Positively charged poly (N-vinyl imidazole) gel-filled loose nanofiltration membranes: Performances and modelling analysis

Resource recovery from RO concentrate using nanofiltration: Impact of active layer thickness on performance

Modelling the removal of monovalent and divalent ions from seawater via nanofiltration is crucial for pre-treatment in seawater reverse osmosis systems. Effective separation of divalent ions through nanofiltration and allowing the permeate containing only monovalent ions to pass through the reverse osmosis system produces pure NaCl salt from the concentrate. However, the Donnan steric pore model and dielectric exclusion assume a uniformly distributed cylinder pore morphology, which is not representative of the actual membrane structure. This study analyzed the impact of membrane thickness on neutral solute removal and investigated the effect of two different methods for calculating the Peclet number on rejection rates of monovalent and divalent salts. Results show that membrane thickness has a significant effect on rejection rates, particularly for uncharged solutes in the range of 0.5-0.7 solute radius to membrane pore size ratio. Operating pressures above 10 bar favour the use of effective active layer thickness over the membrane pore size to calculate the Peclet number. At low pressures, using the effective active layer can lead to overestimation of monovalent salt rejection and underestimation of divalent salt rejection. This study highlights the importance of appropriate Peclet number calculation methods based on applied pressure when modelling membrane separation performance.

Nanofiltration Performance of Poly(p-xylylene) Nanofilms with Imidazole Side Chains

Herein, we report the nanofiltration performance of poly(p-xylylene) thin films with imidazole side chains that were deposited onto commercial polyethersulfone ultrafiltration membranes using a chemical vapor deposition process. The resulting thin films with a few tens of nanometers exhibited water permeation under a pressure difference of 0.5 MPa and selectively rejected water-soluble organic dyes based on their molecular sizes. Additionally, thin flaky ZIF-L crystals (Zn(mim)2·(Hmim)1/2·(H2O)3/2) (Hmim = 2-methylimidazole) formed on the surface of imidazole-containing poly(p-xylylene) films, and the composite films demonstrated the ability to adsorb methylene blue molecules within the cavities of ZIF-L.

The coming of age of water channels for separation membranes: from biological to biomimetic to synthetic

Water channels are one of the key pillars driving the development of next-generation desalination and water treatment membranes. Over the past two decades, the rise of nanotechnology has brought together an abundance of multifunctional nanochannels that are poised to reinvent separation membranes with performances exceeding those of state-of-the-art polymeric membranes within the water-energy nexus. Today, these water nanochannels can be broadly categorized into biological, biomimetic and synthetic, owing to their different natures, physicochemical properties and methods for membrane nanoarchitectonics. Furthermore, against the backdrop of different separation mechanisms, different types of nanochannel exhibit unique merits and limitations, which determine their usability and suitability for different membrane designs. Herein, this review outlines the progress of a comprehensive amount of nanochannels, which include aquaporins, pillar[5]arenes, I-quartets, different types of nanotubes and their porins, graphene-based materials, metal- and covalent-organic frameworks, porous organic cages, MoS₂, and MXenes, offering a comparative glimpse into where their potential lies. First, we map out the background by looking into the evolution of nanochannels over the years, before discussing their latest developments by focusing on the key physicochemical and intrinsic transport properties of these channels from the chemistry standpoint. Next, we put into perspective the fabrication methods that can nanoarchitecture water channels into high-performance nanochannel-enabled membranes, focusing especially on the distinct differences of each type of nanochannel and how they can be leveraged to unlock the as-promised high water transport potential in current mainstream membrane designs. Lastly, we critically evaluate recent findings to provide a holistic qualitative assessment of the nanochannels with respect to the attributes that are most strongly valued in membrane engineering, before discussing upcoming challenges to share our perspectives with researchers for pathing future directions in this coming of age of water channels.

Nano-Interlayers Fabricated via Interfacial Azo-Coupling Polymerization: Effect of Pore Properties of Interlayers on Overall Performance of Thin-Film Composite for Nanofiltration

The supporting layer of nanofiltration membranes is critical to the overall nanofiltration performance. However, conventional supports lack efficient surface porosity, which leads to the limited utilization rate of the polyamide (PA) layer. Herein a double-skin-layer nanofiltration membrane with porous organic polymer nanointerlayers prepared via a two-step interfacial polymerization technique is presented to investigate the effect of the interlayers' pore properties on the performance of the thin-film composite. Nanometer interlayers with different pore sizes are fabricated via interfacial azo-coupling polymerization. The pore properties of the nanointerlayer extremely influence the permeance, where a suitable pore size of 4.22 nm promotes pure water permeance of up to 32.2 L m^-2 h^-1 bar^-1, which is ∼3.8-fold greater than the membrane without an interlayer. However, an interlayer with 0.54 nm pores limits the performance (4.7 L m^-2 h^-1 bar^-1), which is even lower than the unmodified membrane (7.5 L m^-2 h^-1 bar^-1), because of the narrow pores and confined transport mode. However, the confined diffusion rate of amino monomers from the support to interface leads to a thinner PA layer of ∼45 nm and results in high flux. This work provides a facial route for the fabrication of interlayers and facilitate the design of high-performance membrane materials with interlayers.