO) has played an important role in alleviating the problems caused by freshwater shortage and water contamination in recent years. However, issues of low water permeability, reverse solute diffusion, concentration polarization and membrane fouling are still widely present in FO processes. These challenges are the current research focus in exploring novel FO membranes. Fabricating FO membranes from chemically modified commercial polymers is a relatively novel approach and has proven effective in obtaining appropriate FO membranes. This paper focuses on the progress of FO membranes made specially from chemically modified polymer materials. First of all, a brief overview of commercial polymers commonly used for FO membrane fabrication is provided. Secondly, the chemical modification strategies and synthesis routes of novel polymer materials as well as the resultant FO membrane performance are presented. The strengths and weaknesses of chemical modifications on polymer materials are assessed. Then, typical FO applications facilitated by the FO membranes made from modified polymer materials are exemplified. Finally, challenges and future directions in exploring novel polymers through chemical modifications for FO membrane fabrication are highlighted. This review may provide new insights into the future advancement of both novel membrane materials and FO membranes.

High-performance thin-film composite forward osmosis membrane fabricated on low-cost PVB/PVC substrate

The incorporation of the PVB significantly improved the performance of the PVB/PVC substrates based thin-film composite forward osmosis membrane.

Zwitterionic polymers showing upper critical solution temperature behavior as draw solutes for forward osmosis

We investigated the suitability of homopolymer with UCST characteristics as draw solutes for the FO process for the first time.

Synthesis and Application of Organic Phosphonate Salts as Draw Solutes in Forward Osmosis for Oil-Water Separation

The development of suitable draw solution in forward osmosis (FO) process has attracted the growing attention for water treatment purpose. In this study, a series of organic phosphonate salts (OPSs) are synthesized by one-step Mannich-like reaction, confirmed by FTIR and NMR characterizations, and applied as novel draw solutes in FO applications. Their solution properties including osmotic pressures and viscosities, as well as their FO performance as a function of the solution concentration are investigated systematically. In FO process, a higher water flux of 47-54 LMH and a negligible reverse solute flux can be achieved in the PRO (AL-DS) mode (active layer faces the draw solution) using a homemade thin-film composite membrane (PSF-TFC) and deionized water as the feed solution. Among all OPS draw solutes, the tetraethylenepentamine heptakis(methylphosphonic) sodium salt (TPHMP-Na) exhibits the best FO flux at 0.5 mol/kg concentration, which is further applied for the separation of emulsified oil-water mixture. The recovery of diluted OPS solutions is carried out via a nanofiltration (NF) system with a rejection above 92%. The aforementioned features show the great potential of OPS compounds as a novel class of draw solutes for FO applications.