Polymeric membranes for desalination

Membrane-based desalination technologies have evolved to be a paradigm for sustainable separation processes, such as removing certain toxic ions from contaminated water to minimize the discharge of concentrated wastewater to the environment and extracting valuable ion species from saline water with lower chemical and energy consumption.1 According to the driving force of separation, current desalination technologies using membranes can be classified into three aspects: pressure-driven nanofiltration (NF) or reverse osmosis (RO), concentration-driven diffusion dialysis (DD), and potential-driven electrodialysis (ED). Among these separation processes, polymeric membranes play a central role but, unfortunately, suffer from the permeability and selectivity trade-off effect.2 This special issue highlights recent design and synthesis strategies, modeling, and applications on polymeric membranes aiming at breaking the limitation. For NF membranes, Li and colleagues report a zwitterionic copolymer modified polyethersulphone/sulfonated polysulphone (PES/SPSf) membrane for enhanced dye/salt selective separation. Two monomers, positively charged N,N-dimethylaminoethyl methacrylate and negatively charged monomer sodium vinylsulfonate, are copolymerized via the UV-light grafting to form a zwitterionic layer on the surface of PES/SPSf blend membrane. The obtained membranes show a low rejection rate of 6.5% for Na2SO4 and a high retention rate of 97% for basic blue 26, leading to a high dye/salt separation performance over a long continuous operation time of 50 h. For RO membranes, nanomaterials are incorporated into polyamide layers to enhance the water flux, while the desalination performance is compromised by the gaps between nanomaterials and the polyamide matrix. Wang and colleagues employ covalent organic frameworks (COF) nanofibers as the nanofiller, which possess a crystalline structure with large surface areas and well-defined micropores, creating a thinner polyamide layer via controlling the release of the amine monomer. Moreover, the rigid and permanent nanopores of COF provide extra water transport channels. The prepared membranes exhibit an enhanced water flux and a slightly improved NaCl rejection due to the reduced thickness and the additional channels. For DD membranes, Li and colleagues introduce auxiliary groups into the anion exchange membranes (AEMs) by the quaternization of brominated polyphenylene oxide and hydroxyl tertiary amine. In terms of the acid recovery application, the fabricated membranes show an increase in the dialysis coefficient of HCl (UH+) from 0.011 to 0.033 m/h, and the separation factor is above 35.6. To further improve the dialysis coefficient, Lin and colleagues report a porous AEM with high free space volume based on a porous substrate. The membrane is prepared by creating the porous chloromethylated polyethersulfone substrate via the nonsolvent phase inversion, followed by the in situ crosslinking with ethylenediamine and quaternization using N-methylimidazole. The resultant dialysis coefficient and acid/salt separation factor (S) are 4.3 and 2.6 times higher than the commercial DF-120 membrane under the same condition, respectively. For ED membranes, Wang and colleagues integrate the anion-selective electrodialysis (ASED) into a pressure-driven membrane separation process to treat waste leachate. The commercial anion-selective membranes achieve a rejection of 72.7% for sulfate ions and 88.4% for chemical oxygen demand, making the ASED process with a low-energy consumption of 1.85 kWh/m3. As an alternative to the commercial high-cost anion-exchange membrane (i.e., AMX), Bazinet and colleagues deposit a thin layer of crosslinked branched polyethyleneimine (PEI)-based ionomer on top of the porous substrate to prepare a cation-coated filtration membrane (CCFM). The energy-efficient fabrication process and the commercial precursor PEI enable a low cost for the CCFM. The obtained membranes display a higher NaCl demineralization rate and a higher whey demineralization rate; even though they have a lower ionic conductance than the AMX membrane. Ge and colleagues have developed an in situ interfacial polymerization (ISIP) technique to prepare monovalent anion-permselective membranes, where trimesoyl chloride (TMC) is used to generate a crosslinked polyester layer on the methyl diethanolamine quaternized membrane surface. More importantly, the unreacted acyl groups of the TMC are hydrolyzed to create anionic COOH groups. The ISIP enables the formation of a negatively charged dense layer, leading to a high limiting current density and a low membrane resistance of 4.7 Ω cm2. The fabricated membrane shows a high flux of 6.3 × 10−8 mol cm−2 s−1 for Cl− and an increased anion selectivity of ~60 for Cl−/ SO 4 2 − . Recently, capacitive deionization (CDI), a promising desalination technology, has attracted extensive research interest. Pan and colleagues report a Ti3C2Tx MXene/polypyrrole (PPy) composite electrode prepared by the in-situ polymerization of conductive PPy on the surface of MXene nanosheets. The hybrid structure integrates the high conductivity of MXene and the electrochemical capacity of PPy and improves the electrode stability by effectively alleviating the self-stacking of the nanosheets. As a result, the MXene/PPy electrode demonstrates a high desalination capacity of 50.64 mg g−1 and an improved desalination rate of 16.5 mg g−1 min−1. The fundamental understanding of ion transport in ion exchange membranes (IEMs) can contribute to the rational design of IEMs, making their applications broader.3 Kamcev et al. review recent progress in modeling ion partitioning and diffusion in IEMs, relating the IEM performance to the corresponding fundamental membrane properties. The Donnan-Manning model for ion partitioning and the Manning-Meares model for ion diffusion, derived from Manning's counter-ion condensation theory, are discussed in detail and have been widely cited in the IEM literature. Besides, the compassion between similar models for predicting IEM transport properties is conducted. Finally, the unique role of the Donnan-Manning and Manning-Meares models is highlighted. We would like to conclude by thanking all the authors, reviewers, and editorial staff at the Journal of Polymer Science for contributing to this special issue. The authors hope this issue can further stimulate the research interests and efforts of polymeric membrane researchers to address significant challenges in the field of membrane-based desalination technology.