A rational manipulation of the surface structures and properties of thin-film composite membranes is important to optimize their separation performance and service durability. In this study, a facile strategy is reported for fabricating electroneutral loose nanofiltration membranes based on the rapid co-deposition of biomimetic adhesive polydopamine and poly(ethylene imine) (PEI) by using CuSO₄/H₂O₂ as a trigger. Through this strategy, the surface properties and the filtration performance of the membranes can be easily tailored by the addition of PEI and CuSO₄/H₂O₂, as well as tuning the deposition time. UV, XPS, SEM, AFM, zeta potential, water contact angle and nanofiltration measurements were used to investigate membrane performance. Surface characterization revealed that overall enhanced surface properties including low roughness, favourable hydrophilicity, and relatively neutral charge can be achieved after the addition of PEI. The optimum membranes, with 1 h co-deposition of PDA and PEI, show an ultrahigh water permeability (26.2 L m⁻² h⁻¹ bar⁻¹), distinguished rejection for both negatively and positively charged dyes and a high permeation of divalent salts (>90%). Furthermore, the optimum membranes also show excellent operational stability in long-term nanofiltration operations in alkaline solution. This study provides an efficient and facile approach to tailor the membrane properties and structure for loose nanofiltration membranes.

Reverse electrodialysis (RED) represents one of the most promising membrane-based technologies for clean and renewable energy production from mixing water solutions. However, the presence of multivalent ions in natural water drastically reduces system performance, in particular, the open-circuit voltage (OCV) and the output power. This effect is largely described by the “uphill transport” phenomenon, in which multivalent ions are transported against the concentration gradient. In this work, recent advances in the investigation of the impact of multivalent ions on power generation by RED are systematically reviewed along with possible strategies to overcome this challenge. In particular, the use of monovalent ion-selective membranes represents a promising alternative to reduce the negative impact of multivalent ions given the availability of low-cost materials and an easy route of membrane synthesis. A thorough assessment of the materials and methodologies used to prepare monovalent selective ion exchange membranes (both cation and anion exchange membranes) for applications in (reverse) electrodialysis is performed. Moreover, transport mechanisms under conditions of extreme salinity gradient are analyzed and compared for a better understanding of the design criteria. The ultimate goal of the present work is to propose a prospective research direction on the development of new membrane materials for effective implementation of RED under natural feed conditions.