Ionic liquid regulated interfacial polymerization process to improve acid-resistant nanofiltration membrane permeance

Polyelectrolyte nanofiltration membranes for base separation and recovery

Nanofiltration (NF) membranes have the potential to significantly advance resource recovery efforts where monovalent/divalent ion separation is critical, but their utilization is limited by inadequate stability under extreme conditions. "Base separation"-i.e., separating hydroxide from other ions-has emerged as an essential approach in resource recovery, enabling the extraction of multivalent anions (e.g., carbonates and phosphates) from hydroxide-rich streams. There is a particularly high demand for membranes capable of separating carbonates from hydroxide-rich CO2 capture solvents and phosphates from hydroxide-rich adsorbent regeneration solvents. However, conventional polyamide NF membranes degrade during long-term exposure to alkaline conditions, limiting their application in extreme conditions. In this study, alkaline-resistant polyelectrolyte membranes are fabricated by depositing alternating layers of polycation, poly(diallyl dimethylammonium chloride) (PDADMAC), and polyanion, poly(sodium 4-styrenesulfonate) (PSS) to a polyethersulfone substrate. The membranes are tested for hydroxide/carbonate and hydroxide/phosphate separation performance, as well as performance stability during prolonged exposure to highly alkaline conditions. Results indicate that higher feed solution pH improves carbonate and phosphate rejection by promoting ion deprotonation and strengthening electrostatic repulsion from the negatively charged membrane. In contrast, increasing carbonate and phosphate concentrations in the feed solution reduces the rejection due to charge screening. The six-bilayer PDADMAC/PSS membrane removes more than 99 % of carbonates and phosphates while allowing extensive passage of hydroxide at pH 13. Stability tests confirm that PDADMAC/PSS membranes maintain excellent ion selectivity over four weeks of exposure to pH 13 KOH, whereas commercial polyamide NF membranes degrade within one week. These findings highlight the potential for PDADMAC/PSS membranes to advance critical resource recovery efforts, providing a durable and effective solution for applications under extreme conditions.

High performance, pH-resistant membranes for efficient lithium recovery from spent batteries

Cation separation under extreme pH is crucial for lithium recovery from spent batteries, but conventional polyamide membranes suffer from pH-induced hydrolysis. Preparation of high performance nanofiltration membranes with excellent pH-resistance remains a challenge. Here we synthesize a high performance nanofiltration membrane (1,4,7,10-Tetraazacyclododecane (TAD)-1,3,5-Tris(bromomethyl)benzene (TBMB) thin film composite membranes (TFCMs)) with excellent pH-stability through interfacial quaternization reaction between TAD and TBMB. Due to the high stability of "C-N" bonds in TAD-TBMB TFCMs, its separation performance is stable even after 70 days immersion in concentrated acid (3 M H2SO4, HNO3, or HCl) and base (3 M NaOH), which is at least 15 times more stable than benchmark commercial membranes. The membrane shows an overall separation performance (11.3 L m-2 h-1 bar-1 (LMHB), RCo2+: 97% in 2 M H2SO4) due to the size sieving and the intensified charge repulsion, outperforming many of the state-of-the-art membranes. Finally, the TAD-TBMB TFCM remains stable during 30-days continuous nanofiltration of 2 M H2SO4 and leachate (2 M H2SO4, ions: 6.2 g L-1) from spent batteries.

Double charge flips of polyamide membrane by ionic liquid-decoupled bulk and interfacial diffusion for on-demand nanofiltration

Fine design of surface charge properties of polyamide membranes is crucial for selective ionic and molecular sieving. Traditional membranes face limitations due to their inherent negative charge and limited charge modification range. Herein, we report a facile ionic liquid-decoupled bulk/interfacial diffusion strategy to elaborate the double charge flips of polyamide membranes, enabling on-demand transformation from inherently negative to highly positive and near-neutral charges. The key to these flips lies in the meticulous utilization of ionic liquid that decouples intertwined bulk/interfacial diffusion, enhancing interfacial while inhibiting bulk diffusion. These charge-tunable polyamide membranes can be customized for impressive separation performance, for example, profound Cl-/SO42- selectivity above 470 in sulfate recovery, ultrahigh Li+/Mg2+ selectivity up to 68 in lithium extraction, and effective divalent ion removal in pharmaceutical purification, surpassing many reported polyamide nanofiltration membranes. This advancement adds a new dimension to in the design of advanced polymer membranes via interfacial polymerization.

Recent Advanced Development of Acid-Resistant Thin-Film Composite Nanofiltration Membrane Preparation and Separation Performance in Acidic Environments

Membrane filtration technology has attracted extensive attention in academia and industry due to its advantages of eco-friendliness related to environmental protection and high efficiency. Polyamide thin-film composite nanofiltration (PA TFC NF) membranes have been widely used due to their high separation performance. Non-acid-resistant PA TFC NF membranes face tremendous challenges in an acidic environment. Novel and relatively acid-resistant polysulfonamide-based and triazine-based TFC NF membranes have been developed, but these have a serious trade-off in terms of permeability and selectivity. Hence, how to improve acid resistance of TFC NF membranes and their separation performance in acidic environments is a pivotal issue for the design and preparation of these membranes. This review first highlights current strategies for improving the acid resistance of PA TFC NF membranes by regulating the composition and structure of the separation layer of the membrane performed by manipulating and optimizing the construction method and then summarizes the separation performances of these acid-resistant TFC NF membranes in acidic environments, as studied in recent years.

Interface-Confined Channels Facilitating Water Transport through an IL-Enriched Nanocomposite Membrane

Improving the permeance of the polyamide (PA) membrane while maintaining the rejection is crucial for promoting the development of membrane separation technology in the practical water-treatment industry. Herein, a novel metal-ionic liquid (Zn-IL) coordination compound was synthesized by in situ growth to improve the water permeance of PA nanofiltration membranes, using an amine-functionalized IL (1-aminopropyl-3-methylimidazolium chloride, [AEMIm][Cl]) as a ligand to react with Zn(NO₃)₂·6H₂O. Piperazine (PIP) and trimesoyl chloride (TMC) were adopted to prepare the PA layer covering the Zn-IL complex. Due to the unique property of the Zn-IL complex, the Zn-IL/PIP-TMC absorbing force to water was increased, enabling the fast transport of water molecules through the membrane pore channels in the form of free water. The resulting Zn-IL/PIP-TMC nanocomposite membrane exhibited a high permeance of up to 26.5 L m^-2 h^-1 bar^-1, which is 3 times that of the PIP-TMC membrane (8.8 L m^-2 h^-1 bar^-1), combined with rejection above 99% for dyes such as methyl blue.