Tailoring the polyester/polyamide backbone stiffness for the fabrication of high performance nanofiltration membrane

A review on polyester and polyester-amide thin film composite nanofiltration membranes: Synthesis, characteristics and applications

Nanofiltration (NF) membranes have been widely used in various fields including water treatment and other separation processes, while conventional thin film composite (TFC) membranes with polyamide (PA) selective layers suffer the problems of fouling and chlorine intolerance. Due to the abundant hydrophilic hydroxyl groups and ester bonds free from chlorine attack, the TFC membranes composed of polyester (PE) or polyester-amide (PEA) selective layers have been proven to possess enhanced anti-fouling properties and superior chlorine resistance. In this review, the research progress of PE and PEA nanofiltration membranes is systematically summarized according to the variety of hydroxyl-containing monomers for membrane fabrication by the interfacial polymerization (IP) reaction. The synthesis strategies as well as the mechanisms for tailoring properties and performance of PE and PEA membranes are analyzed, and the membrane application advantages are demonstrated. Moreover, current challenges and future perspectives of the development of PE and PEA nanofiltration membranes are proposed. This review can offer guidance for designing high-performance PE and PEA membranes, thereby further promoting the efficacy of nanofiltration.

Tailored design of nanofiltration membranes for water treatment based on synthesis-property-performance relationships

Tailored design of high-performance nanofiltration (NF) membranes is desirable because the requirements for membrane performance, particularly ion/salt rejection and selectivity, differ among the various applications of NF technology ranging from drinking water production to resource mining. However, this customization greatly relies on a comprehensive understanding of the influence of membrane fabrication methods and conditions on membrane properties and the relationships between the membrane structural and physicochemical properties and membrane performance. Since the inception of NF, much progress has been made in forming the foundation of tailored design of NF membranes and the underlying governing principles. This progress includes theories regarding NF mass transfer and solute rejection, further exploitation of the classical interfacial polymerization technique, and development of novel materials and membrane fabrication methods. In this critical review, we first summarize the progress made in controllable design of NF membrane properties in recent years from the perspective of optimizing interfacial polymerization techniques and adopting new manufacturing processes and materials. We then discuss the property-performance relationships based on solvent/solute mass transfer theories and mathematical models, and draw conclusions on membrane structural and physicochemical parameter regulation by modifying the fabrication process to improve membrane separation performance. Next, existing and potential applications of these NF membranes in water treatment processes are systematically discussed according to the different separation requirements. Finally, we point out the prospects and challenges of tailored design of NF membranes for water treatment applications. This review bridges the long-existing gaps between the pressing demand for suitable NF membranes from the industrial community and the surge of publications by the scientific community in recent years.

Alginate Hydrogel Assisted Controllable Interfacial Polymerization for High-Performance Nanofiltration Membranes

The deepening crisis of freshwater resources has been driving the further development of new types of membrane-based desalination technologies represented by nanofiltration membranes. Solving the existing trade-off limitation on enhancing the water permeance and the rejection of salts is currently one of the most concerned research interests. Here, a facile and scalable approach is proposed to tune the interfacial polymerization by constructing a calcium alginate hydrogel layer on the porous substrates. The evenly coated thin hydrogel layer can not only store amine monomers like the aqueous phase but also suppress the diffusion of amine monomers inside, as well as provide a flat and stable interface to implement the interfacial polymerization. The resultant polyamide nanofilms have a relatively smooth morphology, negatively charged surface, and reduced thickness which facilitate a fast water permeation while maintaining rejection efficiency. As a result, the as-prepared composite membranes show improved water permeance (~30 Lm⁻²h⁻¹bar⁻¹) and comparable rejection of Na₂SO₄ (>97%) in practical applications. It is proved to be a feasible approach to manufacturing high-performance nanofiltration membranes with the assist of alginate hydrogel regulating interfacial polymerization.

Phenanthroline-Based Polyarylate Porous Membranes with Rapid Water Transport for Metal Cation Separation

The selective separation of ions in terms of extremely similar size and properties remains an important challenge in water purification. We innovated a kind of porous nanofilm via interfacial polymerization using rigid heterocyclic ligands to achieve high valent cation selectivity and rapid water/ion transport. The interconnected microporosity and uniformly distributed cation-affinitive sites of the ultrathin membranes enabled water permeation (7.5 L m^-2 h^-1 bar^-1), ion permeance of Na⁺ (1.5 mol m^-2 h^-1 bar^-1), and Mg²⁺/Na⁺ permselectivity (2.1) during nanofiltration. The forward osmosis exhibited a prominent water flux of 95 LMH at 1 M NaCl draw solution, which expanded various applications. The polyarylate membranes comprising 4,7-diphenyl-1,10-phenanthroline showed a higher water permeation and ion selectivity than the planar monomers, e.g., resorcinol. A distinct fluorescence responsiveness existed between membranes and cations for the interaction characterization. Host-guest nuclear magnetic resonance (NMR) spectroscopy and solid-state nuclear magnetic resonance spectroscopy characterized the preferential affinitive of divalent/high-valent cations in the interconnected microporous powders; an ultraviolet spectrophotometer characterized the light responsiveness of the porous nanofilms. Such an active membrane has potential applications in selective separation and adsorption of cations, photocatalytic materials, photosensors, and other fields.

Tannic Acid/Fe3+ Nanoscaffold for Interfacial Polymerization: Toward Enhanced Nanofiltration Performance

Conventional thin-film composite (TFC) membranes suffer from the trade-off relationship between permeability and selectivity, known as the "upper bound". In this work, we report a high performance thin-film composite membrane prepared on a tannic acid (TA)-Fe nanoscaffold (TFC_n) to overcome such upper bound. Specifically, a TA-Fe nanoscaffold was first coated onto a polysulfone substrate, followed by performing an interfacial polymerization reaction between trimesoyl chloride (TMC) and piperazine (PIP). The TA-Fe nanoscaffold enhanced the uptake of amine monomers and provided a platform for their controlled release. The smaller surface pore size of the TA-Fe coated substrate further eliminated the intrusion of polyamide into the substrate pores. The resulting membrane TFC_n showed a water permeability of 19.6 ± 0.5 L m^2- h^-1 bar^-1, which was an order of magnitude higher than that of control TFC membrane (2.2 ± 0.3 L m^-2 h^-1 bar^-1). The formation of a more order polyamide rejection layer also significantly enhanced salt rejection (e.g., NaCl, MgCl₂, Na₂SO₄, and MgSO₄) and divalent to monovalent ion selectivity (e.g., NaCl/MgSO₄). Compared to conventional TFC nanofiltration membranes, the novel TFC_n membrane successfully overcame the longstanding permeability and selectivity trade-off. The current work paves a new avenue for fabricating high performance TFC membranes.

Antifouling sulfonated polyamide nanofiltration hollow fiber membrane prepared with mixed diamine monomers of BDSA and PIP

A novel high-flux sulfonated polyamide nanofiltration (NF) hollow fiber membrane was made from the mixed monomers of 2,2′-benzidinedisulfonic acid (BDSA) and piperazine (PIP).